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text-book of botany for colleges.
 
A TEXTBOOK OF BOTANY FOR COLLEGES
THE MACMILLAN COMPANY
NEW YORK + BOSTON - CHICAGO - DALLAS
ATLANTA + SAN FRANCISCO

MACMILLAN & CO., Limitep
LONDON - BOMBAY + CALCUTTA
MELBOURNE

THE MACMILLAN CO. OF CANADA, Ltp.
TORONTO
A TEXTBOOK OF BOTANY

FOR COLLEGES

BY

WILLIAM F. GANONG, Pu.D.

PROFESSOR OF BOTANY IN SMITH COLLEGE

New ork
THE MACMILLAN COMPANY
1916

All rights reserved
CopyrieHt, 1916,

By THE MACMILLAN COMPANY.

Set up and electrotyped. Published August, 1916.

Norwood press
J, 8. Cushing Co. — Berwick & Smith Co,
Norwood, Mass., U.S.A.
PREFACE

Turis book is written in the knowledge that to nearly all
college students an introductory course in Botany is part of
a general education, and not a preparation for a professional
botanical career. The distinction is important because our
existent courses are largely adapted, albeit unconsciously on
our part, to the latter end. The needs in the two cases are
not the same, though the difference is less in matter and
method than in proportion and emphasis. All students alike
need that personal contact with specific realities, and that ex-
ercise in verifiable reasoning, which laboratory courses render
possible. Knowledge, however, is valuable to the specialist
in the proportions of its objective importance, but to the gen-
eral student in the proportions of its bearing on the actions
and thoughts of mankind. In the one case the demands of
the science are paramount and in the other the interests of the
student.

In conformity with its aim, the book gives more attention
to the large and visible aspects of plant nature than to the
minute and obscure. To the general student the things he
can see in the world, and will meet with again, are more im-
portant than those which lie remote from his path, though the
specialist must know both near and remote, because both exist.
Especially the book lays great emphasis upon interpreta-
tion, or the explanation of the “ principle” of things, and the
connections of botanical science with the general body of
knowledge, and man’s direct relations with plants. Indeed
the book may be described as an attempt to present and inter-
pret the humanly important aspects of plant nature in the

v
vi PREFACE

light of our modern scientitic knowledge. While these are
not the matters the specialist needs most to know, I cannot
but think that he also will find advantage in entering upon
his work through this broader portal.

The book is supposed, to be used in conjunction with organ-
ized laboratory work, and to be read for the sake of connecting
the discontinuous though invaluable knowledge won by expe-
rience in the laboratory with the systematized content of the
science, the two being welded thus into one intellectual unit.
This assumption of contemporaneous laboratory work, sup-
posed always to precede the reading, will explain a much
greater generality or abstractness of treatment than would
otherwise be suitable. Since, however, teachers differ much
in their ideas as to desirable sequence and emphasis, I have
treated the various topics in the form of semi-independent
essays, intended to be separately understandable. The method
involves repetition, but permits omission, by sections, where
the material is found overabundant, as it will be for most
students, though it should not prove so for the best.

The fact that the book is prepared for the general student,
whose psychology I have long been studying (when I might
have been better employed, as I know my investigating col-
leagues think), will explain some features not otherwise obyi-
ous. Thus, structure is treated before function, because that
is the more practicable way, even though the reverse is more
logical. Again when the seemingly obvious is elaborated, it
is because experience has shown how different is the aspect
of those matters to the youthful beginner and the mature
specialist. Further, if not all of the newest matters are in-
cluded, it is not necessarily because I do not realize their
scientific importance, but because, in most cases, they seem
either not sufficiently established or not sufficiently prominent
for inclusion in an introductory course. The test of the value
of the book will be found not in whether my colleagues con-
sider it a well-proportioned compendium of botanical fact, but
in whether it leads students to pursue the subject in an inter-
ested and spontaneous spirit.
PREFACE vil

The illustrations are taken from many sources, the best I
could find. I deem it as legitimate to use a good published
picture as a good published idea, of course with due credit;
and, moreover, its use seems such a deserved tribute to its
excellence as its author would desire. Many are taken from
the well-known works of Sachs, Goebel, Kerner, and Stras-
burger, and are so good that none better can be made; and we
should not deprive the student of their use, or waste the labor
of providing inferior new ones, only because through frequent
repetition they have become wearisome to us. Kerner’s work
is issued in translation in this country by Messrs. Henry Holt
& Company, and this firm has given me full permission to use
these pictures, as well as two from Sargent’s Plants and their
Uses, and several from my own book The Living Plant, pub-
lished by them. Also I have used many, by permission, from
publications of The Macmillan Company, and especially from
one of the greatest of botanical publications, the Cyclopedia
of American Horticulture, edited by Professor L. H. Bailey,
who has graciously granted me the privilege of drawing at
will from that work. The Bausch & Lomb Optical Company
have kindly loaned me several cuts from their catalogues illus-
trating apparatus of my own invention made by them. The
new illustrations, comprising about a third of those in the
book, have been mostly drawn by my colleagues in the depart-
ment of Botany at Smith College, —three by Professor Julia
W. Snow, two of the most elaborate by Professor Grace Smith,
several by Miss Helen Choate, and many by Miss Marion
Pleasants. A few of the diagrammatic figures are my own.
These skilled co-workers, with another, Miss Grace Clapp,
have also given me the advantage of their expert knowledge
in a critical reading of the proofs. I am under special obliga-
tion, however, to Miss Choate and Miss Pleasants, who, not only
through their drawings, but also through their constructive
criticisms, have contributed greatly to the merit of the book,
though I claim its faults as wholly my own. To all of these
generous collaborators I express my grateful acknowledg-
ment.
viii PREFACE

Part II, containing the description of the groups of plants,
comprising about 125 pages, is delayed, but is expected to be
ready within a year. It will be issued separately for a time,
but the two parts will also be bound in one volume.

W. F. GANONG.
June 20, 1916.
CONTENTS

INTRODUCTION

CuapTeR I. Tue Score anp VALUE OF Botanical Stupy.

Cuapter Il. Tue DistincrivE CHARACTERISTICS OF PLANTS

THE STRUCTURES AND FUNCTIONS OF PLANTS

PART I

CuapTerR II]. Toe Morpuorocy anp PursioLtocy or LEAVES

CHAPTER
1.
§ 2.
. The cellular anatomy of stems .

. The development of stems and leaves Pen ‘jude

§

g

CO? MP SP 2 “Or
“1D Ove

co

. The distinctive characteristics of leaves
. The structure of leaves

The synthesis of food by light in nies

. The cellular anatomy of leaves.

The characteristics of protoplasm

. The water-loss, or transpiration, from ene
. The adjustments of green tissues to light .

The various forms of foliage leaves

. The forms and functions of leaves other than follape
. The nutrition of plants which lack chlorophyll

The autumnal and other coloration of leaves

. The economics, and treatment in cultivation, of leaves

The uses of the photosynthetic food .

IV. Tue Morpuorocy anpD PayrsioLocy oF STEMS

The distinctive characteristics of stems :
The structure of stems and support of the foliage

The arrangements of Jeaves on stems
The transfer of water and food through abies.

. The growth of stems and other plant parts

ix

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§ 11.
§ 12.
§ 18.

CHAPTER
§ 1.
§ 2.
§ 3.
§ 4.
§ 5.
§ 6.

CONTENTS

The respiration of plants .

The geotropism of stems : : :

The various forms of foliage-bearing stems

The forms and functions of stems not aes. Ww ith
support of foliage .

The monstrosities of stems and leaves :

The economics, and treatment in cultivation, of stems

V. Tue MorrHoLtocy anD PuysrioLocy oF Roots

The distinctive features of roots

The structure of roots

The cellular anatomy of roots

The absorption of water, and other finenote of roots
Osmotic processes in plants

The composition and structure of dette

The self-adjustments of roots to prevailing gendiqons
The additional, and substitute, functions of roots
The economics, and treatment in cultivation, of roots
Summary of the functions and tissues of plants

VI. Tue MorpHotoey anp PuysioLocy oF FLowers .

The distinctive features of flowers

The structure of flowers :

The accomplishment of fertilization < flowets

The nature and consequences of fertilization

The methods and meaning of cross-pollination

Methods of asexual reproduction

The origin and significance of sex

Heredity, variation, and evolution

The methods used by man in breeding better inate
The morphology of flowers

The morphology and ecology of om er eistens

Special forms, abnormalities, and monstrosities of ere
The economics, and treatment in cultivation, of flowers

VU. Tue Morrworocy anp PrysioLtocy or Fruits
The distinctive characteristics of fruits

The structure and morphology of fruits

The dissemination and dispersal of plants

Special forms and monstrosities of fruits .

The nature and cure of plant diseases

The economics and cultivation of fruits

PAGE
162
174
179

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345
345
347
356
366
367
370
CONTENTS

CHapter VIII. Tue Morpnotocy anp PuysioLocy oF SEEDS

§ 1. The distinctive characteristics of seeds :

§ 2. The structure, morphology, and functions of seeds .

§ 8. The suspension of vitality, resting period, and duration
of life in seeds

The germination of seeds .

. The economics and cultivation of desis

. The cycle of development from seed to seed

oy i

OM Qa
a

PART II

THE KINDS AND RELATIONSHIPS OF PLANTS

PAGE

372
872
373

377
381
385
386
ae

 
 
 
 

 
 

 
 

 

 

 

 
 

 

  
A TEXTBOOK OF BOTANY
FOR COLLEGES

INTRODUCTION

CHAPTER I
THE SCOPE AND VALUE OF BOTANICAL STUDY

Tue word Botany came originally from the Greek, where
it meant simply grass, or herbage, especially that of a pasture.
Its meaning, however, has expanded step by step with the
progress of knowledge, until now it embraces every kind of
scientific inquiry about plants, Thus the scope of the word,
as of the science, has indeed become great. In the first
place, plants themselves are wonderfully diverse in appear-
ance, structure, and habits, for they comprise not only the
familiar trees, shrubs, and herbs, with ferns, mosses, and sea-
weeds, but also the mushrooms, molds, yeasts, and germs of
disease and decay. Furthermore, the number of distinct
kinds, or species, is far greater than most people imagine.
/Of plants having flowers, no less than some 133,000 separate
species have already been described and named by botan-
ists, while of the flowerless kinds, which reproduce by spores,
some 100,000 species are likewise known, making 233,000 in
all! It is believed, however, that a good many others re-
main to be discovered, probably enough to bring up the
number of the flowering kinds to 150,000 and of the flower-
less to the same number, making at least 300,000 in all. As
to the kinds of facts which botanists are trying to discover
concerning this multitude of diversified plants, there are
no limitations, because no bounds exist to the intellectual

B Ht
Z A TEXTBOOK OF BOTANY (Cu. I

curiosity of scientific men, nor is there any way of deter-
mining in advance which new facts will prove interesting to
them or important to mankind.

'The study of Botany is pursued for three purposes, —
pleasure, progress, and profit. First, as to pleasure, its
pursuit in any intellectual field is one of the most rational
and elevating of human activities. There are those who
take as much delight in a close personal acquaintance with
plants, or in a clear understanding of their construction and
processes, as others find in a knowledge of literature, history,
art, or the drama; and the one pursuit is entitled to the
same sympathetic approbation as the others. Second, as
to progress, all experience shows that an individual advances
precisely as a race does, — through constant intellectual ef-
fort; and for such exercise there exists no more natural field
than the scientific investigation of the surrounding world, of
which plants comprise the most conspicuous-part. Third, as
to profit, that is clear when one recalls the intimacy of man’s
dependence upon plants for the very essentials of civilized
existence, — for food, shelter, raiment, and medicine, — in
conjunction with the fact that they are readily capable of
improvement under his hand, as attested by the magnificent
flowers, luscious fruits, and nutritious vegetables which he
has developed from insignificant wild ancestors. The fact
that man can make plants serve still better his material uses
would be reason enough, even were there no others, why
he should study them thoroughly.

Thus the science of Botany has a scope far too vast, and
a body of knowledge much too great, for any one mind to
grasp. Therefore it has become subdivided for purposes of
exact investigation. From this point of view, all Botany falls
into four divisions, and they into subdivisions, as follows.
“Ty. Systematic Botany, the oldest and most fundamental
of the divisions, now commonly called Taxonomy, is con-
cerned chiefly with the Ciassirication of plants, that is,
their arrangement in groups in accordance with their relation-
Ca. I] THE VALUE OF BOTANICAL STUDY 3

ships to one another. It includes exact description of the
species, and application of scientific names, which are taken
from Latin, as the principal language of learning. It has
been studied mostly by observation and comparison of the
prominent external parts of plants, especially the flowers
and fruits; and for the convenience of such study, the plants
are preserved in a pressed and dried condition in collections
each called an Herparium. For the use of students and
other workers with plants, the classification, descriptions, and
names of all the plants of a country are embodied synop-
tically in handbooks, commonly called Manuats (or, if
more elaborate, FLoras), so arranged as to enable a student
to find for himself the correct name of a plant previously
unknown to him. An important subdivision of Systematic
Botany is PALEOBOTANY, or the study of the plants which
existed in past ages, as represented in their petrified, or fossil,
remains found in the rocks, — a subject which throws great
light upon the evolution of our present plants from their
remote and very different ancestors.

II. Morpnouoey, second in age of the divisions, is the
study of the parts, or structures, of plants, in comparison
with one another. It therefore bears much the same rela-
tion to the parts of plants that classification bears to plants
as a whole; and it is studied by the same methods of ob-
servation and comparison. When it leads from the large
external to the small internal parts, thus requiring the aid
of the microscope, it takes the name ANatomy, while if it
goes deeper yet, into the minute construction of the ulti-
mate smallest parts (called cells), it is termed CyroLocy, —
the two latter terms together replacing the older term His-
ToLocy. An important phase is Empryrooey, the study ot
the stages in development of the individual before its
birth or germination, all of its stages collectively constituting
its “life-history.”

III. Puystouoey, third in age of the divisions, is precisely
the same study in connection with plants as it is with ani-
4 A TEXTBOOK OF BOTANY (Cm

mals, including mankind, viz., the study of the organic pro-
cesses or functions. It is pursued by the exact experimental
methods of physics and chemistry, and indeed may be de-
scribed as the physics and chemistry of plant life. Dealing
thus with matters of the most fundamental nature, its dis-
coveries frequently prove not only of the highest scientific
interest, but also, as will presently appear, of great economic
importance. One of its phases, that which concerns the
relations of structure and habit to the conditions under
which plants live, has attained to a prominence requiring a
name of its own, viz., EcoLoay, — a term which has largely
absorbed the older word PLANT-GEOGRAPHY, meaning the
distribution of plants in light of its causes. Still more re-
cently another phase of physiology has become prominent,
viz., GENETICS, the experimental study of the facts and
methods of heredity.

IV. Economic Botany, also known as PLant INDUSTRY,
extremely old as an empirical study though very new as a
scientific one, is the investigation of plants with reference
to their improvement for the uses of mankind. It com-
prises a number of well-known subdivisions, viz., scientific
AGRICULTURE, HortTICULTURE, and ForgEstry, with others less
familiar, viz., BacrrrioLocy, the study of disease germs, and
other kinds; PuHarmacoLoey, dealing with drugs; PaTHot-
ocy (PHYTOPATHOLOGY) concerned with the diseases of
plants ; and PLANT-BREEDING, or the systematic attempt to
produce new and superior kinds, — a subject closely inter-
locked with Genetics. Economic Botany is the special
field of Agricultural Experiment Stations maintained by
civilized governments the world around, including the
United States Department of Agriculture and the State Ex-
periment Stations and Agricultural Colleges in this country,
excepting that Bacteriology belongs primarily to the Medical
Schools. The other three divisions, Systematic Botany,
Morphology, and Physiology, are cultivated particularly in
the Universities.
~

Cu. TJ THE VALUE OF BOTANICAL STUDY oO

These divisions, and subdivisions, of Botany are pri-
marily determined by convenience of study, especially with
reference to the methods and instruments employed. Hav-
ing really no natural boundaries, they intergrade and inter-
lock very closely, on which account the progress of one
depends upon progress of the others. Thus, most phases
of Economic Botany are so dependent upon Physiology in
particular, that the greater Experiment Stations, main-
tained primarily for economic research, are well-nigh as
active in Physiology as are the Universities. This case
is typical of the relation which exists everywhere between
economically useful and scientifically abstract knowledge.
The history of civilization has shown that the greater ap-
plications of science to human welfare, as exemplified in
electricity, wireless telegraphy, or the control of germ diseases,
have arisen not from researches directed to secure useful
results, but incidentally as by-products of purely abstract
investigations made in the pursuit of knowledge without
thought of material returns. All experience shows that
knowledge is a unit, of which economically useful knowledge
is only an ill-defined and changing part ; and the surest way to
gain new useful knowledge is first to win new general knowl-
edge, which is possible only through scientific research.
For this reason the student who aspires to become a leader
in any economic pursuit must first make himself master
of its general or abstract knowledge. Such is likewise the
reason for the emphasis laid in education as a whole upon
subjects having no apparent economic utility.

The facts known about plants being so multitudinous,
amounting it must be to millions, and far beyond com-
prehension by any one person, the student may well ask
how it is possible to acquire that general understanding of
plants implied in an introductory course, and textbook, of
Botany. It is simply thus. The diversity of plants, so
extensive and obvious, is really superficial, and rests upon
foundations of similarity, which, deep, obscure, and dis-
6 A TEXTBOOK OF BOTANY (Cu. I

coverable only by prolonged investigation, are relatively
few in number. By utilizing these deep-lying resemblances,
it is possible to link together great masses of facts in gen-
eralized form, and thus bring the principles of botanical
knowledge within the comprehension of one person, who
may then pursue in detail any particular phase which his
pleasure or business may dictate.
CHAPTER II
THE DISTINCTIVE CHARACTERISTICS OF PLANTS

Tue Universe, wrote the great Linnzus in the sonorous
Latin of the ‘Systema Nature,” comprises everything which
can come to our knowledge through the senses. Tue Srars are
very distant luminous bodies which circle in perpetual motion,
and are either Frxep Stars shining by their own light like the
Sun... or Praners deriving their light from the Fixed Stars.

Tue Eartru is a planetary globe, rotating in twenty-
four hours, moving in an orbit around the sun once a year
. and covered by an immense mantle of NaruraLt OBsEcTS

the exterior of which we try to know... . Narvrat Opsects
. are divided into three Krnepoms or Nature, MINERALS,
Piants, and Antmats.... Pianrs are organized bodies

which live but do not feel (or as we say, are not conscious).

Such is the place in nature of plants, which the botanist
is trying to know.

Of these plants there are many distinct kinds or spEcIEs,
probably some three hundred thousand, as noted already.
Each species, however, consists of thousands, or millions,
or perhaps billions, of InprvipuatL plants.

Individual plants, of the familiar kinds, are each composed
of six primary parts, — LEAVES, STEMS, ROOTS, FLOWERS,
FRUITS, and SEEDS. Each part performs a particular pri-
mary function to which it is fitted in structure. In the ex-
panded thin green LEAVES food is made for the plant, under
action of sunlight, from materials drawn from the air and
the soil. The columnar elastic branching stEMs spread and
support the leaves in the indispensable sunlight. The
slender Roots, radiating and ramifying through the soil,

(
8 A TEXTBOOK OF BOTANY (Cu. Il

absorb the water and mineral salts needed by the plant, to
which they give also a firm anchorage in the ground, The
showy and complicated rLowers effect fertilization, which
is requisite in all sexual reproduction. Fruits, whether
dry like pods, or edible like berries, are concerned with the
formation and dissemination of seeds. The compact hard-
coated SEEDS, containing each an embryo plant and food
supply, separating from the parent plant, and remaining for
a time dormant, provide a transportable stage whereby
plants are spread. Thus each of the six primary parts per-
forms a definite function in the economy of the plant as a
whole, and each part is therefore, from the physiological
point of view, an orcan. In addition each of these organs
performs functions connected with its own individual ex-
istence, notably GROWTH, RESPIRATION, and SELF-ADJUST-
MENT to the surroundings.

The external form of these primary parts, visible to the
unaided eye, is correlated with a definite internal anatomy,
revealed by thin sections viewed through magnifying lenses.
Thus studied, the parts are found composed of definite and
symmetrically arranged differentiations of structure called
TISSUES, having each its distinctive position, color, and
texture, and each performing a definite part of the organ’s
function. Thus the veins and green pulp are tissues of the
leaf, as are bark, wood, and pith of the stem, though some
of the latter are further divisible. These tissues in turn,
when viewed by the compound microscope, are found wholly
composed of very small structures called cELus, which ap-
pear as compartments separated by firm walls and holding
various contents. Of these contents the most important is
the PROTOPLASM, a mobile, gelatinous material, the seeming
simplicity of which is belied by its many remarkable prop-
erties. It is really the protoplasm which performs the
functions of the plant, and which builds the cells, and there-
fore the tissues and organs, suited in structure to the work
which is done by the respective parts.
Cu. IT] CHARACTERISTICS OF PLANTS 9

While typical plants all have the same organs, they are
not all alike, but differ greatly in habits, aspect, and details
of structure. Some are TREBs, tall, long-lived, and single-
trunked, forming the canopy of forests. Others are SHRUBS,
shorter and less lasting, branching from the ground, and
forming the typical undergrowth. Others are HERBS, smallest
and shortest-lived of all, soft-bodied and mostly green
throughout, forming the carpet vegetation of the earth.
Then there are plants which grow supported upon others,
the cLIMBERS and EPIPHYTES: and the plants of strange
aspect found in the deserts: and the WATER-PLANTS, in-
cluding the seaweeds: and all of the great number of the
small and simple PARASITES, which occur everywhere amongst
other plants. Some kinds possess organs other than those
we have mentioned, such as TENDRILS, PITCHERS, and
TUBERS, always associated with special habits ; but these parts
prove on comparative study to be mostly transformed leaves,
stems, or roots, though not all special structures have this origin.

The organs develop in the individual plants in definite
predetermined cycles. Every plant normally originates in
a fertilized EGG CELL, as does the animal in an egg. The
egg cell, lying within the ovule inside the flower, is a
microscopic protoplasmic sphere, at first without organs;
but in the course of development it forms a stem and a few
leaves, in which stage it is an EMBRYO within a seed inside a
fruit. When, after dissemination, the seed germinates, the
embryo develops a root, and more stem and leaves, becom-
ing a SEEDLING, and with further repetition of those parts,
ultimately an ApuLT plant. Then it begins reproduction
by developing FLowers, in which sexual cells, EGG CELLS
and SPERM CELLS, are formed and brought together, making
new fertilized egg cells, thus closing the cycle, which is re-
peated in perfect regularity, generation after generation.

Plants are not, however, merely aggregates of parts per-
forming present functions, but include many relics of their
10 A TEXTBOOK OF BOTANY (Cu. II

lives in the past. The evidence seems to show beyond
question that our present species of plants have descended
by gradual evolution from simpler and fewer species which
formerly existed, and which in turn were evolved from still
simpler and fewer kinds, —back, it is possible, to a single
kind which throve in remotest antiquity. In the course of
this evolution, plants have diverged into the many groups,
and groups within groups, expressed in our schemes of classi-
fication. Thus also various features originally distinctive
of one species came to prevail through whole families, and
even persist to the present, often having lost completely
their original significance. It is the aim of botanists to
distinguish between those features which have merely a tem-
porary functional significance and those which are deeply
fixed in heredity. They use the former in the interpreta-
tion of the phenomena of plant life, and the latter as guides
to evolution and classification. Hence botanical study falls
most fundamentally into the two phases represented by the
two Parts of this book.

While the groups and classification of plants will receive
full treatment in Part II, some general knowledge of the more
important of such facts is essential to an understanding of
Part I. The main groups, with their essential character-
istics, are the following.

1. THe Frowertne Pants, the most highly evolved
and therefore often called the ‘higher plants,’ comprise the
great majority of the trees, shrubs, and herbs constituting
the familiar land vegetation. They are distinguished not
only by the possession of flowers, which often are extremely
inconspicuous, but also, and especially, by their seeds, on
which account they are called sc entifically SpERMATOPHYTES,
that is, “seed plants.” While mostly they dwell on the land
with roots in the ground, and make their food in. their
green leaves, some live in water, and some upon other
plants. They are clearly descended from the following
Cu. IT] CHARACTERISTICS OF PLANTS 11

group, which is much older, as shown by fossil remains in
the rocks.

2. THE FERNS AND THEIR KIN, called scientifically Preri-
DOPHYTES or “‘Fern plants,’’ comprise not only the familiar
true Ferns, but also the less prominent Horsetails and Club
Mosses. They have no flowers, but reproduce by small one-
celled spores and a definite though not prominent sexual
stage. They live chiefly on land, have green leaves, and
make their own food. They are mostly undergrowth plants,
though some in the tropics become trees. They have evolved
(it is likely but not certain) from the following group, and
were formerly more prominent than now, having once formed
great forests, the earliest of such vegetation.

3. THE MossES AND THEIR KIN, called scientifically
Bryopuytes or ‘Moss plants,’ comprise the true Mosses
with the Liverworts. They reproduce like the Pteridophytes,
by spores and a sexual stage. They have green leaves and
make their own food, but they rise little from the ground,
on which they grow densely together, thus forming the
simplest carpet vegetation of the earth. They are de-
scended from the Algw, and were probably the first plants
to cover the land.

4. THe Mo.tps aND THEIR KIN, called scientifically
FUNGI, comprise a great number of small or minute plants
most of which are found associated with the disease and
decay of plants or of animals, e.g., mushrooms, yeasts,
molds, rots, rusts, mildews, and bacteria, — popularly known
as microbes or germs. They occur in the most diverse situa-
tions, but always in contact either with living tissues, upon
which they live PARASITICALLY, or else with dead organic
substances, upon which they live sapRopHYTICALLY. They
are most diverse in forms, sizes, colors, and other features,
in accordance with their particular habits, but never show
the green color of the higher plants. They reproduce by
minute spores, which are carried everywhere by the winds,
thus explaining how those plants can occur in so many
12 A TEXTBOOK OF BOTANY (Cu. IL

situations. They are undoubtedly descended, as shown by
many resemblances in structure, from the Algze ; and so close
are their relationships that, from the point of view of classi-
fication, the two groups are properly included in one, called
THALLOPHYTES, though in practice it is convenient to treat
them separately.

5. Tur SEAWEEDS AND THEIR KIN, called scientifically
ALG&, comprise not only the red and brown seaweeds and
“sea mosses” (which are green underneath those colors),
but likewise many green kinds both of salt and fresh water.
They live mostly under water, make their own food in their
fronds, have diverse shapes with different habits, and re-
produce both by simple spores and sexual stages. They are
the simplest and most ancient of the leading groups, and
the one from which the others are descended.

Alge, Fungi, Bryophytes, and Pteridophytes are often
called collectively Cryprocams, because their reproduction
was once thought obscure, while the Spermatophytes are
called PHanERoGAMS, because their reproduction, through
flowers, was considered evident.

It is the primary aim of science to discover, analyze, de=,
scribe, and classify the elemental facts of nature. It is a
secondary aim to explain phenomena with which the facts are
connected, though to all except specialists the explanations
are hardly inferior in interest to the facts themselves. In
this book, while the description of fact always comes first,
explanations follow promptly after. The explanations of
the phenomena exhibited by living plants fall under four
categories. First, a great many features, especially those
connected with the obvious fitness of form and structure
to functions and habits, are best explained, in the opinion
of a majority of biologists, as result of a process of gradual
ADAPTATION of the modifiable plant to the unmodifiable
physical surroundings during the course of evolution. Second,
other features are clearly survivals, of no other present sig-
Cx. II] CHARACTERISTICS OF PLANTS 13

nificance, from ancestral- forms, as noted already under
HEREDITY. Third, plants are still in process of evolution, :
and hence, for causes and by methods still unknown, are con-
stantly developing new features called variations, or better,
—mutTaTions. Fourth, the adaptations, the heredity, and
the mutations of plants are all more or less affected, and
even in some degree directed, by the chemical nature of the
materials they are composed of, and the physical forces playing
upon them from the world in which they live; and on this
account many of their features have a purely incidental, or
mechanical, or, as we may designate them collectively, sTRUC-
TURAL significance. Thus the actual plant embodies the
resultant of the simultaneous action upon it of adapta-
tional, hereditary, mutational, and structural, with some
other minor, factors. It is the task of the botanist to dis-
tinguish and separate the various influences which make the
plant what it is, for which purpose he needs above all an
open mind, a willingness to weigh all forms of evidence, and
freedom from the human but unscientific tendency to adopt
some single favorite viewpoint and explain all phenomena
therefrom. Many matters in science are interpreted dif-
ferently by equally competent investigators, but discussion
and further investigation always bring the truth, for the
recognition of which we have only one test, —it is that
upon which the great majority of competent investigators,
after full and disinterested investigation, agree.

The generalized statements of this chapter are intended
to enable the student to approach his study with better
understanding. We turn now to the concrete facts and
phenomena of plant nature.
 
PARE YL

CHAPTER III

THE MORPHOLOGY AND PHYSIOLOGY OF LEAVES

1. Tur DISTINCTIVE CHARACTERISTICS OF LEAVES

LreAVES are the most abundant and conspicuous of plant
parts, collectively constituting foliage, the most distinctive
part of vegetation. Their essential features consist in their
green color, flat form, and growth towards light. Their
prominence is explained by their function, which consists
in the exposure of green tissue to light, under action of which
the plant forms its food out of water and mineral matters
drawn from the soil, and a gas received from the air. This
function is all the more important because the food thus
formed serves not only for plants, but ultimately for all
animals as well.

Although uniform in their primary function, foliage
leaves show much diversity in various features. In size,
some are almost microscopic, most are a few square inches
in area, and a few are measured in feet. In shape, some are
nearly circular, others almost needle-form, and others of
diverse intermediate gradations. In color, while typically
green, some are gray, white, yellow, or red; and in autumn
they often display a brilliant succession of colors. In tex-
ture, some are flaccid, as in water plants, others almost
leathery, as in evergreen trees, while most are intermediate,
with a flexible-elastic consistency. In duration of life, they
are typically temporary, lasting but one season, and even in
evergreens for only a few years; but cases occur in which the

leaves persist as long as the long-lived stem. In only one
15
16 A TEXTBOOK OF BOTANY (Cu. III, 1

feature do foliage leaves vary little and that is the thickness,
or rather the thinness, of their green tissue, which is nearly
the same no matter what their sizes and shapes.

The thin flat expanse of green tissue, called the BLADE,
is always the essential, and often the only, part of the leaf.
In many kinds, however, the blade is provided with a slender,
cylindrical stalk, called the PETIOLES, various in length even
up to several feet ; and upon it the
blade is adjusted to the light, and
has free play in the wind. In addi-
tion, some kinds possess a pair of
small appendages, one on each side
of the base of the petiole, called
STIPULES, which, though usually
green like the blade, are very diverse
inform. Blade, petiole, and stipules
are parts of a complete leaf, of which
a typical example is pictured here-
with (Fig. 1).

In some kinds of leaves, es-
pecially large ones, the blade is
not all one piece, but is cleft more
or less into divisions, as familiar in

 

Fie. 1.—A leaf of the
Quince, showing blade, petiole, Oak or Maple. The same process

and stipules; reduced. (After

Ga ee continued much farther results in

the formation of separate LEAFLETS,
each with a stalk of its own, as in Rose or Strawberry
(Fig. 37), while the leaflets also may become themselves
subdivided, even more than once, as in some kinds of Ferns.
Such leaves are called compounpD, in distinction from SIMPLE,
the two being distinguishable by the fact that the leaflets
of a compound leaf always stand in one flat plane, while
simple leaves are distributed around a stem, at least at their
bases. JTurther, leaflets have no buds in their axils, but
leaves, whether simple or compound, always do.
While typical leaves, the kinds designated foliage, are
Cu. III, 2] STRUCTURE OF LEAVES 17

thin, flat, and green, and perform the function of food forma-
tion, other kinds exhibit different features and other func-
tions, as familiar, for instance, in tendrils and _ pitchers.
Likewise there are parts which seem to be leaves but are not,
as in case of some flattened stems, and even roots; for leaves,
while the principal, are not the only green parts of plants.

2. THE STRUCTURE OF LEAVES

Typical, or foliage, leaves, despite their external multi-
formity, possess an essentially uniform anatomical struc-
ture, as shown by comparative observation.

The most conspicuous and important part of the leaf, that
in which the food is formed, is the green tissue, called cHLo-
RENCHYMA, which is singularly uniform in thickness, texture,
and color throughout the leaf blade. Its distinctive green
color is not, however, an integral part of its structure, but
a separate and easily removable substance. One has only
to place a leaf in a glass dish, cover with alcohol, stand in
a warm place, and leave for a time, when the green will
come out in a beautiful clear solution, leaving the leaf a
uniform white. This soluble green substance is called
CHLOROPHYLL, and is one of the most important substances
in nature, as will presently appear.

Second in prominence is the system of vEINs, which
ramify everywhere throughout the chlorenchyma. They
are essentially bundles of tubes which conduct materials
into and out of the chlorenchyma. Most commonly they
taper and branch from the base of the blade towards the
margin, simultaneously producing small veinlets which
interlace to a network, as seen very clearly when held up
against the light. In other kinds of leaves, such as Grasses,
the main veins are uniform in size, and run parallel, or
gently curving from base to tip, the veinlets in this case
being minute or even wanting; and such leaves are called
PARALLEL-VEINED in distinction from the former, or NETTED-
VEINED kinds (compare Figs. t-and-2 with 34). If, further,

Cc
18 A TEXTBOOK OF BOTANY [Cu. III, 2

some typical leaf, e.g. from one of our common trees, be
held up against the light and examined with a hand lens,
one can see very clearly that the ultimate meshes of

xe,
BRU EAAC

ee .S
2 j
ue

Fic. 2.— The vein systems of English Ivy
(above) and Silver Poplar; reduced. (From
The Phantom Bouquet, by Edward Parrish, 1565.)

The pictures were drawn from specimens
“skeletonized’ by removal of the chloren-
chyma. A magnifying lens should be used to
render visible the ultimate veinlets.

layer by which all leaves

 

 

the network of vein-
lets inclose little
polygonal areas of
pure chlorenchyma,
into which — often,
though not always,
extend free tips of
the tiniest veinlets
(Fig. 2). This ulti-
mate relation of
veinlets and chlor-
enchyma is impor-
tant, as will later
appear. Thesmall-
est veinlets are
buried within the
leaf blade, but the
larger ones and the
veins which are pro-
gressively thicker
towards the leaf
base, swell gradually
out from the blade
on its under side
until they become
many times thicker
than the ever uni-
form ehlorenchyma.

Third is the Ept-
DERMIS, 2 very thin
and transparent

s are covered, and which often displays

a shining surface when viewed obliquely towards the light.
Cu. III, 3] SYNTHESIS OF FOOD 19

It is practically waterproof, and thus prevents desiccation
of the soft leaf tissues when exposed to the sun and dry air.
While tightly adherent, as a rule, to the chlorenchyma and
veins, it can sometimes be stripped away, if started with a
knife, from leaves of the Lily-like kinds, while from some
of the Houseleeks (or “Live for ever’’) it can be loosened
by pressure of the fingers, and later blown out, as most
children well know. Commonly the epidermis appears per-
fectly continuous and homogeneous, but in exceptional
cases (e.g. Wandering Jew), the hand lens will show, espe-
cially on the under side of the leaves, tiny slit-like pores in-
closedin greener ovals. These slits, called stomata, are always
present, even though rarely visible to a hand lens. They are
real openings, which connect with microscopical Ark PASSAGES
extending everywhere through the leaf, and having great
functional importance, as will soon appear. Also the epi-
dermis, while typically smooth even to shining, often bears
divers sorts of fine hairs or scales, called TRICHOMES, which
give to the leaves a grayish, woolly, or sometimes scurfy
appearance whereby often the clear green of the underlying
chlorenchyma is obscured.

The petioles of leaves, typically cylindrical in form, consist
mostly of veins, with little overlying chlorenchyma; but
they develop commonly some additional strengthening tissue.
The stipules, when present in typical form, have simply the
leaf structure in miniature. A

3. Tue SynTHESIS OF Foop By LIGHT IN LEAVES

The prominence of leaves, in conjunction with their com-
parative uniformity of structure, indicates for them a very
fundamental function in plant life. This is well known to
consist in the formation of food, which, as one of the most
important of all processes in nature, will here be described
somewhat fully.

All leaves are found by chemical tests to contain sugar,
mostly the kind called grape sugar, which occurs dissolved
20 A TEXTBOOK OF BOTANY (Cu. III, 3

in their sap. Under action of sunlight this sugar increases
in quantity, but in darkness it lessens, because removed
through the veins to the stem. Furthermore, in most leaves,
when this sugar increases beyond a certain percentage the
surplus becomes automatically transformed into starch, which
returns again to grape sugar as the percentage thereof once
more falls. Now it happens that starch (unlike sugar) is
readily recognizable by a striking and easily applied test,
viz., addition of iodine in solution, which turns starch dark
blue; and thus we are provided with a convenient means of
proving the increase
of sugar, as manifest
in its transformation
to starch, under action
of light. The experi-
ment is well-nigh
classic, and every
student should see it.
One has only to keep
a thin-leaved potted
plant for a day or
two in the dark (to
cause the disappear-

Fic. 3.— A light sereen for experiments ance of its starch):
in starch formation by leaves; x }. cover part of a leaf,

The star is cut from tinfoil attached to : aetna s
glass, and the box excludes light but admits air, 1 FA ay atop to pre
vent its ordinary func-

tions, with some kind of contrasting light-and-dark screen,
such, for example, as shown in our picture (Fig. 3) : expose the
plant to strong, but not intense, light for two or three hours :
place the leaf in warm alcohol until the chlorophyll is re-
moved: and cover the blanched leaf with a solution of iodine.
Then a striking result appears, for the parts left in light
by the screen all turn dark blue, and the parts which were
shaded remain white, or at most a little browned by the
iodine (Fig. 4). Thus it is clear that the starch, and there-

 
Cu. III, 3] SYNTHESIS OF FOOD 21

fore the sugar, increases in quantity under action of light.
Indeed so exactly quantitative is this relation of light to
starch-formation that, with certain practical precautions,
one may apply a photographic negative to a leaf, and after
exposure to light develop a very fair positive ‘‘blue-print”’
of the picture with iodine.

The increase of the grape sugar in light is found by ex-
periment to add weight to the
plant. Therefore the sugar must
represent not a transformation of
material already present, but a new
construction out of materials drawn
from outside the plant; and all
research confirms this conclusion.
Further, suitable tests always show
that its formation takes place only
in light and only in gréen tissues,
which never occur away from the
light. Its production indeed is the
particular primary function of the
chlorenchyma, wherever found,
whether in leaves, stems, or other
parts, —the leaves being organs
adapted to spread chlorenchyma to Tian ge =A daar treated
light. The formation of the sugar with iodine after exposure to

2 ; light under the screen of Fig.
being thus a process of synthesis 3: x3. The black shading
under action of light, is known as Tepresents dark blue in the

actual leaf.
PHOTOSYNTHESIS.

What now are the materials from which the grape sugar
is constructed ?

The chemical formula of grape sugar is CsHi20., which
means of course that its molecule is composed of six
atoms of carbon, twelve of hydrogen, and six of oxygen.
Now the proportions Hy»O, in this formula recall the
familiar H.O, suggesting that water may be the source
of that part of the sugar, at least of its hydrogen; and

 
22 A TEXTBOOK OF BOTANY (Cu. HI, 3

this hypothesis is fully confirmed by research. The water
is absorbed into the plant from the soil through the roots,
conducted through the stem, and distributed through the
veins to all parts of the chlorenchyma, from which its
immediate evaporation is prevented by the waterproof
epidermis. As to the carbon, that is known to come
not from the soil (for plants can be grown to perfection
in soils, or even in water,
which lackit completely),
but from the air, in which
it exists in the form of
carbon dioxide (COs),
the heavy poisonous gas
which is released by com-
bustion and also by the
respiration of animals.
It is true, this gas is
relatively scarce in the
atmosphere, of which it
comprises only about .03
per cent (3 parts in

* —- 10,000) as compared with
Fre. 5.— Leaves treated with iodine

after exposure to light in air lacking and about 21 per cent of

possessing, respectively, the usual carbon oxygen and 79 per cent
dioxide; x}. The black shading represents - Bs - >
dark blue in the actual leaf. of nitrogen ; but even

this small amount suffices
for the photosynthetic needs of plants, as can be proved in
various ways. Thus, one has only to keep a thin-leaved plant
for a day or two in the dark to free it of starch: remove two
similar leaves and place them in water in two glass chambers
exactly alike except that from one all carbon dioxide has been
removed by a chemical absorbent: expose them thus a few
hours to light: blanch them of chlorophyll: and immerse
them in iodine, when there follows the result pictured here-
with from an actual experiment (Fig. 5). Thus it is clear
that a leaf can make starch, and therefore sugar, if the car-

 
Ca. III, 3] SYNTHESIS

OF FOOD 23

bon dioxide of the atmosphere is available, but otherwise not.
Carbon dioxide cannot pass through the walls of the water-
proof epidermis (at least not in appreciable quantity), but
it enters the leaf through the slit-like openings, the stomata,

the function of which is thus
explained. From the stomata
it moves along the air pas-
sages to every part of the
chlorenchyma.

The formation of grape
sugar from carbon dioxide
and water is expressed by

the following equation, which
exhibits the extremes, though
not the intermediate steps, of
the process.

6 CO, +6 H,O = CeHO¢ +6 Oo_

Now this equation implies
that in the formation of the
sugar, free oxygen pro-
duced in volume precisely
equal to that of the carbon
dioxide absorbed. This theo-
retical deduction can Teadily
be tested by experiment, by
means of appliances pictured
herewith (Figs. 6 and 7); and

1s

 

 

 

 

 

 

Fic. 6.—A simple arrangement
(seen in section) whereby it can be
proved that oxygen is released by
green tissues in light; x }.

The gas released by the water
plant is caught in the water-filled
test-tube supported above, and sub-
sequently tested.

thus the actual production of oxygen, in the indicated vol-
ume, is conclusively proved, and all parts of this photo-

synthetic equation are found exactly true,

It expresses

concisely and accurately one of the greatest of all natural

processes.

The absorption of carbon dioxide and release of oxygen
thus shown to occur in the photosynthetic formation of

grape sugar in leaves explains

the widely known fact that
24 A TEXTBOOK OF BOTANY (Cu. III, 3

plants (really only
green plants in the
light) ‘‘purify the
atmosphere,”’ that is,
remove from it the
noxious carbon diox-
ide released by ani-
mals in their respira-
tion (and by all com-
bustion), and replace
it by oxygen essential
to animal respiration.
Thus is a_ balance
maintained between
the two kingdoms.
The oxygen released
in photosynthesis
represents merely an
incidental by-product
of the process.

The amount of
sugar made in a given
time per unit area
of leaf has been deter-
mined for a number
of plants, and shows,
as would be expected,
much diversity. The
average of these fig-
ures, however,  ex-
pressed in the nearest

 

 

 

 

 

 

 

Fia. 7.—A photosynthometer, by which the
gas exchange in photosynthesis is quantitatively
tested; x 4.

Into the chamber containing the leaves a known quantity of earbon dioxide is ad-
mitted through the stop-cock from the graduated tube above. After exposure to light,
analysis of the gas in the chamber is made by absorption in the graduated tube by aid of
the tavo reagent tubes shown below on the left. The result can beread directly on the grad-
uated tube, as shown on the left, where the approximate 28° indicates the oxygen present
‘at the closeof an experiment in which 10%% of carbon dioxide had been added to the tube.
Cu. III, 3] SYNTHESIS OF FOOD 25

round number, gives us a useful conventional expression, or
constant, for the process as a whole, even though it has no
validity as applied to any particular plant. This coNVEN-
TIONAL CONSTANT for photosynthesis, assuming the usual
conditions of light, is 1 gram of grape sugar per square meter
of leaf area per hour. This amounts to 10 grams per average
working day, or 1500 grams per summer season, for that
area. In the process 750 cubic centimeters of carbon dioxide
are withdrawn from the atmosphere each hour, and the same
volume of pure oxygen returned thereto; and this amounts
to 7.5 liters per day, and 1124 liters per season for the same
area. These figures are for plants out of doors in summer ;
for greenhouse plants in winter they approximate to half
this amount. It- will interest the student to convert these
quantities into the more familiar terms of square yards,
ounces, and quarts; and it will prove better yet if he see
them all actually reproduced before him. Further, for the
sake of those to whom statistics appeal, more figures may
be added. In a season an average leaf produces enough
grape sugar to cover itself with a solid crystalline layer a
millimeter thick, which is 40 times thicker than the chloren-
chyma which makes it; and in the process it absorbs enough
carbon dioxide and releases enough oxygen to form a column
of the same area as the leaf 1.125 meters high; and this is
all of the carbon dioxide in a column of air 3750 meters or
2.4 miles high. To balance the oxygen absorbed and carbon
dioxide released in the respiration of an average man for a
year, there is needed 150 square meters of leaf area working
through the summer; or in other words, to balance his
respiration for a year a man needs all of the oxygen which
would be released in a summer by the walls of a cubical
room of leaf surface 5 meters on an edge.

We have still to explain why both light and chlorophyll
are essential to the photosynthetic formation of grape sugar.
Before the elements contained in the carbon dioxide and
water can be recombined into sugar, they must first be
26 A TEXTBOOK OF BOTANY (Cu. III, 3

separated, in part at least, from their existent unions in
those substances. But both carbon dioxide and water are
very stable compounds, and therefore’ their dissociation or
separation into their constituent atoms requires the applica-
tion of much power, the basis of which is energy. This
energy is known to be supplied by the sunlight, of which
the réle in photosynthesis is thus explained. Now the
energy in the light cannot of itself effect this dissociation
(else obviously no carbon dioxide or water vapor could re-
main in the atmosphere), and accordingly there is also neces-
sary some agency by which the energy in the light can be
applied to the actual work of dissociating or splitting the
molecules of carbon dioxide and water into their constituent
atoms. That agency appears to be the chlorophyll, though
it is not yet certain in precisely what way it accomplishes the
result. Thus the sun supplies the energy for photosynthesis,
and the chlorophyll applies it as power to the actual work.
This is why both are essential.

The study of chlorophyll by aid of the spectroscope shows
that practically only certain red and the blue rays are ab-
sorbed by chlorophyll from the many contained in the
white sunlight; but these are known to be the rays effec-
tive in photosynthesis. Since those rays are absorbed, they
do not come to our eyes from the leaves; but the unabsorbed
rays, those useless in photosynthesis, reach our eyes in a
mixture which collectively gives the sensation of green.
Thus the greenness of vegetation is due to the light rejected
by the chlorophyll after removal of the rays useful in photo-
synthesis.

The photosynthetic formation of grape sugar is often
compared with a process of manufacture carried on by man.
The leaf is the factory constructed for the work: the epider-
mis forms the external walls, giving shelter from weather,
while the chlorenchyma cells are the working rooms, and
the veins, with stomata and air spaces, the passages
for access and removal of materials; the sunlight is the
Cx Hit, 3] SYNTHESIS OF FOOD a7

source of power, and the chlorophyll the machinery by
which it is applied to the work: carbon dioxide and water
are the raw materials, sugar the desired manufactured prod-
uct, and oxygen an incidental by-product. The comparison
while fanciful in details, is correct in essentials.

Grape sugar is, however, not the only food material formed
in the leaves, for they are also the places of construction of
PROTEINS. These are substances of the greatest importance
in plant life, because they constitute the foundational ma-
terial of the living protoplasm. They are composed of the
elements of the grape sugar,—carbon, hydrogen, and
oxygen, — together with nitrogen, sulphur, and phosphorus
derived from mineral compounds absorbed from the soil and
brought to the leaves with the water. Proteins, though
many and diverse, are all constructed from grape sugar
by chemical addition of the other constituents, — nitrogen
first, and the others later. Unfortunately we know little
as yet, despite many researches, as to their exact place of
formation in the leaves,’ whether in the veins or the
chlorenchyma. They occur abundantly in the veins, along
which they are conducted into the stem. Nor is it certain
whether light is essential to their formation, though the
evidence seems to show not, in which case the energy needed
in their synthesis must be supplied by chemical action.
Probably their formation in the leaves is only a functional
convenience based on the simultaneous presence there of
the basal grape sugar and the needful mineral matters,
brought with the water. These proteins, like the grape sugar,
move continuously along the veins from the leaves to the
stems.

The réle of the grape sugar thus formed in leaves is very
fundamental in plant life. First, from it, or from the pro-
teins built upon it, plants build, by minor chemical trans-
formations, their entire structure, and form all of the many
organic materials in their bodies, as will later appear in detail
in a separate section. Second, the energy of the sunlight,
28 A TEXTBOOK OF BOTANY (Cu. III, 4

used in forming grape sugar, does not become obliterated in
the process, but is simply converted into the latent or po-
tential form. Thus the grape sugar becomes a store of

_ potential energy, which is retained through the later trans-
formations, and which can be released and rendered again
active by the process of respiration, as we shall later describe
in full. Grape sugar, accordingly, and its derivatives are the
source both of the materials and the energy used by plants
in their growth and work, or, in other words, are their Foon.
Furthermore, since all animals are dependent upon plants,
either directly or indirectly, for their food, the photosynthetic
grape sugar is the basal food for all animals also.

This use of the term plant food may seem strange to those
who know the common application of the word to the min-
eral salts taken by plants from the soil. The latter usage,
though well sanctioned by custom, especially in connection
with agriculture, is physiologically erroneous. Food, in the
physiology of both animals and plants, is that material from
which the living body is constructed, and energy obtained
forits work. It is because the mineral salts of the soil supply
only an insignificant fraction of the substance of plants and
none at all of their energy that they cannot be considered
plant food, while the name belongs properly to grape sugar,
which supplies both. The popular usage arose before these
matters were understood, but is too firmly fixed to be changed.
No confusion can arise if one takes note of the connection
in which the word is employed.

4. THe CELLULAR ANATOMY oF LEAVES

The actual process of photosynthetic food-formation is
performed in the cells of the leaf, to which we now turn at-
tention. For this study we use the compound microscope,
which is the indispensable tool of the biologist, and one of
the most powerful and perfect of all the exact instruments
which scientific men have invented to extend the range and
precision of our limited senses.
Cu. III, 4] ANATOMY OF LEAVES 29

When the microscope is turned directly upon a leaf, it
shows little, because the tissues as a whole are opaque. But
if from a typical leaf a very thin slice or section be cut across
from surface to surface, it will show under the microscope
the general aspect presented in our picture (Fig. 8). Promi-

é
Ae

Sex

Fic. 8.— A cross section through a typical leaf, that of the European
Beech; greatly magnified. The shaded round and oval grains are green in
the living leaf. (Drawn, with slight changes, from a wall chart by L. Kny.)

    
     

nent in the view are the three tissues of the leaf, — the abun-
dant chlorenchyma, distinguished by the presence of chlo-
rophyll (in the shaded discoid grains of our picture): the
veins, compact and without color (of which a large one
shows, on the left): and the transparent epidermis, which
covers both surfaces. Also amongst the chlorenchyma
can be seen the various irregular and interconnecting azr-
passages. The cells composing these tissues are individually
30 A TEXTBOOK OF BOTANY (Cu. III, 4

visible, — each a compartment inclosed by a wall and con-
taining various contents.

The chlorenchyma cells are inclosed by thin walls, and
contain three kinds of contents. Most prominent of all
are the chlorophyll grains, or CHLOROPLASTIDS, discoid in
form, and uniformly dyed by the chlorophyll, which does not
occur outside them. These chloroplastids have this great
importance, that they are the actual seats of the photo-
synthetic process. Within the same cells occurs also an
inconspicuous, shadowy-grayish, thin-gelatinous material
(shown by a sparse dotting in our picture), the PROTOPLASM,
the living material which builds all the rest. The proto-
plasm, which contains the chlorophyll grains embedded
within it, forms in these cells only a lining to the walls,
against which it is held tightly pressed by the CELL sap.
This sap is water containing sugar and other substances
in solution ; and not only does it fill the whole cavity of the
cell, but is ordinarily under tense pressure, sufficient not
only to hold the lining of protoplasm against the wall, but
also to keep the elastic wall itself somewhat stretched.
The chlorenchyma cells are variously shaped, — spheroidal,
ellipsoidal, ovoid, cylindrical,—as our picture shows.
The cylindrical shape prevails towards the upper surface,
where the ce ls occur tightly packed together, forming the
so-called PALISADE (as distinct from the sPoney) tissue; and
thus.the greater part of the chlorophyll grains are brought
towards the best-lighted surface. This is the reason for
the familiar fact that most leaves show a deeper green color
on their upper than on their lower faces.

When a vein is cut squarely across, as shown in our picture,
its cells appear angular, compact, and colorless. Three kinds
of cells appear in each vein. Firs’, is an outer or sheath
layer forming the BUNDLE-SHBATH, large and thick-walled
with thin protoplasmic lining. When seen in lengthwise
section they are found to be several times longer than wide.
They are most developed on the largest veins, thinner on
Cu: TIT] ANATOMY OF LEAVES ol

the smaller, and very thin on the ultimate veinlets; and their
function appears to be mainly that of conducting sugar from
the leaf into the stem. Second, within this sheath, towards
the lower side, occur many small, angular, thin-walled cells
with protoplasmic linings, which, seen lengthwise, are found
greatly elongated and crossed here and there by distinctive
perforated plates (Fig. 106), though in the veinlets they are
much simpler in structure (Fig. 9). These are the stpve-
TUBES and associated cells, and their function is principally
that of conducting the proteins made in the leaves to thestem.

    

i ee ee

   

Fic. 9.—A leaf veinlet, in longitudinal section, of Fuchsia globosa ;
greatly magnified. Above are the tracheids, and below are sieve tubes and
associated cells, but the sheath cells do not show in the drawing. (From
Haberlandt’s Physiological Plant Anatomy.)

Third, just above the sieve-tubes lie a number of somewhat
larger, angular, thick-walled cells, lacking a protoplasmic
lining; they are found, when seen lengthwise, to run to-
gether into tubes, which are distinguished by characteristic
spiral and other markings (Fig. 101), though in the veinlets
they are only spirally marked elongated cells (Fig. 9).
The function of these tubes and cells, called respectively
DUCTs and TRACHEIDS, is the conduction of water from the
stem to all parts of the leaf. Ducts and sieve-tubes, the
former always above and the latter below, in conjunction
with the sheath cells, make up the veins, which when large
contain many of all three kinds, but when smaller progres-
sively fewer, until finally the ultimate veinlets may consist
of no more than the equivalent of a single duct and a sieve-
tube.

Although every chlorenchyma cell performs photosyn-
32 A TEXTBOOK OF BOTANY (Cu. III, 4

thesis, and therefore must receive water from a duct and
transmit its sugar and proteins to bundle-sheath and sieve-
tube, many of them, as implied in Fig. 8, stand some
distance removed from the nearest veinlet. It is known,
however, that chlorenchyma cells can draw water, and like-
wise pass soluble substances, from one to another, the physical
methods whereof we shall
presently consider. Now
the distances through
which this method is
effective must of course
be limited, and while no
exact measurements
appear to have been
made, it seems highly
probable that the size of
the ultimate areas of
chlorenchyma inclosed by
the veinlets (as noted on
page 18) is correlated

Fic. 10.— Typical epidermal cells, with with the number of chlor-
guard cells, in outline, seen from the sur- enchyma cells which can

face; magnified to same scale. On the left - : e ae
Allium, on the right Sunflower. thus effectively obtain

 

their water, and remove
their sugar or proteins, through one another.
* The cells of the epidermis are rectangular in section,
though when viewed from the surface, they are found vari-
ously shaped, even to lobed and interlocked (Fig. 10). They
contain protoplasm, but ordinarily no chlorophyll (in the
higher plants); and their walls, as proved by chemical tests,
are infiltrated with a special substance called curin, which
renders them waterproof. Especially characteristic of epi-
dermis is the fact that its continuity is unbroken except. for
the stomata, of which a single example appears in our picture
(Fig. 8, also 22). Stomata, however, which provide the
entrance and exit for carbon dioxide and oxygen, are by no
Cu. III, 4] ANATOMY OF LEAVES 33

means mere gaps in the epidermis, for each is flanked by two
special cells called the GUARD CELLS, which close and open
the stomatal slit in ways, and under conditions, later to be
noted.

The picture of our typical leaf (Fig. 8) shows that the
stoma opens into a specially large air space. This space
is continuous with others, and with passages in a con-
tinuous but irregular system which ramifies everywhere
through the chlorenchyma, extending even in thin vertical
passages (not clear in our figure, though shown by suitable
sections) amongst the densely packed cells of the upper, or
palisade, chlorenchyma. Thus every cell of the chloren-
chyma is reached by the air system, and therefore can re-
ceive carbon dioxide from the air; and by the same route
the waste product oxygen is returned to the atmosphere.
The air system is not constructed of cells, but is INTER-CEL-
LULAR, being formed by a splitting and separation of the cell
walls in the course of their development.

The leaf of our picture happens to possess a smooth
epidermis, but where trichomes are present the epidermal
cells can be seen to extend into one-celled, several-celled,
or many-celled hairs, scales, or prickles. Sometimes the
chlorenchyma also has part, as with many prickles, in which
case the structures are called EMERGENCES. Some of the
cells inside the leaf, as shown by a single example in our
picture (Fig. 8), contain crystals, which are excretions, or
matters useless to the leaf and thus disposed of; and such
single specialized cells are called InIoBLasts.

The mechanism of the leaf as a photosynthetic organ for
the production of food sugar from carbon dioxide and water
is sufficiently well known to permit its representation by a
diagrammatic plan, as given herewith (Fig. 11). The student
should now understand the process so well that with a good
section of leaf before him, perhaps aided by our diagram, he
can see it proceeding as clearly in imagination as he could
with the- physical eye were he sufficiently small to wander

D
34 A TEXTBOOK OF BOTANY (Cu. III, 4

at will through the intercellular passages, and view the opera-
tions through the crystalline walls of the cells. Thus he
would see the water streaming in continuous current through
the ducts of the veins to the veinlets, and spreading thence
from cell to cell through walls and protoplasm until it satu-
rates every chlorophyll grain. Simultaneously the molecules
of carbon dioxide are moving in through the stomata and

 

 

Fie. 11.— Plan of the leaf as a photosynthetic mechanism. The chloro-
phyll grains (darkest shaded) are embedded in protoplasm (lighter shaded) ;
the water (horizontal lines) is brought by the duct (which lacks proto-
plasm but has a spirally-thickened wall), and saturates every part of the leaf,
sap-cavities, and walls, except the outer walls of the epidermis; the sugar
(crosses) and proteins (crossed circles) are removed in the protoplasm-lined
sheath and sieve cells; the air-passages ramify to every cell, and open
through the stomata to the atmosphere.

along the air passages, then through walls and protoplasm
to the same chloroplastids. On these green plastids falls a
flood of white sunlight, from which the chlorophyll Stops the
effective red and blue rays, and turns their vibratory energy
against the assembled molecules of carbon dioxide and water,
which are thereby dissociated or shattered into their con-
stituent atoms, with an immediate recombination thereof
imto grape sugar and free oxygen. The molecules of the
sugar, dissolved in the omnipresent water, diffuse from cell
to cell through protoplasm, walls, and sap to the nearest
Cu. III, 5] PROTOPLASM 35

veinlet, of which it enters the sheath cells and there passes
along the veins to the stem, while the proteins in like manner
pass into and along the sieve-tubes. Meantime the mole-
cules of oxygen are moving out of the chloroplastids through
protoplasm and wall to the nearest air passages, and along
them to the stomata and the external air, passing the entering
carbon dioxide en route. The movement of these materials
in their paths is of course impelled by definite and adequate
forces, and the mechanism is capable of continuous action,
which proceeds without break so long as the conditions remain
favorable. Meantime something similar, as to the details
of which we are ignorant, must be happening in the synthesis
of proteins. That is what every green leaf is doing every
bright day through the summer.

5. Tur CHARACTERISTICS OF PROTOPLASM

All study of physiological processes leads directly to pro-
toplasm, the living part of the organism. It is a perfectly
definite material, with distinctive appearance and properties,
and it alone, of all the innumerable materials or substances
in nature, is alive. In Huxley’s famous phrase, protoplasm
is the physical basis of life.

Despite its importance, the protoplasm of plant cells has
an appearance so inconspicuous as to make it most difficult
either to describe or to represent in pictures. Therefore
in order to understand it, one must see the material for
himself in the laboratory.

In most plant cells, as in those of the leaf lately studied
(page 29), the living protoplasm is rendered almost in-
visible by the thicker and denser walls which inclose it.
However, many epidermal hairs have walls so transparent
as to show the protoplasm clearly, in which case the mi-
croscope reveals an aspect like that of the accompanying
picture (Fig. 12). The protoplasm here extends not only
as a lining around the walls of the cylindrical cell, but also
in irregular threads across the sap cavity. Protoplasm in
36 A TEXTBOOK

this state has an appearance

servers agree in likening to a je

 

Fic. 12. — The appearance of the
protoplasm in a typical hair-cell of
a Gourd, as seen projected against
a black background; greatly mig-

nified. (Reduced from Sachs,
Lectures on the Physiology of Plants.)

BOTANY (Cu. III, 5

OF
and texture which most ob-
lly, a rather thin and clouded
jelly, which holds various
small solid bodies, mostly
food grains, in suspension.
Scientifically, its constitution
is described as colloidal. In
the oldest cells it often be-
comes even more thin and
watery than here, though
hardly ever a true fluid;
and the clouded appearance
often vanishes, leaving the
protoplasm nearly transpar-
ent, in which case it is almost
completely invisible unless
killed and dyed by special
stains. In much _ younger
cells, it is more viscous, be-
coming a gelatinous solid;
and in resting seeds and
buds, which have given up
most of their water, it be-
comes even as firm in tex-
ture as dry gelatine or horn.
Since some of the food parti-
cles have a yellowish tint, a
large mass of such proto-
plasm has a distinctly yellow
color, as seen in the young
growing tips of roots, or the
central parts of young ovules.
There is usually an obvious
relation between the condi-
tion of the protoplasm) in

these respects and the function of the cell.
Cx, IIT, 5] PROTOPLASM 37

A characteristic feature of the living protoplasm in plant
cells is its STREAMING, manifest by a steady movement of
the included particles which obviously are carried along
passively by currents of the protoplasm itself. In some
cells, especially the very large ones of certain Algz, the
streaming is so active, even up to 10 millimeters per minute,
that the protoplasm seems literally to rush across the field
of a high-power objective, while in others, and especially in
young cells completely filled by the protoplasm, special
methods are required for its detection; and all intermediate
degrees occur. The streaming is maintained by energy re-
leased from food by the protoplasm, and apparently it serves
to promote the commingling and transportation of substances
throughout the cell.

Thus it is evident that protoplasm possesses no visible
mechanical constitution such as might be anticipated in
so remarkable a material. But what is its real ultimate
constitution or texture, which cannot be as simple as it looks?
The exéeptional interest of this problem has stimulated the
most profound researches, supported by the most refined
methods, but as yet without satisfactory result. It was
formerly thought, from the appearance of material which
had been killed, stained, and sectioned, that the working
protoplasm consists of a tangle of flexible fine fibers holding
the food granules and various fluids in their meshwork.
Later researches, however, seem to show that it has rather
the nature of a foam or emulsion, commonly obscure but
demonstrable by special methods, in which small globules
of various dimensions and different materials are suspended
and held apart by thin films of a certain continuous sub-
stance; while variously intermingled are food granules, and
other small bodies of uncertain significance (Fig. 13). Proba-
bly the usual ground structure of most protoplasm is thus
ALVEOLAR, though it develops fibrous elements on occasion.

Thus the physical structure of protoplasm, in so far as
known, gives little clew to the source of its remarkable
38 A TEXTBOOK OF BOTANY (Cm. TTT. 5

powers. Its chemical composition, however, is more il-
luminating, for research has shown that protoplasm is not
a single substance, but a mixture of many, numbering dozens
in even the simplest known organisms
(Fig. 14). These substances are vari-
ous in complexity, from the simplest
inorganic salts, through the sugars
and other carbohydrates, to the dis-
tinctive proteins, which include the
most highly elaborate and unstable of
natural chemical compounds. The
proteins, indeed, seem to represent the
essential basis of the protoplasm, the
other substances being more or less

Fic. 13.—Protoplasm Secondary or incidental. These many
from the hair cell of sybstances, some of which would react
a Malva, showing with : ¥
‘musual clearness the With one another, obviously cannot
alveolar structure; very exjst heterogeneously intermingled
highly magnified. (Re- Siece
drawn from Bitschi, Within the same solvent, but must
eo Foams and occur in some definite organization.

one Herein, probably, is to be found the
significance of the emulsion or alveolar structure of proto-
plasm, wherein the different substances are kept apart in
their own separate globular compartments by the neutral
continuous substance, which permits, however, upon occa-
sion, those regulated interminglings and reactions upon
which depend the vital phenomena. At least it seems very
clear that most of the physiological powers of protoplasm rest
far more upon a chemical than a physical basis.

This consideration of the chemical constitution of proto-
plasm inevitably raises the question, —is there among its
chemical substances some one which is the distinctive living
substance and to which all the others are subordinate, or
do the vital powers inhere in the organization of the mixture,
no one constituent being itself alive? We do not yet know.
Both views have their advocates. The former fits best w

 
Cu. III, 5] PROTOPLASM 39

the vitalistic conception of organic nature held by some
biologists, and the latter with the mechanistic conception
held by others.

Protoplasm is unique in possessing simultaneously two
sets of properties, physical and physiological. Its physical
properties, — color, den-
sity, weight, hardness, etc.,
— are of course simply the
aggregate of the proper-
ties of its many con-
stituent substances. Its
physiological properties
are those which are pecul-
iar to itself as the living
material. They are mani-
fest most clearly in the
physiological processes of
plants which they make
possible; and we need
here but give, for the

 

Fic. 14.— Portion of the body (plas-
sake of completeness, and modium) of a Slime-mold; x 225. Such
rather for future reference ©t2nisms, which are naked flat masses

A of protoplasm often several square inches
than present learning, the in area, provide ample material for chem-

ical analysis of the substance. (From

mere roll of their names, Biche eth
Sachs, ures.

viz. automatism, regula-
tion, metabolism, mobility, division, growth, irritability,
heredity, variability, morphological plasticity.

All protoplasm originates, and therefore all organisms
arise, in only one way, so far as known, and that is by
growth and division (or reproduction) of preéxisting proto-
plasm. SPONTANEOUS GENERATION, or the formation of
protoplasm anew out of non-living materials, is not known
to occur anywhere in nature; for all supposed cases thereof
when investigated by scientific methods have been found
to be only apparent and not real, as Pasteur was the first to
prove. Thus we can trace back all existent living beings
40 A TEXTBOOK OF BOTANY [Cu. III, 5
in an unbroken protoplasmic succession to the very first
living organism of the earth. As to the source of the pro-
toplasm of that first being we know nothing, though we
have two hypotheses, both of which may be groundless.
One relies upon an original case of spontaneous genera-
tion, even though perhaps never repeated. The other makes
protoplasm itself an evolu-
tion from earlier and simpler
substances, suited to the dif-
ferent earlier conditions of
the earth, and thus carries it
back to an origin contempo-
raneous and equi-causal with
the origin of non-living mat-
ter. The former is rather the
mechanistic, and the latter
the vitalistic view of the
subject.

There remains one very
important characteristic of
protoplasm, and that is its

 

Fie. 15.— A typical example, in

Mistletoe, of the continuity of proto-
plasm by threads through the cell

walls. The walls have been made to
swell in order to render the threads
more clearly visible. (From Stras-
burger, Jost, Schenck, and Karsten,
Text-book.)

organization within the indi-
vidual plant or animal. In
most organisms the proto-
plasm is subdivided into the
microscopically small masses
constituting the cells. This

subdivision, however, is not complete, for suitable methods
always show that through the cell walls run protoplasmic
threads, which, though extremely fine, suffice to keep the
different cells in physiological continuity (Fig. 15); and such
threads seem to unite all of the living cells of a plant into
one protoplasmic system.

Within each cell the protoplasm shows a definite organi-
zation, clearly exhibited in typieal form in our Figure 12, and
represented in principle in our generalized picture, Figure 16.
Cu. III, 5] PROTOPLASM 41

Most abundant, though often not most prominent, is the
gelatinous-mobile cyroruasM, which is clearly the working
part of the cell,— that which transports materials, builds
the wall, produces chemical reactions, and the like. Next in
prominence is the NUCLEUS, a rounded body of denser but
still gelatinous, or colloidal, consistency, lying in the cyto-
plasm. It seems clearly the control organ of the cell, exert-
ing upon the work of the cytoplasm an influence which
guides the building of the organism along the general lines
of its heredity. Inside the
nucleus is often a smaller
NUCLEOLUS, which con-
sists of a store of nutritive
matter used by the nu-
cleus. Third in promi-
nence in most plant cells
come the PLASTIDS, em-
bedded in the cytoplasm,
also of denser gelatinous
consistency, with rounded

 

Fie. 16.— A generalized plant cell, show-
or discoid forms. They ing the constituent parts, in optical sec-

tion.
serve as seats of food for-

mation, the most prominent kind being the chloroplastids.
In some cells also, a fourth protoplasmic structure has been
‘ newly recognized, viz., the very minute elongated bodies
called CHONDRIOSOMES or MITOCHONDRIA, as to the nature
of which, however, we as yet know little.

Such are the protoplasmic parts of the typical plant cell.
In addition, most cells possess a firm wall, built by the
cytoplasm, and composed of a firm-elastic water-permeable
substance called CELLULOSE. The wall has the obvious func-
tion of a support to the protoplasm, which is far too soft to
support itself; and the collective walls of all the cells con-
stitute a firm skeleton for the plant. In young and small
cells the protoplasm completely fills the space within the
wall, but as they grow older and larger, rifts, filled with sap,
42 A TEXTBOOK OF BOTANY (Cu. III, 5

appear in the cytoplasm, and these rifts enlarge and run
together until they form a single great central sap-filled
cavity; and thus the cytoplasm is left as a thin lining inside
the wall, against which it is held tightly pressed by the pres-
sure of the sap. Obviously the arrangement is one which
gives a maximal spread of surface with the minimal amount
of protoplasm; but spread of much surface is an obvious
functional need of an organism which has a mode of nutrition
requiring extensive expos-
ure to light, and a wide
range in the air and the
soil. Within the sap cav-
ity occur also various cell-
contents, —food grains,

ae special secretions, crystals,
ie O C and others, — according to
the respective functions of
the cells.

The details of cell struc-
ture, especially the shape,
size, thickness, and compo-
sition of the wall and the

character of the contents,
prtidoal asset tmmcutines tthe are most diverse in dif-
They are all derivable, by more rapid ferent tissues, though ex-
Foe arte ree thal hibiting usually an obvious
center. With these shapes occur all relation to the particular
degrees of eae of the walls: (Re- functions of the respective
duced from Ganong, The Living Plant.)

parts (Fig. 17). This rela-
tion between structure and function becomes even clearer
when the study is extended to animal cells, which also are
protoplasmic ; for here the cell construction is dominated by
the very different habits of animals, which are freely and
actively locomotive instead of sedentary and passive. The
protoplasm of animals and plants is, however, the same in

all essentials, and the organisms are so different only because

\
2
Y

 
Cu. III, 6] TRANSPIRATION FROM PLANTS 43

of their very different habits, centering especially in their
different ways of acquiring their food.

6. THe Water Loss, on TRANSPIRATION, FROM PLANTS

A special feature of the physiology of leaves, and other
green tissues, is the constant loss of water therefrom to the
air, —a matter which profoundly influences the forms and
distribution of plants. It is called scientifically TRANSPIRA-
TIon, and the student should not permit the resemblance
between this word and respiration to confuse in his mind the
two processes, which are wholly unrelated.

The general fact that much water evaporates from plants is
well known to all who grow them. The rapid wilting of shoots
when cut but not placed in water, is visible evidence thereof.
The water which gathers in drops on the glass covers of ferner-
ies, or on windows in which house plants are kept, has mainly
this origin, though of course it comes partly from wet soil. The
reality of the transpiration from the green parts, as distinct
from evaporation from the soil, can be shown very perfectly
by the arrangement pictured herewith (Fig. 18); for only the
leaves and stem are inside the closed chamber, the pot and soil
being excluded by a special glass plate. Within a few minutes
some water appears on the glass, at first as a faint vaporous
cloud, and later in large drops which run down the sides.
Thus we have a perfect demonstration of transpiration, or
the removal of water as vapor from leaves and young stems.

The precise amount of transpiration can be determined
in several ways, but most accurately by weighing, which
requires potted plants. To secure transpiration without
evaporation from soil and pot, we use the arrangement shown
in our picture (Fig. 19). When a plant thus prepared is
weighed at intervals on a good balance, the transpiration is
determined exactly, and since the cover may be raised and
known quantities of water added at intervals, the experi-
ment may be continued as long as desired. By this method
it is found that living green parts in the light never wholly
44 A TEXTBOOK OF BOTANY [Cu. III, 6

cease transpiration, though its amount may be insignificant,
while it ranges all the way up to above 250 grams per
square meter of leaf area per hour. The conventional con-

 

 

 

 

 

 

Fie. 18. — A conclusive demonstration of trans-
piration; X 4. The bell jar was dry when placed
over the plant. Its bottom is a plate split and
perforated in such a way as to fit closely around
the stem of the plant.

stant (page 25) for
greenhouse plants
is 50 grams per
square meter per
hour by day, and
10 by night, or
30 night and day
together, or 720
grams per 24 hours.
This amounts to
108,000 grams per
season, which
equals a layer of
liquid water all
over the leaf some-
what more than a
decimeter deep;
and presumably
this figure will
prove higher for
plants out of doors
in the summer. If
one can see the 720
grams transpired
in 24 hours stand-
ing in a measuring
glass in the center
of a square meter
of surface, he will

realize better the most striking fact about transpiration, —
its remarkably large amount. All of this water, it must be
remembered, has to be absorbed by the roots from the soil,

and lifted through the stem.
Cu. III, 6] TRANSPIRATION FROM PLANTS 45

Little less surprising than the copiousness of transpiration
is the variability in its amount. Much depends upon the
character of the plant, for, in general, thick-leaved compact
kinds transpire less than thin-leaved open sorts, and hairy
less than smooth kinds, and slow-growing less than quick-
growing, though occasional surprising exceptions to these
rules occur.
But it also
varies greatly
at different
times in the
same plant, as
shows very
clearly when
a plant is
weighed  fre-
quently, or
still better, is
made to write
upon a drum
of a transpiro-
graph (Figs.
20, 21) a con-
tinuous rec- Fic. 19.— A plant prepared for weight-determinations

dof ats of the amount of transpiration; X 1}.

ord oF 1s own A thin aluminum shell covers the pot, and the roof is
transpiration tubber, which may be lifted at will for watering and

: aérating the soil.

day and night

for a week or longer,—the proper arrangements of course
being made to insure that all water loss shall take place from
the plant alone (as in Fig. 19). If simultaneously, whether
by personal observation or by use of recording meteorolog-
ical instruments, records are taken of the conditions of
weather, — temperature, humidity, light, winds, — the reason
for the fluctuations in transpiration is found. For thus it
becomes clear that the rate of transpiration is increased by
light, heat, dryness (of the air), and winds, and is lessened by

 
46

A TEXTBOOK

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OF BOTANY

[Cu. ILI, 6

darkness, cold, humidity,
and calm. This is assum-
ing an ample supply of
water in the soil, under
conditions for easy ab-
sorption, since otherwise,
of course, transpiration is
mechanically checked by
lack of available water.
Thus it is evident that
transpiration is affected
by external influences in
precisely the same way as
evaporation, thereby rais-

Fic. 20.— The Transpiro-
graph; x 4. The plant, pre-
pared as shown by Fig. 19, is
adjusted on a balance in such
a way that when it has tran-
spired one gram of water, that side
of the balance rises and closes
an electric circuit. The current
acts on the electro-magnet (visi-
ble in the picture), which pushes

a pen against the
revolving time drum
(shown by the lines
and letters), and
simultaneously re-
leases from the ver-
tical tube a spherical
gram weight, which
runs through the
outlet tube on the right and
drops into the scale pan. The
latter is thus depressed, breaking
the circuit, which remains open
until another gram of water has
been lost. Compare the record
in Fig. 21.

Such a precise and continu-
ously self-acting instrument. is
typical of those which it is the
aim of plant physiologists to pro-
vide for all of the plant processes.
Cu. III, 6] TRANSPIRATION FROM PLANTS 47

ing the question as to the
relation between the two
processes. While closely
related, they are not iden-
tical, as shown by the
modern studies on RELA-
TIVE TRANSPIRATION, that
is, the ratio between tran-
spiration and the contem-
poraneous evaporation, as
determined by suitable in-
struments. In brief, tran-
spiration is evaporation
affected considerably by
the structure and physi-
ology of the leaf.

The profound effect of
external conditions upon
transpiration has many
important consequences.
Thus, a conjunction in
high degree of light, heat,
dryness, and winds, as
happens at times in our
gardens, can cause wilting
in some plants even when
they have ample soil
water, because the roots
cannot absorb, or the
stems conduct, water as
fast as transpiration re-
moves it. In such cases
a check in the transpira-
tion, by the coming of
night or a spraying by the
gardener, is promptly fol-

5

©

 
48 A TEXTBOOK OF BOTANY [Cu. III, 6

lowed by a revival of the leaves. It is apparently a similar
excess of transpiration over absorption or conduction which,
no matter how abundant the root water, limits the kinds
of plants we can grow in the dry air of our houses; for
house plants, as well known, are not so much those we want
as those we can make grow. It is clearly the defective
absorption by roots, which absorb slowly at low tempera-
tures, in conjunction with excessive transpiration, which, on
bright, dry, windy days in early spring, causes the drying,
browning, and death in ornamental evergreens ; and likewise
a wilting, browning (called wind-burn), and death, in the bud-
ding foliage of deciduous plants. The winter-killing of
shrubs, as we shall see later, is also largely identical in nature.
But the effect of light, heat, dryness, and winds upon tran-
spiration shows most clearly of all in the vegetation of those
parts of the earth where such conditions prevail in conspicu-
ous intensity, —the deserts. For there, as well known, and
represented in pictures in Part II of this book, the thin-
leaved, open types of plants cannot grow at all, and only
those sorts can manage to exist which are compact and
thick of texture, or have other transpiration-limiting fea-
tures. The aggregate effect is the peculiar and even some-
what bizarre appearance characteristic of desert vegetation.
» What now is the physiological meaning of transpiration,
this water-loss which cannot be wholly stopped even though
at times it endangers the existence of plants, and greatly
restricts their distribution? The cellular anatomy and
physiology of leaves give the answer. All chlorenchyma
tissues are continually saturated with water, the direct evap-
oration of which is prevented by the waterproof epidermis.
This epidermis is practically impermeable to the carbon
dioxide required by the leaves in their food-forming function,
and also to the oxygen released in that process ; but the access
and exit, of those gases take place through the stomatal
openings. When these stomata are open for such gas
passage, however, there is nothing to prevent the water of

 
Cu. III, 6] TRANSPIRATION FROM PLANTS 49

the chlorenchyma from evaporating through them, and it
does so. The result is transpiration, which is thus primarily
not a function in itself, but an incidental accompaniment of
the food-forming process. The formation of a given amount
of food requires a definite amount of carbon dioxide, and
this means so much open stoma, and Bertone loss of Pa
in definite mathematical BS
proportions.

The stomata are slit-
like openings which de- 4
velop by separation of ~
the walls of the young &
epidermal cells. In so |
far as the passage of —
gases is concerned, they
might to advantage re-
main permanently open;
but in fact they open and
close, with a proportion-
ate effect upon transpir-
ation. The opening and
closing in each case is ;
produced by action of 2x3 Ee
two neighboring epider- — Fic. 22. — A typical stoma, with guard
mal cells, specialized as cells, of Thymus, seen from the surface,

! : and in cross section. The operation of the
GUARD CELLS (Fig. 22), guard cells is explained in the text. (After
of which the walls are so 4 Wall-chart by L. Kny.)
thickened as naturally to spring the cells together, thus clos-
ing the stoma; but the absorption of more water into the
sap-cavities rounds out the cells and draws them apart, thus
opening the stoma to a slit, a spindle form, or even, at an
extreme, to an almost circular opening. Thus the mechanism
is such that when the cells of the leaf are collectively losing
water faster than it is restored from the stem, the guard cells
tend automatically to close the stoma, checking proportion-
ally the transpiration, while the access of more water to the

E

 
50 A TEXTBOOK OF BOTANY (Cu. III, 6

leaf, permitting renewed turgescence of the guard cells, pro-
duces a reopening of the stoma. One other important con-
dition, however, influences this result. The guard cells, alone
of the epidermal cells, contain chlorophyll, and hence make
grape sugar in light; and a solution of grape sugar, as will
later be shown, draws water osmotically from neighboring
cells, thus increasing the turgescence of the guard cells and
opening the stoma. Accordingly, while the stomata tend to
close with dryness, so to speak, they also tend to open in
light, which is the time when carbon dioxide is needed in the
work of the leaf. These two conditions, however, often oper-
ate antagonistically, producing irregularities in the action of
the guard cells. Thus, while their operation can be viewed |
as adaptive in general, it is not so in detail. In this respect
the stomatal mechanism resembles most other adaptations,
which, because so many other factors are simultaneously
affecting the part concerned, can never be perfect.

Stomata oecur chiefly, and in most plants exclusively,
on the under sides of leaves, in which position a stoppage
of their openings, and therefore of gas passage, cannot be
caused by rain. Against this detriment several adaptations
have been described, though often misinterpreted as a sup-
posed need for promoting transpiration. Stomata vary
much in size, extent of opening, and number, ranging from
0 all the way up to near 500 per square millimeter. Their
conventional constant (page 25) is 100 per square millimeter
of surface, and their aren when extended the widest. possible
would open 7d) of the leaf surface (Fig. 23). It is at first
puzzling to the observer, as it long was to botanists, how,
through so small a total area of opening, a sufficiency of
earbon dioxide can enter and so much water vapor escape.
The explanation has been found in a very curious physical
fact, viz., that the smaller an opening becomes, the more rapid
relatively (not absolutely) is the passage of a gas through it
by diffusion, while such passage is also more rapid through
slit-shaped than through round openings of the same area.
Cu. III, 6] TRANSPIRATION FROM PLANTS dl

Therefore the capacity of the small stomatal openings for
gas passage is far in excess of that implied by their areas.
The matter becomes clearer from another point of view when
we note that an ordinary stoma when open presents to a
molecule of carbon dioxide or water an entrance or exit as
great as a passage seven miles wide appears to a man.

While transpiration is thus primarily an incidental accom-
paniment of photosynthesis, rather than a physiological pro-
cess in itself, it does have
functional value in one
respect. Plants need in
their leaves, and else-
where, certain mineral S : e 2
matters which are ab-
sorbed from the soil; 0 0 0
and these are lifted with
the water, and left in the
tissues by its evapora-
tion. Indeed, the view
has been held in the past a 2 e © 2
that this is the primary
functional meaning of Fic. 23. — Diagram to show the num-

; : i : ber, and extrerme area of opening, of
transpiration, its COpl- stomata, according to the conventional
ousness being considered constant; drawn to scale, 100 times the

true length and breadth.

necessary because of the

great dilution of the minerals in the soil water. Later evi-
dence, however, shows that little relation exists between the
amount of transpiration and the quantity of mineral matters
found in the plant. Furthermore, an important réle has
been assigned to transpiration in the dissipation of the exces-
Sive energy poured into leaves at times by the strongest
summer sun, — an amount sufficiently great to work damage
in the leaf were it not for the cooling effect of evaporation;
and this advantage must be real, even though incidental
rather than adaptive. Thus it seems clear that transpira-
tion is primarily an unavoidable though partially controlled

 

oO
)

 

 

 
52 A TEXTBOOK OF BOTANY (Cue TL 7

accompaniment of photosynthesis, while secondarily it per-
forms the functions of lifting the mincrals into the leaves,
and at times of neutralizing excessive solar action upon
exposed surfaces.

Connected indirectly with transpiration is GUTTATION,
frequent in young herbaceous plants. It occurs at those
times when roots are supplying water forcibly and abun-
dantly, but transpiration is checked. The surplus water is
then exuded through water pores (which are modified
stomata), at the ends of the veins, where it collects in glisten-
ing drops, commonly mistaken for dew. The drops can be
made to appear by experiment, and are often seen in garden
plants on cool mornings after hot nights, or even on warm
humid dull days; while often in cool evenings after hot
afternoons the water drops run down and wet the foliage, as
familiar in Cannas. In some measure related to guttation
is the formation of shell-like ice on the stems of certain
herbaceous “frost plants” in early winter; for the water
freezes as it is forced from cracks in the dying stems.

7. Tue ADJUSTMENTS OF GREEN TissuESs TO LIGHT

Food formation is the first function of plants, and takes
place only in chlorophyllous tissues under action of light.
Accordingly it is natural that plants should exhibit special
adjustments of their green tissues to the sun.

Most prominent of such adjustments is the existence of the
leaf itself ; for the leaf is simply a thin sheet of chlorenchyma
provided with accessory veins, air spaces, and epidermis. In
any typical foliage leaf, as observation indicates and micro-
scopical measurement confirms, the chlorenchyma is_ re-
markably uniform in thickness throughout all parts of the
blade, in which respect it differs greatly from the veins.
Furthermore, the chlorenchyma of all foliage leaves, no
matter whether small, as in Mosses, or great, as in Palms, is
not far from the same thickness. Exact measurements of the
cross sections of many common leaves show that in different
Cu. III, 7) ADJUSTMENTS TO LIGHT 53

kinds the chlorenchyma varies in thickness from .09 to .58
millimeter, with a mean at .179, and hence a conventional
constant at .2 millimeter (Fig. 24). This variation, though
considerable in itself, is yet wholly insignificant in comparison
with the variation in the sizes and forms of leaves, with which
indeed it bears no relation. Leaves of evergreen or leathery
type which seem specially thick, as in Rubber Plant, have no
thicker chlorenchyma, but only a
thicker epidermis, while the swollen
and succulent leaves of Century Plant
or Houseleek really combine the func-
tion of storage with that of food for-

 

mation, and hence fall into another
category. What then determines this
singularly uniform thickness (or thin-

Fic, 24.— The actual
thickness of the chloren-
chyma of leaves, as seen
in cross section.

The upper, one of the
thinnest, is Abutilon:
the lower, one of the
thickest, is Pelargonium :
the intermediate is the
average of many kinds.

(The lines were drawn
accurately by measure-
ment on a very large
scale, and reduced pho-
tographically.)

ness) of all foliage leaves? The spec-
troscope, the instrument by which light
can be analyzed with great precision,
shows that the red and _ blue-violet
rays of the sunlight, effective in photo-
synthesis, are wholly absorbed by a
layer of chlorophyll, as dense as that
in the chloroplastids, a fraction of a
millimeter thick. Accordingly the ordinary chlorenchyma
can perform its function only when spread out in layers much
less than a millimeter thick. If the chlorophyll is less dense,
7.e. if there are fewer granules in the tissue, the effective light
can go deeper, and the green tissue is thicker though paler,
as in young stems. Furthermore, a stronger light can pene-
trate deeper, and hence effectively illuminate a thicker layer,
than a weak light; and it is a fact that the thicker foliage
leaves are those which live exposed to the brightest sun, while
the thinner kinds occur on shaded undergrowth plants.
Second of the adjustments is the existence of the stem, of
which the wide-branching structure carries the leaves aloft
and spaces them out in the light; and this, as will later ap-
54 A TEXTBOOK OF BOTANY (Cu. III, 7

pear, is the primary function of the stem. It is true, not
all leaves thus attain full individual exposure to light, and
many are shaded more or less by others; but within certain
limits this does not matter, for the reason, fully proved by
experiment, that a bright diffused light is quite as effective in
photosynthesis as direct sunlight, which contains in summer
more energy than leaves can utilize.

Third of the adjustments is the presence of chlorophyll in
all practicable lighted parts. While leaves are preéminently
the chlorophyll-exposing organs, this function is by no means
restricted to them, but is shared in lesser degree by young
stems, young fruits, and even parts of the flower, though the
showy corolla and ripe fruits have other colors suited to their
special functions. It looks as though the plant took ad-
vantage of all its surfaces not needed in other functions to
spread to the light such chlorophyll as it can, even though
that be little.

Fourth of the adjustments is the existence in plants of a
remarkable property of turning their green parts to the light,
no matter from what direction it comes. The fact is familiar
in house plants, which turn leaves and stems away from the
darker room towards the lighter window to a degree pro-
foundly affecting their forms, while the same power can be
proved in many striking ways by simple experiments (Fig. 25).
The younger parts of stems bend over until they point
towards the light, carrying with them the young leaves, which
independently set their blades at right angles to the light.
This bending is effected by growth, which becomes more active
on the side necessary to swing the stems to the light, and in
those parts of petioles necessary to swing the blades across
the light. Obviously the light does not effect the bending,
for that is accomplished by the plant through its own dif-
ferential growth; but the growth is made in response to
the greater intensity of the light, which therefore acts
as the stimuLus to the bending. This process is called
PHOTOTROPISM (formerly heliotropism), and it is typical of
--

Cu. III, 7] ADJUSTMENTS TO LIGHT 00

a great many physiologically advantageous adjustments
which individual leaves, stems, roots, flowers, and other
organs of plants make not only toward light, but towards
gravitation, moisture, chemical substances, and other ex-
ternal influences. This very important property of respond-
ing thus to external stimuli is called rrrrasmity (page 39).

 

Fic. 25.— A Fuchsia grown for a week in a box open only on one side;

seen in profile and face view; * }. Traced from photographs.

Though it often simulates intelligent action, for which it is
sometimes mistaken by the beginner in these studies, it has
really no direct relation to the consciousness of animals.
It does, however, correspond closely with the REFLEX ACTION
of animal physiology, each irritable, like each reflex, reaction
being perfectly specific and invariable in a given part to a
given stimulus. Being thus, in any given case, automatic,

these responses are properly describable as SELF-ADJUSTMENTS. ee
56 A TEXTBOOK OF BOTANY (Gr. ITT,.7

The phototropic response of leaves and stems to light, or
of any other parts to a stimulus, involves the codperation of
four factors. First, there exists in the plant an hereditary
property by virtue whereof the plant makes the responses,
which are usually adaptive and evidently acquired in evolu-
tion in the same way as other plant-features. Second, there

 

Fia. 26. — A leaf-mosaic in English Ivy. (After Kerner, Das Pflanzen-
leben.)

is some mode of perception of light by the plant, the quantity
of light needed being extremely small, only enough, indeed,
to make a physical impression upon the sensitive proto-
plasm. Probably most of the protoplasm of leaf and stem
is thus sensitive, though special regions are more so than
others, and various adaptations for concentrating light in-
side specialized perception cells have been described. Third,
there is some method of transmission of an influence from
the perceptive place to a motor mechanism where the actual
response is produced. This influence apparently travels, as
a rule, through the protoplasm of the cells and the inter-
cellular threads (page 40), although special arrangements,
supposed to facilitate its passage, have also been described.
Fourth, there is a motor mechanism, resting usually upon
a differential activity in a growth zone or other growing
tissue, though in more active responses, as in the Sensitive
Plant and Venus Fly-trap (page 76), a quick-acting hydraulic
Cu. III, 7] ADJUSTMENTS TO LIGHT 57

mechanism is concerned. It is easy to recognize in the reflex
actions of our own bodies the corresponding factors and
mechanisms.

Since stems and leaves turn usually towards the stronger
light, one may well ask why the vegetation of the northern
hemisphere does not all bend towards the south. The reason
seems connected with a fact already mentioned, that leaves
cannot use all of the energy in full summer sunlight, while
a strong diffused light is enough for their needs. Apparently
their full power of response is aroused by such diffused light,
which comes about equally from all parts of the sky.

Where many leaf blades grow closely together, they tend
to move out from under one another’s shade, their petioles
bending or elongating in ways which effect this result. Thus
the blades on a horizontal branch of a tree are commonly
brought into one flat plane. The efiect is particularly strik-
ing in Ivies, where the leaf blades become often so evenly
distributed as to suggest the name of LEAF-Mosatc (Fig. 26).

A familiar light adjustment is involved in the so-called
“sleep movements,’ where the leaflets of compound leaves,
as of Clover, Oxalis, Beans, Acacias, Sensitive Plants, droop
or close together in darkness and spread widely apart in
light (Fig. 27). The response to the light stimulus is plain,
but the significance of the
movement in the plant’s
economy is still uncertain.
The leaflets of other plants
exhibit an analogous move-
ment under very intense Fic. 27.— Leaf of a Clover, in“ awake”’
Light: mc which, hey Oke Te mae Caney
together or assume vertical
positions, returning to the horizontal position when the light
is less intense; and this movement has been interpreted as
protective to the leaf tissues against too intense insolation.
A permanent condition of this protective light adjustment,
which, at its perfection, involves a setting of the leaf edges

 
58 A TEXTBOOK OF BOTANY (Cu. IIT, 8

toward the midday sun, produces the ‘“‘ Compass plants,”
of which there are several kinds in addition to the more
famous one of our western prairies. Many other light ad-
justments are also known in nature, not only in leaves and
stems, but also in roots, flowers, and other parts. They
include movements towards, from, and variously across the
line of incident light. In many cases, a distinct functional
advantage to the organism can be clearly perceived, but in
others this is not evident, though here the limitations of our
knowledge may be at fault.

8. THe Varrous Forms or Fourace LEAVES

Foliage leaves are remarkably diverse in their sizes and
shapes, despite their singularly uniform thickness. They all
perform the same function, and
their differences correspond for
the most part with differences
in the habits of the plants
which produce them.

The sizes of foliage leaves
range all the way from almost
microscopic up to that of
Palms and Bananas, several
square feet in area (Fig. 28).
Marshaling sizes against habits
we find in general that the
largest leaves occur upon
plants which have the most
abundant water and warmth,

Poe sae and least exposure to bright
12 to 15 feet high, and bearing the SUN and winds, —in other
largest known simple leaves. (From — words, upon plants exposed to
Balfour, Class-book of Botany.) 5s z

relatively least transpiration.
These conditions are best realized in the shelter of tropical
forests, and there we find the largest leaves, as all pictures
of tropical undergrowth well show (Fig. 29), while the same

 
Cx, ILL, 3} FORMS OF FOLIAGE LEAVES 09

 

Fic. 29.— Primeval tropical forest, in Ceylon. To illustrate the large
size of leaves in the undergrowth. (Reduced from Kerner.)

principle holds good in our temperate flora, as the student
may recall. At the other extreme, very small leaves occur upon
plants which are exposed to the greatest dryness, brightness,
60 A TEXTBOOK OF BOTANY [Cu. III, 8

 

cold, and strong winds, — conditions which make transpira-
tion excessive. These conditions prevail in highest degree
in arctic, alpine, and desert regions, and there we find the
smallest leaves. In our native flora, the same principle is
exemplified in the plants of bogs, which are open cold places,
and in the evergreen trees, which have to withstand the rigors

 

 

Fic. 30.— A view in Hawaii, showing the contrast between tall-growing
compound-leaved and low-growing simple-leaved Palms. (From Bailey,
Cyclopedia of Horticulture.)

of winter. Under conditions intermediate between the ex-
tremes, the leaves are intermediate in size, as our temperate
vegetation as a whole well illustrates. Correlatively, leaves
which grow exposed to similar general conditions approxi-
mate to a similar size, as well shown in our common deciduous
trees, where the leaves of Maples, Oaks, Chestnuts, Lindens,
Poplars, and others are not far from one size, or at least be-
long to the same order of magnitude.

Leaves which are morphologically large sometimes. be-
come physiologically small by compounding of their blades to
separate leaflets (page 16; Figs. 82 and 37). The compound-
Cu. III, 8) FORMS OF FOLIAGE LEAVES 61

ing is oftentimes associated with exposure to strong winds,
as in Palms, where the compound-leaved forms tower high
over the forests, or grow along wind-beaten strands, while
the simple-leaved forms are confined perforce to shelter (Fig.
30); and it is probable that the compound leaves of the
Tree Ferns (Fig. 31) originated in this way. Compounding,

 

Fic. 31.— Alsophila oligocarpa, a tropical Tree Fern, showing the much-
compounded leaves. (From Bailey.)

however, has also other associations. Thus, in the Pulse
Family, it seems clearly connected with the “sleep,” or
drooping at night of the leaves. In submersed water plants,
where it is common, the compounding, by its exposure of
more surface, facilitates the absorption of the carbon dioxide
dissolved in the water (Fig. 32).

While leaf size seems thus largely adaptational, it is
sometimes as clearly structural or hereditary. Thus the
62 A TEXTBOOK OF BOTANY (Cz. III, 8

small size of the leaves of Mosses, despite their occurrence in
protected places, seems structurally determined by the very
imperfect water-conducting system of those plants. The com-
pounding, with the consequent small leaflets, of our under-

     

 

ae

Fic. 32.— Bidens Beckii,
which grows partly im-
mersed in water and bears
simple leaves above, and
compound leaves below the
surface. (After Goebel,
Biologische Schilderungen.)

growth Ferns seems probably an
hereditary survival from  tree-like
ancestors. And other minor factors
enter into these problems.

In shapes, leaves are equally
diverse, seeming to defy classifica-
tion. Yet comparative study re-
duces them to modifications and
combinations of three primary forms,
which are the orbicular, linear, and
ovate.

Orbicular leaves are well typified
by the Garden Nasturtium (Fig. 33),
with its nearly circular blade and
central-standing vertical petiole from
which the veins radiate to the mar-
gin, giving off a network of veinlets.
In this leaf the blade is unbroken,
but in most others a gap or slit runs
from margin to petiole, as illustrated
by the Pelargonium (‘‘Geranium’’),
the difference apparently represent-
ing a different mode of evolution
from ancestral forms which had mar-
ginal petioles. Structurally the orbic-

ular form serves best the leaf function, since it combines
the most green surface with the least lateral spread, and pro-
vides the shortest paths of conduction for water and food
through the blade. Orbicular leaves are found oftenest upon
low-growing or flat-growing plants, where each blade has
room for exposure to light unshaded by its neighbors, as in
“stemless’’ herbs, in creeping vines like Ground Ivy, and in
Cu. III, 8} FORMS OF FOLIAGE LEAVES 63

leaves which float on the water, as with Water-lilies: while
climbing Ivies show the same tendency, usually modified,
however, by marked angularity of form. The full exposure
of the round blades to light is aided by adjustments in the
slender petioles, and it is in such p ants that leaf-mosaics,
mentioned in the preceding section, become the most perfect.

Linear leaves are typified by those of the Grasses, with their

 

Fic. 33.— Leaves approximating to orbicular shape; x 4}. Garden Nas-
turtium, Yellow Water-lily, Pelargonium, English Ivy, Ground Ivy.

slender elongated blades merging imperceptibly into the pet-
ioles, and their approximately equal-sized parallel veins
joined by inconspicuous veinlets (Fig. 34). Such leaves
occur chiefly in dense growths in the most brightly lighted
places, either upright and parallel like the Grasses in meadows
or the Cat-tails along lake sides, in dense radiating heads like
the Bunch-grasses and Spanish Bayonets (Fig. 35), or else in
mats and tufts, as along the branches of our evergreen trees.
At first thought it would seem that such leaves, presenting
their edges rather than their faces to the sun, must be badly
illuminated. Yet their habitual occurrence in the sunniest
64 A TEXTBOOK OF BOTANY (Cu. Ill, 8

places, in conjunction with the daily swing of the sun through
the sky, must insure among them a sufficiency of that bright
diffused light which, as earlier noted (page 54), is fully as
effective in food
formation as direct
sunlight. Further-
more, the crowded
condition of such
leaves tends greatly
to restrict tran-
spiration, without
equivalent check to
the access of carbon
dioxide; and such
an arrangement has
obvious advantage
to plants of limited
water supply.
Ovate leaves are
typified by those
of Lilac (Fig. 36).
The petiole, at the
wl =~ Z | larger end, merges
i \ into astrong midrib
Hh / from which spring
\ "side veins, which in
Fic. 34.— Linear and other parallel-veined turn give rise to a

leaves; X 3. Hyacinth, Banana (small), Thri- yetwork of veinlets.
nax (a Fan Palm), Hucharis, a Grass.

 

This general shape
is the commonest in nature, and associated with the com-
monest condition of leaf existence, viz., that in which the
blades, neither spread out in one plane nor densely crowded
in full sun, are carried aloft and spaced apart on ascend-
ing stems and branches, as occurs in our larger herbs, and
especially in shrubs and trees. This mode of life is essen-
tially intermediate between that associated with orbicular
Cu. III, 8] FORMS OF FOLIAGE LEAVES 65

and that with linear leaves, and the ovate shape approxi-
mates to orbicular at base and linear at tip. It is therefore
quite consistent that when the leaves become more crowded
on the branches, as
in Chestnut and
Beech, the ovate
shape tends towards
linear, resulting in a
spindle form; but
when on the con-
trary the leaves are
more fully spread
out, the ovate tends
towards orbicular,
with the great veins
coming to radiate
from an _ elongated
petiole, as in Red-
bud. The tendency
towards orbicular
goes farther in heart-
shaped leaves, like
Linden and Violet,
and ultimately leads
back to the true or-

 

 

 

 

 

bicular with central- Fic. 35.— Cordyline australis, the ‘‘Dra-
: . cena Palm,” showing radiate heads of linear
standing petiole. leaves. (From Bailey, Cyclopedia.)

Between orbicular,
linear, and ovate forms, there occur all gradations, giving a
great diversity of forms. Many of these have been named
from their resemblance to common objects (e.g. lanceolate,
spatulate, reniform, peltate) ; and such designations find con-
stant use in the descriptions of plants contained in floras and
manuals.

Closely connected with the shapes of leaves is their
VENATION. Orbicular and ovate leaves are typically netted-

F
66 A TEXTBOOK OF BOTANY [Cu. III, 8

veined, that is, have a few prominent veins and many inter-
secting veinlets (Figs. 2, 33, 36). In the typical ovate forms
there is commonly one midrib with a few veins running thence
parallel-diagonal to the margin, and such venation is called

 

Fic. 36. — Leaves approximating to ovate shape; X 3. Lilac, Maple,
Beech, Redbud, Violet.

PINNATE, While in orbicular forms several approxim: itely equal
veins radiate from the petiole, and that is called PALMATRE.
Linear leaves are typically parallel-verned, that 4 is, have many
approximately equal veins running parallel, with the cross
Cu. III, 8) FORMS OF FOLIAGE LEAVES 67

veinlets almost invisible. In some the veins gradually
converge towards tip and base, as in Grasses and many
Lilies ; in others they run out strictly parallel from a midrib,
as in Banana (Fig. 28), while in still others they radiate from

 

Fic. 37.— Typical lobed and compound leaves; x3. Oak, Locust,
High Bush Cranberry, Virginia Creeper, Orange. The single leaflet of the
latter is jointed to the petiole, which in related forms bears two additional
leaflets.

the base, producing a fan shape, as in the Fan Palms (Fig.
34). And of course there occur all gradations and com-
binations.

There is also close connection between the venation, and
the lobing and compounding of leaves. Some kinds become
deeply lobed between their main veins, and therefore PIN-
NATELY LOBED, as in Oak (Fig. 37), or PALMATELY LOBED, as
in Maple. The significance of this lobing is not yet under-
68 A TEXTBOOK OF BOTANY (Cu. ILI, 8

stood, but it seems connected with a tendency of the chloren-
chyma to collect more closely towards the main veins. The
lobing carried farther leads to compounding, which therefore
is either PINNATE, as in Acacia, or PALMATE, as in Virginia
Creeper (Fig. 37); and often the leaflets are themselves
compounded, even more than once, as in some Ferns.
Parallel-veined leaves are rarely lobed or compounded, their
mode of venation being obviously unfavorable thereto.
The number of leaflets in a compound leaf can be very great,
or no more than three, as in Poison Ivy, or even only one, as
in Orange.

Leaves differ also in the character of their margins, which
in some, e.g. Rubber Plant, and most parallel-veined kinds,
are unbroken or ENTIRE,
but in others are sharp-
toothed or SERRATE, €.g.
Rose, and in others yet
otherwise formed (Fig.
38). The differences
seem to have no func-
tional significance, but
represent structural ex-

Fig. 38. — Forms of leaf margins. pressions of the various
ata ways in which the chlo-
renchyma is arranged with respect to the vein endings.

Leaves also display some peculiar forms of tips and bases
(Fig. 39). The prolonged slender tip found in some leaves
of tropical plants has been claimed to act as a ‘drip point,”
effective in removing water from the leaf after rain, thus pre-
venting a long closure of the stomata; but the evidence is
not clear. Some leaves have the base of the blade prolonged
into ear-shaped (AURICULATE) or pointed forms, occasionally
making the leaf arrow-shaped. In some kinds these ex-
tensions grow together around the stem, which accordingly
seems to pierce the blade (PERFOLIATE), while in others two
opposite leaves grow together in similar manner surrounding

 
Cn TET, 3s] FORMS OF FOLIAGE LEAVES 69

the stem (CONNATE-PERFOLIATE). Such features, for the most
part, seem to have a structural rather than adaptational
origin.

The leaves of plants which grow in places where water
is scarce or hard to absorb exhibit several features obviously

 

Fic. 39. — Special forms of tip and basein leaves; x 4. Ficus religiosus,
with ‘‘drip’’ point; perfoliate Urularia; auriculate Magnolia Fraseri; con-
nate-perfoliate Honeysuckle; Caladium.

related to reduction of transpiration. Such are, — reduction
in size, already mentioned; compact or rounded forms, often
storing water, as in Cactus; a very thick epidermis, which
prevents any loss by direct evaporation ; sunken stomata with
an air chamber outside, or else inrolled leaves, with the stomata
70 A TEXTBOOK OF BOTANY (Ca. Ti, 8

in the concavity (Fig. 40), or coverings of hairs or scales
(Fig. 41), all of which arrangements tend to delay the
escape of water without
materially affecting the en-
trance of carbon dioxide:
and a vertical position of
the green tissues, which
lessens the evaporative ef-
fect of the noonday sun
without any effect upon
gas absorption. The collec-

Fic. 40.— Leaf of Hrica, in cross tive result of these features
section; X 280. (from Kerner.)

 

 

is to give the characteristic

grayish condensed aspect to the vegetation of dry places.
The trichomes of plants are indeed remarkable in their

variety, and often in their beauty when viewed through the

 

 

Fie. 41.— Various forms of epidermal hairs and scales (trichomes) found
upon leaves; much magnified. (From Kerner.)

microscope. Diverse functions have been ascribed to them,
in addition to their part in restricting transpiration, but
without convincing evidence. Perhaps they represent a kind
of play of growth forces rather than any adaptational devel-
opment.
Cu. III, 8] FORMS OF FOLIAGE LEAVES 71

A very remarkable form of leaf occurs in the Welwitschia
mirabilis of Southwest Africa, a plant unique in a great many

 

 

Fic. 42. — Wel chia (Tumboa) mirabilis, growing in the desert of Kala-
hari, Africa. The woody trunk, though many years old, is but two feet in
height. (From Kerner.)

 

features (Fig. 42). The leaves, only two in number, grow
at their bases as they die at their tips throughout the long
life of the plant.
Leaves are pro-
duced in buds, but
produce buds in very
few cases. The leaves
of some kinds of Be-
gonia, however, if cut
across the veins, de-
velop buds which
grow into normal new
plants; and gardeners
are accustomed to
propagate those Be-
gonias in that way. In Fic. 43.—The Life Plant (Bryophyllum
dhe: yell incipe: Tite eer Papuan pg are
Plant (Bryophyllum),
the rather thick fleshy leaves regularly produce buds at the
outer ends of the veins (Fig. 43); and these buds develop
freely into young plants when the leaves fall on damp soil,

 
72 A TEXTBOOK OF BOTANY {(Cu. III, 9

or even when they are pinned up against a wall in the house,
as often done for a curiosity. Apparently this leaf is quite
genuine and not a stem in disguise, as one tends to infer.

Finally, one often finds foliage leaves which exhibit ab-
normal features, such as forked, laciniate, crested, or even
pitcher-form blades, or eccentric coloration, or other unusual
features. When extreme, such cases are popularly called
freaks, and in science monstrosities. It happens that mon-
strosities in leaves are closely connected with those in stems,
and accordingly we can most conveniently discuss them to-
gether in a later section.

9. Tot Forms anp Functions or LEAVES OTHER THAN
FOouIaAGE

While formation of food is the primary, and usually the
exclusive, function of leaves some kinds perform  addi-
tional functions, and exhibit corresponding peculiarities of
aspect and structure. Further, in some leaves the new
function comes
to overshadow
the old, and
even to replace
it. In such case
we have a new
organ, though
one which re-
tains evidence of
am its morphologi-
2 eS eal origin in its
FERN I A mode of develop-

Ths

 

 

Fic. 44. — Mesembryanthemum obeconellum, a plant ment, and vari-
which stores water in the pairs of thickened leaves.

(From Goebel.) ous peculiarities

of structure.
The simplest case of an additional function in leaves con-
sists in the storage of water or food, the presence of which swells
the leaves greatly, as in Century Plant, and Houseleek
Cu. III, 9]

(Fig. 44).

SPECIAL FUNCTIONS OF LEAVES 73

The chlorophyll, of course, is all near the surface,

and wanting in the interior cells of the chlorenchyma, which
increase in number and size, and present a translucent aspect
if water is stored, but are opaque if much food is present.

Sometimes the upper parts of the
leaves become true foliage while the
bases alone store food, in which case
these storage parts, after the foliage
has withered away, form collectively a
typical BULB, as in Hyacinth (Fig. 45).
In related plants the specialization
has gone further, making a division
between foliage and storage leaves, in
which case the latter become exclu-
sively food-storing organs, as in the
bulb scales of Lilies (Fig. 46).

Another form of food-storing leaves,
serving also in some cases as foliage
and in other cases not, are the coty-
LEDONS or “‘seed leaves’? of embryo
plants, later to be fully described.

In many kinds of plants, some of
the leaves deviate in minor features
from the typical condition, in which
case they are called collectively
BRACTS. Commonest of all are the
little pale scale-like bracts which stand

 

Fic. 45.— A Hyacinth
bulb, in section. The
outer or storage leaves
are the bases of last
year’s foliage leaves, and
will be replaced, as they
wither, by the bases of
the new leaves surround-
ing the flower cluster.
(From Figurier, Vegetable
World.)

under each flower in a cluster, where apparently they have
no function, but represent foliage leaves in an arrested or
rudimentary state of development ; for it is a constant struc-
tural peculiarity of the higher plants that flowers originate
in the axis of leaves, that is, in the upper angle between
leaf and stem. Likewise little scale-like bracts occur just
below the leaf-like branches of Asparagus and florists’

Smilax (page 195).

In the Linden the bract is much larger

(Fig. 47), and attached thereto is the flower cluster which
74 A TEXTBOOK OF BOTANY (Cu. III, 9

”

grows out of its axil; while later this bract serves as a “sail
against which the wind acts in transporting the seeds. Very

 

Fic. 46.— Various forms of common ‘‘bulbs.’’ Nos. 3, Easter Lily,
4, Jonquil, 6, Lilium pardalinum, and 7, Hyacinth, are true bulbs, 7.e. are
composed mainly of storage leaves. Nos. 2, Colocasia antiquorum, and
5, Gladiolus, are corms, i.e. storage stems. No. 1, Tuberose, is a tuber,
and 8, Lily of the Valley, a rootstock, called a ‘‘pip.’’ (From Bailey.)

 

striking are the cases where the bracts become highly colored;
thus forming the showy part of a‘‘ flower,” as in Poinsettia,
the real flowers of which
are small and inconspicu-
ous. The sepals and petals
of ordinary flowers are also
morphologically leaves, as,
in a slightly different way,
are the stamens and pistils.
Colored bracts and petals
retain mostly the structure
of foliage leaves, excepting
that the chlorenchyma now
holds other pigments in
Fic. 47.—A leaf and Place of the chlorophyll.

the specialized bract in American Another striking case of
Linden. (From Bailey.) P

      

the combination of a new
function with the old is found in the pitchers and other leaf
traps in which insects are caught and digested. They all retain
Cu. III, 9) SPECIAL FUNCTIONS OF LEAVES

NI
ou

 

Fic. 48. — The Pitcher Plant of Northeastern America, Sarracenia pur-
purea; X }.

The frontispiece, reduced, of Barton’s Elements of Botany (2d ed., 1804),
the first great American botanical textbook.
76 A TEXTBOOK OF BOTANY (Cu. III, 9

their chlorenchyma, and the changes are chiefly in form.
Thus our native Pitcher Plant, or Sarracenia (Fig. 48), seems
to represent a leaf in which the margin has grown up around
a central-standing petiole, forming as it were first a saucer,
then a cup, and finally a pitcher. In the Nepenthes, most
elaborate of Pitcher Plants (Fig. 49),
there occurs a partial division of
labor between the pitcher and foli-
age functions, for a very perfect
blade exists in addition to the
pitcher. Doubt. still exists as to
the precise morphology of the parts
in this remarkable leaf, though it
seems most probable that the pitcher
represents a blade transformed as in
Sarracenia, with the lid a special
outgrowth and the seeming blade an
expansion of the elongated petiole,
which often serves also as a tendril.
But we must guard against push-

Fira. 49.— Nepenthes, an jing such homologies too far, be-
East Indian Pitcher Plant; %
xX}. The slender stalk be. Cause leaves and other parts, while
tween blade and pitcher strongly influenced in development
often serves as a tendril. ona

(From Le Maout and by the characteristics of the part
Decaisne, Traité Général de from which they have evolved, are
Botanique.) Se

by no means limited to the charac-
teristics thereof, but. often break loose, as it were, and develop
new features upon their own account. In another well-known
insect-trapping leaf, that of the Venus Fly-trap (Fig. 50), the
morphology is obvious, the petiole becoming expanded much
like the blade.

Another function performed by leaves is that of support
to climbing plants, in which case they form TENDRILS, which
are characteristic organs of most vines. Tendrils are very
slender almost thread-like structures, fitted to twine around
supports, to which they thus attach their plants. In the

 
Cu. III, 9] SPECIAL FUNCTIONS OF LEAVES 77

simplest case, the petiole acts as the tendril, making a
turn around the support, as in our common wild Clematis
(Fig. 51). In other cases, as illustrated by our figures, the
tendril is a trans-
formed leaflet or
leaflets, or else
stipule-like struc-
tures, or even the
entire blade. The
typical tendril
moves about
through the air
until it touches
some object ; then
it bends towards
the touched side,
and, if the object
be of suitableform,
continues the pro-
cess, and makes
several turns
around it (Fig.
52). Then the in-

 

 

the tendril be-
comes twisted to
a double spiral,
drawing the plant =
(ose tthe SG peo eens

 

Fly-trap, Dionega muscipula, a
port, after which plant which catches insects by sudden closure of its
leaf blades; x 3. (From Figurier.)

it develops tough
fibrous tissues, thus forming a strong but elastic bond be-
tween plant and support. In this definite action of tendrils
we have another instance of those automatic self-adjustments
made possible by the irritability of protoplasm (pages 39,
55), this particular form being called THIGMOTROPISM.
78 A TEXTBOOK OF BOTANY iCu. LU, 9

Another special form and function of leaves is represented
in the brown BuD SCALES which enwrap the winter buds of
our trees. They mostly lack chlorophyll, their cell walls
become thick and well cutinized, and often they develop

 

fic. 51. — Forms of leaf tendrils; x 3. Pea, Smilax, Bignonia, Clematis,
Lathyrus Aphaca. The apparent leaves of the latter are stipules.

coatings of resin or hairs; and they fall away as the buds un-
fold. In some kinds each seale is an entire leaf, in others
it is a petiole with blade suppressed (Iie. 53), or it may be a
stipule, as conspicuous in Tulip tree, where together the pair
forms a close-fitting cap (Fig. 57).
Cu. II, 9] SPECIAL FUNCTIONS OF LEAVES 79

Leaves are also often modified to sprnes, especially in
plants of dry places. The significance of spines, however, is
uncertain; for the older
view that they represent a
protection against animal
enemies seems inadequate,
while the newer idea that
they result from a struc-
tural degeneration of leaves
rendered superfluous by
changed habit has not won
acceptance. In the trans-
formation they lose their
chlorophyll and flat form,
and become slender, coni-
cal, and hard. In some
cases each spine represents
a single transformed leat,
as is believed true in the
Cactuses (Fig. 54); in
others they represent the
midrib and two lateral ribs
of a leaf, as in Barberry
(Fig. 55); in Euphorbias,
when paired, they clearly
represent stipules (Fig.
57); while in some tropical
climbers the stipular spines
are very strong downward-

 

 

Fic. 52. — Stages in the twining of a
i _ tendril, of Bryonia; x4. This is a
turned hooks which catch stem tendril, but the method is the

ota er vegetg- same in leaf tendrils. (Drawn, with
firmly upon other \ 8 slight alterations, from a wall-chart by

tion. Errera and Laurent.)

While the blade is the
distinctive chlorenchyma-carrying part of the leaf, the
foliage function is in some cases assumed by petioles or
stipules, the blade being more or less suppressed. Thus, in
80 A TEXTBOOK OF BOTANY [Cu. III, 9

the Australian Acacias, the chlorenchyma is all in the
petioles (called pHYLLOpIA), which are vertically flattened
(Fig. 56), while the much compounded blades distinctive of
Acacias are sup-
pressed. In other
eases the stipules
become enlarged,
aiding the blade in
its function as in
Violets (Fig. 57),
reaching to a size

and form identical

Fic. 53. — Transition from. bud seales to leaf, with those of the
showing the former to be petioles, in Box Elder ;

x}.

 

blades as in Gal-
ium, or replacing
the foliage altogether as in Lathyrus Aphaca (Fig. 51). The
causes of these curious substitutions of functions are mostly
not known, but they are presumably connected with pe-
culiarities in the past history of the plants. For example, it
seems likely that the abandon-
ment of the leaf blade and
transfer of the foliage func-
tion to the petioles in Acacias
represents a mode of adapta-
tion to a climate increasing
in dryness. Leaflets, which
expose much horizontal sur-
face, are out of place in
dry climates, while a single
petiole, flattened vertically,
is better protected against — Pic. 54.— A cluster of spines from
extreme transpiration (page ee emails >; xX #  (After
70).

One cannot but notice the diversity of form, and the
variety of apparent function, in the stipules. In existent
plants they seem to represent no distinctive organ, but

 
Cu. III, 9] SPECIAL FUNCTIONS OF LEAVES 81

rather a kind of morphological entity easily specialized in
diverse directions. Recent investigations have shown that

leaves containing stipules receive from
the stem three sets of veins, from two of
which the stipules are supplied, while
leaves lacking stipules receive but one
set, or vein. Since the original or primi-
tive leaf of our modern trees was appar-
ently three-lobed, the stipules may repre-
sent the two lateral lobes, which became
reduced as the middle lobe developed
into the leaf blade of our existent plants.

Not all paired structures at the bases
of leaves are stipules. In Pereskia, a

re climbing Cactus, the ae

paired hooks whereby
the plant clings to a
support are the first two
spines of an axillary
cluster, and in some kinds
like seeming stipules are °%
simply the first leaf of an

that, for reasons uncertain,

 

Fig. 56.— A

 

&
Je

of Aristolochia the leaf- g 55.— Leaf spines

Barberry ; Ke:

(After Gray.)

axillary branch. In the Telegraph Plant
(Fig. 58), they are leaflets,
than the terminal leaflet; and in this plant
they have further the remarkable property,

much smaller

they are con-

stantly rising and falling, in short jerky

phyllode of an motion suggestive of the arms of the old
> ia: =
Acacia; = 2 semaphore telegraph, — whence of course the

Often a few leaf-
lets of the com- plant’s name.

pound leavesap- Typically, leaves are flat plates of tissue,

pear at the tip.

and in heir various transformations this

plane character is mostly retained. In certain cases, how-
ever, the face of the leaf develops an outgrowth of tissues,

G
82 A TEXTBOOK OF BOTANY (Gm. TTT, 10

a kind of branching
of the face of the
leaf. Such seems the
case in the lid of the
Nepenthes — pitcher
earlier mentioned,
and in the corona,
or crown, of the
petals of some
flowers, notably the
Daffodil (Fig. 230).

Thus we see that
the leaf, though
having a definite
and typical primary
function and struc-
ture, is yet highly
plastic in all of its
features, and ean be
led along many dif-
ferent lines of de-
velopment. Such

Fig. 57. — Special forms of stipules; ™ }. morphological plas-
Euphorbia, paired spines: Galium, with two ticity is character-
opposite leaves simulating a 6-leaved whorl: , . *

Tulip Tree, bud scales: Polygonum, united in a Stic of all parts of
sheath (ochrea) around the stem: Violet, acces- living beings and is
sory foliage. S Oo

 

one of their cistine-
tive properties (page 39). The tracing of such lines of
development is the distinctive province of morphology.

10. Tue Nurrition or Puants Wuicu Lack
CHLOROPHYLL

While most plants possess chlorophyll and make their
own food, there are some which do not. Tf, now, all plant
food is based on grape sugar made in green tissues, how
do these chlorophyll-less kinds secure their supply? The
Cu. III, 10) PLANTS WITHOUT CHLOROPHYLL 83

matter is simple; they take it from green plants, or from
animals which obtain it from green plants. When they take
it from living plants or animals, they are called PARASITES,
the one from which it is taken being known as the Host;
and when they take it from dead plants or animals or decay-
ing remains thereof, they are called sapRopHYTEs. The
difference between
parasites and sapro-
phytes has no par-
ticular physiological
significance, but is
rather a convenience
in our description of
those plants. The
absorbing organs of
such plants are called
HAUSTORIA.

Among the Flower-
ing Plants, the most
familiar parasite is Fic. 58. — The Telegraph Plant, Desmodium
doubtless the Dodder 9¥’@"8: +. It is native to tropical Asia, but

7 . : is grown in greenhouses. (From Figurier.)
(Fig. 59), a relative
of the Morning Glory. Its slender, orange-colored, smooth
stem twines around and among various green herbs in the
fields; and wherever it touches their stems it sends forth
aérial rootlets which penetrate the tissues until they reach
the veins (Fig. 59). Here a connection is established with
both ducts and sieve tubes, from which the parasite can now
draw both water and food. The most familiar flowering
saprophyte is doubtless the Indian Pipe or Ghost Plant
(Fig. 60), the roots of which are believed to absorb the
decaying material of green plants, not, however, directly,
but by aid of a Fungus (Mycorhiza, page 244). Such para-
sites and saprophytes, having no chlorophyll, need no leaves,
which accordingly are reduced to mere scales; and these
persist only as relics of an evolution from chlorophyll-

 
84 A TEXTBOOK OF BOTANY [Cu. III, 10

possessing ancestors. Without leaves, there is small need
for stems, which accordingly are also much reduced in
many of the flowering parasites. An extreme in these
respects is reached in that remarkable flowering parasite,
the Rafflesia of Java (Fig. 61), where the plant consists
solely of asingle gigantic
flower (some three feet
across and the largest
flower known), which,
through a very short
stem and some haus-
torial roots, is parasitic
upon overground roots
of trees.

The Fungi, including
the Bacteria, comprise
many thousands — of
species of parasites and
saprophytes, which ex-
hibit structures having
obvious relation to the
conditions under which
those plants live. Para-
sitic Bacteria mostly
inhabit the tissues of

 

Fig. 59.— The Dodder, Cuscuta Europea; ;
« 34. Itishere parasitic on Willow, on which living plants or animals,

it twines. Note the scale-like minute leaves, ¢,, ee Bo
and the flowers in clusters. On the left is a from which they absorb

section showing the connection of the haus- the nutritive juices di-
Sea ee veins of the host. rectly through the walls

of their very simple
bodies. The true Fungi possess no leaves, stems, or roots,
but consist ordinarily of two parts, — first, a feeding body
called a mycrELium (Tig. 62), composed of numerous fine
white threads which ramify over and through their hosts, or
the decaying materials on which they grow; and second, a
SPOROPHORE which comes out from the surface, and develops
Cu. III, 10] PLANTS WITHOUT CHLOROPHYLL 85

the minute reproductive spores in the air where the winds
can scatter them. Indeed, were it not for the sporophore,
often the presence of the hidden mycelium would never be
suspected. The familiar
mushrooms and molds
have this structure.

Parasites, whether flow-
ering plants or fungi, enter
and penetrate their hosts
by use of digestive fer-
ments, or enzymes, put
forth by the tips of the
entering haustoria. En-
zymes are definite chemi-
cal substances which have
power to digest (7.e. con-
vert into soluble forms)
the cell walls, starches,
and proteins; and these
digested materials are
absorbed into the roots
or mycelium and form
food for the parasite. It
is precisely the same with
saprophytes. Thedamage <& )
done by parasites to their LHe Tf!
hosts is of three sorts,— Fic. 60. — The Indian Pipe, or Ghost
Jirst, the removal of food, ee ae ee tee
thus tending to starve the Bailey.)
host plant; second, the
excretion of injurious or poisonous substances apparently
by-products of the parasite’s own metabolism ; and third,
the disturbance of the growth-control mechanism, resulting
in the production of various monstrosities.

Parasites and saprophytes are relatively small plants, the
majority being microscopic ; and they constitute an insig-

 
86 A TEXTBOOK OF BOTANY (Cu. ET, 10

nificant and inconspicuous part of the earth’s vegetation.
Thus it is clear that their mode of life is far less successful
than that of green plants. There is, however, another
group of organisms of similar habit which has been more
successful in this respect, and that is the animals. They,
too, are parasitic or saprophytic upon plants, but have

 

Fic. 61. — Rafilesia Padma, of Java, parasitic on a root. (From Kerner.)

this advantage, that possessing the power of free locomo-
tion, they are not confined for their food to single hosts, but
can take it from many. :
It might be supposed that in absence of chlorophyll, the
bright colors displayed by some Fungi, notably the brilliant
reds and yellows of poisonous toadstools, perhaps have part
in a food-making process. No evidence for such function
exists, and the significance of those colors is not known.
The student may recall that the Mistletoe, a reputed para-
site, possesses chlorophyll. That plant, however, is only
a half parasite, for while taking water and minerals from the
host it makes its own food in its leaves. There are plants
Cu. HI, 10) PLANTS WITHOUT CHLOROPHYLL 87

which are likewise half parasitic upon the roots of other

plants, as in case of our wild Purple Gerardia.
Insect-catching plants do not belong among parasites,

because they all make their own food. The insectivorous

 

Fic. 62.— The mycelium (threads ramifying in the ground) and
sporophores (above the surface) of a small Puff-ball: x 5.

habit is connected only with the acquisition of nitrogen
compounds, as will later appear.

Finally, there is one other very distinct method of plant
nutrition. Certain Bacteria which live in the soil have
power to make their own food from carbon dioxide and water
entirely without sunlight, the necessary energy for the pro-
cess being derived from chemical energy set free by the
oxidation of substances in the soil. The process is thus
naturally designated CHEMOSYNTHESIS in distinction from
photosynthesis. While occurring at present, so far as known,
in only one group of Bacteria, the method has great interest
for the reason that it suggests a way in which plants may
have made their food in the far-distant times before chloro-
88 A TEXTBOOK OF BOTANY (Cu. III, 11

phyll was developed. The existing chemosynthetic Bac-
teria, indeed, may represent a survival from that ancient
epoch, in which case they are doubtless the most ancient type
of organisms now inhabiting the earth.

11. Tue AUTUMNAL AND OTHER COLORATION OF LEAVES

The distinctive color of leaves is the chlorophyll green,
which most of them exhibit. Other colors, however, occur,
especially in ‘foliage’ and ‘‘variegated”’ plants, and in the
autumnal foliage.

The most prominent of the non-green colors of living leaves
isred. Itis most intense in cultivated plants, such as Japanese
Maples, Copper Beeches, Coleus, Beets, and Red Cabbages.
In all cases, however, the color has been greatly intensified
under cultivation, from a very moderate quantity in the
ancestors of these plants. Little blotches or streaks of
red color are indeed very common in wild plants, as in-
tensive observation, centered on this point, soon reveals.
The color is due to the presence of a red substance, called
descriptively ERYTHROPHYLL but chemically (ANTHOCYAN or
ANTHOCYANIN, which is dissolved in the sap of the cells.
Being thus soluble in water, it is easily removed by hot
water from red leaves, which thereby are left green, showing
that chlorophyll is present in foliage plants, though masked
by the more brilliant and abundant erythrophyll. As to
the reason for its presence, that is greatly in doubt. Prob-
ably it has no functional utility in itself, but represents simply
an incidental product of the complicated metabolism of the
plant.

In some cases, however, a functional utility has been
claimed for erythrophyll. Thus, a great many plants in our
own flora show in the leaves in early spring a blush of red
which later disappears. The claim has been made that here
the red forms a protective screen to the young developing
parts, by absorbing the blue and ultraviolet rays of the
sunlight believed to injure unscreened living protoplasm,
Cu. III, 11] COLORATION OF LEAVES 89

much as the photographer’s ruby light cuts off the same
rays which would spoil his plate in development; and thus
is tided over the time prior to the full formation of the
chlorophyll, which incidentally acts as a sufficient protec-
tion. It has also been supposed that the absorbed light
is converted into heat, and used to warm the young parts
and thus promote their development. The latter explana-
tion would account for the prevalent red color in the mosses
of open bogs, which are notoriously cold places. Various
explanations have also been offered for the deep red of
the under sides of leaves in some tropical plants, and
for the brilliant hues of the toadstools. But the evidence
in these cases does not stand our earlier-cited test for sci-
entific truth (page 13), which shows how much we have
still to learn about some of the commonest phenomena.
The case is quite different, however, with the colors in flowers
and fruits, for here the evidence demonstrates functional use,
as will later appear. A functional use seems also reasonably
clear in the beautiful rose-red Alge called ‘‘sea mosses,”
where the red screen (here, however, not erythrophyll, but
another red pigment) probably aids the underlying chloro-
phyll in a better utilization of the sunlight as altered by its
passage through the sea water.

Second in prominence of the non-green colors of living
leaves is yellow. Indeed, the normal green color of leaves is
not a perfectly pure green, but tends a trifle towards yellow,
which, however, is only rarely pronounced in healthy leaves.
It occurs occasionally in small blotches and stripes in wild
plants, from which it has been much developed under cul-
tivation in some variegated leaves, notably in yellow vari-
eties of Coleus. It is more commonly associated with
waning vitality of the leaf, whether through old age, or
insufficient light, or the action of parasites, or (and above
all) the fall of the leaves in autumn. It is due to the presence
along with the chlorophyll, of a mixture of yellow pigments,
descriptively called xaNTHOPHYLL, and composed chiefly
90. A TEXTBOOK OF BOTANY (Ca. III, 11

of two chemical substances, CAROTIN and XANTHOPHYLL
PROPER, though sometimes additional yellow pigments are
present. Carotin and xanthophyll have the property of
relatively high stability in light, on which account they
show forth in full intensity when the more unstable chloro-
phyll, which is made only while the leaf is in full health,
fades away in the light.

The white colors of leaves represent simply the natural
color of composition of the leaf structure when all colored
pigments are absent. The white is translucent in cells which
contain sap, but is silvery in those which are dead and
filled with air, as in some variegated Begonias. White
areas cannot, of course, form food, and are rare in wild
plants; but they have been greatly intensified in cultiva-
tion, in the striped and variegated foliage of Begonias, fancy-
leaved Caladiums, and Ribbon Grasses. Sometimes the
same leaves contain also areas or stripes of red, thus increas-
ing the variegation, as occurs very prominently in the re-
cently-developed Rainbow Corn.

Various colors appear also in leaves as result of the action
of parasites, either Fungi or Insects. In some cases the color
belongs to the parasite itself, as in the Rust of Wheat leaves,
where it resides in the rusty-red spore masses. More com-
monly it results from damage done to the complicated metab-
olism of the leaf by the parasite, followed by disappearance
of chlorophyll,. and consequent exposure of the yellow
xanthophyll; or the tissues may be killed altogether, and
hence soon display their distinctive decay color, which is
brown. Colors due to injury by parasites may usually
be recognized by a certain abnormal or unhealthy aspect
they give to the leaf, and especially by their wholly irregular
or asymmetrical distribution in relation to the leaf structure.’

Most striking and interesting, however, of all the non-
green leaf colors is the autumnal coloration of foliage, which
constitutes one of the major phenomena of nature. Its
foundation lies in the fact that with waning vitality, brought
Cu. III, 11] COLORATION OF LEAVES 91

on by old age or the coming of autumn, a leaf makesno more
chlorophyll, while that already present fades rapidly away,
permitting other colors which are present to show, and
likewise some new ones to form under the altered conditions.
The rapidity with which chlorophyll can fade in the light is
strikingly shown by the simple experiment of exposing a
fresh alcoholic solution to strong light in contrast with a
control kept in the dark (page 17). In an hour or two the
green color is gone, leaving the solution colored yellow by
the xanthophyll. This experiment shows why leaves turn
yellow in autumn, for the fading of the chlorophyll exposes
the xanthophyll, always present with chlorophyll but far
more resistant to destruction by light. Thus all autumn
leaves are yellow, though some acquire additional colors.
The xanthophyll is easily extracted in a clear solution by
simply warming yellow leaves in alcohol; and it is also ob-
tainable by blanching an alcoholic extract from green leaves,
as just mentioned. As to the function of this widely present
xanthophyll (a mixture of carotin and xanthophyll proper),
that is still unknown, though the constancy of the substances
indicates some important functional utility. Herein lies
another of the problems inviting the future investigator.
Less abundant but more conspicuous than yellow, as an
autumn color, is red, which is due to the erythrophyll (an-
thocyanin) already described. Being soluble in the cell
sap, it is easily removed, in a clear solution, by heating
the red autumn leaves in water. It is indeed worth one’s
while, for xsthetic as well as educational reasons, to extract
the green, yellow, and red pigments in their beautiful clear
solutions, and view them side by side in glass cylinders
against the light; for these are the three which give almost
the entire coloration to all foliage. The erythrophyll origi-
nates in autumn leaves very differently from xanthophyll,
for it is not previously present, but is made during the fading
of the chlorophyll. There is much uncertainty about the
details, but it seems reasonably certain that it results in-
92 A TEXTBOOK OF BOTANY _ [Cu. III, 11

cidentally, as a purely chemical reaction, when certain sub-
stances, of which sugar is certainly one, and tannin is prob-
ably another, happen to be present, and, under the conditions
prevailing in the dying leaf cells, are struck by bright light.
It is the fading away of the chlorophyll which admits
into the leaf a sufficient intensity of light to produce the
chemical reaction. That the light is essential to the process
is suggested by the extra brilliance of the colors in specially
bright climates and seasons, and is proven by the fact that
any leaf which would ordinarily turn red does not do so
if closely covered by another, as may be tested by experiment.
Thus red in these leaves does not replace yellow, which is also
present, but simply outshines it. The reason why some
kinds of leaves turn red, and others only yellow, appears
to be simply this, that some kinds contain the necessary
substances and others do not. It is highly significant in
this connection that the leaves which turn most brilliantly
red, e.g. Maples, Oaks, and Sumachs, are noted either for
their abundance of sugar, or of tannin, or of both.

Next in importance of autumn colors is brown, which
has several origins. In some leaves it is apparently an oxi-
dized product of yellow sap substances called flavone deriva-
tives; in others it results from an oxidation of tannins in cell-
walls when exposed to the light and the air, — precisely the
same kind of photochemical process which turns wood or bark
brown with time. In these cases the color has obviously
no functional utility, but represents a purely incidental
result of the chemical and physical conditions which pre-
vail in the dying or dead tissues. When the browning
takes place not too rapidly, it sometimes combines with
the yellow of xanthophyll into a beautiful golden bronze,
as in some Oaks, though it may later become so intense as
to mask the xanthophyll, which fades slowly, as in Beech.
With the brown, as with other colors, the exact shade is
often determined by the simultaneous presence of other
substances, such as resins, or even by remnants of unfaded
CH LIE ii] COLORATION OF LEAVES 93

chlorophyll, or by air-spaces, hairs, or other structural fea-
tures. In a few cases no brown color appears, and by the
slow fading of the xanthophyll the tissues are left nearly
white, as happens to some extent in our Birches.

All autumnal coloration of foliage rests upon these five
colors, either singly or in combinations, modified somewhat
by other substances, or by the leaf structure. The student
will notice how different they are in their significance to the
plant, for while chlorophyll has a well-known and vastly
important function, and xanthophyll an unknown but prob-
ably important function, erythrophyll and the browns are
mere chemical resultants of the physical and chemical con-
ditions prevailing in dying leaves, and white is the natural
color of the unaltered leaf structure. In autumn leaves,
obviously, none of the colors seem to have any functional
utility to the plants, and autumnal coloration as a whole
appears to represent simply a gigantic chemical incident,
comparable with the blue of the sky and the red of a sunset.
Though thus but an incident, it is a happy one for mankind,
in whose elevated enjoyment of nature it forms a great
factor.

Everybody knows that autumnal coloration is far more
brilliant in some climates and some seasons than others,
thus showing a marked sensitiveness to external conditions.
Something depends on the kinds of plants which constitute
the flora, for plants differ in their susceptibility to the
color changes. Again, the coloration is notable only in those
regions where the transition from summer to autumn is
rather abrupt, and the vitality of the leaves is suddenly
checked while they are still full of sap; and it is relatively
poor in places of gradual transition from summer to autumn
where the leaves lose their sap before dying. It is through
the abrupt check to the vitality of the leaves that early
frosts help the coloring, though they do not cause it, as
popularly believed. In fact, any cause which hastens the
waning of leaf vitality brings on the coloration more quickly.
94 A TEXTBOOK OF BOTANY [Ca. II, 12

Thus with our Maples, the partial splitting away of a branch,
an injury to the bark, or infection by disease, will often pro-
duce the red coloration in the leaves of the injured branch
while the remainder of the tree is still green. Further, a
bright climate is essential to the best coloration, partly be-
cause bright light produces a quicker and fuller fading of
the chlorophyll, and therefore a better exposure of the xan-
thophyll, and partly because the brilliancy of erythrophyll
formation is directly proportional to the brightness of the
light. It is because bright days and frost go together that
the latter is commonly credited with more than its due
in the process. The conditions of the preceding summer,
whether dry or wet, play also some minor part, through
influence on leaf vitality. In general, other conditions
being equal, the brightness of autumn coloration in any given
region is proportional to the clearness of its autumn climate,
while its brightness in any given season is proportional to
the clearness that year. This importance of light explains
why the color is more vivid in climates like that of New
England, where the autumnal skies are prevailingly bright,
than it is in old England, where autumn is a season of mois-
ture and cloud. Finest of all is the coloration in places where
the summer ends abruptly, the autumn is bright, and the
frosts come early, as occurs in Eastern Canada, where some
of us think it is the best in the world.

12. Tur Economics, AND TREATMENT IN CULTIVATION,
oF LEAVES

All cultivation of plants depends for its success upon con-
formity to their physiological peculiarities. It is true,
gardeners and farmers have not had in the past any scien-
tific knowledge of these matters, but through centuries of
experience, consisting in observation and trial and the passing
along of the results, they have reached conclusions nearly
enough correct for all practical purposes. We consider now
the practice of plant cultivation with respect to leaves.
Ca DIT 12) ECONOMICS OF LEAVES 95

Few kinds of plants are cultivated for their leaves alone,
aside from foliage plants, grown in gardens for ornament.
Direct utility is confined to a few which happen to store
food, as in Cabbage, or which contain some palatable relish,
as in Lettuce, Spinach and other ‘“greens,’’ or yield some
special product, like Tobacco, or serve as fodder for cattle,
as in Grasses. Such uses, however, are insignificant in com-
parison with the indirect importance of leaves as the source
for the food and other useful substances which are formed or
stored elsewhere in the plant. For this reason leaves, even
though temporary organs of little direct economic value,
must all be kept in health and good photosynthetic oper-
ation; and thereto is much of our gardening and farming
practice devoted.

For best health, leaves need ample but not too much sun-
light, all the carbon dioxide they can get, plenty of water,
some mineral salts, and air.

In winter, greenhouse plants receive little more than a
fourth of the sunlight of summer, and not enough for their
needs. Hence house plants must be given the very best light
available; and good modern greenhouses are studies in
light-efficiency, embodying the best experience and inves-
tigation in direction of exposure (preferably south or south-
east), pitch of roof, transparency of glass, and slenderness of
frame. On the other hand, the full summer sun contains
not only more energy than plants can make use of, but often
much more than is good for them, particularly if in green-
houses, where they lack the free circulation prevailing out-
doors. On this account it is needful, even in spring, to shade
such houses by curtains, slats, matting, or paint on the
glass. Under light thus tempered greenhouse plants grow
quite as well as in full sunlight, while keeping in better
general health. Similarly, it has been found that some kinds
of crops actually thrive better under some shade, though
this is not wholly a matter of light, but also in part of
protection from hail and strong winds. Thus it is found
96 A TEXTBOOK OF BOTANY (Cu. IIT, 12

profitable to grow Pineapples under slat shading in Florida
and Tobacco under thin cotton tents in Massachusetts ;
while some recent experiments indicate that several common
crops, including Potatoes, Cotton, Lettuce, and Radish
likewise do better under some shade. Corn is one plant
which seems to thrive best without any shade, though it
is to be noted that this plant exposes not the faces but only
slanting surfaces of its leaves to the sun.

The carbon dioxide indispensable to food formation comes
from the air through the stomata; and therefore the leaf
must be kept free from dirt which would clog them. Such
a clogging of the stomata, with consequent starvation of the
leaves, explains the damage now done to hedges along coun-
try roads by the dust thrown by automobiles, and likewise
the death of leaves growing near cement factories, from which
a very fine dust continually radiates. In minor degree
dust is a detriment to house plants, explaining the value
of an occasional spraying or washing by rain, and also the
following advice contained in a recent almanac, — ‘‘Cover
your plants kept in the living rooms with a thin cloth when
you sweep.”’ Not only dust, but the floating spores of plants,
and also the excretions of some insects, close the stomata in
greenhouse plants, and necessitate the frequent scrubbings
which gardeners must give. Fortunately such damage is
minimized by the fact that most leaves have the great ma-
jority, or all, of their stomata upon their under surfaces.

Water is needed by leaves for food-formation, to compensate
transpiration, to hold the soft tissues tensely spread, and for
other purposes; and every gardener and keeper of house
plants knows how essential is an ample supply. In some
cases, however, no amount of water supplied to the roots will
compensate the transpiration from the leaves, because of slow
absorption by roots or transmission by stems. Thus are
explained several familiar phenomena (page 47), viz. the
occasional wilting of garden plants when the soil is not dry,
the limitation in the kinds of plants which can be grown in
Cx. III, 13] USES OF THE PLANT’S FOOD 97

houses, the disastrous browning, wind-burn, and winter-kill-
ing of shrubs. One might think it possible to compensate
these difficulties by supplying water directly to leaves;
but leaves cannot absorb any appreciable quantity of water,
and such benefit as seems to follow spraying is due to the
check in transpiration (page 47). The spraying of plants in
the sun may even bring damage, because drops of water left
on the foliage sometimes act as small burning glasses, which
concentrate the sunlight, kill the protoplasm, and brown the
foliage in spots.

Transpiration from leaves has another connection with
gardening in this way, that seedlings when transplanted con-
tinue to lose water; and since the absorbing roots are
destroyed, the plants always wilt; hence it is best when
practicable to cover them with boxes, etc., to check tran-
spiration until new roots areformed. For exactly this reason
gardeners remove much of the foliage of cuttings before
placing them in the ground to root.

Leaves also need certain mineral matters for chemical uses,
involving the application of fertilizers; and they must have
sufficient oxygen, which means fresh air, for their respiration.
These matters, however, can be considered more conveniently
in later sections.

13. THe Uses oF THE PHOTOSYNTHETIC Foop

It has been said more than once in the foregoing pages
that the photosynthetic grape sugar made in green leaves in
the light is the basal food of plants and animals alike. Here
follows the evidence for this statement.

The photosynthetic grape sugar and the associated pro-
teins move continuously from their places of formation in
the leaves, and pass along the veins into stems, roots, buds,
flowers, fruits, and other parts, every cell of which receives
a share thereof. Within the cells a part of the sugar and
proteins are chemically transformed into other substances,
having definite functions in the plant’s economy. These

H
98 A TEXTBOOK OF BOTANY [Cu. III, 13

chemical transformations are collectively designated as the
plant’s METABOLISM. Functionally, the metabolic changes
center chiefly in the provision of materials serving five ends,
— the skeleton, reserve foods, living protoplasm, special se-
cretions, and respiration.

1. THE PLANT SKELETON. In the great majority
of plant cells, a part of the food sugar is used in building the
cell walls (page 41), which collectively constitute the plant
skeleton. The substance of the walls is primarily CELLULOSE,
a transparent, elastic, water-absorbing material, of which
the filter paper of laboratories is a good illustration, though
cotton and linen are nearly as pure. Chemically its formula
is (C6Hi005),, which means that its molecule is composed of
the combination CsHi90; repeated an unknown number of
times. The combination CsHio0; (not known to occur by
itself) differs only slightly in proportions from the food sugar
(CeHi20, — H2O = CeHiO;), and is clearly transformed
therefrom. The ease with which cellulose absorbs and trans-
fers water has high physiological importance in the interior
of the plant, but would be fatal on the exterior in contact
with dry air. In these outer walls, however, a part of
the sugar (or cellulose) is converted into new substances
called cuTIN and sUBERIN, which are waterproof, and have
a faintly brownish color; and the epidermis which enwraps
the soft parts of plants, and the cork which encloses their
woody stems, have walls of such cutinized or suberized cellu-
lose. Furthermore, this cellulose, while ample in strength
for the construction of small plants, is too yielding for the
building of large ones, which have to withstand great strains
from their weight and the winds. Accordingly, in the
trunks of trees and shrubs some of the sugar (or cellulose) is
converted into a new substance called LIGNIN, which infil-
trates and greatly stiffens the walls without loss of their
power to transmit water; and such lignified walls constitute
woop. The shells of nuts, and some coats of seeds, also
owe their hardness to lignification. And other modifications
Cu. III, 13] USES OF THE PLANT’S FOOD 99

of the walls occur, including the GELATINATION familiar in
the Flax seed, while often the walls are also strongly infil-
trated with mineral matters.

The cell walls of a plant collectively form a continuous
system, somewhat like the cement walls and floors in our
modern buildings. In the compartments (the cells) lives
the protoplasm which builds the whole structure. Thus the
protoplasm, itself too soft and weak to rise from the ground,
can, like man, construct lofty buildings, in the rooms of which
it can dwell in the sun.

It happens that the qualities which fit the cell walls for
their functions in plants make them also useful to man for
many of his needs. Hence he appropriates the elastic cel-
lulose for paper, or, as it occurs in long fibers, for cotton and
linen to make clothing. The waterproof cork serves to stop-
per his bottles. The stiff wood provides a rigid but easily-
worked material which he utilizes, as lumber, for his dwell-
ings, and as cabinet woods, for his furniture, while it serves
minor uses innumerable.

Man makes one other use of cellulose and its derivatives
not represented by any function in the plant, but dependent
on an incidental feature of their chemical composition, viz.
— they will oxidize, or burn, thus providing him with fuel.
This use goes further than appears at first sight, for coal is
nothing but the cell walls of plants which throve in swamps
of the Carboniferous epoch, and in course of long ages, under
pressure and warmth, lost the two gaseous constituents, hy-
drogen and oxygen, retaining only the solid and oxidizable
carbon, which is the substance of coal. A perfect sequence
can be traced from the photosynthetic sugar made in the
green leaves of the Carboniferous plants, first to cellulose,
then in succession, with progressive loss of the gaseous con-
stituents, to lignin, peat, soft coal, and anthracite. The
same qualities which make cellulose burn, make it explode,
in suitable combinations; and hence it is convertible into
high explosives, useful in peace and deadly in war.
100 A TEXTBOOK OF BOTANY (Cu. III, 13

2. THE RESERVE FOODS. While much of the photo-
synthetic sugar is used diréctly as food by the various living
cells throughout the plant body, a large quantity is trans-
formed into reserve materials, which accumulate in special
parts, to be used later in growth, especially that of the next
season. The places of such accumulation are buds, bulbs,
tubers, and seeds; and it is to the presence of these accu-
mulated foods that the swollen form of those parts is due.
These reserve foods are of three general classes, — carbohy-
drates, fatty oils, and proteins.

The Carbohydrates are minor transformations of grape
sugar into substances which retain the food value of the
sugar, though with different physical properties. They in-
clude the sugars, starches, and hemi-celluloses.

Tue Sucars are of several kinds. The photosynthetic
sugar itself is a mixture of two kinds, grape sugar or GLUCOSE
(also called pExTRosE) and fruit sugar or FRUCTOSE, these
two being the simplest and most stable of the sugars. They
have an identical formula, CsH»Os, and differ only in the
arrangement of the atoms within the molecules. Both are
present, the former more abundantly, dissolved in the sap
of practically all plants. The glucose, with some fructose,
accumulates in stems, as in the Sugar Cane, where it con-
stitutes most of the molasses, and in Corn, whence it is
taken for use as the clear syrup called “glucose.’’ Both occur
also in fruits, where, however, the fruit sugar is usually the
more abundant ; and they form also the sugar of nectar, which
is the basis of honey, chief food of many insects. Far better
known, however, is Cane sugar, or SUCROSE (SACCHAROSE),
which accumulates in Sugar Cane, Beets, and the Sugar Maple.
Its formula is Cy2H201, implying a close relation to glucose
and fructose (2 Cs>H120¢6 —H20 = CyH22011), to which it is read-
ily converted back, into a molecule of each, in various ways.
And several other sugars, differing little from these, occur also
in plants, though none are especially prominent. Grape and
fruit sugars can be made artificially in the chemical laboratory.
Cu. III, 13] USES OF THE PLANT’S FOOD 101

The sugars are very nutritive substances, and thus con-
stitute reserve food of the highest value to plants. Their
qualities, however, make them also good food for animals,
which draw freaky upon them. Thus, they form the chief
food of insects, are an important constituent of the fodder
of domestic animals, and give value to the vegetables and
fruits used by man, who, however, goes much further in his
utilization of them, since
he not only systemati-
cally cultivates and im-
proves the plants which
produce them most
abundantly, but also ex-
tracts, refines, and stores
them for his own more
convenient use. Press-
ing out the sweet sap,
he boils away the water,
obtains the sugar in
crystals, and refines
them of impurities, a
process much easier for
cane than grape sugar, Fic. 62 a.— Starch grains (concentrically
for which reason the for- striated) in the cells of Potato; highly

: magnified. (From Figurier.)

mer is common on our

tables, while the latter is there unknown. Grape sugar, how-
ever, has another economic importance, in that it is the
sugar which is fermented to alcohol by the Yeast Plant,
though that organism has the power first to convert other
sugars to grape sugar. From this source comes our entire
store of alcohol, including all of our wines and strong liquors,
as we shall note more fully in the section on fermentation.

THE STARCHES, also, originate in transformations of grape
sugar. Their formula is the same as that for cellulose
(CsHi005)n, with the , signifying a different number. They
are insoluble in the sap, and exist in the plant as solid grains

 
102 A TEXTBOOK OF BOTANY (Cu. II, 13

(Fig. 62 a), having very characteristic forms and markings,
differing with the kind of plant (Fig. 63). Starch is
formed from sugar only in the plastids of the cells, either the
chloroplastids of the green cells, or the colorless leucoplastids

 

Fic. 63.— Typical grains of various starches; highly magnified. Upper
row, Potato, Maranta, Pea, Hyacinth; middle row, Wheat, Oats, Sago,
Smilax; lower row, Canna, Corn, Bean, Oxalis.

The characteristic forms and markings of the grains form invaluable
identification marks in the recognition of adulterations of foods, etc. (Re-
drawn from Ganong, The Living Plant.)

of storage cells; and it cannot as yet be made artificially.
Starch is particularly abundant in tubers (Potato), tuberous
roots (Sweet Potato), bulbs (Lilies and Hyacinths), and es-
pecially in large seeds, to all of which its presence imparts a
dull, white, firm aspect, in marked contrast to the soft trans-
Cu. III, 13] USES OF THE PLANT’S FOOD 103

lucency where sugar is the food, as, for example, in Beets.
Being insoluble in water and therefore not removable in that
form from storage cells, starch must be digested before use,
in which process it is converted by the action of enzymes
back into grape sugar, the change being marked, as familiar
in germinating seeds and growing potatoes, by a transition
from the dull white to a soft translucent appearance.
Starch, stored by plants for their own uses, forms likewise
the best of food for animals, which take what they need, and
like plants digest it by enzymes back |
to grape sugar, in which form it is
transferred for use to all parts of their
bodies. It is the principal constituent
of the ordinary foods of all herbivo-
rous and graminivorous animals. As
for man, starch is by far the most im-
portant of all the food substances
taken by him from plants. This is
sufficiently plain when we recall that S
all of the grains, which constitute the Fi. \G4 = oThiekencd
principal food of the human race, — ae es are Se
Wheat, Corn, Rice, Barley, Millet, and ae (aanhady, oes
others, — consist chiefly of starch. tends into pits persistent
Tus Hemr-cettvLoses are much ™ Pe “ls
less prominent than the sugars and starches. They are
modified forms of cellulose, having the same chemical for-
mula, but with the , indicating a different number. They
oecur as extra layers of the cellulose walls (Fig. 64), espe-
cially in some tropical seeds, which thereby are made heavy
and hard, as well illustrated in the Date seed, or still better
the Ivory Nut,—a large seed of a Palm, hard enough to
serve as imitation of ivory. The hemi-celluloses are easily
digested by plants but only in part by animals. They merge
over gradually to the pectins, or fruit jellies (the ordinary
gelatin being an animal product), which are dissolved out by
hot water in making preserves, and these again merge over

   
104 A TEXTBOOK OF BOTANY (Cu. III, 13

into the gums, like gum arabic, all readily digestible by plants
and animals.

The Fatty Oils come ultimately from grape sugar, through
intermediate stages, including fatty acids. They are really
mixtures of true fats, which are not volatile, and thus differ
from the essential oils, to be considered under secretions.
They are found in a few fruits, such as Olive (yielding olive
oil), but accumulate in quantity in a good many seeds, from
which we obtain Castor oil, Cottonseed oil, Linseed oil, and
some others. They occur usually in small round globules
among other food substances, giving a characteristic oily
luster to sections through such tissues, and, while commonly
liquid, they form sometimes a butter-like solid, as in cocoa-
butter. They are insoluble in water, and hence not movable
through the plant until digested back to the soluble fatty
acids. Chemically they are rather diverse in composition (a
typical formula, that of tri-olein, being Cs37Hio4O¢), but are all
marked by this peculiarity, — that their proportion of oxygen
is very small to that of their carbon and hydrogen.

As with sugars and starches, the fatty oils are also good
food for animals. They are a valuable constituent of the
seeds eaten by animals, including man, who also extracts
and refines them for food and for diverse uses in medicine,
arts, and manufactures. Like the animal fats to which they
are so closely related, their paucity of oxygen makes neces-
sary a large supply of fresh air for their assimilation; but
they yield a great deal of heat, which explains why fats are
so craved in cold climates.

The Proteins are much more complicated substances, form-
ing the most important, even if not the most abundant,
of the reserve foods. While scattered throughout all living
cells, they accumulate chiefly in seeds, where they occur
mostly as solid grains, either scattered throughout the cells,
as in Peas and Beans, or in a special layer just underneath
the husk, as in Wheat and other grains (Fig. 65). There
are hundreds of kinds of named proteins, grouped under
Cx. II, 13]

USES OF THE PLANT’S FOOD

105

certain chemical classes, the chief of which are the ALBUMINS,
material like white of egg, GLUTELINS, in semi-crystalline

grains (Fig. 66), GLOBULINS, fa-
miliar in the gluten of flour
which gives tenacity to dough,
NUCLEO-PROTEINS, the chemical
basis of the chromosomes (the
most important part of the pro-
toplasm), and a great many
others, While ordinarily in
solid grains, they are all digest-
ible by enzymes into soluble
and diffusible forms called PEpP-
TONES and PROTEOSES, and thus
can be moved through the plant.
Chemically they are all very
complex, for to the elements of

—

ee SS Eee ———
eae ray x

 

Fic. 65.—Section across a
grain of wheat, showing the layer
of protein-holding cells under the
husk and outside of the starch-
holding cells; » 180. (From
Strasburger.)

grape sugar there are added small amounts of nitrogen, sul-
phur, and phosphorus, taken with water through the roots;
and it is for this reason that nitrates and phosphates in par-

ticular are so essential to fertility in a soil.

The stages in

Fic. 66.— A cell
from Castor Bean,
showing the protein

grains, of which the
structure is rendered
visible by treatment
with reagents.

 

their formation are complicated, and
only partially known, but it seems clear
that first the nitrogen is added chemi-
cally to the elements of the sugar,
forming amino-compounds or amides
(containing C, H, O, N), with which
later the other elements are combined.
These amides are inconspicuous sub-
stances though widely distributed in
plants, the most common being Aspar-
agin, C;Hs03;N2. There is good reason
to believe that many of the proteins are
built up from a simple combination in

much the same way that we found the starches and cellu-

lose are based on a CsHi.O; foundation (page 98).

These
106 A TEXTBOOK OF BOTANY (Cu. III, 13

proteins, composed of the elements C, H, O, N, 5, [PJ], in
diverse, but always complicated, proportions, form the basis
of flesh in animals; and it is because the seeds of the Pulse
Family (Peas and Beans) contain so much protein that they
approach near to meat in their food value.

Like the carbohydrates and fatty oils, but perhaps even
more than they, the proteins are good food for animals, which
take them in fodder, vegetables, fruits, and grains. To ani-
mals, however, they have this special importance, that while
muscles, nerve substance, and other essential tissues are
composed chiefly of proteins; the higher animals at least
have no power to construct them from simpler substances,
but must take them ready-made from plants, or from ani-
mals which have taken them from plants. It is for, its con-
densed supply of such proteins that meat has such food value,
and it is, of course, for their value as protein-accumulators
from plants on his behalf that man keeps cattle and other
domestic animals which he eats. Unlike the case of the sug-
ars, starches, and fatty oils, however, man does not, because
of practical difficulties, extract the plant proteins and re-
fine them for use, though he can do so when he wishes; but
he usually takes them with the other food materials which
they happen to accompany.

3. THE LIVING PROTOPLASM. The living material,
the most important in all organic nature, has already been
described (page 35). It is chemically a mixture of a great
many substances, but its greater and most essential part is
composed of proteins. The proteins, indeed, have their
great importance as reserve food because they are a step in
the formation of living protoplasm. Some of these proteins
are very complex (one, for example, has the formula
CroHiissNoisOQ2i855, and much more complicated kinds are
known); and they are consequently unstable and_ labile,
changing into other forms with absorptions or releases of
energy which are the foundation of various phenomena of
life. But our knowledge of the chemistry of the living
Cu. Il, 13] USES OF THE PLANT’S FOOD 107

protoplasm is wholly insignificant in comparison with the
magnitude and importance of the phenomena it displays.

4. THE SECRETIONS. These are numerous and di-
verse substances having each a special meaning in the plant’s
economy. Chemically they are as different as well can be.
Some are carbohydrates; others are hydrocarbons (con-
taining carbon and hydrogen only); some contain nitrogen
like the amides; while still others are obvious transfor-
mations from proteins. Some secretions have a perfectly
obvious function; others clearly have some function though
it is not known; but in many cases the substances seem
to represent simply by-products of functional changes, or,
like autumn colors, the incidental result of conditions which
happen to occur in certain parts. Some of them serve well
certain needs of man, who takes them for his purposes, often
extracting and refining them to this end. The principal
classes of secretions are the following.

Tue EssENTIAL Orns, or aromatic oils, best known in
Clove oil, Cedar oil, oil of Lavender, and of ‘‘Lemon Ge-
ranium,”’ and the oil of Orange rind, differ greatly from the
fatty oils in being volatile,
and hence giving odors. They
occur in plants in special cells,
or in special collections of cells
called glands (Fig. 67). They
are the basis of practically all
the odors of plants, including
the fragrance of flowers, to
which they serve to guide
insects in connection with

 

. 3 Fic 67.— A gland, in section,
cross-pollination, later to be containing ethereal oil, in Dic-

In tamnus Frazinella; much magni-

more fully considered. oe eee

leaves they have been sup-

posed to give protection, by their acrid taste, against insect
enemies, or to have other uses, for all of which the evidence
is still insufficient. Chemically they are in part hydrocar-
108 A TEXTBOOK OF BOTANY (Cu. III, 13

bons, or else contain also some oxygen, being formed without
doubt from carbohydrates. Their pleasant odors and tastes
are utilized by man in perfumes and essences, though in
recent times he has been able to dispense with the plants,
and manufacture a great many in his own chemical labora-
tory. But they will always continue to add charm to our
gardens.

Related to the essential oils are some other substances of
considerable importance, of which the most important are
resins, camphor, and caoutchouc. Resins, known to us in
balsam, rosin, pitch, and spruce gum, are formed mostly in
_ special passages, and are particularly abundant in the
Coniferee or Pine Family; but we know little as to their
significance, whether functional or incidental. Man utilizes
their imperviousness to water in various ways. A fossil
resin 1s amber. Camphor is a gum of a special tree, again
of unknown significance, and having well-known uses by
man. Caoutchouc, the basis of rubber, is formed by many
plants, usually in their “milk” (or latex), though its meaning
to the plant is uncertain; but the uses that man makes of
its wonderful tenacity and elasticity need no description.

Tue PicmeEnts are the substances which give the bright
colors to the various parts of plants. They are very diverse
in chemical composition (often including elements addi-
tional to those of carbohydrates and proteins), and in
significance to the plant. Thus chlorophyll (composition
CuH»OeNyMg) has a function already familiar to the stu-
dent, while the ever-associated xanthophyll (composition
CyoH 5602) and carotin (CaoH55) have, no doubt, a function,
though it is unknown. Anthocyanin, called descriptively
erythrophyll (composition, in a typical case, the Cranberry,
CoHe;OpCl) is the basis of the reds, purples, and blues in
plants, yielding red with acid cell sap, and blue with alka-
line. In flowers these and other pigments help to guide in-
sects, and in fruits other animals, for functional reasons
later to be noted; but in other cases they seem to represent
Cu. III, 13] USES OF THE PLANT’S FOOD 109

simply incidental by-products of other processes, as in
foliage plants (page 88), in autumn leaves, in the heart
wood of trees, in the colored saps of roots and stems, and
in the highly colored Fungi, though in some of these cases
investigators have found suppositional explanations of their
presence. These pigments are mostly too unstable in light
to serve any useful purpose to man, unless we consider pleas-
ure a utility, for he takes great delight in assembling them in
gardens. Some pigments, however, are stable, including
a few which lack color in the plant but acquire it on ex-
posure to air (e.g. indigo and madder), making them useful
dyes. But chemists can now make such dyes artificially,
and more cheaply than we can obtain them from plants.

THE ALKALOoIDs are best known to us in Morphine (from
the Poppy), Nicotine (from Tobacco), Quinine (from a tree
bark, Cinchona), Strychnine (from seeds of Nuzx vomica),
Cocaine (from the leaves of a shrub, Erythrorylon Coca) ;
while Caffein or Thein (from Coffee and Tea), and Theo-
bromine (from the Cacao tree) are related, if not actually
inthe same class. They occur mostly in special cells or tubes
(often in the ‘‘milk” system, or latex), but their signifi-
cance to the plant is very uncertain. Some investigators
hold that they are semi-poisonous waste products which the
plant thus isolates, while others have thought that their
powerful bitter tastes form a protection to the plants against
animal foes. Chemically they are composed of C, H, O, N,
thus suggesting a derivation through the amides. They are
all endowed with active properties, which are the source of
their value to man, for, as the list above given will show,
they include some of the most efficacious stimulants and
powerful poisons which are contained in our materia medica.
In fact, the principal plant poisons and our most important
drugs belong in this class. The ptomaznes, those well-known
poisons resulting from the action of Bacteria in animal
tissues, are also alkaloids.

Related to the alkaloids in their active properties are some
110 A TEXTBOOK OF BOTANY {Cu. III, 18

of the substances called GLUCOsIDES, a very large and het-
erogeneous group, probably of diverse significance to the
plant, characterized chiefly by the chemical fact that they
consist of glucose (grape sugar) in union with another sub-
stance. Certain ones give the bitter taste to nut kernels,
and to the bark of many trees, and the peppery taste to
Nasturtium, Water Cress, and some other plants.

Tut ENzyMeEs are the most important of the plant secre-
tions. They are formed in small quantities but large numbers
of kinds in diverse parts of plants, where they are apparently
dissolved in the protoplasm. Chemically they are supposed
to be proteins, but this is not certain, for, while we know their
effects, we hardly yet know the enzymes themselves. This
is because of the great difficulty of extracting them in a pure
state from the complicated protoplasm. Their importance
depends upon the fact that, like the catalyzers of the chemist,
they cause chemical changes in various substances (each en-
zyme but one change in one substance, as a rule), without
themselves entering into the reaction; and on this account
very small quantities of enzymes can change great quantities
of substance. It is apparently by the action of enzymes that
the majority of chemical changes in plants are brought
about. Thus an enzyme called diastase is active in diges-
tion, changing the insoluble starch into soluble sugar both in
germinating seeds and animal saliva; another, called zy-
mase, secreted by the Yeast Plant, changes sugar into al-
cohol and carbon dioxide, as will be described under fermen-
tation ; pase converts fats to soluble fatty acids; pepsin
changes insoluble proteins into soluble peptones both in seeds
and the animal stomach; and so with many others. No
phase of plant chemistry is now of such acute interest and
active investigation as that concerned with the enzymes.

Other secretions are the following. The fruit acids, malic
and citric and others, give the tart taste to fruits, of funetional
utility in connection with dissemination by animals, and pleas-
ing toman. The tannins occur chiefly in the bark of plants,
Ca. III, 13] USES OF THE PLANT’S FOOD 111

where their bitter, astringent taste has been supposed to
protect the trees against rodents and insects, while a certain
antiseptic quality prevents development of parasitic Fungi
and hence decay of the bark. It is the oxidation changes
in these tannins under weathering which give the dark brown
color to old bark. Having incidentally the remarkable prop-
erty of hardening the gelatine in skins, they are utilized
by man for tanning leather, though here again the chemist is
providing artificial substitutes. The plant wares occur as
the ‘‘bloom”’ upon some fruits and leaves, and at times, as
in the Bayberry of the coast, such a wax is abundant
enough to be collected and used for candles, as our forefathers
found; but the meaning of the wax to the plants is not
certain. And-other secretions occur, of more special kind
and mostly uncertain significance.

Rather common in plants are crystals, frequently, though
not always, in cells differing from their neighbors; and
they often exhibit marked
beauty of form (Fig. 68).
They are composed chiefly
of oxalate or carbonate of
lime, and represent not
secretions but excretions;
for they seem to be either
useless by-products of func-
tional chemical reactions,
or else substances brought
into the plant from the soil Fic. 68. — Crystals of calcic oxalate,
with the water, and not ina cell of Begonia; much magnified.
needed in growth. The (“fer Bey)
plant has no continuously-acting excretion system such as
the higher animals possess, but instead accumulates waste
matters in out-of-the-way cells, often in leaves and bark, the
fall of which does incidentally provide an excreting system.

5. RESPIRATION. The photosynthetic sugar has one
other use, not at all inferior in importance to any yet

   
112 A TEXTBOOK OF BOTANY (Cu. ITI, 13

mentioned, namely, a considerable quantity is consumed
in RESPIRATION, whereby energy is set free for the work of
the plant. This important subject will find treatment in
the next chapter, along with plant growth where its mani-
festations are plainest. There, also, will be traced the final
fate of all the plant substances after they have served their
functions, or played their other respective parts, in the life of
the plant.

Thus all of the substances constituting the plant body, —
the skeleton, foods, living protoplasm, and secretions, and
also the materials from which is derived the energy by which
plants do their work,—are built up from the photosynthetic
sugar, either by direct transformations thereof, or with cer-
tain small additions from a few mineral substances taken by
the roots from the soil. Upon these materials made by
plants all animals are dependent for their food, both that
from which they construct their bodies, and that which
yields the energy for their work. Thus the importance of
the photosynthetic sugar, of the green leaves, and of the
photosynthetic process becomes abundantly clear.
CHAPTER IV

THE MORPHOLOGY AND PHYSIOLOGY OF STEMS

1. Tue DIsTINCTIVE CHARACTERISTICS OF STEMS

Stems are second only to leaves in prominence and im-
portance as a constituent of vegetation. They are dis-
tinguished by their tapering-cylindrical, continuous-branch-
ing, radiate-ascendant forms, so constructed as to support
and spread the leaves in the light. This is their primary
function, although, as with other plant parts, some kinds per-
form additional and even substitute functions.

Foliage-supporting stems, even when performing the same
function, differ greatly in their external features. In shape,
their differences center in diverse degrees and methods of
branching, as will later be noted. In size, they range from
minute in small herbs, all the way up to the gigantic stature
of the famous California Redwoods (Sequoia gigantea), over
320 feet tall and nearly 30 feet through, or the Gum trees of
Australia (Eucalyptus amygdalina), even taller though not
so stout. In mere length, however, these stems are much
surpassed by the Rattan Palm, which clambers as a vine for
more than a thousund feet through the tropical woods. In
texture, all herbaceous stems, including the new growth on
trees, are soft-cellular like the leaves, being softest in water
plants, which are supported by their buoyancy in the water.
In trees, however, the stems become firm in various degrees
through softwood and hardwood, even to “ironwood,” as
familiar in lignum vite. In color herbaceous stems are green,
from presence of chlorenchyma, which aids the leaves in food
formation; but older stems, which develop a thick protective

I 113
114 A TEXTBOOK OF BOTANY [Cu. IV, 1

bark, are brown or gray, as the incidental result of the
weathering-decay of the tissues.

Stems differ much in duration, according to the habits of
the plant. Some are ANNUALS, that is, they start from seed,
develop an herbaceous shoot, use their food to make new seeds,
and die, allin the same summer. They abound in our flower
gardens and include most weeds. Others are BIENNIALS,
that is, they start from seed, develop an herbaceous shoot,
store food in some underground part, and die to the ground
in one summer; then they use this food to form a new shoot
which develops seeds and dies completely the second season.
They are familiar in our vegetable gardens, in Beets and
Carrots. Some are HERBACEOUS PERENNIALS, that is, they
act like biennials except that they continue to form a food
supply and develop new shoots and new seeds year after
year. They include most of the favorites of our flower
gardens. Others again are WOODY PERENNIALS, that is,
they do not die back to the ground at all, unless accidentally,
but persist and become woody, so that each season’s new
growth is added upon that of the preceding year, thus de-
veloping shrubs and trees. Then there are some which,
like the annuals, flower and form seed only once in their
lives (monocarpic plants), but take many years in prepara-
tion. Thisis the case with the Century plant, which accumu-
lates food for thirty years or more, then blossoms, forms seed
profusely, and dies; but the same habit is found in other
groups, including even some Palms.

The mode of growth of the woody perennials, whereby each
season’s growth is added upon the preceding, involves none
of the internal limitations of size or age to which animals are
subject. Hence trees continue to grow until stopped by
causes incident to their very size, such as the difficulty. of
transferring a sufficient water supply to great heights, and
the leverage they come to present to the action of storms,
whereby branches are broken, rot Fungi admitted, and decay
begun. Trees fortunately constructed in relation to these
Cu. IV, 2] STRUCTURE OF STEMS 115

conditions can attain to a great size and age. Thus the giant
Redwood is known to exceed two thousand years in age, some
trees now standing being probably older than the Christian
era, while the Dragon Tree of the Canary Islands has been
claimed to live even longer. If, however, mere age is in
question, there are probably much older plants, for the Sphag-
num mosses of peat bogs appear to have had a continuous
growth from the inception of the bogs at the close of the
glacial period, many thousands of years ago.

Stems, like leaves, perform also special functions, when
suitably modified in structure, — forming tendrils, storage
organs, and even foliage, as will later appear. It is easily
possible, for the most part, to distinguish such stems from
leaves, —for stems usually grow from buds in the axils of
leaves, while leaves have buds in their axils.

2. THE STRUCTURE OF STEMS AND SUPPORT OF THE FOLIAGE

The primary function of stems, and their distinctive con-
tribution to the plant’s mode of life, is the support and
spread of the foliage. Therewith, however, are involved
minor functions, notably
conduction of water and
food, with growth, respira-
tion, and self-adjustment to
prevailing conditions.

Typical foliage-support-
ing stems are herbaceous
when young, but commonly
become woody with age.

Herbaceous stems, whether
true herbs or the herbaceous
tips of woody branches, are
typically cylindrical and
BEHEbt, nt pea ce Fic. 69.—A_ typical leaf-bearing
leaves horizontally all stem, of Norway Maple; X }. (From
around. At the tip is a Kerner.)

 
116 A TEXTBOOK OF BOTANY [Cu. IV, 2

bud developing the leaves, which are there small and close,
but which downward are progressively larger and more
widely spaced apart (Fig. 69). The leaves stand usually
upon slight annular swellings of the stem, sometimes ob-
scure and sometimes well marked, called NopEs, which are
separated by smooth cylindrical leafless INTERNODES. In
the axil of each leaf occurs a small bud, the foundation of a
branch, which later de-
velops and bears leaves
precisely in the manner of
the main stem.

In their tissues, herba-
ceous stems are much like
the leaves, as to chloren-
chyma, epidermis, stomata,
trichomes, and peculiari-
ties of color. The veins,
however, do not show to
the eye, being buried within
the cylindrical stem. In
cross sections cut close to
the bud one sees little more

 

Fic. 70.— The tissues of a typical :
herbaceous stem, of the Stock, in cross than the general growth

section; * 55. The cambium is repre- tissue, but farther back ap-
sented by the heavier double line through ; : d
the fibro-vascular bundles, which are PCa&rs some such aspect as

seven innumber. Thecollenchymaisnot that of our picture (Fig.

marked. (From Scott, Structural Botany.) :
70). Beneath the thin

epidermis lies the chlorenchyma, pale green but rather thick,
obviously aiding the leaves in food formation. Centerward
can be seen the cut ends of the veins, called also VASCULAR,
or FIBRO-VASCULAR, BUNDLES, which run lengthwise of the
stem, and have the same general structure, and the same
function of conduction for water and food, as in the leaves.
Commonly they are arranged in a ring, in which case they
enclose a PITH, of loose open texture, often glistening-white
from included air. The pith is especially the storage part of
Cu. IV, 2] STRUCTURE OF STEMS 117

 

Fic. 71.— Generalized sectional drawings, based on the Maple, to show
the tissues of a typical stem. Explanation in the text. Secondary growth
begins in the lower of the longitudinal sections. The cambium is left white.

Fic. 72.— Companion series to Fig. 71, based on a Palm as the other

type of stem.

 

 

(From Sargent, Plants and their Uses.)
118 A TEXTBOOK OF BOTANY (Cu. IV, 2

young stems, though other tissues share in that function. All
of these features are shown with particular clearness in Fig. 71.

 

Fic. 73.— The fibro-vascular
system, showing its nodal branch-
ing, in the young stem of Clematis
viticella. (After Nigeli, from
Strasburger.)

 

In sections taken well back of
the tip, two other tissues appear.
One is a mere line extending
right through the fibro-vascular
bundles, and from one to another
(Figs. 70, 71), uniting them into
one ring, or (since they run
lengthwise) one sheath. This
is the important CAMBIUM, or
growth tissue, which later builds
new tissues on both its outer and
inner surfaces. The other is a
band of whitish-glistening tissue
just beneath the epidermis,
called coLLENcCHYMA. It has a
firm elastic texture, and aids
the young stem to support the
strains imposed by the presence
of the leaves. Its position close
to the outside is typical of the
strengthening tissues of stems,
which are developed upon the
principle of the hollow column
or tube. This principle is
known to engineers as that
which provides the greatest re-
sistance to lateral strains with
the least expenditure of ma-
terial, on which account it is
used by them in many construc-

tions, — most familiar perhaps in architectural columns and

bicycle frames.

The fibro-vascular bundles (or veins) of the stem extend
downward all the way to the tips of the roots, and upward
Cu. IV, 2] STRUCTURE OF STEMS 119

into the buds. Just below the leaves some of the bundles
fork, and each sends one branch, called a LEAF TRACE, into
a leaf, and the second up the stem, as indicated in the
typical example here pictured (Fig. 73), and as can be
seen directly in a translucent stem like that of the Balsam.
This branching and rejoining of the bundles produces the
node, which is thus explained, while thereby the bundles
are united into one great
eylindrical network or
system. In this cylinder
the turning of bundles out
into the leaves results in
gaps just above them; and
around these gaps the new
developing fibro-vascular
cylinders of the axillary
buds establish their connec-
tion with the main cylinder
(Fig. 71).

While in most herbaceous
stems the bundles are so Fic. 74.—Stem of Corn, in cross
arranged as to form a ring ee ee a ice
when seen in cross section,
in others they are scattered irregularly, as illustrated here-
with (Fig. 74). In such cases the bundles anastomose in the
stems and extend out into the leaves in a manner differing in
details, but not in principle, from the methods just described
(Fig. 72). Thus the bundles collectively constitute a con-
tinuous conducting system for water and food throughout
the plant.

The tissues above considered are all formed in the buds,
and belong to the PRIMARY GROWTH of the plant. Later the
cambium, and other growth layers, add new tissues, which
thus belong to the SECONDARY GROWTH.

Woody stems develop from an herbaceous condition,
through stages easily observed in the twigs of our common

 
120

Fig. 75.— Winter
twig of Horse Chest-

nut;

x

 

1

t:

TEXTBOOK OF BOTANY (Cu. IV, 2

trees during the first winter (Fig. 75).
The leaves are now gone, not to reappear
on this part of the stem; but the LEAF-
SCARS remain, marked by a lighter colored
corky layer, in which can be seen the
severed ends of the veins. Each scar of
course stands at a node, sometimes plain,
but often not, just above which is the now
prominent axillary bud, while a_ larger
terminal bud ends the twig. The thin
epidermis has been replaced by a layer of
gray-brown waterproof cork, scattered over
which are the lighter colored warty ex-
crescences called LENTICELS.

The leaf-scars and lenticels need special
comment. Leaves fall from trees because
of the formation of a special ABSCISS-LAYER
of tissue which develops across the base of
the leaf in late summer (Fig. 76). Gradu-
ally this layer closes the free communica-
tion between stem and leaf, though mean-
time the valuable materials of the leaf are
mostly transferred to the stem. Then

 

Iie. 76. — Vertical section through a
twig and petiole of Poplar, showing the
absciss-layer, a.l. (From TF. Darwin,
Elements of Botany.)
Cu. IV, 2] STRUCTURE OF STEMS 121

follows the waning vitality, cessation of chlorophyll forma-
tion, appearance of autumn coloration, and finally, by a
weakening of the
walls of the absciss
layer, the fall of the
leaf itself, the absciss
layer becoming the
corky and waterproof
leaf-scar. The lenti-
cels are physiologi-
cally important
structures, for they
replace the stomata
(which disappear of course with the epidermis), as avenues
of gas exchange between the interior of the stems and the
external atmosphere. This exchange is no longer needed
for photosynthesis, which ceases as cork develops, but is nec-
essary for the respiration of the living tissues within, as will
later be shown. | The lenti-
cels are places where a loose
tissue with  inter-cellular
spaces is formed instead of
the impervious cork; and
this tissue by its growth
partially closes them in
winter and forces them open
the next spring (Fig. 77)!
The tissues of these tran-
sitional stems show very
clearly in cross section (Fig.
Fic. 78. — A cross section through 78). Their most striking
a winter twig of Tulip Tree; 10. feature is the sharp division
The lighter continuous line is the cam-
bium, and the medullary rays are dis- between bark and wood at
tinct. the cambium. The parts of
the bundles inside the cambium have grown greatly, and
show clearly the characteristic forms and texture, while the

 

Fic. 77. — A typical lenticel, of Sambucus nigra,
in section; magnified. (From Haberlandt.)

 
122 A TEXTBOOK OF BOTANY (Cu. IV, 2

tissue between them is reduced to fine radiating lines, which
henceforth are called the MEDULLARY RAYS. These woody
parts of the bundles, called xyLeM, contain the ducts, and
conduct water through the stems. Inside the cylinder of
bundles is the very distinct pith. In the pith is much
starch, which is food for the next season’s growth, though
it occurs also in medullary rays and bark, often in strikingly
symmetrical patterns when set forth in blue by the iodine
test. Outside of the cambium can be seen, though only

 

Fic. 79. — Stages in the healing of a pruned stem. cl indicates callus, a
tissue which precedes the overgrowing bark. (After Curtis, from Duggar,
Plant Physiology.)

imperfectly by hand lenses, the outer, or PHLOEM, parts of
the bundles, which contain the sieve tubes and conduct food
through the stem. The remainder of the bark is composed
mostly of the former chlorenchyma, now fast losing its chloro-
phyll, and known henceforth by its morphological name of
CORTEX, while the temporary collenchyma and epidermis are
being replaced by layers of waterproof cork, made by a cork
cambium, and pierced here and there by the lenticels. All
of these features can be traced very easily in nearly all twigs.

The tissues of plants have a remarkable power of healing
injuries which befall them. Any break in the soft tissues is
healed partially within a few hours, and completely within
Cu. IV, 2] STRUCTURE OF STEMS 123

a few days, by formation of cork layers, often manifest by
their brown color. Where an injury includes the wood, as in
case of broken branches or the pruning of large trees, the
wood itself does not heal, but the neighboring bark, and also
the cambium, gradually overgrows it. In time the cambium
reéstablishes itself over the injury and then continues to make
wood as before (Fig. 79). This power of healing injuries
has high value for plants, since their epidermis and cork
form not only a protection against dryness, but serve also

   

 

Fig. 80. — Cross section through bark and wood of an old Elm tree,
showing abscission of the bark; x 3.

as their first line of defense against the entrance of injurious
parasites, which are ever ready to enter any break in the
tissues of the stem.

With increasing age several new features appear in woody
stems. Sections then show that the outer part of the bark,
which is dead, is cut off from the interior living part by
layers of cork, which form anew each year, much as the
absciss layers form in the bases of leaves (Fig. 80). Asin case
of leaves, also, the valuable materials in the outer bark are
previously removed to the stem. This dead bark becomes
vertically cracked by the pressure of the expanding wood
within, and the resultant fissures replace the lost lenticels
as avenues of gas exchange between the interior of the stem
124 A TEXTBOOK OF BOTANY (Ore TV, 22

and the atmosphere. Further, the outer dead bark steadily
weathers and falls away, either somewhat evenly as in Beech,
or else in great flakes cut off by the cork layers as in Elm,
Hickory, Oak, or in remarkably smooth layers as in Birch,
The inner living part of the bark consists of soft, continuously
growing tissue, together with the phloem parts of the bundles.

In the older stems, both wood and bark are greatly in-
creased in thickness as result of the activity of the cambium,
the growth layer of the stem, which continuously forms
new wood on its inner and new bark on its outer face.
This process goes on indefinitely, making the woody trunk
grow steadily in thickness. The bark, however, is simul-
taneously weathering and peeling away on the outside, and
there comes a time when the rate of this weathering just
about keeps pace with the additions within, thus holding
the bark thenceforth of nearly constant thickness, though
in constant renewal. In the wood only a few outer layers
forming the sap woop, distinguishable by the light color, are
alive, while the HEART Woop, usually much darker colored,
is all dead; and the heart may even decay and vanish com-
pletely, leaving a mere shell of sap-wood, which, however,
suffices, on the hollow column principle, to support the tree.
The cambium forms the ANNUAL RINGS, one each year
(Fig. 80). It is easy to see that the appearance of the
rings is due simply to the contrast between the loose open
texture of the wood formed in spring, when large quantities
of water, carrying with it stored food, are needed for the new
growth of the herbaceous parts, and the close compact growth
of the autumn, when less water, and no such food, are re-
quired.| It is these annual rings which, when cut lengthwise,
give the distinctive, attractive “grain”? to cabinet woods.
Not only do annual rings appear in the wood, but they also
occur in the bark, though here they are difficult to see
(compare Fig. 87), because the tissues are soft, and soon
crushed, and later cut off by the cork layers. Since they
are formed by the cambium, the older layers of bark
Cu. IV, 2 STRUCTURE OF STEMS 125

are outside, in reverse of the condition in the wood, as
shown in principle by our diagram (Fig. 81). The third
new feature consists in the SECONDARY MEDULLARY RAYS
(Fig. 82). They form in the ever-broadening fibro-vascular
bundles, which thereby are kept divided to nearly their
original width. It is hardly correct, however, to speak

 

 

Fic. 81. — Diagram of a cross section of a generalized stem, to illustrate
the interrelations of fibro-vascular bundles, pith, medullary rays, both
primary and secondary, cambium (black), cortex, and cork. Annual rings
in bark and wood of identical age are identically shaded. The extension of
the rings across the medullary rays is not shown, though it is usually plain
in the wood while obscure or absent in the bark.

any longer of separate fibro-vascular bundles, since their
identity has long since been lost in that of the general woody
mass and the bark. .

The medullary rays are an important, and sometimes a
conspicuous feature of the wood. Beginning as plates of
tissue between the originally separate bundles, they are
later developed and multiplied in number as a persistent part
of the wood, in which they serve as avenues of communica-
126 A TEXTBOOK OF BOTANY [Cu. IV, 2

tion between the inner and outer layers. They do not run
far, as a rule, up and down the stem (Fig. 82), no farther
than the distance between the successive forkings of the fibro-
vascular bundles in the original bundle cylinder (page 119).
They are more prominent in some woods than others, and are
especially striking in Oak, where they form the prominent
radial lines so plain
on cross sections, and
the irregular shining
plates for which Oak
is ‘‘quartered”’, that
is, cut longitudinally
in a way to display
them. The Oak has
also ducts so large as
to be clearly visible
to the naked eye, —
whence its conspicu-
ous grain.

Stems exhibiting
clear distinction of
bark, wood, and

pith, having cam-

Fic. 82. — A 4-year-old stem of Pinus sylves- 3
yea ¢ s
tris, with bark partially removed at the cam- bium, annual ring
bium ; magnified. It shows clearly the medullary and medullary Trays,
rays, primary and secondary, and the annual and increasing in-
rings, containing resin canals. (From  Stras-

burger.) definitely in. thick-

ness by secondary
growth, represent the most highly developed type, which in-
cludes all of our common trees and shrubs. Since they grow
by additions of layers to the wood, they are called EXOGENOUS.
The other prominent type has none of the above-mentioned
features, but remains permanently in a primary growth con-
dition with the bundles scattered irregularly throughout the
stem (Figs. 72,74). In the belief, since found erroneous, that
such stems grow by addition of new bundles inside of the

 
Cu. IV, 2 STRUCTURE OF STEMS 127

older, they were named ENDOGENOUS, and the name remains
inuse. This type is characteristic of Grasses, Lilies, Palms,

 

Fic. 83.— Typical exogenous and endogenous stems, in cross section, of
Red Pine and a Palm; x +. (Drawn from photographs.)

and in fact of all plants in the great natural group of the
Monocotyledons, where it is associated with parallel-veined

leaves, and sparse branching.
The contrast between the two
types appears very clearly in our
picture (Fig. 83). The typical
endogenous type does not permit
an indefinite increase in diameter,
for, after the fibro-vascular
bundles first laid down have in-
creased to their full size, the stem
no longer enlarges in diameter,
but only in height, whereby en-
dogenous plants are rendered
extremely slender and graceful, as
Palms and Bamboos illustrate.
The great heights maintained by
such stems with slender diameters

  

Dracena Draco, of the
Islands, an endo plant
which grows indefinitely in diam-
eter. (From Balfour.)

 

rest partly on the yielding elasticity permitted by the long
curving courses of their separate fibro-vascular bundles
128 A TEXTBOOK OF BOTANY [Cu. IV, 3

(Fig. 72), and partly on the perfection to which the hollow-
column principle is carried in their construction, as witness
the Bamboo. Upon the latter feature they depend far more
than do exogenous plants, which find ample support in their
massive solid trunks. ' Some Monocotyledons, however, do
exhibit increase in diameter, for the outer layers of their
stems develop a cambium-like tissue which continues to
form new scattered bundles as long as the plant lives. It is
thus that the great Dragon Tree, though endogenous, can
attain to so great a diameter and age (Fig. 84). In all endog-
enous plants, the seeming bark is nothing other than the
compact outer tissues, darkened more or less by action of
the weather, of which the effects penetrate to some depth.

Striking though the difference appears between the
exogenous and endogenous types of stems, they perform the
same functions with apparently equal efficiency., The
differences between them are therefore not functional, but
depend rather upon their relationships within two dif-
ferent and ancient lines of evolutionary descent. Did we
not know this fact, we might seek long for a functional ex-
planation of differences the significance of which lies only in
heredity.

3. THe CELLULAR ANATOMY OF STEMS

From the tissues of stems, which can readily be recognized
by aid of a hand lens, we turn naturally to consider the
constituent cells, making use of the microscope.

One of the best stems for such study, because of its ex-
ceptionally clear definition of the parts, is that of the Dutch-
man’s Pipe (Aristolochia Sipho), a common vine. Sections
through the terminal bud, or very close thereto, show only
the closely packed, squarish, protoplasm-filled cells which
one soon learns to associate with the embryonic stage of
growth (compare Figs. 92, 162). Such embryonic tissue is
always called MeRIsTeM, whether in buds, growing tips of
roots, cambium, or elsewhere. <A little behind the bud
CuslVe 31 ANATOMY OF STEMS 129

Fic. 85. — The cellular anatomy, in corresponding cross and longitudinal
sections, of a young stem of Aristolochia Sipho, a twining vine; x 50.
In the center, the pith; next three fibro-vas-
cular bundles, showing the xylem with
ducts, the rectangular nucleate cells of

the cambium, and the phloem,
marked by sieve-plates and nu-
cleate cells; next the sclerenchyma
ting, with the starch sheath
just outside; then the cor-
tex, consisting of chlo-
renchyma and collen-
chyma; and finally
the epidermis.

 
 
  
  
  
 
 
 
 
 
 
   
 
 
  
    
   
 
  

  

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130 A TEXTBOOK OF BOTANY (Cu. IV, 3

the cells are found well differentiated, as our picture illus-
trates (Fig. 85). Outside is the single layer of the epidermal
cells, with occasional stomata, not essentially different from
those in leaves. Just beneath lies the zone of collenchyma
cells, of which the thickened angles, elongated forms, and
composition from elastic cellulose explain their function as
the first strengthening tissue of the flexible and elongating
stems. Next comes the chlorenchyma, like that of the leaves,
though with scantier chlorophyll. Its innermost layer con-
tains starch, and constitutes the sTaRCH SHEATH, of which
the function is disputed, some investigators assigning it a con-
ducting function for carbohydrates, while others consider
it a geotropic-perception sheath, as will later be explained.
Next inside comes a very prominent zone of angular, thick-
walled, light-colored, greatly elongated empty cells, found by
tests to be hard and stiff. These are SCLERENCHYMA cells,
the characteristic strengthening cells of plants, found in
diverse situations, and here evidently giving special support
to the young stems of this vine, which stand out laterally
before twining around a support.

Inside the sclerenchyma ring can be seen the fibro-vascular
bundles, which here present an unusually distinct structure.
Each bundle is ovate in cross section, with the point towards
the center, and shows three parts. The xyLEM, or wood,
inside of the cambium, contains the large, somewhat angled,
thick-walled pucts, lacking protoplasm but variously marked
by spirals, pits, and the like. They are formed by the
union of many cells of which the intermediate walls have
been absorbed. Intermingled therewith are smaller cells,
partly woop FIBERS and partly woop PARENCHYMA, having
minor functions in connection with the conduction and
storage of carbohydrate foods. The characteristics of the
xylem are further well illustrated in the diagrammatic
figure 86.

The pHLoEM, or bast, is composed of small thin-walled
elongated protoplasm-containing cells lying outside of, and
Cu. IV, 3] ANATOMY OF STEMS 131

matching, the xylem strands. The larger cells are crossed
here and there by the perforated plates which show them
to be SIEVE-TUBES; and they are the protein-conducting
parts of the bundles, precisely as in leaves (page 31). Inter-
mingled with the sieve-tubes are other slender cells, com-
PANION CELLS, which have something to do with the function
of the sieve-tubes, and BAST PARENCHYMA Cells, in which

, OSs E

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Fie. 86.— Generalized drawing of an exogenous stem, to show the
typical anatomy of the cellular elements; highly magnified. From left to
right, the cork, the cortical parenchyma, starch sheath, bast fibers, phloem
parenchyma, sieve-tube, cambium, ducts with xylem parenchyma, and pith
(From Kerner.)

carbohydrates are conducted, and which, therefore, along
with the wood parenchyma, take the place in stems of the
conducting bundle sheath of leaves (page 30). Often the
phloem contains in addition very long and thick-walled but
flexible fibers, called BAsT FIBERS (Fig. 86), which give stiff-
ness to the stem when sclerenchyma is wanting. It is these
bast fibers which in the flax plant yield us our linen, and
in some trees provide tough strands utilized by savage tribes
132 A TEXTBOOK OF BOTANY (Ca. 1V5<3

for cords, and even a lace-like material serviceable for
fabrics.

The cambiwm of the fibro-vascular bundles lies between
xylem and phloem. It consists of several rows of compact,
rectangular, thin-walled, elongated, protoplasm-filled cells,
having the meristematic aspect which is always associated
with growth (compare Fig. 92). In older stems, lines of cells
in the tissue between the bundles (the beginning of the medul-
lary rays), become converted into cambium, continuous with
that in the bundles, and thus the cambium ring is completed
around the stem. It then forms a perfectly continuous cylin-
drical sheath between wood and bark on trunks, branches,
and roots, and it merges imperceptibly into the meristem of
the buds and root tips, which are thus brought into a single
continuous growth system; but it does not occur in leaves.
Being a growth tissue, and therefore thin-walled, it is easily
torn, which explains why bark is so readily removable from
wood, especially in spring when the cambium is most active
and tender. Indeed, at this season the cambium can be
stripped in long gelatinous sheets from the wood of some
trees, notably white pines. In its growth it divides contin-
uously in its own plane, the cells on its inner face becoming
new xylem elements, and those on its outer face new phloem
elements, while the intermediate cells remain cambium.
In this manner it builds also the medullary rays of both wood
and bark.

Inside the ring of bundles lie the cells of the pith, exhibit-
ing the large sizes and rounded forms associated with storage,
whether of food or water. Among them appear very clearly
the intercellular air-spaces, which can likewise be traced in
other parts of the stem, although it has not been possible to
show them in our small scale drawing (Fig. 85). In reality
they are parts of a continuous intercellular aération system
which extends from the pith along the medullary rays and
through the outer tissues to the lenticels and the exterior
air. In the pith, as in other parts, can frequently be seen
Cu. IV, 3] ANATOMY OF STEMS 133

erystals, which have the
significance already ex-
plained for those of the
leaf (page 111).

Woody stems exhibit
their cell structure very
clearly in sections (Fig. 87).
In the bark can be seen the
flat, continuous, brownish
cells of the cork, made by
a special cork-cambium just
beneath them. The first
cork is usually formed just
beneath the epidermis,
which it replaces; but later
the cork-cambium forms
anew each year at some
distance from the surface,
thus building the layers of
cork which cut off the areas
of bark (Fig. 80). The
wood shows clearly the
various cells of the xylem
and medullary rays (Fig.
87), as likewise the cellular
construction of the annual
rings, with the contrast
between the loose open cells
of the spring wood and the
compact growth of the pre-
ceding autumn.

 

Fic. 87. — A segment, in cross section,
of a stem of Linden. From without inward are epidermis (here unusu-
ally persistent) ; cork; cortex (the starch sheath not shown); phloem, con-
sisting of alternating layers of bast fibers (lighter) with sieve and paren-
chyma elements; cambium; xylem, showing three annual rings, with large
ducts, wood cells, and (on the sides) medullary rays; and pith. The view
shows one complete fibro-vascular bundle, three years old. (Drawn from
a wall-chart by L. Kny.)
134 A TEXTBOOK OF BOTANY (Cu. IV, 3

Some stems present special cellular features, of which the
most striking is the LATEX system. This latex, the milky
juice of plants which contains so many diverse substances
of uncertain significance (page 108), is found mostly in long,
slender, closed tubes branching irregularly through the
softer tissues, and even the
wood of stems, leaves and
other parts (Fig. 88).

The Aristolochia, and
other stems just men-
tioned, are all exogenous.
The endogenous type pre-
sents some, though no
great, cellular differences.
Thus, as exemplified in the
Corn (Fig. 89), the bundles,
of course lacking cambium,
present each a very distinct
strand of thin-walled, regu-
larly-arranged phloem,

es alongside of which is the
ite, ie les sige Magis strand of xylem, distin
Kerner.) guished by very large ducts
and commonly a great air
passage; while the apparent bundle-sheath encircling the
bundle has been found to develop by extension from the
xylem. In such stems there is no distinction of pith,
medullary rays, and cortex; but all are merged together
in one pith-like general ground tissue (Fig. 74).

In the foregoing description of the structure of stems, we
have considered only one type of fibro-vascular bundle, —
the kind having parallel strands of phloem and xylem.
Many other types and sub-types, however, occur, as well as
many special forms of arrangement of the bundles within
the cylinders and in relation to the leaves. It has recently
been found that these morphological features of stem

 
Cu. IV, 4] DEVELOPMENT OF STEMS 135

anatomy are very stable in heredity, thus making them good
guides to the evolutionary history and present relationships
of plants. This important phase of investigation is now in

 

Fic. 89.— A single bundle in Corn (one from those shown in Fig. 74),
in cro ; X 130. s points to the strand of phloem; m and sp are
ducts which, with the intermediate cells, form the xylem; / is an air space
containing a ring, a, from a duct; vg indicates the sheath around the bundle.
(From Strasburger.)

 

active and successful development, but is somewhat too
special in method for further consideration in an introduc-
tory course.

4. Tue DEVELOPMENT OF STEMS AND LEAVES FROM BUDS

Stems and leaves originate together in buds, though it is
more exact to say that the embryonic condition of a stem
with its leaves constitutes a bud.

Most familiar of all are the winter buds of trees, in which
the bud proper is enwrapped within corky brown scales,
often with accessory linings of hairs or coatings of resin, as
136 A TEXTBOOK OF BOTANY [Cm IV,4

the Horse Chestnut, for instance, well illustrates (Fig. 75).
The scales, which are modified leaves of the preceding
year’s growth, though prominent, are not an essential part
of a bud, having only the temporary function of protecting
it over winter, after which they fall. The scales are lacking
from all summer buds and some winter ones. The really
essential feature of a bud is the embryonic stem composed
of meristem or active growth tissue, together with the

 

Fic. 90. — A head of Lettuce, in section, illustrative of bud structure ;
<4. (From Bailey.)

nascent leaves which grow out laterally therefrom. The
lower and older leaves of a bud commonly overlap and cover
the upper and younger for a time, but later open out to begin
their work; and in herbaceous plants a perfect gradation
is often apparent between the nascent leaves in the bud
and the full grown leaves of the stem.

Buds are of all sizes, from too small to be seen without the
aid of a lens, up to several inches in diameter, as in Palms.
A Cabbage or head of Lettuce is essentially a gigantic bud,
and offers a particularly favorable illustration of the essentials
of bud structure; for a section made lengthwise through its
Cu. IV, 4] DEVELOPMENT OF STEMS 137

center shows very clearly the characteristic tapering stem,
with the series of leaves in all stages of development
(Fig. 90).

Within the buds the leaves are arranged in various ways,
either overlapping, or each folded by itself on its midrib, or
inrolled from margins or tip. The arrange-
ments are called collectively VERNATION, and
have importance in descriptive taxonomy.

The most prominent, and commonly the
largest, buds are those which are TERMINAL
on the main stems and branches, and which
continue the stem growth. More abundant
are the axILLARY buds which develop in the
upper angle between leaf and stem, and are
nearly as numerous as the leaves themselves,
at least in exogenous plants. The functional
reason for the usual occurrence of buds in this
position is found, no doubt, partly in the near-
ness to the source of food indispensable for
their development, and partly in the favorable
structural opportunity to make connection
with the main stem in the gap left in the
fibro-vascular cylinder above the leaf base
(page 119). Inafew plants, of which Tatarean
Honeysuckle and Red Maple are examples
(Fig. 91), more than one bud occurs in each
axil, either side by side, or one above another, | Fis. 91.—A
the extra buds being called accEssory. ee
Finally, while in many plants no buds other accessory buds ;
than terminal or axillary occur, in others they pe rae
develop in almost any position, especially at
some place of injury; and these so-called apDVENTITIOUS
Bubs produce the branches in irregular positions, as often
seen in Willows and some other woody plants. Not all
irregular branching, however, results from adventitious
buds, for axillary buds often remain latent for years, becom-

 
138 A TEXTBOOK OF BOTANY (Gay IVc4

ing deeply buried under the bark, and yet finally give origin
to branches.

Where the terminal bud is notably prominent, as in Horse
Chestnut (Fig. 75), the axillary buds are largest just below
it, and progressively smaller farther back. Such a terminal
bud unfolds its flowers, stem, and foliage very quickly and
makes no more growth in length that season, though the upper
axillary buds may develop into branches. Such is DEFINITE
ANNUAL GROWTH. Where the terminal bud is relatively small,
as in Elm, it continues to grow and produce new leaves and
axillary buds all summer, and new branching takes place from
the lower new buds. Such is INDEFINITE ANNUAL GROWTH.

The occurrence of a bud in the axil of every leaf gives the
plant a great surplus, of which few ever develop into branches,
though all are capable of so doing. Generally speaking,
those nearer the outer ends of the branches, and therefore
nearest the light
and free space,
are the ones
which develop,
though if the ter-
minal, or outer
axillary, buds be-
come destroyed,
whether by frost,
insect ravage, or
experimental de-
sign, the next

 

Fie. 92. — A bud, of unusually elongated form, lower develop in
ofa water weed, Elodea canadensis, in exterior view their pla ces.
and section, showing the development of leaves; e
* 150. (After L. Kny.) Since all are

capable of — de-
velopment, it is evident that some factor controls them
collectively, either inhibiting the development of some or
stimulating that of others, —a subject to which we shall
later return under Growth.
Cu. IV, 5] ARRANGEMENTS OF LEAVES 139

The mode of formation of stem and leaves within buds
is revealed by longitudinal sections. In an illustrative case
(Fig. 92), one can see very clearly the blunt conical end of
the stem, composed of small tightly-packed cells in process
of formation through new cell-divisions, while backwards
the cells are evidently beginning to elongate with the
lengthening of the stem. The first visible trace of a leaf is
found in the enlargement of a superficial cell, which soon
divides; the resultant cells again divide and, including both
epidermis and cortex, gradually push out in a flat projection,
—the leaf. These leaves enlarge steadily, thus making a
perfect gradation from those just appearing to those fully
formed. As they develop, the nodes on which they stand,
at first close together, are carried apart by lengthening of
the internodes, and the tissues gradually pass from the
meristematic, or embryonic, to the differentiated or mature
condition. In this process, however, the areas of tissue in

the axils of the leaves remain meriste-
matic, thus originating the axillary i.
buds.

 

 

5. THE ARRANGEMENTS OF LEAVES
ON STEMS

Leaves develop upon stems not at
haphazard, but in definite math-
ematical order. This  definiteness
of arrangement, called scientifically
PHYLLOTAXY, while sometimes obscure,
is often strikingly manifest to the eye.

When two leaves occur at a node,
they are always OPPOSITE to one an-
other, and each pair stands at right ( \
angles to the pairs above and below, :
thus forming four ranks on a vertical = Fie. 93. — The opposite
stem, as the Coleus of our gardens, “72ngemen) oF

: : : illustrated by a museum
and the Mint family in general, well model.
140 A TEXTBOOK OF BOTANY [Cxe IV.-5

illustrate (Fig. 93).

When the stem is not vertical, how-

ever, the leaf blades swing around phototropically on their

Fie. 94. — The whorled
arrangement.

 

petioles until they face upward, in a
sort of mosaic, towards the light
(page 57), thus obscuring the op-
posite arrangement, which, however,
can always be seen where the petioles
join the stems. It is important to
remember that phyllotaxy is a matter
of the place of origin of leaves upon
stems, and has little to do with the
positions which the leaf blades ulti-
mately assume. In some kinds of
plants, not two, but three, or more,
leaves occur at each node, forming
a WHORL, in
which case the
leaves com-
monly cover
the gaps. be-
tween those

above and below, as occurs in many
plants of the Lily family (Fig. 94),
The arrangement is particularly plain
in the relation of the petals to the
sepals in most flowers. Often, how-
ever, it is rendered imperfect by
twisting of the stems, variation of
number of leaves in the whorls, and
other less obvious causes.

When only one leaf occurs at each
node, they fall collectively into a
spiral, and the leaves are said to be
ALTERNATE. In the simplest case

   

ia. 95. — The alternate,
} spiral, arrangement.

the successive leaves stand one-half way around the circum-
ference of the stem from those next above and below, thus
Cu. IV, 5]

ARRANGEMENTS OF LEAVES

141

forming two vertical ranks and bringing the third leaf over
the first (Fig. 95), as well manifest in Corn and other

Grasses. In other cases, the leaves stand
one-third of the circumference apart,
forming an obvious spiral, bringing the
leaves into three vertical ranks with a
fourth over a first (Fig. 96), as in Sedges,
which, correlatively, have triangular
stems. The next of the arrangements
actually found is that where the leaves
stand two-fifths of the circumference
apart (Fig. 97), in which case the spiral
must turn twice around the stem before
a leaf, the sixth, is reached over the first,

_
—

rt
|

4

     
 

  

\
)\
)

( |
}

\

Se
Fic. 97. — The alter-

nate, 2 spiral, arrange-
ment.

five vertical ranks
resulting. This is
the commonest of
the alternate  ar-
rangements. It is
very clear in the
Apple, and in many
common plants,
though at times, in long stems, its regu-
larity is disturbed by some twist of the
stem. It underlies the prevalence of the
number five in the plan of most flowers, as
the one-third and one-half arrangements
underlie the numbers three and four in
others. The next arrangement is that
of three-eighths (Fig. 98), found in the
Holly. The next is that of five-thir-
teenths (Fig. 99), found in the House-
leek, which forms the familiar rosettes,
while in Pine cones and other such struc-

Fic. 96. — The alter-
nate, 4 spiral, arrange-
ment.

tures, arrangements of eight twenty-firsts, and even thirteen
thirty-fourths and twenty-one fifty-fifths have been deter-
142 A TEXTBOOK OF BOTANY [Cu. IV, 5

mined. These fractions primarily express the angular diver-
gence of the leaves from one another around the stem, but
secondarily the numerator shows the
number of turns made by the spiral in
reaching a leaf directly over any given
one, while the denominator expresses the
number of leaves in such a complete turn.

It is not only true that these fractions
are actually found in phyllotaxy, but
also a fact that they are the only ones
which ordinarily occur, the exceptions
being rare, and following an analogous
plan. Furthermore, when a stem having
one of these fractional systems becomes
twisted, the leaves are always brought
into the next system above or below.
When, now, the fractions are arranged
in sequence, —

 

Iria. 98.— The al-
ternate, 2 spiral, ar-
rangement.

Ni-K

5. 78.
13 2

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o

some remarkable relations among them
come out, — viz. in all cases after the first and second, the
numerators and denominators are each the sum of the two
preceding, while each numerator is the
same as the denominator next before the
preceding. This curiously related series,
which as a mathematical abstraction is
known from its discoverer as the Fibo-
nacci series, finds actual physical expres-
sion not only in phyllotaxy, but also in
sane other phenomena of nature. Pa yeh ees,
Che significance of phyllotaxy has been. of Houseleek, showing
diversely interpreted. Some botanists aoe
have explained it as adaptive, thinking ~ i
it must give to clusters of leaves the best aggregate ex-
posure to ight. But such reasonableness as this theory may

 
Cu. IV, 5] ARRANGEMENTS OF LEAVES 143

seem to possess in case of the opposite arrangement and the
lower fractions of the spirals vanishes in case of the higher
systems, which are inappreciably different in so far as leaf

 

exposure is concerned; while, moreover, the ultimate ex-
posure of leaf blades is determined chiefly by their own photo-
tropic movements on their petioles, with little or no regard
to the plan of their origin. Later studies, however, seem to
show that phyllotaxy originates in the construction of buds,
as an incidental re-

sult of the order in

which the nascent <
leaves develop in

=
a f=,
feet ay

     

 

relation to one an-
other upon the cone
of embryonic stem
tissue. This order
of development, in
turn, seems to be
connected with con-
ditions of mutual

 

 

 

 

 

 

pressure of the form- OO
ing leaves upon one OX RES

another in buds of eae
different shapes, this :

  
  
 

 

on of
in the

t Statior

pressure manifesting
itself very  differ-
ently in slender
buds, which mostly produce the opposite system and low
fractions, as compared with the broad or flat buds, which
chiefly produce the higher fractions. Apparently the leaves
originate regularly and successively in the lines of least
resistance in the differently shaped buds,—and the result
is phyllotaxy. Herein we seem to have a particularly clear
case of one of those purely structural factors which were
earlier mentioned as having a part with adaptation and
heredity in determining details of plant form.
144 A TEXTBOOK OF BOTANY [Cu. IV, 6

Since new buds, which give origin to new branches, are
axillary to leaves, the branching of plants should correspond
with their phyllotaxy. This, indeed, is true in principle,
as shown by young twigs; but as plants grow older the
regularity of their branching becomes greatly disturbed by
irregular shading and diverse natural accidents. Flowers
always originate from axillary buds, and hence clusters of
flowers also exhibit the plans of phyllotaxy. This becomes
especially striking when flowers are condensed closely
together in heads, as in the Composite family ; and thus is
explained the wonderful phyllotactic symmetry of Dahlia
flowers, and of the head of a Sunflower in ‘‘seed”’ (Fig. 100).
Other structures which show such symmetry strikingly well
are cones of various trees, plants of compact growth, like
the Mamullaria of the Cactus family, and various rosette
plants. In all of these cases the primary spiral is difficult
to trace because of its condensation; but incidentally there
arise a number of secondary and tertiary spirals, and these
it is which become so strikingly evident.

6. THE TRANSFER OF WATER AND Foop THROUGH PLANTS

A secondary function of stems is the conduction of water
from the roots to the leaves, and of food from the leaves to
the roots. We now consider the method of these important
processes.

In the lower plants, the Algze, Fungi, and Bryophytes,
composed altogether of parenchyma cells without any, or
with only arudimentary, system of veins, both food and water
are passed directly from one cell to another. The process is
a slow one, and in land plants prevents any great develop-
ment of size, as the very low growth of all Bryophytes, or
Moss plants, exemplifies. The higher plants, however, both
Flowering plants and Ferns, have developed veins, or vas-
cular bundles, which permit the comparatively rapid trans-
fer of both water and food through long spaces of stem, thus
rendering possible the growth of those plants to tall trees.
Cu. IV, 6} TRANSFER THROUGH PLANTS 145

Of the two currents in the fibro-vascular bundles, the water
current is by far the more voluminous, because of the great
demands of transpiration. Some of the largest trees require
each day literally tons of water, which must be raised one,
two, or three hundred feet, and in rare cases still higher, into
the air. To raise a given amount of water to a given height
requires the expenditure of a definite amount of energy,
no matter whether done quietly by a tree, noisily by an
engine, or laboriously by human effort. It has been calcu-
lated that the amount of work done, and energy required,
to raise the water used by a large tree during twenty-four
hours is approximately the same as that expended by a
person in carrying three hundred large pailfuls of water up a
ten-foot flight of stairs, — that is, a pailful every two minutes
through a ten-hour working day. It is the botanist’s prob-
lem to explain the source of the energy whereby such great
quantities of water can be raised to such heights against
gravitation in small tubes which entail a great deal of friction.
In other words, what are the forces which impel the rise of
the sap in trees?

The water in passing along ordinary stems moves chiefly
in the xylem part, especially the ducts, of the fibro-vascular
bundles. This can be proven by experiment, for if an her-
baceous stem be cut and stood in water dyed with some
obvious color, e.g. red ink, and then later, as the first traces
thereof appear in the younger parts, the stem be sectioned at
different heights, the ducts will be found filled with the red
fluid, which is also diffusing outward to the neighboring
tissues. In a tree the water runs only in the younger xylem,
z.e. the outer and younger rings of the white sap woop, and
the colored HEART woop has no part in the process. Thus is
explained the fact that many kinds of trees can lose their
heart wood by decay without detriment to water conduc-
tion, as also the familiar fact that in tapping Maple trees
for their sap, it is useless to bore more than an inch or two

into the wood.
L
146 A TEXTBOOK OF BOTANY [Cu. IV, 6

The water-conducting vessels are of two sorts, — first,
elongated single cells, called TRACHEIDS, and second, tubes,
called pucts or TRACHE® (Fig. 101), which are formed from
many cells of which the
intermediate walls have
been absorbed. Trache-
ids occur often intermin-
gled with ducts; they
form the ends of the
xylem part of the veins

    
   

QOOQD:

               

 

 

 

 

 

 

 

 

  

 

 

 

\

\),
3 A in leaves, and they make
L 6 ie up wholly the secondary
zm growth of Pines and other
4 S \4 a coniferous woods (Figs.
Hy Ye iE 102-4). Ducts develop
4 S “al fA usually from a single row
Al eZ of cylindrical cells by
r Z absorption of the inter-
4 Cl I A mediate walls ; but some-
F OV |Z Z times many rows of cells
© a are involved, in which
& az case the duct becomes
| re large and visible to the

 

 

   
 

eye, as in Oak and some
[Aj vines, the single-row type
being usually invisible
Fic. 101. — Generalized drawings of without a lens. Though
typical tracheal elements ; highly magni- tubular in structure,
fied. From left to right, a fiber-tracheid ; ;
pitted and spiral tracheids; spiral and ducts are never unlim-
pitted ducts, which show end walls and ited in length ; many are
remnants thereof. (From Strasburger.)
not more than a few
inches, few exceed a few feet, and the longest, which occur
in some vines, are only a few yards in length. In all known
cases, however, the ends of ducts and tracheids are in con-
tact with others of like sort, and the intermediate walls are
so constructed, with guarded thin areas, as to permit a ready

Qs =
SS,

    

fa

 

 

 
Cx. IV, 6} TRANSFER THROUGH PLANTS 147

passage of water moving at the ordinary rate, while resisting
any forcible rush of the water along the stem under suddenly
developed pressures. Thus the tracheids and ducts form
water-conducting systems of unlimited length, even though
the length of the individual elements is restricted. These
thin areas, however, exist not only at the ends of the ducts,
but throughout _ their
lengths, where some-
times they appear as
bordered pits in an
otherwise thickened
wall, as is very charac-
teristic of the conifer-
ous wood (Fig. 104)
or else as the meshes
of a reticulation: or

 

   

 

 

 

 

ir

Fic. 102. — Cross
wood of Pine;

section through the
highly magnified.

as thin parts between
spiral or annular thick-
enings (Fig. 101),—all
of which distinctive
arrangements represent
different ways of com-

The cells are mostly tracheids with bor-

dered pits, visible in the walls. Note the
medullary rays and the abrupt transition
from autumn to spring growth. In the
autumn wood is a resin canal. The line
in all of the walls is the middle lamella, i.e.
a plate representing the wall first formed
before thickened by additional layers, and
somewhat different in chemical and physi-

cal composition from the latter. (Reduced

bining a thickening of ©¢
from Cavers, Practical Botany.)

the walls with the
presence of thin places through which water may move to
other ducts or tracheids, or to neighboring tissues. The
annular and spiral markings are usually found in ducts or
tracheids of the primary growth, in which elongation is still
in progress, while other kinds occur in the secondary growth,
where elongation has ceased. Both tracheids and ducts,
when mature, are without protoplasm, forming non-living
tubes. Their mechanism, as a water-conducting system, is
shown in our diagrammatic figure 105.

The all-important question as to the forces by which the
water is lifted through the ducts has been answered by in-

vestigators in several different ways. In earlier times it
148 A TEXTBOOK OF BOTANY (Cu. IV, 6

was thought that the water ascends by capillarity, precisely
as oil rises in a wick, or water in a towel; but experiments
have proven that water cannot thus rise in wood more than
a few feet. Also it has been held that atmospheric pressure,
by which water is raised in a pump to a height of thirty-two
feet, would explain it, the greater height reached in trees
being supposed to
result from the fact
that intermixed air
makes the water in
the ducts much
lighter than with-
out it; but further
study has shown

   

 

ein ee 5 z :
‘ei A (Of2 OF POreioleRieid: VEE that neither are the
Fic. 103.— Radial section (7.e. parallel with conditions in the
a medullary ray) of the Pine of Fig. 102. plant suitable for

A medullary ray runs across the tracheids, ee

with the younger and outer end, containing pro- the oper ation of
toplasm, on the left. Outermost, on the left, are atmospheric pres-
sieve cells showing sieve plates on the walls; next 3
is cambium; then tracheids of the spring wood, ‘Ure, nor would it
showing the characteristic bordered pits; then suffice in very tall
autumn wood containing a resin canal. Com- =
parison with Fig. 102 will show the construction trees. Also it has
of a bordered pit, across which extends a thin been argued that

and flexible plate. (From Cavers.) he spain Saeed
We r ascends

in the walls of the xylem by a process of tmbibition, due to
the attraction of wood for water, in precisely the same way
that water passes into wood across the grain; but experi-
ments have shown conclusively that the water goes through
the cavities, not the walls, of the ducts. Still later it was
claimed that living cells, in wood parenchyma or medullary
rays, which accompany the ducts, act as a means of pro-
pulsion of the water upward, each living cell absorbing water
from the upper end of one duct and forcing it into the lower
end of one higher, somewhat on the analogy of tiny force
pumps; but experiments seem to have proven that the water
still rises when all living cells are killed by poisons. Finally,
Cu. IV, 6] TRANSFER THROUGH PLANTS 149

most recently of all, a new and striking explanation has been
offered, with much experimental support, to the effect that
the water rises by traction, i.e. is drawn up in long threads,
as if solid, by forces acting in the leaves. This matter needs
somewhat fuller explanation.

In the next chapter it will be shown that the forces of
osmotic pressure, operating in the roots, draw water from
the soil and give it a start up the

: 5 AVY S| es
stem; also the same forces in the ANY Fy Gg i
leaves draw water in the same Fey | | A 83
way from the ducts into the leaf * A) #| t fy
cells. Now it is found that the f Ai 4 :
forces thus exerted by the leaf i ig of
cells are amply powerful to lift } 9 | | f y Ny
the water to the tops of the tall- | | Bf \ ‘
est trees if only the water in the LY PERS Whe

RAE LORIE IA

 

 

ducts would hold together in ' ;
iret Th h . pes Fic. 104. — Tangential sec-
threads. e new theory Main- tion (i.e. at right angles to a

tains that the water does thus aes ng’ of the re Ws
mets . gs. 102-3, i 7
hold together, as if in solid oe porter
threads, by virtue of its own | The tracheids, with; their
‘ ; fas bordered pits, are plain, as are
internal cohesion, a property the cut ends of the medullary
which is manifest in part in the rays, of ee — a
surface-tension familiar to all "™ on) (from Cavers.)
students of physics. Everybody knows that a large water-
drop hanging free from the under side of a glass plate can
be lifted with the plate, and it seems clear that the water
could be lifted in much larger masses if lengthened out to
very thin threads, as it is, of course, in the ducts. The
water thus pulled into the leaves by the osmotic power of
the leaf cells is removed from those cells by the still greater
power of evaporation (transpiration), the energy for which
is supplied by the heat of the surroundings. Thus, on this
theory, it is really the energy of evaporation which raises the
water in tall trees. But while evaporation is the principal,
it is not the only source of energy available, for obviously
150 A TEXTBOOK OF BOTANY (Cu. IV, 6

any power which will draw water into the cells will lift the

water-columns in the ducts. This

 

 

 

 

 

 

 

 

 

 

 

Bar

Fic. 105.— Plan of the stem as a con-
ducting mechanism, arranged as in Figs. 11
and 166, with similar signs for protoplasm,
water, sugar, and proteins. On the left is the
pith, and then, in order, two ducts, a sieve-
tube, phloem parenchyma, cortex, and cork
with lenticels.

result follows from
various chemical or
physical processes in
which water is ab-
sorbed ; and such se-
eretory actions are
believed to explain
the lifting of the sap
in the spring before
the leaves are de-
veloped. This ex-
planation is not yet
universally accepted,
many botanists still
holding that the liv-
ing cells along the
stem are the chief
factor in the process.

It must, of course,
be true that the
greater the height of
a tree, the greater
the difficulty of rais-
ing a sufficient
transpiration supply
against the increas-
ing hydrostatic re-
sistance, and the fric-
tion within the small
ducts. Thus a limit
is imposed to the
height of trees, which
potentially can grow

upwards indefinitely, but actually have heights approxi-
mately fixed for each kind. Our ordinary deciduous trees,
Cu. IV, 6] TRANSFER THROUGH PLANTS 151

after reaching their heights, tend to spread out laterally and
become flat topped; and the dead branches which occasion-
ally occur above their green summits represent certain ones
which were able to exceed the ordinary limit in an especially
wet summer, but died in a dryer one. Trees which attain
to the greatest heights are apparently such as have par-
ticularly favorable structural relations to conduction or
transpiration.

It is well known that in spring the sap exudes readily from
injuries in many kinds of plants. Thus Grape Vines if
pruned too late in the season will “bleed’’ very copiously,
and large drops of sap often fall upon sidewalks from broken
twigs or bark of shade trees; and the flow of sap in the Maple
in spring is a well known phenomenon. These are evi-
dently cases in which the sap is forced out by osmotic pres-
sure in the roots, complicated, however, by osmotic pres-
sures in the stem, and by expansion and contraction, under
varying temperatures, of the air which occurs in the stem.
The internal pressure developed by such expansion explains
why the flow is greatest on a warm day after a cold night.

The transfer, or more technically, TRANSLOCATION, of food
through the plant occurs only in solution in water. In
general the food has three paths. First, it may pass directly
from cell to cell through protoplasm and walls by the power
of diffusion, later to be studied. This is the sole method in
the lower and simpler plants, and is that by which the food is
removed from the chlorenchyma cells of the leaf to the near-
est veinlets (page 32). It is also the method by which the
food spreads from the ends of the veins to the growing cells
in all parts of the plant. Second, in woody plants in spring
the food stored in the roots or lower part of the stem is trans-
ferred into the ducts (along the medullary rays from the
bark), where it is rapidly lifted to the growing leaves with the
rising water current. This explains the presence of sugar
in the sap of Maple and other trees in spring, though later
in the season the sap is nothing other than soil water. Third,
152 A TEXTBOOK OF BOTANY [(Cu. IV, 6

the translocation occurs along
special cells and vessels of
the veins. Thus the carbo-
hydrates, as already explained
(page 31), move along the vein
sheath in the leaves, and along
parenchyma cells, chiefly of
the phloem but partly of the
xylem, in older bundles. All
such conducting cells are elon-
gated, possess plain walls, and
contain protoplasm. The pro-

 

 

Fic. 106. — The sieve plate, on
surface and in section, of a typical : j
sieve-tube of Squash; x 400. teins have their clear paths of

Note that the protoplasm extends : a Rete
Sa Hiab. Rican TimanEn WieSbeene conduction in the sieve-tubes,

ings in the plate. (From Stras- of which this is the distinctive
bores) function. The sieve-tubes are
elongated, thin, and smooth-walled cells, arranged in lines,
with their intermediate walls perforated (Fig. 106); and they
have always a lining of living pro-
toplasm. Associated with them are
certain companion cells, which have
seemingly a part, though an un-
known one, in their function. So
far as known, the proteins move
along the sieve-tubes solely by dif-
fusion from the places of greater to
places of lesser abundance. The
extreme slowness of this method,
however, suggests that the living
protoplasmic linings of the sieve-
tubes may act in some way to force
the movement, though there is no
evidence thereof.

Thus it is true in general that Via. 107, — The result of
the movement of water in stems is COnstiction by a label wire

: : ona growing shrub. (From
chiefly in the wood, and the move- Bailey.)

 
Cx: EV, 7] GROWTH OF PLANTS 153

ment of food is in the bark. Accordingly, if the bark be
constricted, thé passage of food downward from the leaves
is stopped, and its accumulation produces a swelling above
the constricting object. This happens in botanical gardens
where labels are attached by wire, or in shade trees which are
strengthened by iron bands placed around them (Figs. 107,
155). The swelling may, however, go so far as to make the
bark over-arch the constriction and establish a new connec-
tion beyond it, thus burying band or wire completely.

7. THE GrowTH oF STEMS AND OTHER Puant Parts

It happens that stems exhibit the principal phenomena
of plant growth more clearly than any other parts. Ac-
cordingly we may best consider that important subject
at this place.

The growth of the higher plants differs from that of the
higher animals in this, that while animals develop a single
set of organs which serve throughout life, plants exhibit a
constant and indefinite repetition of a few primary organs,
—leaves, stems, roots, flowers, fruits, and seeds. But the
general mode of growth of an individual leaf, stem, root,
or other part is much the same as that of an animal organ.

The growth of each individual organ or part, whether of
animal or plant, exhibits three stages, which often overlap,
but commonly are more or less separate. They are, first,
DEVELOPMENT, or formation in the embryonic state: second,
ENLARGEMENT, or increase in size, and third, MATURATION,
or ripening to full functional efficiency. The difference
between the stages comes out very clearly in case of the
spring vegetation. First, as everybody knows, the flowers
and leaves which unfold in early spring were all formed or
developed the preceding season, and existed over winter
folded compactly away in buds, in which they can easily
be found. The microscope shows that most of the cells of
those parts are perfectly formed, though small and filled
with food. Second, in the early spring the buds, absorbing
154 A TEXTBOOK OF BOTANY [Gay IVF

water from the stem, swell and open, and the leaves and
flowers push forth and rapidly enlarge to their full size,
as the familiar Horse Chestnut so strikingly illustrates.
The microscope shows that this expansion is chiefly effected
by a great increase in the size of the previously-formed
cells, of which the large cavities are now empty of food, but
tensely filled with water. That this enlargement is really
brought about by absorption of water is very well proven by
the fact that many such greatly-enlarged parts if thoroughly
dried in an oven are found to be no heavier than in the
original buds. This is especially clear in embryos, which,
often germinating to a size a dozen or more times that which
they had in the seed, actually lose dry weight in the process.
The functional reason for this rapid spread of a little dry
substance upon a great deal of water is plain; the plant mode
of life requires a spread of green surface as early as possible.
Third, the enlargement nearing completion, the parts mature,
acquiring the final details of their coloration, thickening, and
other features connected with the more effective performance
of their functions. In this state they continue until their
function is completed, after which they die and vanish
through decay.

The development, or formation (as distinct from enlarge-
ment), of new parts takes place in meristematic cells (Figs.
92, 162), which divide more rapidly in some places and direc-
tions than in others. These divisions are apparently effected
by the cytoplasm of the cells, which is controlled in the pro-
cess by the chromosomes of the nuclei, as will appear more
fully in connection with reproduction.

We turn now to enlargement, the most striking and familiar
phase of growth. So rapidly does it proceed in the stems of
some plants out of doors in spring that its progress is visible
from day to day. It is said that the growth of Bamboo in
the tropics is so rapid as to be directly perceptible at times to
the unaided eye, reaching often to more than two feet within
twenty-four hours. Ordinarily, however, growth proceeds
CHV; 7] GROWTIL OF PLANTS 155

so slowly as to need the microscope (specially arranged hor-
izontally for the purpose), or else some other magnifying

 

 

 

Fie. 108. — An auxograph in operation; x ?.

The thread from the tip of the plant passes over the smaller of two
united wheels, and is kept just taut by the weight of the pen, which
hangs on the end of the thread passing over the larger wheel. The pen
rests against the cylinder, which is turned once an hour by a clockwork
in the closed case. The growth spiral marked by the pen is exaggerated
in the cut.

device, to make it apparent. A very effective instrument
for this purpose, called an aAUXOGRAPH (meaning “growth
writer’’), is shown by our picture (Fig. 108). The arrange-
 

 

A TEXTBOOK OF BOTANY (Cu. IV, 7

ment is such that the growth of the plant permits
a magnifying wheel to turn, and thereby a pen to
fall along a cylinder which revolves once an hour,
and upon which, accordingly, the pen marks a spiral
line crossing any vertical line once an hour. Thus
is obtained, night and day without break, an auto-
graphic record of the plant’s growth, an example of
which, precisely traced, is given herewith (Fig. 109).

As shown by such an instrument, the growth of
any individual part, such, for example, as the
flower-stalk of some bulbous plant, exhibits always
two striking features. First, as our record well
shows, there are many marked fluctuations in the
rate. Second, aside from the fluctuations, one can
always see that the rate of growth, instead of being
uniformly rapid from start to finish, exhibits a slow
beginning, a rise up to a culmination where it is
most rapid, and then a gradual fall away to cessa-
tion as the part approaches maturity. This mode
of enlargement, which apparently results inciden-
tally from the way the cells expand, is called the
GRAND PERIOD. It is apparently characteristic of
the growth of all individual parts, viz. of single in-
ternodes of stems, of leaves, flowers, fruits, and
really (though not apparently) of roots. In struc-
tures composed of many unit parts, however, as in
a stem with a number of internodes, the grand
periods of the parts often overlap, and thus yield

Fra. 109. — The complete record, obtained by the auxograph
of Fig. 108, of the growth of a flower stalk of Grape Hyacinth,
from its appearance above ground until the completion of blos-
soming. It is reduced photographically, from the S-times mag-
nification on the record papers, to the scale of the actual growth.
It is also inverted from the record papers and therefore stands
in the true position of the growth. Each space on the vertical
line marks one hour, and the heavy horizontal lines indicate
noon of each day. The lines which run together in the greatly
reduced cut are perfectly distinct in the original record.
Cu. IV, 7] GROWTH OF PLANTS 157

collectively a continuous record. The grand period is
most familiarly manifest in the opening out of the spring
vegetation, in which, after a slow expansion of the buds, the
actual opening takes place very rapidly, and full size is soon
reached. In general the spring vegetation opens out on the
crests of the grand periods of the parts concerned.

As to the minor fluctuations of the growth records, they
are found by experiment to have precisely the meaning
which one naturally ascribes to them, viz. they are con-
nected with the weather. If careful comparison be made
between rate of growth and the contemporaneous mete-
orological conditions, the following general results become
evident.

1. Higher temperature promotes growth, and lower checks
it. This fact is of course sufficiently familiar, for everybody
knows that plants grow faster in warm weather and slower
when cool. The reason thereof is this, — growth involves
a number of physical and chemical processes, all of which
are directly promoted by heat. However, so far as plants
are concerned, there are limits to its favorable action, be-
cause above ordinary temperatures heat begins to act in-
juriously upon the protoplasmic constituents, especially
the susceptible proteins, which easily coagulate. Each
plant has a minimum temperature below which it does not
grow at all, an optimum temperature at which it grows fast-
est, and a maximum temperature beyond which it ceases to
grow. The conventional constants for these cardinal points,
for our common plants collectively, are 5° — 30° — 40° C.
The matter is illustrated very graphically when plants are
grown in a differential thermostat (Fig. 110), in which they
can be made to plot their own curve, so to speak, of their
growth in relation to temperature. By use of this instru-
ment the three points may be determined for any given
plant, and thus it is also shown that in general the points
range higher in tropical plants, lower in the arctic kinds, and
intermediate in those of temperate regions. Since the
158 A TEXTBOOK OF BOTANY (Cu. IV, 7

cardinal points are ingrained in the protoplasm of particular
species, it is clear why tropical plants do not thrive when
taken abruptly to the arctics, and vice versa. The points,
however, are alterable somewhat with time, as manifest in
acclimatization.

2. Humidity in the atmosphere (distinct altogether from
the water supply at the roots) promotes growth, while
dryness checks it. Byer pad knows that plants grow best,
on the whole, during close, ‘‘muggy’’ days, the kind most
uncomfortable to us; and such days are called ‘growing
days’’ by some farmers. The reason thereof is chiefly this,

 

 

 

Fic. 110. — Effect of different temperatures upon Oats grown in a dif-
ferential thermostat, an oceans evenly heated from the right side and
cooled from the left; x ;',

that the enlargement of young parts is forced by the in-
ternal pressure of water (the osmotic pressure explained in
the next chapter); this internal pressure is lessened by re-
moval of much water through high transpiration caused by
dry air, but is maintained with low transpiration accompany-
ing humidity of the atmosphere. Other things being equal,
the growth of vegetation is more rapid in regions of moist
climate, and slower in dry ones, as forests and deserts respec-
tively illustrate. The matter, however, is complicated by
the matter of water supply to the roots, and other consid-
erations. :

3. Light affects plants complexly. Indirectly, of course,
it is essential to green plants through its relation to food sup-
ply. Directly, however, it has no great effect unless very
strong, when it tends 0 check the growth of most plants,
which can thrive best under some shade, as we have earlier
noted (page 95). The records of auxographs show very
Cx. IV; 7] GROWTH
clearly that most plants,
when temperature is the
same, grow faster at night
than in daytime; and this
fact is familiar to garden-
ers. This greater growth
at night, however, is chiefly
a result of the fall in tran-
spiration whichaccompanies
darkness, but secondarily
there seems also involved
some release from a direct
check imposed upon growth
by very bright light. In
such a case the light is sup-
posed to act through the
unfavorable chemical influ-
ence of the blue rays upon
the living protoplasm, — an
action which, in forms like
the Bacteria, where the pro-
toplasm is unshielded by
chlorophyll or other color-
ing matters, makes strong
light actually germicidal.
But if plants are continu-
ously exposed to too little
light, they tend to “draw,”
as the gardeners say; that
is, they become pale, in
obvious partial starvation,
and elongate greatly at ex-
pense of other growth (Fig.
111). This elongation has
been commonly supposed
to represent an adaptive

OF PLANTS 159

 

Fie. 111. — Effect of different in-
tensities of light upon the growth of
Peas; x3. a, in darkness; 6, in +
light; c, in full light. While for a time
the growth in darkness is much the
greater, the extra bulk is chiefly water,
there is loss of dry weight, and the
plant ultimately dies of starvation.
(From Duggar.)
160 A TEXTBOOK

OF BOTANY (Cu. IV, 7

self-adjustment, tending to raise the green tissues into better
light conditions, when the plants happen to start in dark

 

 

 

 

Fig. 112.— The place of elonga-
tion in growth of a typical stem of
Lysimachia vulgaris, as shown by
the spread of marks at first evenly
spaced; X 3. (Modified from Errera
and Laurent.)

places.

4. Many minor condi-
tions likewise affect growth,
notably special gases in the
atmosphere, peculiar condi-
tions of soil and water,
electrical currents, or baro-
metrical pressure. Some of
these conditions accelerate
growth, while others retard
it, though mostly in minor
degree, and often in devi-
ously indirect ways.

All growth requires, in
addition to warmth and hu-
midity, a supply of water,
food, and air. The water,
absorbed by the roots, is
needed for the swelling of
the young parts. The food,
whether made in the green
leaves or absorbed from
some other source, is needed
partly as building material
for the enlargement of cell
walls, and partly as a
store of energy for effecting
the plant work. The air,
more properly the oxygen,
is needed to release the
energy in the food through
the action of respiration, a
fundamentally —indispensa-

ble process, considered in the following section. As by-
Ca LV; (7 GROWTH OF PLANTS 161

products of respiration, both carbon dioxide and heat are
released in growth.

All of the phenomena here described in connection with
stems appear also in leaves, roots, and other parts, as shown
by use of a suitably
modified auxograph, or
by the measuring mi-
croscope. All parts are
found to exhibit the
grand period, and the
fluctuations in relation
to the weather; and == ! ris
the only appreciable NER AIS L TY
differences in their SCH OS
modes of growth are
associated with their M
differences ii constic: Fie. 113. =the place of expansion in

growth of a typical leaf of English Ivy, as

tion. Stems grow by shown by the spread of the marks evenly
“made on the small leaf at the right; x 4.

    
  

the progressive elonga-
tion of their internodes, which, forming in the bud, go
through their grand periods in succession; and thus is pro-
duced the familiar appearance
which has been compared with
the opening out of the joints of a
telescope (Fig. 112). The leaves,
quite differently, expand pretty
evenly throughout their structure
(Fig. 113), going each through its
grand period, as do flowers and
: I fruits. The roots, on the other
"Se hand, grow almost solely near
iM their tips (Fig. 114), their indi-
Dn. alee ene Saas n k vidual cells passing very quickly
elongation in growth of a typ- through the grand period, on
ical root of Corn, as shown by Which account roots seem as a
the spread of evenly spaced
werk whole to grow evenly. This
M

 
162 A TEXTBOOK OF BOTANY (Cu. IV, 8

mode of growth for roots is fully explained by facts in their
physiology and structure, as will later be noted.

8. THe R&sSPIRATION OF PLANTS

This process has received frequent mention in the foregoing
pages, and in ways which imply much importance. Since
its phenomena are more clearly manifest in connection with
growth than elsewhere, it may best be considered at this
place. Respiration and photosynthesis are without question
the two most important of all physiological processes.

A demonstration of typical plant respiration is the fol-
lowing. In the chamber of a respiroscope (a ‘respiration
demonstrator’’), like that pictured in Figure 115, there is
placed a handful of germinating seeds, and the instrument
is stood in a warm, darkened place for twenty-four hours;
then water is poured down the thistle tube, and the air of
the chamber thus forced out goes bubbling up through
limewater placed in the cylinder. The limewater speedily
turns milky, thus proving by this familiar test that carbon
dioxide was present abundantly in the chamber. If the
student should ask the natural question whether the carbon
dioxide known to be present in air would not account for
this result, the answer is given by the other or ‘control’
chamber, the air of which, sent through similar limewater,
leaves it quite clear. In fact the quantity of carbon dioxide
present in so small a volume of air is insufficient to show any
effect by this method. It is therefore evident that carbon
dioxide has been produced by the germinating seeds. If,
further, a sample of the gas is withdrawn from the chamber
containing the seeds and chemically analyzed, as can be
done very easily, these additional facts appear. First, some
of the oxygen originally present in the air has disappeared,
and various evidence shows it has been absorbed by the
seeds. Tn starchy seeds, like Outs, the oxygen absorbed and
the carbon dioxide produced are approximately equal in
volume, though often this RESPIRATORY RATIO is different.
Cu. IV, 8] RESPIRATION OF PLANTS 163

Second, not only carbon dioxide but also water is released
by the germinating seeds. Thus in the growth of seeds,

 

 

 

 

 

 

 

 

Fic. 115. — A respiroscope; X }. While the seeds are germinating, the
chamber is kept closed by the smaller rubber stopper and clamp, though
these are removed in applying the test.

oxygen is absorbed and carbon dioxide and water are
released. This, however, is precisely what occurs in the
respiration of animals, including mankind. The process in
164 A TEXTBOOK OF BOTANY [Cu. IV, 8

the seeds is in fact respiration, identical in all essentials
with that familiar in animals.

If, now, other plant parts be placed in the respiroscope
chamber, — growing bulbs, roots, buds, flowers, fruits, leaves,
stems, —in fact any growing plant parts, and if the same
tests and analyses as before be applied, then the general re-
sult is always the same. It is true, there are cases in which
oxygen seems to be absorbed without release of carbon
dioxide, and cases in which carbon dioxide seems to be
released without absorption of oxygen, with all intermediate
gradations. These exceptions, however, from the point of
view of respiration, are not real, for in the former case the
carbon dioxide is known to be retained and used in forma-
tion of special substances, and in the latter the oxygen is
taken, not from the air, but from compounds composing the
plant tissues themselves. And here is one further point of
the first importance, fully proved by respiroscopes. Respira-
tion occurs in general not only in tissues which are growing,
but in all tissues which are alive. In brief, respiration is
found to occur in all living and working parts of plants and
animals alike. Its amount depends, as a rule, upon the
activity of the parts, being directly proportional thereto.

But what of green leaves, which were earlier shown to
perform an opposite process in their photosynthesis, absorb-
ing carbon dioxide and releasing oxygen? If, with green
leaves in the chamber, the respiroscope be kept in the light,
then indeed no carbon dioxide shows in the test; but if the
instrument be kept in the dark, then that gas is yielded
abundantly. This does not mean that respiration stops in
the light, but only that then it is surpassed by photosyn-
thesis, which for the same area of leaf is, on the average,
about twelve times more active than respiration. This fact
explains why photosynthesis, even though in progress only
a part of the time, can purify the air, despite the unceasing
respiration of both plants and animals.

Respiration is little affected by darkness or light. Ac-
Cu. IV, 8] RESPIRATION OF PLANTS 165

cordingly, since photosynthesis is completely dependent
on light, there must exist a certain light intensity, at which
the photosynthesis and respiration of a leaf exactly balance
one another. This must happen twice a day, morning and
evening, and perhaps during very dark days. In such case
it is probable that the two processes use reciprocally one
another’s products, and neither of the gases passes into or
out of the leaf. It is because photosynthesis stops, while
respiration continues, in the dark, that plants are unhealth-
ful in sleeping rooms at night, though in truth the effect is
quantitatively small, else one could not camp overnight in
the woods.

In respiration the oxygen comes from the air; but the car-
bon of the carbon dioxide, and the hydrogen of the water
come from the food, in which they were incorporated, in the
original grape sugar, by photosynthesis. Since respiration
thus withdraws solid material from the body, it is always
accompanied by loss of weight, and, unless compensated by
addition of food, it ends in emaciation and starvation.

Viewing the process now in the large, and centering atten-
tion upon the end substances involved, we find that respi-
ration may be expressed in an equation as follows, —

C;.H»Os + 605 = 6 CO. +6 HO.

This equation, which may be termed the respiratory equation,
is the exact reciprocal of the photosynthetic equation (of
page 23).

The exchange of gases here described is not, however, the
important feature of respiration, but merely an incidental
accompaniment thereof. The central fact, and explanation
of the great physiological importance of the process is this, —
that it releases the energy latent in potential form in the
food, — which energy, set free at precisely the points of need,
supplies the power which does the work of the plant in its
growth and other processes. Respiration is in fact identical
in nature with combustion, which, in a steam engine, pro-
166 A TEXTBOOK OF BOTANY (Cu. IV, 8

duces a union of oxygen with the carbon in coal, whereby,
with formation of carbon dioxide, energy is released i
the form of heat; and this heat expands the steam which
drives the engine and does the work. Both respiration and
combustion are essentially energy-releasing processes, acting
alike through oxidation of carbon; for itis one of the most
fundamental of cosmic facts, that whenever and wherever
carbon is allowed to unite chemically with oxygen, energy is
set free, and can do work, whether in the cells of a plant,
the muscles of a man, or the boiler of an engine. Respiration
is in principle nothing but a slow regulated combustion
within the bodies of plants and animals under protoplasmic
machinery capable of turning its energy into work.

What now is the original source of the energy thus con-
tained in the food? Energy exists in nature in two forms,
kinetic or active, like heat, light, and electricity, and potential
or latent, as in wound springs, raised weights, explosives,
and unoxidized chemical substances in general. In fact,
potential energy exists in all unsatisfied affinities, physical or
chemical. When kinetic energy is used to wind springs,
lift weights against gravitation, or separate substances from
close chemical unions, it goes thereby into the potential
form, and thus remains until the spring unwinds, the weight
falls, or the chemical unions again take place, during which
processes the potential energy again becomes kinetic, and
can be used to do work, as clocks, water power mills, and
artillery illustrate. The kinetic energy of the sunlight in
photosynthesis originally dissociated the carbon dioxide and
water into their constituents, thereby passing into potential
form. The oxygen was released into the air, but the carbon
and hydrogen remained in the food, and thus were trans-
ferred throughout the bodies of plants and animals, carrying
the store of potential energy, which, on chemical access of
oxygen in respiration, again becomes kinetic and does work.
The process has an accurate parallel in the case of the storage
battery, now familiar in the modern automobile. A charge
Cu. IV, 8] RESPIRATION OF PLANTS 167

of electricity, kinetic energy, is sent into the battery, and
there forcibly dissociates certain stable chemical compounds
into simpler substances; whilst these substances remain
apart the energy is latent, ready to come forth once more
in kinetic form (again an electrical current), when, by the
closing of a circuit, the substances are allowed to re-combine
into the old compounds. Food is nothing other than a stor-
age battery, charged in green leaves by the sun, and discharged
in the body by respiration.

Thus it is plain why oxygen is necessary in respiration,
and therefore why both plants and animals have need of
“air.” The parts of plants above ground have an ample
supply admitted through stomata and lenticels along the air
passages; but the case is different with roots, for often the
air is expelled from the soil by the presence of water. This
is why wet soils must be drained, — not to remove water, but
to introduce air. Where roots live continually in water, as
in bogs and swamps, the air supply is usually obtained
through large passages extending down through the stems
from the leaves; and it 1s the presence of these ample air
passages which gives the soft spongy texture to so many
marsh plants.

That respiration is indispensable to all plant processes in-
volving growth and movement can be proven very strikingly
by methods which deprive the parts of oxygen, while leaving
them otherwise uninjured and ready for work. This is ac-
complished by use of an instrument called the anoryscope
(“without-oxygen demonstrator”) shown in our picture
(Fig. 116).

The energy released in respiration is mostly applied to
various kinds of work involving motion, — the circulation
of protoplasm, the enlargement of cell walls, and the like;
and on the completion and cessation of motion it is con-
verted into radiant heat. Thus all growing, and even all
living, parts are somewhat warmer than their surroundings.
A thermometer thrust into the opening flower of a Calla
168 A TEXTBOOK OF BOTANY [Cu. IV, 8

or a Jack-in-the-Pulpit will rise two or three degrees, and in
some tropical plants of the same family, two or three times
as much; while in any living parts the release of some heat

 

 

 

 

 

 

 

 

 

Tia. 116. — An Anoxyscope; x 7. The two tube-chambers are exactly
alike except that from the one on the right all oxygen is removed by a
chemical absorbent (pyrogallate of potash), which is replaced by pure
water in the one on the left. In the case here shown, the soaked oats placed
on wet moss in both chambers have grown well in ono case but not in the
other. The instrument permits many analogous experiments.
Cu. IV, 8] RESPIRATION OF PLANTS 169

may be proven by suitable methods, as explained with our
picture (Fig. 117). It is this same respiration heat, intensi-
fied and regulated, which yields the “blood heat”’ of animals.

It will now be worth while to summarize the contrast be-
tween photosynthesis and respiration, as is possible in a
table.

Photosynthesis Respiration
Occurs only in green plants Occurs equally in all plants
and animals

Takes place only inthechloro- Takes place in all living

phyll grains protoplasm
Proceeds only in light Proceeds in both light and
darkness
Constructs food Destroys food
Increases weight Lessens weight
Absorbs carbon dioxide Releases carbon dioxide
Releases oxygen Absorbs oxygen
Forms grape sugar from car- Reduces grape sugar to car-
bon dioxide and water bon dioxide and water
Stores energy Releases energy

Thus respiration destroys the product of photosynthesis,
but in the process develops the driving power for plant and
animal life.

There is a form or phase of respiration so important as
to demand special treatment, and that is FERMENTATION.
If some Yeast (which is a unicellular microscopic plant, to
be fully described in Part II of this book), in the form of
ordinary compressed Yeast, be placed in a flask with water
and sugar, and stood in a warm place, within a few minutes
the mixture begins to ‘‘work”’ or ferment, producing many
fine bubbles which rise through the liquid and form a froth at
the top. If, from this flask a bent outlet tube be led under
limewater (Fig. 118), through which the escaping gas can rise,
then the limewater will turn very milky, proving the gas
to be carbon dioxide. After fermentation has ceased, the
170 A TEXTBOOK OF BOTANY (Cu. IV, 8

 

 

 

 

 

 

 

Fie. 117. — A ‘Caloriscope; x ¢. the two
Dewar bulbs have double walls with a vacuum
between (as familiar in ‘‘Thermos’’ bottles), and
hence prevent loss of heat. The respiring tissue to
be tested is divided into two parts, and placed in
the two. flasks, but is immediately killed in one.
Thus the delicate thermometers are made to record
within a few hours the degree of heat released by
the respiration of the living tissues.

 

liquid in the
flask is found to
contain a con-
siderable quan-
tity of alcohol,
together with
small amounts of
other substances
known to be
formed by the
activity of the
Yeast, which all
this time is in
vigorous growth
and multiplica-
tion. If certain
mineral salts, es-
sential to the
metabolism of
the Yeast, are
present, the pro-
cess continues
until the sugar
is exhausted, or
else until the in-
creasing quanti-
ties of alcohol in-
hibit the further
growth of the
Yeast. Thus, un-
der experimental
conditions, the
Yeast plant in
its growth  fer-
ments sugar to
carbon dioxide
Cu. IV, 8] RESPIRATION OF PLANTS 171

and alcohol, though it is ultimately overcome by the alcohol
it produces. As to the significance of fermentation, all evi-
dence unites to indicate that it represents primarily the
Yeast’s form of respiration, exaggerated usually by unnatural
experimental conditions. Shut away from the air, the Yeast
is unable to take oxygen from that source, and has to resort
to the supply contained in the
grape sugar, into which this plant
has the power to convert cane
sugar or starch. By action of an
enzyme, the sugar is broken up,
whereupon its carbon unites with
oxygen from the same molecules
into carbon dioxide, with the usual
release of energy at the moment
of union. The remainder of the
molecule falls as a natural chemi-
cal incident into alcohol, accord-
ing to the following equation, —

C.H»O; = 2 CO; + 2 C.H,O.

Thus in the formation of the
carbon dioxide, a process essen- _ Fic. 118. — Arrangement
taal to the Yeast, the aleghol is (2 Sepemres oF ee
incidentally and, so to speak, in- mentation; x 3. Explanation
evitably formed. nes

That fermentation is fundamentally the Yeast’s respiration
is attested by many lines of evidence, including the existence
of intermediate steps between fermentation and ordinary
respiration. Thus Peas, which ordinarily respire in the usual
way, can also form carbon dioxide, with incidental production
of alcohol, when derived of all free oxygen; and this power
is probably rather widespread, though in limited degree,
among plant tissues which have imperfect access to air.

The production of carbon dioxide and alcohol by fermen-
tation renders the process of great importance toman. Thus

6

 

 

 

 
172 A TEXTBOOK OF BOTANY [Cu. IV, 8

we use the carbon dioxide released by Yeast for raising bread,
not, it is true, because it is carbon dioxide, but because it is
a gas which can be released conveniently all through the
dough, from which, along with the alcohol, it is driven out
by heat in the baking. Far more extensive is our use of
fermentation for production of alcohol. Thus the fermenta-
tion of grape juice yields wine, and of germinating barley,
beer; and any germinating grains can be fermented in like
manner. Fermentation, however, can never yield alcohol
stronger than that which inhibits the growth of the Yeast,
viz. not over 15 per cent; and for stronger solutions we must
resort to distillation. This process depends on the fact that
the boiling point of alcohol is lower than that of water (78°
against 100° C.), on which account it is possible, by suitable
regulation of the heat, to cause most of the alcohol to evapo-
rate without much water, the vapor being collected and con-
densed. In this way are obtained the stronger alcoholic
liquors, — whisky, rum, gin, brandy, etc., which consist of
approximately 50 per cent alcohol, and 50 per cent water,
together with various flavoring and other substances, some-
times naturally occurring in the distillate and sometimes
artificially added. The strong alcohol used in medicine and
the arts (about 95 per cent) is obtained by repeated distilla-
tion, while pure, or absolute, alcohoi is secured only by use of
chemical methods which remove the last traces of water.
Fermentation is a phase or form of respiration, and a
special form of fermentation is ppecay. Everybody now knows
that decay is caused principally by the activity of micro-
organisms, Molds and Bacteria especially. Accordingly we
can preserve organic substances indefinitely by any methods
which inhibit the action of those organisms, as do heat, poi-
sons, and strong solutions of sugar and salt. When these
Molds or Bacteria reach suitable solutions of organic sub-
stance, whether in plant tissues or animal bodies, they act
precisely as does the Yeast in the solutions of sugar, — viz.
deprived of an adequate air supply (some kinds indeed never
Cu. IV, 8] RESPIRATION OF PLANTS 173

use the air, and even are fatally affected by its presence), they
break up the molecules of organic substance, and therefrom
obtain materials for their respiration and growth, simul-
taneously forming various by-products. Usually they re-
lease carbon dioxide in their respiration or fermentation,
but sometimes they form other gases which happen to possess
the offensive odors familiarly associated with decay. Some
kinds of organisms start the decay, which other kinds con-
tinue; and thus in successive steps the most complicated
organic substances are reduced gradually back to the car-
bon dioxide, water, nitrogen, and mineral matters from
which they were originally constructed. There is always
at each stage some kind of organism ready to utilize any po-
tential energy remaining in the organic substance, until’
finally all is exhausted. This is the ultimate fate of all or-
ganic substance, which, formed photosynthetically in
leaves from carbon dioxide and water and mineral salts, is
converted back to those substances, either by the respiration
of the higher plants and animals themselves, or else by the
respiration of the micro-organisms of decay. Thus the
carbon dioxide, water, nitrogen, and mineral matters with-
drawn from the general circulation of nature and locked up
for a time in the substance of plants and animals are all re-
turned in time to their source. But on this withdrawal and
return hinge the visible phenomena of life.

The formation of incidental by-products by Molds and
Bacteria in their respiration and growth has important prac-
tical consequences to other organisms. Thus parasitic Fungi
produce such by-products, whereby their host plants are poi-
soned and damaged, often to complete destruction. The
Bacteria associated with disease often produce violently poi-
sonous products, collectively called toxins ; and it is these sub-
stances, and not any direct injury done by the Bacteria them-
selves, which cause death from bacterial diseases in man, other
animals, and some plants. This fact underlies the methods
of combating those diseases by use of antitoxins and the like.
174 A TEXTBOOK OF BOTANY (Cu. IV, 9

9. THe GBHOTROPISM OF STEMS
When seeds are germinating, it is always found that no
matter in what positions they happen to lie, the sprouting
roots grow over to point downwards, while the new stems as
regularly grow upwards (Fig. 119). Hf, while these parts
are still young, the germinating embryos are overturned, the
roots and stems grow around into their former directions.

 

Fic. 119. — Grains of Corn, fastened to vertical corks, with the root

ends all pointing centerwards, and there germinated; x3. The cork on

the left was kept fixed in this position, but that on the right was kept re-
volving, clockwise, on the clinostat (of Fig. 121). Sinee the roots grow much
faster than the shoots, the latter were added to the drawings after the
former. Note that on the fixed cork the roots point downward and the
shoots upward, while on the revolving cork both roots and shoots continue
the general directions in which they happen to start. (Drawn from photo-
graphs.)

Later, the side stems and roots as they develop assume
positions which are horizontal, or nearly so; and they like-
wise, if overturned, grow again into those directions. If a
young plant in a flower pot be laid on its side, its tip will
be found next day to have grown around into the upright
position; and if the pot be completely inverted, then a day
later the tip will again be found upright, now in exact. re-
verse of its original position in relation to the pot (Fig. 120).
These adjustments occur in darkness as well as in light, which
shows that they are not phototropie (page 54), though like
Cu. IV, 9] GEOTROPISM OF STEMS LZ

phototropism, they are growth movements, as experiment
proves. If the plant be kept revolving upon a clinostat, how-
ever (Fig. 121), then
no such responses can
occur, and the parts
continue to grow in the
directions they happen
to have at the start.
Everybody knows how
stiffly upright are the
Fir trees, and how
remarkably horizontal
their branches (Fig.
122); and this is as
true on the steepest
hillsides as on level
ground, showing that
the upright position is
not in any way deter-
mined by the slope of
the surface from which
the trees grow. The
facts here cited are
typical, and represent
a very widespread at-
tribute of the higher
Cet ae ee
grow mM such manner individual leaf blades have taken positions

as to swing their main @Pproximating towards horizontal. (Drawn
from a photograph.)

 

roots straight down,
their main stems straight up, and their side roots and stems
at definite angles to the up-and-down direction.

The up-and-down, and the horizontal, directions on the
earth’s surface are determined by a single factor, viz. gravi-
tation, to which, accordingly, plants show remarkable ad-
justments in their growth, —- a property called GEOTROPISM.
176 A TEXTBOOK OF BOTANY (Cu. IV, 9

It is clear that plants in growing geotropically are not forced
into those positions by the gravitational attraction, for even
if the young roots were pulled downward by gravitation,
this force could obviously not push the young stem upward,
or the side roots and branches out horizontally. In fact,
geotropism bears the same relation to gravitation that

 

 

Fie. 121. — A Clinostat; x 4. A powerful clockwork in the case on the
right turns the central spindle continuously once in fifteen minutes. It
can be used in any position. Thus plant parts can be kept revolving in any
desired plane, whereby the action of a stimulus can be made equally all-
sided instead of one-sided, — the most convenient method of neutralizing
a one-sided stimulation. The instrument is much used in the study of
irritability.

phototropism does to light (page 54); just as the light
neither pulls the stems towards it, nor pushes the roots
away from it, nor forces the leaves across it, but acts simply
as a guiding stimulus to the plant’s own assumption of those
positions, so gravitation neither pulls nor pushes the roots
and stems into the positions they take, but acts simply as a
guiding stimulus to the plant’s own growth. This action
of gravitation as a stimulus, and not directly as a force, ex-
plains why parts grow as readily and perfectly away from
it or across it as towards it.
Cu. IV, 9] GEOTROPISM OF STEMS 177

Seeking a connection between geotropism and the plant’s
physiology, we can find no direct relation, such as shows so
clearly with phototropism in the relation between light and
photosynthesis. It is true, under the influence of geotro-
pism the roots are guided down into the soil, and the shoots -
up into the air, and therefore to the positions essential to
their functions ; but, as one immediately recalls, those parts
would take those positions without any geotropism, — for
the shoot would be guided upward by phototropism and
the roots downward by hydrotropism (an adjustment to
moisture later considered). Geotropism, therefore, produces
exactly the same results in shoots and roots as would photo-
tropism and hydrotropism. But there is this difference, that
the light and moisture are variable, and often (especially the
light) absent altogether at times when the plant is in growth
and needing guidance, while gravitation is perfectly con-
stant at all times. It seems wholly probable, therefore, that
this invariability of action through all external conditions
is a reason why plants employ gravitation as well as light
and moisture in guiding the growth of their shoots and
roots. Precisely the same principle is illustrated in our
human affairs where we regulate our daily actions, our
risings, our meal times, and our occupations, by the clock,
instead of trusting to the height of the sun, often obscured,
or our appetites, often spasmodic. In similar manner the
sailor steers by his compass, even when sun or stars are
visible.

It must not, however, be inferred that growing towards
gravitation or away from it are inseparable from the nature
of a main root and main stem respectively. Thus, some
main stems have the transverse or horizontal geotropism,
as in ground vines. It is for this reason that Ground Pine,
our creeping Ferns, and Solomon’s Seal, keep their main
stems down upon or just under the surface of the ground,
instead of sending them upwards. Moreover, the geotropism
of parts can change during life, as familiar in cases’ where a

N
178 A TEXTBOOK OF BOTANY (Cu. IV, 9

side branch of Spruce or Fir turns upward, and takes the
place of the main stem, on destruction of the main terminal
bud. All evidence shows that the geotropism of any part is
not in the least inherent in its morphological nature, but is
correlated with its habit of life.

Stems and roots are by no means the only geotropic parts.
Thus, in the experiment of the inverted potted plant, per-
formed in the dark, not only does the stem grow upward,
but most of the leaf blades become horizontal, through in-
dependent readjustment upon their own petioles (Fig. 120).
Something similar occurs naturally in the undergrowth of
woods, where, in the evenly-diffused light, many kinds of
leaf blades become strikingly horizontal. Flowers and
fruits are often perfectly geotropic, as will later appear in
connection with their functions. The lower forms of plants
are likewise susceptible to geotropism; and the horizontal
growth of the bracket Fungi upon trees, and the upright po-
sition of the Mushrooms and Toadstools are thus guided.

While geotropism thus guides the growth of plants in their
various parts, its effects of course are intermingled with
other irritable responses. Thus, when light acts from one
side on a geotropically upright plant, the position which the
plant actually takes is a resultant between its tendency to
respond both ways; and its roots and other parts are in-
fluenced in similar manner. The fact is, the plant tends
to guide its growth as a whole, and assume its general form,
under the guidance of gravitation, thus establishing a gen-
eral arrangement of parts, but this ground form is readily
modifiable in details by other influences.

It is interesting to note the way in which the plant appears
to perceive, so to speak, the direction of action of gravita-
tion. There is much evidence to show that in special cells
of root and stem tips, the starch grains, which are relatively
hard and heavy, press down by their weight on the sensitive
protoplasm, and thus give the sense of direction by which
the growth processes are controlled.
Cu. IV, 10] FOLIAGE-BEARING STEMS 179

10. THe Various Forms or FoutaGk-BEARING STEMS

Stems which perform the same primary function, of sup-
port to the foliage, can yet differ greatly in aspect and
structure. These differences are connected with different
habits.

The simplest form of foliage-bearing stem is found where
a vertical, cylindrical, unbranched trunk bears one set of
leaves towards its summit, as do most Palms (Fig. 30).
Such stems, which may be termed coLumNaR, characterize
crowded vegetation, as in the jungles of the tropics and
the meadows of temperate regions. They often attain
great heights in proportion to their diameters, in which
case they exhibit very perfectly the hollow-column principle
of construction (page 118). Thus the Bamboo, practically
columnar though it bears small branches, presents an actual
hollow column of such great strength as to give it high
value for building and other uses in the tropics.

The second form of stem is found where the leaves are
spread along slender horizontal branches radiating from a
columnar-conical trunk. In such cases the younger and
shorter branches are above and the older and longer below,
thus producing the very perfect cone-shape displayed in our
Fir trees when free to grow without interference (Fig. 122).
This form of stem, called botanically EXCURRENT, is associ-
ated particularly with the evergreen habit, in which the leaves
persist and do some photosynthetic work in the winter.
Evergreen trees are especially characteristic of high latitudes
and altitudes; and it has been argued that the conical
form, with the upper branches progressively covering the
lower except near the tips, which readily yield to a weight,
renders such trees largely immune against damage by the
snow and ice of the climates in which those trees most
abound. When such trees are well spaced apart, the cone
form is often strikingly perfect from ground to summit ; but
when crowded together in forests, their lower branches are
180 A TEXTBOOK OF BOTANY [Ge EVs 10

overshaded, die, and fall away, after which the old bases
become buried as knots by the ever-increasing layers of the
wood. Thus in time the cone of foliage comes to stand at

 

 

 

 

 

Fic. 122. — Abies venusta, a Fir, showing the typical conical form.
(From Bailey.)

the summit of a branchless columnar trunk which yields
our most valuable timber. Such stems do not show the
hollow-column principle of construction, except incidentally
when the heart wood is removed by decay, for the mode of
growth is such as to build a stout solid column.
Cu. IV, 10] FOLIAGE-BEARING STEMS 181

The third and most highly developed form of stem is found
where a swelling dome of foliage is held outspread by a
system of stems radiating, tapering, and branching from a
relatively short central trunk, as well developed in our
common deciduous forest trees, — Oak, Maple, Elm, and
others (Fig. 123). Since the trunk thus melts away, as it
were, to the twigs, the form
is called DELIQUESCENT. It
permits a great lateral
spread as well as high eleva-
tion of the foliage, and is
the most effective of the
forms in the exposure of
great numbers of leaves to
light; and correspondingly
it is the dominant forest
type the world over in both
temperate and tropical re-
gions. The damage which
would be entailed in win-
ter on such widespreading
forms by accumulation of Fic. 123. — Sugar Maple, in a
snow and ice on the leaves pasture, _ showing the deliquescent
is obviated by the shedding LOBE oR Ee
thereof in the autumn. Here, also, a remarkable symmetry
of form is developed where space is sufficient, as Maples in
old pastures, Elms in meadows, or lawn trees attest (Fig.
124). In such cases the foliage has almost the form of a
sphere, or an ellipsoid, or a rounded cone. A crowding in
forests, however, produces the same effect as in the excurrent
type, excepting that the branches here tend to reach up to
a low dome or even flat top, as can be seen very clearly
when a deciduous forest is viewed from some mountain.
The effect is particularly striking where the dark spires of
evergreens break through the rolling plain of the deciduous
foliage.

 
182 A TEXTBOOK OF BOTANY (Cu. IV, 10

Vertical stems are racially symmetrical, or alike all around,
both externally and internally. When horizontal or oblique,
however, as with branches, they are often excentric, having a
greater thickness of wood on the lower side, in obvious ad-
justment to the support of the branch against the leverage
imposed by its increasing length and weight. In such cases,
also, there is frequently a thick-
ening in the lower angle be-
tween branch and stem, on
the principle of a bracket,
which is present in even
greater development at a place
of much greater strain, viz.
at the angle where a verti-
‘al. trunk joins spreading
roots. Here the bracket often
amounts to a buttress, as well
shown in old Elms, and even
more strikingly in some trees
which grow in the tropics.
> The principle of resistance to

Fria. 124. — The symmetry ofa Strain explains also the form
lawn tree, the weeping Birch. of the main branches in the
(From Bailey.) 7 '

deliquescent type, for com-
monly they rise almost vertically from the trunk, and turn
gradually outward, becoming vertical again at the foliage-
bearing tip.

Stems have not only to support the great mass of the foli-
age and also their own considerable weight, but must like-
wise resist lateral pressure from winds, which exert great
power against the foliage and therefore strong leverage on
the stems. Corresponding thereto is the tough-elastic
texture of the stems, whereby they are enabled to yield to
winds in a manner to shed off their force, as one can see in
any great trees in a storm. Where strong winds prevail in
one direction during the season of growth, a tree may be held

 
Cu. IV, 10] FOLIAGE-BEARING STEMS 183

so much of the time in the leeward position that it acquires
a permanent set that way (Fig. 125), though the result is
complicated by the greater transpiration, and consequent
less growth, on the windward side. The leverage of the
winds is felt most at the base of the trunk, which explains
the need for the buttresses above mentioned. There is
evidence to show that these buttresses, like the brackets
and excentric growth of the branches, develop in irritable
self-adjustment to the stim-
ulus of the strains there felt,
in precisely the same way
that leaves and stems turn
phototropically to light, or
stems hold themselves up-
right in adjustment to grav-
itation.

Between stems and
branches no structural dif-
ferences exist, the word
“branch,’’ as we use it, . Tic. 125.— A yellow Birch, ex-
being merely an abbrevia- posed to winds from one direction
Hoa for branches at the eee - ee season. (Drawn

rom a photograph.)

stem.” For the most part

all of the branches of a given plant are structurally alike, but
sometimes they are not. Thus in fruit trees, some branches
make extremely little growth in length each year, while their
buds form flowers and fruits with the least possible stem ;
and such branches are the familiar FRUIT-sPpURS. Again,
some of the branches on a plant may be limited in growth
and assume flat forms, as in cladophylla elsewhere described
(page 195), the remaining branches having the ordinary
form. An even more familiar case of special branches is
found in flowers, which are morphologically modified branches
including sexual parts. In a few cases, trees form a certain
absciss-layer across the bases of some of their young branches,
producing the result of a natural pruning.

 
184 A TEXTBOOK OF BOTANY (Cu. IV, 10

While the upright self-supporting condition is typical in
foliage-supporting stems, modifications thereof occur in
connection with special habits. Most prominent are CLIMB-
ERS, which make use of trees, rocks, walls, and other supports
to lift their foliage to the light. Being thus supported, they
need no great thickness and remain slender, devoting their

 

Fic. 126. — A typical epiphytic Orchid, showing aérial roots, and the
pseudobulbs, or storage stems, from which spring true leaves. (Reduced
from Kerner.)

material to increase in length. Some simply clamber over
other plants, as in case of the Rattan Palm already men-
tioned (page P13) or the many great lianas of the tropics, or
the Clematis of our woods. Such plants possess hooks (Rat-
tal’ tavining petioles (Clematis, Fig. 51), or other arrange-
ments preventive of slipping from the supporting vegetation.
Others, forming our principal vines, cling to a support, either
by tendrils, as in Grape and Passion Vine (Fig. 136), or by
Cu. IV, 10] FOLIAGE-BEARING STEMS 185

adherent disks, as with Virginia Creeper, or by disks on the
ends of aérial rootlets as in the Ivies which grow upon
buildings (Fig. 180). Others are twiners, and wind their
very slender stems around the support, as do Morning Glory
and Dutchman’s-pipe. Some special forms of irritability are
concerned in the climbing movements. Thus, vines which
climb against walls have the stems negatively phototropic,
and thus are kept against the surface to which their
roots adhere.
All climbing
stems remain
slender, form-
ing new wood
but slowly, and
possess, as a
rule, very large
ducts.

From the
climbing to an
epiphytic habit
there is every
gradation in

tropical vege-
tati E Fie. 127. — Aechmea miniata var. discolor, typical of
atlon. PI- the funnel-form epiphytes. (From Bailey.)
PHYTES are

plants which have no connection of their own with the ground,
but live supported ‘towards the light upon others, without
being parasitic. Very few occur in the flora of temperate
regions, aside from a few stray Mosses, Lichens, and other
low forms, but most tropical Orchids, some Ferns, and
many members of the Pineapple Family, including the
“Long Moss”’ of the South, are typical epiphytes ; and they
often cover the branches of tropical trees in great variety and
profusion (Fig. 126). Their mode of life is peculiar, and many
striking adaptations thereto have been described by those
who have studied them in the tropics. Their attachment to

 
186 A TEXTBOOK OF BOTANY (Cu. IV, 10

the supporting plant is precarious, and they remain compact
with very short stems often concealed completely by crowded
leaves. Their water supply comes from the rain which wets
the bark on which their roots grow ; but a few possess methods
of collecting the rain in funnel-shaped cups formed by their
leaves (Fig. 127). All
epiphytes, indeed, show
marked water-conserving
features, including thick-
ened epidermis, sunken
stomata, storage tissues,
and other features associ-
ated with plants which
must stand frequent dry-
ness (page 69). Their
supply of mineral matters
is such only as they can
derive from the decaying
vegetation amongst which
they live, and much of it
comes from the bark into
which they send their

     

Ny i roots. Some kinds, how-
yy (i) X ever, collect among their
} 5 \ : leaves the bark, twigs,

‘ : flowers, ete., which fall

Fic. 128. — An epiphytic Fern, Platy- i : y
cerium grande, possessing two kind of from above, while others
fronds, — ordinary (drooping) and humus- possess leaves so adjusted

collecting (upright); x 4. (From Goebel.) ‘
to the supporting trunks
as to form half cups in which bark and other materials
streaming down with the rain are caught and held, later
decaying to a humus from which both water and mineral
matters are readily absorbed (Fig. 128). And many other
interesting features, some structural and some self-adjustive,
are known to accompany the epiphytic habit. From the
penetration of dead bark for rain water to a penetration of
Cx. IV, 10] FOLIAGE-BEARING STEMS 187

living stems for their soil water, the step would seem easy for
roots; and thus has probably originated the half-parasitic
habit represented in the Mistletoe. Thence it is only a
short step further to a connection with the food supply of
the host plant, and a completely parasitic habit. It is prob-
able that the parasitism of the flowering plants has mostly
originated in this way.

Like climbing stems in many respects are creeping or
trailing stems, such as those of Partridge Berry and Ground

 

Fic. 129. — The rhizome, or rootstock, with ascending shoots, of a
Sedge; x }., (From Le Maout and Decaisne.)

Pine. Since the ground supports them, they remain slen-
der, and simple in structure. This habit merges over imper-
ceptibly into that where the stems run, not on the surface
but just beneath it, as in some Ferns and the Grasses; and
remarkable self-adjustive adaptations have been described
whereby the stems are kept at a constant depth. This habit
is best developed in the Grasses and Sedges, where the slender
underground stems branch and interlock so profusely as to
form the familiar turf, from which rise short vertical stems
bearing the foliage (Fig. 129). When thus underground, the
stems lose their green color and acquire the aspect of roots,
188 A TEXTBOOK OF BOTANY (Cu. IV,10

whence their botanical name of RooTstocKks; but they are
always distinguished by the presence of nodes and rudi-

 

Fic. 130. — Stolon of Black Raspberry. (From Bailey.)

mentary scale-like leaves. Such rootstocks often accumu-
late food, thus tending towards new organs, which we may
best consider in the following section.

There also occur a kind of traveling stems. The very
slender woody stems of the Brambles bend over and touch

 

ia. 131. — Sempervivum soboliferum, showing typical offsets.
(rom Iserner.)

the ground at their tips, where they take root; and thus the
plants form dense and ever advancing thickets (Fig. 130).
Cu. IV, 10] FOLIAGE-BEARING STEMS 189

Some plants develop both upright and reclined stems, the
latter, called srotons, lying close to the ground, as in
Hobble-bushes, descriptively named. Short leafy stolons,
called orFsETs, are formed by some plants of compact
growth like the Sempervivums, which thereby spread out-
ward in a continuous
growing mat (Fig.
131). Very long and
slender stolons, evi- —
dently adapted to
spreading the plant,
are called RUNNERS,
as familiar in the
Strawberry.

The flowering
plants are typically
land dwellers, but in
course of their evolu-
tionsome kinds have
returned to a life in
the water, — e.g.
Water-lilies and a
great many of the
Waterweeds. The
stems of such plants
are buoyed up by
the water, which
thus supplies the
support for the foli-
age, in correspond-
ence wherewith the stems are weak and soft, serving rather
as cords to retain the leaves than columns to lift them.

Some flowering plants live also in deserts, into which they
have been forced in the course of evolution. The scarcity
of water entails on such plants great reduction of surface,
leading in the most typical cases, like the Cactus, to aban-

 

Fic. 132. — Fucus vesiculosus, the common
brown Rockweed; * +. (From Figurier.)
190 A TEXTBOOK OF BOTANY (Cu. IV, 10

donment of the leaves and the assumption of photosynthesis
by the compact, rotund, water-storing, ribbed stems, which
possess Inany structural features connected with restriction
of transpiration (Fig. 141). The difference in aspect and
structure between forest plants, desert plants, and water
plants shows how profoundly plant form is affected by water
supply. In accordance, indeed, with this relation to water,
most plants fall under three well-recognized groups, the
desert plants being called xpRopHyTES, the water plants

    
  

_ Fic. 133.— The Giant Kelp, Mac-
rocystis pyrifera, which grows up-
wards of 200 feet long. (From
Le Maout and Decaisne.)

HYDROPHYTES, and the intermedi-
ate or ordinary plants MESOPHYTES.
The mesophytic is of course the
best condition for plant life, and reaches its highest perfec-
tion in the rank growths of the tropical forests and jungles,
though it is nearly as well attained in the deciduous forests
of temperate regions.

The primitive water plants, the Alge, in their highest
development are distinguished by a THALLUs, familiar in
the fronds of brown Rockweeds (Fig. 132) and the red Sea-
mosses. The thallus is neither leaf nor stem, but rather a
more primitive structure from which leaf and stem have not
yet differentiated. Some of the greater Alge, as for ex-
ample the giant Kelp of the Pacific (Fig. 133), have de-
veloped a distinct leaf and stem structure, though it by no
means represents the evolutionary ancestor of the shoot of
the higher plants.

The term sHooT is used in connection with the flowering
plants to designate stem and leaves collectively.
Cu. IV, 11] SPECIAL FUNCTIONS OF STEMS 191

11. THe Forms anp FUNCTIONS OF STEMS NOT CONNECTED
WITH SUPPORT OF FOLIAGE

As with — other
plant parts, stems
are not limited to
the one primary
function in adapta-
tion to which they
seem clearly to have
been evolved, but
perform also others,
which sometimes re-

 

Fig. 134.—Solomon’s Seal,  Polygonatum
multiflorum; X 4. Each “seal”? marks a fallen
shoot, and a year’s growth of the rootstock.

place the original (From Strasburger.)
function. Thus are

produced new organs, with distinctive aspect and structure.
The most frequent additional function of stems is storage

 

Fic. 135. — A typ-
ical corm, composed
mostly of stem, of
Crocus. (From
Figurier.)

of food or water. All woody stems store
food over winter, but since ample room
therefor exists in the ordinary tissues, —
in pith, bark, medullary rays, and parts of
the fibro-vascular bundles, —such stems
exhibit no external evidence of the storage
function. Some stems, however, do show
marked swellings resulting from storage of
food and water, as especially clear in the
pseudobulbs of epiphytic Orchids (Fig.
126). Storage of food is commonest in
underground stems or rootstocks, which
thereby are given a swollen aspect, as for
example in Solomon’s Seal (Fig. 134),
where a new piece of food-filled stem,
producing a new shoot, is made each year.
Similar arrangements are found in Iris,
Trillium, and others, and reaches an ex-
treme in the corm of Crocus (Fig. 135),
192 A TEXTBOOK OF BOTANY (Cu. IV, 11

where the nearly globular storage stem is commonly mis-
taken for, and called, a bulb (page 73). All of these
stems produce roots, and also give rise to the foliage;
but cases occur in which food-storage completely displaces

f

the foliage-supporting function,
and also the production of roots.
Then we have a new organ, ex-
emplified in the common potato,
the stem nature of which is
attested by the eyes, which are
axillary buds subtended by small
scale leaves. Such an organ,
rotund with accumulated food,
and composed mostly of thin-
walled rounded storage cells of
the greatly developed pith and
cortex, is called a TUBER, of which
many forms occur among plants.

Another important special func-

   

tion of stems is represented in
tendrils, which have the same

Fic. 136. — Tendrils, from
axillary buds, in a Mexican elongated slender forms, move-
Passiflora. : :
Compare also Fig. 52. The ments through the air, thigmo-

tendrils of Grape Vine and all tropic twining about a support,
of the Gourd family (Squash, , eee ; eas
Wild Cucumber), represent the anc splra 5 1ortening, AUTeady
main stem, the further growth cleseribed in leaf tendrils (page
taking place from the axillary 5. Nee Es = ie
Igdien Chl tere Cea} 77). Passion Vine, Wild Cu-
cumber, and Grape Vines have
stem tendrils (Fig. 136), which are more abundant and
perfect in form than leaf tendrils, perhaps because support
is a more natural function of stems than of leaves.
Stems also become transformed into spines, which are
sometimes very large, as in Honey Locust (Fig. 137). The
Cu. IV, 11] SPECIAL FUNCTIONS OF STEMS 193

single spine of the Cactus-like Euphorbias is a stem, really
the persistent and hardened flower-bearing branch. As in

case of leaves, however,
the significance of these
spines is uncertain (page
79).

Support of the flowers,
which mostly stand out
in the light, is another of
the special functions of
stems. Flower © stalks
are usually slender-cylin-
drical, nodeless, and leaf-
less, though sometimes
they bear bracts (page
73). An elongated stem
ending in a single flower
or small cluster, espe-
cially if starting directly

 

Fic. 137.—Spine, a branch
developed from an axillary bud,
in Honey Locust; x h.

from the ground, as with Adder’s-tongue or Violets, is called
a scape; a flower stalk from the axil of a leaf is called a

 

Fie. 138. — Rubus  squarrosus,
shrub in which the foliage function is
assumed by the stems and _ petioles;
much reduced. (From Wiesner.)

oO

PEDUNCLE, and in clusters
each separate stalk is a
PEDICEL. A typical flower
stalk consists really of one
internode, bearing at its
top several nodes merged
together in one enlarged
RECEPTACLE which = sup-
ports the floral parts (page
271).

The most striking of the
new functions assumed by
stems is found in the re-
placement of leaves as
foliage. In the simplest
194 A TEXTBOOK OF BOTANY [Cm IV, 11

case the stem acquires more chlorophyll, shown by a deeper
green color, thus supplementing better the work of the

  

5

Fic. 139.—Muehlenheckia platy-
clada; * %. (From Goebel.)

nodes and the persistent
small leaves. Still more
striking are the cases in
which flattened stems, in
this case branches, be-
come limited in growth,
and assume characteristic

leaves; but in others the leaves
are reduced in size almost to dis-
appearance, leaving the foliage
function wholly to the slender-
cylindrical stems and _ petioles.
In others the stems become
flattened, thin, and green like
the leaves, as in the familiar
greenhouse plant Muehlenbeckia
(Fig. 139), the stem nature of
which, despite its deep green
color, is proven by the prominent

 

Fic. 140. — Leaf-like  cladophylla
(branches) of Butcher’s-Broom, Ruscus
Hypoglossum, in the axils of bracts, and
bearing leaves and flowers: x 4. (After
Kerner.)

 

leaf shapes, to such a degree that their stem nature would
hardly be suspected at all, were it not that they grow from
Cu. IV, 11] SPECIAL FUNCTIONS OF STEMS 195

the axils of small scales which are morphologically leaves,
os exemplified in the familiar “Smilax” of the florists. The
utcher’s-Broom of Europe is similar in general, but has
vis further interesting feature, that on the face of the
CLADOPHYLL (as such leaf-like branches are called), occurs
a small though genuine leaf, bearing in its axil a flower
cluster (Fig. 140). The apparent leaves of the common
“Asparagus Fern’ likewise
are branches, of which several
occur in the axil of each
scale-like leaf. In clado-
phylla the stems have be-
come foliage without other
function.

 

Fia. 142. — Rhipsalis Houl-
letii; x 4. The seeming leaves
are flattened stems, morpho-
logically equivalent to a form
like the Echinocactus of Fig.

 

Fic. 141. — Echinocactus, a 141, with the ribs reduced to
typical globular ribbed Cactus. 2and flattened. (From Riimp-
(Originally after Engelmann.) ler, Die Sukkulenten.)

The functions of foliage and storage are combined in the
succulent stems of Cactus and other plants of dry places.
Such stems, which store principally water absorbed during
the rainy season, become swollen to cylindrical, or even
“almost globular forms, while the entire leafless surface bears
ample chlorenchyma, with stomata through the thick epi-
dermis (Fig. 141). Many of these plants possess vertical
196 A TEXTBOOK OF BOTANY (Cu. IV, 12

ribs, which have the effect of increasing the spread of green
surface without a proportional increase of transpiration,
which, of course, is the ever-present danger to plants of dry
places (page 69). These ribs vary much in number, from
many to few, and even in some cases to two, when the struc-
ture approximates closely in appearance and function to a
single leaf (Fig. 142). Thus is presented still another exam-
ple of the attainment of the same functional end by a dif-
ferent morphological route.

The explanation of such remarkable morphological-physi-
ological overturnings as are presented by the cladophylla
is probably to be found, as with similar anomalies in leaves,
in a devious course of evolution through conditions and
habits very different from, those now distinctive of these
plants.

12. Tue MonstTrRosiITiIEs oF STEMS AND LEAVES

It often happens that individual parts of plants grow so
differently from their usual. method as to attract attention
and be designated “freaks.” Scientifically such cases are
called ABNORMALITIES, or if extreme, MONSTROSITIES. Aside
from their interest as curious things needing explanation,
they are scientifically important for the light they throw
upon the methods of plant development.

First, it must be noted that not all peculiar growths are
properly monstrosities, for many result from purely mechani-
cal causes. Thus, when a stem is encircled by a rigid ring
(e.g. supporting iron band or wire attachment of a label),
it becomes thereby constricted in its further growth, and
swells greatly above the obstruction, because of the ac-
cumulation of food stopped in its downward passage through
the bark (Fig. 107). Precisely this cause produces great
spiral ridges on trunks gripped by twining vines. Again,
different parts of the same plant often become grown or
grafted together, because crushed or rubbed against one an-
other when young. In this way twin fruits are sometimes
Cu. IV, 12] MONSTROSITIES OF STEMS 197

produced, though others are true monstrosities resulting
from partial fission of one. Oranges sometimes exhibit a
segment very different in color and texture of skin from
the rest; but these are a special incident of grafting, as else-
where explained (page 211). Strawberries which remain
hard, shrunken, and green on one side are merely individuals
which did not receive enough fertiliz-
ing pollen (page 279). And other
peculiarities of like sort, more or less
obvious in origin, occur in various
plant parts.

Of true stem monstrosities perhaps
the most common are FASCIATIONS.
These are cases in which the usually
cylindrical stem with its single ter-
minal bud becomes a flattened stem
with several imperfectly separated
terminal buds, as occurs at times in
Asparagus (Fig. 143), Hyacinths, and
other herbs, and in Forsythia and
Barberry among shrubs. A striking
example, seemingly in a fruit, but
really in a stem, occurs in the Pine- Ae eae aag een
apple figured herewith (Fig. 144). shoot of Asparagus, which
Fasciations are much more common is , 2°rmally cylindrical ;
‘ A ; . x4. (Drawn from a pho-
in cultivated than in wild plants, and tograph.)
sometimes can be propagated; as, for
instance in the Crested (7.e. a fasciated) Cactus (Fig. 145),
while a crested form of Celosia gives us the Cockscomb of
our gardens, and a related condition in leaves produces the
feathered fronds of the Pearson Fern, — a new variety of the
plain Boston Fern. Fasciations are evidently caused by a
partial fission of one meristematic growth center into several.
In some cases the result follows an injury by insects, but in
such cases it cannot be propagated ; in others it seems clearly
due to internal causes of still unknown nature, affecting the

 
198 A TEXTBOOK OF BOTANY (Cu. IV, 12

meristematic tissues or the reproductive cells, and these are
the kinds which it is possible to propagate, and thus preserve

Fic. 144.—
A Pineapple,
fasciated to
an unusual
degree. It is
flattened in
the plane that
isvisible; x}.
The Pineap-
ple is mostly
stem covered
with  coales-
cent small
ovaries and
bracts.
(Drawn from
a photo-
graph.)

 

in our gardens. The first step towards a fasciation would
be a bifurcation, sometimes seen
in the fronds of Ferns, and in some
double fruits, 7.e. in Orange (Fig.
146).

Closely related to fasciations are
cases of unregulated bud develop-
ment, most familiar in the Bird’s-
eye Maple. The eyes are knots,
that is, buried branches, developed
from a mass of adventitious buds
which start on the side of a trunk
of a Maple, presumably as a result
of some injury (page 137), and in
their growth about keep pace with
the expansion of the trunk. An-
other prominent case is found in
“ Witches’ brooms” (Fig. 147),
those dense masses of slender twigs

 

Fic. 145. — Greatly fasci-
ated, or crested, Echinocactus.
(From Rimpler.) found on the upper branches of
Cu. IV, 12] MONSTROSITIES OF STEMS 199

Spruces and some other trees. Here, instead of the usual
development of a few buds with inhibition of others, many
or all of the buds on the
branches affected develop
equally, and more or less
independently of the
others. It is known
that this condition is
produced by the pres-
ence of a parasite, the Fria. 146. — A twin-fruit, of Mandarin
obvious effect of which oe x}. (Drawn from a photo-
is to paralyze the mech- ee
anism of growth correlation by which the buds are ordinarily
controlled.

Closely analogous to these cases in buds is the unregulated
growth of tissues. Thus, the large burls or gnarls which ap-

 

 

Fig. 147. — A typical Witches’ Broom, caused by an cidium, a Fungus, on
a branch of Fir. (From Kerner.)
200 A TEXTBOOK OF BOTANY [Cu. IV, 12

pear on old Elms, especially near the bases of the lower
great branches, are composed of complexly contorted and
twisted masses of wood, often beautifully grained when
sectioned and polished. They are formed by areas of cam-
bium, which, instead of keeping their places and parts in the
regular fibro-vascular cylinder, proceed to grow profusely,
and thus are thrown out into irregular folds. A less extreme
case is found in Curly Birch, and in some other irregularly
grained hardwoods highly valued in fancy carpentry. In
some cases such growths
are apparently started by
injurious strains, which
would explain their fre-
quency at the bases of
great branches; and very
likely they represent areas
in which the growth-con-
trol mechanism has been
ruptured by the strain.
It is interesting to note
that a close analogy exists
between these burls and
the troublesome tumors

 

Fic. 148.— A Wooden Flower, or
Wooden Rose, on a leguminous plant; : :
x4. The parasite which induced it was Which form in the human

a flowering plant, Phoradendron. (From

Engler and Prantl, Pflanzenfamilien.) body, for the latter also

are formless growths re-
sulting from continued operation of the growth energy of
the tissues after the control stimuli have been inhibited,
usually as result of some strain or other accident. Other
burls, however, with various kinds of knotty growths, are
started by presence of parasites, which also inhibit the
usual control, presumably by chemical action. Of this
nature is the remarkable ‘wooden flower,’ sold to tourists
in tropical America (Fig. 148). It is nothing but 2 stem in
which a parasite has inhibited the growth control over a
limited area, leaving that part free to grow as it happens.
Cu. IV, 12] MONSTROSITIES OF STEMS 201

Related to these peculiarities of tissue development are the
TORSIONS, or close twistings sometimes found in plant tissues,
either stems or fruits. They are often prominent on trees
standing in burnt woods, or on fence rails, where the layers
of wood form closely wound spirals.

Rather striking, and not uncommon, are PROLIFERATIONS,
well illustrated in the cases where a leafy shoot projects
from the tip of the fruit in Pear or Straw-
berry (Fig. 149). In Roses the stem occa-
sionally grows up through the center of a
flower and produces another, thus making
a ‘‘two storied”’ flower (Fig. 150), while
two-storied fruits, of similar origin, occur
occasionally in Apples. An incomplete
case is represented in the Navel Orange,
where the stem grows up between the seg-
ments of the fruit, and bears a smaller
orange, not, it is true, on the top, but
within the top of the main one. This case
is also of interest as showing that such
monstrosities can be propagated, for all
Navel Oranges are reproduced by grafting.
Stems, and therefore the stalks of flowers py¢. 149 — Pro-
and fruits, can potentially elongate indefi- liferous Pear. (From

: Siac iaore 5 Balfour.)
nitely, and some special inhibitory influence
must ordinarily check their growth in flowers and fruits. It is
apparently the occasional failure, presumably by some acci-
dent, of this inhibitory stimulus, which results in proliferations.

Among the commoner monstrosities are SUBSTITUTIONS
of one part or feature for another. Most people know that
green Roses occur; and a variety is grown in Botanical
Gardens on which the flowers are well-nigh as green as the
leaves. Formerly such cases were considered ‘‘reversions,”
the petals being supposed to have returned to the state of
green leaves from which they were evolved. They seem
rather, however, to result from a substitution of chlorophyll

 
202 A TEXTBOOK OF BOTANY (Cu. 1V, 12

for the usual color substance, of which the formation is
inhibited by some accident. We sometimes find the oppo-
site phenomenon, where the
floral color is thrown into
leaves, as happens with some
Tulips, in which the upper-
most leaf of the flower-stalk
takes the color of the flower.
Genuine reversions no doubt
do occur; and perhaps we
have a case in the occasional
appearance of leaves upon the
smooth sides of Apples and
Cucumbers, this part of the
fruit being morphologically
stem. Sometimes Potatoes
appear above ground in the
axils of the leaves, evidently
because food material destined
for the underground tubers

 

Fie. 150.

Proliferous — Rose. ‘ :
(From Masters, Vegetable Tera- becomes diverted into axillary

tology.) buds.

 

There can be little doubt that with increasing knowledge
we shall learn to control such substitutions, and various other
stimuli which produce special growths upon plants. Thus the
horticulture of the future will surely include some practice
whereby palatable and nutritious growths, on the analogy of
aerial tubers and galls, will be produced at will upon the
leaves or stems of plants.

Several forms of monstrosities are distinctive of leaves.
Rather common is the formation of a cornucopia-like pitcher,
instead of a flat blade, as happens in Pelargoniums, Cabbage,
and others (Fig. 151). Here the bases of the leaf blade seem
to unite or graft together over the petiole at an early stage,
and remain united during the subsequent growth. The
case has an interest in showing one way in which pitchers
Cu. IV, 12) MONSTROSITIES OF STEMS 203

may have originated in the Pitcher Plants (page 76). Also
distinctive of leaves is a peculiar monstrosity called PHYL-
LOMANIA, propagated in a green-house variety of Begonia,
where the stem or petioles produce a great number of very
minute, but otherwise well-formed blades (Fig. 152). Here
the form-factors which shape the blade, whatever they are,
evidently have spread all over the plant. An extremely fine
division of the leaf blade, closely following the veins, some-
times occurs, and can be propa-
gated: and such is the origin
of the “laciniate”’ or finely
cut leaves of some cultivated
trees and shrubs.

Not properly monstrosities,
though usually associated and
intergradient therewith, are
GALLS. Typical examples
occur in the bright red round
swellings on Oak leaves, which,
when opened, are found to
contain the larva of an insect
(Fig. 153). A common form
upon stems is the familiar
globular swelling of the stem
in Golden Rods. They are o
formed by the plant tissues BR Pee pepies
after an insect has laid an egg
therein, though we do not yet know the precise nature
of the stimulation which controls their development. The
growing insect feeds upon the leaf tissue, then makes its
way out and escapes. The advantage of the arrangement
to the insect is plain, but its meaning to the plant is still
problematical. Hundreds or thousands of such galls are
known, constant in form for the same kind of insect on the
same kind of plant. Some are large, some small, some rough
or hairy, some smooth, some on leaves and some on stems, and

 
204 A TEXTBOOK OF BOTANY [Cu. IV, 12

   

 

 

/ : ! i iy

Fic. 152. — Begonia phyllomaniaca, which produces many small leaves
over leaf and stem. (From Bailey.)

some involve both, as in case of the Willow Roses, — those
rose-like masses of shortened leaves often seen on the ends of
Willow stems.

 

Fic. 153. — Typical galls, with the Insects, of Oak; slightly reduced.

On the left a leafy ‘Oak-apple,”’ and on the right the insect in cocoon
and adult stages. In the center, an Oak gall, and on the right, lower, the
same cut open, showing the larva of the insect. (From Thomé, Text-book
of Botany.)
Cu. IV, 13] ECONOMICS OF STEMS 205

A very close relation exists between monstrosities, and
those extreme variations called in horticulture sports.
In fact a sport, the foundation of some of our most valuable
varieties of cultivated plants, as typified, for example, by the
Navel Orange, is probably nothing other than a monstrosity
which has originated from internal and not external causes,
and which can be propagated.

Monstrosities occur, of course, in the other plant parts,
notably flowers and fruits, and along with our description
thereof we shall consider still further their causes.

13. Tue Economics, AnD TREATMENT IN CULTIVATION, OF
STEMS

As with other plant parts, stems possess structures and
contain substances suited to their functions and_ habits.
These materials, however, happen to meet certain needs of
man, who accordingly appropriates them for his purposes.

The size, composition, and tough grain of the great
trunks built by trees for support of their foliage fit them ad-
mirably for innumerable domestic and manufacturing utili-
ties. Nature has supplied lumber and cabinet woods in
great abundance and variety, but not so great .as man’s
increasing needs; and he is driven perforce to conserve,
augment, and improve the supply through scientific forestry.

Likewise from stems he obtains material for paper, not now
as in old times from consolidated strips of herbaceous pith
(papyrus), but from cellulose fibers (rag or linen papers),
and from the lignified elements of the xylem. These he sep-
arates by grinding, or else by use of chemicals which dissolve
the middle lamelle (page 147), and then felts them together
to a pulp which is compressed between rollers to the familiar
thin sheets. Also he uses tough bast fibers for threads,
notably in case of Flax, which he weaves to cloth, giving
linen, though cotton has a very different origin, as will
later appear. Both bast fibers and sclerenchyma strands
20

a

A TEXTBOOK OF BOTANY (Cu. IV, 13

are utilized as hemp, or other cordage. Likewise the bark-
cork has uses dependent on its waterproof qualities.

From the stores laid down by plants in their stems man
derives many foods, either directly through some vegetables
or indirectly through fodder plants. Most of his sugar
comes from the main stems of the Sugar Cane, and a little
from Maple, and some starch from Sago Palm, while special
storage stems, like potatoes, yield him specially rich harvest.
And likewise from stems he draws drugs, dyestuffs, tanning
substances, resins, rubber, and almost innumerable other
materials, having in the plant distinctive meanings which
involve properties happening to serve some human
purpose.

Man’s command over the resources of Nature rests not
alone upon his direct appropriation and use of materials
which plants happen to offer, but also upon his power to
multiply their quantity and improve their quality by culti-
vation. That part of cultivation which consists in conform-
ity to the plant’s physiological peculiarities (page 94) is
comparatively simple with stems, involving no special hor-
ticultural or agricultural practice, doubtless because of the
relatively simple and mechanical part taken by stems in
the plant’s economy. But the other phase of cultivation,
viz. improvement, which always depends on the wtil/zation
of potentialities which the construction or composition of
the plant happens to offer, has some important applications
in stems, especially in connection with pruning and grafting.

PRUNING consists in the removal of some parts of a plant
for the benefit of the remainder. Its very possibility de-
pends on two leading facts. First, branches are practi-
cally all repetitions of one another, and hence are not in-
terdependent ; and accordingly any particular ones may be
removed without damage to the rest. Second, any injuries
made in living tissues of plants not only heal quickly, but
the bark gradually overgrows and permanently covers large
areas of dead tissues, as already described (page 122, Fig. 79).
Cu. IV, 13} ECONOMICS OF STEMS 207

If pruning is done in winter or early spring, the injuries heal
largely before the first rush of the valuable sap.

There are four principal uses of pruning. First, parts
affected with disease which might spread to sound parts
can be removed. Second, some desired shape can be given
ornamental or fruit trees by removing growth in undesired
directions. This practice merges over imperceptibly into
the clipping of plants forcibly to desired shapes, as practiced
with hedges or with evergreen plants in the Topiary work
of formal gardens. Third, more space and light can be
insured to a few branches, in place of a mediocre exposure
to many, thus promoting the development of fine individual
flowers or fruits. Trees and shrubs not only form many
more buds than ever develop, but develop many more
branches than is good for them all. By a form of pruning,
viz. disbudding, it is possible to develop the wonderful great
exhibition types of Chrysanthemum.

The fourth use of pruning is the most important of all,
especially in orchards, — viz. to produce more formation
of fruit and less of leaf and stem. The possibility of gaining
this end by pruning depends on the fact that in plants (as
also in animals) a certain reciprocal balance exists between
the reproductive and the vegetative parts, such that any
check to either promotes the other, — and the fruit, of course,
is a part of the plant’s reproductive mechanism. In a state
of nature, all woody plants form only enough reproductive
parts for their needs, and, as a phase of their competition
with one another for light and space, throw the remainder
of their energy into growth of stem and leaf. The human
fruit-grower, however, does not so much wish his trees to
become big as to bear plenty of fruit; and by pruning
away much stem and leaf, he can turn the plant’s energy
into more copious formation of fruit. Thus the cultivated
Grape Vines, left to themselves, produce long leafy canes
bearing few clusters of Grapes ; but when thoroughly pruned,
they produce little cane but many fine clusters. Of course
208 A TEXTBOOK OF BOTANY (Cu. 1V, 13

such pruning must be done with discretion, for in the last
analysis the production of fruit depends upon the work of
leaves and stems; but the aim of the pruner is that optimum
balance at which only enough food is sent to stem and
leaves to insure moderate growth for the next season, while
all of the remainder goes into fruit. Naturally the best
pruning requires judgment, skill, and technique, which
are acquired only by a combination of natural aptitude with
long and interested practice.

There are other minor uses of pruning for special purposes,
of which an example is the root-pruning said to underlie the
production of the remarkable dwarf trees of the Japanese.
By the consequent restriction of water and mineral matters,
the entire development of the plant is restrained without
other alteration of its characteristics.

Even more important than pruning in the utilization
of the natural potentialities of stems is GRAFTING, or, as the
entire art comprehensively is now often called, GRAFTAGE.
It consists essentially in this;—a piece of stem, called a
CION, or SCION, of some valuable variety of plant is inserted
into the stem of another, which is usually a less valuable but
more hardy kind, called the srock, in such manner that
the cambium tissues can unite. In these cases cion and
stock grow together as one organism, which through life, no
matter how large the plant becomes, retains below the
union the hardy roots and other characters of the one, and
above the union the special good qualities of the other.
The possibility of grafting depends upon the capacity of
the cambium of related plants thus to unite; and its value
depends upon the permanent retention of the characters of
the cion substantially unaltered,

In practice only closely-related kinds can be grafted to-
gether, presumably because of chemical incompatibility in
the protoplasm of more distant relatives. Further, only
exogenous kinds will unite, because the joining of the cam-
bium is the central feature of the process; and much of the
Cu. IV, 13] ECONOMICS OF STEMS 209

technique of grafting centers in making good contacts of
cion and stock, and in holding the parts together until their
permanent union is effected (Fig. 154). Grafting is mostly
done in very early spring, when the tissues are resting, but
are soon to become active. Later, as the tissues awaken, they
knit together, the wound heals over, and thereafter they
grow as one plant, without need of further attention, except
that for a time care must
be taken to remove any
shoots which spring up
from the stock, for these,
with their greater vigor,
may draw all sap from
the cion and cause it to
perish. Ideally the pro-
cess is simplest when cion
and stock are the same
diameter ; but very small
twigs can readily be
grafted upon very large
stumps. Naturally an
elaborate technique and, e154 > Mstiation of the method
great special knowledge prepared; next, two cions inserted in a
appertain to the subject. ‘#04: on the Tish, the asing of the

Grafting is practiced for trance of Fungi. (From Bailey.)
three principal reasons :

First, and most important, it permits both the preserva-
tion and the multiplication of valuable kinds of plants which
appear as BUD sports, but which neither transmit their good
qualities through seed, nor strike root from cuttings, and
hence, except for grafting, would be lost. Bud sports, which
are related to monstrosities (page 205), are individual branches
which show in their development some striking difference
from others on the same plant. Most of our best varieties
of Apples, Pears, Oranges, and other fruits, have originated
in this way, and are perpetuated only by grafting. Indeed,

P

 
210 A TEXTBOOK OF BOTANY [Cu. IV, 13

grafting may be defined from this point of view as a process
of fitting a set of ready-made roots upon kinds of plants
unable to make any of their own.

Second, grafting can be used to produce certain desirable
changes in minor qualities of the cion, though no essential
features can thus be altered. An earlier or later time of
blossoming or fruiting of a tree, a better adjustment to a
particular soil or climate, advantageous dwarfing or enlarg-
ing, resistance to root parasites, even in some small degree
an improvement in color or size may be wrought in the cion
by grafting on a suitable stock. All such features, however,
seem to depend upon the sap, which of course is supplied by
the roots of the stock. The more essential characters are
seated in the protoplasm, and remain unaltered by grafting,
since the protoplasm, unlike the sap, does not pass from
stock to cion, but remains separate in the two.

Third, curious effects in plant form are obtainable by
grafting, as when a dozen or more varieties of Cherries are
made to grow on one tree, or bizarre constructions are pro-
duced by the grafting upon one stock of many forms of Cacti,
which happen to graft extraordinarily well.

The older books upon horticulture frequently mention
GRAFT-HYBRIDS, of which the most famous is Cytisws Adam,
produced by grafting between yellow-flowered and purple-
flowered shrubs, and itself preserved by grafting. It shows
diverse comminglings of yellow and purple in the flowers,
but not an intermediate color. In a true hybrid, produced
by the crossing through fertilization of two parents of dif-
ferent races or species, the color is that of one parent or the
other, or else has an intermediate shade, but is never a
mosaic of the two colors, as in this plant. However, modern
research has shown that Cytisus Adami is no hybrid at all,
but a mixture of the tissues of the two parents, such a com-
bination being now called a cutma@ra. It has been found
possible to produce these chimeras artificially by so manip-
ulating the grafting that a part of a bud of the cion unites
Cu. IV, 13] ECONOMICS OF STEMS 211
with a part of a bud of the stock, in which case the resultant
bud has the tissues of the two parents intermingled in diverse
ways. Such chimeras, accidentally produced, are not un-
common in Oranges, or even in Apples, which sometimes have
one segment of skin differing sharply in color or texture from
the remainder.

An important economic aspect of stem structure is in-
volved in the new practice of tree surgery. In order to pre-
serve valuable trees, it is now customary not only to prune
away branches seriously affected by disease,
but also to clean out cavities thus caused,
and fill them with cement, in imitation of
the methods successfully practiced by
dentists with teeth. Experience, however,
is hardly justifying earlier expectations, for
such cement-filled cavities, though seem-
ingly at first satisfactory, often decay next

 

Fia.

155. — A

the cement, which shrinks slightly in setting
and allows sap to exude and Fungi to enter.
Besides, the rigidity of the cement fits
badly with the elasticity of trees which
must sway in the wind, while its weight in
some positions is a serious strain upon thin
cylinders of wood. A promising, though

good and a bad way
to strengthen a
weak crotch of a

tree. Better yet,
in many cases, is
the use of a chain
between two bolts
instead of the single
long bolt. (From
Bailey.)

rather expensive substitute, is a filling of

wooden blocks set in an elastic, antiseptic material like tar.
In other details tree surgery has made real progress, e.g. in
the supporting of weak branches by chains and bolts, the
former of which permit a free motion in the tree, while the
latter prevents that choking of the bark which follows the
use of encircling bands (Fig. 155). The subject is still in
the developmental stage, on which account it offers a tempt-
ing field to incompetent practitioners, and even impostors,
against which type of ‘‘tree-surgeons”’ the owner of trees
must be upon guard.
CHAPTER V
THE MORPHOLOGY AND PHYSIOLOGY OF ROOTS

1. THe Distinctive Features or Roots

Roots are typically underground parts which spread
through the soil and absorb therefrom the water and mineral
matters needed by plants, while simultaneously providing
a firm anchorage for the stems which rise in the air. Thus
roots have a distinctive primary with a prominent secondary
function. Though diverse in forms, and occasionally per-
forming additional or substitute functions, they are less
multiform in these features than leaves or stems, no doubt
because of the more homogeneous environment under which
they dwell.

Typical soil roots extend from the base of the stem, and
continuously radiate, branch, and taper down to a fibrous
size. Taking all angles from vertical to horizontal, they form
collectively a mass suggestive of some shoots, but inverted
(Fig. 156). Unlike shoots, however, they are rarely sym-
metrical, because mechanical irregularities in the ground,
and self-adjustments to the uneven distribution of water,
air, and mineral salts, greatly alter their shapes, making
actual root systems extremely irregular. The radiate form,
so distinctive of soil roots, enables them to reach a large
volume of soil, while also providing the best anchorage
against the all-sided strains to which stems are exposed; but
there also occur cases in which a single main root continues
the stem vertically downward, the lateral roots being very
much smaller. Such a Tap roor (Fig. 157) is rare in trees
but common in herbs, as familiar in Dandelion and others,

212
Ca. Vy 1 FEATURES OF ROOTS 213

where often it is used for storage of food. That the mass
of roots keeps towards the surface, especially in the largest
plants, is due in part to the need for aération, and in part to
the increasing hardness of the soil with greater depth.

In size, roots bear close relations to shoots, for it is clear
that the shoot takes the lead, so to speak, in determining the
form and habits of the
plant, and _— secondarily
produces a corresponding
quantity of roots. No
matter what the size at
the trunk, all roots end in
the delicate white tips de-
voted to absorption and
growth; and in correlation
with this uniform function,
performed under compara-
tively uniform conditions,
the tips of typical soil roots
are not far from one size.

In texture, roots vary
from woody-hard in trees
(the wood, indeed, of roots
being often harder and
more compact than that of
the stems) down to the
softness of meristem in
eae aes. The fibrous Fie. 156. — A typical root system, of
parts are tenaciously tough, Corn. (From Bailey.)

—a quality which is evi-
dently connected with the fact that the anchorage function
of the roots falls largely on the fibers.

In color, roots are white at their growing tips, that being
the natural color of meristematic tissue. Farther back they
are brown, from the development of protective cork; and
in older parts they are very dark from the action of the

 
214 A TEXTBOOK OF BOTANY (Cu. V, 1

soil on the bark. Sometimes, when exposed to the light,
young roots will turn red, apparently through formation of
erythrophyll, which may have any of the meanings already
explained for that substance (page 88).

In duration, roots conform to the plants which produce
them, being annual, biennial, or perennial, and either herba-
ceous or woody.
Unlike — shoots,
however, roots
drop no parts,
for the growing
tips develop
without break
into the older
and thicker,
and finally the
woody parts.

Roots are pro-
duced from
stems, most
commonly and
typically from
the lower end of
the first stem
formed by the

Fie. 157. — A typical tap root, of Dandelion. embryo plant;
(From Bailey.) but sometimes

they develop

from other parts, particularly from the nodes where these
happen to touch the ground. Further, many kinds of
plants, like the common ‘Geraniums,’” which do not
naturally produce roots from their stems, can be made to
do so from slips or cuttings, though this is impossible with
most kinds. Sometimes, though rarely, roots produce stems,

 

as in Locusts and Apple trees, which send up sucKERs from
their roots at a distance from the trunks.
Cu. V, 2] STRUCTURE OF ROOTS 215

True soil roots are found only in the Flowering Plants and
Ferns. The lower land plants (the Bryophytes, or Moss
plants) have substitutes in large hair-like RHIzoIps. The
Alge need no roots, since they absorb through their whole
bodies, though the Rockweeds have attachment organs,
somewhat like roots in aspect. In the Fungi no roots occur,
although their slender absorbing mycelial threads (page 84)
possess certain characteristics of root hairs.

While soil roots are primarily organs of absorption and
anchorage, they also perform other functions, becoming
storage organs, spines, climbing organs, and even foliage, as
will presently be noted.

2. THE Structure or Roots

The principal features of root structure can be seen very
well in the root system of some garden herb or house plant
carefully lifted and washed free —
of adherent soil. Observation of
such material shows that the
entire root system of a plant is
continuous, without any trace of
such nodes as occur in the stem.
Each part is typically cylindrical,
though often forced by the soil to
other shapes. The branching is
very irregular, in marked con- ie
trast to the phyllotactic sym- — Frye. 158, — Cross section of
metry of the shoot, but answering the fibrous part of a young root

ae . of a Bean, Phaseolus multiflorus.
to the composition of the soil; (From Sachs.)
but in some seedlings the first side
roots appear in vertical rows corresponding to the fibro-
vascular bundles which enter the roots from the stem, — e.g.
in Bean seedlings four such rows occur. AlF new branches of
roots originate deep in the tissues, in contact with the fibro-
vascular bundles, whence they make their way out through
the overlying tissues, partly by the solvent action of diges-

 
216 A TEXTBOOK OF

BOTANY [Cu. V, 2

tive enzymes, and partly by mechanical rupture, as a later

picture illustrates (Fig. 164).

This method of origin contrasts

greatly with that of leaves, which arise as surface swellings
in the bud, while the origin of branches is intermediate in

Fic. 159. — A typical root tip, of Radish ;
magnified.

 

nature. The vein, or
fibro-vascular, system
of roots is in perfect
continuity with the
systems in stems and
leaves. The separate
fibro-vascular bundles
of young roots, clearly
visible in sections by
aid of a hand _ lens,
differ little from those
of the stem, although
the fibro-vascular sys-
tem of roots as a whole
is more strongly con-
densed towards the
center, often obliterat-
ing the pith (Fig. 158).
Thus, while stems ap-
proximate, as we have
seen (page 118), to the
hollow-column _ princi-
ple of construction,
roots are built rather
on the plan of cords or
cables. The difference

is obviously correlated with the different kinds of strains
the two parts have to bear; for, while stems are exposed
to great lateral strains from the winds (and, on the non-
vertical parts, from their weight), against which the hollow
column is most effective, the roots are exposed only to
pulling strains, in resistance to which the solid cable is best.
Cu. V, 2] STRUCTURE OF ROOTS 217

The most highly developed roots, those of our exogenous
trees, show three distinct though intergradient parts, —
viz. the slender white tips, the elongated fibers, and the
thick woody parts.

The tips, best seen in material grown for the purpose in
moist air or moss, show really four parts (Fig. 159). First,

 

Fic. 160. — The root-hair zones and growth zones in some common roots ;
x4. From the left, Pea, Radish, Corn, Lupine, and, below, Oats. The
seeds were germinated in moss behind sloping glass plates.

the actual end of the root consists of a RooT cap, formed
from behind by the very delicate growth tissue, to which
it acts as a protection in. the advance of the root through
the soil. Second, just behind the root cap lies a yel-
lowish spot, which marks the GrowINne point, the place
218 A TEXTBOOK OF BOTANY (CH. Ve 2

of formation of all new cells by which the root increases in
length, the color being that of the abundant living proto-
plasm showing through the transparent walls. Third,
just behind the growing point lies a short smooth zone, which,
though little prominent, has yet this great importance, that
it is the GROWTH ZONE, or place of enlargement to full size
of the new cells formed in the growing
point. The growth of the root in length is
wholly confined to this zone (though new
cells cause an increase in diameter farther
back), in great contrast to the conditions
in stems, where the growth occurs through
several expanding internodes (Figs. 112,
114). Fourth, just behind the growth zone
comes another, differing greatly in length in
different plants and under different condi-
tions, the ROOT HAIR ZONE (Fig. 160). The
ROOT HAIRS thereon show remarkably well,
especially through a lens, in roots germi-
nated from seeds in moist air, though they
have no such regularity of shape in the soil
(Figs. 161-2). In the former material the
hairs radiate very evenly outward, forming
collectively a sort of nimbus along the root;
and they are obviously forming anew in
front, going each through its grand period,
Fic. 161.—Rad- and dying behind. Thus the zone moves
ae me along as a whole just behind the advancing
soil. (FromSachs.) tip. The funetion of the hairs is well
known; they provide the great surface

necessary for the absorption of the water when it is scant
in the soil. They pass this water through the cortex to
the ducts, which extend all the way from this zone to the
leaves. We can now see a reason why the entire growth
of the root in length takes place in advance of this zone,
for any growth behind the hairs would obviously tear them

 
 

 

Fic. 162. — Longitudinal sections through a root of Corn, at the growing
point, growth zone, and hair zone; highly magnified.

The scale of the drawing is not large enough to permit the representation
of all of the details mentioned in the text.
220 A TEXTBOOK OF BOTANY (Cu. V, 3

from the root. In cross sections one can see the fibro-vascular
bundles lying so closely towards the center as greatly to re-
strict the area of the pith, or even to obliterate it altogether,
though there is always a relatively thick cortex (Fig. 163).

The tips of the soil roots of different plants are far more
uniform in size, and especially in diameter, than are the
leaves and young stems,—of course because of the more
uniform environment presented by the soil. Exact measure-
ments show that in ordinary plants, the roots at the growth
zone vary in diameter from .3 to 1.07 mm. with a mean at
.67 mm., while the side roots vary from .19 to .79 with a mean
at .53, giving a conventional constant of .6 mm. for the diam-
eter of root tips in general. This size bears without doubt a
relation to the conditions of water absorption by the roots,
analogous to the relation of leaf-thickness to light (page
33), though the precise factors have not yet been deter-
mined.

Backwards the young white tips merge gradually into the
familiar brown, fibrous roots. Cross sections thereof show
the formation of a corky bark, the beginning of a secondary
growth in thickness of the bundles (in exogenous kinds),
and a general aspect of toughening of the tissues; for this
is the part of the root which seems to take much of the strain
of the anchorage function.

In herbaceous plants, as a rule, the roots remain fibrous,
but in shrubs and trees they grow continuously in thickness
by the activity of the cambium, quite after the manner of
the stem. Thus they develop a distinct bark and wood,
with annual rings, medullary rays, and other features already
familiar in stems. Indeed, except for their underground
position, such roots are practically stems.

3. THE CELLULAR ANATOMY oF Roots

As with other plant organs, the cellular anatomy of roots
is linked so closely with their functions that the two recipro-
cally throw light upon one another.
Cu. V, 3] ANATOMY OF ROOTS 221

A very thin section cut longitudinally through the tip
of an ordinary root, e.g. of Corn, presents under the micro-
scope the aspect here pictured (Fig. 162). Close to the coni-
cal end stands out the growing point, distinguished by its
many small, densely-packed cells, which are squarish in
section, thin-walled, and filled with the all-important
protoplasm. This is the place of cell-formation for the en-
tire tip of the root, the new cells being made by division
from a small central group, after which they absorb nourish-
ment and enlarge to the original size. In front these new
cells are constantly forming the root cap, becoming larger
and empty near the outside, where they are continuously
abraded away by the forcible passage of the root through the
soil. Backwards, in the growth zone, the cells hold the reg-
ular ranking in which they are formed, but grow rapidly
larger, especially in length, while keeping their thin walls,
to which the protoplasm comes soon to form only a lining.
Each individual cell, in fact, immediately after its formation,
goes through a grand period of enlargement (page 156), soon
reaching its maximum size; and this explains how the
growth zone follows so closely behind the growing point.
Here also can be seen the beginning of the cellular differentia-
tion of the fibro-vascular bundles, while the intercellular
aération system also is plain, though it does not appear in
our drawing. Backward the growth zone merges impercep-
tibly into the hair zone. The hairs originate as slight swellings
from the outer walls, and grow rapidly longer until they
attain the familiar tube form. In this zone appear also the
striking fine spirals of the ducts, of which the mode of for-
mation is clearly apparent in good sections. The end walls
in a long line of superposed cylindrical cells break down,
under action of digestive enzymes, while simultaneously
the spirals begin to appear as local thickenings of the
walls.

These sections show further that the outermost layer
of cells of the root possesses no breaks or openings of any
222

description, the walls being everywhere continuous.

A TEXTBOOK OF

BOTANY (Cu. V, 3

This

absence of stomata is perfectly explained by the habits of

 

Fic. 163. — Cross section of a root of
a Bean, Vicia Faba, just behind the hair
zone; X SO.

The four strands of xylem meet in the
center, obliterating the pith, while the
strands of phloem stand separately be-
tween the arms of the cross thus formed.
Between xylem and phloem can be seen
the developing cambium, which presently
begins to form néw xylem inside of the
phloem, thus originating bundles of the
ordinary stem type. Surrounding the
fibro-vascular system is the endodermis,
and outside thereof the very wide cortex.
(Fibro-vasecular system after L. Kny, the
remainder drawn from nature.)

roots, which have no
chlorophyll and need no
cutinized epidermis. The
oxygen used in the respira-
tion of the roots passes in
solution directly through
these walls, which are
uncutinized.

Cross sections bring out
several additional features
(Fig. 163). Here can be
seen more distinctly the
innermost layer of the
thick cortex, called the
ENDODERMIS (Fig. 163),
the exact morphological
equivalent of the starch
sheath of stems (page 130).
In the roots, however, the
walls of this layer are
partially cutinized, espe-
cially on the radial parts,
for reasons not yet under-
stood. Also there appears
a notable difference in the
arrangement of the young
fibro-vascular bundles
compared with the stem.
The xylem, recognizable
by the very large size
of the ducts, and the
phloem, distinguished by
the smaller angular form
of the sieve tubes, do not

as
Ca. V, 3] ANATOMY OF ROOTS 223

stand in-and-out from one another but alternately, or in
different radii. This arrangement, found in all roots, has
been viewed as adaptive, in removing the phloem out of the
path of transfer of the water from root hairs to ducts; and
support is given this supposition by the fact that immedi-
ately behind the hair zone the arrangement is abandoned,
for the new xylem
and phloem made by
the developing cam-
bium stand in-and-
out from one another
as in stems. The
method by which
the cambium makes
the transition from the
one arrangement to
the other is easily un-
derstood by aid of
the figure. Endog-
enous roots do not,
of course, form a cam-
bium, but have sepa-
rated closed bundles
as in their stems.
Just behind the hair Fic. 164. — Longitudinal section of a root
3 of Corn, showing the origin of a side root;
zone the cambium highly magnified.
begins the secondary ee ah root ae es in eunbant Bes : a
: ro-vascular bundle, and ‘‘dissolves”’ its way
increase in thickness, out, by action of enzymes, to the surface.
by addition of xylem
from its inner and phloem from its outer face, precisely as with
stems. Farther, back along the root, one can see here and
there in cross sections the mode of formation of the new side
roots, which come from the fibro-vascular bundles, and make
their way to the surface, as already described (Fig. 164).
In the thick woody parts of the roots of shrubs and trees
the cellular anatomy is nowise essentially different from

 
224 A TEXTBOOK OF BOTANY [Cue Vy 4

stems. Indeed, except for the relics of their early root
anatomy deeply buried within their tissues, and their some-
what greater compactness of texture, such roots are stems,
both structurally and physiologically, despite their under-
ground position.

4. Tur ABSORPTION OF WATER, AND OTHER FUNCTIONS
or Roots

Typical roots perform one primary function, viz. absorp-
tion of water and mineral matters; one secondary function,
viz. anchorage for
the stem; and one
or two minor func-
tions supplemen-
tary to these.

Water is the
most necessary of
all the materials
absorbed by plants,
in which it is used
for six purposes.
First, it forms an
essential — constit-
uent of the photo-
synthetic food
(page 21). Second,
it constitutes a
large proportion of
the composition of

Fie. 165. — Typical root hair, and cortical cells, plants amounting
in a longitudinal section of Radish. (After a wall I pe 2
diagram by Frank and Tschirch.) (as, shown by com-

parative weighings
of fresh and dried material) to more than 90 per cent in
most herbaceous parts. Third, it holds the soft parts tensely
spread by igh sap pressure within the cells. Fourth, it is
a necessary solvent for the many chemical reactions in

 
Cu. V, 4] ABSORPTION BY ROOTS 225

progress in plants, such reactions rarely occurring except
in solution. Fifth, it provides a medium of transport,
in form of solution, for substances through the plant. Szzth,
it is needed to compensate the incessant loss by transpira-

tion. These are the
reasons why plants
must have plenty of
water.

The water used by
ordinary plants is
wholly absorbed
through their roots,
and none is taken
through leaves or
stems. Further, the
actual absorption is
known to take place
in the young parts of
roots, and mainly
through the root hairs.
The hairs are thus
effective, not through
any special power de-
nied to other cells of
«the young root, but
simply through the
great surface they
spread. It is because
these hairs, tightly
adherent to the soil,
are mostly torn away
when roots are lifted
from the soil, that
plants commonly wilt
on transplanting, and
recover only after

Q

 

 

 

 

 

 

Fic. 166.— A plan of a root as an absorbing
mechanism, arranged as in Figs. 11 and 105,
with similar signs for water, protoplasm, and
sugar. At the tip the growing point; at the
left, pith; a duct; two rowsof cortex; the root
hairs. Note that hairs and cortex contain
protoplasm and sugar, but the duct contains
neither.
226 A TEXTBOOK OF BOTANY (Cu. V, 4

new tips and hairs have again made connection with the

 

 

 

 

 

 

 

Kia. 167. — A pressure gauge at-
tached to a root for the measurement
of sap-pressure; x }

The rise of the mereury in the long
tube above the levelin the reservoir bulb
gives the sap-pressure in ‘‘atmospheres.”’

water supply.

Each root hair is a cell,
possessing a cellulose wall
lined by living protoplasm
(Fig. 165) and a sap con-
taining various substances,
especially sugar, in solution.
The hairs are in close con-
tact with particles of soil,
and bathed in the surround-
ing water (Fig. 169). In
the root they are in con-
tact with the cortical cells,
which likewise have cellu-
lose walls, protoplasmic
linings, and sugar-contain-
ing sap; and the cortical
cells in turn are in contact
with the ducts which have
no protoplasmic linings. A
typical example of this
absorbing system is shown
by an earlier picture (Fig.
159), while its mechanical
construction is illustrated
by our diagrammatic Fig-
ure 166.

The water in the ducts,
while sometimes containing
sugar and the like, is ordi-
narily nothing other than
soil water, with some min-
eral matters in solution.
Furthermore, this water is
forced into the ducts by
Cu. V, 4] ABSORPTION BY ROOTS 227

the cortical cells under considerable pressure, as manifest to
the eye when a suitable pressure-gauge is attached to the
cut stump of an active plant (Fig. 167). Thus tested, potted
plants will often show a root pressure, 7.e. a pressure of water
in the ducts, sufficient to raise water over thirty feet, while
some trees show two or three times as much. This pressure
is not enough to raise water to the tops of the tallest trees,
but it does give the sap a good start up the stem, after
which it is lifted to the leaves by the forces we have earlier
considered (page 147). This root pressure, however, is the
source of the “bleeding” of broken or pruned stems in the
spring, and also of guttation.

What then is the nature of the power by which the root
hairs absorb water and give it so forcible a push up the
stem? Evidently the water absorbed by the hairs and passed
through the cortical cells must pass through walls and proto-
plasm, which are membranes, and through the cell solutions,
which, for simplicity, we can consider as solutions of sugar,
their most prominent constituent. Such absorption is
known in physics under the name osmosis, and so important
a part does osmosis play, not only in absorption of water,
but also in other physiological phenomena, that the student
should not fail to make its acquaintance through experiment.
Any simple device in which a membrane, e.g. a piece of
parchment, separates a sugar solution from water, will
serve the purpose; but a specially convenient arrangement
is represented in the osMoscoPsE shown in Figure 168. When
a solution (molasses is a very convenient solution of sugar)
is placed in the parchment tube, which then is immersed
in water, the solution will rise in the vertical tube at a
distinctly visible rate. If instead of water a solution
weaker than that in the parchment tube be used, the result
is the same, though the rise is slower. If the water be
placed inside and the solution outside, there is no rise, but the
tube soon empties, shrinks, and collapses. These phenom-
ena are typical, and the osmotic process may be generalized
228 A TEXTBOOK OF BOTANY [(Cu. V. 4

thus, — whenever a solution and water, or two solutions of
different strengths, are separated by a membrane which they

[1

 

 

 

Fie. 168. — An osmo for
the demonstration of osm Sede

The larger jar contains water, the
tube inside is parchment paper, and
the dark liquid is molasses. When
this liquid has risen to the top of
the open tube, it can be dropped
back to level by opening the stop-
cock of the reservoir-funnel.

  

can wet, there is always a
movement from the weaker to
the stronger at a rate propor-
tional to the difference in
strength.

In the foregoing experiment,
though the solution rises in
the tube, some also escapes
into the water, as shown by
its color when molasses is
used. From the root hairs,
however, no sugar escapes to
the soil. When we seek a
structural reason for this dif-
ference, we find that the root
hair possesses a protoplasmic
lining, which has no counter-
part in the tube. It is, how-
ever, entirely possible to make
up from certain common
chemicals, and supply to the
parchment tube, a lining which
in this respect acts like the
protoplasm, viz. it permits
water to enter, but no sugar to
pass out; and such “artificial
cells” are often constructed in
botanical laboratories. Thus
we see that membranes exist
which permit both water and
sugar to pass (PERMEABLE
membranes), while others per-
mit only water to pass (SEMI-
PERMEABLE membranes). This
Cu. V, 4] ABSORPTION BY ROOTS 229

difference is vastly important in both plant and animal
physiology.

It is perfectly clear that the water passes osmotically
into the root hair cells, and thence to the cortical cells,
which have solutions as strong as the hairs, or stronger. In
small simple plants, especially the Moss plants, the water
moves thus from cell to cell, throughout the plant. But
where ducts are present, as in all of the Flowering Plants and
Ferns, the water passes from the innermost cortical cells
into those ducts, and does so as pure water, and not as a
sugar solution. Why does water leave the cortical cells, when
it enters the similarly constructed hair cells? In a physical
machine it would not do so; the cortical cells would absorb
water from the ducts, instead of giving it out to them,
precisely as in case of the hairs and the soil. Herein we
face a still unsolved problem of plant physiology. Several
methods are imaginable, though none have been proven;
but there is little doubt that the explanation will be found
in some simple chemical or physical change controlled by
the living protoplasm. Presumably the method is dependent
on the relatively great thickness of the cortex in all ab-
sorbing roots; and it may prove that each cortical cell
contributes a little towards breaking the osmotic hold
on the water, the codperation of many being therefore
essential.

In the experiment described a few pages earlier the solu-
tion was free to rise. What happens when the tube is
closed? In this case pressure always develops, first stretch-
ing, and then bursting the cup, unless very strong; and if a
suitable gauge be attached, the pressure can be measured.
The results are surprising, for with cells specially built for
great strength, and the use of strong sugar solutions, osmotic
pressures have actually been measured in excess of 24 atmos-
pheres, that is, 360 pounds to the square inch, which is more
than the pressure in most steam boilers; and we know that
greater pressures occur. In cells of the higher plants the
230 A TEXTBOOK OF BOTANY (Cx Vand

pressures are much lower than this, usually not more than
10 to 20 atmospheres, though in the lower plants, especially
some Molds and Bacteria, there is reason to believe that the
pressures rise often far above the 24 atmospheres just men-
tioned.

Such striking and important phenomena as osmotic ab-
sorption and pressure demand explanation, which, however,
cannot yet be given with certainty. A close quantitative
relation exists between osmotic pressure and gas pressure,
on which account some investigators have considered them
identical, holding that a substance in solution is virtually
in the state of a compressed gas, and exerts a gaseous pres-
sure. Others, however, maintain that nothing moreis involved
than the adhesive affinity of the sugar, or other dissolved
substance, for the water, — the substance confined within
the membrane drawing and holding the water which
can pass the membrane freely. The most probable explana-
tion makes it a result of the checked diffusive power of the
dissolved substance, which cannot escape through the mem-
brane, though the water can enter. As to the passage of
water, and (in case of some membranes) dissolved substances,
through membranes which seem perfectly solid, that clearly
occurs between the ultimate structural units of the mem-
brane, whether molecules or other units. But the subject is
too recondite for further discussion at this place.

The mineral matters needed by plants are compounds
which contain the following seven elements, — viz. ni-
trogen (which plants cannot absorb from its uncombined
state in the air, and therefore must obtain from compounds
in the soil); sulphur and phosphorus, integral constituents
of proteins, and therefore of living protoplasm; potassiwm,
needed for incidental processes in connection with the forma-
tion of carbohydrates; calciwm, a neutralizer of injurious
substances; magnesium, an integral constituent of chloro-
phyll, with ron, incidentally necessary in some way to the
formation thereof. These elements all occur in mineral
Cu. V, 4] ABSORPTION BY ROOTS 231

salts dissolved in the soil water with which they are absorbed
into the plant. Though other mineral matters are also
absorbed, only those which contain these elements are
invariably essential; and if we add the three elements, car-
bon, hydrogen, and oxygen, we have a list of ten elements,
indispensable to the life of the higher plants.

Not all of the mineral salts dissolved in the soil water are
absorbed equally by plants, or in the same proportions by
different plants; but in how far this seeming ‘selective
power”’ of roots is merely incidental to their physical and
chemical constitution, and in how far it acts adaptively to
the needs of the plant, is still uncertain. Probably, as in
most such phenomena, something of both is involved.

Such is the method of the primary function of roots, that
of absorption. The second function, anchorage of the plant
in the ground, is chiefly mechanical and comparatively simple.
Against the lateral strains upon stems from the action of
winds, a suitable resistance is provided in the radiating
disposition of the roots, with their tough cord, or cable,
type of construction. There is good reason to suppose that
roots subjected to the greatest strains may become thicker
and tougher in adaptive self-adjustment thereto, in the
very same way that our own muscles grow stronger through
exercise.

In addition to the two functions which roots perform as
their peculiar contribution to the economy of the plant as a
whole, they have also certain others essential to their own
individual well-being, — notably respiration and growth.
Respiration in roots has precisely the same method and
meaning as in other parts of the plant (page 165). Roots,
accordingly, require air, and this need has a dominating in-
fluence upon many features of their habits and structure.
In plants which live in bogs, marshes, swamps, and other
places of standing water, the air is usually transferred to the
roots from the leaves along the intercellular air system,
which in such cases is specially developed. By ordinary
232 A TEXTBOOK OF BOTANY [CHE Vee 9

roots, however, air is absorbed from the supply contained in
the porous soil. Roots have no stomata, or other openings
in their equivalent for an epidermis; but the air in the soil
becomes dissolved in the water, and goes in solution through
the saturated walls into the cells of the root, from which it
passes to the air spaces, where it re-collects in the gaseous
form and thus reaches other parts of the root. The carbon
dioxide produced in respiration diffuses out to the soil by
exactly the reverse process. It is because of self-adjustment
to a more abundant air supply (aérotropism) that most of the
roots of great plants do not commonly penetrate far into the
ground, but keep close to the surface. This is also the reason
why trees commonly die when their roots are deeply buried,
as sometimes happens in grading around new buildings.

Protection of roots against desiccation, the ever present
danger to leaves and stems, is effected incidentally by their
position within the damp ground. Thus it is possible for
the young tips to dispense with a cutinized epidermis, which
would be inconsistent with their absorptive function. The
older roots develop a bark, but it is thin as compared with
that of the stems.

5. Osmotic PRocESSES IN PLANTS

The absorption of water by roots is only one of several
important plant processes in which osmosis has part. It is
important to recall that osmosis is a physical process, though
living protoplasm may regulate the conditions of its opera-
tion: that it occurs wherever in Nature two solutions of
different strengths are separated by a membrane they can
wet: that in such case there is always a movement from the
weaker to the stronger solution: that the movement in-
volves both solvent and dissolved substance in case of per-
meable membranes, but the solvent only in the semi-permeable
land: that the stronger solution will swell and rise if free, but
when confined will develop pressure which can become very
great. Also its rate is directly proportional to temperature.
Cu. V, 5] OSMOTIC PROCESSES 233

The most striking utilization of osmotic pressure by plants
consists in the maintenance of the form and rigidity in leaves,
young stems, flowers, and other soft herbaceous parts. So
small is the percentage of solid matter in such tissues (not
over 10 per cent, with 90 per cent of water), and so thin and
flexible the cell walls, that they cannot alone sustain their
own weight, as shown by their collapse in wilting. These
herbaceous parts are held tensely stretched and outspread
in their characteristic forms by the osmotic pressure of their
sugar-containing sap inside the thin-walled cells, the needful
water being supplied from the ducts. That herbaceous tissues
owe their stiffness to osmotic turgescence may be proven
conclusively by the simple experiment of immersing them
in a solution having a greater osmotic strength than the sap,
in which case of course an osmotic movement out of the
cells will take place. The result is always a collapse of the
tissues even more striking than wilting produces. It is true
the experiment works badly with leaves and stems, because
the waterproof epidermis almost prevents osmotic move-
ment; but the effect is perfect in parts without epidermis,
such as strips cut from Potatoes or Beets. These become
soft and flexible after only a few minutes’ immersion in
strong sugar or salt solution, although comparison strips are
rendered stiffer and harder than ever by immersion in
pure water. Not only do such tissues become flaccid by
wilting or immersion in strong solutions, but they also
shrink in area, thus proving that the tense cells are held
actually stretched by the osmotic pressure within them.
The stiffness which pressure of water can give is familiar
also in fire-hose.

Equally important is the réle of osmotic pressure in growth,
for it supplies the mechanical power whereby the newly formed
cells expand in size, often against much resistance of the
overlying tissues. The young cells osmotically absorb
water, and the resultant pressure stretches the wall, in
which new cellulose is continuously laid down by the proto-
234 A TEXTBOOK OF BOTANY (Cu. V, 5

plasm until the cell is full-grown. By use of the same power
roots force and enlarge for themselves passages through
hard soil, even prying aside stones in the process; and by
the same power they disrupt masonry and lift curbstones in
streets. So essential is osmotic pressure to growth, and
hence so indispensable is adequate water to growing plants,
that any marked scarcity of water, or rapid removal thereof
from the plant, always checks its growth. This is why the
growth rate of a plant always falls, other things being equal,
when transpiration becomes active, and vice versa: why
plants tend to grow faster at night than in daytime: and
why growth usually is checked with the sunrise.

The question must now occur to the student, whether
osmotic pressure can ever become so great as to strain if not
burst the plant cells. This does in fact sometimes happen.
Thus some fruits, notably Plums, in warm moist weather
occasionally burst, from this cause, on the trees. In Tomato
plants, watery blisters are sometimes formed osmotically,
producing a kind of ‘‘physiological disease’”’ called Oedema.
Most kinds of pollen (the small yellow grains producing the
male cells in flowers), when placed in water, swell and burst,
of course to their destruction. This result would be caused
by the rain were it not that in most flowers the pollen is
well protected therefrom by its position, or other arrangements,
as will later be noted (page 295). A case of protective ad-
justment against excessive osmotic pressure seems involved
in the starch formation in leaves. In green leaves in the
light, as the student will recall, the appearance of starch is
always preceded by the formation of sugar, the starch being
formed only after a certain concentration of the sugar has
been reached. The starch, however, is always re-converted
to grape sugar when the concentration again falls, and thus
is translocated into the stem. Now this seemingly useless
formation of starch finds an explanation in the fact that while
grape sugar exerts osmotic pressure, starch exerts none.
The conditions are all consistent with the supposition that
Cu. V, 5] OSMOTIC PROCESSES 235

as the concentration of the photosynthetically-formed sugar
approaches a quantity which might exert injurious action on
the cell, the surplus is converted automatically into starch.
The insoluble proteins found abundantly in sieve-tubes have
presumably a like explanation, as has the cane sugar found
in some leaves intermingled with grape sugar, for cane sugar,
weight for weight, exerts only about half the osmotic pressure
of grape sugar. In this latter fact, indeed, is probably found
the reason why cane sugar is so much more common a storage
form than grape sugar, as Sugar Cane, the Maple tree, and
Sugar Beets illustrate. The fact that such changes, easily
effected by plants, can produce so great a difference in osmotic
properties may help to explain how the water is released
from the cortical cells of the roots (page 229).

A striking and important feature of osmotic phenomena
in plants is this, — that the living protoplasm lining the
cells can act either as a permeable membrane, permitting both
water and dissolved substances to pass, or as a semi-perme-
able membrane, permitting only water to pass, or can act
at one time as one kind and at another as the other. These
various movements, complicated by the nature of the
many chemical substances present, and by special phenom-
ena of diffusion, solution, imbibition, and like molecular
processes, explain, on a purely physical basis, many of the
most important phenomena in plant physiology.

Aside from the living plant, many osmotic phenomena in
plant tissues are familiar in our daily experience. When
shrunken currants or raisins are immersed in water, es-
pecially if heated in cooking, they swell tensely, — for there
is sugar in their cells. Berries cooked with little sugar swell
and burst (though expanding air confined in the tissues also
plays a part) ; but cooked with much sugar, as in preserving,
they collapse. Dry sugar placed on fresh strawberries soon
becomes a sirup, while the berries soften and shrink. The
osmotic explanations are all obvious. We place cucumbers
and celery in cold water to crisp them, that is to make their

 

 
236 A TEXTBOOK OF BOTANY (Cu. V, 5

soft cells more tense and explosive; but warm water is not
used because it tends to fill the air spaces and thus deaden
the explosions. Sugar and salt are effective preservatives of
fruits and meats, though not in themselves deadly to: the
living organisms (germs) which cause decay; and the fact
that those substances must be used in great strength sug-
gests the explanation, that they inhibit the activity of the
germs by osmotically robbing them of water. Beans or
rice are cooked more quickly and perfectly if not salted until
nearly done, and indeed if placed in water too strongly
salted at the start may refuse to swell at all. The sensation
of thirst which follows the eating of much sugar or salt has
apparently this basis, that those substances withdraw water
from the stomach, thus causing the thirst sensation. The
student will be able to cite other examples of osmotic phe-
nomena in daily experience. /
Closely connected with osmosis, of which it is part, is
DIFFUSION. When the molecules of a substance are re-
moved beyond the range of one another’s cohesive attrac-
tion, as in a gas or a solution, they acquire an active
back-and-forth motion from the kinetic energy of the
heat waves reaching them from the surroundings. Thereby
they strike and rebound from one another, and hence are
worked outward, exerting pressure if confined, but spreading
indefinitely if not. Accordingly by diffusion any substance
as a gas or a solution always tends to work away from places of
greater to places of lesser concentration, and away from a place
where it is being produced, and towards a place where it is
being absorbed, each substance diffusing in general as though it
alone were concerned. Familiar phenomena of gaseous dif-
fusion occur in the spread of odors through a house, of floral
fragrance through gardens, and of smoke through the air;
while solution diffusion is illustrated by the spread of ink
or sugar through water. This isthe way that carbon dioxide,
in photosynthesis, passes from the great reservoir of that gas,
the atmosphere, through the stomata and along the air-
Cu. V, 6] STRUCTURE OF SOILS 237

passages to the places of use in the chlorenchyma; and the
way in which the oxygen as released passes outward along
the same passages and stomata. It is also the method by
which sugar and proteins made in chlorenchyma cells pass
from cell to cell until the veins are reached, and then along
sieve tubes and sheath cells to places of storage or use in
stems or roots. It is probably also the ultimate source of
osmotic pressure, which is diffusion pressure (page 230). No
matter, however, what the details may be, the energy of
diffusion is in all cases the same, — heat from surroundings.

Two other physical processes important in plant physi-
ology must here receive mention. Cell walls, if of cellulose
or lignified but not if cutinized, absorb water forcibly by
IMBIBITION, which rests fundamentally upon adhesive affinity
between wall and water. A familiar manifestation occurs
in the warping of boards, which occurs as result of access of
water from one side, or its removal from one side by heat.
Likewise certain dry cell walls can absorb water as vapor
from the air, even producing forcible swelling and move-
ments of the structures concerned; and such HYGROSCOPIC
phenomena occur in connection with the dissemination of
seeds, and elsewhere, as will later be noted.. The other pro-
cess 1S CAPILLARITY, that power by which water rises or sinks
in small passages according to whether it wets them or not,
the energy being furnished by forces of tension within the
liquid itself. Capillarity, however, plays but minor part
in the physiology of most plants, though it has an indirect
importance through its influence on the movements of water
through soils.

6. THE CoMPOSITION AND STRUCTURE OF SOILS

Roots have most intimate connections with soils, which
must therefore be considered in connection with root physiol-
ogy. Besides, soils have high interest on their own account,
and because of their importance in agriculture.

Soils are far more complex than they look, having no less
238 A TEXTBOOK OF BOTANY (Cu. V, 6

than six primary constituents, viz. pulverized rock, water,
air, humus, dissolved substances, and micro-organisms.
These are by no means intermingled without order, but have
relations to one another which result incidentally in a
kind of crude structure.

Putverizep Rock. This constitutes the great bulk,
fully 90 per cent, of ordinary soils. It is derived from the solid
crust of the earth either by chemical decay of the rock or
else by mechanical attrition. Attrition occurs by force of
moving ice, as In glaciers (which have ground the surfaces of
most northern countries), or else of running water, as in
rivers, which forever are grinding the bowlders in their beds
to fine silt. Thus we find every gradation, from great
bowlders down through gravel and sand to silt and the finest
clay. Under the microscope any soil presents the aspect
of rough-angular fragments of rock, variously colored, and
more or less crystalline. The weight and mutual pressure
of these rock particles provide the resistance needful in the
anchorage function of roots, while their irregularity in size
and shape, forbidding a tight packing together, insures the
open irregular spaces through which water and air can
circulate in the soil. These features are well shown in our
generalized drawing (Fig. 169).

Water. This comes second in abundance though first in
importance of the soil constituents. It furnishes the en-
tire supply to ordinary plants, which can take none through
their leaves or stems. It comes into the soil either direct
from the rain or else by way of capillary movement up from
lower levels. It is sometimes so plentiful as to saturate a soil,
that is, fill its spaces completely, as occurs temporarily in all
soils after drenching rains and permanently in bogs and
swamps. Such a standing, or HYDROSTATIC, condition of the
water is not beneficial to ordinary plants, because, while
supplying far more than they need, it displaces the air essen-
tial to the respiration of the roots. As this too plentiful
water drains or dries away, however, the larger spaces be-
 

Cu. V, 6] STRUCTURE OF SOLLS 239

come emptied, and refill with air, though the water still
lingers in the smaller passages and angles in the CAPILLARY
condition. Such a soil is moist, and its combination of
water and air provides the very best conditions for roots,
though one that is nowhere constantly found. It is the
condition represented in our drawing (Fig. 169). As the

SSG
Vv

 

Fic. 169. — A generalized drawing of a section, highly magnified, through
a good soil and a portion of a root with root hairs.

The soil particles are cross-lined, the water is concentrically-lined, the
humus is black, and the air spaces, in the soil, are left white.

water is further removed, by evaporation and root absorp-
tion, some moisture continues to cling tenaciously in thin
films around the particles of soil, from which it is removed
with greater and greater difficulty the thinner the films become.
Upon these Hycroscopic films plants must depend for their
supply during much of the time; and it is apparently for
absorption from them that the root hairs, flattened tightly
against the soil particles, are especially fitted (Fig. 170).
The hygroscopic water films have an important relation
with the soil particles. Not only do the films cling very
240 A TEXTBOOK OF BOTANY [Cu. V, 6

closely to the particles, but they are themselves, through
internal cohesion and surface tension, tenaciously strong ;
and thus they are brought into a state comparable with
stretched rubber. On the other hand, the water molecules
are extremely mobile within the films, as if they were the
best ball bearings. From this combined tenacity and
mobility of the films, it results that when water is with-
drawn from any part of the soil, whether by root hairs or
‘ by evaporation, the films directly affected

draw upon the others with which they are
connected, and these upon others, so that
the draft is thus made over a considerable
distance. Hence a plant is not dependent
for its water supply upon the soil with
which its roots are in actual contact, but
can draw from a far wider area. This ex-
plains why a house plant dries out the soil
of the pot uniformly ; how Cactus and other
desert plants draw from great areas, growing
well spaced apart; and why deep homogene-
xs ee ous soils, like those of the prairies, supply
root hair in the Water so evenly to crops. Furthermore, since
soil, showing its the water films have in general the «same

intimate contact 5 : ‘
with soil par- thickness regardless of the size of the soil

 

ticles; x 240 particles, a fine soil can retain more water
(about). (After : ;
Straspurser)) than a coarse one, which is why clay holds

more water than sand.

Arr. Thisforms the third in abundance of the constituents
of ordinary soils, and is the source of the indispensible oxygen
for the respiration of most roots. It fills the irregular spaces
not occupied by water between the rock particles (Fig. 169) and
is ordinarily continuous with the atmosphere above ground.
In places of permanent hydrostatic water, ike swamps, the
air is excluded, and only such plants can there live as have
large air passages to the roots from the leaves, or are able to
absorb dissolved oxygen directly into their submerged bodies
Cu. V, 6] STRUCTURE OF SOILS 241

from the water. It is in order to introduce air into such
soils that we drain them preparatory to growing crops.

When air stands long in a soil, it loses part of its oxygen
and accumulates carbon dioxide from root respiration. Ac-
cordingly it is better for plants that this vitiated air should be
expelled at intervals, and replaced by a fresh supply. Such
a result accompanies soaking rains; and the keeper of house
plants does well to imitate the method by giving the plants
an occasional thorough soaking, and allowing them to dry
out in large part between times. Such treatment is much
better than a frequent addition of small amounts, for the
latter method does not effect renewal of air.

Humus. This comes fourth in abundance of ordinary soil
constituents. It comprises the dark-colored vegetable matter,
mostly the remains of decaying roots, which to the eye of an
expert is so characteristic a mark of a good soil. A mixture
of humus with sand and clay constitutes Loam, the best of
garden soils. The proportion of humus in soils varies greatly,
from almost none through an optimum amount (represented
in our picture (Fig. 169), to a very great deal, as in MucK,
which owes its black color thereto. Bogs consist almost
wholly of a kind of humus, called Prat, which only partially
decays, and therefore accumulates. The value of humus in
a soil, from the plant point of view, is four-fold. It lightens,
or opens, a soil, thus increasing its aération capacity; it
helps to retain moisture, being very absorbent; it adds
substances, by its decay, to the soil solution, some beneficial
and some harmful, though our knowledge of these matters is
scanty as yet; and most important of all, it supports
numerous micro-organisms, which play a first réle in soil
fertility.

DIssoLvED Susstances. In the soil water occur many
dissolved substances, and therefore it becomes a SOIL SOLU-
TION. Though profoundly important to plant life, the actual
quantity of such substances present is relatively small, even
the richest soil possessing only a small fraction of 1 per cent al-

R ’
242 A TEXTBOOK OF BOTANY (Cu. V, 6

together. Most important are the mineral salts necessary in
the nutrition of plants, and therefore commonly, though not
quite correctly, called “plant foods” (page 28). They consist
In compounds of nitrogen, sulphur, phosphorus, magnesium,
iron, potassium, and calcium, having the uses in the plant
already described (page 230). They come into the soil so-
lution chiefly through chemical disintegration of the rocks
which contain them, but to some extent through action of
living organisms, as will be further described a page or two
later. These natural sources of supply are sufficient in
case of wild plants, which, by decay, return their substance
to the ground; but under cultivation, where great quantities
of mineral matters are annually removed with the crops,
some are apt to run short and-must be replaced artificially,
which is accomplished through fertilizers. The mineral
salts which usually first become scarce are compounds con-
taining nitrogen, phosphorus, and potash; and since all
three are abundant in barnyard manures, we can see the
agricultural value thereof. Nitrates, phosphates, and potash
salts, obtained from other sources, are also used commonly
as fertilizers. Such, at least, is the older and, among
farmers, still prevalent belief as to the role of fertilizers in
the fertility of land. But of late some leading investigators
have advocated a different view, based on the claim that the
soil solution supplies all of the mineral salts which plants ordi-
narily need, even on much-cropped land, the fertilizers
finding their use chiefly in the neutralization of other un-
favorable conditions in the soil.

The functional use of the different mineral salts to plants
is inferred from various lines of evidenee, but chiefly from
the results of WATER CULTURE (Fig. 171). Many herbaceous
plants can be grown from seed to maturity with the roots in
water, their well-developed aération systems providing suffi-
cient oxygen to their roots. By using pure (distilled) water
as a basis, it is possible to supply to a plant all of the neces-
sary mineral salts except some given one, in which ease the
Cu. V, 6)

STRUCTURE OF SOILS

243

peculiarities of the resultant plant give a clew to the réle of

that substance.

In addition to the mineral matters the soil solution con-
tains small amounts of diverse organic substances, partly

beneficial to plants and
partly injurious. They
are mostly set free by the
decay of humus, which
was originally living
tissue containing — pro-
teins, carbohydrates, and
other classes of sub-
stances ; but some appear
to be formed as excretions
of living roots. It was
an old belief, long aban-
doned but now revived
with new evidence, that
roots excrete substances
injurious to themselves,
though commonly harm-
less to other kinds; that
the accumulation of such
substances tends to poison
a soil for the plants which
produce them; and that
soils rendered barren by
long use of one crop are
not exhausted of neces-
sary mineral salts,
commonly supposed, but
are poisoned by the ac-
cumulation of these excre-
tions.

as

But these matters

 

 

Fic. 171. — Typical illustration of the
methods and results of water culture;
x Tb.

The plants are Buckwheat. To dis-
tilled water in the middle jar were added
all of the mineral salts needed by the plant;
to that on the left, all except potassium ;
to that on the right, all except iron. In
the latter case the upper, less shaded,
leaves are white, not green, in the plant.
(Originally from works of Pfeffer.)

are still in debate, and their deci-

sion must await further evidence.

MIcR0-ORGANISMS.

Last in prominence, though not in
244 A TEXTBOOK OF BOTANY (Cu. V, 6

importance, of the soil constituents are certain minute liv-
ing organisms, viz. Fungi, Bacteria, and Protozoa.

Fungi, of certain small kinds, develop in contact with the
tips of the roots of many plants, particularly such as live
in much humus, weaving around them a close cover of my-
celial threads, which replace the root hairs (Fig. 172). This
MYCORHIZA, as it is named, ab-
sorbs water and mineral matters
which it transmits to the roots;
and there is some reason to be-
lieve that it also absorbs solu-
ble organic matters set free in
decay of the humus but useful
again to the plants. The associa-
tion seems clearly beneficial both
to fungus and flowering plant;
and accordingly we have here
one of the cases where two dif-
ferent organisms derive benefit
from their association, a condi-

Bee ea ete tion called symprosis. Some
on the root of European Beech ; kinds of soil Fungi seem also to
Ae RS co Bs have thesame powers as Bacteria,

e entire root tip, back to :
beyond the hair zone, is com- next described, in relation to soil
pletely and closely covered by a nitrogen.

felted mass of mycelial threads, ;
which extend also into the soil. Bacteria, already known to the

Ce Pemiont Teepe) student as the smallest and
simplest of living organisms, are abundant and of many kinds
in all soils; but the most important are those which effect
NITRIFICATION and NITROGEN FIXATION. Nitrogen, a con-
stituent of the protoplasm, is one of the substances most
indispensable to plants; but although it composes four
fifths of the atmosphere, the higher plants are unable to take
it from that source, and have to rely upon compounds ab-
sorbed in solution through the roots. The presence of
mineral salts containing combined nitrogen is therefore one

 
Cu. V, 6] STRUCTURE OF SOILS 245

of the most important, perhaps the most important, factor
underlying soil fertility. Moreover, the supply needs con-
stant renewal to compensate for loss by drainage and removal
from the land with the crops. Now it happens that some
kinds of soil Bacteria have the power to change certain nitrog-
enous substances, nota-
bly ammonia, common
in soils but not usable by
the higher plants, into
other nitrogenous  sub-
stances, notably nitrates,
readily usable by -those
plants; and such nitri-
fication of soils, while it
only transforms, and does
not add nitrogen com-
pounds, is yet an impor-
tant element in soil fer-
tility. Further, there are
other kinds of soil Bac-
teria which possess the
power to take free nitro-
gen from the air and
incorporate it into com-
pounds in their own
bodies ; and such nitrogen
fixation, on decay of their — Fie. 173.— Typical root nodules (or tu-
bodies, adds nitrogen to ee ae x: (Drawn
the soil, and is the chief
source of supply in soils of that indispensable substance.
Both kinds of Bacteria live in the humus, or at least are de-
pendent thereon for most of their food, in which fact lies the
principal reason for the association of humus with good soils.
The nitrogen compounds formed by these Bacteria become
ultimately dissolved in the soil solution, whence they are
absorbed by the roots of higher plants. In a few families,

 
246 A TEXTBOOK OF BOTANY [Cu. V, 6

however, and conspicuously the Pulse family, the relation is
more direct, for the nitrogen-fixing Bacteria live in the
tissues, in the nodules so familiar on the roots of Beans
and Peas (Fig. 173), to which the compounds are thus sup-
plied with minimal loss. There is obvious connection be-
tween this economical arrangement and the fact that the
seeds of Leguminose are richest of all plant products in
nitrogenous substances, particularly proteins, thus coming
nearest to meat in food value.

The importance of nitrogen-fixing Bacteria in soil fertility
has of course suggested the attempt to enrich poor soils by
adding the suitable Bacteria thereto. Many attempts have
been made to this end, but while successful as laboratory
experiments, they have not as yet achieved importance in
practice.

To complete the subject of nitrogen acquisition by the
higher plants, we should note that such has been held to
explain the insectivorous habits of the pitcher plants and
others which trap insects (page 76). The plants which
capture insects digest the bodies thereof, and absorb into
their own tissues the resultant substances, which of course
are particularly rich in nitrogenous materials. In general,
the insectivorous plants are found in places where the
nitrifying Bacteria of soils are unlikely to be found, — our
Sarracenias and Sundews in bogs, the Venus Fly-trap in
sand, and the Nepenthes on the trunks of trees.

Protozoa are minute one-celled animals, typified by the
creeping Amoeba. They abound in rich soils, the fertility
of which they are now claimed to influence. It is found that
any methods of treatment, by heat or poisons, which kill
these Protozoa but not the Bacteria, produce increased
fertility; and since it is likely the Protozoa feed upon
Bacteria, the inference is drawn that the destruction of the
former permits increase in numbers of the latter, with pro-
portionally better nitrification and nitrogen-fixation. Here
again, however, we must await further evidence.
Cu. V, 7] SELF-ADJUSTMENTS OF ROOTS 247

7. Tur SELF-ADJUSTMENTS OF Roots TO PREVAILING
CONDITIONS

Roots possess in remarkable degree that property of in-
dividual adjustment to the peculiarities of their immediate
surroundings, such as was earlier described in the photo-
tropism of leaves and the geotropism of stems.

Geotropism, indeed, is no less characteristic of roots than
of stems. The first root which issues from the germinating
seed always grows over to point directly downward, no
matter in what position the seed happens to lie (Fig. 119).
It is described as positively geotropic, or PROGEOTROPIC,
the main stem being negatively geotropic, or APOGEOTROPIC.
The secondary or side roots possess transverse geotropism,
growing out horizontally, or nearly so, and are described “as
DIAGEOTROPIC. The tertiary roots, however, those which
grow from the side roots, are hardly geotropic at all, and
therefore respond more freely to the other influences next to
be mentioned. The adaptive explanation of such geotropic
growth is obvious, for thus the main root is brought in the
quickest way to the water supply, essential to the further
growth of the young plant ; the side roots are spread at angles
which take them into the widest area of soil, while giving
them angles advantageous to their anchorage function;
and the tertiary roots are left free to wander wheresoever
the materials needed by the plant are most abundant.

Especially characteristic of roots is their HYDROTROPISM,
or sensitive adjustment to moisture in the soil. Roots
not only grow towards soil moisture, but branch and grow
more profusely in moist than in dry places. <A practical
exemplification thereof is found in the filling of drain pipes
by tree roots (Fig. 174). The adaptive explanation of
hydrotropism is sufficiently clear; since the primary func-
tion of roots is the absorption of water, they need to find the
most abundant supply. The actual operation seems to
be this, — the tip of the root is sensitive to differences in
248

A TEXTBOOK OF BOTANY

(Cu. V, 7

the quantities of moisture coming from different directions ;
it transmits a suitable influence to the growth zone; this
zone swings the tip over towards the moister side until the

 

 

 

Fra.

174. — Masses of roots, of a
Willow, which filled drain pipes.
The rod between them is a meter
stick,
The apparent cross folds mark the
collars where the tiles were joined,
(Drawn from a photograph.)

cross-marked in decimeters.

 

stimulus is even all around;
then the root continues its
growth in that direction. It
is important to note that
leaves and stems, neither of
which absorb any water, are
not in the least hydrotropic.
A third self-adjustment of
roots takes them towards air,
—A®ROTROPISM. Other things
equal, roots grow
towards the places in the soil
where air is most plentiful.
At first sight it would seem
that hydrotropism and aéro-
tropism must neutralize one
another, since in general much
water in the soil means little
air, and vice versa. In fact,
however, a complete satura-
tion of the soil gives more
water than plants can make
use of, just as we have found
that full summer sunlight gives

being

more light than can be used
by leaves (page 56); and it
is towards the optimum com-
bination of water and air, best
for root life, that are
guided by their aérotropism,
when their hydrotropism is
satisfied. It is because this
optimum combination of water

roots
Cn: V7] SELF-ADJUSTMENTS OF ROOTS 249

and air, with mineral matters, occurs commonly in drain
pipes that roots are prone to enter and fill them. The
adaptive explanation of aérotropism is of course very plain ;
it is found in the need that all roots have for air (7.e. oxygen),
indispensable to their respiration, which underlies all of
their growth and work.

Aérotropism is really but one phase of CHEMOTROPISM, or
self-adjustment to particular chemical substances, of which
several forms are known. Thus, some roots grow towards
a greater supply of the mineral substances they specially
absorb, though their behavior in this respect is not always
consistent, nor is it well understood. They show also several
minor ‘‘tropisms,”’ of which TRAUMATROPISM, or a turning
away from injurious contacts or substances, is best known.

All of these tropisms are typical cases of irritability, the
equivalent of reflex action in animals (pages 55, 176). The
response is not forced by the moisture, air, etc., but simply
guided thereby, the work of turning being done by the plant.
These phenomena in roots are especially interesting because
the place of perception of the guiding stimulus is usually
the growing point, while the place of response is the growth
zone just behind it. Thus we have an arrangement com-
parable with that in animals, where special sense organs
receive the stimuli, and a separate muscular system makes
the responses.

In most plants the young main root is so strongly geotropic
that it can be deflected only a limited amount from the
vertical position by other influences, but the tertiary and
later formed roots have so weak a geotropism that they
respond to other stimuli very freely. And here must
naturally arise this question: What happens in cases where
two or more different stimuli act simultaneously upon the
same root from different sides? In some few cases the
stimuli seem to influence one another’s action, but in general
the root attempts to respond to them all. The position that
is actually taken is then a resultant, depending upon the
250 A TEXTBOOK OF BOTANY [Cu. V, 8

directions of the stimuli and the relative sensitiveness of the
roots thereto.

The student will be interested to read at this point the
fine passage which closes Darwin’s book ‘The Power of
Movement in Plants.’’ He should keep in mind, however,
the fact that a thread of simile and fancy runs through the
paragraph, to the matters of which our modern science now
gives a somewhat more mechanistic interpretation.

8. Tuer ADDITIONAL, AND SUBSTITUTE, FUNCTIONS OF
Roots

While the great majority of roots have the typical forms
and functions already described, there are some which
perform additional and even substitute functions, with
corresponding modifications of structure.

As in case of other plant organs, roots which perform the
typical functions yet exhibit marked diversity of form,
usually in clear correspondence with different habits. Thus
the difference between the tap root and a mass of fibrous
roots (page 212) is of this nature. Again, the relative im-
portance of the absorbing and anchorage function varies
much, the latter being highly important in great trees, and
almost negligible in low herbs, especially such as have under-
ground rootstocks; and corresponding differences in struc-
ture are manifest. The only roots of the low-growing Bryo-
phytes are the great root hairs, or RHIzoIDS, effective in
absorption, but obviously having little utility, as there is
little need, for anchorage. The depths to which typical
roots descend vary also, for while those of swamp and bog
plants keep near the surface, obviously in relation to air
supply, those of some desert plants reach at least to sixty feet,
as in the common desert shrub called Mesquite, evidently
in adjustment to the water supply. And other differences
are revealed by intensive study, — some most reasonably
explained as adaptive, others as hereditary, and others as
structural or incidental. Yet the diversity presented by
Cu. V, 8] SPECIAL FUNCTIONS OF ROOTS 251

such roots is insignificant in comparison with the corre-
sponding diversity in leaves and stems. The explanation
is found no doubt in the fact that the conditions of life
underground are much more uniform than conditions of life
in the air.

The commonest additional function of roots is storage,
mostly of food, but partly of water, which occurs in all
degrees, from so little as not to affect perceptibly the root
shape, up to the pro-
duction of a rotundly-
swollen organ. The
storage is oftenest in
tap roots (which per-
haps originated in this
way), as familiar in
Carrots, Beets, and
Turnips. In | other
cases side roots are
specialized, as exempli-
fied in Dahlia and Sweet
Potato, where they be-
come tuberous (Fig.
175), and would be
hard to distinguish

 

Fig. 175. — Typical tuberous roots, of
from stem tubers, were Dahlia; x}. (From Strasburger.)

it not for the absence

of eyes. Anatomically, as functionally, stem and root
tubers are closely alike. As with stems, when starch is
principally the food the texture is hard and white (e.g.
Turnip): when sugar, it is softer and translucent (e.g. Beet) :
and when water, as in some desert plants, it is markedly
succulent and almost transparent.

Those flowering plants which have returned in their evolu-
tion to a life in the water (e.g. Water Lilies and Pondweeds)
exhibit naturally a marked reduction in the root system.
Their roots are smaller in size and usually lack both hairs
252 A TEXTBOOK OF BOTANY (Cu. V, 8

and the cap, though in some the cap is imitated by a pocket-
like cover, and the hairs by slender side roots. The reduc-
tion leads even to complete disappearance of roots in some
immersed Pondweeds, which absorb through their leaf and
stem surface, precisely as did their far-distant ancestors,
the Alge. Herein we have a clear case of the widespread
tendency for parts
rendered useless by a
change of habit to
disappear gradually,
often after lingering
long in a rudimentary
condition.

There is in some
plants another and
very different correla-

Fic. 176. — Jussiwa repens; X 4. The swol- tion between root
len structures are roots, composed chiefly of structure and water
aérenchyma ; when young their tips reach the i Ss
surface and they become filled with air, which habit. Some com-
later they supply to the parts under water. mon herbs when
(After Goebel.)

 

grown in wet places
develop at the contact of air and water a loose open tissue
involving large intercellular spaces (AERENCHYMA), which
seem to transmit air to the under-water parts. In a half-
floating water plant called Jussiea repens (Fig. 176), some
of the roots develop this tissue immensely, becoming enlarged
to conspicuous white spindles which rise vertically to the
surface, whence they evidently take air for use of the under-
water parts. Thus we have roots with a new substitute
function, that of aération. The Bald Cypress, a prominent
tree of the southern swamps, develops from its roots remark-
able projections, or ‘‘ knees,’ which commonly reach the water
surface, and are so constructed as to suggest their use as aérat-
ing organs for the roots, though such function has been denied.
Other structures of analogous sort are found in Mangroves
and elsewhere, as described in works upon water plants.
Cu. V, 8] SPECIAL FUNCTIONS OF ROOTS 253

In a few kinds of plants the roots are A®RIAL, that is, fitted
to live in the air, temporarily or permanently; and they
exhibit corresponding modifications in structure. The
simplest case is illustrated by Corn, where roots grow out on
the stem a little above ground and run diagonally to the
soil, there acting as props to the stem, as well as organs of
absorption. This arrangement is much farther developed
in some tropical plants, notably the Screw Pine, or Pandanus
(Fig. 177), where these roots come to form almost the whole
support of the plant, the main stem remaining small. Similar
roots, but more irregularly placed,
cause the Mangroves of tropical
shores to form their dense thickets.
There are tropical plants, belonging
mostly to the Fig family, of which
the seeds germinate high up in the
crotches of trees to which they
happen to be carried; thence the
growing plants send down aérial
roots which, on reaching the ground,
thicken to trunks so robust that
often they strangle the supporting
tree, leaving the strangler a several- Fic. 177. — Pandanus,
trunked tree in its place. Analogous ae apeits
effects occasionally are seen in our
own woods where trees have started on top of moss-covered
bowlders, excepting that here the bowlder does not vanish.
By similar aérial roots, put down vertically from horizontal
branches and later developed to trunks, a single Banyan
tree is enabled to spread to a many-trunked grove covering
several acres (Fig. 178). There is no difficulty, of course,
in understanding how roots can form trunks, since every-
where behind their young tips all roots are practically stems
in their structure (page 220).

The extreme of the aérial habit is attained in some roots
which never reach the ground. Thus, in the epiphytic

 
254 A TEXTBOOK OF BOTANY (Cu. V, 8

Orchids (Fig. 126), familiar in all greenhouses, while some of
the roots penetrate the crevices of decaying bark and there-
from absorb both water and mineral matters, others hang
down free in the air. The latter display a distinctive,
swollen, whitish aspect, the thickness being due to the pres-
ence of many epidermal layers of loose, empty cells. Into
these the water from rain is easily absorbed and thence

 

Fic. 178. — The Banyan, Ficus religiosa, of India.
All degrees of development of the descending aérial roots appear in the
view. The tree is a small and very open one. (From Balfour.)

transferred to the fibro-vascular system, though the popular
belief that such roots can absorb water as vapor from the air
seems unfounded. In some kinds of Orchids the aérial roots,
hanging in the light, show traces of chlorophyll, while
in a few tropical kinds (Fig. 179) the roots become fully
green, flatten to almost leaf-like thinness, and completely
assume the photosynthetic function in place of the leaves,
which are reduced to mere rudiments. Herein we have
indeed a remarkable case of a complete substitute function
in roots, and the one we would least expect, — that of foliage.

Aérial roots, of other forms, act as supports to climbing
plants. Thus the true Ivies (e.g. the English Ivy and, of
course, the Boston Ivy) put forth from the shaded sides of
Cu. V, 8} SPECIAL FUNCTIONS OF ROOTS 255

their stems large numbers of short roots, tough in texture
and adapted to dryness, which adhere at their tips to stones
or other supports (Fig. 180). In some tropical climbers, e.g.
species of Ficus (Fig. 181), elongated aérial roots grow out

 

Fic. 179. — Teniophyllum Zollingeri, an epiphytic Orchid in which the
aérial roots have become flattened and have assumed the function of foliage.
A young flower stalk shows towards the left. (From Goebel.)

horizontally, somewhat like tendrils, until they touch a
support, around which they turn thigmotropically, thus
securing a firm grasp; and it is interesting to note that
such roots are able to swing in the horizontal plane by virtue
of a marked LATERAL GEOTROPISM. Aérial roots also occur
256 A TEXTBOOK OF BOTANY (Cu. V, 8

upon the trunks of Tree Ferns, which they sometimes cover
completely with their stiff brown threads, though the signifi-
cance of their presence is not clear.

A very striking modification of roots is found in the absorb-
ing roots, or HAUSTORIA, of parasites. The common Dodder,
for example, a parasitic twiner (Fig. 59), puts out these
roots wherever it touches the host plant, and they, by use of
digestive enzymes, penetrate the
host, and attach themselves to
the fibro-vascular bundles, from
which they absorb nourishment.
The haustorial function is simi-
lar in other parasites, including
the parasitic Fungi, though here
the absorbing structures are my-
celial threads, not true roots.

Roots are transformed to spines
in some Palms, though the signifi-
cance of these structures is here
no plainer than in case of leaves
and stems (pages 79, 192). Also,

Fic. 180, — English Ivy, Some other, though minor, special
showing the aérial roots by functional uses and_ structural
ee ee modifications of roots have been

described.

Though not matters of function, certain other special
matters about roots may conveniently be noted at this
place. Thus roots, like some stems, show anomalies of
structure often very puzzling, as, for example, the appear-
ance in Beets which simulates annual rings. These rings are
due to the fact that the cambium, after forming a certain

 

amount of phloem and xylem, ceases to grow; thereupon a
new cambium arises Just outside the first cylinder, grows for
a time, isitself replaced, and so on, many times. Again, roots
can shorten in length, especially in some perennial herbs,
which thus are kept below ground despite the yearly growth
Cu. V, 9] ECONOMICS OF ROOTS 257

‘

of a “crown” at the top. The shortening is effected by «
forcible expansion of the shorter cross axes, and hence a con-
traction of the longer vertical axes, of the cortical cells. An
incidental result thereof is
the formation, very marked
in some tap roots, of cross
wrinkles or folds, which
are thus explained.

Roots, like leaves and
stems, exhibit abnormali-
ties and monstrosities,
though in less number and
diversity than other parts,
again no doubt because of
the comparatively uniform
conditions of life under-
ground. On the other
hand roots are especially
subject to an influence
from which leaves and
stems are comparatively
free, viz. the forcible im-
position upon them of
flattened, contorted, or
other peculiar shapes, of-
ten amusingly or gro-
tesquely Jmuvetive of fa- Fic. 181 — A Ficus climbing by aid of
miliar objects, by the rocks aérial roots. (From Kerner.)
and other impediments
among which they grow. Such growths are often unearthed
and displayed as curiosities, or used in “‘rustic’’ carpentry.

 

 

9. Tuer Economics, AND TREATMENT IN CULTIVATION, OF
Roots

The most important economic uses of roots depend on
the stores of food they contain, especially in the case of
Ss
258 A TEXTBOOK OF BOTANY [Cu. V, 9

biennials and herbaceous perennials, which store their food
perforce underground. Beets, Carrots, Turnips, Radishes,
and Sweet Potatoes are the most familiar examples. Not
all, however, of the farmer’s “root crops” are roots, for
some are stems, as in potatoes, though this purely morpho-
logical distinction has no importance whatever in economics,
and very little in physiology.

Because root-absorption is osmotic, and therefore requires
a soil solution much weaker than the sap of the root hairs,
it injures plants to over-fertilize them ; for fertilizers are solu-
ble, and thus increase the strength of the soil solution. But
the matter is also complicated by chemical relations, and
the stimulation given to growth of soil Fungi and other
organisms.

Because roots need air for their respiration, wet or
clayey soils must be drained for our crops, often at great
trouble and expense. Yet water also is necessary, and must
be conserved for dry times. The art of drainage consists
in the maintenance of a beneficial balance between water
and air throughout the growing season. It is because roots
find this balance so excellent in drains, which therefore they
tend to fill, that gardeners must take care where they place
plants having specially hydrotropie roots (Willows, Poplars,
Elms). Where a tree happens to grow on ground which
must be graded to a higher level, a wall holding back the soil
from the trunk will often permit enough aération of the
larger roots to save the tree, though frequently it does not.

Like other parts, but perhaps more than they, roots need
warmth when growing. This is why hot beds are used in
the spring, — the heat being developed by the respiration of
organisms producing fermentation or decay in the manures
which are used. The same value inheres in “bottom heat”
supplied through pipes in the soil, sometimes used in forcing
greenhouse plants.

Any osmotic process, which root absorption is, proceeds at
a rate directly proportional to temperature. Consequently
Cu. V, 9] ECONOMICS OF ROOTS 259

a low temperature permits only a slow water absorption,
which explains the damage often done to trees and shrubs in
early spring by warm weather and high winds while the
ground is still cold, if not frozen; for the resultant wither-
ing, or windburn, and browning is caused by fatal drying
through excessive transpiration unsupported by sufficient
water supply (compare page 48). Much winter-killing of
plants is due to the same cause, 7.e. a similar exceptional
loss of water through lenticels while the ground is still frozen.
Thus it becomes a part of good gardening so to place and
treat susceptible plants, by selection of sheltered situations
or suitable coverings and the like, that they cannot be ex-
posed to high transpiration conditions while the soil is chilled.

The very different powers of plants to strike roots from
cuttings or slips has important consequences in gardening.
Some, e.g. the common house ‘“‘Geraniums,”’ strike root
very easily; others, e.g. most ornamental shrubs, do so with
some difficulty, and only when aided by special treatments ;
while still others, e.g. our common fruit trees, will not do
so at all. The causes of the differences are not fully known,
but in general plants of succulent texture, with soft fibro-
vascular system and plenty of stored food, strike root most
easily. The matter is practically important in two ways,
—first, plants which strike root from cuttings can thus
be propagated rapidly and cheaply, and second, special
varieties or sports can thus be preserved and multiplied
indefinitely. The usual treatment of cuttings conforms
perfectly to root physiology. The skilled gardener cuts
usually just below a node, because the roots start most
readily from nodes; he removes much of the foliage, be-
cause too much transpiration would dry ‘out the tissues
before the new roots could replace the supply; he puts the
cuttings first in sand, because ample air is the first requisite
for the vigorous growth of new roots, but when rooted, he
transplants them to loam for a better water and mineral

supply.
260 A TEXTBOOK OF BOTANY [Cu. V, 9

Nurserymen are accustomed to transplant their trees and
shrubs two or three times before sale, in order to force root
formation near the stem. Thus more young roots are pre-
served when the plant is lifted for shipment, and it has better
chance for recovery when again planted out. This is a reason
why nursery-grown trees and shrubs usually survive transplan-
tation so much better than those brought from the woods.

Much farming practice, ancient and modern, finds explana-
tion in root physiology, though there is not agreement on all
details. The addition of fertilizers, formerly interpreted as
a replacement of needful mineral salts removed with the
crop (page 242), may perhaps represent another method of
neutralizing unfavorable soil: conditions introduced by root
excretion and decay. The ancient practice of letting a soil
he fallow (or idle), for a period, may restore the fertility
either by giving time for the diffusion of more mineral salts
from lower levels, or through the removal by drainage or
oxidation of the injurious substances formed by the roots.
The rotation of crops may derive its value either from the
different demands made by different crops upon the mineral
supply of the soil, or from the fact that the organic materials
added by roots are usually not injurious to other kinds of
plants. There is, however, no question as to the reason for
the value of the old practice of green-manuring, that is,
plowing Clover, and other crops of the Pulse family, into the
ground. These plants, with nitrogen-fixing Bacteria in their
own roots, are very rich in nitrogen compounds which are
thus added to the soil. Plowing is primarily of benefit in
loosening a soil both for penetration of roots and admission
of air. Sub-soil plowing carries the air still deeper, while
likewise raising more mineral matters to the upper layers.
The value of fall plowing, leaving the soil exposed for the
winter, may possibly lie partly in destruction of the Protozoa
which are supposed to destroy the nitrifying Bacteria (page
246). Cultivating a soil, in the gardening sense, consists in
roughening the surface; it breaks the homogeneity of the
Cu. V, 10] SUMMARY OF PLANT TISSUES 261

soil, lessens the continuity of the water films (page 240),
and hence checks the free movement of soil water to the
surface and its loss there by evaporation. The method is
important in dry farming, of which the first problem is the
conservation of soil moisture.

10. SuMMARY OF THE FUNCTIONS AND TISSUES OF PLANTS

Having considered separately the principal tissues of
plants, it will now be well to summarize them together
to show their connections and the systems they form.

The functions of plants are performed by their proto-
plasm, which is subdivided into cells. When these cells
are specialized to a particular function in considerable num-
bers, we call them collectively a tissue. Organs are enlarge-
ments or extensions of the plant body of such shape and
position that they bring the tissues devoted to a function
into advantageous relation with some external condition
crucially affecting that function, —as witness the leaf in
its relations to light. Thus organs do not so much perform
functions as permit tissues to perform functions in advanta-
geous relation to external conditions essential to their
operation.

While some functions are performed almost wholly by the
protoplasm inside the cells, others involve great specializa-
tion of the shape, thickness, and other features of the cell
walls.

With respect to the tissues involved, all functions fall
into four classes, as follows:

I. Functions performed in tissues requiring special posi-
tions and constituting morphological systems.

1. Prorecrion against unfavorable external influences,
notably dryness of the air and entrance of parasites. The
function requires external position, continuity, and a water-
proof and ‘‘germ-proof” structure. Tissue, EPIDERMIS,
replaced later by cork, covering stems and leaves, but merg-
ing on young roots to the root hair layer. This physi-
262 A TEXTBOOK OF BOTANY (Cu. V, 10

ologically-determined tissue has become fixed as a distinct
morphological tissue system, called the DERMAL system
(Fig. 182).

2. Puorosynruesis, or formation of food under action
of light. This function requires a superficial position, but
protection by the epidermis, and hence comes immediately
beneath the latter, which is transparent. Tisswe, CHLOREN-
cuyMa, which covers young stems, and extends out into flat
projections, the leaves; in older stems ceases to be green,
merges into bark, develops cork cambium and layers of
cork in replacement of the vanishing epidermis, and grows
to allow for expansion of the stem; in roots it merges over
into the cortical layer. Its innermost layer forms a special
STARCH-SHEATH in young stems, or ENDODERMIS in young
roots, having perhaps a perceptive function. This tissue
has become fixed as a distinct morphological system called
the corTEX (Fig. 182) or CORTICAL SYSTEM.

3. AssorPTion of water, mineral matters, and carbon
dioxide. This function requires an external position and
freedom from interference of the epidermis. Tissue, for
water and mineral matters, an external hair-layer on the
roots, continuous with the epidermis of stem and leaves, and
a thick cortical layer, continuous with the chlorenchyma.
The absorption of carbon dioxide by the cortex involves
no special tissues, but depends on stomatal breaks in the
epidermis, and intercellular air spaces, which are especially
large in chlorenchyma.

4. Transpiration, incidental to gas absorption, involves
no tissues, but the need for its control explains the presence
of the regulating guard cells of the epidermis.

5. Conpucrron of water and food through the plant. It
requires a position in contact with the water-absorbing
and food-making cortex, and an elongated tubular con-
struction. Tissues, XYLEM, comprising DUCTS and asso-
ciated elements: PHLOEM, comprising SIEVE TUBES and as-
sociated elements: in leaves the BUNDLE SHEATH, — the
Cu. V, 10]

 

 

 

 

 

SUMMARY OF PLANT TISSUES 263

 

  

Fic. 182. — Vertical section through a
generalized plant, to show the typical ar-
rangement of the tissue systems.

Outside is the dermal system (double
lined), formed by the dermatogen region in
the bud, and providing the protective epi-
dermis on leaf and stem, and the hair layer
on the young root. Next is the cortical
system (crosses), formed by the periblem
region of the bud, and providing the chloren-
chyma of leaf and young stem, with cortex
of bark and young root. Next is the central
cylinder, or stele, formed by the plerome
region of the bud, and providing the vascular
bundles, with the pith (coarse dotted).
The bundles include food-conducting phloem
(cross lined), and water-conducting xylem
(diagonally lined), with cambium (fine dotted)
between them. The growing points of roots
and stems (also fine dotted) with the cam-
bium which unites them constitute the
growth system.
264 A TEXTBOOK OF BOTANY (Cz. V, 10

three forming vascuLAR BUNDLES. These bundles unite
frequently with one another into a cylindrical fibro-vascular
system, which, repeated in all plants, has become a dis-
tinct morphological system called the CENTRAL CYLINDER, or
STELE. This cylinder runs continuously throughout stems
and roots, and into the branches (Fig. 182), but not into the
leaves, which receive only separate individual bundles there-
from; for leaves consist simply of flattened masses of cortex,
with epidermis, into which extend individual bundles from
the central cylinder.

6. Growry, or development of new tissues, primary,
secondary, and general. Primary growth requires terminal
position, as in buds and growing tips of roots, while secondary
growth requires a position among the tissues to be contin-
uously formed, as with cambium, which builds xylem and
phloem. Tisswe, MERISTEM, which is primary in buds and
root tips, and secondary in cambium; but meristem itself
can arise anew in young tissues, as in case of CORK CAMBIUM,
the growth layer of some endogenous stems (page 128), and
roots like the Beet (page 257). The growth of tissues is not,
however, confined to meristem, but can take place in young
tissues by a general cell division, e.g. in expanding bark,
fruits, etc. Collectively the meristematic tissues form a
GROWTH, OF MERISTEMATIC SYSTEM, which, in the exogenous
type of structure, is continuous throughout the plant (Fig.
182).

Epidermis, cortex, and central cylinder, comprising tissues
so fundamentally important in the life of the typical higher
plants, have been repeated so long in evolutionary history
that they have become fixed as morphological systems which
are now regularly laid down in distinet tissue layers by the
primary meristem of buds and root tips. Thus the tissue
layer from which the epidermis develops is called pERMATO-
GEN, that of the cortex PERIBLEM, and that of the central
cylinder pLEROME. The plerome comprises a cylinder of fun-
damental or parenchymatous tissue wherein are embedded
Cu. V, 10] SUMMARY OF PLANT TISSUES 265

certain strands of PRocamBrum, which develop cell for cell
into the vascular bundles, while the tissue inclosed by them
forms the prrn, that between them the MEDULLARY RAYS,
and that outside them the prricycie. If the procambium
cells all turn into xylem and phloem, we have a closed bundle
of the endogenous type, but if some of it turns into cam-
bium, we have open, continuously-growing bundles of the
exogenous type. In such cases the cambium_ usually
extends across the medullary rays and forms a continuous
cylinder.

Il. Functions performed in tissues requiring special posi-
tions but not forming morphological systems.

7. Repropucrion, which is asexual or sexual. If asexual,
the reproductive bodies are usually separable portions of thal-
lus or shoot. If sexual, the reproductive bodies, the mother
cells, spores, and sexual cells (later to be described), originate
in areas of primary meristem persistent on certain branches
(flowers). It requires a superficial position for fertilization
and dissemination, and hence the reproductive tissues are
developed in the cortex.

8. Suprorr of the foliage against strains of weight and
winds. It requires, upon well-known mechanical principles,
a tough, fibrous construction in a tube-form as near the
exterior as practicable ; but, to permit movement of air, water,
and food in and out through stems and roots, it is neces-
sarily discontinuous, and hence mostly disposed in strands.
The needs for this function vary greatly with the habits
of plants. Tisswes, COLLENCHYMA, just beneath epidermis,
SCLERENCHYMA in cortex or pericycle, BAST FIBERS in phloem,
WOOD FIBERS in xylem, while special strengthening SCLEREIDS
occur in some exposed leaves. Because of the frequent
presence of fibers with the vascular bundles, especially in
herbaceous plants, these bundles are commonly designated
fibro-vascular, even if no fibers are present. Collectively
these tissues are sometimes described as forming a MECHAN-
ICAL SYSTEM, though it has no morphological identity.
266 A TEXTBOOK OF BOTANY (Cu. V, 10

III. Functions performed not in special tissues but in special
regions of the tissue systems.

9. Srorace of food and water. No special position is
requisite; hence it occurs in any available regions of pith,
cortex, medullary rays, and other parts, which often, by
multiplication of the cells, produce swollen tubers, bulbs, etc.

10. Secretion and excretion. No special position is
requisite, but usually occurs in special cells, as the 1p10-
BLASTS, or collections of cells forming glands, resin pas-
sages, etc. The LATEX system includes tubes ramifying
throughout the other tissues (page 134).

11. Dissemination. Involves no special tissues, but
modifications, with special outgrowths of hairs, wings,
hygroscopic walls, etc., of superficial tissues, epidermis,
and cortex, as described in a later section.

IV. Functions performed in all living cells.

12. Merasorrsm, or chemical changes (apart from photo-
synthesis and respiration) involved in the life processes.

13. Resprrarron, or release of energy by oxidation. Re-
quires usually the access of free oxygen, which is effected
by development of an INTERCELLULAR AIR SYSTEM (AERATION
SYSTEM) ramifying everywhere throughout the plant, and
opening to the atmosphere through stomata or lenticels.
There occurs in some cases a special development of tissues
with large air passages, called AERENCHYMA.

14. Serr-apsusrmMenr to immediate surroundings. Re-
quires perception of the external stimulus, transmission to
a motor zone, and a motor response mechanism. For all
of these phases special structural features have been de-
scribed, usually special modifications of tissues, as in case
of the starch sheath (page 130).
CHAPTER VI
THE MORPHOLOGY AND PHYSIOLOGY OF FLOWERS

1. Tue DistTinctivE FEATURES OF FLOWERS

So prominent and distinctive a part of the plant is the
flower, that to most people the study of flowers and the
study of Botany are practically identical. The error of this
belief is sufficiently attested by the proportions of the chap-
ters in this book. Functionally the flower is the plant’s
principal organ of reproduction, being especially devoted to
effecting fertilization, the central feature of sexual reproduc-
tion. It is in connection with this function that flowers
have developed the beautiful colors, attractive fragrance,
and striking forms to which they owe their esthetic charm.
There seems indeed no limit to the variety they present in
these respects.

In color, flowers taken collectively display by far the great-
est variegation found anywhere in nature; and somewhere
among them could probably be matched any hue of the
chromatic scale. Yet some true flowers lack any special
color, being green like the foliage, as with Grasses, Birches,
and Pines, which plants indeed are not popularly known to
have flowers at all. Whatever the bright colors, they have
usually this feature in common, that they contrast markedly
with the background, being oftenest white or yellow if seen
against foliage, blue or red if raised above it, and white if the
flowers open at night. Furthermore, showy flowers usually
stand out well beyond the foliage, and more than that, so ad-
just themselves as to face the brightest light. Thus flowers
seem especially fitted to attract the eye, as indeed we shall

presently find that they are.
267
268 A TEXTBOOK OF BOTANY [Cu. VI, 1

Flowers are also distinguished by odors, which, however,
do not occur in all, perhaps not in the majority, at least so
far as our comparatively dull sense can perceive. When
present, they:are usually pleasing, or fragrant, to us, though
a few, like Skunk Cabbage, are positively repulsive. Con-
trary to a popular belief, fragrant odors and bright colors do
not commonly go together, at least not in wild plants; for
in general the most fragrance occurs in flowers of which the
colors are not especially visible, while odors are unusual in
plants of exposed habit, — in meadows, roadsides, or prairies.
Thus odors supplement a certain defect of color in making
known the presence of flowers.

In size, flowers range very widely. The typical size in
those which occur separately would perhaps exceed somewhat
an inch across; but thence they range down to almost mi-
croscopic dimensions, as in certain small floating water weeds,
and upward to the truly gigantic proportions of the tropical
Rafflesia, in which a single flower is more than three feet
across (Fig. 61). Everybody knows that some connection
exists between flowers and insects, and it is true in general,
as we shall presently see, that a relation exists between flower
size and insect size. Between flower size and conspicuous-
ness, there also exists a correlation in this way, that while
the larger flowers are commonly solitary, the smaller occur
massed together into clusters, acquiring thus a collective
prominence, the perfection of which is found in the com-
posite heads of the Daisy, Sunflower, and others of their
family.

In shape, flowers are strikingly multiform. Simplest are
those which have no showy parts at all, but only the incon-
spicuous sexual parts. Of the more familiar kinds, some
are regularly concave, like Buttercups and Apple blossoms,
with the sexual parts in the center, but thenee they range
to such elongated forms as the Fuchsia, and, becoming ir-
regular, produce bizarre effects in the Orchids, even to a
degree simulative of insects or other unrelated natural ob-
Cu. VI, 2 STRUCTURE OF FLOWERS 269

jects. Between these shapes and real insects there exist
certain relations presently to be noted.

It is also characteristic of flowers to be fleeting, for com-
monly they last but a few days, after which they wither and
fall. Sometimes indeed they keep fresh but a few hours, and
it is only very rarely that their substance is sufficiently firm
to persist after drying, though this does occur in the kinds
we call “Everlasting.” Not the entire flower, however,
perishes with the showy structures, for the central parts
persist and grow gradually to the fruit which contains the
seeds. Indeed, it is the most normal and characteristic
feature of the flower that it precedes the fruit. The popular
idea that the flower is in some way essential to the production
of seed is thus correct.

In case of the flower, as of other plant parts, the popular
and the scientific conceptions are by no means coincident.
The botanist includes under the term any structures, no mat-
ter how minute and obscure, which have part in the pro-
duction of seed, and excludes from the term any structures,
no matter how flower-like, which have no such function.
Thus, the so-called ‘‘moss flowers’? are not botanically
flowers at all, and much less are ‘“‘ wooden flowers,”’ “flowers
of tan,’’ and some other objects to which the name is fanci-
fully applied.

2. THE STRUCTURE OF FLOWERS

Despite their striking external multiformity, flowers are
comparatively simple and uniform in their mode of con-
struction.

A typical simple flower, such for example as the Peony
(Fig. 183), has six or seven distinct kinds of parts.

Outside is the caLYx, composed of a whorl of five green and
leaf-like sepats. In the unopened bud they form a close
protecting cover to the parts inside them, wherein consists
obviously their function. Usually they are somewhat
triangular in shape, opening out in a star form, but often
270 A TEXTBOOK OF BOTANY [Cu. VI, 2

they are rounded (as in the Peony), or else elongated, or
otherwise shaped. Typically green like leaves, they some-
times assume both the color and shapes of the next inner
parts, the petals, as with Anemone and Four-o’clock. Usu-
ally persisting for a time in the opened flower, they some-
times fall off as it opens, as in Poppies. Commonly composed
of separate sepals (POLYSEPALOUS), the calyx is often one
piece (GAMOSEPALOUS), forming a saucer-, cup-, urn-, or tube-
shaped structure, from the summit of which the free sepals
project. Oftenest five
in number, the sepals
may be two, three,
four, six, or more, in
lessening frequency.
The student may
easily confirm all of
these matters for him-
self, and extend them,
in any garden or green-
house.

Next inside the

Fig. 183.— A typical flower, of Peonia calyx comes the co-
peregrina; X 4. on : ti

Some of the sepals and petals have been ROLLA, formed, in the

removed in order to show clearly the stamens, Peony, of a whorl of

a, and the pistils, gy. (From Strasburger.)

five brightly-colored
PETALS. Collectively they open out in a way to display a disk
of color surrounding the sexual parts ; and herein, as will later
appear, consists their function, —that of showing to insects
the position of those parts. The separate petals are here
broadest towards their tips, with narrow bases; but from
this typical condition there are wide deviations. The bases
are extended into greatly elongated stalks, as in Carnation ;
or their tips are pointed, elongated, cleft, fringed, and vari-
ously formed, as the flowers of any greenhouse or garden il-
lustrate ; while the most remarkable spurs, hoods, and other
structures occur, as in Larkspur and Columbine (Fig. 207).

 
Cua. VI, 2] STRUCTURE OF FLOWERS 271
In fact it would seem as though every shape that fancy can sug-
gest must be embodied in the shapes of the corollas of flowers.
In conformity with their fleeting character, they are delicate
in texture, showing clearly through a lens or the microscope a
leaf-like anatomy of veins and
cortical cells, —the latter held
tensely expanded by the os-
motie pressure within. Typ-
ically composed of separate
petals (POLYPETALOUS), the
corolla is often, like the calyx,
one piece (GAMOPETALOUS),
forming a saucer-, cup-, urn-,
or tube-shaped _ structure,
from the summit of which
spread the free petals, as
Primrose (Fig. 201), and
other garden flowers illus-
trate. Usually standing di-
rectly on the enlarged end
of the flower stalk, or RECEP-
TACLE, the petals often stand

 

on the calyx, as in Garden
Nasturtium. As to number,
petals vary like the sepals,
being usually, though not al-
ways, the same number as
they. Not infrequently the
petals are wanting altogether,
in which case the sepals often
replace them in color and
function.

Fic. 184. — A typical simple ovule,
of Polygonum divaricatum, in section ;
185.

e, egg cell, in the embryo sac,
which lies in a mass of tissue, the
nucellus; fun is the funiculus or stalk
by which the ovule is attached to the
ovary; at and wi are the integu-
ments, developed from the funiculus,
and leaving an open micropyle; cha,
the chalaza. (From Strasburger.)

Sometimes the sepals are likewise absent, and

in this case the flower is very inconspicuous unless color
is supplied by bracts beneath the flower (page 74), as in
Poinsettia. As to the variety of colors displayed by corollas,
we have spoken already.
272 A TEXTBOOK OF BOTANY [Cu. VI, 2

Next inside the corolla comes the whorl of sTAMENS.
Each consists of a slender cylindrical stalk, the FILAMENT,
bearing at its tip an enlarged rounded yellow anruEer. A
lens will show that this anther opens by longitudinal slits,
allowing the escape of a yellow powder (each grain of which
is a cell), called ponteN. Pollen contains the male sex cells
of the plant, which later fertilize the female sex cells in the
ovules, as presently to be noted. The filaments are some-
times short, even to disappearance, but again are elongated
almost to thread-like, as in Night-blooming Cereus. Often-
=~ est cylindrical, they are

. sometimes flat and even
2 } petal-like, as in Water
\ lilies, where it is difficult
VW to say whether the fila-
fae ments are petal-like or

Fre. 185.— Generalized forms of petals bear the anthers.
ovules, showing the principal arrange- ‘
ments on the stalks; magnified. The anthers, typically

Left, upright, orthotropous, form like younded, are sometimes
Fig. 184; middle, inverted on elongated
attached stalk (raphe), anatropous ; right, greatly elongated, or
curved over to inversion, campylotropous. forked, or otherwise re-
(Reduced from Strasburger.) 3

markably shaped, while
they open invery diverse ways. Usually forming a direct exten-
sion of the filament, they are sometimes balanced on the point
thereof, as familiar in the larger Lihes. Commonly quite
separate from one another (POoLYADELPHOUS), they some-
times form one picce at their bases (MONADELPHOUS), as
in some of the Pulse family, and Abutilon. While typically
they stand directly on the receptacle, they are often on
corolla or calyx. Their existence is fleeting like that of the
corolla, with which they commonly fall. While often
numerous, as mm Peony, Buttercup, and Rose, they are
commonly limited in number, being typically the same num-
ber, or double the number, of the petals, and therefore
oftenest five or ten, or else less frequently three or six, or four

 

or eight, though other numbers occur with lesser frequency.
Cu. VI, 2] STRUCTURE OF FLOWERS 273

When the same in number with the petals, the stamens
usually alternate therewith, though they stand opposite in
Primrose (Fig. 201). Flowers also oecur without stamens,
or rather, to be exact, with the stamens and pistils in
separate flowers.

Beyond the stamens, and occupying the center of the
flower, come the PISTILS, made up of parts called CARPELS,
which, like sepals, petals, and stamens, are morphologically
leaves. The Peony here pictured (Fig. 183) shows two
pistils composed each of a carpel; and of one carpel each are
the many pistils of Buttercup and Strawberry, while Peas
and Beans have one pistil composed of one carpel. Most

 

Fic. 186. — Typical forms of nectaries. From the left; spurs of Toad-
flax, receptacular swellings of Grape, spurs of Columbine, scales on the
petals of Ranunculus. (From Bailey.)

commonly, however, there is one pistil composed of several
united carpels. Whether stmmpLe (of single carpels), or
COMPOUND (of several carpels united), the pistil has typically
a rounded hollow base called the ovary, tapering upward
to a short cylindrical stalk called the styLE (very short in
the Peony), ending in a roughened area called the sticma.
When the ovary is opened, it is found to contain a number
of small rounded whitish bodies called ovuLEes, within
each of which, in a special sac, lies a female sex cell, called
the Eaa ceLy. The functions of the parts of the pistil are
plain; the stigma receives the pollen containing the male
cells, the style supports the stigma in a position suitable
for recelving the pollen, and the ovary protects the delicate
ee
274 A TEXTBOOK OF BOTANY (CH: VI; 2

ovules in which the egg cells are fertilized by a method
to be fully described in the following section.

The ovules are the most important parts of the pistils.
A typical ovule when mature shows the structure represented
by Figure 184. Innermost is the relatively large EMBRYO
sac (large enough to be seen by the naked eye in sections
of very large ovules), containing much protoplasm which can
be recognized by its yellowish-brown color. In the sac he
also several small cells (Fig. 190), amongst which is the one
of greatest importance, usually the
largest, the EGG cELL. This egg cell
when fertilized grows gradually to an
embryo plant within the embryo sac.
The embryo sac is imbedded within
a rounded mass of tissue called the
NUCELLUS, which in turn is inclosed
by one or two INTEGUMENTS. These
grow up to surround the nucellus from
the stalk or FuNIcULUs, though they

never inclose the nucellus completely ;
eon nem a Snap- for an opening called the MICROPYLE
aragon, a typica. irregular S
(bilabiate) flower; x 3. is always left for the entrance of the
ne Maout and ollen tube (Fig. 190). The funiculus

is sometimes straight, but oftener is
elongated and grown to the rest of the ovule, the resultant
ridge being called the rapHE. In this way the ovule is swung
into positions facilitating the entrance of the pollen tube into
the micropyle (Fig. 185). Through the funiculus runs a
vein which conveys food into the ovule as far as the meeting
place, called the cHanaza, of nucellus, coats, and funiculus,
whence it passes by diffusion to the various parts.

Typically rounded in form, the ovary is often elongated,
cylindrically as in Peas, flattened as in Beans, or variously
angular. The style is almost wanting in the Peony, but in
some flowers is clongated even to thread-like, as conspicuously
in the silk of the Corn, while at times it is flat and quite petal-

 
Ca. VI, 2 STRUCTURE OF FLOWERS 2705

like, as in Iris (Fig. 199). The stigma, while frequently flat,
or rounded, is elongated variously, and even branched, some-
times to almost a feather-like degree, as in some trees (Fig.
197). The ovules, while typically few, are sometimes but one
to acarpel, though often they are many, as with Poppies, in
which case they stand in regular aoe
masses or rows upon supporting
ridges, usually prominent, called
PLACENT#. Unlike the other
parts, the pistils are not fleeting
but persistent structures, for,
after fertilization, the _ pistils
grow normally into fruits, and
the ovules into seeds; at least
this is true of the ovaries, though
commonly the styles and stigmas
fall with the petals. While typ-
ically the pistil or pistils stand
on the receptacle separate from _ Fis. 188.— The structure of
a pollen grain, and germination
the other parts, frequently the of the pollen tube, in Lilium
s a Martagon; x 200.
other parts stand upon theovary, Ee a ete ee a
bringing the latter below and _ tative nucleus, k, and a generative
outside of the showy part of cell, m, which produces the two
: sperm nuclei; B, the same grain,
the flower, as with Apple and _ in another view, about to germi-
oa g s nate; C, the germinated grain
Fuchsia ; and | in this case the with its tube, the growth of which
ovary 1S described as INFERIOR, is controlled by the nucleus k ;
as: contrasted with the ordinary 7 ced of the tube, yath tts cep
Eis sperm nuclei, formed from the cell
SUPERIOR condition. m, as it nears the egg cell. (From
Three other less prominent 5t'asbureer.)
parts appertain to flowers: the receptacle, nectaries, and
bracts.
The RECEPTACLE is the tip of the floral stem, usually en-
larged to a club-shape where it bears the floral parts, though
g Pp p , g
sometimes it develops specialized forms, as will later be
noted in the section on morphology. NEcTARIES excrete,
often in shining drops, the nectar upon which insects feed.

 
276 A TEXTBOOK OF BOTANY (Cu. VI, 3

Typically they occur at the bases of the petals, either as
glandular swellings, or else as shallow cups, as seen so con-
spicuously in the Crown Imperial, where they hold huge
glistening drops; and sometimes they are parts of the re-
ceptacle (Fig. 186). Often they are wanting, in which case
the nectar is usually secreted from the base of the petals
without special glands. Bracts are leaves belonging to
the stem below the flower, but often codperating in the con-
struction or function of the flower. Sometimes they consti-
tute the conspicuous color-bearing part
(asin Poinsettia or Bougainvillea) ; but
again they are merely accessory sepal-
like appendages, as in Strawberry, or
else they replace the sepals in both
form and function, as in Sunflower
and others of the Composite family.

Typically a flower is symmetrically
radiate in form around a central axis,
such kinds being called REGULAR, as
with all of those we have thus far
mentioned. Butmany are IRREGULAR,

Fic. .189.—A pollen With a special tendency to form a two-
grain germinating on a lipped structure, as in the Mints and
Ea een Snapdragon (Fig. 187), while this char-
acter attains a very extreme development in the Orchids.
Its significance will soon be explained.

Various parts of the flower can be wanting to such a de-
gree that pistils only or stamens only remain, thus forming
PISTILLATE and STAMINATE flowers respectively. All that is
really essential to a flower is the possession of a stamen or a
pistil. A flower which contains both is said to be PERFECT, and
one which has also petals and sepals is said to be COMPLETE.

 

3. THE ACCOMPLISHMENT OF FERTILIZATION BY FLOWERS
Everybody knows that flowers are essential to the produe-
tion of seed, though not everybody knows in what way.
Cu. VI, 3] FERTILIZATION IN FLOWERS 277

Their function consists in effecting FERTILIZATION, the union
of male and female sex cells, without which process seed
does not form. The accomplishment of fertilization is the
primary function of the flower.

Fertilization in flowers involves three stages, two of them
preliminary and subsidi-
ary to the third and
crucial one, —viz. pol-
lination, growth of the
pollen tube, and fusion
of the sex cells.

The pollen grains are
developed in the anthers,
and develop within them-
selves the male, or SPERM,
cells (Fig. 188). The
transport of the ripe
pollen from anthers to
stigmas, through a space
sometimes small but fre-
quently great, is called
POLLINATION. It is not
effected by any power ; ;

sys Fic. 190.— A typical ovule, of Narcissus,
within the plant, but by ready for fertilization, in section; much
some external agency,— magnified.

Near the upper end of the embryo sac

mostly by wind in the lie three cells, of which the larger is the egg

inconspicuous flowers cell. The pollen tube is shown entering
‘ ‘ ’ the micropyle. (Drawn from a wall chart

and by insects in con- jy Dodel-Port.)

spicuous ones.

Pollination accomplished, the growth of the pollen tube
begins. Into the roughened, sugary-adhesive, epidermless
surface of the stigma (Fig. 189) there grows from the pollen
grain a slender, delicate, thin-walled tube, in which can be
seen the distinctive living protoplasm. This tube, carry-
ing the two sperm nuclei near its tip, grows down through
the tissues of the style, dissolving for itself a way by aid of

 
278 A TEXTBOOK OF BOTANY [Cu. VI, 3

enzymes and absorbing the digested products for use in its

own further growth;

and commonly the tube dies and

withers behind as the forward part advances. Thus it
reaches and enters the ovary, and, taking a direct path,
approaches an ovule which it enters by the micropylar

 

Fic. 191.— A generalized
pistil and ovule, in section,
showing the mechanism of fer-
tilization described in the text.

opening; thus it reaches the rela-
tively large sac, the embryo sac,
which every ovule contains (Fig.
190), and within which, near the
micropyle, lies the EGG CELL. This
mechanism of fertilization is repre-
sented in principle in our general-
ized picture (Fig. 191). Thus are
the sperm cells brought to the im-
mediate vicinity of the egg cells.
The final, and really the essen-
tial, stage in this process consists
in the fusion of the two sex cells
which is thus effected (Fig. 192).
One of the two male nuclei (the
fate of the other will appear later
in connection with the fruit), with
probably some surrounding cyto-
plasm, moves out of the pollen
tube into the egg cell, presumably
opening the way by action of
enzymes. For a time the egg cell
exhibits two nuclei ; but they move
together, touch, and then gradually
fuse together into one and fertili-

zation is complete. The result is a cell containing a nucleus
derived from the union of two nuclei from different parent
cells; and this appears to be the central and essential feature

of all fertilization.

After fertilization the sepals, petals, stamens, nectaries,
styles, and stigmas, their functions evidently accomplished,
Cu. VI, 4]

ordinarily wither and fall away,
leaving only the ovary on the re-
ceptacle. Then this ovary grows
into a fruit, the ovule into a seed,
and the fertilized egg cell into an
embryo plant. In case, however,
no fertilization is effected, the parts
of the flower usually persist some-
what longer than otherwise, though
no fruit, seed, or embryo is
formed; but presently all parts,
including the ovary, wither and
fall. This is the way in which
flowers are essential to the pro-
duction of seed.

4, Tur NaTuRE AND CONSE-
QUENCES OF FERTILIZATION

Fertilization in flowers, as the
preceding section has shown, cen-
ters in the fusion of the male and
female nuclei within the egg cell;
for pollination and the growth of
the pollen tube are merely the
mechanism for bringing the sex
cells together. Fertilization occurs
in the reproduction of nearly all
plants and animals, and while the
mechanisms for bringing the sex
cells together are as diverse as
possible, the central feature of the
fusion, especially of the nuclei, is
always the same. Thus this fusion

NATURE OF FERTILIZATION

279

 

Fic. 192.— The fusion of
the sex cells, somewhat gen-
eralized from a typical case;
X 375.

A, the end of the pollen
tube (D of Fig. 188), contain-
ing two sperm nuclei, sk; B,
the same tube in contact with
an embryo sac, en; C,a sperm
nucleus, sk, has entered the
egg cell, the nucleus of which,
ek, it has approached; D, the
sperm nucleus, sk, has lost its
elongated form and become
rounded like the egg nucleus,
with which next it fuses com-
pletely. (Reduced from Stras-
burger.)

act of fertilization runs as a thread of structural and physi-
ological identity almost throughout the plant and animal
kingdoms, binding plants and animals together in this
280 A TEXTBOOK OF BOTANY (Cu. VI, 4

one fundamental feature, despite their diversity in other
respects.

So certain, indeed, is the identity of the sex cells through-
out animals and plants that of late some botanists have
adopted from zoélogy the terms BGG and SPERMATOZOID,
and use them instead of the older terms egg cell and sperm
cell. The latter words are retained in this book, because while
indicating sufficiently the morphological relations, they
are more closely associated with the study of plants. Egg
cells and sperm cells are called collectively GERM CELLS.

We examine first the cytological basis of the fusion of the
germ cells. The student will recall that the protoplasm
in a living cell is differentiated into the cytoplasm, which is
clearly the constructing part, the plastids, which have
chemical functions, and the nucleus which seems to act as
a control center of the work and development of the cell
(page 41, Fig. 16). Now as to the nucleus, its essential
part is its CHROMATIN, which lies embedded in the gelatinous
protoplasm constituting most of its substance. This chro-
matin is a distinct chemical substance, colorless jin the
living cell, but possessing a remarkable power to absorb
colored stains (whence of course its name); and it ordinarily
exists in fine granules strung together in threads which are
much intertangled (Fig. 193, 1). This is its usual, or resting,
state, but when the cells are about to divide, in reproduction
or growth, the chromatin threads disentangle themselves
and collect into definite elongated bodies called curowo-
soMEs (Fig. 193, 7). The number thereof is always definite
for each kind of plant or animal, though varying widely
for different kinds, — all the way, in fact, from two to more
than one hundred. Whatever the number, however, there
is good reason to believe that they keep their identity even
in the resting state, so that the chromosomes which come
out of the seeming tangle are identically the same ones which
went into it. Now in the fusion of the nuclei of the germ
cells, the sperm nucleus passes over its chromosomes to the
Cu. VI, 4] NATURE OF FERTILIZATION 281

egg nucleus, as shown diagrammatically in Fig. 194. It is
true, the chromatin is in the resting state at this time, so
that the chromosomes cannot be distinguished; but at the

 

 

 

 

Fic. 193.— Stages in typical cell division in growing tissue, somewhat.
generalized.

1, Resting cell. 2-5, The chromatin collecting into chromosomes, which
become definite in number and outline. 6, Splitting of each chromosome
lengthwise into two. 7, 8, The chromosomes growing shorter and thicker.
9, 10, They collect in the equatorial plate of the forming spindle. 11, 12,
13, Separation of the pairs of chromosomes. 14, 15, 16, Formation of the new
nuclei, return of the chromosomes to the chromatin threads, and formation
ofanew wall. Thus are produced two new cells from division of the original
one. Next these daughter cells grow to the full size of the parent cell,
after which they again divide in the same manner. (After Strasburger.)
282 A TEXTBOOK OF BOTANY (Cn. VI, 4

very first division of the fertilized egg cell its nucleus dis-
plays a number of chromosomes precisely equal to that which
we know the sperm and egg nuclei possess together, while
in some cases it is found possible to identify the two sets.
All evidence seems to indicate that this bringing together
of the chromosomes (or chromatin) from two parent cells
into a single nucleus is the principal (and perhaps only)
functional end of fertilization. The accompanying com-
mingling of the cytoplasm appears not to be important,
and indeed in some cases seems not to occur, only the nucleus
of the sperm cell passing into the egg cell.

Thus we are brought to ask what may be the significance

 

Fie. 194. — Diagram to illustrate the performance of the chromosomes
in the fusion of the germ cells.

The two chromosomes from the pollen or male parent are white, and those
from the egg cell or female parent are black.

of the chromosomes. Our knowledge of them is still im-
perfect, but this much seems clear, — that they are the phys-
ical basis of heredity, the carriers of the factors which cause
the new individual to develop in general like its parents.
The chromosomes do not themselves build the cells out of
which the organism is constructed, for that is clearly done
by the cytoplasm; but in some way, still unknown, the
constructive work of the cytoplasm seems guided by the
chromosomes, which thus must contain the plans, so. to
speak, of the new structure. Furthermore, and this is im-
portant, the evidence indicates that the set of chromosomes
contributed by each nucleus in fertilization is complete, that
is, capable of guiding the construction of a complete organism
Cu. VI, 4 NATURE OF FERTILIZATION 283

substantially like that which produced it. Thus the nucleus
of the fertilized egg cell contains chromosomes in duplicate, 7.e.
two complete sets, each capable of reproducing an individual.

The subject becomes clearer as we consider the events
which follow fertilization. The fertilized egg cell at once pro-
ceeds to divide. First the chromatin threads collect them-
selves into definite chromosomes, equal in number to those
contributed by the two nuclei, and indeed, there is little
doubt, individually identical therewith. Then each chromo-
some splits lengthwise very regularly into two (Fig. 193),
whereupon, by action of a spindle-shaped mechanism,

 

Fic. 195. — Diagram to illustrate the performance of the chromosomes
in the division-of the fertilized egg cell.

On the left the same egg cell shown in Fig. 194, its chromosomes being
derived from the two parents; on the right the two cells resulting from
its division.

one of the half chromosomes in every case is drawn to one
end of the cell, and the other half to the other, thus dividing
every one of the original chromosomes equally between the
two ends of the cell. Then a wall forms across the cell be-
tween them, and the two sets merge back each to its chroma-
tin tangle. Thereafter these two half-sized cells absorb
nourishment and gradually grow to the full original size, the
chromatin included. Presently these new cells themselves
divide, and the chromosomes which differentiate from the
chromatin tangles seem identical with those which went
into them, and therefore are the grown-up halves of the
284 A TEXTBOOK OF BOTANY (Cu. VI, 4

original male and female set (Fig. 195). Then follow
other like divisions, always by the same method, though
presently proceeding more actively in some places than
others, — with the result that first an embryo, then a seed-
ling, and finally an
adult plant is pro-
duced. When, now,
the cells of the adult
are examined, their
nuclei are found
each to contain ex-
actly the same num-
ber of chromosomes
as did the fertilized
egg cell from which
the plant has de-
veloped; and_ fur-
thermore it seems
reasonably certain
that these chromo-
somes of the adult
are the exact indi-
vidual descendants
of those in the fer-
tilized egg cell, and
therefore of those
contributed by the

Fic. 196.— The pollination of Vallisneria original parent male
spiralis, a common water plant; x1}. (After a
Kerner.)

 

 

 

and female cells.
This phase of our
subject appears clearly in a later diagram (Fig. 219). Thus
the adult would have in every cell of its structure chromatin
matter, that is, heredity material, derived from both of its
parents. This arrangement helps us to understand how
an individual can resemble either father or mother in any
feature of its structure. ,
Cu. VI, 4] NATURE OF FERTILIZATION 285

There comes, however, a break in the regularity of this
chromosome division. It occurs when the adult plant is
forming its own sex cells (pollen and embryo sac). At this
time one cell division, called the REDUCTION DIVISION, yields
to the new cells only half the number of chromosomes which
had prevailed through the body.
The functional significance of the
reduction is perfectly clear, for if
the uniting sex cells contained the
full number of chromosomes, ob-
viously the number would double
in every generation, to their ulti-
mate enormous multiplication,
whereas by the reduction division
the number is kept constant. The
details of the reduction division are
complicated and not wholly under-
stood, but it occurs in such a way
as to give each sex cell one complete
set of chromosomes instead of the
double set which all body cells pos-
sess. These chromosomes, however,
(and this is a point of great con-
sequence in heredity), do not repre-

 

teinients : Fic. 197. — Flowers of
sent individual chromosomes which he Hazel (Corylus Avellana) ;

occurred in the body cells, but are <4. The staminate flowers
ns 1 fr th : es hang in two large catkins,
reconstructed from them in such a anq° the pistillate flower

way as to include some material stands near the top of the

stem. F Balfour.
from the father set and some from “°™ ‘7™ a

the mother set, in combinations which apparently are due
only to chance, and never the same in any two. This mat-
ter is illustrated by the diagram of Figure 219, and is
apparently identical in every respect in plants and animals.
Thus the principal consequence of fertilization seems to
consist in the introduction of complete sets of paternal and
maternal heredity-carrying chromosomes into every cell
286 A TEXTBOOK OF BOTANY (Cu. VI, 5

throughout the body of each individual, though whether
this is the functional aim, or only an incidental result of
fertilization, is uncertain.

What now is the significance of fertilization, or, in other
words, of sexual reproduction? Fertilization is by no means
essential to reproduction, since many kinds of low organisms
lack it, while even the higher plants possess many methods
of purely vegetative, or asexual, reproduction, as a later
section will make clear.
The asexual method is
thus not only possible,
but it is actually much
simpler and easier of ac-
complishment than the
sexual. Why then the
overwhelming — predomi-
nance of the sexual

Og fay oto method? Unfortunately

      

YD we have not yet any cer-
Fic. 198.— Various forms of pollen tain knowledge upon this
grains; magnified. point. It has commonly

The 3 three-lobed grains in the row on Been believed: ¢t mdi
the left are Pine, the two lateral lobes PCCM Deheved that indi-

being air-filled bladders. The roughened yiduals produced by fer-
forms are mostly carried by insects, in the ae i . i Seae
hairy bodies of which the various projec- tilization are more vari-
tions become caught. (Reduced from able than those produced
Kerner.)

by asexual methods, and
that such variability gives advantage in competitive evo-
lution. Yet some investigators hold other views, and the
matter is one on which we must await further evidence.

5. THE Mertuops aNnp MEANING or Cross-POLLINATION

It was said in a preceding section that the transfer of pollen
from the anthers to the stigmas of flowers is effected, as a
rule, not by action of the plant itself, but by some external
agency, notably wind and insects. The matter, however,
goes farther than this, for the floral arrangements are such
Cu. VI, 5) METHODS OF CROSS-POLLINATION 287

that the transfer is usually not between anther and stigma
within the same flower, but between anthers and stigmas of
different flowers, —and oftener than not between different
plants, of course of the same species. This CROSS-POLLINA-
TION, as it is called in contrast with CLOSE-POLLINATION
within the same flower, is a matter of much botanical conse-
quence.

In the simpler kinds of
plants, eg. the low Alge
(page 12), the physiological
equivalent of cross-pollina-
tion results incidentally from
the fact that the free-swim-
ming sexual cells (or GAm-
ETES), ‘usually cast out into
the water by many plants of
one kind together, inter-
mingle and unite promis-
cuously. Of course in such
cases a union may result be-
tween cells from the same ;
plant, as doubtless often sth

 

occurs, though in higher forms Baas eA Na
there is reason to believe that
chemical attractions and re-
pulsions between the gametes

tically.

The stamen, somewhat to the
right of the center, is under the style ;
the stigma is the upper side of the
small projection from the latter.

: From Le Maout and Decaisne.)
compel crosses between differ- ¢

ent plants. In the Ferns, and some other plants having
free-swimming male cells, a cross is insured through the
fact that the sperm and egg cells produced by the same
plant are not usually mature at the same time.

Some flowering plants are pollinated on the water, no-
tably our submerged pond weeds, eel-grasses, etc. A typical
case is the fresh-water eel-grass, Vallisneria spiralis (Fig. 196).
Growing in masses together, these plants produce pistil-
late flowers which float by long stalks at the surface,
288 A TEXTBOOK OF BOTANY (Cx. VI, 5

and staminate flowers, which become loosened and rise to the
surface; here they float about until they touch a stigma of
the floating pistil, when fertilization is effected in the usual
way. Afterwards the pistil is drawn deep under water, and
held there during ripening by the spiral coiling of the stalk.
Such water-pollinated flowers are mostly so inconspicuous
as not to be popularly recognized as flowers at all. They
exhibit, indeed, no other floral characteristics than the
possession of the comparatively obscure, even though vastly
important, stamens and pistils, although some kinds possess
rudimentary petals and sepals as relics of their evolutionary
history. Some water
plants, however, no-
tably the Water-lilies,
retain their showy
flowers pollinated by
insects.
In the flowering land
Fic. 200.— Dichogamous flower of Scrophu- plants the simplest
laria nodosa; X 2. agency of pollination
Left, a flower with ripe stigma but unripe. : -
stamens, shown better in the section (middle is the wind. Wind-
figure); right, a flower with ripe stamens pollination occurs in
but withered stigma. (After Warming.)
most of our trees, e.g.
Elms, Birches, Oaks, Pines; in some shrubs, e.g. Alders;
and in a few herbs, — notably Grasses, including the
Corn. <A typical case is the Hazel (Fig. 197), in which
the long dangling catkins are clusters of staminate flowers,
while the pistillate flowers are so few and inconspicuous as
to require special search even by the trained eye of the bot-
anist, and would hardly be recognizable at all were it not for
the relatively prominent feathery stigmas. When ripe, the
pollen, easily shaken from the catkins by a touch, is wafted
about on the lightest breezes, so that some of it comes into
contact with the stigmas, though of course an overwhelming
preponderance is wasted. This case illustrates the typical,
though not invariable, features of wind-pollinated flowers,

 
Cu. VI, 5) METHODS OF CROSS-POLLINATION 289

which are these: first, they are inconspicuous, for they
lack colored corolla, odor, and nectar, such prominence as
they possess being due simply to their abundance, or to the
yellow color of their anthers. Second, the male blossoms
far exceed in number the female, in obvious correlation with
the wastefulness of this method. Third, the stigmas are
large, often branching-feathery, thus spreading a greater net
for the drifting pollen. Fourth, the flowers are borne in
ways to insure free passage of the pollen without inter-

 

Fic. 201. — Dimorphic flowers of Primrose; x 2.
Pollen from the upper stamens will develop on an upper stigma, and from
lower stamens on a lower stigma, but not other ways. (From Bailey.)

ference by presence of leaves. Thus the flowers unfold
before the foliage in the spring, as with most of our trees,
or else they are lifted beyond or above the leaves, as with
Pines and Grasses. Fifth, the pollen is found, on micro-
scopical examination, to be hight in weight and to spread large
surface, which is even increased, as in Pines, by extension
into large empty bladders (Fig. 198). Sith, the stamens and
pistils are commonly borne in separate flowers, often upon
different plants, thus preventing close- and insuring cross-
pollination. While these characters are typical in wind-
pollinated flowers, they are not invariable; for wind-polli-
U
290 A TEXTBOOK OF BOTANY [Cu. VI, 5

nation merges over gradually to insect-pollination, the floral
structure being also intermediate, as some Maples and Wil-
lows illustrate.

The most prominent of the agencies of cross-pollination,
however, are insects, to the utilization of which the most
typical flowers are fitted. The characteristics of insect-
pollinated flowers are
these. First, the flower
is conspicuous to the
eye, through a showy
corolla usually colored
in contrast to the back-
ground and set prom-
inently forth in the
light; and often its
presence is indicated
also by strong odor.
Second, nectar is com-
monly present in the
base of the flower, and
when absent, is replaced
by more abundant
pollen which some in-

Fic. 202.— The cleistogamous flowers of Betts particularly de
the common Blue Violet ; reduced. mire. * Third, stamens
sche omoasnons Gowers a on mus and pistils are usually
flower, and b a pod containing seeds. Ordi- present in the same
nary flowers, of natural size, on the right. flowers. Fourth, the
(From Bailey.) p ;

shape of the flower is
such that an insect in probing for nectar leaves some
pollen on the stigma, and receives some by contact with
the anthers. Fifth, the pollen grains are commonly angu-
lar, roughened, or adhesive (Fig. 198). Observation,
supplemented by experiment, proves that insects visit
flowers for the nectar, their usual food: that they are
guided in general to the flowers by the bright spots of

 
Cu. VI, 5) METHODS OF CROSS-POLLINATION 291

contrasting color, although odor is often more potent in this
respect ; and that in probing about in their active way for the
nectar, their hairy bodies brush pollen upon the stigmas and
recelve a new supply from the anthers. All of these matters
the student can confirm for himself in any garden in summer.

At first sight it would seem that insects must effect close-
rather than cross-pollination, but such in fact is not the
case. Any one can see that insects in gathering nectar
usually visit flower after flower and plant after plant of the
same kind as long as
these are plentiful ;
and since their
bodies possess a large
pollen-carrying ca-
pacity, it must usu-
ally happen that any
given stigma, even if

 

pollinated from the
same flower, be-
comes also pol-
linated by other
plants from the
abundant mixture
on the body of the
is found to be the
pollen which effects
pollen tubes being
the latter case.

tirely sterile to their own pollen.

Fic. 203. — Flower of Salvia pratensis, pol-

linated by a Bee; x 4.

Right, the flower ready for pollination, show-
ing position of stigma and stamens. The stigma
is touched by the insect in entering, and later
the stamens are brought down on its body by
operation of a hinged-lever arrangement. (From
Wiesner.)

insect. In such cases, as a rule, it
foreign and not the flower’s own
the fertilization, the growth of the
more rapid in the former than in

Flowers, indeed, are known which are en-

Furthermore, in some

plants close-pollination is prevented by mechanical arrange-
ments, as with the Iris (Fig. 199), where the stigma is upon
a shelf struck by the insect when entering but not when
leaving the flower. Still more common are the cases in
which the stamens and pistils are not ripe at the same time,
as with Scrophularia, an arrangement called DicHOGAMY
(Fig. 200). And there are plants, of which the Primroses
292 A TEXTBOOK OF BOTANY [Gm Vib

are the most prominent, in which pollen and stigmas are of
two reciprocally corresponding kinds, though the two never
occur in the same flower, an arrangement called DIMORPHISM
(Fig. 201). These and other arrangements bring it to pass
that cross-pollination is in fact the rule in showy flowers. It
is not, however, invariable, for with many kinds of flowers,
especially of the simpler sorts, close-pollination is perfectly

 

Fic. 204. — Cypripedium Calceolus.
The insect can enter only by the front or upper opening, and leave only
by a side opening; thus it must pass in succession the stigma and anthers.
(From Kerner.)

possible, and is actually self-effected, if no foreign pollen
be present. Still further, there exist a few flowers, of which
some Violets are examples (Fig. 202), in which self-pollination
is not only the rule, but the invariable rule; for the flowers
never open and the anthers shed the pollen directly upon
the stigmas, an arrangement called cLeistoaamy. All such
plants, however, possess also showy flowers, cross-pollinated
in the usual way.
Cu. VI, 5) METHODS OF CROSS-POLLINATION 293

Not only does the structure of the typical showy flower
exhibit remarkable fitness to cross-pollination by insects,
but this fitness is carried to degrees which have evoked the
wonder and admiration of long generations of loving ob-
servers of plants. The simplest condition is found in regular
flowers like Buttercups or Apple blossoms, where almost
any kind of insect may equally well alight in the shallow
basin, and, busily seeking the
nectar, effect pollination. This
is likewise the case with the
Composite, — the Dandelions and
Daisies and Sunflowers, and that
sort. In irregular flowers, such
as the Larkspur and Mints (Fig.
203), the arrangements are such
that only Bees and lke insects
ean reach the nectar in the elon- ils
gated spurs or tubes; and those y,

PR

i

 

are the principal insects which
visit such flowers. In these
flowers, as elsewhere, the me-
chanical arrangements are such
that the visiting insect must take
a path which insures cross-pol-
lination. In some Orchids, espe-
cially the Lady’s Shipper (Fig.204),
the insect has to enter the flower

Fie. 205.— A flower of Yucca
Whipplei, being pollinated by a
Pronuba moth; X +.

The insect deliberately col-
lects pollen from one flower, car-
ries it to the stigma of another,
and there presses it securely
down. It then lays an egg in
the ovary of that flower, and its
larva feeds on some of the seeds,
which would not develop without
the pollination. (From Kerner,
after work by Trelease.)

by one opening which the stigma guards, and leave by another
over which hangs an anther. In Orchids, indeed, the fitting
of floral form to insect shape and habit has become wonder-
fully exact, so that in some cases only a single species of
insect can pollinate the flower, the adjustment between the
two being carried remarkably into details. These, how-
ever, are but few of the great variety of arrangements pre-
sented in this relation between flowers and insects, which
include even a case of deliberate and purposeful pollination
294 A TEXTBOOK OF BOTANY (Ons Vie '5

by the insect (Fig. 205). All of these matters are described
in detail in several works devoted to the subject. The
student will find that to the original investigators, these re-
markable adjustments between flowers and insects seem most
reasonably explained as result of a gradual process of adapta-
tion of one to the other during the course of their evolution.

Insect-pollination prevails far more widely than any other
method, and moreover is characteristic of the highest kinds
of plants. A reason for its superiority over wind-pollination
consists obviously in its greater economy and efficiency, for
in the one case the
pollen is simply cast
forth and its access to
a stigma left to chance,
while in the other the
pollen is carried di-
rectly from anthers to
stigmas. A phase of
this economy — mak-
ing the most, so to

 

 

Fia. 206. — Marcgravia nepenthoides, polli- ; sues
nated by humming birds; much reduced. speak, of insect visits
In the pouched nectaries below the flower, — explains the pres-
there is secreted abundant nectar, in probing
for which the hovering birds bring their heads ©NCe of stamens and
successively against the flowers. (After H. pistils in the same
Miller.)

flowers.

While insects are overwhelmingly the most important,
they are not the only animal cross-pollinators of flowers.
Some kinds of large bright flowers are regularly pollinated
by small nectar-eating birds, especially humming-birds
(Fig. 206); and others, growing close to the ground, are
pollinated by snails, which are attracted by a succulent tis-
sue formed in the spike among the flowers. But insects,
from their combination of small size, active habits, and
nectar diet, make the most effective cross-pollinators.

As with other plant organs, flowers have not only their
primary function, which they perform as their contribution
Cu. VI, 5) METHODS OF CROSS-POLLINATION 295

to the economy of the plant as a whole, but also certain sup-
plementary functions essential to their own nutrition or
safety. Thus pollen is commonly liable to injury by water,
i.e. rain, through osmotic absorption, as earlier shown
(page 234) ; but many flowers are completely inverted, thus
shedding the rain, as in
Columbine (Fig. 207), or
the petals overarch the
stamens, or scales and
hairs prevent access of
raindrops, or other ar-
rangements occur. Also
the nectar is attractive
to insects too small, or
unadapted by habit, to
effect pollination, no-
tably ants, against which
there must needs be pro-
tection ; and a good many
corresponding adapta-
tions have been claimed
in flowers, — the closed
throat of the Snapdragon,
which bees can open but
ants cannot, the adhesive
glands on the calyx of
Plumbago, the hairs in
the throat of many flowers, Fic. 207.—The Columbine, Aquilegia
and a good many others, “7"aieyrh showing the inverted poston
for further accounts of

which the student must turn to the special works. Of
course, as with other organs, various hereditary and struc-
tural factors also enter into the construction of particular
flowers, which therefore can by no means be explained in
detail upon the basis of adaptation to insect-pollination
alone.

 

Hiyi/||0
we)
296 A TEXTBOOK OF BOTANY (Cu. VI, 5

Not only a structural but also a physiological fitness to
the accomplishment of pollination occurs in flowers. Thus,

 

Fie. 208.— A _ cluster of
Bellflowers, set in one-sided
light, showing phototropism of
the flowers. (Drawn from a
photograph.)

most flowers are very strongly
phototropic, aS one can see very
easily on any sunny bank or in
borders near buildings, and as can
be proven very strikingly by ex-
periment (Fig. 208). Thus they
are brought, and their faces set, in
the most conspicuous positions.
Again, many flowers, especially
those which present a_ special
alighting position to insects, are
very strongly geotropic (diageo-
tropic), and thus, no matter what
accident may befall the flower
stalk, the individual blossoms keep
their positions for the visiting in-
sect (Fig. 209). The tubes and
coronas of Narcissus and Daffodils
are kept thus in their horizontal
positions, as can be proven by tying
a young flower stalk down hori
zontally. Again, many flowers do
not open before rain, or else close
on its approach, and some few
flower-clusters turn upside down,
seemingly in adaptation against
damage to the pollen by water
(page 234). Thestimuli concerned
are not in all cases clear, nor are
the weather predictions always
accurate, though in this they per-

haps succeed as often as our human forecasters with all their

exact instruments. Also, various movements of stamens
occur, partly effected mechanically, as in Mountain Laurel
Cu. VI, 5) METHODS OF CROSS-POLLINATION 297

(Kalmia), partly irritably, as in Barberry, and diversely in
some Orchids, though the exact significance of these and
other like phenomena is still matter of debate.

oie
eee

me :

Ne Bey Bd Pa on \
: Te IN
ba a,

\

 

| ,

 

Fia. 209.— Clusters of Larkspur, showing the diageotropism of the flowers.
The tip of the larger cluster was tied down at an early stage of its devel-
opment. (Drawn from a photograph.)

There remains one leading question: Why these elaborate
arrangements for cross-pollination when close-pollination is
298 A TEXTBOOK OF BOTANY (Gn Va 6

so much simpler, as cleistogamous flowers attest? There
is good evidence, based on direct experiments by Darwin,
that more and stronger offspring are produced by cross-
pollination than by close-, meaning a cross between different
plants, for a cross between different flowers on the same
plant is little if any better than close-pollination. Thus an
advantage must inhere in cross- over close-pollination, which
means of course in cross- over close-fertilization, pollination
being only a preliminary mechanical step to fertilization.
This superiority, in light of the well-known evolutionary
principle of the survival of the fittest, amply explains the
prevalence of cross-fertilization in plants, and its exclusive
use in the higher animals. But why is cross- superior to close-
fertilization? As to this, there is still much doubt, but we
can gain some light by translating the matter into terms
of the chromosome mechanism.  Close-fertilization must
necessarily give to the offspring two sets of chromosomes
identically alike, precisely as in case of asexual reproduction
(page 300), while cross-pollination gives to the offspring two
sets of chromosomes which differ somewhat. Hence cross-
fertilization has the same superiority over close- that close-
has over none; or to put the matter in another way, cross-
fertilization is apparently necessary in order to realize the
full benefits of fertilization. As to the advantages of fer-
tilization over asexual reproduction we have already spoken
(page 286). One must regret that such fundamentally im-
portant matters cannot as yet be satisfactorily explained,
but they cannot. The future, however, will surely yield
their interpretation.

6. Mertuops or Asexual REPRODUCTION

While most reproduction in plants involves fertilization
and sex, there is much that is purely asexual. Sexual and
asexual methods are by no means mutually exclusive, but
commonly exist together in the same plants, either side by
side or in a form of alternation. ;
Cu. VI, 6] ASEXUAL REPRODUCTION 299

The only plants which reproduce exclusively by asexual
methods, so far as known, are the very simplest Alge and
the Bacteria. Their single-celled bodies divide across into
two; the halves grow to full size and fall apart (Fig. 210) ;
and thus we have asexual reproduction by division. One
might think also that the higher Fungi should be included
in the asexual category; but newer studies seem to be
showing that these plants possess a form of fertilization.

In addition to division, the asexual methods include
specialized vegetative bodies, potential vegetative parts, and
asexual spores.

1. Specialized vegetative bodies. These are mostly of the
nature of separable buds, formed on thallus or shoot and
later set free, when they
grow directly to new
plants. Such bodies are
found in Algze and Fungi,
especially the Lichens,

and reach fuller devel- Fie. 210.— Asexual reproduction, by
opment in the higher division, of Pleurococcus, a green Alga

which grows upon tree trunks; much
groups. Thus, some magnified.
Liverworts produce in The smaller cells thus formed soon grow
: to full size.
special cups on the thallus
many symmetrical flat bodies called gimme (Fig. 211), which,
washed out or blown to some distance, produce there new
plants. Some Ferns produce on the margins of their fronds
little buds, which sprout and continue their growth when
the leaves are dropped. Identically the same feature is
found in the Life-plant, or Bryophyllum (Fig. 43), which
has buds on the margins of its leaves, and thus can produce
new plants when the leaves fall on the ground. Essentially
the same principle is involved in the formation of little
plants at the ends of the runners in the Strawberry, as earlier
described (page 189). Some waterweeds form compact win-
ter buds, which separate and float away to start new plants
in the spring. The larger Lilies produce in the axils of their

  
300 A TEXTBOOK OF BOTANY [Cu. VI, 6

leaves little seed-like bodies (Tig. 212), which really are com-
pact buds though they separate and give rise to new plants.

 
 

Similar in nature, albeit very different in ap-
pearance, are the little bulblets which so many

liliaceous plants produce as
outgrowths from their main
bulbs. This is a great
profit to our gardening, for
the possibility of our bulb
beds and borders depends
upon the existence of these
bulblets, which are simply

 

separable buds.

Fig. 211.—The thallus of a Liver- 29. Potential vegetative

“2.

wort, Marchantia; natural size.
The cups bear small vegetative gem- parts.

Some plant parts

me, of which one is shown enlarged. not specialized for repro-

duction can yet serve in-
cidentally that function. Thus the brittle twigs of Willows,
if blown or drifted to suitable places, take root and grow to

(From Kerner.)

new trees. Many kinds of creeping root-
stocks, or even roots, will produce new
plants when severed, as exemplified by
the pertinacious Couch Grass of our gar-
dens. The most important phase of such
reproduction, however, is found in the
power possessed by many plants to strike
root from cuttings placed in the ground,
whereby they produce full and_ perfect
plants, even though they never reproduce
naturally by this method ; and many plants
which will not strike root from ordinary
cuttings can yet be made to do so by
devices well known to skilled) gardeners.
This potentiality of vegetative reproduc-

tion, rarely or never exercised in most of these plants, is
rich in consequences for practical gardening.

 

Fie. 212. —A
seed-like separable
bud on the stem
of a Lily. (From
Railey.)
Cu. VI, 6] ASEXUAL REPRODUCTION 301

3. Asexual spores. A spore is typically a single-celled
body which can grow directly into a new plant. Many of
the simpler Algze throw out into the water tiny spores which
swim freely about by aid of small cilia, and hence are called
ZOOSPORES (Fig. 213); they resemble closely the gametes
later described (page 303), but grow without any fusion
into new plants. Higher Algz also produce such spores,
which are thrown off to
drift with the currents,
as will be described in
Part II of this book.
Most of the Fungi pro-
duce asexual wind-scat-
tered spores, usually in
very great number, and
minute as the dust,
either on the gills, as
with Mushrooms, or in
special long-stalked
spore cases, as in Molds
(Fig. 214), or in other
equivalent ways.
Liverworts and Mosses
produce spores in their

Fic. 213.— Zodspores, asexual unicel-

capsules, and Ferns in ! : n
ae a lular reproductive bodies, of Alge; highly
the “fruit dots” on the magnified.

under sides of their The forms are typical. All swim by action
of the hair-like cilia, — towards the cilia.

 

fronds, as will later

more fully appear. In all of these groups except the Fungi,
and even obscurely in them, occurs also a sexual reproduc-
tion with fertilization, as already described, while moreover
there exists usually, and perhaps always, a marked alter-
nation between the sexual and asexual methods. This
latter subject merges into the so-called ALTERNATION OF
GENERATIONS, a matter of very great morphological inter-
est, to which we shall return in Part II.
302 A TEXTBOOK OF BOTANY (Cu. VI, 7

A very special and rare form of asexual reproduction is
found in PARTHENOGENESIS, 7.e. the development of an egg
cell to a plant without fertilization. This is known in
relatively few plants, which include especially the Composite
family, though it is frequent in the reproduction of Insects.
Closely related thereto are cases of POLYEMBRYONY, in
which cells of the embryo sac other than the egg cell de-
velop into apparent embryos,
which, however, are really
vegetative structures of the
nature of branches. Both
parthenogenesis and poly-
embryony are too rare to
play any appreciable part in
plant reproduction as a whole,
and have their chief interest
as showing the plasticity of
natural processes.

A question must arise at
this point as to whether any
differences exist between the
sexually and asexually pro-
duced offspring of the same
plant. It seems clear that
in general the asexually pro-

Fie. 214,— The spore cases ana Guced individuals are less
spores of a common Mold; x38. variable in their characters
eo eee than the sexually produced,
although there are notable exceptions. At all events it is cus-
tomary among gardeners to employ asexual propagation when
they wish to retain a variety true to some valued character, but
to use sexual or seed reproduction when trying for novelties.

 

7. THE ORIGIN AND SIGNIFICANCE OF SEX

In the preceding sections, the two fusing sexual nuclei,
the male and the female, have been treated as functionally
CH VI 7] SIGNIFICANCE OF SEX 303
alike and contributing the same in kind to the offspring.
Such is the conclusion indicated by modern research. If,
now, the two sexes thus contribute equally to the constitu-
tion of the offspring, where lies the essential difference be-
tween them, or, more exactly, why does sex exist at all?

For light upon this question we turn to the sexual methods
and structures presented by the different groups of existent
plants, from the lowest to the highest. These will be found
described in full in Part II of this book, but for our present
purposes may be sum-
marized as follows. ‘

I. There are plants of

simple kinds, notably

Bacteria and some low

Alge, which reproduce Ex

solely by division, that is, @>X-)}

the single cells constitut- = “._.\———

 

 

ing the adults simply di-
vide across and grow to

full size (Fig. 210), pre-
cisely as do meristematic
cells already described in

Fie. 215. — Gametes of the simple Alga
Protosiphon, in process of fusion; highly
magnified. On the right a complete
“zygote.”

the higher plants (page
299). Here is represented a stage of reproduction in which
there is neither fertilization nor sex.

II. There are several known Algie, of grade somewhat
higher than those just mentioned, in which the plants are
all alike, and produce small reproductive cells called GAMETES,
likewise all alike, and provided with swimming appendages.
These gametes are thrown out into the water, where, swim-
ming freely about, they come together at haphazard and
fuse, uniting their nuclei, quite in the manner of the fer-
tilization of the higher plants (Fig. 215); and from the re-
sulting cell a new plant develops. Here is evidently repre-
sented a stage in which fertilization occurs, but without
any difference between the sexes.
304 A TEXTBOOK OF BOTANY (Cu. VI, 7

Ill. The Rockweeds, the common brown seaweeds so
prominent on sea coasts at low tide, and some other Alge of
higher grade than those mentioned under II, produce two
kinds of reproductive cells, one relatively large, round, and
without swimming appendages, the other small, elongated,
and adapted to swim freely (Fig. 216). Both kinds when
ripe are thrown into the water, where the large cells float
passively about while the small cells swim to them and fuse
with them, quite in the manner of fertilization in the higher
plants; and this fertilized cell grows into a new plant.
Wecall the larger the BGG CELL,
or EGG, and recognize it as
female, and the smaller the
SPERM CELL or SPERMATOZOID,
and recognize it as male; and
herein we have a clear case of
the existence of sex. Consid-
ering, now, the nature of the
differences between the two sex
cells, it is evident that the egg
cell owes its great size to the
ne ee large supply of food it contains,
cells, one of whichentersandeffects this food being used in the de-
ana a S00: CHedragen velopment of the new plant un-

til it can make its own supply ;
and since it is thus large and clumsy, so to speak, its capacity
for free locomotion is diminished, and even the attempt is
abandoned. The sperm cell, on the other hand, consists
of little more than a nucleus, with only enough cytoplasm to
construct an efficient swimming apparatus. Here, as in
the higher plants, the two nuclei appear to contribute through
their chromosomes exactly alike to the offspring, and it seems
clear that the difference between the two cells consists.in a
division of labor with respect to two subsidiary features
of reproduction, viz. the bringing of the sex cells together,
and the provision of food for the resultant offspring, — one

 

 
Cu. VI, 7] SIGNIFICANCE OF SEX
cell assuming wholly the one function, and
the other the other. No differences occur
in the plants which produce these cells,
excepting in the parts immediately con-
nected with the formation of cells of such
different sizes. Thus we have a stage in
which there is a clear distinction of sex, but
only in the sexual cells themselves, and it
arises not from any fundamental matter of
difference in contribution to the constitution
of the offspring, but in a secondary matter
of division of labor in connection with the
mechanism of fertilization, and the nutrition
of the resultant embryo.

IV. The higher, or Red, Alge
have a complicated reproduction
under which we can recognize the
essential fact that the egg cell,
naked as in the lower kinds, remains
permanently attached to the parent
plant, upon which it is fertilized by
a much smaller floating sperm cell,
and from which the resultant
growth is supplied with food (Fig.
217). Thus we have a stage, not,
it is true, exactly represented in
living forms, but presumably once
occurring in kinds now extinct,

 

wherein the egg cell remains at-
tached to the parent plant, on
which it is fertilized and by which
the resultant equivalent of an em-
bryo is supplied with food.

V. The stage just described is

the highest attained by the Alge.

In the simplest land plants, the

x

Fic. 217.— The egg cell,
attached to a fragment of
frond, of Nemalion multi-
fidum, a seaweed; xX 700.
Extending from the egg cell
is the long-projecting “ tri-
chogyne,”’ adapted to receive
the small floating sperm cells,
of which two are attached.
(After L. Kny.)

Bryophytes and Ferns,
306 A TEXTBOOK OF BOTANY [Cu. VI, 7

the egg cell is no longer naked, but, in obvious correla-
tion with the danger which would attend the exposure of
its delicate, succulent substance to dry air, is inclosed
within a protective cover, so constructed that when the egg
cell is ready for fertilization and the surrounding conditions
are favorable, the cover opens, and not only permits, but
facilitates, the access of the free-swimming sperm cell, which

 

Fic. 218. — Sexual reproduction of a Fern; x 240.
The structures occur on the under side of the sexual or prothallus stage,
close to the ground. On the left, a section of the prothallus in which the
egg cell is buried and covered by the tubular ‘‘archegonium.’’ On the

right, the free-swimming sperm cells escaping from the ‘‘antheridium.
When the sex cells are ripe, the access of water causes both structures to
open; the archegonium releases into the water a substance (malic acid)
very attractive to the sperm cells, which swim towards it, and enter the tube,
when the first to reach the egg cell fuses therewith and effects fertilization.
(After L. Kny.)

itself develops in a special structure (Fig. 218) ; and then the
developing embryo is supplied with food by the parent
plant. Here is evidently represented still another stage in
the evolution of sex, in which have been developed, above
the earlier differences, special and different structures, which
protect the sex cells in ways to facilitate the access of the
free sperm cell to the fixed egg cell.

VI. The highest development of sex in plants is repre-
sented in the construction of the flower, as described in an
Cx. VI, 7] SIGNIFICANCE OF SEX 307

earlier section (page 269). Here fertilization is effected, not
in water by a free-swimming sperm cell, as in all earlier
stages, but in the air by wind- or insect-carried pollen grains
from which the pollen tubes carry the sperm cells to the egg
cells. In correspondence with the dry and exposed sur-
roundings, the egg cell is deeply buried within the body of
the parent plant, — within an embryo sac, inside an ovule,
enclosed by an ovary, while the pollen occurs in closed an-
thers. Now the mode of transport of the pollen, by external
agencies, requires that the anthers, with some part of the
ovary fitted to receive the pollen, be accessible to wind and
insects; and such is the function of stamens and pistils.
Accordingly these parts, specially fitted to bring the sex cells
together, constitute physiologically the sexual organs of the
plant, even though on morphological grounds this designa-
tion has been denied them. Here is evidently represented
still another stage in the evolution of sex, consisting in the
presence of sexual organs, fitted to effect union of the sex cells.

VII. In most plants the stamens and pistils are borne
close together in the same flowers, which are said to be
PERFECT (Or HERMAPHRODITE). In some cases, however, like
Birches and Oaks, they are borne in separate flowers on the
same plant, when they are said to be Monacious. In any
case only the stamens and pistils show structural differences
connected with the different sexes of the cells they produce,
and the plant itself shows no trace of sex. In a few kinds of
plants, however, the staminate and pistillate flowers are
borne upon separate plants (are Diaciovs), in which case
the plants are somewhat naturally, though not quite cor-
rectly, described as male and female. Ordinarily there is
no determinable difference, aside from the flowers, between
such plants, but occasionally, as in Date Palm, some Wil-
lows, and a few others, there is a marked difference in as-
pect between staminate and pistillate individuals, thus giving
a structural basis to the terms male and female as applied
to plants. Here, however, is the limit reached by plants in
308 A TEXTBOOK OF BOTANY [Cu. VI, 8

their sexual differentiation, though the higher animals have
gone a little farther, for in them the male and female
sex cells are always borne by different individuals, which
are distinguished, not only by their very different sexual
organs, but also by correlated differences in habits, occupa-
tions, dispositions, aspect, stature, and other visible features.

Thus, in summary, there runs throughout all sexual dif-
ferentiation the one constant thread of the fusion of the
two sex cells, which brings together the parental chromo-
somes in equal contribution to the constitution of the off-
spring. It is only the mechanisms subsidiary thereto which
vary. These mechanisms originate in a way to imply that
the sexes were originally alike, and the differences between
them arose through a division of labor, at first between the
sex cells and later between the individuals which produce
them, in connection with two subsidiary features of sexual
reproduction, — viz. effecting the union of the sex cells, and
nourishing (and later protecting) the embryonic offspring.
Even in the highest plants and animals, sex seems to mean
no more than this difference, developed to such a degree as
to produce structures, organs, and even individuals, fitted
to the respective parts taken by the sex cells. It is indeed
possible that other factors are also involved in the result,
but if so, they are obscure, while these are obvious.

8. Herepiry, VARIATION, AND EvoLuTIon

The matters considered in the preceding sections lead
naturally to others expressed in the title of this section.
They are largely of theoretical, though very fundamental
nature. Although in the past largely speculative in treat-
ment they are now the subject of profound experimental
researches, the conclusions of which apply equally to plants
and animals.

Heredity is the resemblance of an individual to its an-
cestors. Variation is the difference between an individual
and its ancestors. It is easy to see how, granting the chro-
Cu. VI, 8] HEREDITY AND VARIATION 309

mosome mechanism earlier described, heredity should oc-
cur. Indeed, on this basis, offspring should repeat. their
ancestors exactly, and the scheme leaves no room for vari-
ation at all.

The student will note the phrase “like its ancestors,”
not ‘“‘like its parents.”’ It is a matter of popular knowledge
that family characteristics often skip a generation, or several
for that matter; and children thus show features of their
grandparents intermingled with those of their parents.
Our knowledge of this subject is now firmly grounded, thanks
to the labors of Mendel and his many modern successors in
experimental biology. As a result it seems clear that the
characters or features which make up an individual, and
which are built by its cytoplasm under control of its chromo-
somes, are not indefinite in number and kind, as popularly
imagined, but are definite in both respects. In other words,
an individual consists of a definite, though great, number
of ultimate irresolvable unit characters, of which it forms
a kind of mosaic. Furthermore, each such unit character
is apparently represented in the chromosomes of all of
the cells by some kind of determiner which controls the
construction of that character by the cytoplasm, though
whether this determiner be some material carrier, some kind
of register, some form of model, some type of enzyme, or
some other entity, is not known. Accordingly, the ferti-
lized egg cell, and every body cell formed therefrom, having
its two sets of chromosomes, must contain two sets of all
the determiners necessary to construct that kind of organ-
ism; or in other words every kind of character of an organism
is represented in duplicate in every one of its body cells, one
determiner being contributed by each parent (see the dia-
grammatic Fig. 219). Now arises the question : How do these
duplicates behave with respect to one another during the
development of the cell, and what determines which one is
to direct the cytoplasmic construction, and thus determine
the character, in any particular case? On this matter Men-
310 A TEXTBOOK OF BOTANY (Cu. VI, 8

 

   
   
  

ME
fe" Cy
\

»
\\

   

Fic. 219.— A diagram to illustrate the principle of the chromosome
mechanism of heredity.

The triangular masses of cells are adult individual plants, or animals,
male and female, developed from the parental germ cells shown below, and
forming above their own germ cells, which are uniting in pairs into fertilized
egg cells. In the nuclei of the individuals are the chromosomes, reduced
for simplicity to two, and composed of determiners, reduced for simplicity
to four, a black determiner being assumed to be always dominant to a
white one. For example, we may take a triangle to mean height of stem,
black meaning taller and white shorter; circle, color of corolla, black darker,
white lighter; square, shape of leaf, black longer, white rounder; diamond,
texture of stem, black rougher, white smoother. Thus the two individuals
would be taller, longer-leaved, darker-flowered, rougher-stemmed, though
having both the capacity to transmit the other qualities, as shown in
their germ cells.

Two such individuals as here pictured, being externally alike though
differently constituted in their chromosomes, are described in the technical
language of genetics as phenotypically identical but genotypically different ;
and, having both dominant and recessive determiners, are heterozygous for
all characters. They can, however, as the diagram shows, produce offspring
which contain only the dominant or the recessive determiners for certain
characters, that is, are homozygous for those characters.

 
Cu. VI, 8] HEREDITY AND VARIATION 311

del was the first to obtain exact knowledge, which has been
confirmed and greatly extended by others. Using different
varieties as parents, he was able to trace the separate char-
acters in their hybrid offspring, and thus he discovered that
the rule in such cases is this, — the matter does not depend
upon chance, but one of the two determiners regularly prevails
over the other (is DOMINANT, in his phrase), and shows its in-
fluence in the developing cell, while the other is latent (RE-
CESSIVE, in his phrase), and without visible effect. This is
the way in which parental characters can lie unseen and
latent in the body, thus in our common but erroneous phrase
“skipping a generation.”

There is, however, much more in the subject than this.
As already explained (page 285), when the adult individual
forms its own new sex cells, the number of chromosomes,
and therefore of determiners, is halved by the reduction
division, but in such manner as to give to each new sperm
or egg nucleus one complete set. This set is taken partly
from the father set and partly from the mother set, the
combination apparently being made wholly at random, as
manifest by the fact that the different sexual cells of the
same individual differ greatly in the make up of their com-
binations (see Fig. 219). Thus it happens that every sexual
or germ cell contains a determiner for each character from its
father or its mother, but never from both, a fact called
technically “‘the purity of the germ cells.” It is also true
that, for any given character, about as many germ cells
carry the father determiner as carry that of the mother.
Now if two individuals of the same kind breed together, as
imagined in our figure, and if the union of the germ cells
is left simply to chance, as seems to be true, then there
follows, so far as each single character is concerned, a very
remarkable and important result, which can most simply
be described by use of our diagram. Thus, if we center our
attention upon color of corolla (the circles with black,
dominant, and white, recessive), we find that four and only
312 A TEXTBOOK OF BOTANY [Cu. VI, 8

four modes of fertilization are possible ; a black from a male
nucleus may unite with a black from a female, or a black from
a male with a white from a female, or a white from a male
with a black from a female, or a white from a male with a
white from a female. Thus we can have four kinds and
only four, of fertilized egg cells, one containing two black
determiners, one containing two white determiners, and
two containing a black and a white. In other words, the-
oretically 1 of all the offspring of this couple will have the
black character only, the white being eliminated entirely from
their bodies and those of all their offspring if they breed only
with their own kind; + likewise will have the white character
only, the black being eliminated out of them and their off-
spring if they breed with their own kind; and two }’s, that
is 4, of the whole will have the black and white both in their
own bodies, and can transmit either to their descendants,
although, as black is dominant to white, they will themselves
show only the black character, the white being latent or re-
cessive. Thus of all the offspring ? will show the dominant
black and { the recessive white, though of the 2, 2 have the
white latent. The arrangement is represented for a single
character in Fig. 220. This fact was discovered by Men-
del in hybrids, but of course is equally true in principle for
ordinary offspring from parents of the same variety. It has
been found to hold true very widely, even though not uni-
versally, in a great many kinds of plants and animals; and
it is the central feature of MENDEL’s Law, now one of the most
prominent matters in all Biology.

For the sake of the study of the principle we have re-
duced our subject to the utmost degree of theoretical sim-
plicity. In fact, however, matters are never so simple, and
commonly are vastly complex, in actual life. Thus, the
law only holds true as an average of high numbers, its oper-
ation being often obscured by chance with small numbers ;
characters and determiners are not few in number, but
many, even to hundreds and thousands; similar forms are
Cu. VI, 8] HEREDITY AND VARIATION 313

not likely to breed together repeatedly unless compelled by
experiment, though the same result is effected in some
plants which pollinate themselves; characters are not passed
along singly, but commonly a number together in loose
aggregations; determiners seem to exert certain influences
upon one another directly; and there are yet other compli-
cations. Hence in Nature the law is not manifest to obser-
vation, though discoverable by experiment; but it operates

O x ®

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COCO 09008 00086 @0080

Fig. 220. — Diagram to illustrate Mendel’s Law of the segregation of
characters in heredity, using a single character of Fig. 219.

If germ cells having the dominant character (black circle) breed with
others having the recessive character (white circle), then all of their off-
spring show only the dominant character but carry the recessive character
latent (black circle with white center). If these forms breed together, their
offspring will show the distribution of characters represented in the diagram,
— one-fourth will be pure dominants and one-fourth pure recessives, while
the remainder are dominants carrying the recessive character latent. If
those in this generation breed only with their own kind, the result in the
next generation is as shown in the diagram; and thus indefinitely.

as a steadily working principle which runs as a kind of
guiding ‘thread through all heredity, while coming to view
now and then in such phenomena as “skipping a genera-
tion,” elimination of characters from a race, and other
less obvious matters.

Thus, on the basis of our knowledge of the performance of
the chromosomes in reproduction, in conjunction with
Mendel’s law, heredity must rest upon the transmission of
determiners which, existing in each species in a certain
number, are distributed in different combinations in the
314 A TEXTBOOK OF BOTANY [Cu. VI, 8

different individuals. Expressed otherwise, and somewhat
fancifully, individuals are simply temporary kaleidoscopic
combinations of the various determiners belonging to the
species, the act of reproduction, especially the reduction di-
vision and subsequent fusion, providing the new turn of the
kaleidoscope.

Thus much for heredity, which means the resemblances of
individuals to their ancestors. What now of variation, which
means the differences? The chromosome mechanism ex-
plains heredity well, but not variation. Indeed the mechan-
ism seems to leave no room for variation, since by its oper-
ation all individuals are simply combinations of determiners
which preéxist. Yet variation is as real a fact as heredity,
for organisms do change with time, as proven by comparison
of living plants and animals with their fossil ancestors.

The conception of variation, however, needs definition,
for some apparent variation is not at all important in evolu-
tion. Thus, individuals are often strongly altered in their
development by their conditions of life, — insufficient or
peculiar food, ete., and also often become altered by self-
adjustment to the conditions of their immediate surround-
ings, as we have noted already under various phases of
uritabiity. But such changes (called FLUCTUATIONS) are
known not to be hereditary, that is, they affect the cyto-
plasm but not the determiners in the chromosomes. The
variations (called GENETIC VARIATIONS, or MUTATIONS),
which produce hereditary alterations in organisms, must
affect the determiners, either by interpolating new ones,
or by altering the character or relations of those already
present. Yet while such mutational variation undoubtedly
exists, we have no knowledge as to how it arises or in what
way it affects the determiners. Indeed the origin of varia-
tion is the great crucial problem. of present-day Biology,
though it will be settled, and before long, by the experi-
ments now in progress. It is the watching understand-
ingly for the answer to such deep questions which gives to
Cu. VI, 8] HEREDITY AND VARIATION 315

the study of science its great charm, and it is the chance to
find the answer one’s self which gives to scientific investiga-
tion its matchless zest.

That the organisms which now exist on the earth are
different from those which formerly existed, and that these
organisms are fitted to the conditions under which they
live, are two facts which have long been known to scientific
men, who have explained them in different ways. Thus
Linnaeus, and most others of the earlier naturalists, be-
lieved that the new kinds were each suddenly created, and
in very exact fitness to the surrounding conditions, by an
omnipotent Creator. This doctrine is known as SPECIAL
CREATION. It did not, however, stand the test of advancing
knowledge, for ample evidence seemed to show that existent
kinds of organisms have developed out of earlier kinds;
and it seemed reasonable to suppose that in course of this
development the organisms and their parts became adapted
to their environments. This is the meaning of EVOLUTION.
All modern research has tended to confirm its correctness.

The fact of evolution is one thing, and the method whereby
it has come about is another; and the explanation of its
method has been for a half century the foremost problem
of philosophical biology. Two great leading solutions have
been offered for the problem. Lamarck, a French zoologist
who was active a century ago, argued that the changes which
are known to occur in individuals, either directly by action of
the environment or by self-adjustment thereto, are trans-
mitted to the next generation and there re-appear; and
that thus a character can be intensified generation after
generation until a new kind or species results. This is the
view of the TRANSMISSION OF ACQUIRED CHARACTERS. Trans-
lated into terms of the chromosome mechanism, it would
mean that any change in a character of an individual or-
ganism, which of course affects the cytoplasm of the cells
concerned, can become registered or represented in some way
in the determiners in its germ cells. Now of such a result
316 A TEXTBOOK OF BOTANY (Cu. VI, 8

there is not only no known evidence, but such evidence as
we possess seems wholly against its occurrence, with possible
rare exceptions which hardly affect the general principle. All
evidence seems to show that while alterations in the deter-
miners alter the organism, the reverse is not true.

A second solution, and the most famous, is that of Darwin,
who was active in his work somewhat over a half century ago.
He argued that a spontaneous variation of all features of
organisms is constantly in progress; that only a few of the
many varying individuals can survive; that such variations
as happen to lie in a direction which fits the organism to its
environment will help that organism to survive in com-
petition with those having a less favorable direction ; that
the offspring of the surviving organism will inherit the
variation; that some will vary in even higher degree; and
that thus in time the variation can accumulate to a degree
which makes its possessor not only a new kind but better
adapted than its ancestors to those particular conditions.
Thus Nature acts to select certain characters, and the view
is known as NATURAL SELECTION. Translated into terms of
the chromosome mechanism, this means that the determiners
are not stable entities, but exist in a state of unstable equi-
librium such that they can produce characters in greater or
lesser degree of intensity. As a matter of fact most of the
evidence we have accumulated upon this point seems op-
posed to the idea that the determiners are thus unstable,
and many investigators deny them all variability. More
recently, however, some apparently incontrovertible evidence
has been found which points to an inherent instability of
the determiners or unit characters, and their modifiability
by selection; and the Darwinian coneeption of evolution
by selection of such variations will probably prove correct
in the end.

A modification of Darwin’s explanation of the method of
evolution is that of De Vries, a Hollander still actively work-
ing. He maintains, on the basis of observational and ex-
Cu. VI, 9} METHODS OF PLANT BREEDING 317

perimental evidence, that some new kinds or species of or-
ganisms originate not slowly and gradually from other kinds,
but suddenly, —even so suddenly as in one step from parent
to offspring. Such new steps are supposed to be not frequent,
but occasional, long periods of stability alternating with
short periods of change. Upon forms thus originating
natural selection operates to preserve the best fitted kinds.
The species which thus originate, called often ELEMENTARY
species, differ really, though only slightly, from those which
give rise to them; and several mutational steps are needed
to make such markedly different species (LINNHZAN SPECIES),
as the older naturalists associated with that word. This
view is known as that of Muration. ‘Translated into terms
of the chromosome mechanism, it means that the determiners,
after long periods of transmission in stable form, suddenly
alter, apparently not by the interpolation of new ones so
much as by spontaneous sudden change in the old. But
the evidence on this matter is still in debate.

9. THe Meruops usep BY Man IN BREEDING BETTER
PLANTS

Everybody knows that our most valued varieties of farm
and garden plants— our grains, fruits, vegetables, and
flowers —do not occur wild, but have been developed by
man from inferior wild kinds. Our principal grains, Wheat
and Corn, have been so far improved from their wild an-
cestors that. we know only doubtfully what those were.
Our best known fruits, Apples, Pears, and Oranges, are
incomparably superior to the original kinds in size, flavor,
and other qualities we value. Among vegetables, the
Cabbage, Cauliflower, Brussels Sprouts, and others, most
diverse in aspect, are all known to have been derived in
gardens from a very simple little strand plant of western
Europe. In flowers, a great many of our garden favorites
have been improved from their wild states to a degree
which would render the relationship unsuspected were it
318 A TEXTBOOK OF BOTANY (Cu. VI, 9

not for our historical records. Most remarkable of all, and
perhaps the acme of man’s developmental accomplishments,
is the Chrysanthemum, in which, from two little simple
wild plants, smaller than our common field Daisies, have
been developed all of the great variety of distinct types, and
all of the superb individual specimen plants seen in our
horticultural exhibitions, culminating in single plants over
sixteen feet across and bearing fifteen hundred blossoms, and
in single blooms over twenty inches in diameter. We
consider now the methods by which man has achieved
these results.

New varieties originate under cultivation, but not as a
direct result thereof. High cultivation can supply the con-
ditions for the best development of individual plants or a
given crop, but the improvement is not hereditary, and
therefore does not yield new kinds, which we acquire in only
three ways, — by SELECTION OF VARIATIONS, PRESERVATION
OF SPORTS, and HYBRIDIZATION.

1. Selection of variations. Both experience and experi-
ment attest that plants of the same variety growing side by
side, whether wild or in gardens, present many differences,
or variations, from one another; further, that some of these
variations are hereditary, though many are not; and still
further, that by persistent selection generation after gen-
eration of the plants displaying a given variation (e.g. size
in a grain, red color in a flower), and the use of their seeds
in growing the next crop, there results in time a variety in
which the given feature is far more prominent and prevalent
than in the original form, and moreover comes true to seed.
It is true that much of such selection now practiced upon
highly developed varieties of plants, whether grains or flowers,
appears to consist simply in the assembling together of the
plants which already possess the variation in high degree,
and is not accompanied by any actual intensification
thereof. In other words, selection may effect the zsolation
rather than the development of a variety. But an intensifi-
Cu. VI, 9] METHODS OF PLANT BREEDING 319

cation of variations must sometime and somehow occur, else
we could never have obtained our multiform and multi-
chrome Chrysanthemums from their comparatively uniform
and simple wild ancestors ; and the variation once intensified,
by whatever method, could be isolated to a variety by se-
lection. This method of improvement by selection is slow,
but is favored by use of great numbers of plants, and by the
fact that plants vary more rapidly and extremely under
cultivation than in the wild state. In this indirect way,
indeed, cultivation does promote the development of new
varieties.

2. The preservation of sports. Occasionally some one bud
on a plant will produce a branch having leaves, flowers, or
fruits strikingly different from those on the rest of the plant,
such a feature being called a sport. If, now, that particular
branch be propagated by cuttings or by grafting, the new
feature holds true; and thus the plants which contain
it can be multiplied indefinitely. The Red, or Copper,
Beeches, familiar lawn trees, originated in a single red-
leaved branch on an ordinary Green Beech, and have
since been propagated and multiplied by grafting. The
Navel Orange, which is seedless, and further distinguished
by the small accessory Orange within its upper end
(page 201), originated in a sport branch upon an ordinary
Orange tree, and has been preserved and spread by bud-
ding (a form of grafting). Indeed, most highly developed
fruits have originated thus; somebody has found them as
sports upon more ordinary kinds, and preserved them by
grafting. If the sporting branch cannot be propagated by
cuttings or by grafting, the sport cannot be preserved at
all, for bud sports are not reproduced by their seeds, which
produce only the original form. Sometimes, however,
SEED SPORTS appear, in which case the sports come true to
seed and can thus be propagated, as in case of some fruit
trees and a few garden herbs.

The mode and causes of origin of sports are unknown.
320 A TEXTBOOK OF BOTANY (Cu. VI, 9

They occur in all degrees, from barely perceptible to very
striking, from useless to valuable, and from ugly to attrac-
tive, — only those which appeal in some way to man’s in-
terests being noted and preserved. They are clearly in the
nature of extreme variations, which merge over also to mon-
strosities (page 205) ; and, whatever the case with bud sports,
no distinction is apparent between seed sports and those
mutations or hereditary variations upon which selection
works.

3. Hybridization. When two parents belong to different
varieties or species, their offspring are called HYBRIDS, and
the process of making such crosses is called HYBRIDIZATION.
Only closely related kinds of plants or of animals can be
hybridized, presumably because the process requires a cer-
tain degree of chemical similarity in the complicated pro-
toplasm. To make the cross in plants, the pollen from a
flower of one parent must of course be transferred to a
stigma of a flower of the other parent, which process is usually
effected by aid of a fine brush. It is also indispensable to
prevent the access to that stigma of any other pollen, in-
cluding the plant’s own. This end is accomplished by re-
moving the anthers before they are ripe and covering the
flower completely with a gauze bag which excludes cross-
pollinating insects.

Hybrids show four distinctive characteristics important
in plant improvement. First, hybrids are apt to be larger
and finer plants than their parents, although, owing to the
operation of Mendelian segregation, this feature is not pre-
served in the next generation. It may, however, be kept
by use of cuttings or grafting. Second, entirely new fea-
tures, not apparent in cither parental line, may appear,
seemingly not simply as a result of mixing two ancestral
strains, but through a kind of sporting induced by the dis-
turbance incident to the wide crossing. Third, a given
undesirable character may be bred completely out of a race
and replaced by a better, on the principle of Mendelian
Cu. VI, 9} METHODS OF PLANT BREEDING 321

segregation (page 312), which applies in full force to hybrids,
where indeed it was discovered. Fourth, two, or more, de-
sirable qualities belonging to different varieties may be
brought together and permanently combined in a single
variety. Theoretically this is the highest utility of hy-
bridization, and its practice the highest form of plant breed-
ing.

Hybridization is, however, by no means so simple in
practice as in principle. It is often very difficult to accom-
plish mechanically; many plants which one desires to
hybridize fail to set seed with one another’s pollen; new
features are as likely to. be useless as desirable; hybrids
designed to combine certain good qualities are as likely to
combine others which are bad; the reproductive power of
hybrids is usually poor; and many other difficulties make
hybridization a slow and difficult method of effecting de-
sired improvements in plants. Nevertheless, in the hands
of skilled breeders, it is the most important of the three
methods of plant improvement, and is actually yielding most
valuable results, especially in the breeding of grains.

It was earlier said that cultivation, though it makes better
plants and crops, does not produce new varieties. Indi-
rectly, however, it helps to that end; for under cultivation
plants vary and sport far more profusely and widely than
when wild, — apparently because of their better nutritive
conditions, in conjunction with the stimulative effect of new
surroundings, and perhaps the removal of old restraints.
Further, it is possible, by devices of cultivation, to intensify
the rapidity and degree of variation, though not to direct its
character ; and skilled breeders can thus ‘‘ break the type,”’ in
their phrase, as a foundation for new varieties. It is also
of course true that the greater the number of plants grown,
the greater the chance for the appearance of new and de-
sirable variations ; and this method of growing plants in vast
quantities is one of the “secrets of success’? of the best
known of present-day plant breeders, Luther Burbank.

¥
322 A TEXTBOOK OF BOTANY [Cu. VI, 10

By a combination of the methods here described, our
cultivated plants have been developed from their wild
ancestors. Obviously the process is a kind of evolution, in
which man’s needs or fancies play the part of the selecting
and preserving agency. The methods do not include any
way of originating any desired feature; all we can do is to
select, preserve, and intensify such features as nature
offers.

In earlier times most, or all, of man’s improvements in
plants were without plan or forethought, his selection being
made upon features which pleased him, or seemed profitable,
at the moment; and it is only because in general he has
continued to be pleased by the same things that our culti-
vated plants have been brought to their present high de-
velopment. In modern times, however, much of the im-
provement is accomplished by expert workers who proceed
with deliberate forethought and a definite aim in mind.
This is typical plant breeding, to which we may confidently
look for great triumphs in the future.

10. THe MorpHoLtocy or FLOWERS

Although the flower is physiologically a distinct organ of
the plant, having its own primary function of effecting fer-
tilization, its structure shows obvious morphological relation
to leaves and stem.

The sepals of flowers are commonly green, and so leaf-like
in origin and anatomy as to permit no doubt that they, at
least, are morphologically identical with leaves. Besides,
the most perfect gradations occur from sepals through bracts
to the green leaves of the stem (e.g. Calycanthus). Petals,
also, despite their difference in color, have a perfectly leaf-
like development and anatomy, with an occasional complete
gradation to sepals (e.g. Cactus flowers); so that they too
are morphologically leaves. As to the stamens, the fila-
ments correspond to leaves in all the morphological test
points, including a transition to petals (e.g. in Water-lilies),
Cs. VI, 10) MORPHOLOGY OF FLOWERS 323

so that they likewise are leaves, of a linear or needle-like
sort. The anther, however, answers to nothing in a leaf,
and we hold it in reserve for a moment. In the pistil
each carpel has the leaf origin and anatomy, its development
being such that it infolds with the upper surface inward
(Fig. 221). Where the edges of the infolded leaves grow to-
gether, the tissues are enlarged, forming placente (Fig. 222),
upon which stand the ovules, while the
tips of these leaves become prolonged and
modified to styles and stigmas. The ovules,
however, do not answer to anything in a
leaf, and we reserve them, like the anthers,
for the present. The receptacle is very
clearly a stem, enlarged at the tip to
bear the other floral parts. Sepals, petals,
stamens, and carpels all stand in whorls,
which, as with whorls of green leaves on
the stem, regularly alternate (page 140, and
Fig. 94), while other relations of phyllotaxy
occur in these parts. Furthermore, as with
ordinary leaves and stems, flowers originate
in buds, which are either terminal or axillary.

 

Fic. 221.— Dia-
grammatic repre-
sentation of the
mode of union of

Thus the typical simple flower consists mor-
phologically of a branch, of limited, or
determinate, growth, containing whorls of
modified leaves borne close together at
the end of a stem, and surrounding two en-
tirely different kinds of structures, anthers

three carpellary
leaves into a one-
celled ovary. The
united edges form
the placente, on
which the ovules
are borne. (After
Gray.)

and ovules.

We turn now to examine the morphological nature of
anthers and ovules, which involves the relations of flowers to
the reproductive structures of the lower kinds of plants. It
happens, unfortunately, that not all of the stages which
must have existed in the evolution of the flower are now
represented in existent plants; but, as will be shown in
detail in Part II of this book, enough of the stages survive
324 A TEXTBOOK OF BOTANY (Cu. VI, 10

to indicate the general course of that evolution. Thus we
can trace the anthers and pollen grains back without any
serious break to SPORANGIA (or SPORE CASES) and SPORES
(the kind called microsporangia and microspores) of the
highest. flowerless plants, each anther being a composite
microsporangium and each pollen grain a microspore.
We can trace the ovules back in the same way to MEGA-
SPORANGIA and MEGASPORES (Fig. 223), each nucellus being
a megasporangium, and the embryo sac a megaspore, while
the integuments are a special new outgrowth from the
stalk of the sporangium. We can, however, trace these
parts still farther back to an origin in a single kind of sporan-

 

Fic. 222. — Diagrams to illustrate, in cross section, the various ways in
which carpels, here five in number, unite to form compound pistils and
placente.

First, carpels all separate; second, united like Fig. 221, giving parietal
placente; third, infolded to the center, like the first but grown together,
giving central placentse; fourth, like the third, but with the partitions
wanting, giving free central placenta.

gium and spores, such as we find in the Ferns, where they
occur in the brown sori, or ‘“‘fruit dots,” on the backs of the
fronds (Fig. 224), and we can even trace them, if we choose,
back into the Algze. Thus we see that pollen grains with
the anthers, and embryo sacs with the ovules, are mor-
phologically equivalent to the spores and spore cases of the
lower plants, and are therefore far older than the other
parts of the flower. Hence a flower consists morpholog-
ically of stem, leaves, and sporangia with their spores. Or,
since the spores are the more important as well as the older
parts, we may say that morphologically a flower consists of
spores together with stem and leaves specialized to aid in
their reproductive function.
Cu. VI, 10} MORPHOLOGY

This identification of pollen and
ovules with the spores of the lower
plants at once throws light on two other
features of floral structure. rst, the
megasporangia and microsporangia of
the flowerless plants occur in close asso-
ciation with, or upon, certain leaves,
somewhat modified accordingly, called
SPOROPHYLLS (Fig. 223); and it seems
clear that stamens and pistils are the
lineal descendants of the sporophylls.
As to petals and sepals, it is not yet
certain whether they represent ancient
sporophylls which have lost their spo-
rangia, or green leaves independently
specialized, though the latter seems
most probable. Second, the pollen
grains and embryo sacs (the ancient
spores) are not themselves the sex cells,
but develop the sperm cells and egg
cells through intermediation of some
cell divisions which have no apparent
meaning under existent conditions (Figs.
188, 190, and full account in Part IT).
Now in the lower plants the spores
are not sex cells either, but they pro-
duce special and often elaborate struc-
tures (including the prothallus stage
of the Ferns, the thallus of the Liver-
worts, and the whole body of the
Mosses), upon which the sex cells are
developed; and it is the reduced pro-
thallus, or equivalent, of the lower

OF FLOWERS

325

 

Fie. 223. — The fruit-
ing strobilus of Selagi-
nella inequifolia, a
Pteridophyte; x 12.

On the left, micro-
sporangia containing
several microspores; on
the right megasporangia
containing four mega-
spores. The sporangia
stand upon sporophylls.
(From Sachs.)

plants which persists as the seemingly meaningless cell divi-

sions within the pollen grain and embryo sac.

Thus while

ovule and embryo sac, with anther and pollen grain, are parts
326 A TEXTBOOK OF BOTANY (Cu. VI, 10

of the flower, the prothallial cells
of both embryo sac and_ pollen
grain, together with egg cell and
sperm cell belong to a new gen-
eration.

These morphological matters
are certainly complicated and
difficult at first to grasp in detail.

Fig. 224.—Sorus of a fern, They can be made clearer, how-
at Gloss “Secon, Somme tHE) rae by aid of a table or dia-
stalked sporangia containing
spores; magnified. From these gram which will exhibit their
Scar Bes ess relations in light of their evolu-
pollen grains of flowers. (From tionary origin, and of the con-
2 Dern? nections of the reproductive with
the nutritive parts; and such a diagram is presented on
the opposite page.

We have now traced the flower back to its morphological

 

 

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and absence of the whorls.

They represent cross sections supposed to be made through the widest
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flower (Staphylea), and the second is apetalous (Beet). Below, the first is
asepalous and apetalous (Saururus), the next is staminate only (Willow), and
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328 A TEXTBOOK OF BOTANY (Cu. VI, 10

foundation, but have still to trace it upward through a
remarkable morphological elaboration.

Typically the flower has sepals, petals, stamens, and carpels
(Fig. 225), but these may be absent in various degrees, making
the flowers apetalous, asepalous, pistillate, or staminate, all
of which terms are self-explanatory.

Typically all of the whorls have the same number of parts,
as in the phyllotaxy of leaf whorls on the stem (page 140,
Fig. 94). That number is oftenest five (Fig. 226), no doubt
beeause of the predominance of the 2? system of phyllotaxy
(page 141); next most often it is three, connected with the
2 system; while less often it is four, presumably connected

Fig. 226.— Diagrams of typical flowers, to illustrate the principal

numerical plans. Constructed as in Fig. 225.
5-plan, Oxalis; 4-plan, Fuchsia; 3-plan, Lily.

with the } system; and these are the only numbers which
prevail through flowers. This relation to phyllotaxy, by
the way, shows how purely structural and little adaptational
is the numerical feature of floral structure. Any of the four
,Whorls may deviate from the number characteristic of the
flower. Thus Poppies have but two sepals, Monkshood has
but two petals, Orchids have but one or two stamens, and
Peas have but one carpel. As to the stamens, they are some-
times fewer, but often are more numerous than the typical
number, especially in simple flowers pollinated by many
insects, such as Roses and Buttercups. The carpels, onthe
contrary, rarely exceed the typical number (though they do
so in both of the plants last mentioned), but oftener than
Cu. VI, 10] MORPHOLOGY OF FLOWERS 329

not are less than the prevalent number, being commonly
three in a 5-part flower, or even only one, as prevails through
the great Pulse family (Fig. 227). In general a diminution
in number accompanies increasing efficiency in function, and
marks a higher grade in evolution. Thus the Composite
family (that of the Sunflower and Chrysanthemum), the
largest plant family, and the one which stands highest of
all in plant evolution, has five sepals (when any), five petals,
five stamens, and

one carpel. fam \ Joe oS
As the floral leaves, Ce 2 Cg \\
especially the sepals ‘ & : ic @ 8
and petals, develop S
and broaden in the Ne yi X ce ay,
bud, their edges be- i Sata we Seeeinnd
come variously dis-
posed with respect

to one another. In
some flowers these

0

mJ

exactly matching to-

gether without any

overlapping, as in the

sepals of Fuchsia Fic, 227.— Diagrams of typical flowers, to
: é illustrate deviations from numerical symmetry.

(Fig. 226), an ar Constructed as in Figs. 1225, 226. Above,

rangement called Stellaria and Cassia; below, a Composite

(Helenium) and Primrose.

VALVATE. In others

the edges regularly overlap spiralwise, as in the petals of

Fuchsia, an arrangement called convoLuTE. Oftenest they

overlap in such manner that some parts have both edges

under, some both over, and some both ways, an arrangement

called ImBricaTE (Primrose in Fig. 227). These arrange-

ments, called collectively #sTIVATION, often persist in the

open flowers, though sometimes so lightly as to be easily dis-

arranged by a touch or the wind. They are apparently due

to a combination of phyllotactic and developmental factors.
330 A TEXTBOOK OF BOTANY (Cu. VI, 10

Typically, and usually, the floral whorls alternate, as in
the case of leaves on the stem (page 140). Most of the
exceptions are only apparent, as in the Lily family (Fig. 226),
where a whorl of six stamens seems to stand opposite a
whorl of six petals or sepals (e.g. Lily of the Valley); but
in reality whorls of sepals and petals, here alike, and two
whorls of stamens regularly alternate. In case of the
Primrose, where five stamens stand opposite five petals
(Fig. 227), it is likely that another set of five stamens, which
would make the alternation perfect, has vanished in the
course of evolution. Indeed, two whorls of stamens are
more frequent, and perhaps more ‘‘typical’”’ than one.
The usual lesser number of carpels, of course, destroys the
alternation in their case.

Typically the sepals, petals, stamens, and carpels all
stand separate and distinct upon the receptacle, precisely
as do leaves on the stem; but sometimes each whorl forms a
single structure. Thus the calyx, as earlier noted (page 270),
is often one structure at base, and even to near its top, while
sometimes it forms a tube with only small teeth on its free
margin, e.g. Phlox. It was formerly supposed that such a
calyx is formed by a union of the lower parts of the sepals,
the tips alone remaining free, on which account it was called
GAMOSEPALOUS (united sepals) in distinction from POLYSEP-
ALOUS applied to the separate condition. This view, how-
ever, finds no support in the development of the indi-
vidual flower, where no such union of parts takes place;
for, in fact, the sepals originate and grow separately for a
time, and then are lifted by the growth of a continuous ring
of leaf-like tissue, which gradually elongates to the tubular
part of the calyx. It is possible that in course of their
evolution the sepals have become united, as the older view
held; but it is equally possible, and much more in accord
with the method of their present development, that only the
free tips represent the original separate leaves, while the
tubular part is a new development, just as we know the
  

Y?

  

   

  

f 4
y

Fic. 228. — Diagrams of typical flowers in vertical section, showing the
various relations of calyx, corolla, stamens, and carpels, as interpreted by
their development from the buds.

Receptacle is dotted; floral tube is lined lengthwise; carpels are lined
crosswise. The parts in broken linerdo not fall in the median plane in a 3-
plan flower.

Upper based on Scilla; next on Hyacinth; next on Snowdrop; lower on
Narcissus.

 

9

331
332 A TEXTBOOK OF BOTANY (Cu. VI, 10

external tube, or corona, of the Daffodil to be. Precisely
the same is true of the gamopetalous corolla, and also of the
monadelphous stamens, although in cases where the stamens
are united, as in the Composite, these anthers do actually
grow together although they originate separately. As to
the carpels, where two or more unite into a single pistil,

 

Fic. 229. — Fuchsia speciosa, showing the raceme of morphologically
specialized flowers, with inferior ovary, and both petals and stamens raised
on the calyx tube. (From Bailey.)

the case is quite clear, for they always originate separately
in the bud, and later actually grow together as they develop.

The mode of fusion of the carpels determines the place of
the placentzee and the number of compartments (unfor-
tunately called cells) in the ovary. Thus in the Pulse family,
illustrated by the familiar green Pea, only one earpel is
concerned, and it infolds with a single parietal placenta
(Fig. 227). When two or more carpels unite to one pistil,
Cu. VI, 10) MORPHOLOGY OF FLOWERS 333

they may grow together in any of the ways shown in
Figure 222, producing parietal, central, or free central pla-
cente, with one or several compartments.

Typically each of the four whorls stands directly on the
receptacle independently of the other three; but remarkable
interrelations of the whorls also
occur in various flowers, as repre-
sented diagrammatically in Fig-
ure 228. In some cases the
calyx and corolla together form
one structure, called PERIANTH,
upon which stand the stamens,
as in the Hyacinth, while vari-
ous other combinations occur.
Formerly such cases were inter-
preted on the supposition that
the different whorls were united,
or adnate, to one another from
the receptacle upward; but
here also the development of
the flower favors another inter-
pretation, viz. that the tube
which the parts occupy in com-
mon has developed in precisely
the same way as the tube of the ,

Fig. 230. — The Daffodil, Nar-
corolla or calyx, — not by &® cissus Pseudo-Narcissus, showing
union of originally free parts, the large corona, an outgrowth

j from the sepals and petals. (From
but as a new growth inter- Bailey.)
calated between the free struc-
tures and the receptacle. Especially striking is the con-
dition of inferior ovary (page 275), where sepals, petals,
and stamens stand upon its top (third flower, Fig. 228).
This arrangement was formerly interpreted on the sup-
position that the calyx (and therefore also the corolla
and stamens) was united or adnate to the ovary all the
way up from the receptacle below; but here also the

 
334 A TEXTBOOK OF BOTANY [Cu. VI, 10

development of the flower favors a different interpretation,
viz. that the receptacle grows up in cup-shaped form, carry-
ing upon its top the four whorls, of which the carpels come
simply to close in the roof of the ovary, as represented in
the lower diagrams (Fig. 228). In case of the Apple, the up-
growing receptacle appears to have inclosed the set of carpels,
represented by the core. Yet these distinctions of floral
parts have in reality no great weight,
since as the flower becomes special-
ized the former sharp distinction
between stem and leaves, and even
that. between receptacle and floral
tube, tends to disappear. This
consolidation of the parts of the
flower goes still farther in cases
like Fuchsia, where the floral tube
stands upon the ovary, and upon
the tube stand sepals, petals, and
stamens (Fig. 229); and it reaches
perhaps its perfection in the Orchids
where even the stamens and pistil
form one mass.

Typically the sepals, petals, sta-
mens, and carpels follow the method
of leaves in their development, and,

Y like leaves, branch readily in their
see oe oe ome plane, but rarely out of it. Yet
(From Bailey.) the floral parts do at times produce

special outgrowths from their faces,
as In case of some nectaries, the scales in the throats of
some Pinks, and the remarkable ‘crown of thorns” in
the Passion flower. Somewhat similar in origin is the corona
of the Narcissus, a structure which in the Daffodil (Fig. 230)
surpasses in size and prominence even the regular floral
tube itself.

In such features as these outgrowths, and in many of the

 
Cu. VI, 111 MORPHOLOGY OF CLUSTERS

339

other facts of progressive consolidation and _ specialization
of parts above described, we see that the flower is by no
means closely bound by its former leaf and stem nature, but
has acquired in large measure its own morphological inde-
pendence. It is therefore in effect a morphological member
as well as a physiological organ of the plant.

11. Tor MorPHotocy AND EcoLtocy oF FLOWER CLUSTERS

The conspicuousness of flowers,
especially of the smaller kinds, is
greatly augmented by their aggre-
gation into clusters. There is
more, however, in the subject than
this, for clusters often exhibit a
specific individuality, with distinc-
tive new characters of their own.
In wind-pollinated kinds, where
showiness has no functional value,
the clusters have apparently no
more than a structural significance,
as a convenience of development.

Each flower originates in a bud,
representing morphologically a
spore-bearing determinate branch
(page 323); and flower buds, like
leaf buds, are usually either termi-
nal or axillary. Now every possible
gradation is found between a con-
dition in which solitary flowers are
scattered along stems in the axils
of green leaves and that in which
numerous flowers are massed densely
together with the leaves reduced to
insignificant bracts or wanting al-
together. Where the solitary con-
dition ends and a cluster begins is

 

Fie. 232. — Eremurus
himalaicus, showing a rac-

emose spike
(From Bailey.)

of

flowers.
336 A TEXTBOOK OF BOTANY

 

Fic. 233.— Button Bush, Cephalanthus
occidentalis, showing the head of flowers.

(From Bailey.)

[Cu. VI, 11

largely an arbitrary mat-
ter, determined in practice
by whether leaves or
flowers are more promi-
nent in the mass. In
many, perhaps most,
cases, however, there is no
difficulty in distinguishing
a cluster, because it ex-
hibits a sharp transition to
the leafy stem; and this
distinctness constitutes
the first step in the indi-
viduality of the cluster.
The simplest clusters
are those in which a con-
tinuously growing stem

produces a flower in the axil of each reduced leaf, the older blos-
soms being thus below and the younger above, —and often the

lower become fruits while the
upper are still buds. Such a
cluster, commonest of all
kinds, is called a RACEME
(Fig. 229). In marked mor-
phological contrast therewith
is the crme (Fig. 231), in
which a terminal flower closes
the growth of the stem, and
the new flowers appear from
buds progressively lower
down. The two types, called
respectively INDETERMINATE
and DETERMINATE,
spond exactly with the defi-
indefinite annual

corre-

nite andl

 

Fie, 234. — Corymb of Cherry.
(From Figurier.)

growth of stems, earlicr described (page 138).
Cu. VI, 11] MORPHOLOGY OF CLUSTERS 337

Both racemes and cymes often become compound by the
branching of the main flower stalks, and the two types occur
intermingled in the more complicated clusters, such as the
pyramidal tuyrsus of the Lilac and Horse-chestnut and the
much looser PANICLE of the Meadow Rue, and most of the
loose-topped Grasses. In the other direction, the clusters
become very compact. Thus racemes sometimes have so
many flowers on such short
stalks as to form collectively
aSPIKE (Fig. 232), as familiar
in Mullein, while if bracts in
a spike are more prominent
than petals, as so commonly
occurs in wind-pollinated
trees, we have a CATKIN,
familiar in Birches (Fig. 197)
and “pussy willows.”’ If the
main stem remains short,
bringing the flowers all close
together, the cluster is a HEAD,
as familiar in Clover and
Button Bush (Fig. 233).

The clusters thus far noted
are little more than aggrega- NE
tions of similar flowers, but Fie. 235.— A typical umbel, of
more highly developed kinds oe ee
show a marked approach to
the aspect of single large flowers. The tendency is first
manifest in the production of flat-topped clusters. Thus,
if the main stem and the stalks of the lower flowers of a
raceme all elongate at about the same rate, there results
a flat-topped corrmp (Fig. 234). When, further, the
main stem remains still shorter, or undeveloped, and the
flower stalks have all about equal lengths, there results a
characteristic UMBEL (Fig. 235), a very common form of
cluster, and one which prevails through, and has given name

Z

   
338

to, a large family of plants, the

A TEXTBOOK OF BOTANY (Cu. VI, 11

Umbellifere. Both

corymbs and umbels also become branched or compounded.
Still more advanced in evolutionary rank are those clusters

     
 

“STERILE
PORTION OF
SPADIX

RY STERILE
FLORETS

Fira.
The spadix, with
flowers, of an
Arum; the large
showy spathe is

236. —

removed. (From

Cavers.)

in which there is found a division of labor
with respect to the functions of reproduction
and conspicuousness. In some clusters the
conspicuousness which shows the flower to
insects is given by bracts greatly developed,
as with the Calla and Jack-in-the-pulpit,
where the single showy bract or SPATHE acts
functionally like a corolla, leaving only the
function of pollination to the little incon-
spicuous flowers arranged on a fleshy spike
called a spaprIx (Fig. 236). Bracts also form
the showy parts of the flat-topped clusters
of comparatively inconspicuous flowers in
Poinsettia and Flowering Dogwood. Still
more highly developed are those clusters in
which this division of function occurs be-
tween the flowers themselves. Thus, in the
wild Hydrangea and its relatives, the inner
flowers of the flat-topped compound cyme
remain inconspicuous, and the showiness of
the cluster is due to the petals of the outer-
most flowers which have developed very
greatly (Fig. 237), losing entirely in the
process their reproductive parts. It is these
outer NEUTRAL flowers which have been de-
veloped in cultivation to form the fine great
showy pyramidal clusters (thyrsi) of our lawn
Hydrangeas. This arrangement reaches its
highest development in the family Com-
positze, where, in forms like the Sunflower,

the outer row of the flowers (the so-called RAY FLOWERS) in
the dense, flat-topped cluster develop greatly their corollas
which make the whole showy parts of the head, but lose their
Cu. VI, 11] MORPHOLOGY OF CLUSTERS 339

stamens and often also their pistils in so doing ; while simul-
taneously all of the interior flowers (the pisk FLOWERS)
remain comparatively inconspicuous and devoted entirely
to pollination. So far, indeed, does the resemblance to

 

Fic. 237. — Flower cluster of Hydrangea Bretschneideri, a compound
corymb with showy neutral flowers.
Lower left; certain details of the fruit. (From Bailey.)

large single flowers proceed that even a calyx-like structure
(called INVOLUCRE) is developed from bracts, these collective
features giving the clusters so much the aspect of smgle
flowers that they are popularly thought to be so. The resem-
blance, indeed, appeals even to insects, which visit and
340 A TEXTBOOK OF BOTANY (Gu: VE. 12

pollinate the clusters in precisely the same way as they do
single flowers. These heads in the Composite represent
the highest evolutionary development of ciusters.

12. SpecraL Forms, ABNORMALITIES, AND MONSTROSITIES
oF FLOWERS

Although leaves, stems, and roots often perform functions
and have forms very different from those which are pri-
mary and typical in those organs, flowers have hardly any
additional or substitute functions, doubtless because of
their high degree of specialization to their primary function.
On the other hand, flowers far surpass all other organs in
the abundance of their abnormalities and monstrosities,
presumably because their much greater complication of
structure allows more opportunity therefor.

Abnormal or monstrous flowers, those which deviate in some
unusual or eccentric way from the conditions usual in that
kind, are apt to occur in any bed, especially in gardens, —
for they are more frequent under cultivation.

The monstrosities occur in all possible parts. Sepals are
found, either singly or the whole whorl, entirely leaf-like in
size and appearance, even to complete compounding in
some Roses. Also they occur so petal-like in color and form
as to resemble a seemingly two-storied flower, as in ‘‘ Hose in
hose”? Primroses. Petals act in many strange ways, even
turning leaf-green in some monstrous Roses. They are
especially prone to multiply much in number, giving us
double flowers, of which a great many kinds can be propagated,
and occur in our gardens. Stamens are sometimes completely
petal-like ; sometimes bear ovules in their anthers instead
of pollen; sometimes are completely replaced by carpels.
Carpels often fail to unite their edges, thus leaving the ovary
open; and they become in various degrees leaf-like. Some-
times the ovary contains anthers with pollen instead of
ovules, and sometimes the ovules are replaced by tiny
green leaves. The receptacle also acts diversely, its most
Cu. VI, 12] MONSTROSITIES OF FLOWERS 341

frequent abnormality consisting in a continued growth right
up through the center of the flower, above which it produces
a second flower, or else a leafy branch, as already described
in connection with stems (page 201). Sometimes two or
more of these abnormalities are combined in a single flower,
in which case we have a genuine, and often an extreme,
monstrosity (Fig. 150). One or more of the whorls may be
absent though normally present, or present when normally
wanting; and any or all may become altered in color,
multiplied in number, or converted entirely into a bunch of
green leaves. Regular flowers become diversely irregular,
and irregular kinds perfectly regular. Also flowers, especially
their pistils, become malformed to galls under insect stimula-
tion (page 203). It is surprising how many and diverse are
the abnormalities which appear when one’s attention is
directed to these matters, and how many are described
and pictured in the special works devoted to the subject. Of
the latter the most famous and instructive is the classic
“ Vegetable Teratology” by Masters, which the student will
do well to examine.

Not only structural, but physiological abnormalities occur,
as for example in cases where the “‘resting-period”’ (page 378)
is wanting, and the flower opens in autumn instead of the
next spring, as happens with exceptional Strawberry blossoms
and flowers of shrubs. Of course such flowers are destroyed
by frost without chance to form seed. Sometimes the ab-
normality, especially in extreme monstrosities, occurs only
in a single flower, in which case it is usually not hereditary
and cannot be propagated, just as with fluctuating varia-
tions (page 314). But sometimes all of the flowers on one
branch or one plant exhibit the feature, in which case it
can usually be propagated like a sport, which indeed it
really is, — both bud sports and seed sports of this kind
occurring. Hence we have in our collections the permanent
strain of the “Hose in hose’ Primrose; in our greenhouses
we have a green Rose propagated as a curiosity; and in
342 A TEXTBOOK OF BOTANY (Cu. VI, 12

our gardens we have double flowers in an extreme abundance,
the doubling in some cases being due to the transformation of
stamens to petals, and in others to a multiplication of petals.
Thus it is plain that no line can be drawn between variations
and abnormalities, sports and monstrosities.

We should now note somewhat more fully the causes of
monstrosities, as to which we have little exact knowledge,
though some good circumstantial clews. It was once be-
lieved that they are mostly reversions to a simpler ancestral
condition, but further knowledge has shown that they are
usually reversions to a simpler structural condition. They
are chiefly due to disturbance in the growth control mecha-
nism. The development of any organism and its parts
depends upon three sets of factors: First, there is the supply
of matter and energy contributed by the metabolism of the
plant, and as these are supplied to every living cell, all
parts have thus the power and the impulse to grow without
dependence upon the others. Second, there is the guidance
of the development of the particular parts, exercised in
some way by the chromosomes through the cytoplasm, and
partly determined by heredity and partly by responses to
external stimuli. Third, there is correlation between the
different parts of the plant such that the power and impulse
of each part to grow far more than it does is kept in restraint
and subordinate to the development of the organism as a
whole, as witness the case of buds, sometimes forty times
more numerous than are permitted normally to develop
(page 138). As tothe mechanism of this correlation we have
as yet no idea, though it is clear that the physical path of its
operation lies through the protoplasm which is continuous
from cell to cell. Now monstrosities can often be traced to
a failure in operation of some one of these sets of factors,
but they seem oftenest to result from a failure in the third,
caused by mechanical damage to the path of conduction (as
incase of burls, page 200) or by chemical paralysis through
action of parasites (Witches’-brooms, page 198). When the
Cu. VI, 13] ECONOMICS OF FLOWERS 343

control mechanism becomes inoperative while the growth
energy is still forcing forward the growth of the part, then
the part seems to be controlled by whatever structural con-
dition happens to be strongest at the moment.

13. Economics, AND TREATMENT IN CULTIVATION, OF
FLOWERS

Flowers, unlike the five other primary plant parts, have
few economic uses, aside from the beauty they give to our
gardens. That, however, is surely a utility of civilization,
and besides it maintains great business interests in seed
firms and nurseries which supply ornamental flowers, trees, and
shrubs. In a few cases perfumes are extracted from flowers,
which also supply the nectar elaborated by bees into honey.
But otherwise their direct uses are insignificant.

Turning to the cultivation of flowers, we find some features
of gardening practice dependent on their physiology.

Since showy flowers are cross-pollinated by insects, those
who grow seeds or fruits for market find it well to keep
Bees, best of cross-pollinators, in their gardens, or even
their greenhouses, where crops of Tomatoes or Cucumbers
are forced for early market. It is true the pollination can
be effected artificially by use of fine brushes, as often done
for special purposes; but Bees are more economical. In
another way this relation of insects to flowers affects practical
interests, for if the blossoming time of our fruit trees, Apples,
Pears, and others, falls cold and wet, the insects are not active
and pollination is only partial, which is one cause of poor
fruit years.

The reciprocal balance, already described (page 207),
between vegetation and reproduction, makes it possible for
gardeners to promote flowering by checking the stem and
leaf growth, either through withholding fertilizers, by root
pruning, or by other devices known in the business. Pruning,
in orchards, has chiefly this use, as earlier noted (page 207).
These methods, however, have strict limitations, and are
344 A TEXTBOOK OF BOTANY [Cu. VI, 13

effective only in skilled hands. Theoretically the best
results would be attained when a plant has been stimulated
to vigorous vegetative growth until a large reserve of food
has accumulated, and then is checked in its stem and leaf
growth.

Flowers are prone to wilt when cut, even if placed imme-
diately in water, because they now lack the root pressure
which helped their supply. Moreover, their evaporation
current through the cut ducts draws into the latter various
micro-organisms which here find such congenial conditions
for growth that they fill the passages and stop the water.
The devices for preserving the freshness of flowers are ad-
justed to neutralize these conditions. Thus, everybody knows
that flowers keep best in cool, moist, shaded places, — be-
cause evaporation is there checked; and florists keep their
Roses before sale in refrigerators for this reason. On the
other hand, a frequent changing of the water, clipping away
the lower and often discolored ends of the stems, the addi-
tion of a little salt, dipping the cut ends for a moment in
hot water, charring the ends in a flame, — all of them devices
recommended by different people for preserving particular
kinds of flowers, — have in one way or another the effect of
antagonizing the organic growths in the ducts, thus keeping
the passages open. It is said that white flowers last longer
after cutting than colored kinds, which perhaps is connected
with the fact that they absorb less sunlight than colored
kinds, and hence suffer less evaporation from their tissues.
Florists have still another device, useful in some cases, de-
pending on the fact that since petals usually fall immediately
after fertilization, flowers last longer if that is not effected.
Fertilization can be prevented by removing the anthers
from all flowers as soon as they open. This is commonly
practiced with large Lilies.
CHAPTER VII
THE MORPHOLOGY AND PHYSIOLOGY OF FRUITS

1. Tue DISTINCTIVE CHARACTERISTICS OF FRUITS

THE word Fruit has far wider significance in scientific than
in popular language, for to the botanist it includes any
structure which has part in the development of seeds, no
matter whether edible or not, or what the aspect it presents.

Most fruits are the ripened ovaries of flowers, from which
all other parts (excepting of course the receptacle) have
fallen away, though occasionally some of the other floral
parts persist, and become incorporated with the ripening
ovary. There are fruits, however, which have no connection
with ovaries, as in berries of Yews and cones of Pines, though
in such cases other structures replace the ovaries in function.

The ovary, as a rule, withers and falls with the other parts
of the flower unless pollination occurs; but after pollination
the ovary develops to a fruit, the ovule to a seed, and the
fertilized egg cell to an embryo. Thus pollination acts as
the stimulus to fruit formation, the arrangement being
obviously advantageous in preventing the waste of good food
material upon fruit and seed if no embryo is formed to be
protected and disseminated, — and no embryo is formed
without fertilization.

Fruits display well-nigh as great a diversity in their visible
features as do the other plant organs. They fall rather
naturally, however, into two great classes, — dry fruits, like
pods, and fleshy or edible fruits, like berries.

In size, fruits are almost microscopic in some very small

plants, and vary thence upward to the great double Coco-
345
346 A TEXTBOOK OF BOTANY (Cu. VII, 1

nut, a foot or two in diameter, and weighing some thirty
pounds. The largest fleshy fruit is probably the Jack fruit
or Durian of the tropics, often mentioned by travelers.

In shape, fruits are diverse as possible, though tending to
rounded forms like the ovaries from which they are developed.
Sometimes they answer very closely to the shape and aspect
of a single seed, to such a degree as to be commonly mis-
taken therefor.

In texture, the difference between dry and fleshy fruits
becomes very manifest. In dry fruits the walls of the ovary
are parchment-like or woody, as in most pods, or even al-
most ivory hard, as in some nuts and fruit pits, while in
fleshy fruits the ovary walls become soft, pulpy, nutritious,
and palatable, as we, and other animals, know very
well.

In color, the two classes are likewise contrasted. The
dry fruits are mostly brown or gray, like bark, indicating that
their color has no bearing on their function, and is simply
that which happens to be natural to ripening woody tissues.
The fleshy fruits, on the other hand, are mostly bright colored,
—red, yellow, purple, and sometimes white, — in marked
contrast to their respective backgrounds. Such colors we
naturally assume to indicate a functional connection with a
seeing eye, — an assumption which proves to be true, as a
later section will indicate.

The fruits, of botanical terminology, include some struc-
tures which are popularly rated as vegetables, notably Cu-
cumbers, Pumpkins, and Squashes. These, however, are
forms of fleshy fruits, as their whole structure attests.

Fruits produce seeds in diverse numbers from one to many
hundreds. Dry fruits which contain several seeds open or
dehisce to allow their escape, but fleshy fruits, no matter how
many their seeds, remain closed, the seeds being released
in other ways which we shall presently consider.

As in case of other organs, popular terminology is some-
what uncritical. Thus the ‘‘fruit-dots’”’ of Ferns have no
Cu. VII, 2 MORPHOLOGY OF FRUITS 347

connection with fruits; ‘Cedar apples’ are only a Fungus
product; and the “fructification” of Fungi refers only to
their spore masses.

2. Tur Structure and MorpHouocy oF FRUvITS

The structure and morphology of fruits are largely de-
termined in the ovaries from which they originate, — fruits
being primarily ovaries further developed and specialized.
The particular features of the fruit have usually an obvious
connection with the method of dis-
semination of the seeds, —the accom-
plishment of such dissemination
being commonly a function of the
fruit.

The structural features of the
ovaries — walls, partitions, number
of compartments and _ placentee —
can usually be recognized clearly,
and in the same relative connections,
in the fruits, while the DEHISCENCE,
or opening through which the seeds
escape, likewise follows as a rule Fis. 238. — Pods of Col- |

a : umbine. (From Bailey.)
some morphological lines of the
ovary. Deviations in these features, however, often occur,
and can usually be traced to a connection with the method
of dissemination.

The fruit structure is clearest in dry fruits. Thus a typical
fruit of the simplest sort is represented in the pod of Colum-
bine (Fig. 238), which is developed from an ovary of one
carpel, bearing one row of seeds; these are arranged along
a parietal placenta, formed where the edges of the carpellary
leaf unite, and the pod in dehiscence simply dis-unites those
edges. In the Green Pea, however, of precisely the same con-
struction, the pod dehisces both by disuniting the edges and
also forming a new split along the back or midrib of the car-
pellary leaf. Pods originating in two or more carpels like-

 
348

 

Fic. 239. — Pod of a
Poppy; x t.

It stands at the sum-
mit of a long stiff stalk.

A TEXTBOOK OF BOTANY

[Cu. VII, 2

wise usually dehisce by disuniting the
joined edges, though sometimes they
split also down the carpellary midribs.
Frequently, however, the dehiscence
follows no morphological line in the
ovary, but occurs in new and independ-
ent positions connected with a par-
ticular method of dissemination. Thus,
in the capsules of Poppies new openings
arise around the tops of the fruits and
in Purslane the capsule splits right
across without any regard to morpho-
logical lines (Fig. 239); in some of the
Mustard family the carpels mostly split
away as valves from the placentz, which
persist for a time as a framework (Fig.

240); and other arrangements also occur, some of which
prevail throughout families in ways to show that large
structural and hereditary factors enter along with adapta-

tion into the construction of fruits.
structural
the dry fruits

aggregate
tures,

On the basis of their
fea-
are

classified and named as FoL-
LICLES, LEGUMES, SILICLES,
etc., these distinctions hav-
ing importance in connection
with the taxonomy of plants.

The only dry fruits which
do not dehisce at all are
those which contain but a
single seed, as typified by the
little AKENES of the Straw-
berry and Buttercup, com-
monly supposed to be seeds
(Pig. 241). They are in fact
functionally seeds, both in

 

Fie. 240.—Honesty, Lunariaannua,

in which the persistent partitions of
the pods
(From Bailey.)

form shining plates;

x}
Cu. VII, 2] MORPHOLOGY OF FRUITS 349

dissemination and germination, the ovary wall serving simply
as an additional pit-like coat. A very important form of
single-seeded indehiscent fruit is the grain ai 242), dis-
tinguished particularly by the fact that seed
coat and ovary wall are grown completely
together, thus making the structure so seed-
like that only the botanist knows its true ;
morphological nature. As its name implies, ae eee
this fruit is characteristic of the grains, — (akenes) of Butter-
Corn, Wheat, Oats, etc. Nuts also are ve poe ae
commonly one-seeded, though here we meet

with morphological complications, both as to the original
number of the ovules and the nature of the shell.

While in general the construction of the fruit answers
closely to that of the ovary, some exceptions occur, indicating
that the fruit has a certain morphological
independence of its own. The development
of new dehiscence lines is one instance
thereof. The number of compartments, or
cells, is usually the same in ovary and fruit,
but sometimes partitions disappear, or new
ones develop; while we find also such
changes as the formation of four little nut-
lets (prevailing throughout the Mint family)
from a two-celled ovary. Not infrequently

Fic. 242. — A_a several-celled ovary produces a one-celled
eee aoe and one-seeded fruit, as in most of our com-
the embryo, R, G, mon nuts (Fig. 243), in which an occasional
ee a development of a second seed gives us the
ovary coat, 7; x4. philopena variety.

- oe an In many cases other parts of the flower

persist and are incorporated with the ovary
into the fruit, contributing to its functional effectiveness.
Thus the style, usually deciduous with the petals and
stamens, persists in Clematis, where it forms the very con-
spicuous plume (Fig. 244). In the Composite family, the

 

 
350 A TEXTBOOK OF BOTANY (On Vile 2

so-called pappus, a structure on the ovary usually interpreted
as morphologically calyx, persists as hooks, plumes, and
other analogous structures (ig. 256). Furthermore, wholly
new structures also develop from the ovary
wall, usually in obvious adaptation to
dissemination. Thus many small weeds
develop hooks or adhesive glands, making
their ‘‘seeds”’ cling tight to the clothing

of the stroller in autumn fields. Very
ce aes Tien prominent are the flat wings which de-
showing development velop on the Maple (Fig. 245), the Elm,
Gis Gey end the Ash.

Fleshy fruits also exhibit, though less
clearly, the signs of their origin from ovaries. They possess
two features not found in dry fruits, — viz. bright and con-
trasting colors, and seeds which are usually protected in
some way against injury by digestion when eaten; for, as
will appear in the following section,
fleshy fruits are eaten and their seeds
thus disseminated by animals. The
simplest fleshy fruit is the BERRY, in
which the wall of the ovary, whether
carpels or receptacular cup, develops
into the pulp, while the seeds have
stony coats, as well exemplified in
the Grape, and also in Cranberry and
Blueberry. Closely related is the
stone fruit, or DRUPE, wherein the
outer layers of the ovary wall ripen
to the soft pulp, while the inner layers
form the hard stone, which consti-
tutes the most effective protection to
the seed, as so typically illustrated in
the Cherry, the Plum, or the Peach (Fig. 246). The fruits
just mentioned, by the way, show on one side a depressed
line which indicates the original joining of the edges of the

 

 

Fic. 244.— Fruit of Clema-
tis. (Irom Bailey.)
Cu. VII, 2 MORPHOLOGY OF FRUITS Bol

single carpel from which each fruit is developed. In the
fleshy fruits of the Apple and Pear type, the receptacle grows
up and incloses the carpels (the core), forming a type called
the pomME, the receptacular nature of
which is further attested by the obvious \
remnants of persistent sepals. In some
of the largest Gourp fruits, like the
Pumpkin and Squash, the outer wall is
hard and only the inner part becomes
edible, while in the related Watermelon
it is chiefly the placentae which form the
pulp, as is likewise true in Tomato and
Cucumber. As to the method of protec-

tion of the seeds in large fruits like the ee ae ae
Apple, Watermelon, and Orange, that Bailey.)

will presently be mentioned.

In the fruits just described the pulp results from the spe-
cialized ripening of carpel, or receptacular ovarian wall, or
placente ; but it may develop from other parts also. Thus
in the Strawberry the edible part of the fruit is wholly the
receptacle, which bears the many seed-like
akene fruits. In the Wintergreen berry
the pulp is largely calyx; in the Yew
berries it is an extra seed coat (for Yews
have no ovaries), called an arty. In the
Orange, which is a kind of huge berry
with a separable skin, the pulp is con-
stituted from hair-like structures developed
from the inner walls of the carpels.

Ria, 246. Drape In considering the various morphological
of Cherry. Thestone origins of the pulp one cannot but ask why
ear (From one plant forms it in one way and another

so differently. As to this we have little
exact knowledge; but circumstantial evidence indicates
that here, as elsewhere, evolution moves along lines of least
‘resistance, the pulp in any given case being made from

 

 
302

A TEXTBOOK OF BOTANY (Cu. VII, 2

that part which was already most nearly pulp-like in its

structure.

The fleshy fruits thus far described are all stmpLe, that is,
composed of a single pistil; but AGGREGATE and MULTIPLE

 

Fic. 247, — The
Mulberry, made
up chiefly of the
ripened calyxes of
a cluster of flowers ;
x kh (From Figu-
Tier.)

fruits also occur. Thus, while in Strawberry
the pulp is the receptacle on which stand
the many dry akenes, in the nearly related
Raspberry the receptacle forms no part of
the fruit, which is made up of the many
separate aggregate carpels ripened to little
drupes; while in Blackberry both drupelets
and receptacle are included. Further, in-
stead of a single flower a cluster may form
a single large MULTIPLE FRUIT. This is the
case in the Mulberry (Fig. 247), in which
the pulp is chiefly calyx, and also in the
Pineapple, where not only the ovaries, but
also the bracts and main stem of a large
cluster of flowers ripen to the single coales-
cent fruit mass. <A different form of

multiple fruit is that of the Fig, where the flowers are
arranged inside a hollowed receptacle (Fig. 248). Somewhat

in the nature of a multiple fruit also is
the cong (Fig. 249) of Pines, Spruces,
and that family. This form of fruit
belongs to the Gymnosperms, or naked-
seeded plants, which have no ovaries
but usually bear their seeds on the
bases of overlapping scales which col-
lectively make up the cones.

The particular feature of pollination
which acts as the stimulus to fruit
formation is known. The pollen tube,
as it reaches the embryo sac (page 278),
contains normally two sperm nuelci, of
which one always fertilizes the egg cell.

 

V'ta. 248. — A Fig fruit.
(From Bailey.)
Cu. VII, 2 MORPHOLOGY OF FRUITS 353

The other also enters the embryo sac, and moves towards its
center, where it fuses with the principal nucleus of the embryo
sac itself (Fig. 250). This fusion nucleus, with surrounding
protoplasm, soon divides and forms the beginning of the
endosperm or food substance later
used by the developing embryo.
Thus this so called “ double fertiliza-
tion”’ acts as the stimulus to endo-
sperm formation, and endosperm
formation seems clearly to act as the
stimulus to seed and fruit formation.
Incidentally this double fertilization
involves another important conse-
quence, in that the endosperm, like
the embryo plant, contains chromo-
somes from the pollen parent, of
which it can thus show some char-
acteristics. A conspicuous case
thereof is found in Corn, where red
grains, the result of a red endosperm
showing through the grain coats,
appear on white ears after pollination
by ared variety. This phenomenon, Fic. 249. —A_ typical
called XENIA, Was very puzzling until {0'" ae ee Mech
its real nature was discovered. Above, on right, a scale

While ordinarily a fruit does not Sith ovles; Or Met ccale
form, or set” ing the gardener’s separating with them to
phrase, unless fertilization has oc- a ia ee
curred and an embryo is formed, cases
are known in which the fruit develops without the presence
of embryos in the seeds, a condition called PARTHENOCARPY.
In most such instances pollination is essential, as indeed
would be expected from the réle of double fertilization ; but
in a few cases, notably some Figs, even pollination is not
essential, and fruit formation follows on dower formation
without any known special stimulus.

2A

 
354 A TEXTBOOK OF BOTANY (Cu. VII, 2

The development of ovaries into fruits involves often a
great increase in size, as notable in the gourds. Herein is
involved not simply an enlargement of cells already present

in the ovary, but abundant new cell formation from the

 

Fie. 250. — Double fertilization
in the embryo sac of Liliwm Mar-
tagon, generalized.

One of the two sperm nuclei, sp,
is shown in contact with the nucleus
of the egg cell, ov; the other sperm
nucleus, sp2, is in contact with the
embryo sac nucleus, eh. The cells
s, and go, called synergide, and the
three marked a, called antipodal
cells, represent an inheritance of
the thallus of lower plants. (After
Strasburger.)

sac, forms the new embryo.

parenchymatous, but not spe-
cially meristematic, tissue, —a
method identical with that by
which the bark of trees and
the chlorenchyma of leaves are
enlarged.

While the ovary is develop-
ing to the fruit, the ovule is
developing to the seed. The
coats of the ovule harden,
with some changes, to the seed
coats; the micropyle becomes
sealed with corky tissue; the
formation of the endosperm
tissue is completed ; and (most
important of all) the fertilized
egg cell develops into the
embryo plant.

The development of the
embryo from the fertilized egg
cell may best be traced in a
typical case, illustrated in Fig-
ure 251. First the egg cell
divides, and then the resultant
cells divide, for a time in a
line, forming a SUSPENSOR, the
end cell of which, thus brought
well out into the embryo

This so-called INITIAL CELL

divides, as shown in our figure, and divides again until

there is formed a multicellular globular structure.

Then

growth becomes more active at special points, there forming
Cu. VII, 2] MORPHOLOGY OF FRUITS 355

the ‘‘seed-leaves”’ or coryLEDONS and the HYPOCOTYL, as
shown in our figure. At this stage the embryo consists
mostly of meristematic tissue, though an epidermis is well
formed, and the fibro-vascular system begins to appear, while

     
 

  
 

tr
ne

|
rH 1
[H
a)
Se

Fic. 251. — Stages in the development of a typical embryo, of Rape
(Brassica Napus); < 250.

The embryo sac, in part, is shown on the left; the egg cell was at its lower
end, and has grown into the suspensor, with the initial cell at the top. Above,
left, the 8-celled stage of the embryo, with suspensor, shown with protoplas-
mic contents. Below, middle, are cross sections of the globular stages of the
embryo. Right, a nearly formed embryo, with two cotyledons, the sus-
pensor not yet absorbed. (After L. Kny.)

 
 

the foundation for a root develops next the suspensor.
Meantime the endosperm is developing until it surrounds
the growing embryo, the suspensor becoming absorbed ;
and the two together finally fill the embryo sac. This
is the state of the embryo in some seeds (page 374) when
ready for germination, but in other kinds the embryo con-
356 A TEXTBOOK OF BOTANY [Cu. VII, 3

tinues to grow and (by action of digestive enzymes) absorbs
the endosperm, and even the nucellus, so that finally it comes
to fill completely the seed coats and contains all of the seed
food within itself. The shape of the embryo is correlated
with that of the embryo sac, being short and straight in
some, variously elongated and curved or bent in others, often
to extreme degree. Whatever the shape, however, it fol-
lows from the mode of development of the embryo that
the root end of the hypocotyl lies next to the micropyle.

3. Tue DISSEMINATION AND DISPERSAL OF PLANTS

Many features in fruits show obvious connection with the
dissemination of seeds, though fruits are by no means thus
completely explained. Properly, DISSEMINATION means sim-
ply the scattering of seeds from the parent plant, but the
term is often employed more broadly, even to an equivalence
with DISPERSAL, which means the spread of plants over
the earth’s surface. Many parts besides fruits are here
concerned, so that we may best consider the subject as
a unit.

That plants which have in themselves no power of free
locomotion such as animals possess can yet spread very
widely and quickly is shown by the familiar case of weeds,
which, introduced into a new territory, often overrun it
before man becomes aware of the danger. A striking pres-
ent-day instance is familiar in the deadly Chestnut disease,
a Fungus with wind-carried spores, which, introduced from
Asia into eastern America about ten years since, has already
spread through several states. Animals could hardly
spread faster.

The physiological necessity for some method of dissem-
ination is amply obvious, for if all spores or seeds produced
by a plant were to germinate where formed, or on the ground
directly beneath, the resultant congestion would prevent
normal development of any of the plants. Green plants
need room to spread their foliage to the sun, and parasites
Cu. VII, 3} DISSEMINATION OF PLANTS 357

need new host plants; so that plants, in a manner like
animals, must spread in search of food. Accordingly, dis-
semination is not a mere incident of plant life, but a neces-
sary function.

The general methods of plant dissemination and dispersal
fall under some six heads as follows:

1. Independent locomotion. While none of the familiar
land plants have any such power, it is frequent in the lower
Alge. Thus, as will appear more fully in Part II of this
book, some of the simplest kinds work their way over the
bottom of ponds by aid of protoplasmic threads, while others
jerk their bodies along by sudden vibrations. More famil-
iar and typical, however, are the zodsPoRES possessed by
many Algz (page 301, Fig. 213), which can swim by action
of vibratory cilia in a manner so animal-like as to have
originated their name. The Slime Molds, or Myxomy-
cetes (page 38, Fig. 14), living out of the water but in damp
places, can creep over wet surfaces in a manner identical in
aspect and method with that of the animal Amceba.

2. Extension through growth. As described in earlier sec-
tions (pages 187-9), many plants send off stems along the sur-
face of the ground or beneath it, in the forms called stolons,
offsets, runners, and rootstocks, which take root at their
tips and there form new plants, after which the old con-
nection with the parent often withers away. The Straw-
berry offers a familiar and typical example of this mode
of spread; the suckers which spring from the ground in the
vicinity of fruit trees, often from old roots, are other ex-
amples; but it reaches perfection in the Grasses, especially
the familiar Couch Grass of the gardens. Of course all
such plants have likewise a dissemination by their seeds,
their spread by growth-extension being additional and often
incidental. In fact all gradations in this method are
found from cases clearly incidental or accidental up to
those which seem clearly adaptational, in which fact is prob-
ably embodied a leading principle of evolution, viz. that
358 A TEXTBOOK OF BOTANY [Cue VIE

adaptational features frequently, perhaps mostly, arise by
the development of features originally incidental.

3. Waftage by winds. This is the commonest of the
methods of dissemination, and the most efficient for wide
dispersal. Fruits and seeds often develop
a large spread of surface in proportion to
bulk, thus giving a hold to the wind which
wafts the seeds to a distance. Often this
feature is a ‘wing,’ a thin flat plate de-
veloped from the ovary, as in Maple or
Elm (Fig. 252), or from the seed-coat, as
in Bignonia (Fig. 253), or from part of a

gees aa scale, as in Pine (Fig. 249), or from a
of American Elm, bract, as in Linden (Fig. 47). That the
containing one seed; plants here mentioned are all trees is not
enlarged. (From net 3 as
Bailey.) mere coincidence but typical of the fact

that winged fruits or seeds are almost
confined to trees or high-climbing shrubs, while the same
is true of kinds having very flat pods, as in Locusts.
On the other hand, the fruits and seeds of herbs and low-
growing shrubs more often have hairs or plumes, either in
great profusion, as in Cotton (Fig. 254), or in terminal tufts,
as in Milkweed (Fig. 255), or in parachute-like arrange-
ments, as in Dandelion (Fig. 256) ;
or in yet other ways. Such structures
give a hold to the wind, which per-
mits not simply a lateral transport
of the seeds before they reach the
ground, as occurs and suffices in the
case of trees, but a lifting action — Fre. 253. —Seed of Big-
whereby even light breezes raise the ae
seeds to a height whence winds may
varry them far. It is notable that plants of this kind, as
witness the Dandelion, are among the most widespread and
abundant of plants. The plumes are of diverse morpho-
logical origins, being special outgrowths from seed coat

 

 
Cu. VII, 3] DISSEMINATION OF PLANTS 359

(Cotton), ovary (Willow), calyx (Composite), persistent style
(Clematis), and aborted flower stalks (Smoke Bush). Here
also, it is likely, the general principle of modification along
lines of least resistance explains the
particular cases.

Another mode of wind dissemina-
tion is found in the “tumbleweeds.”’
In these the flower cluster, as in some
members of the Parsley Family, or
else the entire plant, as in Russian
Thistle, becomes detached, incurls
the branches, and thus is rolled over
open ground, scattering the seeds as
it travels. Such plants are espe- Pye. 254, — Seed of Cot-

cially characteristic of open plains ton, with the important
long hairs; x +. (From

country. The Rose of Jericho, of Fisurier.)

the Scriptures (Fig. 257), is said to

have this habit, thus giving a concrete meaning to the scrip-
tural phrase ‘‘blown like stubble before the wind.”

It is also the wind which scatters, though indirectly, the
small, rounded, smooth seeds formed in so many kinds of pods
upon long stalks, such as the Poppy (Fig. 239) ; for such seeds
seem to be shaken forcibly
from the pods when struck by
strong gusts in autumn and
winter. It has been claimed
that the pods have such form
and angles of exit as to guide
the seeds well away from the
plant.

Finally (in so far as we can
take space to discuss this phase
of our subject), the wind effects
dissemination of very minute spores or seeds without any
special arrangements. The method rests on the fact that
as a body decreases in size, its bulk diminishes far faster

 

 

Fig. 255. — Seed of Milkweed ;
x t.
360 A TEXTBOOK OF BOTANY (Cu. VII, 3

than its surface, on which account very minute bodies present
so much surface in proportion to their weight that the lightest

 

Fic. 256.— Fruit of
Dandelion, containing
one seed; xX 2. (From
Le Maout and De-
caisne.)

breezes can waft them into the air and
carry them indefinitely. This is why
dust floats in the air, and with the dust
float the very minute spores of innumer-
able plants, of Ferns, Mosses, and many
kinds of Fungi, — Mushrooms, Molds,
Mildews, Blights, and the resting spores
of Yeasts and Bacteria; and thus is ex-
plained the remarkable spread of those
ubiquitous organisms. The seeds of
some tropical Orchids are small enough
to be spread by this method, especially
as their surface is enlarged by presence
of small bladders.

Dissemination is the basis of dispersal,
in which the wind is the most effective

agent. Thus small plumed seeds as well as spores, which
can be lifted by light breezes, are carried over vast terri-
tories by great gales, even from island to island, and conti-

nent to continent.

4. Flotage by water. The currents of rivers and oceans are
of course important agencies of dissemination and dispersal.

This result is often incidental, as when
twigs of Willows, or even small plants
washed out in time of flood, are carricd
down stream and left to take root on some
emerging bank. Many fruits or seeds
ordinarily scattered by wind float also on
water, and thus are earricd by rivers,
which transport likewise the separable
winter buds of many Water weeds. There
are striking cases in which seeds, them-
selves heavicr than water, possess arrange-
ments whereby they are kept buoyed up

stattca); ‘

 

lie. 257. = The
Rose of — Jericho
(Hicrochuntica Ana-
1

(From Bailey.)
Cu. VII, 3] DISSEMINATION OF PLANTS 361

on its surface. Thus the White Water Lily seed has a
loose, air-holding extra coat (Fig. 258) and some Sedges
have loose-textured carpels, which form efficient floats.
Some seeds have flat, corky expansions, as in Iris, while
others float by virtue of the unwettable
surface they possess. The Nelumbium
(the Lotus of ancient literature and art)
has a broad, buoyant receptacle (Fig.
259), out of which the seeds are said to
be dropped from time to time in its
decay. Very striking is the case of the — Fra, 258. — Seed of
Coconut (Fig. 260), in which the ovary Water Lily, with air-
: ‘ holding coat (aril).
develops to a great air-holding husk com- (After Gray.)
posed of material specially resistant to salt
water, with which arrangement is correlated the fact that this
plant has been carried by ocean currents and the winds all
around the tropical seas. Ocean currents, indeed, contest with
winds for the first place among agencies of plant dispersal.
5. Carriage by animals. Very important as agents of
dissemination, and even of dispersal, are animals, which are
effective in two different

 

First, many fruits or seeds
possess structural arrange-
ments by virtue whereof they
are held attached to the bodies
of animals, and are finally
dropped far from their places
of formation. Especially com-
mon are hooks, which become
Fic. 259. — Floating receptacle of entangled in wool or fur,
Lotus (Velumbium) ; x }. where they often remain until

the hair is shed. Such hooks

have diverse morphological origins; they are developed
bracts of the flower head in Burdock (Fig. 261), and out-
growths from the ovary wall in Cocklebur (Fig. 262).

   

Hpi G Nan aa a

Ny
A iy

\ ‘, f | Vy y s

       
362

A TEXTBOOK OF BOTANY

(Cu. VII, 3

Sometimes they are gigantic in size, as in the Martynia of

the western plains (Fig. 263).

 

Fra. 260, — The Coco-
nut, in section; reduced.

It shows the air-hold-
ing husk, the hard shell
(black), endosperm or
“meat ”’ (cross-lined),
and central cavity con-
taining sap, or ‘'milk.”
Below in the endosperm
can be seen the small
embryo, which lies just
under, and comes out in
germination through one
of the “‘eyes.”’ (From Le
Maout and Decaisne.)

Everybody knows how abun-
dantly the weed seeds cling to our
clothes after walks in the fields in the
autumn; and they cling mostly by
hooks. The same result follows from
the presence of adhesive coverings to
fruits or seeds, as especially common in
epiphytes or parasites, for instance,
the Mistletoe. In these cases the
sticky seeds adhere firmly to the feet
of perching birds until brushed off by
contact with rough parts of some
tree, the adhesiveness then serving to
attach the seeds to the tree upon which
they must grow. Adhesive seeds occur
also in some water plants, which thus
become attached to the feet or feathers
of wide-ranging water birds, many of
which travel so widely as to render
those plants cosmopolitan. Thus birds
come next after winds and ocean cur-

rents as agents of plant dispersal.
Second, fleshy fruits, with their edible, palatable pulp and

their bright,

contrasting colors,
easily found and eaten

are
by aninals,

through the bodies of which the seeds,
variously protected against injury from
digestive juices, pass uninjured, and
thus are dropped far from their places
of origin. This seems very clearly the
functional significance of edible colored
fruits in nature, all lines of evidence
converging upon this explanation. In

 

Fic.
Burdock
(From Ixerner.)

261. — Head of
fruits; x 4.

this way the smaller forms of fleshy fruits, especially the
diverse forms of berries and the smaller drupes, are scattered
Cu. VII, 3] DISSEMINATION OF PLANTS 363

chiefly by birds, which are the most important disseminators
of seeds by this method. The larger fruits, Apples, Oranges,
Gourds, and multiple fruits, are eaten in
part by birds and in part by mammals.

The pulp has very diverse morpho-
logical origins, as earlier noted (page 351),
and the protective coats to the seed are
either the seed coat or else a pit formed Fra. 262. — Fruits of
from ovary wall (page 350). The claim oo ee
has been made that the seeds of the
largest fruits (those too large to be swallowed whole) mostly
find their protection in the slipperiness of the coats, which
prevents their being actually swallowed at all, though they
are carried some way with the fruit.

Some seeds have bright colors without pulp, as in Abrus
beans, and those of the common Magnolia; and such are
said to be swallowed by birds, which perhaps mistake them
for something edible, or else simply desire their bright
attractiveness, as trout take the sportsman’s fly. Some
seeds and fruits which bear striking resemblances to insects
are supposed by some ob-
servers to secure dissemi-
nation on that account,
though this is not certain.
Recently it has been claimed
that the many small seeds
provided with CARUNCLES,
z.e. little rounded projec-
tions filled with nutritive
food substances (Fig. 270),
are very effectively dis-
seminated by ants, which
carry off the nutritive bodies, and incidentally the seed
therewith. Also snails and even fishes have been claimed
to carry seeds.

The greatest results in the dissemination and dispersal of

 

 

Fic. 263.— Seed pod of Martynia lutea;
x4. (From Le Maout and Decaisne.)
364 A TEXTBOOK OF BOTANY ICH VIE 3

plants are accomplished by man himself, who carries plants
deliberately all around the world for his own purposes,
and also spreads them accidentally through his commerce
and travel. Much of his deliberate dispersal has effect
only so long as his watchful care persists, for without it the
most of his carefully grown crops would soon be exter-
minated by the return of the native vegetation.

6. Projection by spring-release. In the dissemination of
plants by aid of winds, water currents, and animals, the
fruits and seeds are
wholly passive, and
the results are secured
by the presence of
structures which make
the dissemination an
inevitable result of the
natural and ordinary
operations of those
agencies. There is,
however, one way in
which plants effect an
active dissemination
by forees developed
within themselves, —
namely, in the hurling
of seeds through the
air by the action of suddenly released spring mechanisms.
Thus in Violets (Fig. 264), the pod so ripens that the carpels
press harder and harder upon the smooth seeds held in
an angle between them, until suddenly the pressure over-
comes the friction and the seeds are shot to a distance,
much as one may shoot a smooth bean from between the
pressed fingers. Again, in the Vetches, bands of tissue in
the pod so ripen under tension as to bring such a strain
on the sutures of dehiscence that they suddenly rupture
and shoot the seeds forth in every direction (Fig. 265).

 

Fra. 264. — Seed pods of a Violet, forcibly
projecting the seeds. (From Kerner.)
Cu. VII, 3] DISSEMINATION OF PLANTS 365

The Witch Hazel hurls forth its seeds in similar manner, as
does the Acanthus, the Castor Bean, and the West Indian
“Sand box,” which bursts with a
noise described as like a pistol shot.
In some fruits it is the ripening not
of dry, but of osmotically turgescent
tissues, which produces the explosive
result, as in case of the Impatiens,
called also descriptively ‘‘ Touch-
me-not,’’ and the so-called ‘Squirting
Cucumber”’ of the Levant (Fig. 266).
Many accessory adaptations in
fruits, connected more or less directly
with dissemination, have been de-
scribed by various observers, although
not always accepted as genuine by
others. The Kenilworth Ivy and the
Chinese Primrose both grow in their ee ee Paine
native homes upon steep rocky hill- Lathyrus vernus, bursting
sides or cliffs; both have phototropic @24 throwing its seeds.
(From Kerner.)
flowers, but their seed pods turn
away from light, thus bringing the seeds into clefts of the
rocks, whereby the plants are kept at the same elevations.
Dandelions and
other plants hold
their flower buds
close to the
ground = until
their flowers are
ripe, then raise
them until after
pollination, then
draw them down
once more until
the fruits are

 

 

Fic. 266. — The Squirting Cucumber (E£cbalium :
Elaterium). ripe, and then
366 A TEXTBOOK OF BOTANY [Cu. VII, 4

raise the plumose fruits well up into the winds; and water
plants have analogous ways of drawing their pollinated
flowers under water, and later releasing the ripened fruit or
seeds (page 288). Several minor parts in connection with
dissemination have been ascribed to hygroscopic tissues,
with their power of strong movement (page 237), including
the sudden release of elastically set tissues, certain creeping
movements of fruits along the ground, and others, most of
them probably only incidental.

Nowhere among plants, excepting in connection with cross-
pollination, appear so many features of structure which
have been interpreted as adaptations. Our sketch of the
subject does it little justice, but is all that our space per-
mits. The student can find ample additional detail in the
books especially devoted to this subject.

4. SprcraL Forms anp MOonstTRosiTiEs oF FRUITS

Fruits, like flowers, but unlike leaves, stems, and roots,
seem not to perform functions other than the one which
is primary and typical to them; and conversely there seem
to be no special organs of plants which can be traced to a
morphological origin in a transformed fruit. But of ab-
normalities and monstrosities, fruits show a good many,
mostly having connection with similar features in flowers.
They have largely been treated, however, in earlier sections,
and need only be reviewed at this place.

Mechanically caused effects, simulating monstrosities, are
found in Strawberries or Raspberries, where one side of the
fruit remains hard and green as result of a failure of com-
plete pollination (page 197) ; and an Apple or an Orange dis-
playing a clean-cut segment of different skin is a chimura,
resulting from grafting (page 211). Some twin fruits are
also a product of natural grafting when very young, soft-
tissued, and tightly pressed together (page 196), though some
twin fruits, as in Partridge Berry, are perfectly normal and
usual.
Cra. VII, 5) NATURE OF PLANT DISEASES 367

Among genuine monstrosities, twin fruits, which are rather
common, result from a partial fission of one (Fig. 146 and
page 198). A genuine fasciation, producing several distinct
lobes, occurs, in highly cultivated Strawberries, and also
in Pineapples, as earlier pictured (page 198). Two-storied
fruits also occur as result of proliferation (page 202), though
sometimes a leafy branch occurs in place of the second fruit ;
and the Navel Orange is an imperfect case. Apples and
Cucumbers occasionally produce bracts on their sides (page
202). Sometimes the carpels do not all unite in the pistil,
but some remain separate, as is said to occur rather often in
Oranges. Ears of Corn sometimes have tassels of male
blossoms on their upper ends, and sometimes branch at the
base. And other monstrosities less common are met with
of which some can be propagated.

5. Tue Nature anp CurRE or PLANT DISEASES

Plants are subject to many diseases, which appear perhaps
in fruits more prominently than elsewhere.

Plant diseases, which are studied very thoroughly in agri-
cultural institutions under the name of PHyTOPATHOLOGY,
or simply PatHo.ocy (page 4), fall into three general cate-
gories, — those caused by parasitic Fungi, those of purely
physiological origin, and those of which the nature is still
unknown.

Parasitic Fungi, including Bacteria, cause the great ma-
jority of the worst plant diseases, notably Rusts, Blights,
Smuts, Mildews, Wilts, Rots, Cankers, Crown Galls, Black
Knots, Scabs, Spot and Blotch diseases, and other special
diseased growths, the description of which will be found in
Part II of this book. The very small spores by which these
Fungi reproduce are carried everywhere by winds, and when
thus brought to a suitable host plant they germinate, and
the haustorial threads enter the plant body either through
the stomata, the water pores, or some injury of the surface,
or even at times, though rarely, directly through the epi-
368 A TEXTBOOK OF BOTANY (Cu. VIT, 5

dermis. Then the feeding threads, or mycelia, branch every-
where throughout the tissues of the host, which they pene-
trate by aid of enzymes secreted by their tips; while the
walls and protoplasm and foods thus digested are absorbed
and used in the further growth of the mycelium. Sometimes
the Fungus takes no great tolland the host continues to live;
but often the damage proceeds even to the death of the host.
It is not solely, often indeed not principally, by the robbery
of food that the parasite works the injury, but by poisonous
substances, by-products of its metabolism, released in the
tissues of the host. Thus parasitic Bacteria release into
the bodies of animals, including man, the poisonous toxins
which are the real cause of death from bacterial diseases
(page 173); and in analogous manner parasitic Fungi cause
damage and death to large areas of tissues, with resultant
formation of brown or black spots, which often spread until
the host’s nutrition falls below the necessary minimum, and
it dies. In addition, Fungi also cause a kind of paralysis
of the growth-control mechanism of their hosts, resulting
in the formation of many kinds of monstrosities (page 199),
especially diseased woody growths. In some cases the Fun-
gus attacks parts which are not vital, such as the heart wood
of trees; but others attack the most essential parts, as in
case of the Chestnut disease, where the Fungus destroys the
living cambium. In any case the Fungus sooner or later
sends out to the surface of the host its spore-producing struc-
ture, from which the ripe spores are wafted by the wind, this
“fructification,” as it is sometimes called, being often the only
part of the parasite visible on the outside of the host.

Physiological disturbances include SUN scALDs, where the
sensitive tissues are injured by too much sunlight; @pEMma,
an osmotic disease (page 234); cHLoRosis, or failure of
chlorophyll to develop, producing unhealthy mottled leaves ;
and a number of others. One might almost include death
from old age in this category, although plants differ much
from animals in this relation (page 114).
Cn. VU, 5) NATURE OF PLANT DISEASES 369

Unknown causes, apparently not parasites or any known
kind of physiological disturbance, produce the important
diseases known as Peach Yellows, the Mosaic Disease of
Tobacco, the Curly Top of Sugar Beets, and others, these
names sufficiently describing the characteristic symptoms.
Perhaps a wholly new type of disease-cause awaits dis-
covery.

It is not usual to include insect ravages among plant
diseases, but rather to reckon them in a category of pests,
although their effects come often very close to those of the
true diseases, and they are combated by much the same
methods. Insects do damage in three principal ways:
first, they eat the leaves of plants of which the food-making
power is thus damaged. Second, they bore into and feed
upon fruits or other parts, which thereby are rendered re-
pulsive even if not extensively injured. Third, they lay their
eggs in plant tissues under conditions which promote the
formation of galls, already described (page 203). Insects are
chiefly combated by poisons, either in sprays, as against the
Elm Beetle, or else in gas, as in the fumigation of green-
houses, or of fruit trees under specially made tents.

The great number, variety, and ubiquity of plant diseases,
together with the excessive loss they entail every year upon
agricultural and horticultural interests, have long stimulated
their practical study, with a resultant development of elab-
orate methods for combating them. In the past these
methods have mostly centered in the cffort to find a sub-
stance fatal to the parasite and not to the host, and such is
the principle of spraying plants with solutions like the well-
known Bordeaux mixture. Another method, of which the
importance has more recently become manifest, consists in
determining and applying to plants the conditions requisite
for keeping them in fullest vigor, for the condition of bal-
ance between parasite and host is often such that a healthy
individual can resist while a weakened one cannot. This
principle becomes clear in connection with the ventilation

23B
370 A TEXTBOOK OF BOTANY [Cu. VII, 6

of greenhouses or storehouses for fruits or root crops. Fungi
as a rule, in correspondence with their small size and para-
sitic mode of life within tissues, can stand bad air much bet-
ter than the higher plants, which are far larger, and physi-
ologically adjusted to more space and air. Accordingly,
bad ventilation always gives to Fungi an advantage over
their hosts, which explains why plants rot more freely in
unventilated than ventilated places.

In our own times an attack has been made on the disease
problem along a very different line, viz. the effort to breed
immune varieties. The variability of plants is manifest in
susceptibility to a given disease as in other features, some
individuals of a kind being very susceptible and others much
less so. By a systematic selection of the latter individuals
it is found possible to breed immune races. Though the
problem is by no means so simple in practice as in prin-
ciple, marked success has already attended the method, of
which we are sure to hear more in the future.

6. EcoNomics, AND CULTIVATION, OF FRUITS

The economic uses of fruits, apart from their seeds, which
will be considered later by themselves, center almost wholly
in the food value of the edible kinds, especially berries,
pomes, and gourds. It is notable, however, that fruits as
food are rather luxuries than necessities, having a relatively
sight food value though great palatability. This com-
bination, of course, comports perfectly with their function
of seed dissemination by agency of animals, where the
function does not require that animals shall be nourished,
but only attracted. The dry kinds of fruits have hardly
any uses to man, the opium derived from young Poppy pods
being perhaps the most prominent economic product.

As to the cultivation of fruits, the physiological and struc-
tural methods concerned have mostly been covered inci-
dentally in the earlier sections. Such are the pollination of
flowers by insects to insure fruit setting; pruning to divert
Cu. VII, 6] ECONOMICS OF FRUITS 371

plant energy from superfluous vegetation to optimum fruit
formation ; grafting to preserve and multiply a desirable new
variety or sport; spraying and other like methods, including
the use of poisonous gases, to combat parasitic fungi and
insects; plant breeding for the development of better or im-
mune new kinds; and ventilation, of greenhouses or store-
houses, to aid the larger plants and fruits in resisting dis-
ease Fungi. Fine fruits are commonly wrapped individually
for transport, chiefly to prevent the breaking of the skin,
which not only injures their appearance, but breaks their
first line of defense against Fungi.
CHAPTER VIII
THE MORPHOLOGY AND PHYSIOLOGY OF SEEDS

1. Tue DISTINCTIVE CHARACTERISTICS OF SEEDS

SEEps form the transportation stage by which plants,
otherwise sedentary, are spread outward from their places
of origin. They are relatively small parts, which separate
of themselves from the parent plants, are so constructed that
they may be kept long alive and carried afar, and then
when planted, produce individuals like those which produced
them. Living under diverse conditions, and transported
by various agencies, they differ much in their various ex-
ternal features.

In size, seeds vary from almost dust fine in some tropical
Orchids (much smaller than the scriptural mustard seed),
all the way up to the Coconuts, the mean or average size
lying, however, much nearer the former than the latter
dimensions.

In shape, seeds are most diverse, from approximately
globular to elongated, variously angled, and extremely flat,
the particular shape being apparently connected in some
cases with the method of dissemination, in others with the
shape of the embryo, and in others with less evident factors.
Often the shape is complicated by the development of
special outgrowths, — plumes, hooks, ete., — having con-
nection, as in case of fruits, with dissemination.

In color, seeds are sometimes bright, chiefly red or yellow,
as are some fruits, and in other cases are mottled, sometimes
in ways which simulate the markings of insects; and all of
these features may have connection with dissemination

372
Cu. VIII, 2] MORPHOLOGY OF SEEDS 373

(page 356). Far more common, however, are the simple
brown and gray colors such as all ripening tissues assume,
as a purely chemical incident, where no functional reason
for special color exists. This is the case with all wind- and
water-disseminated seeds, and those thrown by spring
mechanisms.

In texture, seeds are almost invariably hard, sometimes
extremely so, as in the Ivory Palm. The hardness results
from three causes: the almost complete absence of water
from the tissues, the very thick coats, and the frequent
large proportion of cellulose food. They soften very greatly,
however, in germination.

Many seeds are difficult to distinguish externally from
small fruits, especially when the latter are one-seeded.
The difference is purely morphological, consisting in the
presence of the ripened ovary as a kind of additional coat ;
but functionally such one-seeded fruits are identical with
true seeds.

As with other plant organs, there are various structures,
popularly thought to be seeds which are not, such as the
little black, hard-walled bulblets in the axils of Lily leaves
(Fig. 212). There is no such thing as ‘‘ Fern seed,”’ the mis-
named ‘‘fruit dots’? on the under sides of fern fronds con-
taining only spores, a very different kind of structure. Spores
have no embryo, that is, no ready-formed young plant, which
all true seeds contain. Indeed the possession of an em-
bryo is by far the most distinctive mark of a seed.

2. Tue Structure, MorpHoLtoey, AND FUNCTIONS OF
SEEDS

Despite a wide variety in their external features, seeds
possess in common certain primary parts, three and some-
times four in number, which of course are those essential
in their function.

First in importance is the EMBRYO, which is simply a
formed but unborn plant, with its development held for a
374 A TEXTBOOK OF BOTANY  [Cu. VIII, 2

time in suspension. It has always a small unjointed stem,
the HypocotyL, which bears at one end the foundation for
a root, at the other the foundation for a bud, and close
to the latter one or more ‘“‘seed leaves,”
or COTYLEDONS (Figs. 267-9). These
cotyledons, which vary from leaf-thin to
hemispherically thick, are oftenest two,
less frequently one, and sometimes
several, as prevails in the Pine family.
While the cotyledons are commonly
viewed as morphologically leaves, modi-
aieoer = Bas fied by their peculiar position and func-
of Lima Bean, as it tions, there is doubt as to whether they
ee ee oe really originated as leaves, or are the
moval of one cotyle- descendants of special organs by which
kena Oe asia embryos originally absorbed food direct
from the parent plants. The bud _ be-
tween the cotyledons is mostly undeveloped in the seed,
but in some large, well-developed embryos, it produces visible
leaves, in which case it is called the PLUMULE (Fig. 267).
Second in importance is the nutritive Foop, which is sup-
plied by the parent plant, and consists chiefly of starch, oils,
and proteins, in a dry and concentrated condition. In some
kinds this food occurs in a special tissue,

 

 

  

fs eS
called ENDOSPERM, surrounding the em- ( ee
bryo, which therefore lies embedded ee
within it (the so-called aLBUMINOUS

seeds, Fig. 270); but in others the food V

is contained inside the embryo itself, Fis. 268.— Embryo

: ee of Morning Glory, with

mostly in the cotyledons which are then onecotyledonremoved:

conspicuously enlarged ; and the embryo ™ > pene oe ae
ms plumule,

now completely fills the space within the

seed coats (EX-ALBUMINOUS seeds, Fig. 271). The endosperm
originates in the embryo sac simultaneously with the embryo
(page 354), and the two develop step by step together until
they fill the embryo sac, and even (through the absorption
Cu. VIL, 2]

MORPHOLOGY OF SEEDS

375

of the nucellus by the endosperm) all of the space within the

seed coats.
nation of which the embryo absorbs the
endosperm through its cotyledons. In
the ex-albuminous seeds, however, this
absorption of the endosperm occurs
before germination, and this is the mean-
ing of the difference between the two
kinds. It is in correlation with this
further stage of development that ex-
albuminous seeds have so often a
plumule, while albuminous kinds have
only the undeveloped foundation of a
bud.

Third of the parts are the SEED COATS.
Oftenest there is but one, which is thick,
hard, and woody, and has the obvious
function of protecting the embryo against
injury during the period of dissemina-
tion. Sometimes there is also an inner
coat, then usually
membranaceous, and
less often an addi-
tional outer coat,
called an aRiL, which
is generally loose

Such are the albuminous seeds, in the germi-

 

Fig. 269. — Grain of
Corn, in longitudinal
section; < 3.

At the right is the
embryo, showing plu-
mule, primary root,
and hypocotyl. In the
latter can be seen the
fibro-vascular system
extending into the large
SCUTELLUM, which
forms a haustorial or-
gan for absorbing the
endosperm, —ew (looser
texture) and eg (more
compact texture). It
is doubtful whether the
cotyledonisrepresented
by the scutellum, by
the sheath leaf of the
plumule, or by both to-
gether. (From Goebel.)

 

Fig. 270. — Albumi-
nous seed, of Castor
Bean, in section; X 2.
The embryo lies em-
bedded in endosperm ;
below is a caruncle.

from the others and has obvious con-
nection with dissemination, as in cases
earlier mentioned, 7.e. the Yew berries
(page 351) and the Water-lily seeds
(page 361). There is some structural
connection, not yet fully understood,
between these arils and the little insig-
nificant and seemingly functionless swell-

ing called the STROPHIOLE, occurring near the hilum in
some seeds, and the much larger cARUNCLE (Fig. 270), an
376 A TEXTBOOK OF BOTANY (Cu. VIII, 2
appendage which contains nutritive substances apparently
having a functional meaning in connection with dissemi-
nation (page 356).

Seeds show many structural relations
with the ovules from which they develop,
precisely as do fruits with their ovaries,
though it must not be inferred that all
such features in seeds and fruits are
simple persistences of ovule or ovary
characters. It is equally possible that
some have originated in seeds or fruits
and worked back in evolution into ovules
and ovaries.

Every seed shows on its coat a tiny

 

Fie. 271. — Ex-albu-
minous seed, of Apple;
x 4.

The embryo, show-
ing clearly the coty-
ledons and hypocotyl,

fills completely the pit, sometimes differently colored, which
a inside the seed js the persistent though now sealed

MICROPYLE, or opening through which
the pollen tube entered the ovule (page 278). This of
course has no connection with the much larger scar, called
the HiLum, left where the seed breaks away from its stalk
(Fig. 272). Where ovules are turned over on their elongated
stalks, which are grown to the coats (page
272), the arrangement persists, in the seeds,
which show a marked ridge, or raphe. The
position of the chalaza of the ovule often
is manifest in a marked chalazal angle in

 

 

the seed. : 2. — Seed
3 of a pansy; X 5.

Appendages, when present, whether hairs, Below and facing

plumes, hooks, or others, are direct out- 2 the ae is the

; a ulum ; at the point

growths from the seed coat, and have (invisible) is. the

obvious function in connection with dis- ™icropyle; — along

semination, as already discussed (page 356).
Outgrowths of the same kind occur often
on ovaries which contain only a single seed,

in which case one can tell only by dissection whether

ovary wall is present or not.

the side on the left
is the raphe; and
at the top is the
chalazal angle.

an
Cu. VIII, 3] VITALITY OF SEEDS 377

Seeds apparently present no transformations into struc-
tures of other function, and few abnormalities or monstros-
ities. The principal peculiarity of this kind consists in
POLYEMBRYONY (page 302), or the production of more than
one embryo to a seed. The additional embryos have di-
verse morphological origins, resulting oftenest from a budding
of nucellus cells into the embryo sac the structure taking
very perfectly the embryo form; but they grow also from
other cells inside the embryo sacs, and from other embryo
sacs contained in the same nucellus. The embryos them-
selves often show a monstrosity in POLYCOTYLEDONY, the
production of cotyledons in more than the normal number.

3. THE SUSPENSION oF VITALITY, RESTING PERIOD, AND
Duration oF Lire IN SEEDS

The primary seed function of serving as the disseminative
stage of the plant involves a number of physiological features,
of which the more prominent are indicated in the foregoing
title.

The value, or necessity, of a SUSPENSION OF VITALITY
during dissemination is qu:te obvious, since the embryo
plant while in transit, and hence for considerable periods of
time, is perforce exposed to great dryness, intense light,
destructive chemicals, etc.; and these conditions are in-
consistent with that continuous interchange of oxygen,
water, and food essential to the ordinary life of plants.
As to the actual physical method by which the suspension
of vitality is insured in seeds, that seems to rest primarily
upon dryness, the greater part of the water being allowed
to escape without replacement during the ripening of the
seed. Since water is the indispensable solvent for chemical,
and the vehicle for physical, operations underlying growth and
other processes, its gradual withdrawal slows the processes
down, apparently evenly and without injury, until finally
a point is reached at which they are barely in action, —
precisely as engines may be slowed, by withholding of power,
378 A TEXTBOOK OF BOTANY  [Cu. VIII, 3

to a scarcely perceptible motion. Indeed, so slow are the
life processes in ordinary dry seeds that, as tested by the
most important and typical process of them all, viz. res-
piration, they are not actually demonstrable by even the
very refined methods of research which have been applied
to the problem. Accordingly some investigators have main-
tained that the processes are actually suspended, as an en-
gine may be stopped, all ready to start again when suitable
conditions are supplied. But the collective evidence, in-
direct as well as direct, seems rather to indicate that the
processes never stop completely so long as the seed remains
capable of germination.

The extent to which the conditions of life in seeds differ
from those of ordinary active life is attested by the extremes
of temperature they can endure without injury. Thus well-
dried seeds can be kept some time above the boiling point
of water (100° C.) without damage, though active embryos
would be killed very quickly by an exposure to only 60° C.
Again, seeds have been kept for days surrounded by liquid
air, at a temperature of — 194° C., and then have germinated
freely, though active embryos would perish at 0° C. And
seeds can endure some other untoward agencies in like man-
ner. It is the same with the thick-walled resting spores of
Fungi and Bacteria.

The RESTING PERIOD, also called DELAYED GERMINATION,
of seeds, is less familiar, but equally important. Some seeds
of wild plants will germinate as soon as mature, if given
favorable conditions of moisture and warmth; most. kinds,
however, first remain quiescent for days, weeks, months, or
even years. [Hssentially the same phenomenon appears in
the buds of trees and shrubs, for if twigs are brought into the
warm greenhouse and placed in water, most buds will not
start at all before February, though later, under precisely
the same treatment, they will open and display their
flowers to perfection. Bulbs and tubers (e.g. potatoes) act
in a similar manner. It is true that some individual flower
Cu. VIII, 3] VITALITY OF SEEDS 379

buds, like some individual seeds, will start in the fall; but
such cases are clearly abnormalities or variations, due to
failure of the control mechanism to operate (page 342) ; and
the result is always fatal. It is thus evident that the resting
period is not simply an incident of seed and bud life, but is
obligatory, so to speak, under natural conditions, though it
can be shortened artificially in a good many cases. The
functional value, or necessity, of the resting period is obvious,
since it tends to prevent the germination of seeds and open-
ing of buds in warm times of late autumn or winter, when sub-
sequent freezing must inevitably kill the new growth. As to
the physical basis of the resting period (the method by which
it is enforced on the seed), that seems to be diverse. In
some cases it is known to depend upon the embryo, con-
sisting in a slow ‘“‘after-ripening,” 7.e. formation of enzymes,
acids, or other essential substances; but in other cases it
has been proven to depend upon the character of the seed
coats, which are so constructed as to prevent the admission
of oxygen or of water, both indispensable to germination, —
the inhibition continuing until the coats are ruptured by de-
cay. It is of course a necessary corollary of this explanation
that in such cases germination will be prompt if the seed
coats are artificially broken ; and such is found by experiment
to be true and has long been known to nurserymen and
gardeners. Thus, they break Peach pits with a hammer, open
Canna seeds with a file, and bruise or break the coats of
others in diverse ways, thereby greatly hastening the germi-
nation of those kinds.

While the seeds of most plants have a resting period,
cultivated plants seem mostly to lack it. Thus, we grow
Corn, Beans, Peas, and other crop plants in our laboratories
in autumn from seeds of that summer. This peculiarity,
indeed, sometimes brings loss to the farmer, since in excep-
tionally warm wet autumns, grain is apt to germinate in the
ear in the standing crop, to its very great damage. The
resting period has presumably been lost from cultivated
380 A TEXTBOOK OF BOTANY  [Cz. VIII, 3

plants through its complete disuse during the many cen-
turies of their cultivation by man, who has attended to the
safety of the crop himself and directed his selection to quite
other qualities.

The DURATION OF LIFE, or VIABILITY, in seeds is most
various. Every one who works with a garden knows that
some kinds keep good for only one season, while others last
two or three; and methods exist for testing the viability in
cases of doubt. There are kinds which must germinate the
summer they are formed, or not at all; and this is true of
Elm, Willow, and Poplar, — trees which form their seed
early in spring. Most kinds, however, wild as well as cul-
tivated, if kept dry and cool, remain viable for one, or two,
perhaps three years, though beyond that period the number
of kinds which survive steadily wanes with advancing years.
Tests made on seeds taken from dated museum or herba-
rium collections have shown indubitable germination in seeds
eighty-seven years old, with a possible case over one hun-
dred and twenty years. It is interesting to note, by the
way, that these extreme longevities occur in seeds possessing
thick hard coats. As to the reported germination of seeds
taken from the wrapping of mummies, or from ancient tombs,
hundreds or thousands of years old, it is not confirmed by
the exact methods of science, while on the other hand there
is ample evidence that seeds are often introduced fraudu-
lently into such places.

What then actually ends the viability of such seeds? If
they can live so long in the inert state, why not indefinitely ?
The very fact, by the way, that all die, and mostly within a
few years, is presumptive evidence for the view that the life
processes are not in suspension, but only slowed down. The
death of the seed comes gradually, and without any visible
external sign, in most cases at least; and it clearly is not
due to exhaustion of food or like kind of cause. Here, how-
ever, our knowledge ends. Possibly the loss of water can
proceed to a fatal degree; perhaps the accumulation of waste
Cu. VIII, 4] GERMINATION OF SEEDS 381

products of the slow metabolism within the tightly-sealed
seed coats poisons the embryo; and it may be that the slow
coagulation of the proteins destroys the essential constitu-
tion of the protoplasm. Between these possibilities, and
perhaps others, the future will decide.

4. Tur GERMINATION OF SEEDS

The seed, its resting period completed, germinates on
access of water, air, and warmth. The water it needs to
expand its parts; the air is necessary for its respiration;
which is very active in all growth; the warmth is required
to accelerate the many physical and chemical processes in-
volved. As to light, that has no influence, direct or indirect,
in most cases, though special seeds are known which will not
germinate in light, and others which will not germinate with-
out it, doubtless for reasons incidental to some peculiarity
of their metabolism.

In germination we can distinguish some seven stages.
First, most seeds, though not all, swell greatly throughout,
often to more than double their dry size, by absorption
of water, which enters partly by imbibition and partly by
osmosis. As these words imply, the absorption is forcible,
and thus seeds can lift considerable weights in the ground or
break strong containers under experiment.

Second, the seed coats are broken, no matter how thick
and strong, by the pressure from within. In some the
rupture is irregular; in others, it follows definite lines cor-
responding with angles or depressions of the coats. Some
very striking special arrangements to this end are known
(Fig. 273).

Third, the digestion of the food substances begins. The
insoluble starches, oils, and proteins are converted by en-
zymes into soluble sugars, fatty acids, and peptones, as
manifest to the eye in the change from opacity to trans-
lucency, and a softening of the seed. Then the digested food,
absorbed by the cotyledons in albuminous seeds, though
382 A TEXTBOOK OF BOTANY  [Cu. VIII, 4

already within them in ex-albuminous kinds, is ready for
translocation, and use in the growing parts of the embryo.

Fourth, the end of the hypocotyl of the embryo, lying next
the micropyle, now pushes forth, and as soon as clear of the
seed coats, grows geotropically over to point downward,
developing meantime the root at its tip. This root is a
new growth, and not a transformation of the hypocotyl, as
students are prone to suppose. Then, if the seed, as is
usual with wild plants, lies on the sur-
face of the ground, the root begins to
enter the earth. No sooner does the root
start into the soil than (from small seeds
at least) it sends out a radiating ring or
collar of root hairs which take firm hold
on the rock particles. Thus is provided
a resistance, without which further
growth might rather lift the seed from
the ground than force the root into the

ae ee soil. In some other seeds, such as Flax,
nating seed of Pump- such resistance is provided by a muci-
sy ee “pee” laginous coat which gums it, so to speak,
opment of which the tothe ground. Practically all embryos,
seed coat is forced open. gs the first act of their development,
(From F. Darwin.)

thus secure access to the water supply
which is indispensable to their further development.

Fifth, on the basis of the anchorage secured by the pene-
tration of the root into the earth, the hypocotyl now begins
to make such growth movements, too complex for easy de-
scription but readily shown in our pictures (Fig. 27+), as
cause the withdrawal of the cotyledons from the seed coats,
and their subsequent elevation, when they open out to the
light. In cases, however, like Peas and some Beans, where
the cotyledons are apparently too thick to serve later as
effective foliage leaves, they remain in the ground, while
the plumule issues from between them, and grows geotropi-
cally upward.

 
Cu. VIII, 4] GERMINATION OF SEEDS 383

 

 

Successive stages in the germination of the Lima Bean, from the seed to the fully germinated embryo ; x 4.

OU)

Fic.
384 A TEXTBOOK OF BOTANY  [Cu. VIII, 4

Sith, the parts which rise in the light, especially the
cotyledons and plumule, as they issue from the seed coats,
begin to turn green, and, by the time they are spread open
at the top of the young stem, have their full quota of chloro-
phyll, in obvious preparation for the manufacture of new
food.

Seventh, the enlargement of hypocotyl, cotyledons, and
plumule proceeds by absorption of water until all of the
cells laid down in the embryo are fully expanded, at which
time, with the root firmly fixed in the ground, the young
stem is erect with the cotyledons fully green and expanded.
Germination is now complete, and the germinated embryo
is ready to continue development, with formation of new
parts, into a seedling. It is true, the formation of new
leaves and buds does not always await the completion of
the expansion of embryonic parts, but in principle at least
there is this distinction between germination and the sub-
sequent growth of the seedling.

If a fully germinated embryo be compared point by
point with one from a resting seed, as may best be done with
some of the compact succulent kinds like Cactus, the fol-
lowing differences appear. First, except for the root and the
chlorophyll, the germinated embryo possesses nothing really
new. Second, it has become many times larger, even to
twenty or thirty times. Third, again excepting the root, it
has usually few new cells, the enlargement having consisted
chiefly in the increase in size of those already developed.
Fourth, the cells are now all apparently empty (except for
water) instead of densely packed with solid food, thus
explaining the watery translucency of the germinated
embryo as contrasted with the white opacity of its unger-
minated condition. Fifth, its dry weight, determined by
comparative weighings of oven-dried material, is actually
less, showing that the far greater bulk consists chiefly of
water. Thus it is clear that germination consists primarily
in the great expansion through water absorption of the
Cu. VIII, 5] ECONOMICS OF SEEDS 385

close-packed cells of the original embryo, the food being
used partly in the formation of the root and partly in the
enlargement of cell walls. Evidently the functional point
of the process is found in the great spread of green surface
thus quickly achieved by the use of a relatively small amount
of solid material. The value of the spread of surface in this
case is obvious, for the young plant has to begin as early as
possible the acquisition of its own photosynthetic food supply.

5. Tur Economics AND CULTIVATION OF SEEDS

Among all of the parts of plants, seeds stand preéminent
in direct utility to man. This of course is because they
include the grains, Corn, Wheat, Rice, Barley, Rye, and some
others, together with the leguminous crops, Beans, Peas,
Millet, which collectively make up the greater part of the
food supply of mankind. These seeds contain rich stores of
starches, oils, and proteins, originally laid down by plants
for the use of their embryos, and now taken for his needs by
man, who has been able through long centuries of cultivation
and breeding to greatly increase their yield both in quantity
and quality. Of a different kind is one other great economic
use of seeds, viz., the fibrous hairs developed by the Cotton
seed as its disseminative mechanism (by wind) yield the
cotton of commerce (Fig. 254).

The grains, as earlier noted (page 349), are fruits as well
as seeds, the seed coat and ovary wall being grown together
into one structure which constitutes the husk. The husks
are removed in milling white flour, but retained in graham
flour, which is the more nutritious because it includes the
layer of protein-storing cells which form the outermost part
of the food in the grain (Fig. 65).

The agricultural and horticultural treatment of seeds
appears to offer nothing peculiar, the various principles of
cultivation and breeding being the same as with other parts.
There is, however, one economic matter peculiar to seeds,
in connection with their viability. Since nothing in the

2c
3386 A TEXTBOOK OF BOTANY  [Cu. VIII, 6

aspect of a seed tells whether it is still alive or not, or what
percentage of a given quantity is alive, the purchaser of seeds
is at the mercy of a dealer unless he can himself make test of
viability. For such tests various methods have been devised,
the most simple and direct of which is that of placing a
given number in folds of blotting paper kept wet, dark, and
well aérated, and noting the percentage which germinates.

6. Tue Cycite or DEVELOPMENT FROM SEED TO SEED

Having studied the six primary parts of plants with respect
to their structures and functions, it remains to consider
their successive appearance in that cycle of development
through which every individual passes. It is possible to
break the cycle for study at any desired point, but in prac-
tice we may best start with the germinating seed. The facts
having already been considered in detail, we can best review
the subject in a way to bring out its general principles.

The seed contains a well-formed embryo plant, provided
with stem, rudiments of root and bud, and cotyledonary
leaves, all enwrapped with a store of food substance inside
protective coats. In germination the seed absorbs water,
swells, and bursts the coats; the stem pushes forth its
lower end, which grows over geotropically downward
and enters the ground. Meantime its tip is developing a
root, which, on contact with the soil, puts forth many root
hairs, whereby it absorbs osmotically a sufficiency of water.
No sooner is the root secure in the ground than the stem
makes growth bendings which first withdraw the cotyledons
from the seed coats, and then lift them geotropically upward
until they open out to the light on the tip of the vertically
straightened stem. Meantime the whole plant is swelling
rapidly in size through absorption of water, and turning
green over stem and leaves by formation of the chlorophyll
so essential to its future welfare. Thus the fully GERMINATED
EMBRYO how stands rooted in the ground and erect in the
sun, to which it spreads a large surface of chlorophyll. In
Cu. VIL, 6] CYCLE OF DEVELOPMENT 387

this process all of the food supplied by the parent plant has
been used; and thenceforth the new plant must depend en-
tirely upon its own physiological powers, for the exercise
of which, however, it is now fully prepared.

The successive stages in the developmental cycle of plants,
while distinct in principle, largely overlap in practice, so
that even before the completion of germination, the young
plant has commenced the activities of its next, or seedling,
stage. With the spread of its chlorophyll in light, it begins
to acquire a new food supply of its own, which forms a
basis for further development. The root now begins to send
out branches, diageotropically guided either horizontally or
at definite angles from the vertical main root, though these
directions of growth are soon disarranged by obstructions in
the soil. Meantime the plumule bud, between the cotyledons,
is continuing its development, forming in symmetrical order
new leaves, which, at first small and tightly appressed to
the stem, later gradually open out until they present their
full faces to the sun. Simultaneously there is continuous
increase in size, and the formation of suitable firm support-
ing and other needed tissues. Thus is attained the stage of
the SEEDLING.

Gradually the seedling passes into a stage which in case
of trees is called the saptine. In the roots new branches
spring from the secondary roots, not at definite places or
angles, but guided hydrotropically and chemotropically
towards the moistest and richest parts of the soil, where
they develop more profusely, thus making the root system as
asymmetrical as the soil is irregular in texture. Meantime,
while the leaves are still in the embryonic stage, new buds
develop in their axils, and later, after those leaves have
passed their maturity and fallen, grow out into branches
which bear new leaves in precisely the same manner as does
the main stem. These branches, guided diageotropically,
grow out at definite angles with the vertical main trunk,
and, possessing also the same symmetrical phyllotactic ar-
388 A TEXTBOOK OF BOTANY (Cu. VIII, 6

rangement as the leaves, tend to build stem-and-leaf structures
very symmetrical in plan. Meantime also the special tissues
which give strength and meet other needs are continuing to
develop in places required by stress or other demand.

In this stage appears the striking seasonal cycle imposed
on all plants outside of the tropics by the extreme alter-
nation between summer and winter. The swmmer alone has
the warmth to permit full vital activity in plants, and ac-
cordingly is the season of green vegetation, accumulation of
food, and development of new parts. In the autwmn prep-
aration is made for the winter, and accordingly that is the
season when fruits are ripened, buds are enwrapped in their
scales, leaves are cut off and dropped, and tissues are par-
tially dried; while the attractive colors of fruits and the
varied hues of dying leaves make it a time of bright color
in vegetation. The winter is the season of enforced dor-
mance, when the dried tissues of plants, approaching the
conditions in seeds, remain almost inactive within their
nearly sealed wrappings, which display no colors other than
their incidental grays or browns. The spring is the season
of unfolding, when the ready-formed parts, amply supplied
with stored food, absorb copious water, enlarge, burst their
wrappings, and push forth green leaves to make new food,
and bright flowers to effect fertilization; and all vegetation
wears the soft colors of the new-forming tissues. This is
the four-part seasonal cycle through which our perennial
plants pass every year as long as they live.

The next stage of the developmental cycle is the ADULT.
It is not distinguished from the sapling by attainment of any
fixed size, for plants (unlike animals) continue to grow, by
formation of new parts, as long as they live. Nor is it
marked by any change in the mode of formation of roots,
buds, or leaves, which continue to be made in the same gen-
eral way. It is true, a gradual loss of the youthful sym-
metry accompanies advancing age in trees and_ shrubs,
partly because of the interference of the over-plentiful
Cu. VIII, 6} CYCLE OF DEVELOPMENT 389

branches with one another, partly because of accidents, and
partly because of phototropic and other self-adjustments.
The real mark of adult age is the beginning of sexual repro-
duction. After the young plant has attained a considerable
growth, presumably accumulating food in reserve, some
of the axillary buds, precisely alike in position and mode of
formation to those which have been producing leafy branches,
begin to produce flowers,—that is, specialized determinate
branches containing reproductive spores which develop the
sexual cells. As to the nature of the stimulus which leads
the plant thus suddenly to convert certain of its branch
buds into flower buds, or more exactly, to develop reproduc-
tive spores with the correlated floral structures, we have as
yet no exact knowledge, although the influence of various
external factors is clearly apparent. Having once begun to
produce the flowers, the plant continues to make them, just
as it makes leaves, branches, and roots, as long as it lives.
The central parts of these flowers are pollen grains and em-
bryo sacs, which in turn develop the two kinds of sex cells.

The next stage in the cycle includes fertilization. The
floral parts are essentially organs functionally fitted to effect
union of the sex cells, — and a union usually between two
different parental strains. By utilization of the motive
power of winds, insects, etc., the pollen containing the sperm
cell is transported from its place of formation to the vicinity
of the deeply-buried egg cell, after which the growth of a
pollen tube brings egg cell and sperm cell together into a
single FERTILIZED EGG CELL.

The next stage is that of the development of the fertilized
egg cell into an embryo. The sngle cell, lying in the
embryo sac, begins at once to divide and to grow, then
divides again and grows farther, and thus, under guidance
of influences partly hereditary and partly environmental,
it gradually assumes the form of the many-celled embryo,
with its stem and cotyledons. Meantime the endosperm or
food substance is forming around the embryo, and the hard
390 A TEXTBOOK OF BOTANY [Cu. VIII, 6

seed coats are developing around both. Thus is reached the
stage of the fully formed empBryo within the seed.

The final stage is that of dissemination, performed by the
sEED. A considerable time often elapsing either before
transport or during that process, with simultaneous
exposure to extreme conditions, the seed goes into a resting
condition with all of its processes reduced to a minimum,
and with provision against premature germination. Then,
separating from the parent plant, it becomes transported
by wind, animals, or other locomotive agency, acting upon suit-
ably developed mechanisms, to a distance sufficient to per-
mit the free development of its plant without interference
with the parent. Having attained a suitable place, its
resting period ended, and water, air, and warmth sup-
plied, the seed germinates. But with germination the
cycle is closed. If the term cycle seem inappropriate, since
the return is not to the same seed, then the simile of the
spiral, winding back to the same starting line, may better
express the process,
INDEX

Figures in heavy type indicate pages on which illustrations occur.

Abnormalities, 196.

Abrus, 363.

Absciss-layer, 120.

Absorption, 262; by roots, 224,

Adaptation, 12.

Adhesive seeds, 362.

Adult, 388.

Aération system, 132, 266.

Aérenchyma, 252, 266.

Aérial roots, 254, 256.

Aérotropism, 232, 248.

Astivation, 329.

After-ripening, 379.

Agriculture, 4.

Air, in soils, 240.

Air-passages, 19, 29.

Air system, 33.

Akene, 348, 349.

Alcohol, production,
101.

Alge, 12; Red, 305.

Alkaloids, 109.

Alternation of generations, 301.

Alveolar, 37.

Anatomy, 3, 8.

Anchorage by roots, 231.

Animals, nutrition, 86; seed carriage,
361.

Annual rings, 124.

Annuals, 114.

Anoxyscope, 167, 168.

Anther, 272.

Antheridium, 306.

Anthocyanin, 88; composition, 108/

Antitoxin, 173.

Ants, in dissemination, 363.

Apogeotropic, 247.

Appendages, 376.

Archegonium, 306.

Areas of chlorenchyma, 32.

Aril, 351, 375.

Aristolochia, anatomy, 129.

Asexual spores, 301.

172; source,

 

Asexual vs. sexual propagation, 302.

Asparagus, fasciated, 197.

Automatism, 39.

Autumn coloration, 88;
external conditions, 93.

Auxograph, 155, 156; record, 156.

effect of

Bacteria, 84, 244, 368; nitrifying,
244; nutrition, 84; in soils, 244,

Bacteriology, 4.

Bailey, L. H., Cyclopedia, 60.

Bald Cypress, 252.

Balfour, Class Book, 58.

Bamboo, 127, 179.

Banana, 58.

Banyan, 253, 254.

Bark, abscission, 123.

Barton, Botany, 75.

Bast, 130; fibers, 131, 265; paren-
chyma, 131.

Beet rings, 256.

Begonia phyllomaniaca, 204.

Berry, 350.

Biennials, 114.

Bignonia seed, 358.

Bird’s-eye Maple, 198.

Birds, as cross-pollinators, 294;
dissemination, 363.

Black Knots, 367.

Bleeding, 151, 227.

Blights, 367.

Blood heat, 169.

Blotch diseases, 367.

Bordeaux mixture, 369.

Botany, definition, 1;
subdivisions, 2.

Bracket, on stems, 182.

Bract, 73, 271, 276; colored, 74; of
Linden, 74; in Poinsettia, 74.

Branch, 183.

Bryophyllum, 71; 299.

Bryophytes, 11; low growth of, 144.

Bud, accessory, 137; adventitious,

in

study, 2;

391
392

137; anatomy, 138; axillary, 137;
defined, 135; on leaves, 71; of
Palm, 136; scales, 78, 80; sepa-
rable, 300; sizes, 136; sport, 209;
terminal, 137; unregulated de-
velopment, 198; winter, 135.

Bulb, 73, 300; forms, 74.

Bulblet, 373.

Bundle-sheath, 30.

Burbank, 321.

Burdock head, 362.

Burls, 199, 200.

Bursting pods, 365.

Bitschli, 38.

Button Bush, 336.

Cabinet woods, 205.
Cactus, 189.
Caffein, 109.
Caloriscope, 170.
Calyx, 269, 351.
Cambium, 118;
growth from, 124.
Camphor, 108.
Cankers, 367.
Caoutchouc, 108.
Capillarity, 148, 237.
Carbohydrates, 100; value, 100.
Carbon dioxide, absorption by plants,
22.
Carotin, 90; composition, 108.
Carpel, 273, 351.
Caruncle, 363, 375.
Cavers, Botany, 147.
Cedar apples, 347.

described, 132;

Cell, contents, 42; definition, 8;
division, 281, 283; initial, 355;
sap, 30; shapes, 42; structure,
41; wall, thickened, 103.

Cellulose, 41; composition, 98;
uses, 99.

Cement, in trees, 211.
Central cylinder, 264.
Chalaza, 274.
Chalazal angle, 376.
Chemosynthesis, 87.
Chemotropism, 249.
Chestnut disease, 356.
Chimera, 210, 366.
Chlorenchyma, 17, 29, 262;
32; thickness, 53.
Chlorophyll, 17, 108, 386; composi-
tion, 108 ; function, 25; spread, 387.

areas,

INDEX

| Chloroplastids, 30.

Chlorosis, 368.

Chondriosomes, 41.

Chromatin, 280.

Chromosome mechanism of heredity,
310.

Chromosomes, 280;
significance, 282.
Chrysanthemum, 318.

Cion, 208.

Cladophylla, 196.

Clambering stems, 184.

Classification, 2, 10.

Cleistogamous flowers, 290.

Cleistogamy, 292.

Clematis fruit, 350.

Climbers, 9, 184.

Clinostat, 174, 176.

Close-pollination, 287.

Cluster, 268; of flowers, 335.

Cocaine, 109.

Cocklebur fruit, 363.

Coconut, 345, 361, 362, 372.

Collenchyma, 118, 265.

Colors of leaves, 88, 90; brown, 92;
green, 88; non-green, 88; red, 88;
white, 90; yellow, 89.

Columbine, 295; pods, 347.

Companion cell, 131.

Compass plants, 58.

Conduction, 262; of carbohydrates,
152; of proteins, 152.

Cone, 352; 353.

Constriction of stems, 152.

Conventional constant, 25.

Convolute, 329.

Copper Beech, 319.

Cordage, 206.

Cordyline, 65.

diagram, 282;

Cork, 261; cambium, 264; de-
scribed, 133; uses, 99.
Corm, 191.

Corn bundle, 185; stem, 119.

Corolla, 270.

Corona, 82, 332, 333.

Cortex, of roots, 220.

Cortical system, 262.

Corymb, 336, 337.

Cotton, 205; seed, 359.

Cotyledons, 73, 355, 374.

Crested forms, 197.

Cross-pollination, 286, 287; meaning,
298,

 
INDEX

Crown Galls, 367.
Cryptogams, 12.
Crystals, 33; in plants, 111.
Curly Birch, 200.
Curly Top, 369.
Cutin, 32, 98.
Cuttings, 259.
Cyme, 334.
Cypripedium, 292.
Cytisus Adami, 210.
Cytology, 3.
Cytoplasm, 41.

Daffodil, 333.

Dandelion fruit, 360.

Darwin, 316.

Darwin, F., 120.

Decay, nature, 172.

Dehiscence, 346, 347.

Dermal system, 262.

Dermatogen, 264.

Desert vegetation, 48.

Determiner, 309.

Development, 153;
described, 154.

De Vries, 316.

Dextrose, 100.

Diageotropic, 247.

Dichogamous flower, 288.

Dichogamy, 291.

Differential thermostat, 157.

Diffusion, 236; described, 236.

Dimorphic flowers, 289.

Dimorphism, 292.

Dicecious plants, 307.

Disbudding, 207.

Diseases of plants, nature, 367.

Dispersal, 356.

Dissemination, 266, 356.

Distillation, 172.

Division, 39, 299.

Dodder, 83, 84, 256.

Dodel-Port, 277.

Dominant, 311.

Double fertilization, 354.

Dragon tree, 115, 127, 128.

Drainage, 167.

Drip point, 68, 69.

Drupe, 350, 351.

Dry farming, 261.

Duct, 31, 122, 130, 146, 262; length,
146.

Duggar, Physiology, 123.

cycle, 9, 386;

 

393

Durian, 346.
Dust, on plants, 96.

Ecology, 4.

Economic botany, 4.

EKeg, 280.

Egg cell, 9, 273, 274, 278, 304; fer-
tilized, 389.

Elementary species, 317.

Elements essential to plants, 230.

Elm fruit, 358.

Embryo, 9, 373, 374; development,
355; germinated, 384, 386; plant,
386; sac, 274.

Embryology, 3.

Emergences, 33.

Endodermis, 222, 262.

Endogenous, 127; growth, 128.

Endosperm, 374.

Energy, kinetic, 166; potential, 166.

Enlargement, 153; described, 154.

Enzyme, 85; description, 110.

Epidermal cells, 32.

Epidermis, 18, 29, 261; cells, 32.

Epiphyte, 9, 185; funnel form,
185.

Epiphytic, Fern, 186; Orchid, 184.

Erica leaf, 70.

Errera and Laurent, 160.

Erythrophyll, 88; formation,
functions, 88.

Essences, 108.

Essential oils, 107.

Evolution, 13, 308, 315.

Excretion, 266.

Exogenous, 126.

Extension through growth, 357.

9s:

Fall plowing, 260.

Fallow, 260.

Fasciated Asparagus, 197; Echino-
cactus, 198; Pineapple, 198.

Fasciation, 197, 367.

Fatty oil, 104; as food, 104; kinds,
104.

Fermentation, 169; demonstration,
171; equation, 171.

Fern, plants, 11; reproduction, 306 ;
seed, 373.

Fertilization, 277, 279; double, 354;
significance, 286.

Fertilizers, rédle, 242; use, 260.

Fibonacci series, 142. -
394

Fibro-vascular, bundles, 116, 118;
system, 118.

Fig, 352.

Figurier, Vegetable World, 73.

Filament, 272.

Films, of water, 239.

Fir tree, 180. _

Fleshy fruits, dissemination, 362.

Flora, 3.

Floral diagrams, horizontal, 326, 328,
329; numerical plans, 328; verti-
cal, 331; the whorls, 326.

Flower, cleistogamous, 290; colors,
267; complete, 276; dichogamous,
288; dimorphic, 289, 292; dura-
tion, 269; disk, 339; economics,
343; features, 267; function, 8;
geotropism, 297; hermaphrodite,
307; insect pollinated, 290; ir-
regular, 276, 293; monstrosities,
340; morphology, 183, 322; neu-

tral, 338, 339; odors, 268; per-
fect, 276, 307; phototropism,
296; pistillate, 276, 285; polli-
nated by bee, 291; preservation,
344; ray, 338; regular, 276;
staminate, 276, 285; structure,

269; typical, 270; wind-pollinated,
288.

Flowering plants, 10.

Fluctuations, 314.

Fodder, constituents, 101; plants,
206.
Foliage, autumnal coloration, 90;

support, 265; variegated, 90.

Follicle, 348.

Food, 28, 374; of animals, 112;
reserve, 100; synthesis, 19.

Forestry, 4, 205.

Freaks, 72, 196.

Frost plant, 52.

Fructification, 347.

Fructose, 100.

Fruit, acids, 110; aggregate, 352;
defined, 345; dehiscence, 346;
dots, 324, 373; dry, 345; econom-
ics, 370; features, 345; forma-
tion stimulus, 352; fleshy, 345;
function, 8; monstrosities, 366;

morphology, 347; multiple, 352;
relation to ovary, 345; simple, 352;
spurs, 183; twin, 196, 199; two-
storied, 367.

 

INDEX

Fuchsia, 332.

Fucus, 189.

Fungi, 11, 84; colors, 86; damage
by, 368; nutrition, 84; parasitic,
367; in soils, 244.

Fusion nucleus, 353.

Fusion of germ cells, 280.

Galls, described, 203 ; typical, 204.
Gamete, 287, 303.

Gamopetalous, 271.
Gamosepalous, 270, 330.
Gelatination, 99.

Gemme, 300.

Generation, skipping a, 311.
Genetic variations, 314.

Genetics, 4.

Genotypically, 310.

Geotropism, 174, 175, 296; function

of, 177; lateral, 255; of Mush-
rooms, 178; of roots, 174; of
stems, 175.

Gerardia, Purple, 87.

Germ cell, 280; fusion, 280; purity
of, 311.

Germination, 381; delayed, 378;
of Lima Bean, 383; movements,
382; of mummy seeds, 380;
of pollen, 275; stages, 381.

Giant Kelp, 190.

Gland, ethereal oil, 107.

Globulin, 105.

Glucose, 100.

Glucoside, 110.

Glutelin, 105.

Gnarls, 199.

Goebel, Schilderungen, 62.

Gourd, 351.

Graft-hybrids, 210.

Graftage, 208.

Grafting, 208, 209, 371; results, 210.

Graham flour, 385.

Grain, 317, 385; Corn, 375;
portance, 385; structure, 349.

Grand period, 156, 157; described,
156.

Grape sugar, formula,
plant, 27.

Gravitation, effects on plants, 175.

Gray, Botany, 16.

Greenhouse construction, 95.

Green-manuring, 260.

Greenness of vegetation, 26,

im-

21; réle in
INDEX

Growth, 39, 264; definite annual,
138; described, 153; control
mechanism, 342 ; effect of humidity
on, 158; effect of light on, 158,
159; effect of temperature on,

157, 158; of general tissue,
354; grand period in roots, 221;
indefinite annual, 138; of leaves,
161; primary, 119; of roots,
161; secondary, 119; of stems,
160.

Guard cells, 33, 49, 50, 262; oper-
ation of, 49.

Gum, 104.

Gum tree, 113.

Guttation, 52.

Gymnosperm, 352.

Haberlandt, Anatomy, 31.

Hair-like structures, 351.

Hairs, 70.

Half parasite, 87.

Haustorium, 83, 256.

Head, 336, 337.

Healing of injuries, 123, 206.

Health in plants, 369.

Heart wood, 124, 145.

Heat of respiration, 168.

Heliotropism, 54.

Hemi-cellulose, 103.

Herb, 9.

Herbarium, 3.

Heredity, 10, 13, 39, 128, 285, 308;
defined, 308, 314.

Heterozygous, 310.

Hilum, 376.

Histology, 3.

Hollow column, 180.

Homozygous, 310.

Honesty, 348.

Hooks, 361.

Horse Chestnut twig, 120.

Horticulture, 4.

Host, 83.

Hotbeds, 258.

Houseleek, 142.

House plants, 48, 241.

Humus, described, 241, 243.

Huxley, 35.

Hybrid, 320.

Hybridization, 318; inethod, 320.

Hydrangea, 339.

Hydrophyte, 190,

 

395

Hydrotropism, 177; described, 247.

Hygroscopic phenomena, 237; tis-
sues, 366, p

Hypocotyl, 355, 374.

Idioblasts, 33.

Imbibition, 148, 237.

Imbricate, 329.

Immune varieties, 370.

Improvement of plants, 2.

Indian Pipe, 83, 85.

Inhibitory influence, 202.

Initial cell, 355.

Injuries, healing, 122.

Insect-pollinated flowers, 290; char-
acteristics, 290.

Insects as cross-pollinators, 290.

Integuments, 274.

Intercellular air system, 33, 266.

Internode, 116.

Involucre, 339.

Iodine test, 20.

Tris flower, 287.

Ironwood, 113.

Irritability, 39, 55.

Ivory Palm, 373.

Jack fruit, 346.
Jussicea, 252.

Kerner, Pflanzenleben, 57.
Knees, 252.

Knowledge, 5; useful, 5.
Kny, L., 133.

Laciniate, 203.
Lamarck, 315.

Latex, 108, 109; system, 134; sys-
tem, described, 134.

Lathyrus Aphaca, 78, 80; pod,
365.

Le Maout and Decaisne, Traité, 76.

Leaf, anatomy, 28, 29; arrange-
ments, 139; auriculate, 68, 69;

axil of, 73; of Bidens Beckii, 62;
characteristics, 15; compound, 16,

67; connate-perfoliate, 69; eco-
nomics, 94; entire, 68; as a
factory, 26; functions, 7, 72;

linear, 63, 63; lobed, 67; margins,

68; morphological plasticity, 82;
mosaic, 56; netted-veined, 17,
66; orbicular, 62, 63; ovate, 64,
396

66; palmately compound, 68;
parallel-veined, 17, 66; perfoliate,
68, 69; pinnately compound, 68;

pitchered, 202, 203; plan, 34;
scars, 120; serrate, 68; shapes,
62, 68; simple, 16; — storage
function, 72; structure, 17; ten-

drils, 76, 78; thickness, 16; trace,
119; typical, 16; venation, 18.

Leaflet, 16.

Leaves, arrangements, 139; alternate,
140, 141, 142 ; opposite, 139; varie-
gated, 89; whorled, 140.

Legume, 348.

Leguminose, vs. Bacteria, 246.

Lenticel, 120, 121; described, 121.

Lettuce bud, 136.

Life history, 3.

Light, adjustment,
justed, 57; rédle
screen, 20.

Lignin, 98.

Linden, bract, 74; bundles, 133.

Linen, 205.

Linnean species, 317.

Linneus, 7, 315.

Lipase, 110.

Liverworts, 11.

Loam, 241.

Locomotion, 357.

Long Moss, 185.

Lumber, 205.

leaves ad-
plant, 26;

ook
in

Mangrove, 253.

Manual, 3.

Maple fruit, 351.
Marcgravia, 294.
Martynia, 362, 363.
Masters, Teratology, 201.
Maturation, 153.

Mechanical, effects, 196; system,
265.

Mechanistic. conception of nature,
39, 40.

Medullary ray, 122, 265; descrip-

tion, 125;-seeondary, 125.
Megasporangium, 324.
Megaspore, 324.

Mendel, 309.

Mendel’s Law, 312, 313.
Meristem, 128, 264.
Mesembryanthemum, 72.
Mesophyte, 190.

 

INDEX

Metabolism, 39, 98, 266

Microorganisms, 243.

Micropyle, 376.

Microscope, 28.

Microsporangium, 325.

Middle lamella, 147.

Mildews, 367.

Milkweed seed, 359.

Milky juice, 134.

Mineral salts, 230, 242; use, 28.

Mistletoe, 86, 187, 362.

Mitochondria, 41.

Mobility, 39.

Molds, 11.

Monadelphous, 272.

Monocarpic plants, 114.

Monocotyledons, 127.

Moneecious plants, 307.

Monstrosities, 72, 196; cause, 342;
of flowers, 340 ; of stems and leaves,
196.

Morphine, 109.

Morphological, diagram, 353;
ticity, 39.

Morphology, 3; definition, 82.

Mosaic disease, 369.

Moss, flowers, 269; plants, 11.

Muck, 241.

Mulberry, 352.

Multiple fruit, 352, 363.

Mutation, 13, 314, 317.

Mycelium, 84.

Mychoriza, 83, 244.

plas-

Natural selection, 316.

Navel Orange, 201, 205, 319, 367.
Nectar, 343.

Nectary, 275; forms, 273.
Nelumbium, 361.

Nematlion, 305.

Nepenthes, 76, 246.

Nicotine, 109.

Nitrates, 242.

Nitrogen fixation, 244,

Node, 116.

Nodules, 245.

Nucellus, 274.

Nucleolus, 41.

Nucleo-protein, 105.

Nucleus, 41.

Nursery plants, 260.

Nut, 349.

Nutrition without chlorophyll, 82.
INDEX

Oak, quartered, 126.

Oats, temperature effect on growth,
158.

G£dema, 234, 368.

Offsets, 188, 189.

Oil, Castor, 104; Cottonseed, 104;
Linseed, 104; Olive, 104.

Orchid, pollination, 293;
372.

Osmoscope, 227, 228.

Osmosis, danger, 234; described,
227, 232; explanation, 230; uses
in plants, 233.

Osmotic, phenomena, common, 235;

seeds,

pressure in growth, 233;  pres-
sures, 229; processes, described,
232.

Outgrowths from petals, 334.
Ovarian wall, 351.

Ovary, of Buckeye, 350; compart-
ments, 349; compound, 273;
described, 274; inferior, 275;
simple, 273; superior, 275; union
of carpels, 323.

Ovule, 273, 323; described, 274;
forms, 272; to seed, 354; struc-

ture, 271, 277.
Oxygen, release by plants, 23.

Paleobotany, 3.
Palisade tissue, 30.
Palm, 60, 127, 136.
Palmate venation, 66.
Pandanus, 253.
Panicle, 337.

Pansy seed, 376.
Paper, 205.

Parasite, 9, 11, 83; damage, 85.
Parasitic Fungi, 367.
Parrish, 18.
Parthenocarpy, 354.
Parthenogenesis, 302.
Pasteur, 39.
Pathology, 4, 367.
Peach Yellows, 369.
Pearson Fern, 197.
Peat, 241.

Pectin, 103.

Pedicel, 193.
Peduncle, 193.

Peg, of Pumpkin, 382.
Pepsin, 110.

Peptone, 105.

 

397

Perennials, 114;
woody, 114.

Perfumes, 108.

Perianth, 333.

Periblem, 264.

Pericycle, 265.

Permeable membrane, 228, 235.

Petal, 270; outgrowths, 334.

Petiole, 16.

Phanerogams, 12.

Pharmacology, 4.

Phenotypically, 310.

Phloem, 122, 130, 222, 262.

Phosphates, 242.

Photosynthesis, 262; amount, 25;
definition, 21; vs. respiration, 169.

Photosynthetic equation, 23.

Photosynthometer, 24.

Phototropic response, nature, 56.

Phototropism, 54, 296; in Fuchsia,
55.

Phyllodia, 80, 81.

Phyllomania, 203.

herbaceous, 114;

Phyllotaxy, 328; described, 139;
origin, 143.

Physiological disturbances, 368.

Physiology, 3, 5.

Phytopathology, 4, 367.

Pigments, 108.

Pine, cross section, 147; radial sec-
tion, 148; stem, 126; tangential
section, 149.

Pineapple, fasciated, 198.

Pinnate venation, 66.

Pistil, 273 ; generalized, 278.

Pistillate flower, 276, 285.

Pitcher Plant, 75, 76, 203.

Pitchers, 9, 74.

Pith, 116, 132, 265.

Placenta, 275, 323, 347, 351; dia-

gram, 324.

Plant, adult, 9; breeding, 4, 371;
breeding, methods, 317; definition,
7; diversity, 5; food, use of term,
28; foods, 242; geography, 4;
Industry, 4; insect catching, 87;
primary parts, 7; skeleton, 98;
spraying, 97; transplanting, 97.

Plants, numbers, 1.

Plastid, 41.

Platycerium, 186.

Plerome, 264.

Plowing, 260.
398

Plume, 358.

   

 

Plumule, 374.

Pollen, 272; germination, 275, 276;
grains, 286; injured by water,
295,

Pollination, 277, 370.

Polyadelphous, 27:

Polycotyledony, 377.

Polyembryony, 302, 377.

Polypetalous, 271.

Polysepalous, 270, 330.

Poppy, 348.

Potentialities, utilization, *

Preservation of sports, 318, 319.

Pressure gauge on root, 226.
Procambium, 265.

Progeotropic, 247.

Projection of seeds, 364.
Proliferations, 201, 367.
Proliferous Pear, 201; Rose, 202.
Propulsion of water, 148.
Protection, 261; of roots, 232.

  

Protein, 104; composition, 27; as
food for man, 106; grains, 105;
kinds, 105; layers, 105; where
made, 27.

Proteose, 105.

Prothallus, 306.

Protoplasm, 30; alveolar structure,
38; appearance, 35, 36; char-
acteristics, 35; chemical compo-
sition, 38; composition, 106;
constitution, 37; continuity, 40;
definition, 8; organization, 40;
properties, 39; streaming, 37;
texture, 36.

Protozoa, in soil fertility, 246; in
soils, 244.

Pruning, 122, 370; uses, 206.

Pteridophytes, 11.

Ptomaines, 109.

Puffball, 87.

Quinine, 109.

Raceme, 336; determinate, 336;
indeterminate, 336.

Rafflesia, 84, 86; 268.

Rainbow Corn, 90.

Raphe, 274, 376.

Rattan Palm, 113, 1S4.

Receptacle, 193, 271,
Recessive, 311,

275, 351.

 

INDEX

Reduction division, 285.

Redwoods, 113, 115.

Reflex action, 55.

Regulation, 39.

Relative transpiration, 47.

Reproduction, 265; asexual, 298;
in Fern, 306; sexual, 389.

Resin, 108.

Respiration, 111, 112, 266; amount,
164; described, 162; in roots, 231;
vs, combustion, 165.

Respiratory ratio, 162; equation,
165.

Respiroscope, 162, 163.
Resting period, 341, 377, 378;
ture, 379.
Reversions, 201.
Rhizoid, 215, 250.
Rhizome, of Sedge, 187.
Rock, pulverized, 238.
Rockweed, 189, 304 ;
Root, aération, 2
256, 257; anatomy,
anchorage function, 250; cap, 217,
221; crops, 258; cross section,
215; distinctive features, 212;
distorted, 257; in drains 248 ; du-
ration, 214; economies ex-
eretions, 243; as foliag 254,
255; function, 7; growing point,
217, 221; growth zone, 218, 221;
hair, 218, 224; hair in soil,
240; hair zone, 218, 221; hairs,
use, 225; length, 250;  longi-
tudinal section, 219; need for
air, 258; of Orchids, 254; origin,
pressure,
of, 232; prun-

plan of, 225;

protection

208; selective power, 231;

, 247; shortening,

special functions, 250; as

256; as storage organs,

216% structure,
215; system, typical, 213; tip,
218, 217; tip, diameter, 220; tip,
of Radish, 216.

Rootstock, 187, 188.

Rose, green, 201, 341;
359, 360.

Rotation of crops, 260.

Rots, 367.

Rubber, 108.

Rubus squarrosus, 193,

na-

sex cells, 304.
8; aérial, 253, 254,
220, 222;

 

 

 

ing,
self-adjustments,
257 ;

spines,
251; — strains,

of Jericho,
INDEX

Russian Thistle, 359.
Rust, 367 ; of Wheat, 90.

Saccharose, 100.

Sachs, Lectures, 36.

Sand-box, 365.

Sap, rise in trees, 147; theory of
ascent, 149; wood, 124, 145.

Sapling, 387.

Saprophyte, 11, 83.

Sargent, Plants, 117.

Sarracenia, 75, 76, 246.

Scabs, 367.

Scape, 193.

Science, aim, 12; applications, 5.

Scion, 208.

Sclereids, 265. -

Sclerenchyma, 130, 265.

Scott, Botany, 116.

Seasonal cycle, 388.

Seaweeds, 12.

Secretion, 266.

Secretions, 107.

Seed, 390; albuminous, 374, 375;
characteristics, 372; coat, 351,
375; condition of life, 378; du-

ration of life, 377, 380; economics,
385; ex-albuminous, 374, 376;
function, 8; plants, 10;  pro-
jection, 364; structure, 373.
Seedling, 9, 387; of Radish, 218.
Selection of variations, 318.
Self-adjustments, 55, 266.
Semi-permeable membrane, 228, 235.
Sempervivum, 188.
Sepals, 269.

Sex, cells, fusion, 278, 279; estab-
lished, 304; origin, 302; origin,
summary, 308; in plants, 307;

stages in development, 303.
Sexual organs, 307.
Shade, growth under, 95.
Shoot, 190.
Shrubs, 9.
Side roots, origin, 223.
Sieve, plate, 152; tube, 31, 131, 152,
262.
Silicles, 348.
Skeleton of plants, 98.
Skunk Cabbage, 268.
Sleep movements, 57, 61.
Slime-mold, 39, 357.
Slips, 259.

 

399

Smuts, 367.
Snapdragon, 274.

Soil, composition, 237; cultivation,
260; solution, 241; structure,
238, 239.

Solomon’s Seal, 191.

Sorus, 326.

Spadix, 338.

Spathe, 338.

Special creation, 315.

Species, 7.

Spectroscope, 53.

Sperm, 277; cell, 9.

Spermatophytes, 10.

Spermatozoid, 280, 304.

Sphagnum, 115.

Spike, 335, 337.

Spines, 192, 193, 256; Barberry, 81;
Echinocactus, 80; morphology,
79; significance, 79.

Spongy tissue, 30.

Spontaneous generation, 39, 40.

Sporangium, 324, 325.

Spore, 324, 373; asexual, 301; cases
of Mold, 302; dissemination, 360.

Sporophore, 84.

Sporophyll, 325.

Sports, 205, 319; preservation, 318,
319; seed, 319.

Spot diseases, 367.

Spraying, 371.

Squirting Cucumber, 365.

Stamen, 272; irritable, 297.

Staminate flowers, 276, 285.

Starch, as food for man, 103; for-
mation under light screen, 21;
formation vs. osmosis, 234; for-

mation in presence of CQn:, 22;
grains, 101; grains, typical forms,
102; kinds, 101; sheath, 130.

Stele, 264.

Stem, anatomy, 128, 129, 131;
characteristics, 113; columnar,
179; as conducting mechanism,

150; creeping, 187; deliquescent,
181; economics, 205; endogenous,
127; excurrent, 179, 180; exog-
enous, 127 ; function, 7, 53 ; general-

ized diagram, 125; herbaceous,
115; special function, 191; stor-
age, 191; structure, 115; sym-
metry, 181, 182; tissues, 116;

tissues, generalized, 117; tissues,
400 INDEX
herbaceous stem, 116; trailing, | Topiary work, 207.
187; traveling, 188; typical leaf-] Torsions, 201.

bearing, 115; various forms, 179;
woody, 119.

Stevens, 119.

Stigma, 273.

Stimulus, 54; perception, 178.

Stipule, 16; morphology, 80; special
forms, 82.

Stock, 208.

Stolon, 188, 189.

Stoma, 19, 49, 262; clogging, 96;
diagram of number and area of
opening, 51; number, 50; position,
50.

Storage, 266; battery, 167.

Strasburger, Textbook, 40.

Streaming of protoplasm, 37.

Strophiole, 374.

Structural features, 13.

Strychnine, 109.

Style, 273.

Suberin, 98.

Subsoil plowing, 260.

Substitutions, 201.

Sucker, 214.

Sucrose, 100.

Sugar, 100; cultivation, 101; kinds,
100; Maple, 181.

Sun scalds, 368.

Sundew, 246.

Sunflower head, 143.

Support of foliage, 265.

Suspensor, 354.

Symbiosis, 244.

Systematic botany, 2.

Teniophyllum, 255.

Tannin, 110.

Tap root, 212, 214, 250.

Taxonomy, 2.

Telegraph Plant, 81, 83.

Tendrils, 9, 76, 192; mode of oper-
ation, 79.

Thallophytes, 12.

Thallus, 190.

Thein, 109.

Theobromine, 109.

Thigmotropism, 77.

Thyrsus, 337.

Tissue systems, diagram, 263.

Tissues, definition, 8; healing, 122;
summary, 261.

 

Toxin, 173.

Traches, 146.

Tracheid, 31, 146.

Traction, 149.

Transfer, of water and food, 144.

Translocation, of food, 151.

Transmission of acquired characters,
315.

Transpiration, 43, 262; amount, 43;
constants, 44; demonstration, 43,
44; effect of external conditions,
47; effects, 96; fluctuations, 45;
plant prepared for study, 45;
record, 47; reduction, 69; rdle,
49; significance, 51.

Transpirograph, 45, 46.

Traumatropism, 249.

Tree, 9; crotch, supported, 211;
height, 150; lawn, 181; surgery,
211.

Tree Fern, 61.

Trichomes, 19, 70.
Tropical undergrowth, 59.
Truth, test for, 13.
Tuber, 9, 192.

Tubercles, 245.

Tuberous roots, 251.
Tulip Tree, cross section, 121.
Tumbleweed, 359.
Tumboa, 71.

Tumors, 200.

Twin fruit, 196, 199, 367.
Twiners, 185.

Umbel, 337.
Unit character, 309.

Vallisneria, 284, 287.
Valvate, 329.
Variability, 39.
Variation, 308;
selection, 318.
Vascular bundles, 116, 264.
Vegetables, 206; vs. fruits, 346.
Vegetative, bodies, specialized, 299:
parts, potential, 300.
Veinlet, 31.

defined, 308, 314;

Veins, 29, 118; of leaf, 17; netted,
17; parallel, 17; use, 144.
Venation, 65; netted, 65; palmate,

66; parallel, 66; pinnate, 66.
INDEX

Ventilation, 369, 371.

Venus Fly-trap, 76, 77, 246.
Vernation, 137.

Viability, 380; tests, 386.

Violet, seed pods, 364.

Vitalistic conception of nature, 39, 40.
Vitality, suspension, 377.

Warming, 288.

Water, capillary, in soils, 239;
flotage by, 360; hydrostatic, 238 ;
hygroscopic films, 239; in seeds,
377; in soils, 238; uses in plants,
224,

Water culture, 243; described, 242.

Water Lily seed, 361.

Water plants, 9, 61,
252.

Wax, 111.

Weeping Birch, 182.

Welwitschia, 71.

Whorl, 140.

Wiesner, 291.

Wild Geranium, 334.

251; roots,

 

401

Wilting, 48.

Wilts, 367.

Wind, effects on trees, 183; pol-
linated flowers, characteristics, 288 ;
pollination, 288; waftage, 358.

Windburn, 48, 97, 259.

Wing, 358; on fruits, 350, 351.

Winter-killing, 259.

Witches’ brooms, 198, 199.

Wood, 130; fibers, 265; grain, 124;
parenchyma, 130.

Wooden Flower, 200.

Xanthophyll, 89, 90, 91;
tion, 108.

Xenia, 353.

Xerophyte, 190.

Xylem, 122, 130, 222, 262.

composi-

Yeast, 169.
Yucca, pollination, 293.

Zoospores, 301, 357.
Zymase, 110.

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