1 MAIN L! BIOLOGY LIBRARY / * fr v* '"* . *kiM o .-a H "3 ll Bf GENERAL BOTANY FOR UNIVERSITIES AND COLLEGES BY HIRAM D. DENSMORE, M.A. \v PROFESSOB OF BOTANY AT BELOIT COLLEGE, BELOIT, WISCONSIN WITH ORIGINAL ILLUSTRATIONS BY THE AUTHOR AND BY M. LOUISE SAWYER, M.S., FORMERLY INSTRUCTOR IN BOTANY AT BELOIT COLLEGE GINN AND COMPANY BOSTON NEW YORK CHICAGO LONDON ATLANTA DALLAS COLUMBUS SAN FRANCISCO BIOLOGY LIBRARY G ENTERED AT STATIONERS' HALL COPYRIGHT, 1920, BY HIRAM D. DENSMORE ALL RIGHTS RESERVED 320.10 HEPT, lUrfc. GINN AND COMPANY PRO- PRIETORS BOSTON U.S.A. PREFACE This textbook is an outgrowth of the author's long experience in giving to college students an introductory course in botany. It has been used by the writer, in a briefer typewritten form, with several classes at Beloit College and by botanical instruc- tors in three other institutions. It has thus been adapted to the needs of students as a result of practical experience and criticism. The author's aim in writing the book has been to furnish the student with clear statements, properly related, of the essen- tial biological facts and principles which should be included in a first course in college botany or plant biology. It has been assumed that the text would be supplemented by lectures and readings to adapt it to particular needs in different institutions. Such topics as relate to the economic aspects of botany have consequently been treated concisely, with the idea that they would be elaborated and extended variously by different instruc- tors. In all cases, however, the biological principles underlying such practical aspects of botany have been supplied in the text. The author hopes, therefore, that the book will furnish both students and instructors with a helpful, connected statement of the more important facts and principles of modern botany. Content and use. The subject matter of the text is divided into three parts, which are so arranged that they may be used together, as the basis for a year's course in general botany, or separately, for courses of one term or one semester in length. Part I is intended to present the biological aspects of plant life from the standpoint of structure and function, on the basis of studies of the higher and more familiar seed plants. Three main themes are considered in this part of the text, namely, the relations and adjustments of the higher plants to other organisms v 498431 vi GENERAL BOTANY and to the inanimate forces and materials of their environment ; the cellular structure of plants and its relation to growth, repro- duction, and the anatomy of woody and herbaceous plants ; the phenomena of reproduction, and their relation to crossing, hybridi- zation, and plant breeding. The author has used this portion of the text, with some selections from Part II, as a basis for a semester course in plant biology. Parts I and II could, with equal advantage, be combined as the basis for a course on the higher plants during the latter portion of the college year. Part II deals with the morphology, life histories, and evolution of the main plant groups. In the chapter devoted to the fungi emphasis is placed upon the nature of enzymes and fermenta- tion and upon the relations of these processes to parasitism, disease, and decay. This chapter also gives an introduction to the important aspects of fungi related to plant diseases and plant pathology. In the treatment of the higher spore-bearing and seed plants the author has given nearly equal attention to the evolution of structure and to reproduction, instead of placing the main stress upon reproductive features, as is often done in elementary textbooks. In the parts relating to structure the teachings of the newer anatomy are followed. This method of treatment has been found to be simpler pedagogically, and more in accord with modern botanical knowledge, than the usual presentation based upon the older anatomy. Part III is intended to serve as an introduction, to field work and to the study of the interesting biological and economic aspects of a few important families and species represented in the spring flora. To this end considerable space is allotted to the study of trees and their importance to man. The main problems of forestry are emphasized concretely in connection with the life of a few selected species of forest trees. The herbaceous species of the monocotyledons and the dicotyledons are also treated from their biological and economic aspects, in order to indicate, if possible, the point of view from which additional species may be studied. PREFACE vii A brief chapter on plant associations is appended to the studies of families and species, in order to stimulate thought and observation along these lines. No attempt has been made to present plant ecology in a formal manner, since the entire treatment of families and species in Part III is ecological in its nature and thus presents the elements of ecology in a concrete mariner in connection with field studies. Part III is therefore not primarily taxonomic, but rather biological, economic, and ecological in its point of view, and is thus in harmony with the treatment of plants in the first two parts of the text. Distinctive features. Plants are presented throughout the text as living, active organisms, comparable to animals and with similar general physiological life functions. The purely scientific and descriptive portions of the text are directly linked with the theoretical and economic aspects of biology which are of immediate human interest. Thus, the cellular structure of organisms is directly related to growth, >to the structure and life of trees, and to the economic value of wood ; the chapter on vegetative and sexual reproduction is followed by a presentation of the essential facts and theories relating to hybridization, breeding, and evolution. The topic of evolution, however, is not presented as a theory by itself, but rather as nature's method of plant breeding and improve- ment, closely related to similar processes carried on by man. The chapters on plant physiology are summarized and closely correlated with the seasonal life of such common plants as the bean, clover, and locust. Physiological processes are thus made directly and concretely applicable to the seasonal life of well- known plant species. More space is devoted to the cell, mitosis, and cell structure than is usually accorded such topics in an elementary textbook. Two reasons seem to the author to justify giving this larger space to cellular biology. The first is derived from experience in presenting these subjects to beginning students, who usually manifest more interest in them than in most other aspects of viii GENERAL BOTANY botany. The second relates to the increasing importance of first- hand knowledge of the cell and mitosis to a proper understand- ing of the modern literature and popular discussions concerning genetics, heredity, and breeding. This part of biology is also of fundamental importance in psychology, physiology, and sociology. In case teachers do not care to give much time to laboratory work on mitosis, the outline and figures of the text should furnish a valuable basis for class discussion of this important topic. The presentation of plant structure from the viewpoint of modern anatomy is also new in an elementary textbook, but such treatment is justified by the author's experience in teach- ing this aspect of the subject to beginning students in botany. It is hoped also that the outline figures and the simple treat- ment in the text will enable instructors not familiar with this phase of botany to present the subject from the standpoint of modern plant anatomy. The laboratory directions, which are printed under separate cover, are intended to stimulate interest and observation without giving detailed guidance in laboratory work. Acknowledgments. In closing the author desires to acknowl- edge his indebtedness to former students and assistants, as well as to other botanists and friends, who have materially aided him in the completion of this book. First acknowledgements are due to Miss M. Louise Sawyer, instructor in botany at Knox College and a former student and instructor at Beloit College. Miss Sawyer's criticisms and sug- gestions were invaluable in the early stages of writing the text and while it was being tested with classes in Beloit College. In the making and reproduction of the original drawings Miss Sawyer deserves equal credit with the author for whatever of originality or helpfulness they may be found to possess. She reproduced all of the original drawings in ink and made the camera drawings on the cell, mitosis, and anatomy from slides in the author's laboratory. The drawings on the pollen tubes and sperm atogenesis in Iris are taken from an original paper published by Miss Sawyer. PREFACE ix Special acknowledgment is also made for the privilege of using in this text various illustrations from Bergen's " Founda- tions of Botany," Bergen and Caldwell's " Introduction to Botany " and " Practical Botany," and Bergen and Davis's " Principles of Botany." Acknowledgments are due to Dr. H. S. Conard of Grinnell College for the critical reading of the preliminary manuscript of Parts I and II and for criticisms and suggestions based upon the use of the text, in typewritten form, with his own classes. The author is also indebted to Dr. E. C. Jeffrey of Harvard University for some of the microphotographs of woody and herbaceous stems used in the text, and to Dr. W. J. V. Osterhout of Harvard for valuable suggestions. CONTENTS PART I. BIOLOGY OF THE HIGHER SEED PLANTS SECTION I. PLANTS AND THE ENVIRONMENT CHAPTER PAGE I. THE RELATIONS OF PLANTS TO THE ENVIRONMENT . . . 3 II. THE FORM AND ADJUSTMENTS OF THE PLANT BODY TO THE ENVIRONMENT ............. 14 SECTION II. CELL STRUCTURE AND ANATOMY III. THE CELLULAR STRUCTURE OF PLANTS ....... 45 IV. HISTORICAL SKETCH (TiiE CELL AND THE CELL THEORY) 54 V. GROWTH AND CELL DIVISION ..... ...... 60 VI. THE STRUCTURE AND FUNCTIONS OF STEMS, ROOTS, AND LEAVES .......... ....... 83 SECTION III. PHYSIOLOGY VII. NUTRITION AND SEASONAL LIFE OF PLANTS ..... 117 VIII. THE RELATION OF PLANTS TO WATER . . ..... 137 SECTION IV. REPRODUCTION IX. VEGETATIVE AND SEXUAL REPRODUCTION ..... . 154 X. PLANT BREEDING AND EVOLUTION ......... 174 XI. HISTORICAL DEVELOPMENT OF BOTANY AND THE BIOLOGI- CAL SCIENCES ................ 212 PART II. THE PLANT GROUPS XII. THE ALG^ . . ... . . . ......... 219 XIII. THE FUNGI ....... , . . ....... 242 XIV. BRYOPHYTES (LIVERWORTS AND MOSSES) ...... 287 XV. PTERIDOPHYTES (FERNS, EQUISETA, AND CLUB MOSSES) . 297 XVI. GYMNOSPERMS ................ 321 XVII. ANGIOSPERMS (DICOTYLEDONS) ..... . . . . . 337 xi xii GENERAL BOTANY PART III. REPRESENTATIVE FAMILIES AND SPECIES OF THE SPRING FLORA CHAPTER XVIII. DESCRIPTIVE TERMS 353 XIX. TREES, SHRUBS, AND FORESTS 3gg XX. HERBACEOUS AND WOODY DICOTYLEDONS . 396 XXI. MONOCOTYLEDONS ......... 416 XXII. PLANT ASSOCIATIONS . . ....:*. 435 INDEX . 447 GENERAL BOTANY PART I. BIOLOGY OF THE HIGHER SEED PLANTS SECTION I. PLANTS AND THE ENVIRONMENT CHAPTER I THE RELATIONS OF PLANTS TO THE ENVIRONMENT Plant biology, or elementary botany, is chiefly concerned with the structure and activities of plants and with their relations to the living and lifeless objects and forces which surround them and constitute their environment. It is evidently impossible at the outset of our study to present more than a general survey of the relations and functions of plants in nature, but it is hoped that such a survey will stimulate the interest of the student in the larger aspects of botany and will serve to give him a new appreciation of the great importance of plants in the world of living things. In order to present this more comprehensive idea of plant life clearly and concisely we shall need to consider first the peculiar relations of the world of vegetation to inanimate nature, including the soil, the air, and such forces of its en- vironment as light and gravity. This discussion can then be logically followed by considering the relation which plants sustain to animate nature, including man and other animals. With this brief introduction we may proceed directly to the consideration of these topics. THE INANIMATE ENVIRONMENT The forces. Light and gravity, which impinge upon the bodies of plants from without, are used by the higher plants in such a way as to secure the advantageous placing or ad- justment of their leaves, roots, stems, and flowers in the soil 3 GENERAL BOTANY and the air. Fig. 1, a, illustrates this placing of the organs of a young, growing plant in the positions usually assumed by them, and we shall find that these positions are, in part at least, directly traceable to the effects of environmental forces acting on the plant body. We may note in the figure that the main taproot grows vertically downward toward the center of the earth, but that the lateral, or secondary, roots assume quite a different position, growing at more or less definite an- gles from the pri- mary taproot. The main stein likewise grows in a direc- tion exactly oppo- site to that of the main root, as it should do in order to perform its as- signed task of dis- FIG. 1. Two seedlings of the scarlet-runner bean a, a seedljng growing in a normal position with its organs properly adjusted to light and soil ; b, the stem (hypocotyl) has adjusted itself hy curvature in response to gravity act- ing as a stimulus playing the leaves to the sunlight and the flowers to wind and insects. The student has doubtless noticed also that the leaves of plants may assume the normal horizontal position illustrated in the figure, or, in the case of plants before a window, the leaf blades may take an oblique position, so that they face the maximum light. If the plant represented in Fig. 1, a, had been grown in a flowerpot laid on its side, as in 6, or if it had started its growth on a steep hillside, the roots, stem, and leaves would have turned so as finally to assume the most favorable relation to sunlight, air, and soil. Botanists have demonstrated the fact, which we shall explain more in detail later, that growing roots, RELATION OF PLANTS TO ENVIRONMENT 5 stems, and leaves are sensitive to such forces as light and grav ity, and that they have the ability to respond to them by definite growth movements which are dependent upon the nature of the organ concerned. This sensitiveness and response enables the organs of plants to adjust themselves to their environment by placing themselves in proper positions for the absorption of foods and energy. For example, the root system secures a wide distribution in the soil, since its primary and lateral roots are able to respond to the gravity stimulus by growth in different directions with reference to the stimulating force. Similarly, the main stem and its branches respond to gravity and light, and leaves are able to adjust themselves to light coming from different angles. Plants are, therefore, sensitive organisms which sustain a defi- nite relation to the forces of their environment, and they are able to use these forces in assuming the various attitudes and positions which are most advantageous to them in a given place and time. The energy of the sunlight is also absorbed and transformed into heat and manufacturing energy, to be used in the making of sugar, starch, and cellulose by the green portions of plants. The materials. Green plants are likewise able to use the material substances of the air and the soil as no other organ- isms can do, and thereby to build up food for themselves and for the animal world, for we shall find that all animals are ultimately dependent upon plants for sustenance and continued existence. This peculiar relation of green plants to the inorganic sub- stances of their surroundings is easily understood from the diagrammatic drawing (Fig. 2). The root system of the plant represented in Fig. 1, a, has been admirably distributed through a wide area of soil by means of the sensitiveness of the roots to gravity. From this soil the roots absorb water (composed of hydrogen and oxygen (H 2 O)) and soil salts containing the various substances, such as nitrogen, sulphur, phosphorus, and potash, which the plant uses in building its own foods and its living substance. The leaves take in carbon dioxide (CO 2 ) from the air, and practically 6 GENERAL BOTANY all parts of the plant body may absorb oxygen. These life- less substances of its environment are therefore continually streaming into the plant from its surroundings through its roots, stems, and leaves. The striking and characteristic thing about the green plant is that it can take these simple, life- less substances of the air and the soil and construct foot! from them both for itself and for animals, and that, like animals, it can build these inert foods into living substance, endowed with the unique and characteristic properties of life. Within recent times chemists have been able, in the chemical laboratory, to compound such food substances as sugar and even simple forms of nitrogenous substances or proteins; but no chemist has yet been able to do what the plant does daily, namely, to convert lifeless matter into living matter. The power of green plants to make organic compounds is due to the fact that they have a green substance, chlorophyll, within their tissues which enables them to absorb and use the energy of light in \the making of starch and sugar. When the streams of carbon dioxide pour into the leaf from the air, they meet there the water derived by the roots from the soil. In the presence of the green pigment, chlorophyll, the living substance of the leaf is then able to utilize the outside energy of the sun and unite the carbon of the inflowing carbon dioxide (CO 2 ) with the hydrogen and oxygen of the water (H 2 O) to make sugar and starch. The sugar is then combined with the nitrogen, sulphur, and phos- phorus, derived from soil salts which are absorbed by the roots, into nitrogenous foods and ultimately into the living substance of the plant body. A part of these foods, or, more probably, the living sub- stance itself, is then broken down by union with the oxygen which enters the plant at various points in the plant body and brings about oxidation, or respiration, as in our own bodies. This oxidation process resembles somewhat the oxidation which occurs when coal is burned in a furnace or in a stove, and results in the formation of cell energy and certain waste products in the plant comparable to the heat, gases, and ash produced by the combustion of coal and wood in a stove. This waste material RELATION OF PLANTS TO ENVIRONMENT 7 is often cast out of the plant in much the same form in which it came into it, namely, carbon dioxide and water ; but more fre- quently these substances are used over again to rebuild living substance and plant food. The energy released by oxidation is used by the plant in its vital processes of growth, reproduction, and repair. The peculiar relation which nonliving matter in the form of foods sustains to the living matter composing the bodies of animals and plants may be expressed in various ways. Cuvier and Huxley put this relation in a striking manner by comparing living beings to a whirlpool. The whirlpool is permanent, but the particles of water which constitute it are constantly changing. Those which enter it on the one side are whirled around and temporarily constitute a part of its individuality ; and as these leave it on the other side their places are made good by newcomers. It is undoubtedly true that the green plant illustrates the whirlpool conception more correctly than any other organism, since it alone of all living forms has the power of converting the simple lifeless compounds of the earth and air into living matter. When we study the next topic, we shall see that animals and man are not so intimately related to inorganic nature as green plants are. THE ANIMATE ENVIRONMENT Most plants are intimately associated with other plants and also with certain forms of animal life which affect their lives in various ways and so constitute a part of their natural environment. Thus, a plant on a lawn, in a meadow, or in a forest is closely surrounded by its neighbors, which compete with it for light, air, and soil space in which to expand and obtain food. Tall plants shade shorter ones, and closely matted plants, like grass, are likely to occupy the soil space to the exclusion of less social plants. In a forest the competition of trees is always evidenced by the death of those which fail to keep pace with 8 GENERAL BOTANY PLANT BODY INCOME neighbors in the upward growth toward the light. A tree developing in the open always has a more symmetrical form and a fuller leafage than one of the same species growing in a forest. In these and various other ways the environment of any given individual or species of plant is vitally affected by neighboring plants of the same community or society. Animals are also factors in the plant's environment in that they tend to injure or destroy plant life for food or for protec- tion. Some plants, like the cacti and thistles, are armed and thus protected against the higher forms of animals ; but even these ex- ceptional forms are subject to attack by smaller forms, which may infest the roots or bore into the stem or leaves and either 1. ENERGY 2. INORGANIC MATERIALS d. OXYGEN FIG. 2. Diagram illustrating the nutrition of a green plant At the extreme left the income in energy and inorganic materials is shown ; within the circle the main organic substances constructed within the plant body are in- dicated. At the extreme right the outgo in energy and inorganic materials is indicated ; within the circle the energy and waste products of respiration (destructive process) are shown kill or injure them. So, while the plant draws upon its inanimate envi- ronment for energy and foods, it rmist needs compete, in a hard struggle for its existence, with the other living organisms, including both plants and animals, by which it is surrounded. These living forms constitute its animate environment. RELATIONS OF PLANTS TO ANIMALS Plants help man and other animals by giving them food and protection and by creating an environment favorable to their needs and comfort. Plants are also of the greatest indus- trial and commercial importance on account of the food, shelter, RELATION OF PLANTS TO ENVIRONMENT 9 OUTGO 2. ORGANIC FOODS and energy which they furnish for the maintenance and welfare of man in the home and in the industries. Food relations of plants and animals. The food relations of plants to man and animals can be most easily understood by com- paring the. income and outgo of a common green plant with that of a man, or of an animal similar to man in its feeding habits. For this comparison Figs. 2 and 8 may be used. The cir- cle in each figure may be taken to represent the plant or animal body con- taining the living INCOME substance that both does the feeding and produces the energy for the living or- ganisms. These fig- ures illustrate the fact that the ani- mal bears the same general relation as the plant to lifeless matter, to the pro- duction of energy, and to the elimina- tion of wastes. Life- less matter, in the form of foods, bread, butter, and meat, is taken into the body, digested and assimilated by the living substance of the animal body, and converted temporarily into living matter as illustrated in the figure. The living matter thus produced from lifeless foods is then oxidized, and this process yields energy for the ordinary animal activities. The waste materials produced by this oxidation are then cast out of the body in the form of carbon dioxide, water, and nitrogenous wastes ; and there is some loss, of energy with the heat that escapes from the body. A real difference between the animal and the plant, however, lies in the nature of the lifeless matter taken in. In the animal 3. INORGANIC MATERIALS a. WATER b. OXYGEN C. SALTS FIG. 3. Diagram illustrating the nutrition of an animal The income, outgo, and products of constructive and destructive processes are shown as for the green plant in Fig. 2. Compare Figs. 2 and 3 and consult the text for further discussion 10 GENERAL BOTANY the bread, butter, and meat (which are taken as representative of animal foods) are real organic food materials, which have been previously built up from raw, inorganic materials by green plants. Thus, bread, as we all know, is made of flour, which con- sists of finely ground kernels of wheat. The starch and the smaller quantities of nitrogenous gluten of which flour is largely composed were built up, as we have already learned, in the leaves of wheat plants from carbon dioxide, water, and soil salts, absorbed from the air and the soil. After being constructed in the leaves, they were passed into the wheat kernels as reserve food for the growth of the embryos in the seed when the seed should germinate. This ready- made food of the green plant is seized upon by animals for their maintenance. In a similar manner the meat which con- stitutes the food of some animals is ultimately traceable to a plant origin, since even carnivorous animals (meat eaters) derive their food from herbivorous animals (plant eaters). For example, cats live on mice, and the mice get their food from grain or other plant parts ; and cattle, pigs, chickens, and fish, which are the most common sources of meat for man's use, depend upon grass, hay, grain, and water plants for their food supply. All of these animals would starve in a comparatively short time without the continued supply of food which green plants build from carbon dioxide, water, and soil salts. This general relation of the lifeless world of matter and force to the living world of animals and plants is illustrated diagrammatically in Fig. 4. It is only necessary to add to the explanation already given the well-known fact that the death of animals and man is frequently caused by colorless plants known as bacteria. Entire crops are also often de- stroyed by parasitic fungi such as the rusts of grains, and all decay is induced by these colorless fungous plants. This decay finally converts the bodies of the dead animals and plants into gases and "other chemical substances, which filter into the soil and form a part of the lifeless matter which is absorbed as raw food material by green plants. The most familiar RELATION OF PLANTS TO ENVIRONMENT 11 instance of this fact is that of the farmer fertilizing his fields with fertilizers composed of the remains of plants and the excrements of animals. We have now completed the general survey of the relation of green and colorless plants to that part of nature which INCOME OUTGO I. FROM SUN ENI GREEN PLANT FIG. 4. Organic food cycle What three classes of organisms are represented as concerned with the construction and use of foods in the world ? What part does each play in the maintenance and consumption of its food supply ? What distinctive relation has the green plant to the maintenance of the world's food supply? constitutes their environment. The details of the picture will be filled in as we proceed in later chapters with the structure and work of plants. Plants and the environment of animals. We are not accus- tomed to think of the earth as it would be without the plant life which both clothes and beautifies it. The protective, en- vironmental function of plants is perfectly obvious, however, without lengthy comment or discussion. The great forests form a necessary environment for hundreds of species of animals, ranging from the larger forms, which make their homes in 12 GENEKAL BOTANY the trees, to the smaller and humbler forms, like insects, which inhabit the crevices of decaying logs and bark and the leaf mold created by the decay of fallen leaves. Winter to these creatures of the forest is less severe than it would be in an open and barren region, and summer brings an abundance of food, with protection from heat. Similarly, an intimate knowl- edge of the life of our common birds, insects, fishes, and mam- mals would reveal the all-important influence of plants on the environment of these animals in the meadows, lakes, and streams which they inhabit. The great deserts, likewise, are habitable by a limited number of animal forms, including man, on account of the protection and food supplied by the cacti and other desert plants which have become adapted to such arid regions. On account of these environmental relations of plants to man, the culture of certain kinds of plants which are useful for food and for lumber and for ornamental purposes has assumed a larger and larger place in. industry and commerce as civilization has advanced. Industrial and commercial relations of plants. The great importance of modern industrial botany has led to the estab- lishment, by our national and state governments, of important departments for the investigation and culture of plants. In these departments hundreds of expert botanists are studying those aspects of plants and plant culture which are deemed to be of the greatest importance to man. Agriculture, and its relation to plants, is one of the great lines of endeavor connected with practical botany. Forestry, although not yet so fully developed in our own country, bids fair to receive an increasing amount of attention as our forests become depleted and the supply of lumber decreases. Plant breeding, or the production of better kinds of fruits, grains, and ornamental plants, is receiving wide attention and interest at the present time. As a result of this great activ- ity on the part of individuals and of our national and state governments, many improved kinds of food and forage plants have already been bred, and the future promises even greater improvements in both agricultural and horticultural varieties. RELATION OF PLANTS TO ENVIRONMENT 13 Plant breeding, aside from its great practical importance, is of equal theoretical interest to students of evolution and genetics, since the practical breeding of plants is based upon theoretical principles which form the basis for generalizations in these subjects. The study of plant diseases, or plant pathology, is also of the greatest practical interest to the growers of all kinds of plants. It has been found that plants are not only subject to diseases produced by such colorless plants as rusts and smuts of grain and tree-killing fungi but that they are also subject to bac- terial diseases, as is the case with man and other animals. To eradicate these diseases and save the great losses produced by them annually, our government has now organized a very large department of plant pathology. It is not necessary to point out in greater detail the importance of plants in nature and the consequent interest and importance attached to the study of plant biology and botany. SUMMARY 1. Plants use the forces of their environment light, heat, and gravity in adjusting their organs properly, in the air and in the soil, for the absorption of raw food materials. 2. Green plants are unique among living objects in being able to build foods for themselves and for animals out of simple chemical substances which occur in the air and soil. 3. Green plants are therefore intermediates between inorganic, lifeless nature and the more highly organized animals. 4. Plants are also important to animals and man in forming a proper environment for their protection and welfare. 5. Plants are of the greatest importance to man industrially and commercially. CHAPTER II THE FORM AND ADJUSTMENTS OF THE PLANT BODY TO THE ENVIRONMENT We learned in the previous chapter that the organs of the plant body roots, stem, leaves, and flowers were so arranged and adjusted to the materials and forces of the environment as to secure abundant food and energy for the maintenance of life. This necessary arrangement and adjustment of plant organs to their surroundings is secured in part by an inherited plan of architecture, which governs the formation and growth of organs in the embryo and in the adult organisms, and in part also by powers of movement, called tropisms, by which growing plant organs place themselves in proper relations to soil, light, and air. THE FORM AND PLAN OF THE PLANT BODY If we observe the plant body of most plants, we shall see that it consists of a main stem and root, which constitutes its central axis, and of lateral organs in the form of leaves, flowers, branches, and lateral roots. More careful observa- tion of such a plant will also show that the lateral organs are not placed on the main axis in an indefinite manner, but that they have a definite order and arrangement inherited from a long line of plant ancestors. It will soon become evident also that by this inherited plan of the plant body the organs are so related to each other and to air, light, and soil as to make the plant as a whole a good working organism in its efforts to secure food for sustenance and growth. These facts will become more and more obvious as we proceed to study the relations of leaves, branches, and secondary roots to each other and to the main axis. 14 THE PLANT BODY 15 Blade FIG. 5. Body plan of the lilac and apple a, lilac twig with cyclic leaf arrangement ; b, apple twig with spiral leaf arrangement. Compare Fig. 5, and b, with Fig. 7, a and b The, leaves of plants (Fig. 5) spring from definite points on the main axis, which are called nodes, and the nodes are separated by definite spaces, or in- tervals of stem, called internodes. The leaves also grow from definite points at the nodes and are so arranged on adja- cent nodes that a leaf at one node never stands immediately above the ] ea f at t ] ie no( J e ^-^ J below it. By this ar- rangement the shading of the green tissues of one leaf by another above it is avoided. The leaves on a stem are therefore arranged in mathematical order, which usually conforms to one of two types of arrangement, namely, the cyclic arrangement and the spiral arrangement. In the cyclic leaf arrangement two or more leaves occur at each node, and the leaves of adjacent nodes alternate with each other. In such plants as the lilac and catnip two leaves are placed opposite each other at each node, alternating with the FlG 6 Leaf arrangem ent on pairs of leaves at the nodes immedi- a lilac twig seen from above ately above and below them (FigS. Note the alternating leaf pairs, 5, ', and 6). The entire leafage of such a stem is thus arranged in four vertical rows of leaves up and down the stem, each row being separated from its neighbor by an angle of 90. Each leaf in 16 GENERAL BOTANY any one row is therefore separated from tne leaf immediately above or below it in the same row by two internodes of the stem. If one observes a stem or a branch from above, the four rows of leaves are plainly visible, and the entire upper portion of the shoot has the appearance of Fig. 6. In the diagrammatic draw- ings (Fig. 7), in which the leaves, nodes, and internodes are repre- sented as if projected on a plane, the circles represent nodes and the intervening spaces internodes. The shaded central portion is meant to represent the small leaves and very short internodes of the terminal bud. Outside of this central shaded area the interned al spaces widen gradually toward the pe- riphery of each figure to repre- sent the gradual FIG. 7. Diagrams showing cyclic and spiral leaf lengthening; of arrangements the internodes on a, cyclic leaf arrangement of the lilac, reduced to one plane. , ,, The circles represent nodes, and the spaces between inter- a s ^em trom its nodes. The leaves are seen to be in four longitudinal rows apex to i^S base, along the stem. 6, spiral leaf arrangement of the apple, ,p, . , , reduced to one plane. Nodes and internodes as in a InlS lengthen- ing of the lower internodes as the leaves grow in surface is of advantage in preventing shading of the lower leaves by large leaves above them. The cyclic leaf arrangement (Fig. 7, a) is thus seen to be well adapted to the exposure of leaves to sunlight without interference or shading by their neighbors on the same shoot. In the spiral leaf arrangement (Figs. 5, >, and 8) only one leaf is placed at a node, and the leaves spring from the nodes so as to effect a spiral distribution along the twigs and young shoots. Since any given leaf is always separated from a leaf above or below it in the same straight line by two or more internodes (Fig. 7, 5), according to the type of spiral arrangement in any particular case, the same advantages as regards sun exposure THE PLANT BODY 17 Axillary flowering branc/i are secured by the spiral as by the cyclic leaf arrangement. The spiral arrangement of leaves is often distorted by growth and is therefore difficult to trace on mature shoots. It is consequently more clearly evident at the apex of a branch, where the twisting effects of growth are less evident. Since the buds and lateral branches arise from the axils of the leaves of the season, it is evident that the entire branch system must follow either the spiral or the cyclic plan upon which the leaves are arranged (Fig. 8). The general form and leaf exposure of an adult plant will therefore be largely de- termined by the above body plans and by the manner in which they are worked out through Axillary leafy branch- -^Secondary roots -Primary taproot FIG. 8. Body plan and arrangement of organs in the buckwheat (Fagopyrum) Note that the branches spring from the angle (axil of the leaf) between the leaf and stem, and are there- fore disposed spirally on the stem, like the leaves. The adjustment of leaves and roots to light and soil is evident growth as the plant matures. The root systems of plants are less definite in their arrangement than the branch sys- tems, probably owing to the fact that their soil environment is more uniform and less exacting than the forces and elements to o which the aerial portions of the plant body are subjected. Thus, there may or there may not be a primary central taproot cor- responding to the central stem axis aboveground. The secondary 18 GENERAL BOTANY roots usually arise in an orderly succession, but there are no definite nodes and internodes. We shall learn later that the successful distribution of roots in the soil is determined in most plants more largely by their powers of adjustment through growth movements than by their arrangement on any fixed body plan. The relation of the body plan of aerial plant parts to the ultimate form of some common herbaceous and woody plants will now be considered. The form and development of herbaceous plants. In a number of herbaceous plants the terminal bud grows rap- idly during the early part of the season and produces a stout central stem, while the lateral buds remain dormant and thus fail to produce lat- eral branches to any consid- erable extent. The leaves FIG. 9. Body plan and pyramidal form spring from the nodes either of an herbaceous plant The ultimate form of this cineraria plant and the exposure of its leaves to light are de- termined largely by its spiral body plan, the alternation of nodes and internodes, and the continued growth of the terminal bud. Pho- tograph by Fuller, from Cowles's " Ecology " in spiral or in cyclic order, the lower leaves being usu- ally somewhat larger and hav- ing longer petioles than the upper and younger ones. The result of such growth is a plant of pyramidal form (Fig. 9), in which the entire leafage is admirably exposed to sunlight. The root system of such plants is variously arranged, but without the definite plan of the aerial parts, owing to the fact that the main root has no definite nodes from which lateral roots spring. Since, however, the soil salts and water are usu- ally distributed equally on all sides of the r6ots, and since there is no danger from shading, it is evident that roots do not need THE PLANT BODY 19 the more accurate plan of the stem, leaves, and branches. In other herbaceous plants the lateral buds may all produce lateral branches, and in that case the plant assumes a symmetrical pyramidal or rhomboidal form like the ragweed (Ambrosia), aster, and Russian thistle. All gradations in symmetry and reg- ularity of form between the erect and the pyramidal or rhom- boidal types mentioned occur in herbaceous plants, according to the relation between the growth of the main axis and the later branches. Where great irregularity in the growth of the lateral branches occurs, the body plan is often obscured in the adult plant but is evident in youth and in the arrangement of leaves and buds on the branches. The form and development of trees. Among trees there is the same general relation between the body plan, the growth of buds and branches, and the ultimate form of the tree and its adaptation to the forces and materials of the environment as we have noted above in herbaceous plants. THE ERECT TKEE TYPE The pines. In erect tree types like the common pine (Fig. 10) the main axis terminates each season in a twig which bears the buds for the next season's growth. These buds consist of a vig- orous terminal bud, which continues the elongation of the axis, and of from three to five vigorous lateral buds clustered at the base of the terminal bud. The remaining buds of the terminal twig are latent and rarely if ever grow into lateral branches. When spring arrives the vigorous apical bud elongates and produces the terminal twig of the season with its spirally arranged scale and needle leaves. Meanwhile the vigorous lat- eral buds elongate and produce a whorl of branches which are separated from each other by inconspicuous internodes. Since these branches arise from buds which are really arranged spirally in the axils of minute scale leaves, they are called false whorls, to distinguish them from true whorls, which could only arise on trees with a cyclic body plan. The newest false whorl of each season is separated from the false whorl of the previous season, 20 GENERAL BOTANY 'alse whorls immediately below it, by that portion of the terminal twig which bore latent buds and is hence devoid of branches (Fig. 10, ) Since but one false whorl of branches is produced each season in the manner described above, the trunk of an adult pine tree presents a series of false whorls of branches from base to apex, separated by smooth portions of the trunk (Fig. 10, 6). These smooth, branchless por- tions of the trunk are not internodes, as they are sometimes thought to be, but rather rep- resent those portions of the terminal twigs of each season where the latent buds failed to produce branches. Since the branches at the apex of the tree are younger and so shorter than those toward the base, the result is a pyramidal tree with a strong central excur- rent trunk. The needle leaves and reproductive cones are always produced by buds at the ends of the lateral branches, and on account of their spiral arrangement they are so placed as to be admirably exposed to the sunshine, which aids the leaves in their food- making, and to the wind, which helps to scatter the winged seeds of the pine cones. The body plan and the method of bud growth, therefore, combine in the pines to produce a tree of great beauty and of nice adaptation to environmental conditions. False whorl FIG. 10. Pine trees illustrating the erect type a, a young pine ; b, a mature pine. Observe the ex- current trunk, the false whorls of branches, and the pyramidal form. Consult the text for the main fac- tors concerned in the seasonal growth (1-7) and the development of these external features of the pine THE PLANT BODY 21 SPREADING TREE TYPES The elm. In trees of the spreading type, like the elm, poplar, oak, and hickory, the same general plan of development can be traced as that outlined for the pine, except that the terminal bud of the main axis is replaced by a lateral bud after a few years, so that no main central excurrent trunk is contin- ued throughout the life of such trees. In the elm the terminal bud , is replaced each I season by a lat- eral bud, which produces a main central excurrent trunk for a few years (Fig. 11, e). Ultimately, as in all such trees of the spreading type, a few lateral branches gain the FIG. 11. Growth of the American elm, an illustration of the spreading type of trees ascendancy and The letters from left to right show several stages in the de- form all of the velopment of the elm. The ultimate form is determined by body plan, the method of bud growth, and pruning effects. The corresponding letters on each figure indicate the vigor- ous (and so successful) branches produced each season. For further discussion consult the text spreading crown of the adult tree (Fig. !!,/> So long as the central axis continues to grow in length, its method of growth, as well as that of the vigorous lateral branches, follows the general plan already outlined for the pine. The ter- minal twigs at the ends of the main branches produce each year 22 GENERAL BOTANY clusters of vigorous buds, as in the pine. In the elm the sub- terminal bud of each cluster early replaces the true terminal bud and continues the growth of the main axis of the branch or, in the young tree, of the central trunk. Two or three vigorous buds below the subterminal one form vigorous lateral branches corresponding to the false whorls of the pine. These vigorous lateral branches consti- tuting a false whorl in the elm are not, how- ever, so numerous as in the pine and are sepa- rated by longer inter- nodes (Figs. 11 and 12). In the elm and other sim- ilar trees some of the buds on the terminal twigs of the season below the terminal cluster of vigor- ous buds usually form small dwarfed twigs, while others remain la- tent. These smaller lat- eral twigs are ultimately self -pruned, owing to the fact that they are shaded by their more vigorous competitors, so that the mature portions of the trunk and branches ultimately present a condition similar to that which exists in the pines, with alternating false whorls of lateral branches and naked segments of the central axis. The false whorls in the elm are represented, however, by not more than two or three successful branches in a whorl, instead of by many as in the pine. Often, as the elm grows older, a single vigorous branch grows, each season, just below the bud which continues the main axis, FIG. 12. A mature elm tree illustrating the spreading habit The numbers on the main trunk and branches are similar to those in Fig. 11 THE PLANT BODY 23 so that a forking appearance is produced in the upper branches and the terminal twigs of the crown. In other hardwood trees, like the hard maple and Carolina poplar, false whorls of branches "are formed, resembling very closely those of the pine in distinctness and in the number of branches in a whorl. The leaves of the season in the elm and other hardwood trees are disposed as in the pine, at the ends of the terminal twigs of the season. In the spreading type of tree they present an im- mense surface to the sun for photosynthesis, while their spiral or cyclic arrangement secures to them adequate light without the danger of overlapping. SUMMARY From the above accounts, illustrated by the development of the pine and the elm, it is seen that three main factors determine the ulti- mate forms of trees and the successful display of their foliage, fruits, and seeds. These factors are the body plan, the unequal growth of buds and branches, and pruning effects due to competition in the crown of the tree. Of these factors the unequal growth of the buds is certainly the most important, since by this factor the pyramidal or the spreading form of the tree is determined, as well as the nature and disposition of the false whorls of vigorous lateral branches. In the spreading-tree types the use of the term false whorls is only permissible in order to make clear the close similarity which exists between the mode of annual bud growth in the pyramidal pines and that in the spreading types of deciduous trees. In herbaceous plants the same general principles obtain in the development of the mature plant as in trees, and the forms assumed by them correspond, as we have seen, to the erect pyramidal type and the spreading type of the pine and the elm. ADJUSTMENTS TO THE ENVIRONMENT BY TROPISMS It has just been shown that the inherited plan of the plant body of our common plants is favorable to the proper plac- ing of leaves, roots, and steins for the absorption of food materials and energy from the soil, air, and sunlight. It will be quite evident, however, to the critical observer of plants, 24 GENEEAL BOTANY Cotyledon that the growth of branches and leaves at regular nodal inter- vals, and their cyclic and spiral arrangements on the main stem and its branches, are not all that is necessary for the proper adjustment of these organs to the environment. This fact is most strikingly illustrated in the growth of young plants from the seed, where the parts of the embryo, originally folded in the seed, must gradually expand and adjust them- selves to the air, light, and soil for the absorption and use of raw food ma- terials. The growth and development of the embryo and seedling of the common pea fur- nishes a good il- lustration of the movements which take place in the adjustment of a young plant to the new condi- tions presented to it as it emerges from the seed and the soil. The seed of the pea is composed of a dense outer covering, or seed coat, inclosing the embryo proper, which entirely fills the seed coats. This embryo consists of two fleshy cotyledons, or food-storing leaves, which comprise the bulk of the embryo. Between the two cotyledons are found two or three minute leaves, constituting the first bud of the young plantlet, and a stemlike body, the hypocotyl, which bears the cotyledons and the plumule. These parts are all shown in their proper relation in the early stages of germination of the pea (Fig. 13, a), just after the plumule and hypocotyl have broken through the seed coat. FIG. 13. Stages in the development of a seedling of the garden pea a-c, emergence of the embryo from the seed and of the plumule from the soil; c-e, erection of the plumule and growth of the lateral roots ; /, mature seedling with leaves and roots adjusted to light and soil THE PLANT BODY 25 As the embryo emerges from the seed the plumule is curved and the leaves are seen to be borne on the first internode of the stem above the cotyledons, which is known as the epicotyl. The hypocotyl has meanwhile elongated and is continued as the pri- mary root. The curved condition of the plumule protects its deli- cate leaves from being broken off as it pushes up through the soil. The curvature is not due to gravity but to unknown internal causes. As soon as the embryo emerges from the soil, the plumule, now composed of several nodes and internodes, straightens and finally assumes an erect position. As the leaves expand they respond to the stimulus of light and take up a favorable posi- tion for the reception of the maximum amount of light for the manufacture of starch. The roots, meanwhile, respond to gravity in such a manner that the lateral roots grow out at an angle from the vertical taproot and so permeate a large area of soil from which to draw water and soil salts. The mature seedling (Fig. 13,/), through these various ad- justing movements, is thus admirably adapted to securing from the environment the energy, gases, water, and salts necessary to its growth and development. It is quite evident that the above movements and changes in the position of the various organs of the growing seedling are quite independent of the division of the stem into nodes and internodes and of the cyclic or spiral arrangement of the leaves. These structural arrangements are, however, closely cor- related to the adjusting movements, so that the architectural plan and the unfolding movements work together for a better final adjustment of the organs in the older seedling. Mature plants do not usually manifest such definite and obvi- ous movements as seedlings, and yet there is abundant evidence in adult plants of all kinds that the ultimate position of their growing organs, and their more perfect adjustments to the environment, are brought about by a correlation of body plan with adjusting movements which are caused by external and internal stimuli. 26 GENERAL BOTANY One of the principal reasons why such adjusting movements are necessary is the fact that the foods and energy which must be absorbed by green plants for their maintenance are usually quite unequally distributed in the air and soil about an indi- vidual plant. The plant must therefore turn its parts toward the most favorable source of supply to reap the maximum benefit of any given condition. Thus, the two sides of a plant before a window are exposed to very unequal intensities of light, and in 6 FIG. 14. Phototropic response in leaves of Nasturtium tropaeolum a, a plant grown under normal illumination in a greenhouse ; 6, the same plant after exposure for six hours to lateral illumination. From Cowles's " Ecology " order to receive enough of the sun's energy for effective work in making starchy foods it must turn or swing its leaves in such a manner that their broad green surfaces shall be exposed di- rectly to light coming in from the outside. We are all familiar with the fact that plants under such circumstances are able to turn the entire leaf surface from the normal horizontal position to an oblique position (Fig. 14), so as to face the sources of maximum light supply. Under these circumstances the separa- tion of the leaves by internodes, and their cyclic or spiral arrangement, is still effective in preventing shading, but the nicer adjustment of the leaves to unequal light exposure on the two sides of the organ is made through the power of the leaves to respond to light acting as a stimulus and to turn their THE PLANT BODY 27 faces toward the most abundant light supply. In a similar manner, roots which are growing in soil where the amount of water or food varies on different sides of the root system are able to turn toward the maximum food or water supply, as when roots grow into old wells and water pipes, attracted by the excess of moisture. These turnings in response to stimuli are called tropisms. It is not surprising, therefore, that plants, probably on account of their stationary habit, have developed a wonderful sensitive- ness to the forces and agents of their environment, which enables them to adapt themselves and their fixed architectural plan to the variations in their surroundings which might otherwise harm them. Investigation has shown that plants are sensitive to gravity, sunlight, moisture, soluble substances in the soil, and various other stimulating forces and materials. Indeed, this ability to adjust their organs is more marked in plants than in most animals. While plants are sensitive to a great variety of stimuli in their surroundings, certain forces and agents are more prominent as directing stimuli than others, and these will therefore receive the most attention in the following discussion. Stimulus and response. Plant stimuli, as indicated above, are usually the external forces and materials of the- environment. Any difference in the intensity of such external stimuli or in the direction of their application to a plant organ is capable of bringing forth a response in the form of growth, food building, adjusting movements, or even the death of the organism. We are here concerned only with that form of response to stimuli that results in definite movements which adapt the plant more effectively to the daily and seasonal changes in its surroundings. A moment's consideration will enable the student to realize the great difference between the higher plants and the higher animals as regards both the reception of an external stimulus and the method, or mechanism, by which the two classes of organisms respond to stimuli. The higher animals are furnished with special sense organs, such as the eye for the reception of light and the ear for sound ; in plants the entire surface of a leaf or 28 GENEBAL BOTANY a stem usually receives the stimulus of light or of a mechanical agent. The nearest approximation to sense organs in plants are certain groups of cells in root tips, and in the stem tips of some particular plants, which are endowed with the property of re- ceiving gravity or light stimuli. In general, however, plants have no special sense organs with nerve endings, like animals, with which to receive external stimuli. The mechanism, or method of response to stimuli, is likewise very different in the higher plants and the higher animals, since plants have no muscles, attached to a jointed skeleton, for effect- ing movements. The most common method of response in plants is that of unequal growth on the two sides, or on regularly alternating sides, of an organ which has been excited by an external stimulus. Thus, in the pea seedling (Fig. 13) the erection of the epicotyl from its curved position was effected by the greater elongation of the tissues on the concave side of the epicotyl than on the convex side. The horizontal spread of the leaves is likewise brought about by the more rapid growth of the tissues on one side of the young leaves than on the other. The curvatures of roots and stems, with which we shall have to deal later, are also brought about in the same manner. This method of securing movements in plants by growth is necessarily slower than the corresponding move- ments of animals, which are results of nerve stimulation and mus- cular contraction, but it is nevertheless well suited to the nature of stationary organisms in which a new set of leaves and new growths of stems and roots are produced each year. FIG. 15. A compound leaf of the bean with pulvini The pulvini appear at the base of each leaf- let. Note also a large pulvinus at the base of the main petiole. After Sachs THE PLANT BODY 29 Petiol Petiole In the case of roots the older portions of the root systems are already fixed in their position by the surrounding solid earth; but the new roots which grow from the old ones each spring are enabled to move and penetrate into new soil regions in order to absorb foods and water. In a similar manner each new annual crop of leaves can adjust itself to the conditions con- fronting it in the season in which it must do its work. Flow- ers are likewise able to adjust their positions so that they may secure the visits of pollinating insects, and fruits assume positions suitable for dissem- ination by wind or animals. The older parts, which have lost the power of growth, and therefore of movement, thus become supporting, storing, and conducting organs of plants, while the annual growth of new roots, leaves, and flowers enables the or- ganisms to adjust them- selves to any ordinary change which may take place in the environment, and thus to adapt themselves" to the usual seasonal changes and other requirements of their surroundings. Special motor organs exist in some plants, however, which enable certain organs to execute more rapid and exact move- ments than those described above. Examples of such motor organs are found in the leaves of peas and their near relatives. These special organs are termed pulvini (singular, pulvinus) (Figs. 15 and 16). They are highly modified portions of the petioles at the base of each leaflet arid often at the base of the petiole itself, where it joins the main stem or a branch. The quick curvature of these pulvini serves to swing the leaves into positions which enable them to secure a better adaptation to light or a more complete protection from too great loss of water. Internal and unknown stimuli also affect plant move- ments, in some instances quite as profoundly as the external FIG. 16. Pulvini of the bean Magnified. After Sachs 30 GENERAL BOTANY stimuli discussed above. Thus, the arching of the epicotyl of the pea seedling, already referred to, is not due to external stimuli, but to internal conditions which bring about unequal growth of the organ on two opposite sides, resulting in an adaptive curvature. Some of the adaptive movements of flowers and fruits are apparently due to similar internal con- ditions which appear to affect the organism like external stimuli. These latter are, however, special cases applying to particular plant groups or species and do not affect the general principle that plant movements are usually effected by unequal growth in response to external stimuli. In designating the kinds of stimuli and the nature of the response of plant organs to them botanists have adopted certain terms which are useful in designating stimulus and response as applied to any given organ of a plant. Thus, organs may be said to be protropic when they grow toward the source of the stimulating force, and apotropic when they grow away from such a source. Protropic and apotropic organs will evidently have their main axes parallel with the direction from which a given stimulus acts. Diatropic organs, on the contrary, do not grow toward the source of a directive force, but place their main axes at some angle to the direction of a stimulating agent. If we apply these definitions to the pea seedling (Fig. 13), the main root would be protropic and the stem apotropic with reference to gravity. The secondary roots would, however, be diatropic with reference to gravity, and the leaves diatropic with reference to light. If it is desired to combine the name of the stimulus with the nature of the response, other terms .may be combined with protropic, apotropic, and diatropic. Thus, organs stimulated by gravity are said to be geotropic; if stimulated by light, they are designated as phototropic. Progeotropic, apogeotropic, and diageotropic may therefore be used to indicate the responses to a gravity stimulus, on the one hand, and prophototropic, apophototropic, and diaphototropic to indicate the responses to a light stimulus, on the other. Similar combined terms are used in connection with other stimuli, but these need not be considered in our brief account. TPIE PLANT BODY 31 The gravity sense. It will be of special interest to the stu- dent at this point to learn something of the method by which some of the great botanists of the past laid the foundation for our present understanding of the nature of plant movements and of the relation which exists between external stimuli and the response of the plant to them. For this purpose the classi- cal experiments of Thomas Andrew Knight, Julius von Sachs, and Charles Darwin on the gravity sense of plants and its relation to plant movements may well serve as an illustration. Gravity is the most universal and constant external force acting upon the living plant world, and it is not surprising, therefore, that of all the outside forces this is found to be the most potent in directing the adjustments of plant organs to their environment by movements or tropisms. The general nature of the response of plants to gravity is suggested by the fact that the stems and roots of all plants take up the same position with reference to the earth's center at all points on the earth's surface. Thus, plants on opposite sides of the earth are found to have the main root growing toward the earth's center and the main stem away from it. Likewise, growing plants which have been prostrated by storms, or which happen to grow on steep hillsides, always tend to place their stems in a vertical position with reference to the earth's center. This suggests the general law that roots re- spond to gravity by growing toward the earth's center, while stems tend to grow in the opposite direction in response to the same stimulus. This general law was first tested out in 1806 by Thomas Andrew Knight, an English botanist, who conceived the idea of substituting centrifugal force for gravity, to see how roots and stems would respond to other forces than gravity. Knight attached boxes, in which young plants were growing, to the cir- cumference of rapidly rotating wheels. When the wheels were rotated rapidly enough, he observed that the roots grew toward the circumference of the wheels, with the acting centrifugal force, while the stems grew toward the center of the wheels, or against the acting force. If the wheels were rotated less rapidly, the 32 GENERAL BOTANY roots and stems took up an intermediate position which was a resultant of the response of "the plant to the two forces, gravity and centrifugal force, acting separately. Knight concluded, therefore, that roots and stems respond to centrifugal force acting as a stimulus. His simple appliances are now replaced by more perfect pieces of apparatus, on which disks can be rotated with extreme rapidity and accuracy. If kernels of germinating seeds of corn are placed on such a disk FIG. 17. Diagram illustrating the principle of an experiment by Thomas Andrew Knight (1806) a, position of seeds, roots, and plumule (stem) at the beginning of the experiment ; 6, position of root and plumule (stem) after rapid rotation. Further discussion in explanation of a and b in the text in the positions indicated in Fig. 17, a, and the disk is then rotated for twenty-four hours, the elongating root and stem will gradually assume the positions indicated in b. The roots will all grow toward the circumference of the wheels, while the stems will all grow toward its center. By employing a force which he could control and modify, Knight was thus able to show that the root and the stem could be caused to curve and to take up various positions as a result of their response to this force acting as a stimulus. Since seeds germinating in the soil seemed to behave toward gravity as they did toward centrifugal force in his experiment, he concluded that gravity, acting as a stimulus, directed the growth of the root toward the earth's center and the stem away from it, and that THE PLANT BODY 33 the opposite positions assumed by these organs on the earth's surface were due to gravity acting as the stimulating agent. It is a matter of common experience that if germinating seeds are placed horizontally in soil or with the root pointing upward and the stem downward, the growing organs turn and adjust themselves to grav- ity, as they do to centrifugal force on rotating disks. Lateral organs such as branches and lat- eral roots have also been found to be gov- erned very largely, in the position which they finally assume, by gravity, although other forces are often influential in the ulti- mate adjustment. Knight's early con- FlG< lg> Di agrams illustrating the principle of an elusions were proved experiment by Julius von Sachs (1879) to be correct by the great German botan- ist Julius von Sachs in 1879. Sachs used a different method for proving that grav- a, the position of germinating seeds of corn on a disk, which is then slowly rotated for several hours in a vertical position ; b, the positions assumed by roots and plumule (stem) after twenty-four hours of rotation ; c, the position of germinating seeds of corn on a station- ary disk at the beginning of the experiment; d, the positions assumed by roots and plumule (stem) after twenty-four hours without rotation. Consult the text for a discussion of this experiment ity acts upon plants as a directive stimulating force. He placed growing seedlings on slowly rotating vertical wheels or disks (Fig. 18). As the wheels revolved, the stimulus of gravity continued, but the effect of gravity on the growing stem and root was practically eliminated, since it acted for too short an interval of time on any given side of these organs to secure a reaction in the form of a curvature. Plant organs usually have to remain in a position of stimulation from thirty minutes to several hours in order to 34 GENERAL BOTANY result in curvature. If, therefore, seedlings are rotated so that opposite sides of the stem and root are alternately placed in a position of stimulation for shorter periods than are required for a reaction, the organs will fail to respond to gravity. The time dur- ing which an organ like a root must be continuously stimulated on one side in order to secure a reaction is called presentation time. In Sachs's experiment the presentation time to gravity was too short in any given position of the growing stem and root tip to secure curvature, and these c~~^ } a organs therefore grew in any (/* i direction in which they were placed at the beginning of the rotation. This proved definitely the effect of grav- Fio.19. An experiment by Charles Dar- ^ aS a direc t iv ^ force in win, designed to locate the sensitive the growth of roots and portion of the root to gravity stems in the normal vertical a, uninjured roots of the bean extended position. It will thus be Seen horizontally for twenty-three hours and , T ^ . , , , thirty minutes; b, root tips of the bean that Knight and Sachs em- touched with caustic and extended hori- ployed quite different metll- zontally for the same length of time as , . , . those in a. After Charles Darwin ods 111 their experiments. Knight aimed to substitute another force for gravity in order to note its effect on root and stem growth, while Sachs sought to neutralize and so eliminate the effects of gravity. Knight's experiment proved that roots and stems respond to an external force which can be controlled and its effects therefore proved, while Sachs's experiment showed conclusively that gravity was necessary for the downward growth of roots and the upward growth of the stem. Darwin's work entitled " Tho Power of Movement in Plants " greatly extended the observations already made on the sensitive- ness of the root to gravity and other stimuli. His most important contribution to the subject was the elaboration of the idea, already discovered by Sachs, that the sensitive zone is located in the very apex of the root. Fig. 19 illustrates Darwin's method of locating the sensitive, or perceptive, zone of the root in the root tip of the common garden bean ( Vicia faba). Darwin's THE PLANT BODY 35 conclusion, given in a summary of his chapter on the " sensi- tiveness of the radicle (root) to contact and other irritants," indicates the nature of his contribution to the general subject of the response of the root to various stimuli, including gravity, in its natural growth through the soil. The peculiar form of sensitiveness which we are here considering is confined to the tip of the radicle for a length of from 1 mm. to 1.5 mm. When this part is irritated by contact with any object, by caustic, or by a thin slice being cut off, the upper adjoining part of the radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend away from the side which has been irritated. Some influ- ence must therefore be transmitted from the tip along the radicle for this length. The curvature thus caused is generally symmetrical. The part which bends most apparently coincides with that of the most rapid growth. The tip and the basal part grow very slowly, and they bend very little. Considering the several facts given in this chapter, we see that the course followed by a root through the soil is governed by extraordi- narily complex and diversified agencies, by geotropism acting in a different manner on the primary, secondary, or tertiary radicles ; by sensitiveness to contact, different in kind in the apex and in the part immediately above the apex ; and apparently by sensitive- ness to the varying dampness of different parts of the soil. These several stimuli to movement are all more powerful than geotropism, when this acts obliquely on a radicle which has been deflected from its perpendicular downward course. The roots, moreover, of most plants are excited by light to bend either to or from it ; but as roots are not naturally exposed to the light, it is doubtful whether this sensitiveness, which is perhaps only the indirect result of the radi- cles' being highly sensitive to other stimuli, is of any service to the plant. The direction which the apex takes at each successive period of the growth of a root ultimately determines its whole course ; it is therefore highly important that the apex should pursue from the first the most advantageous direction ; and we can thus understand why sensitiveness to geotropism, to contact, and to moisture all re- side in the tip, and why the tip determines the upper growing part to bend either from or to the exciting cause. A radicle may be com- pared with a burrowing animal such as a mole, which wishes to penetrate perpendicularly down into the ground. By continually 36 GENERAL BOTANY moving his head from side to side, or circumnutating, he will feel any stone or other obstacle, as well as any difference in the hardness of the soil, and he will turn from that side ; if the earth is damper on one than on the other side, he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his downward course and to burrow to a greater depth. Darwin's contributions to our knowledge of the sensitiveness of the root tip have been confirmed by later researches and have done much towards elucidating our ideas concerning the nature of stimulus and re- sponse as applied to plants. Fig. 20 illus- trates the principle of some ingenious experi- ments by Czapek, who confirmed Darwin's gen- eral idea that the sen- sitiveness of the root to gravity is located mainly in the root tip. Czapek forced roots to grow into bent tubes, as illustrated in the figure, so that the last millimeter of the root was at right angles to the body of the root. Such roots, when placed in the position indicated in Fig. 20, a, produced a curvature like that in b. Root tips placed in a position like that of c, however, failed to curve in response to gravity, as indicated in <#, since the sensitive tip was in the normal progeotropic position. When, therefore, the terminal millimeter of the root tip is placed in a position of stimulation (Fig. 20, a), curvature of the root occurs ; but when the terminal millimeter is placed vertically ( These sp a ce s serve in thickening in such a case as this many instances for the passage of air through the plant and are called intercellular spaces. If sections similar to the above are cut from living tissues, as from a root (Fig. 27), the same general cellular structure will be observed as that seen in sections of dead tissues. The cell cavities of the living cells will, however, appear to be filled THE CELLULAR STRUCTURE OF PLANTS 47 Cell WCL ll Cytoplasm -Nucleus " 'Vacuole with a semifluid, viscid substance, not unlike the honey in a honeycomb in consistency and in general appearance. This sub- stance, which fills the cell cavities of all living plant cells, is the living substance of the plant body, to which Von Mohl first gave the name protoplasm. If sections of roots or of other living parts of plants are properly stained, this living substance, proto- plasm, within the cell cavities will be found to be composed of a darker central body, called the nucleus, and a less dense part surrounding the nucleus, called the cytoplasm. For convenience in designating the parts of a living cell the entire mass of living proto- plasm within one cell cavity is termed the protoplast, which, as we have already seen, is composed of two distinct parts, the cytoplasm and the nucleus. The protoplast in young f * / The figures are designed to illustrate the plant Cells (a) usually bears a C ell parts and the gradual formation of vacuoles. a, a young cell with small vacu- oles ; b-d, progressive vacuole formation as it occurs during the enlargement of a cell by growth. Consult the text for further 'Nucleus Nucleolus Cytoplasmic c d FIG. 27. Camera drawings, greatly magnified, of root tip cells containing cytoplasm, nuclei, and vacuoles discussion somewhat different relation to the other parts of the cell from what it does in older cells (b-d). In young cells it fills the cell cavity with a dense mass of protoplasm, while in the older cells it contains water in the form of water drops, called vacuoles. As the cells grow, these water drops, or vacuoles, enlarge and unite until they finally accumulate as a large central water drop, vacuole, or sap cavity in the center of the cell. In such instances the solid protoplast of the young cell becomes gradually forced outward as the cell grows by the accumulation of water in the large central vacuole. The cyto- plasmic portion of the protoplast then forms a thin layer, lining the cell wall and inclosing the central vacuole. It is now prop- erly called the cytoplasmic sac (d). The nucleus in these older 48 GENERAL BOTANY cells with a large central sap cavity is either suspended in the center of the cell by fine cytoplasmic fibrils, or strands, extend- ing from. the nucleus to the cytoplasmic sac, or it lies embedded in the cytoplasmic sac next to the cell wall. In any case a dense envelope of cytoplasm surrounds the nucleus. The cell wall of all plant cells is formed by the living proto- plasm, or protoplast, which finally occupies the cell cavity sur- rounded by the secreted cell wall. In the development of a plant from a fertilized egg cell the first wall is secreted by its proto- plast as a protective envelope for the living protoplasm. All subsequently formed walls are laid down between the two halves of the protoplast in a dividing cell. The^ wall first formed is aygjjiin and delicate and is composed of pectose. Upon this re is immediately deposited a whitish substance called cellu- r ose, which is closely related to starch and sugar in its chemical composition. Bleached celery is composed largely of the cellulose which forms the cell walls, and a bleached celery stalk will give the student a good conception of the general appearance and nature of this cell-wall substance of plant cells. Although the first wall is always thin, it may become greatly changed in its character as the cells of the plant body differ- entiate to form the plant organs and tissues. These modifications of the cell wall are brought about by two distinct methods: first, by cell-wall thickening ; second, by changes in its chemical character. The thickening of the cell wall may be effected by the de- posit of new layers of cellulose on the primary thin wall by the cell protoplast, in much the same way as a new layer of plaster might be added to the wall of a house ; or the new wall substance may filter into the cell wall and be deposited between the cellulose particles of the original cell wall. The thick-walled cells which form the supporting layers of stems and leafstalks (Fig. 26, b and the main areas of the root tip shaded ; b-e, camera . ni i r drawings of cells from the various regions of the root meristem Cells Which a s indicated in the figure. Note the changes in form lie just back of it. The and size of the cells. as growth proceeds. The nuclei , grow with the cells, but they occupy a proportionately Outer cells OI the cap smaller part of the cell cavity as the cells enlarge are large, thin-walled, and loosely joined together. Their lighter color is due to the large vacuoles which nearly fill the cell cavity of each cell. These outer cells are being constantly worn off as the tip of the root is forced through the soil by the elongation of por- tions of the root above or behind the cap cells. The inner cells merge insensibly into the cells of the meristem above them and, 64 GENEBAL BOTANY like the latter, have the power of active cell division, thus pro- ducing new cells to replace those worn off at the apex by contact with the particles of soil. There is no danger, therefore, that the root will be deprived of its protective cap, even though it is con- stantly losing its outer cells by death and abrasion from the soil. The meristem layer is one or two millimeters in length and is composed of a mass of cells which fit into the upper, concave sur- face of the rootcap. Its cells are more or less cuboidal in form, with dense cytoplasm and large nuclei. The meristem layer is therefore readily recognized in stained preparations, both by the small size and regular form of its cells and by its darker color (Fig. 32, a), due to the activity with which the dense protoplasts of the meristem cells take up and retain artificial stains. The nuclei are also large in proportion to the size of the cells and often show clearly the heavily stained chromatin which makes up the bulk of the nuclear protoplasm. Cell division is not so frequently observed in the meristem cells as in those of the elongating zone, although this process is one of the distinguish- ing characteristics of the meristem. The reason for this seems to be the extremely short period of time required by these cells to complete the complicated process of cell and nuclear division. The elongating zone 1 occupies about four millimeters of the length of the root immediately back of the meristem zone. Its cells are distinguished from those of the meristem region by their lighter color in stained preparations and by their greater average length. The lighter color of its cells is due to their less dense cytoplasm as compared with that of the meristem cells, and to the gradual accumulation in their proto- plasts of absorbed water in the form of vacuoles, which increase in number and size toward the upper limit of the elongating zone. We consequently find that the cells in the lower half of the elongating zone resemble closely those of the meristem, into which they graduate insensibly at the junction of the two cell areas, while the cells of the upper half are longer and lighter-colored, with large vacuoles. 1 The upper part of the elongating zone, in which the cells are beginning to differentiate into permanent tissue, is here designated as maturing. GROWTH AND CELL DIVISION 65 The permanent zone merges gradually and indistinguishably into the elongating zone below it and into the mature portions of the root immediately above it. In general, however, its cells are characterized by their great length and large central vacuoles, or sap cavities. The cyptoplasm in the longer cells is usually in the form of a delicate cyptoplasmic sac, in which the nucleus is embedded. The nuclei appear to be rela- tively smaller in these cells, but accurate measurements indicate that they usually maintain a slow growth as long as the cells increase in size, and that they do not di- minish in volume in the cells in which growth is completed. Elongation. In a growing root the meristem layer gives rise to hundreds of new cells. As these new cells are formed part of them remain meristem cells, while others in the upper portion of the meristem begin to elongate and form a part of the elongating zone, as shown in Fig. 34. At the FIG. 34. A diagram illustrating the method of elongation of a root a, shows the main zones of the root shaded as in Fig. 33, a ; in 6, the same zones are shown after the root has elongated. Note that the meristem and elongating zones remain of the same length in a and 6 as growth proceeds same time the cells in the upper part of the elongating zone are reaching their definite size and will add a new disk of cells to the length of the permanent zone of the root. It will therefore be clear that by this process the permanent part of the root becomes continually longer by successive additions from the elongating zone, while the length of the latter zone is kept constant by a similar number of cells derived from the division of the meristem cells. By these two processes namely, cell division and cell elongation the root thus grows in length and its tip advances through the soil. 66 GENERAL BOTANY 7. GROWTH OF OTHER PLANT ORGANS The same phenomena which we have outlined in the growth of the root obtain in the growth of all other plant organs, including leaves, buds, and stems. Cell division and cell enlargement are the two main phenomena of all growth which are demonstrable by scientific methods. In the case of plants the enlargement of cells in growth seems to be mainly due to the absorption of water and the inflation of the cells by osmotic pressure. By this means very rapid growth can take place without the expenditure of the energy necessary to manufac- ture great quantities of living sub- stance with which to fill the cell cavities of the expanding cells. In the slower-growing animals the water-inflation method of cell growth does not exist to any such extent as it does in plants, since the latter need to adapt them- selves quickly to the changing seasons, which necessitates the rapid production of extensive root and leaf surfaces for the absorption of water and gases at the opening of each warm season. The growth of leaves in spring is a good illustration of this aspect of growth in plant organs. A large tree will produce in a few days in spring many square yards of leaf surface ; this involves the production and inflation of thousands of new cells. It is obvious that the tree would be wholly unable in this short time to manufacture enough new protoplasm to fill the tremendous space caused by the expansion of the cell cavities of its leaf cells. The water-inflation method is therefore both economical and neces- sary in all rapidly growing plant organs, such as leaves, stems, and roots. FIG. 35. Growth of leaves of the lilac The spread of the squares indicates a uniform growth of the leaf over its entire surface GROWTH AND CELL DIVISION 67 The place where growth takes place varies in different organs. In most leaves (Fig. 35) growth is uniform over the entire leaf surface, although in the case of long, narrow leaves it may con- tinue longer at the base than at the apex. In stems growth is more localized than in leaves ; it continues for a longer period and extends over a larger portion of the stem than it does in the root tip just studied. In herbaceous stems (Fig. 36) growth may continue at the apex of the main shoot or its branches for several weeks or for the entire growing season, as in the root. This growth, like that of the root, is due to an active meristem at the stem tip, within the apical buds, in which active cell division fur- nishes new cells for con- tinuous growth in the elongating zone, which, as indicated above, may extend over several-inter- nodes at the apex of the stem. FIG. 36. Growth in length of a root of corn and of the stem of a bean seedling; The spread of the markings in b and d indicates the places of greatest growth. Observe the greater area over which growth takes place in the stem as compared with the root In woody stems growth in length takes place in terminal or in lateral buds, as in herbaceous stems. The buds of woody stems, however, usually contain most or all of the parts of the next season's shoot formed in advance, so that its growth in the spring is completed more quickly than in herba- ceous stems, and the plant is thus able to expand its leaves in a comparatively short time when conditions favorable to spring growth occur. In the lilac (Fig. 37) the bud a is composed of the leaves of the next season, arranged on the very short axis of the future branch. The internodes separating the successive 68 GENERAL BOTANY -teams -Bud scales - pairs of leaves at the nodes are very short, as shown in the long section of the bud b. The meristem terminates the axis of the bud, all of which will constitute the elongating zone when growth first starts in the spring. The expansion of the inter- nodes by growth separates the leaves (c and d), which at first grow more rapidly on their inner than on their outer surfaces, and so unfold and spread out horizontally to the light. As growth proceeds the cells within the lower internodes reach their definite length and thus add new segments to the permanent portions of the young stem, as in the case of roots. In this permanent portion the cells differentiate and the main tissues of the mature stem are formed. Be- fore growth in length ceases, the buds for the next season become apparent and proceed to form the young leaves for the growth of the following spring. Indeed, the growing points, or meristems, for these buds are apparent, in the lilac, in the wintering bud in the axils of its rudimentary leaves. In the autumn, therefore, when the leaves fall, the lilac shoot of the season presents the aspect shown in Fig. 38, 5, with all the buds laid down and protected by bud scales. In all these changes incident to growth the cells and organs of plants are subject to the effect of environmental forces. A certain amount of water and heat, varying with the type of organism, is necessary for growth. Light usually has a retarding effect on growth, although this too is a variable factor in its influence. In general it may be said that extremes Leaves , ud scales FIG. 37. Bud structure and growth in the lilac a, 6, surface and sectional views of a bud in the resting condition ; c, d, similar views of the same bud in spring when it first begins to grow GROWTH AND CELL DIVISION 69 of heat, cold, moisture, and food supply are detrimental to growth, while mean conditions in the case of any given factor are likely to be favorable to it. SUMMARY Growth in plants comprises three main phases or stages. The first is that of cell division, in which the cells of the entire structure, or of specialized parts termed meristems or cambiums, undergo rapid multiplication by mitosis. The second phase is that of enlargement, during which the new cells formed by mitosis in- crease greatly in size, and so cause the growth of the organ or part con- cerned. The third is the phase of differentiation and maturation, in which the enlarged cells are modified to form the dif- ferent tissues of the per- manent plant organs. Plants differ from animals principally in the second and third phases of both cell and organ growth. Plant cells increase in size very largely by the absorp- tion of water, which collects inside the cells in the form of water drops, or vacuoles. In animals the increase in the size of the cells is largely due to actual increase in the amount of living substance. The differentiation and maturation stages of cell growth are also necessarily different in plant and animal cells, on account of the dif- ferent kinds of permanent tissue formed in plant and animal organs. Plant organs differ in their methods of growth according to the nature of the organ or part and the length of time during which enlargement continues. Organs which have a short period of growth, FIG. 38. Lilac twigs in summer and winter conditions and b represent the season's growth of the bud shown in Fig. 37 70 GENERAL BOTANY like leaves, fruits, seeds, and tubers, usually grow more like animals, by a uniform increase in the size of cells throughout the entire structure. Cylindrical organs, on the contrary, like roots and stems, which have periods of enlargement often extending over many years, have definitely located growing masses, or layers, of cells, called meristems and cambiums. These meristem and cambium cells renew their cell divisions each season, in the case of perennial organs, and contribute new cells, which then pass through the second and third phases of growth to form new tissues and organs. The apical meristems which terminate the roots, the stems, and the branches have al- ready been studied. The cambiums are cylindrical layers of meristem cells lying between the wood and the bark in steins and roots and between the outer cork and inner bark. These cam- biums enable roots FIG. 39. Diagrams illustrating the method of elon- gation of roots and herbaceous stems a, 6, elongation of the root tip illustrated as in Fig. 34; c, (/, similar method of representing the elongation of a herbaceous stem like the bean (Fig. 36). Consult the text for further discussion and stems to grow in diameter and also to form new annual tissue layers for protection and for the con- duction and storage of foods, water, and soil salts. The apical meristems also enable plants to send out new leaves and roots each season for the absorption of raw food elements and the compounding of these *raw materials into organic foods. The above general statements relative to the growth of stems and roots by means of apical meristems and cambium layers will be more readily understood by reference to the following diagrammatic figures: Fig. 39, a and b, is a repetition of Fig. 34, illustrating the method of growth of roots in length by means of an apical meristem. Fig. 39, c and d, illustrates in a similar manner the growth of a herbaceous GROWTH AND CELL DIVISION 71 stem, represented in Fig. 36, e and d. In the latter cases the meri- stem is practically identical in form and structure with that of the root tip. It does not, however, produce a protective rootcap, since it is here protected by the enveloping leaves of the terminal bud. Growth in length takes place in the stem, as in the root, by the continuous transformation of cells produced by the division of tl o meristem cells into cells of the elongat- ing zone. Later these become permanent tissue and form the mature nodal and internodal tissue of the older portions of the stem. At the same time a cylindrical cambium layer, in the region indicated by the dark lines in the figure, begins to divide to form the new water-conducting and food-conducting tissue of the vascular, or woody, cylinder of the plant. Fig. 40 represents the elongation of a woody stem by a large terminal bud. In this instance the gen- eral structure of the meri- stem and the method of elongation are the same as in the herbaceous stem. The differences between the two arise from the fact that the herbaceous stem has a more continu- ous growth throughout the season than the woody stem, and from the fact that the buds of the herbaceous stems con- tain only a small part of the season's growth in miniature, while those of the woody stem contain all of the nodes, internodes, and leaves for the next year, laid down in advance. When such a bud (a) begins to grow in the spring, the tissues of the elongating zone, laid down the year before, produce the terminal twig of the sea- son (c), while the meristem produces a new bud, shown in c, resem- bling the mother bud (a) in structure and function. The tissues of the elongating zone in a are already partially differentiated, but FIG. 40. Three diagrams showing method of elongation of a lilac bud a, bud in winter condition, similar to Fig. 37, b ; b, elongation stage in spring, similar to Fig. 37, d; c, permanent condition, similar to Fig. 38. Meri- stem, elongating, and permanent zones shaded as in Figs. 33, 34, and 39 72 GENERAL BOTANY with their great increase in length, b and c, they gradually form the tissues of the bark, cortex, and woody cylinder of the fully formed twig (c). While this extension and differentiation is pro- ceeding, a new cylindrical cambium, formed within the wood cylinder, adds new conducting tissue to the stem in a manner to be explained later. From the above discussion it is apparent that roots and stems grow in essentially the same manner, with slight modifications due to the nature of the particular organ and its environment. Growth also plays a prominent part in the movements of plant organs and in the ultimate form of the plant body, as shown in an earlier chapter. THE CELL AND CELL DIVISION MINUTE STRUCTURE OF THE CELL Before proceeding to discuss the complicated process of cell division it will be necessary to review briefly the structure of the cell and to point out certain details of cell structure which we have not heretofore consid- ered. In the brief discussion which follows, the term resting cell will be used to indicate the con- dition of a cell when it is neither dividing nor in active preparation for cell division. Fig. 41 is a camera drawing of a resting cell of a root " . . tip, in which it may be seen that the protoplasm a root tip in the f> j r resting stage of such a cell presents the aspect of a mesh, or Note the distributed network, inclosing light spaces. In the living condition of the dark state fa Q meshwork would be made up of liv- chromatin net . ing cytoplasm, and the light spaces would correspond to minute vacuoles. This appearance of the living substance in artificial preparations has received various interpre- tations, which cannot be discussed in an elementary textbook. The simplest conception for the beginning student is to regard the living substance as spongy in structure, resembling the struc- ture of a fine bath sponge, with lamellse, or plates, of living pro- toplasm surrounding minute spaces filled with cell sap. Such a spongework, when cut in thin sections, would appear like the GBOWTH AND CELL DIVISION 73 network of the cell protoplasm in our figure. The nuclear wall, or membrane, is a delicate bounding membrane which in the liv- ing condition of the cell is composed of cytoplasm. Within this membrane the nuclear protoplasm is seen to have essentially the same structure as the cytoplasm, except that the bounding walls of the meshes in the nuclear protoplasm are heavier and more granular and stain more deeply in permanent preparations. This deeply staining protoplasm of the nucleus is called chromatin (from chroma, meaning " color ") on account of its avidity for various stains used in preparing cells for microscopic study. The chromatin substance is very essential in cell division and in reproduction, and its distribution in the nucleus of the resting cell is thus of extreme importance. In addition to the chromatin network the figure shows a conspicuous nucleolus surrounded by a characteristic nuclear vacuole. Under high powers of the micro- scope the nucleolus appears to be suspended in this vacuole by delicate strands of nuclear substance which connect the nucleolus with the chromatin network, or spongework. With this prelimi- nary review of the minute structure of a resting cell we may proceed to consider the stages of cell division. CELL DIVISION MITOSIS Function. We have learned that the cells in the meristem .and the elongating zones of growing root and stem tips increase in number by cell division and that the cells thus produced grow into the mature organs and tissue of the plant. The same thing happens also when the single egg cell of the plant forms a many- celled embryo by division. The embryo then differentiates and grows into a new organism. The multiplying, or increase in number, of cells in an organism for purposes of growth is there- fore the most obvious use of cell division in plants. Although this production of new cells appears at first sight to be the main object of mitosis, biologists in recent times have called attention to the remarkable precision with which the chro- matin substance of the nucleus is divided and to the complicated mechanisms of mitosis by which this equal division is effected. 74 GENERAL BOTANY The goal of cell division, therefore, is probably not simply a division of a mother cell into two essentially equal daughter cells but also the distribution of exactly equivalent masses of chromatin, called chromosomes, to each daughter nucleus of the newly formed cells. We shall learn that this equal division and distribution of the chromatin substance to all the cells of a com- plex organism is the basis for the modern theories concerning reproduction and inheritance. We may therefore anticipate with interest the discussion of these important and complicated proc- esses, concerned with the division of the cell and the nucleus, which are comprehended under the general term mitosis. Process. It is customary, for the sake of clearness, to describe the various processes of mitosis under certain phases or stages (Fig. 42). These phases, taken in the order of their occurrence, are the prophase, metaphase, anaphase, and telophase. The stu- dent should bear in mind, however, that the processes included under the above phases are continuous processes, and that one phase graduates insensibly into the next phase, which follows it in orderly sequence. Some of these phases, such as the meta- phase, are undoubtedly longer than others, which are passed through more quickly. This is indicated by the fact that in prepared slides certain phases are much more prominent than others, for the probable reason that their longer duration makes it easier to fix a larger number of nuclei in these stages. All the phases of mitosis are passed through rapidly, however, and the entire process of cell and nuclear division never occupies more than a few hours. The prophase is the preparatory phase of nuclear division, during which changes take place in the nucleus and the cell preparatory to the equal division of the chromatin substance, which is the principal goal of nuclear division. These prepara- tory changes involve two structures which play an important part in the ultimate division of the nucleus. These structures are the chromatin, which we have learned is a permanent portion of the nucleus, and the spindle, which is a temporary structure apparently designed as a framework on which the chromatin gathers and finally becomes distributed in equal amounts to the GROWTH AND CELL DIVISION 75 daughter nuclei. Although the changes which the chromatin undergoes during prophase are coincident with the building of the spindle, it is easier and clearer to discuss the chromatin changes and the origin of the spindle separately. We are already FIG. 42. Drawings illustrating the main stages in mitosis a-e, prophase ; /, metaphase ; g, anaphase ; h-i, telophase ; j-k, cell division ; I, daughter nuclei familiar with the fact that the chromatin in the nucleus of a rest- ing cell is in the form of a net, or meshwork. In early prophase, when the nucleus begins its preparation for division, it may be noted that the chromatin granules begin to accumulate along definite portions of the resting network, thus giving rise to denser masses of chromatin at certain points (a). These dense masses of chromatin, which are at first irregular, ultimately assume a 76 GENERAL BOTANY more definite form and arrangement within the nuclear cavity. In some instances they seem to form a continuous band of chro- matin substance, which is called the spirerne (ft). In other cases definite rodlike masses, called chromosomes (c), emerge directly from the nuclear network. In either case the conversion of the network of the resting nucleus into the definite masses known as chromosomes is most easily understood by supposing that after the chromatin granules have condensed to form either the spireme or the isolated chromosomes, the remaining portions of the original network break down and leave the chromosomes free. As soon as the chromosomes are formed in the above manner they shorten and thicken and finally take up a definite position at the periphery of the nuclear cavity next to the nuclear membrane. The chromatin rods are now ready for the equal division of their substance preparatory to the formation of daughter nuclei. Since the further changes in the chromo- somes during prophase are concerned with the spindle, they will be described in connection with the following discussion of that structure. The spindle which is concerned with the equal distribution of the chromatin substance in nuclear division may for convenience be termed the first spindle, to distinguish it from the second spin- dle, which is concerned with the building of the cell wall which divides the cell as a whole. In such cells as those found in grow- ing root tips the first spindle makes its appearance as two fibrous masses of cytoplasm at the opposite ends, or poles, of a nucleus in prophase (c). The fibers, which are composed of living cytoplasm, become more conspicuous as the spindle continues its forma- tion at either pole of the nucleus. Finally two half spindles are formed, each with a conical apex and a broad base. The base of each half spindle fits over one pole of the nucleus like a skull- cap, and the fibers composing these half spindles appear to extend from its apex, or pole, to the nuclear membrane (c?). When the half spindles are fully formed, the nuclear membrane gradually disappears, beginning at the poles of the nucleus, and the half spindles elongate across the nuclear cavity. They ultimately unite to form a complete single spindle, with a bulging equatorial GROWTH AND CELL DIVISION 77 region and conical poles (e). The spindle fibers are supposed finally to pass through the nuclear cavity from pole to pole of the complete spindle. When the spindle began to form, the chromo- somes occupied the periphery of the nuclear cavity next to the nuclear membrane. As the half spindles elongate across the nuclear cavity with the disappearance of the nuclear membrane the chromosomes appear to be pushed into the equatorial region of the nucleus between the two half spindles, forming what is some- times called the equatorial plate. The chromosomes of this equa- torial plate ultimately move toward the outside of the completed spindle and become arranged in a definite radiate manner at its periphery, attached to the outer spindle fibers. On account of their radiate appearance at this stage it is often called the mother star stage (/). The particular fibers to which the chromosomes are attached are called the traction fibers, since, as we shall learn, they appear to contract and separate the two halves of each chromosome in the next stage of mitosis. The remaining fibers of the spindle are termed the central spindle fibers, or supporting fibers. With the completion of the spindle and the arrangement of the chromosomes in the mother star stage the events of the pro- phase are completed and those of the metaphase_are ushered in. The metaphase stage (/) is concerned with the equal longitu- dinal division of the chromosomes into half chromosomes and with the separation of these half chromosomes preparatory to the mi- gration of the chromosomes to the poles of the spindle to form the daughter nuclei. In this process each chromosome splits through- out its entire length into two equal halves, each of which appears to be attached to a traction spindle fiber. The half chromosomes then begin to separate, as though pulled apart by the shortening traction fibers. The final separation of the half chromosomes marks the end of the metaphase stage. The anaphase is the stage (g) in which the half chromosomes continue their migration toward the poles of the spindle, where they finally arrange themselves in a more or less radiate manner resembling somewhat the mother star stage of metaphase. This radiate arrangement of the daughter half chromosomes is there- fore called the daughter star stage and marks the end of anaphase 78 GENERAL BOTANY and the beginning of telophase (7i). During their migration to the poles the chromosomes assume various forms in the nuclei of different plants and in those of the same plant in different kinds of cells. In the root tip the chromosomes during anaphase are usually greatly elongated and often hooked like a shepherd's staff, while in the germ cells of the same plant they are more often V-shaped and greatly shortened and thickened. In nuclei in root tips the traction fibers appear to be attached to the bent chromosomes at the curve of the hooked chromosomes, while in the germ cells they are attached at the point of the V. It should perhaps be stated that there is no evidence that the traction fibers actually contract and pull the chromosomes to the poles, beyond the facts of their apparent attachment and the peculiar appear- ance of the chromosomes at this period, when they look like plastic rods being pulled poleward. It is quite possible that the chromosomes move to the poles by virtue of their own inherent power of movement, or else by attraction exerted at the poles during anaphase. The telophase (A, i) is the final phase of mitosis and includes the organization of the daughter nuclei and the division of the cell into two daughter cells. The formation of the daughter nuclei occurs after the daughter star stage, which marks the close of anaphase. The daughter chromosomes, in the stage immediately following the daughter star arrangement, draw together and adhere to form a dense mass of chromatin at either pole of the nucleus. A new nuclear membrane is now formed around each chromatin mass by the cytoplasm. Within each daughter nucleus thus initiated a nuclear vacuole arises ; the chromatin mass be- gins to loosen up, and the outlines of the daughter chromosomes reappear. The chromosomes then begin to spread out and unite by new anastomosing branches of their chromatin substance, while vacuoles appear in increasing number within each chromosome. By the formation of these vacuoles and new anastomosing branches the chromosomes are soon reduced to the form of a network, or meshwork, quite similar to that of an ordinary resting nucleus. With the growth of the chromatin net the nuclear sap increases in volume and apparently inflates the nuclear membrane, which GROWTH AND CELL DIVISION 79 .thus augments the size of the nuclear cavity. In the nuclei of root tips and similar vegetative plant parts the two daughter nuclei finally reach the stage of perfect resting nuclei, with chromatin net, nuclear sap, and nucleolus. The nucleolus grows gradually with the chromatin net, but its origin is still obscure. It should not be overlooked by the student that the above proc- esses, which lead to the formation of the new chromatin net of the daughter nuclei by vacuolization and anastomosis of the daughter chromosomes, is exactly the reverse of the processes by which the chromatin net of a resting nucleus is transformed into chromosomes. Condensation of a chromatin net to form the chromosomes of the mother nucleus is always followed by expan- sion of the chromosomes to form the resting net of the daughter nuclei in the vegetative cells of plant organs. In germ cells a slightly different procedure is usually manifested at certain stages in their division processes. O A Cell division is initiated by the formation of a dense spindle (/) which we have called the second spindle, in the space occupied by the central spindle fibers of the first spindle during anaphase and early telophase. It soon becomes barrel-shaped (&), with very dense outer peripheral fibers, which stain heavily with cytoplasmic stains. This second spindle unites the forming daughter nuclei, and has for its function the formation of the new cellulose wall, which completes the separation of daughter nuclei and the protoplast of the mother cell into two daughter cells. The new separating cellulose wall is secreted by a dense cell plate of cytoplasm, which apparently arises as thickenings of the fibers of the second spindle at the center of each fiber. These fiber thickenings increase in size and finally unite to form a solid cytoplasmic disk, or plate (/), extending across the equator of the spindle. The spindle at the same time increases in diameter and stretches entirely across the cell from wall to wall, dividing its protoplast into two equal parts. The cell plate then splits, the split beginning at its center and extending to the junction of the cell plate with the cellulose walls of the mother cell. The new cell wall is formed by the deposit or secretion of cellulose particles between the halves of the cell 80 GENERAL BOTANY plate. When the wall is complete, the halves of the cell plate form the outer layer of cytoplasm of the newly formed daughter cells, next to the new cell wall. Meanwhile the meshed struc- ture of the cytoplasm appears between each daughter nucleus and the new cellulose wall, which marks the completion of cell division and the formation of the two daughter cells. When one considers that all of the above phases of mitosis are necessary for the formation of each pair of new cells in a growing organism, some conception is gained of the immense constructive activity going on in every growing plant or animal. REDUCTION DIVISION AND REPRODUCTION The method of cell division, which we have traced above in the cells of root tips, obtains in all vegetative parts of plants, including roots, stems, and leaves, and is hence called vegetative FIG. 43. Diagrams designed to show the difference between vegetative and reduction (heterotypical) mitosis , ordinary vegetative mitosis in root-tip cells with equal division of chromosomes, as in Fig. 43 ; b, mitosis with reduction of chromosomes to one half in each daughter nucleus. Further discussion in the text cell division (Fig. 43, a). In this type of mitosis the chromosomes which are formed in a cell in prophase (7) line up on the equa- tor in metaphase (2) and split longitudinally to form daughter chromosomes. These daughter chromosomes then migrate to the poles of the spindle (&) and form the daughter nuclei of two new daughter cells (4). By this method of mitosis all cells of the vegetative plant body are supplied with an equal number of chromosomes. GROWTH AND CELL DIVISION 81 In all of the higher plants a' different kind of cell division, called the reduction division (Fig. 43, 6), occurs in the spore mother cells which give rise to the spores from which the gametes, egg and sperm cells, are ultimately derived. In such a spore mother cell - for example, the mother cell of a pollen grain the chromosomes become associated in pairs in prophase (7) and are thus arranged on the equator of the spindle as double chromosomes in metaphase (2). These paired chromosomes do not split in metaphase as they do in vegetative mitosis, but the Egg cell cell Seedling plant FIG. 44. A diagram illustrating the history of the chromosomes in the develop- ment of a plant Note the following important facts brought out in the diagram: The chromosomes are three in number in each gamete ; they are doubled in the zygote by fertilization ; in the succeeding vegetative mitoses the number of chromosomes remains the same as in the zygote cell. Consult the text for a discussion of this figure two chromosomes of each pair separate as whole chromosomes and migrate to opposite poles of the spindle in anaphase (3) to form the chromosomes of the new daughter nuclei (4*). As a result each daughter nucleus receives one half of the number of chromosomes contained in the original mother cell, and so one half of the num- ber contained in all of the vegetative cells of the plant body. This method of cell division is termed reduction division, and it is of the greatest importance in reproduction and in the life his- tory of plants. In the formation of spores in the higher plants the daughter nuclei divide at once, by the ordinary vegetative method without reduction (^), to form four nuclei, which remain associated in groups of four called tetrads (#). Each member 82 GENERAL BOTANY of the tetrad then becomes a spore, from which the gamete cells, egg and sperm, are ultimately derived. These spores and the gametes derived from them will consequently have the reduced number of chromosomes characteristic of the reproduc- tive cells of the higher plants. Fig. 44 illustrates the relation of the reduction division to sexual reproduction and the development of a plant organism. In the figure the male and female gametes, which have been derived from the spores with the reduced number of chromo- somes, are represented as having three chromosomes each. When these unite in fertilization, a zygote cell is formed which contains six chromosomes, or the sum of the chromo- somes of the male and female gametes. The divisions of the zygote cell which follow in the development of the embryo and seedling are of the vegetative type represented in Fig. 43, a, and hence each cell of the seedling will be furnished with six chromosomes. This number remains constant also in all the vegetative cell divisions which occur during the development of the adult plant. It is now known that every species among the higher plants has the same chromosome history as that sketched above, in that the chromosomes are always doubled when fertilization occurs and are reduced to one half the vegetative number in the spores and the gametes. A similar reduction division occurs in animals during the formation of the egg and sperm cells. It is probable also that all of the lower plants have a reduction division at some point in their life history which corresponds to that described above for the higher plants. It is easily seen that if reduction in the number of chromosomes did not occur at some point in the life cycle of each individual organism, the chromo- somes would ultimately become innumerable in the cells of all of the higher plants. Reduction division is also supposed to have an important bearing on the method of inheritance of parental characters, which will be discussed in a later chapter. The great precision and regularity with which the vegetative and reduction cell divisions are carried out in the life of each organism is a sufficient guaranty of their fundamental biological importance. CHAPTER VI THE STRUCTURE AND FUNCTIONS OF STEMS, ROOTS, AND LEAVES WOODY STEMS GROSS STRUCTURE The most evident function of the stem is that of displaying advantageously the leaves, flowers, and fruit for the performance of their proper functions. The main stem is also an intermediary between the roots and the leaves, and as such it performs impor- tant functions in the storage of reserve foods and in the trans- portation of water and soluble food materials. We shall soon learn that in its external features and in its internal structure the stem is a living, active organ in which structure and function are admirably correlated. In our study of the woody stem we shall consider first its gross external features and then its more minute internal structure. It may interest the student to know also that woody stemmed plants are now regarded by some botanists as the forerunners of the soft-stemmed, herbaceous plants of the present day. It is there- fore appropriate to reverse the usual order of presentation and consider the woody stem first. External features. Fig. 45 represents the external features of a shoot of a lilac which has been produced by a season's growth, as described in the last chapter. In the specimen selected for the illustration, both the main shoot and the smaller lateral shoot have two lateral buds which have replaced the terminal bud. This latter condition is the more common one in the lilac, except in the sucker shoots which spring directly from the roots of the old plants. In the figure it may be seen that the fallen leaves have left definite scars, the leaf scars, below each lateral ' 83 84 GENERAL BOTANY lateral bud*, at apex of stem bud, and that the lower limit of the season's growth on both the main and the lateral shoots is marked by a ring of bud-scale scars. The brown bark which gradually covers over the early green bark of the growing shoot is also seen to be broken by minute pores, the lenticels, through which air penetrates from the outside to the internal tissues of the shoot. Carbon dioxide and water vapor are also eliminated from the lenticels as they are from the stomata of leaves. These same structures, together with the wood cylinder, will appear in the sections of the stem to which we will now turn our attention. Internal structure. The gross structure of the shoot is represented in Fig. 46 as it appears in a transverse section. The epi- dermis and the brown bark are not distinguishable in gross sections and appear as a single brown layer cov- ering the outside of the entire section. This layer is designated as the brown bark, within which is the green bark, composed of cells which contain green chloro- phyll ; within the green bark there is an inner lighter layer, called the inner bark, or phloem. The wood cylinder is clearly marked, but the cambium, or growing layer, cannot be clearly distinguished from the inner lighter bark. Its position at the junction between the bark and the wood is indicated by a line. The pith occupies the center of the section. Within the wood cylinder two annual rings of wood are represented as having been formed. The wood rays are also shown extending Year's growih m length FIG. 45. External features of a lilac twig in winter In the lilac the terminal bud dies early and is not usually present in mature twigs. The spring growth is produced from the last two lateral buds formed at the end of the twig of the season, as in the figure STEMS, ROOTS, AND LEAVES 85 as radial lines in the wood. The annual rings arise from the cambium layer and increase the diameter of the stem annually, thus stretching and cracking the bark jacket of older trees. The wood of each annual ring is divided into two zones, called spring wood and summer wood. The spring wood is distin- guishable by being porous ; that is, it is made up of cells with larger openings, which give greater porosity to the spring wood than to the summer wood. These larger cells, or pores, are the cavities of the large water ducts, which are usually more numerous in spring wood, since it is formed when a large sup- ply of water is needed for the growth of buds, leaves, and flowers. The summer wood, on the contrary, is made up of thick-walled cells (which are smaller than those of the spring wood) and has fewer water FlG ducts. Its chief functions for the stem seem to be mechanical and storing. Since the last-formed and denser summer wood of one season abuts upon the first-formed porous spring wood of the next season, the junction line of the annual rings is usually clearly marked. The width of the annual ring for any given season is determined in part by the age of the tree and in part by external conditions. Periods of drought or the loss of leaves by frost or by insects check the annual growth and may even result in a double ring of wood in one season if growth is resumed in the latter part of the summer after such a checking process. Most trees also grow slowly in extreme youth or old age, while growth is most rapid in the middle life. Gross structure of a lilac shoot two years of age The limits of the pith, wood, bark, and epider- mis are shown in the sector of the stem at the left. The principal tissue layers in each of the above subdivisions are shown at the right in the upper and lower sectors 86 GENERAL BOTANY In sections cut across more mature stems of trees like the oak and the alder the above layers will be found to be considerably changed in their relative width and structure (Fig. 47). In such sections the outer, brown bark is much thicker and is often cracked or seamed by the great increase in the diameter of the central wood cylinder. The green bark has also usually disappeared in mature stems and forms a thin layer of crushed cells be- tween the outer corky bark and the inner light bark, or phloem. Cork bark Skeletal tissue (fibers and sderenchyma) Phloem Cambium Summer wood Spring wood Wood ray FIG. 47. Transverse section of an oak stem eight years old. Diagrammatic The central darker dotted portion is heartwood ; the outer wood layers, with alter- nating light spring wood and dark summer wood, belong to the sap wood The wood cylinder is also differentiated into an outer light area of sapwood composed of several of the latest-formed annual rings of growth, and of a darker central portion, the heartwood, made up of the first-formed wood in the center of the tree. The sap- wood contains living cells in the form of wood rays and wood parenchyma and is active in the conduction of water and food and in the storage of food reserves. The heartwood is dead and serves a purely mechanical function in supporting the tree. Its cell walls are impregnated with various substances which change their color and increase their strength. STEMS, KOOTS, AND LEAVES 87. The wood rays are more numerous than in young shoots and vary greatly in length and width according to their position and the time of formation. This is due to the fact that as the wood increases in circumference new rays are started from the cam- bium at various points year by year. The new rays supply a Transverse Transvers Annual ring Radial section Oblique -tangential section a b Quarter -sawing c Straight grain d avy grain f FIG. 48. Various cuts of wood drawn to show the grain and other structural features Consult the text for an explanation of these figures larger food-storage system for the tree, which, with the disap- pearance of the pith and cortex, comes to be lodged more and more in the wood rays and wood parenchyma of the wood cylinder. The pith usually disappears very early and is marked by a dark spot in the center of the tree. The above tissues and tissue layers have a very different appearance when observed in wood blocks or in thin sections cut in 88 GENERAL BOTANY transverse, radial, and tangential sections. Fig. 48 illustrates the appearance of the annual rings and of the spring and summer wood as they appear in transverse and longitudinal sections of stems four and five years of age. In the transverse cuts at the upper end of Fig. 48, a and >, the spring and summer wood appear as in the oak (Fig. 47), but in the radial (a) and oblique (5) sections they present a very different appearance. In the radial section the spring and summer wood appear as narrow FIG. 49. Sections of wood of bird's-eye maple (Acer saccharum) Transverse section at the left, radial section in the center, and tangential (curly grain) at the right in the figure. The curly grain is due to irregularities in the growth of the annular rings. Photograph furnished hy the United States Forest Service and wide vertical lines or bands, while the wood rays run across the grain as darker horizontal bands. In the oblique-tangential section the rays have a similar appearance in the upper semi- radial portion of the section, but in the lower tangentially cut portion they show as vertical lines of varying height and width. This difference in appearance in the wood rays is due to their shape and their radiate arrangement in the tree trunk. The " silver grain " in finished woods, notably in quarter-sawed oak, is due to the wide wood rays, which have a shining, STEMS, ROOTS, AND LEAVES 89 bandlike appearance when viewed in radial sections. Fig. 48, ) is usually composed of a single layer of cells without green chloroplasts, which inclose and pro- tect the more delicate mesophyll cells over the entire surface of the leaf. Its cells are distinctive in that their outer walls are covered with a waxy secretion which makes them almost com- pletely impervious to water and air. The epidermis thus forms an effective protection against excessive evaporation from the mesophyll cells, which would wither and destroy the leaf. The lack of green chloroplasts in the epidermis also enables the sun- light to penetrate easily its translucent cell Avails and illuminate the green mesophyll cells below it. The stomata are highly modified cells of the epidermis which regulate to a certain extent the flow of gases into the leaf and the evaporation of water from the internal air spaces between the mesophyll cells. Each stoma is composed of two guard cells containing chloroplastids, between which there is a narrow slit, or pore, averaging about .008 mm., or ^-Q-Q of an inch, in width (Figs. 62, b and c). Although the pore between the guard cells is so very small, ample provision is made for the exchange of gases with the external air and for the evaporation of water from the leaf, since it is estimated that the number of stomata in the epidermis of common leaves averages from 100 to 700 per square millimeter. Stomata are usually open in daylight, when the mesophyll cells are manufacturing sugar and starch, and arc partially or wholly closed at night, during the period of leaf inactivity. The exact mechanism for the control of stomata is not thoroughly understood, although the guard cells are known to be sensitive to light. The mesophyll cells vary, in size, form, and arrangement, in different kinds of leaves. In the more typical horizontal leaves (Fig. 62, a) the mesophyll cells of the upper surface next to the epidermis are greatly elongated, with their long axes per- pendicular to the leaf surface. On account of their palisade- like form and arrangement this layer of mesophyll cells is termed the palisade layer. In vertical or erect leaves, such as those of narcissus and many lilies, the palisade layer often extends entirely around the leaf as a continuous layer beneath STEMS, ROOTS, AND LEAVES 115 the epidermis. The remaining cells of the mesophyll are called spongy cells, since they are more loosely arranged than the palisade layer, with large intercellular spaces like the canals and pores of a sponge. This canal system of intercellular spaces within the mesophyll of the leaf contains air, with water vapor and gases. Since the composition of the air within the leaf differs from the external air in the relative amount of water and gases contained in it, we may properly designate it as the Epidermis ./*= Palisade parenchyma Midcein Spongy parenchyma rw Gtianl cell Air Guard cells C FIG. 62. Microscopic structure of a leaf of the milkweed (Asclepias) a, a transverse section of a milkweed leaf; 6, a diagrammatie-drawing illustrating the structure cf the guard cells ; c, the guard cells of the milkweed enlarged internal atmosphere of the leaf. The gases and water of this internal atmosphere come in contact with the external atmosphere outside the leaf through the stomata, with which the intercellular spaces of the leaf are directly connected. All of the mesophyll cells are also living cells, in which green chloroplastids are embedded in a living cytoplasmic sac surrounding the large central water vacuole. The chloroplastids are thus placed at the outside of the cell, where they are exposed to sunlight and to the constituents of the internal atmosphere. The importance of these internal arrangements of the leaf will be more fully appreciated when we consider its function in starch manufacture and in the evaporation of water. The veins are composed of two kinds of tissue cells; namely, supporting and conducting cells. The thick-walled supporting 116 GENEEAL BOTANY cells flank the veins above and below to prevent the collapse of the conducting cells and to strengthen the entire framework of the leaf. The conducting cells are in the form of vascular bun- dles, which are continuations of the vascular bundles of the stem and root. They have the same general character as those of the stem, except in the smaller veinlets, where the conducting tissue is reduced to a few cells. The vein tissue thus puts the manufacturing mesophyll cells in direct connection with the main axis of the plant. SECTION III. PHYSIOLOGY CHAPTER VII NUTRITION AND SEASONAL LIFE OF PLANTS PHOTOSYNTHESIS AND RESPIRATION The student has now become familiar with the form of plants, and with their adjustments to the environment. He has also studied the structure of the organs and tissues of the plant body, upon which a proper understanding of its various functions is based. The following discussion will therefore be confined to the main physiological processes concerned with nutrition ; namely, photosynthesis, respiration, transpiration, assimilation, and the digestion of foods. PHOTOSYNTHESIS If a green leaf which has been exposed to bright sunlight for a few hours is bleached with alcohol and then tested for starch with iodine, the green mesophyll areas will be tinted blue or bluish black. This will be found to be due to minute starch grains, which are formed in the green chloroplastids of the mesophyll cells during the exposure of the leaf to the sunlight. The starch grains are formed in such numbers throughout the green tissues of the leaf that the latter is tinted, owing to the reaction of the iodine on the abundant starch. This simple experiment indicates the principal function of the leaves, which is to manufacture sugar and starch for the plant. This sugar and starch then becomes a basis for the manufacture of other foods, such as the fats and the nitrogenous foods, or proteins. It is probable that the leaf is the main center for the compounding of these nitrogenous foods from the sugar which 117 118 GENERAL BOTANY it manufactures within its green mesophyll cells and the nitrog- enous salts which come up to the leaf from the roots in the water stream. The leaves, through their green cells, are there- fore the manufacturing organs for the other members of the plant body, which are dependent upon them for nourishment. The term photosynthesis means literally the uniting, or com- pounding, of substances by means of light. When applied to the work of a green leaf it signifies the making of sugar from the simple raw materials, carbon dioxide (CO 2 ) and water (H. 2 O), by means of energy supplied to the leaf by sunlight. These raw materials for photosynthesis are supplied from the soil through the roots and stem and from the air through the stomata. The carbon dioxide is absorbed by diffusion into the internal atmos- phere of the leaf from the external air, and is then taken up by the chloroplasts located in the mesophyll cells surrounding the intercellular spaces of the leaf. The water which combines with the carbon dioxide, although ultimately supplied by the roots, is immediately absorbed for sugar-building from the cell sap of the mesophyll cells. Since carbon dioxide, like other gases, tends to move from a point where it is abundant, or concen- trated, to a point where it is less abundant, we must picture it as constantly flowing through the stomata into the leaf during the day, to take the place of that which is absorbed from the in- ternal atmosphere of the leaf in the intercellular spaces by the mesophyll cells for the making of starch and sugar. The excess of oxygen which is liberated during photosynthesis likewise dif- fuses out of the leaf or is partly used in the process of respira- tion, which goes on both day and night in the leaf as in other living parts of plants. One of the first products of photosynthesis is undoubtedly sugar, but the excess of sugar produced, which is not used by the living cells for growth and repair, is usually transformed into starch within the plastids of the leaf cells themselves. The excess of sugar and starch formed in the leaf is later transported in the form of sugar into the special storage tissues of the stem, roots, fruits, and seeds. During the day this excess of starch accumulates in the leaf cells, as can be demonstrated by testing NUTRITION AND SEASONAL LIFE OF PLANTS 119 the leaf with iodine, but it is finally transported back into the other organs of the plant, either for immediate use or for storage. The process of photosynthesis is not fully understood, but it is supposed to involve the union of carbon dioxide and water to form carbonic acid (CH 2 O 3 ). Under the influence of sunlight and chlorophyll the carbonic acid is reduced to form a com- pound, possibly formaldehyde (CH 2 O), which is then multiplied or condensed into sugar (C 6 H 12 O 6 ). A part of the original molecule of carbonic acid is at the same time given off in the form of free oxygen, which represents the excess of that gas not needed for the building of sugar molecules. The reduction of the carbonic acid is accomplished, in some manner which is not fully understood, by the sunlight acting upon this substance in the presence of the green pigment (chlorophyll) of the leaf plastids. It is estimated that under ordinary circumstances this decomposition would require the production of energy " equiv- alent to 1300 of heat," and yet the green leaf, through the agency of chlorophyll, is able to do this work without high temperatures or elaborate machinery. The importance of photosynthesis to both plants and animals can hardly be overestimated, since its first products, sugar and starch, form the basic food for plant and animal nutrition. Not only are these products the chief forms of reserve foods in the special storage organs and cells of plants but they also function as the most important material around which other kinds of organic foods are constructed. Thus, the water stream from the roots brings up soil salts and deposits them in the mesophyll cells of the leaves. The nitrogen, sulphur, and phosphorus of these soil salts is then combined with the sugar molecules formed by photosynthesis into nitrogenous foods, such as the gluten of wheat and other forms of protein food material. Without the sugar furnished by photosynthesis this formation of available protein food, upon which both plants and animals depend for sustenance, would not be possible. We know also that the fats and oils of such seeds as flax, hemp, and castor beans are derived from sugar in some unknown manner, and that they are recon- verted into sugar and starch during the germination of such 120 GENERAL BOTANY seeds and the growth of the embryo. Fats and oils, therefore, like nitrogenous foods, are primarily derived from the products of photosynthesis. All organic food for both plants and animals is thus composed of the products of photosynthesis, or of food substances which have been built around these products. In a similar manner the skeletal and supporting structures of the plant in the form of cell walls are immediately derived from sugar and starch formed by photosynthesis. The cellulose which forms the bulk of the cell-wall substance of plant tissue is closely related to starch in chemical composition and is undoubtedly constructed from sugar molecules by the living protoplasm of plant cells. This cellulose framework of the plant body also comprises the bulk of the fuel, in the form of wood, coal, and combustible oils, upon which mankind depends, either directly or indirectly, for heat and light. We see, therefore, that a large part of the food supply for the organic world, the skeletal struc- tures of plants, and the energy derived from fuel in the form of light and heat are dependent upon the process of photosynthesis by green plants. RESPIRATION The process by which living organisms secure energy by oxi- dation for carrying on their life activities is termed respiration. Unlike photosynthesis, respiration is not confined to the cells of the plant body which contain green chlorophyll, but takes place in every living cell of the organism. On account of its similarity to combustion the respiratory process is most easily understood by beginning students when compared with the burning of wood or coal in a stove or a furnace. When thus compared it is found that respiration and combustion are alike in that free oxygen is absorbed and energy is liberated, together with certain waste products which depend for their complexity upon the nature of the substance oxidized. In the burning of coal the oxygen unites directly with the pure carbon of the coal, and the products are heat energy and carbon dioxide, which may be represented by the following general equation : Coal (C) + oxygen (O 2 ) = carbon dioxide (CO 2 ) 4- energy NUTRITION AND SEASONAL LIFE OP PLANTS 121 In the burning of wood the cellulose of the wood-cell walls and. the stored sugar, starches, and nitrogenous foods are chem- ically more complex than coal. As a consequence of this com- plex chemical character of the substances oxidized or burned the final products are more complex than in the burning of coal, and iii addition to the energy released we have also carbon dioxide, water, and other substances given off or thrown down as by-products. In a somewhat similar manner we may picture the respiration or oxidation processes which go on in the living cells of germinating seeds or in other active cells of the plant body of a growing plant. This combustion of more complex compounds may also be represented by a generalized formula as follows : Sugar 4- oxygen = carbon dioxide + water + energy (C 6 H 12 6 ) 6 (0 2 ) 6 (C0 2 ) 6 (H 2 0) In the living cells of plants the compounds which are finally oxidized are very complex and therefore yield more complex final products than is the case in the oxidation of coal or wood. Some investigators believe that the sugars and similar, substances are directly oxidized in living cells, much as they-are in a piece of wood containing these substances as reserve foods or as con- stituents of cell walls. Others suppose that the protoplasm, or living substance, is gradually decomposed during the respiratory process and that oxygen plays an important part in the process, which results in the release of energy and in the production of carbon dioxide, water, and nitrogenous wastes ; for example : Proteins 4- oxygen .= carbon dioxide 4- water -f nitrogenous wastes 4- energy It appears, therefore, that while the energy released is com- parable in combustion and respiration, the final waste products are more elaborate in the oxidations which take place in living cells than they are in ordinary combustion, for the reason that more complex compounds, possibly including protoplasm itself, are decomposed and partially oxidized in plant respiration. 122 GENERAL BOTANY Another striking difference between the oxidations in the living cells of plants and animals and the combustion of coal or wood is found in the temperatures at which the two processes take place. In animals respiration goes on at the normal tem- perature of the animal body, which does not exceed 100 Fahren- heit. The temperatures at which combustions are made possible are known to be much higher than that at which living matter could continue to exist. The above comparison of the combustion of coal or wood and respiration in living organisms may be summarized as follows : Respiration is like combustion in that oxygen is necessary for both processes and both processes yield energy and carbon dioxide, or energy, carbon dioxide, and water, as final by-products. It also differs from combustion in the complexity not only of the compounds broken down but of the waste products which result from the oxidation processes. The function, or use, of respiration is much the same as combustion in an engine, in that it liberates energy which can be used directly or can be transformed so as to furnish power for work of various kinds. In the living cells of seeds, leaves, stems, or roots the energy released by respiration is used for making new protoplasm, for cell division, for protoplasmic streaming, and for other vital processes necessary to the life of the plant organism. This energy, as the student will recognize, is different in origin and in function from the external energy absorbed from sunlight by the chlorophyll and used in photosynthesis for the building of sugar and starch. The latter energy enables green plants to make sugar, which forms the basic organic food for all plant and animal life, while the energy released by the respira- tory process is necessary for maintaining the vital processes of all cells, whether they are furnished with green chlorophyll or not. The mechanism of plant respiration is very different from that of animals in that the plant is not furnished with lungs and an elaborate blood system for absorbing and distributing oxygen to the living tissues of the plant body. The living cells which we have observed in wood rays and in the wood proper are NUTRITION AND SEASONAL LIFE OF PLANTS 123 comparable to the living muscle and nerve cells in animals in their need for a certain amount of oxygen for respiration, but there is no definite circulating system in plants to carry this oxygen to these cells from the external air. In plants the oxygen moves through the intercellular system, which penetrates to all parts of the plant body, by the slow process of gas diffusion. The oxygen enters the plant through leaf stomata and through the lenticels which we have already observed in the bark of twigs and stems. The slow diffusion of oxygen suffices, however, for the produc- tion of sufficient energy for the less active tissues of higher green plants. In the case of more active plants, like bacteria and yeasts, and in the active tissues of growing buds and flowers, plant cells often equal or exceed animals in the energy of the respiratory process. Comparison of respiration and photosynthesis. These two vital processes of plants are often confused, owing to the fact that the same gases are involved in both processes and that they may go on at the same time in one organ, as, for example, the green leaf. Photosynthesis is a food-building process in which carbon di- oxide is absorbed from the external air by cells containing green chloroplasts and combined with water to make sugar and starch. During this process the excess oxygen, contained in water and carbon dioxide, which is not needed for making sugar, is liberated as free oxygen into the intercellular system of the plant. This absorption of carbon dioxide and the accompanying liberation of oxygen can only go on in the daytime, when the chlorophyll can absorb the sun's energy for the photosynthesis process. No gaseous exchange, therefore, which is due to photosynthesis can go on at night or in darkness. Respiration is exactly opposed to photosynthesis in its need for and use of oxygen and carbon dioxide. In this process oxy- gen is absorbed, carbon dioxide is liberated, and energy is formed by all living cells of the plant body, regardless of whether they contain chloroplasts or not. Furthermore, oxygen is absorbed and carbon dioxide is liberated by plant cells at all times of day and night as long as they live and need energy for maintaining their vital functions. 124 GENERAL BOTANY It will occur to the student that the oxygen liberated by photosynthesis during the day may be 'used by the green leaf cells for respiration, and that the carbon dioxide liberated in respiration may likewise be built by photosynthesis into sugar and starch. While this is true, it does not change the funda- mental distinction between the two processes as to their nature and use in the living plant organism. In addition to photo- synthesis there are other processes, notably fermentation and what is termed anaerobic respiration, which are closely related to normal, or aerobic, respiration. These processes will be con- sidered more appropriately, however, in connection with the life of the fungi, in a later chapter. DIGESTION AND ASSIMILATION DIGESTION The nature of the digestive process is the same in both plants and animals, and consists in the conversion of foods from an in- soluble into a soluble condition fitting them for circulation and final assimilation by the tissue cells of the body. The starch which is stored in leaves, wood rays, tubers, and seeds is a good illustration of an insoluble food which must be converted into soluble sugar by digestion before it can be circulated or used by the living cells of growing parts. In like manner the gluten or protein of wheat and the fat of seeds like flax and the castor bean are insoluble and unusable until they are digested at the time of seed germination to form soluble proteins and fats for the growth of the embryo. The place where digestion occurs is very different in the higher plants and animals. Since there are in plants no specialized digestive organs like the alimentary canal of animals, digestion takes place in the cells of storage organs in any part of the plant body where reserve foods exist. In the mesophyll cells of leaves digestion is probably going on at all times, since starch formed by photosynthesis is continually being converted into sugar for immediate use by the leaf cells or for transport along the veins and the phloem of the stem to the wood-ray cells and other storage tissues of the stem. In germinating seeds NUTRITION AND SEASONAL LIFE OF PLANTS 125 digestion takes place in the cells of the storage endosperm or in the embryo itself, where reserve foods were laid down during the growth of the seed from the ovule. Digestion may therefore be said to be distributed throughout the plant body, and for the most part takes place within the cell cavities in the higher plants instead of being localized in a special digestive tract. The agents of digestion are the so-called enzymes, or ferments, familiar to us in the secretions of the digestive glands of man, such as the saliva, gastric juice, and intestinal secretions. In plants these ferments are usually formed by the protoplasts in the cells where digestion goes on (Fig. 63) ; but in some seeds, like the grasses and cereal grains, special glandular layers of secreting cells exist where digestive ferments are formed and then excreted into neighboring tissue cells for digestive purposes. The method of digestion is chemical in nature and consists, in most cases, in adding water to the insoluble food molecules, thus rendering them soluble, as in the conversion of reserve starch to sugar: Starch + water -f diastase ferment = sugar + diastase (C 6 H 10 6 ) n H 2 (C 12 H 22 U ) In this process, which is termed hydrolysis, the exact r61e of the ferment is unknown, since it is not apparently destroyed or diminished by the process, as indicated in the formula. ASSIMILATION The conversion of foods rendered soluble by digestion into the living protoplasm of plant cells is termed assimilation. Since this conversion is impossible unless the food is in the proper state, digestion and assimilation are closely linked processes in plant nutrition. SEASONAL LIFE OF ANNUALS, BIENNIALS, AND PERENNIALS We shall now attempt to apply to the seasonal life of a few common plants the principles of nutrition already laid down. In order to make this application as comprehensive as possible, plants will be selected which live under quite different conditions 126 GENERAL BOTANY and thus have a very different organization and mode of life. We shall thus secure not only a summary of the principles already learned but also a fundamental study of the relations which plants sustain to their environments. AN ANNUAL: THE GARDEN BEAN The bean plant is typical of the most abundant and common forms of plant life that live in medium conditions on land. It is likewise representative of the so-called annual plants, which com- plete their life cycle in a single season and then die down, leaving the seed as the wintering and hibernating structure to perpetuate the race the next season. The life of an individual bean plant for a season (Fig. 64) will thus give us a general idea of the seasonal life and activities of common annual land plants. Food storage. As indicated above, the seed is the wintering stage of the bean plant and is composed of an embryo plantlet in which is stored an abundance of food for the growth of the embryo until it becomes self-supporting. This food, however, is stored up in the bean in the form of solid grains of starch and protein. Fats are also stored in a condition unsuitable for imme- diate use by the embryo. In order that the growing embryo plantlet may use this solid food, therefore, it must first be trans- formed into soluble foods by digestion. Digestion and respiration. Digestion in the bean is not very different from the same process carried on in a growing animal fed upon beans, whole or ground into meal. In the case of the animal the digestive juices are poured into the digestive tract and mixed with the food in the stomach and intestine ; in the bean seed the cells of the cotyledons in which the food is stored secrete the digesting substances or ferments, which transform the solid starch and protein into soluble sugar and protein. The fat is likewise ultimately transformed into sugar before being used by the growing plantlet. In Fig. 63 these facts are graphi- cally illustrated in connection with the cotyledons of a growing bean seedling. In such a seedling the life processes are unusually active, as in animals. The reason for this is that growth in the NUTRITION AND SEASONAL LIFE OF PLANTS 127 plant involves the building of new living substances, the forma- tion of new cells by innumerable cell divisions, and the expansion of cells in the plant body by means of absorbed water. Respiration is also very active in germinating seeds and growing plants, as can easily be demonstrated if germinating Transpiration Respiration Photosynthesis Cortex Phloem (food path) \Xylem (water path) Pith Absorption Water Sails-- Oxygen' Respiratiort FIG. 63. The main physiological activities of a bean plant illustrated diagrammatically seeds or growing seedlings of beans are placed in a thermos bottle under proper precautions. A thermometer thrust among them will quickly demonstrate the evolution of heat, which is an index of the respiratory activity of the growing plant parts. Photosynthesis and migration of foods. The leaves as they unfold begin to absorb carbon dioxide and the energy of sun- light. The roots also absorb water and soil salts, which pass up 128 GENERAL BOTANY the ducts and are combined by the leaf with the carbon dioxide from the air to form starch, sugar, and nitrogenous foods. As long as the plant is young and growing this leaf-made food will be used for immediate growth, but as the leafage increases, an excess of food will be formed daily over and above that used for the immediate needs of the organism. This excess of food, stored in the leaves during the periocf of active photosynthesis by day, is digested by the leaf cells at night in the manner already indicated for the cotyledons during seed germination. The soluble sugar and protein thus formed then moves down the phloem portion of the veins and of the vascular bundles of the stem of the bean plant by the process of osmosis, which is the physical method of movement of all soluble foods in plants. As they move downward in the phloem of the main vascular bundles they are absorbed along the way by the cortex cells and also pass horizontally along the wood rays toward the pith, where, as we have seen, food is often stored. In an annual plant, like the bean, little food is permanently stored in the stem since it is used mainly for seasonal growth and for the production of seeds. As soon as the bean flowers begin to develop they form centers of great activity in growth, especially during the formation of the pollen and the young seeds, or ovules. The food stream then begins to be diverted to the flowers, in consequence of the growth activities going on in the developing anthers and ovules. As soon as fertilization has taken place the young seeds begin to form endosperm, and this process necessitates a constant supply of soluble sugar and pro- tein. Since osmosis takes place from points of greater concen- tration to those of less concentration for any given substance, the cells of the growing cotyledons of the bean must needs convert the sugar and soluble protein into insoluble starch and protein grains in order to reduce the concentration of solu- ble foods in their water vacuoles ; otherwise the flow of food toward these cells would soon cease. This conversion of soluble sugar into insoluble starch is done by leucoplastids in the cells of the cotyledons, while storage protein granules are formed in the general cytoplasm of the cells. Fats and oils also seem to be NUTRITION AND SEASONAL LIFE OF PLANTS 129 made from sugar in small quantities by the general cell cyto- plasm. When the seeds are fully formed and stored with food, they are shed from the mother plant and begin their long winter rest preparatory to starting the new bean plant of the next season. When winter comes on, the living substance of the plant body is killed by frost, and the cellulose framework falls to the ground and is gradually converted into carbon dioxide and Inflorescence Seed (winter rest) Adult plant FIG. 64. The seasonal history of the annual bean plant Spring Seedling germination growth water by fungi and by bacterial ferments. The seeds, meanwhile, are furnished with resistant seed coats and rest safely upon or in the soil until the next spring, when they germinate and pro- duce a new generation of bean plants. Assimilation. The conversion of food into living protoplasm is constantly taking place as the bean plant grows and pro- duces new organs and tissues. This conversion of food into living protoplasm is called assimilation. A considerable bulk of cellulose in the form of- new cell walls is also formed to serve as the skeleton, or framework, for the mechanical support of the living substance of the plant body. 130 GENERAL BOTANY Movements. During these various internal activities the external organs of the growing plant are actively engaged in adjusting themselves to the environment ; but this indispensable phase of the plant's life has already been dwelt upon at suffi- cient length and need not be reviewed at this point. SUMMARY We see, therefore, that during its brief seasonal life the bean plant carries on a complex series of physiological activities quite comparable to those maintained by the bodies of animals and human beings (Fig. 63). Water and crude salts are absorbed from the soil by osmosis and are transported through specially differentiated ducts to the manufacturing mesophyll cells of the leaves. Carbon dioxide and oxygen stream in through stomata, circulate through the intercel- lular spaces, and are absorbed by the living cells for food construction and respiration. Food is made by photosynthesis and is temporarily stored during the day in .the leaf cells ; this food is then digested at night by active digestive ferments and is transported through the phloem and wood rays to points of special activity in growth or to permanent storage tissues in the seeds. Respiration and assimilation are always going on in all living tissues day and night, but they are especially active in floral parts during the period of their formation and in the germination of seeds in spring. The seeds are the special- ized resting and wintering portions of bean plants which are adapted to carry the plant over inclement periods for which the more delicate working plant body is unfit, and in them, therefore, rests the assur- ance of a new generation of bean plants in each successive season. A BIENNIAL: THE WHITE SWEET CLOVER If we compare a biennial plant like white sweet clover with that of the annual bean plant described above, we shall find the seasonal and physiological history of the biennial quite different from that of the annual. The general physiological processes con- cerned with seed germination, the absorption and movement of water, photosynthesis, and respiration will be similar in the two plants ; but the seasonal history and the handling and storage of foods are quite unlike in the two, on account of the biennial NUTRITION AND SEASONAL LIFE OF PLANTS 131 habit of the white sweet clover. In such a plant the first season is devoted largely to the manufacture and storage of food, and this process takes place for the most part in the more or less fleshy taproot. The processes concerned with the migration of food into this taproot and its storage in specialized storage tissues are identical with those already described for the bean. The special storage portions of the root are found in tho very broad wood rays and cortex, which become gradually filled with starch and protein Second (Photosynthesis and reproduction) FIG. 05. The seasonal history of the biennial white sweet clover (Melilotus) during the late summer and autumn, after the plant has reached its growth for the season. During this period also the buds are laid down on the upper broad crown of the main taproot for the early growth of stems and leaves in the spring of the second season. These buds and the taproot then pass the winter in the condition shown in Fig. 65. The following spring, when these buds start to grow, wood-ray cells convert their reserve food by digestion into soluble sugar and protein, which move at first horizontally along the rays, then upward in the phloem into the expanding buds. In these buds the concentration of soluble foods in the cells is kept low by its constant conversion into new cellulose walls and new protoplasm for the cells and tissues formed in the growing leaves and internodes. The food streams 132 GENERAL BOTANY are thus able to flow continually from the root cells to the bud cells until the new stems and leaves are formed for the season. When these are fully grown, the entire energy of the biennial clover plant is devoted to the production of flowers, fruits, and seeds. It is not necessary for us to trace the physiological processes involved in the formation and maturing of these structures, since they are exactly like those which have already been recounted in the production of seed by the annual bean plant. When the second season is ended, the biennial sweet clover plant dies and, like the annual bean plant, intrusts to its seeds the formation of new individuals with the opening of the next growing season. It is thus seen that the storage root and buds carry the plant over the first winter period of its biennial exist- ence, while the seed with its reserve food and embryo is the part which successfully endures the second winter. The advantages and disadvantages of the biennial habit as exemplified in the clover will be discussed in the summary following the outline of the life of perennial plants. PERENNIALS The plant body in perennial plants continues its life from year to year, varying in the length of its existence with the kind of plant and the nature of the surroundings. Since the same plant body continues to live through several seasons, we should expect that perennial plants would manifest distinct adaptations to seasonal changes which are not necessary in annuals and are less marked in biennials. In discussing the life of perennials, therefore, particular stress should be laid on those characteristics which are connected with the perennial habit. The seasonal life of the herbaceous perennial for the first two years is like that of the white sweet clover. The difference between the two is that the roots and underground stems of the perennial, when once, established, serve for storage and the pro- duction of aerial shoots for many years instead of for the second season only as in biennials. Many plants live from year to year NUTRITION AND SEASONAL LIFE OF PLANTS 133 by means of an underground stem which is the perennial part of the plant. Each year an aerial annual part, like the stem of the bean, is sent up to construct food and to produce flowers and fruit. During the warm months of summer the green leaves and stem of this aerial part manufacture and distribute foods to the perennial storing underground stem and to the de- veloping flowers, fruits, and seeds. The summer life of these peren- nials, therefore, is like that of the annuals and of the biennials. But when cold weather comes on, the aerial portion of the perennial dies down, and the protected un- derground por- tion stored with food lies dormant until spring, when it again sends up an aerial annual shoot for food-making and reproduction. In the case of grasses some of the old annual foliage often survives the winter, but the really effective aerial and annual leafage is produced each season from an underground stem, or rhizome, which serves the func- tion of storage and hibernation during inclement periods. In perennial woody plants (the trees and shrubs of the temperate zones) (Fig. 66) the aerial plant body has become adapted to changing seasons and differences in climate, so that it does not die down, as in herbaceous perennials, with each inclement seasonal FIG. 66. The seasonal history of a perennial woody plant, the locust (Eobinia) 134. GENERAL BOTANY period. This adaptation is secured by the woody character of the stem, by the development of a protective cork jacket of bark, and, except in pines and their relatives, by the seasonal shedding of the delicate leafy portions of the plant with the advent of frost or drought. In summer a woody plant is essentially a mesophyte, that is, it is adapted to medium conditions of water and temper- ature, while in winter, or during drought in dry regions, it par- takes of the character of a xerophyte, that is, a desert plant. On account of the great size and long life of woody plants, especially trees, the seasonal life and the entire history of such a plant pre- sents an interesting contrast to that of the other plants thus far described. The following brief sketch of the life of an apple tree will suffice to introduce the student to the characteristic life of woody perennials. The first five or six years of the life of an apple tree are devoted entirely to the building of a massive trunk with its great feeding and absorbing root system and its extended branches for the sup- port of an enormous leaf surface. The main trunk and branches, as well as the large roots, here become the main storehouses into which the excess of food is annually passed and stored in the cortex, the rays, and the living cells of the wood and phloem. Each spring digestion takes place in these storage centers, and food migration occurs along the usual channels to the grow- ing parts of the tree. The wood rays supply food to the cambium directly, the phloem carries food upward to the swelling buds and downward to the growing root tips, while later movements take place in all directions to growing cells in the phloem and the wood. In this way the apple tree expands its leafy shoots and forms new roots each spring for the season's work. When the tree is well established, reproduction begins with the annual production of flowers, which results in the formation of seeds and fruit. After the reproductive phase begins in the life of a tree, all excess food produced in the first months of each season by the leaves, and a part of that previously stored in the trunk and branches, goes largely into the forming of fruit. In the apple tree, as in the sweet clover, the food streams are therefore diverted from the storage tissues in the stem, the branches, and the roots, NUTRITION AND SEASONAL LIFE OF PLANTS 135 along the flower peduncles, into the ovaries and seeds which are to form the apple crop for the current season. In addition to the sugar and other soluble foods which enter the fruit and seeds water must also be absorbed, particularly by the rapidly expand- ing cells of the apples, since we know that the enlargement of cells is almost entirely dependent upon water for the growing water vacuoles. If the trees are prevented from overproduction of fruit in any one season, they may repeat essentially the same reproduc- tive history each season for a number of years, and this is one of the methods now being used in the scientific fruit culture for securing a uniform annual crop. If the trees overproduce in one season, they must rest for a time in order to store up new reserves in the trunk and branches for producing fruit in abundance. After the fruit is matured, the apple lays up a store of food in the usual manner for its next season's spring growth of buds and roots. The ripening of the fruit and seeds, and the shedding of the leaves with the advent of frost, close this interesting and active seasonal life of the apple tree, which may be taken as typical of the life of our common trees and shrubs. SUMMARY We have now seen that the origin of annuals, biennials, and per- ennials from the seed involves the same essential physiological proc- esses. These processes include the active digestion and circulation of reserve foods, respiration for the production of growth energy, and, finally, adjusting movements for the proper placing of roots, stems, leaves, and flowers in the soil, air, and sunlight. All of these plants have also an active summer period during which food is manufactured, flowers are produced, and fruits and seeds are matured. The differences between annuals, biennials, and perennials consist largely, therefore, in the length of life of the plant body and the consequent necessity of adapting this vegeta- tive body to seasonal and climatic changes. In this adaptation of the plant to the environment, annuals have a certain advantage in maturing their seeds in one season, since the seed is a favorable structure for distribution and for withstanding cold, drought, and other environmental factors unfavorable to plant life. The annual death of the plant body is therefore of no consequence as far as 136 GENERAL BOTANY perpetuation of the life of the species is concerned. The plant body, being of importance for the warm season only, can consequently be delicate and small, so that the entire energy of the organism may be concentrated on the reproductive structures. Annuals, on the other hand, are not able to hold their ground against such perennials as dandelions and grass, since these plants retain a position once gained and spread out vegetatively from year to year. Annuals are therefore good immigrant plants, which find new places and occupy them temporarily. They are able to do this by means of their seeds, which are produced abundantly each year and are readily disseminated by wind, water, and animals ; but in the end they are usually crowded out of their places by the hardier and longer- lived perennial plants. Biennials have an advantage in special- izing the first season on the production and storage of a large amount of food and in devoting this food storage during the second season to the maturing of fruits and seeds. The biennial habit is especially adapted to regions with recurring dry and wet seasons. In such localities the rainy season, which is usually short, is suffi- cient for the production of a new plant body and the storage of a rich food supply. During the dry season such plants lose their leaves and hibernate in the form of underground rhizomes, bulbs, or tubers. Dur- ing the second rainy season flowers, fruit, and seeds are produced, by means of which the species is preserved and disseminated. Herba- ceous perennials are also adapted to such climatic changes as those indicated above, and have the additional advantage of the perennial habit. In the temperate regions of the United States, perennials also represent the dominant herbaceous types, since they easily adapt themselves to medium, dry, and wet situations and hold the territory once gained from the less enduring annuals. Trees and shrubs, although adapted to endure great variations in the environment, are not the equals of the herbaceous perennial grasses and allied plants in adapting themselves to wide ranges of climate and soil. This is shown by the fact that the great deserts, the plains, and the high mountain areas are not their usual habitats. The prophecy has therefore been made that the future vegetation of the earth will be derived from the herbaceous perennial type of plants. The tree type had its origin in the remote past, as our coal deposits testify. It may have given rise to the herbaceous per- ennials of to-day, and it may succumb in the future to the younger and more progressive herbaceous perennial and its offspring. CHAPTER VIII . THE RELATION OF PLANTS TO WATER THE MECHANISM OF ABSORPTION OSMOSIS The particles of the soil from which roots absorb water and soil salts are surrounded by delicate films of water (Fig. 69) in which the dissolved portions of the soil necessary to plant life are held in very dilute solution ; namely, from .0001 to .03 per cent. In order to understand the method by which roots absorb this soil water and its dilute salt solutions, the student must first understand something of the laws of osmosis, upon which all absorption and much of the movement of fluids in the plant depend. A simple experiment illustrated in Fig. 67, a, will serve to give the necessary data for understanding the application of osmosis to the movement of water and soil salts into and through the plant. Fig. 67 shows a parchment tube which is not unlike a root hair in its form and in its osmotic properties. If now the parchment tube has been filled with a strong solution of common salt before being placed in the distilled water, the results of osmosis will shortly begin to be manifest to the observer. The water will be seen to rise slowly in the glass tube, until a column several feet in height is attained. At the same time it will be found, by chemical analysis or by the taste of the water in the jar, that minute quantities of salt have flowed out of the parchment tube into the pure water. In the above experiment we have illustrated the essential facts regarding osmosis, or the diffusion of substances in solution through an osmotic membrane which separates two solutions of different composition. In such cases the substance (for example, salt) dissolved in a liquid (for example, water) is called a solute, 137 138 GENERAL BOTANY Glass tube Cork and the water in which it is dissolved is termed a solvent. The solutes are usually said to be of a certain concentration, which means the relative amount of the substance dissolved in the water (the solvent) in a unit of volume. In the experiment, therefore, if the salt and water are assumed to occupy equal portions of the space in the diffusion shell, the salt is of greater concentration inside the shell than outside in the distilled water, where it would be nil; and the water is greater in amount per unit of space in the jar than inside the shell. Neglecting for the moment the physi- cal explanations for the movements of the solvent (water) and the solute (salt), we have seen that each tended to flow from a point of greater to a point of less concentra- tion, and this result may be taken as a common law, or tendency, of substances of a liquid nature and of different concentration separated by a membrane. Since, however, the parchment membrane in the experiment allowed the water to pass in freely, being permeable to it, and hindered the outgo of salt, the result was a great increase in the volume of the water in the parchment tube, and a corresponding increase of pressure (called osmotic pressure) which tended to overdis- tend the tube. As the glass tube furnished an easy exit for the water, and a relief, as it were, from the osmotic pressure in the parchment tube, the water rose against gravity, thus giving rise Solvent FIG. 67. Experiments in osmosis and root pressure a, a diagram illustrating an experiment in osmosis ; 6, a diagram of an experiment illustrating the exudation of water from a cut stem THE RELATION OF PLANTS TO WATER 139 to a water column in the glass tube. It is quite probable that the real cause of the forcible inflow of water into the parch- ment tube, and of the osmotic pressure thus developed, may be found in the attraction which the wall of the tube and the parti- cles composing the solute have for the water molecules them- selves. We are not here so much concerned, however, with the physical explanation of osmosis as we are with its results, which we need to study in order to understand the work of the plant in absorbing and circulating water and foods. These results, as we have indicated above, are, fiest, the tendency of solvents and solutes to equalize through a separating membrane, and, second, the development of a considerable osmotic pressure within a closed membrane into which an excess is thus induced to flow. ABSORPTION BY ROOTS The absorption of soil water and soil salts by roots is governed by processes very similar to those indicated above in the experi- ment with a parchment tube. The root hair, which is the most important absorbing portion of the root, is a tubular extension of a single epidermal cell (Fig. 68). Like most plant cells it is furnished with a delicate cell wall and a lining cytoplasmic sac composed of living protoplasm. The center of the root-hair cell is occupied by the water vacuole, containing a solution of organic acids, salts absorbed from the soil, and in many instances sugar, all dissolved in the water of the vacuole. The cell wall is per- meable to mosfc substances, but the cytoplasmic sac resembles closely th& parchment membrane of a parchment tube in being more permeable to the water than to the solutes dissolved in it. It differs from the parchment membrane in being composed of living substance and in being thus able to control to a certain extent its permeability to substances outside in the soil and also within its water vacuole. For instance, in sugar beets the cells of the root are able to retain from 14 to 18 per cent of sugar within the water vacuoles of the root cells, while no sugar exists in the soil water in which the roots are bathed. Beet roots at the same time allow minute quantities of soil salts, amounting 140 GENEKAL BOTANY on the average to from .0001 to .03 per cent, to pass into the water vacuoles of the root cells from the soil by osmosis. In general, absorption by root-hair cells is undoubtedly to be explained as an osmotic .process following the laws already laid down regarding the movement of solvents and solutes through a parchment membrane. The actual phenomena of absorption and of movement of water and soil salts through the root will be more readily understood by reference to Fig. 69, which indi- cates a portion of a long section of a root, showing root hairs, a portion of the cortex, and water ducts. The lower root hair is repre- sented surrounded by the soil, which is made up of soil par- ticles (solid black), water films (concen- tric lines), and air (light spaces sur- rounding the soil particles). If now nitrogen in the form of a nitrate is in solution in the soil water in greater concentration than it is in the water vacuole of the root hair, the laws of osmosis already enunciated will insure the inflow of the needed nitrogen salt into the root-hair cells, and thence, by the same physical law, into the cortex cells which surround the duct. Water will likewise tend to flow from the soil water into the water vacuoles of the root hair and root cortex cells as long as these water vacuoles contain more solutes, and so less water per unit of volume, than the soil water outside. The result will be a continued flow of certain soil salts into the root hairs from the soil, and a great pressure developed inside of the root hair and cortex cells by the forcible inflow of large quantities of water into the water vacuoles of these cells. This FIG. 68. The structure of root hairs A, a transverse section of a root with hairs; B, a single hair with adhering soil particles THE RELATION OF PLANTS TO WATER 141 Palisadea cells ,Epidermi mini cells latter fact may be practically demonstrated by the student in an experiment similar to that illustrated in Fig. 67, b. If the stem of a proper plant be cut off close to the root, as in the figure, and a glass tube be fitted over the cut end of the stump by means of rubber tubing, water will soon begin to well out of the ducts, which have been opened by cutting the stem. This water, in an active plant such as a coleus or a begonia, will often rise to a height of several feet in a small glass tube, or to that of from 40 to 50 feet in the case of some trees. This phenomenon (erroneously called root pressure) is par- tially explained by osmosis, but the ultimate explanation is as yet unknown. In nature the outflow of water from wounds usually occurs in the spring before the leaves unfold ; it ceases as soon as the leaves expand and begin . active transpiration. The soil salts taken into the roots by osmosis move with the water into the ducts and up the stem to the leaves, where, as we have already noted, the salts are combined with the sugar, which results from photosynthesis, to form the basic nitrogenous foods for the entire plant. Nitrogenous foods are undoubtedly formed also in other living cells of the plant body in the same manner ^as in the leaves. Cytoplasmic sac Ducts aier ~\ ., > i films &# Pa> FIG. 69. The path of water in the plant The lower portion of the figure shows the structure of the soil and the relation of root hairs to the soil particles and to the water films; the upper portion illustrates the con- nection of the leaf tissues with the ducts of the stem 142 GENERAL BOTANY TRANSPIRATION AND WATER ASCENT Transpiration is a term used to indicate the loss of water from leaves and other exposed organs of the plant. It differs from the evaporation of water from a free water surface in that it is controlled by certain structural features of the epidermis and bark, which greatly restrict the loss of water from these organs. Thus, Sachs estimated that a given area of sunflower leaves evaporated only about half as much water as a similar area of free water surface. Transpiration, like evaporation, is also controlled by external conditions of the atmosphere, such as temperature, humidity, and air movements. Although the loss of water takes place from all exposed parts of plants by evaporation, the term transpira- tion is usually understood to apply to the loss of water from leaves, where the greatest amount of evaporation takes place. In the following discussion, therefore, leaf transpiration will be mainly considered. Leaf transpiration. The same structural features which we have* already noted as important in the gaseous exchanges con- cerned with photosynthesis and respiration are also important in transpiration. The delicate mesophyll cells of the leaf (Fig. 62, a) are surrounded by a system of intercellular spaces which open out into the external air through innumerable stomata. There- fore the water which is supplied to these mesophyll cells from the veins tends to evaporate from their cell Avails into the inter- nal air within the intercellular spaces of the leaf, from which it diffuses, like a gas, into the external air through the stomata. If this evaporation of water is too great, the leaf wilts and the plant is in danger. It is thus seen that the need for structural adaptations in the leaf to facilitate gaseous exchange is often an element of danger, since they may also lead to an undue loss of water by transpiration. Therefore leaves often effect a compro- mise in their structure between the need for gaseous exchange and that of controlling water loss which might endanger the life of the plant. Some of the important structural adaptations designed to control transpiration are the following: THE RELATION OF PLANTS TO WATER 143 Leaves in dry climates, in a location where soil water is not readily available, are wont to be smaller than in regions and localities where water is abundant. This contraction of the leaf results in a diminution in size of the intercellular spaces, which thus reduces the danger of excessive evaporation into the intercellular spaces from the mesophyll cells. The outer walls of the epidermal cells may also become greatly thickened and coated over with a waxy secretion called the cuticle. This pre- vents all loss of water except through the stomata. The stomata in most plants are also able to limit the amount of transpiration by effecting a closure when the loss of water from the mesophyll cells is not balanced by that received from the veins. This opening and closing of the stomata is partly explainable on phys- ical grounds, but is not as yet fully understood. Then again many plants, like the mullein, have leaves in which the epidermal cells grow out into a thick coating of hairs which prevent loss of water (Fig. 72). These are only a few of the innumerable structural devices for controlling the excessive loss of water from leaves by transpiration. Control. The external factors which control transpiration are light, heat, humidity, air currents, and the available water in the soil. Light affects transpiration largely through its control of the stomata, which, as was stated above, are usually open during sunlight and closed in darkness. Temperature plays an important role in water loss on account of its effect on the leaf tissues and on the water content of the air. If the leaf tissues are heated by the sun's rays, the result is an increased evapora- tion from the mesophyll cells and an acceleration of diffusion from the internal atmosphere of the leaf through the stomata into the external air. This external air will also take up more moisture when heated than when cool. These facts are con- firmed by experience with plants grown in warm, dry living rooms in the home, where the greatest care must be exercised to prevent them from wilting on account of the excessive loss of water. It is a well-known fact also that plants in humid regions lose very little water by transpiration, on account of the high relative humidity of the air. For the same reason there is 144 GENERAL BOTANY very little transpiration during rain or fog, when the humidity of the air approaches 100 per cent, while the transpiration is less on a moderate day, with the humidity at 70 per cent, than it is on a dry day, with the humidity at 50 per cent. Air currents, by removing the water as fast as it evaporates from the stomata, are also important factors in causing excessive evaporation, espe- cially when vegetation is parched by a combination of low humid- ity and high winds. In this case the external coatings of hairs, already mentioned, is an important factor in preventing loss of water, since it helps to maintain a cushion of moist air over the entire leaf surface, protected from evaporation by the hair layer. It is evident also that the amount of water available from the soil may modify transpiration from the leaves through their tendency to wilt as soon as evaporating overbalances absorp- tion, and so to cause the closure of the stomata, with a conse- quent check on transpiration. It is quite probable also that the leaf cells have some control over their own loss of water in a vital way, although the nature of this control is only indicated by recent experiments, which need elaboration and confirmation. Water ascent. The path of water ascent has already been ex- plained as occurring in the great water ducts which form a part of the conducting and supporting vascular system of the plant. The forces necessary to accomplish the task of lifting water from the roots of tall trees to the crown can best be appreciated after a brief statement of the volume of water transpired and the rate at which it moves up the ducts in the wood of plant stems. The volume of water exhaled from the leaves of ordinary plants is indicated by the rate of transpiration from the leaves. Ganong estimates that the average daily transpiration from a square meter (10J square feet) of leaf surface is 50 grams per hour in daylight and 10 grams per hour in darkness. A birch tree with 200,000 leaves is supposed to give off from 300 to 400 kilograms (from 660 to 880 pounds) of water on a hot day in summer. Sachs estimates that a sunflower plant the height of a man would evap- orate from 800 to 1000 cubic centimeters (about 1 quart) of water from its leaves on an average July day. The rate at which this water moves up the ducts varies in different plants, as the THE RELATION OF PLANTS TO WATEK 145 following figures, derived from the experiments of Sachs, will show. Sachs states that a particle of water may travel as much as 100 centimeters (40 inches) per hour in the vessels of woody plants. He found that in one species of willow the water moved 85 centimeters per hour, in corn plants from 30 to 42 centimeters, in the sunflower 70 centimeters, and in the grape 98 centimeters. With these facts in mind we may now turn to the forces available in the plant for transporting these great volumes of water, at the rates indicated, from the roots to the crown of woody and herbaceous plants. In herbaceous plants it is conceivable that capillarity, or the rise of water in the ducts clue to root pressure, might accomplish the work involved in water ascent. It has been found, however, that capillarity is not effective for this purpose in tubes as large as the ducts of our common plants, and that root pressure, while important in the spring, before transpiration begins, is practically in abeyance during the periods of vigorous transpiration. In tall trees these same forces would be much less adequate than in the smaller herbaceous plants. As indicated above, no theory has ever been advanced to explain satisfactorily all the aspects of water ascent, but there are certain known physical factors which should be mentioned as furnishing a partial explanation for the phenomena. These factors, taken together, constitute what is often called the cohesion theory for water ascent. The cohesion theory is based upon the cohesive power of small columns of water, which is known to be very great when subjected to a straight pull and is variously estimated at from 10 to 150 atmos- pheres. If this cohesion of a water column applies to the water in the condition in which it exists in the ducts and stems of plants, a scientific explanation of water ascent is conceivable on this basis. The entire column of water in a tree trunk would then act like a rope or chain and could be drawn upward as a whole if a sufficient lifting force were applied at the top. This lifting force is believed by the advocates of the cohesion theory to exist in the osmotic suction of the mesophyll cells bordering on the veins of a leaf. As these cells evaporate water into the intercellular spaces of the leaf the cell sap becomes concentrated 146 GENERAL BOTANY in them, and a higher osmotic pressure is thus developed. This concentration of cell sap and the increased osmotic pressure pro- duce a suction force on the less concentrated water content of the adjacent cells of the veins. The result would be the with- drawal of sufficient water from the vein cells at the top of the water column to supply the mesophyll cells, which would cause a corresponding deficiency of water at the base of the water column in the roots. This deficiency in the roots would then be supplied by absorption from the soil and by the forcible filtration of water into the ducts by the cortex cells of the root. This is in brief the conception of the -cohesion theory of water ascent, which, while not entirely satisfactory, has the virtue of dealing with known physical principles. Two of the physical principles involved namely, leaf -cell suction and filtration, due to osmotic pressure in the cortex cells of the root are known to be opera- tive in the plant. The third principle of cohesion, while valid for water columns in general, may not be applicable when applied to water columns as they exist in the ducts of plants. The importance of water ascent in plants is also a question con- cerning which there is considerable difference of opinion. Some writers consider transpiration and water ascent as real functions of the plant, designed to supply water and soil salts to the living mesophyll cells of leaves as well as to other cells of the plant body. Others regard it as a dangerous process which is necessi- tated by the structural adaptations of the leaf for photosynthesis and respiration. The great intercellular spaces of the leaf and the stomata are certainly a menace to the plant when considered from the viewpoint of conservation of water. We shall doubtless find that both conceptions of transpiration are partly true. ECOLOGICAL RELATIONS OF PLANTS TO WATER Mesophytes. The plant structures thus far described are those which pertain to plants living under medium conditions of moisture and temperature. Such plants are termed mesophytes, and they include the great plant populations which inhabit most of the temperate regions of the earth. THE RELATION OF PLANTS TO WATER 147 From the above discussion it will be evident to the student that the amount of water available for the use of the plant must exercise a profound effect upon its form and structure. This available water is dependent also, in the case of land plants, first, upon the amount of water in the soil available for the roots and, secondly, upon the atmospheric conditions, such as temperature FIG. 70. Mesophytic vegetation Meadow vegetation occuring in zones, herbaceous plants of mint (Monarda) in the foreground, scrub and woodland in the background. After Clements and relative humidity, which determine the amount of evapora- tion of water vapor from the leaves. If the amount of available water in the soil is abundant and the atmospheric conditions are such as to restrict excessive evaporation, a medium type of plant life is developed, which is usually designated as mesophytic. The mesophytes include the broad-leaved forest trees, shrubs, and herbs with which we are most familiar, and constitute the typical vegetation of the great forests of the temperate and tropical zones as well as that of the more productive lowland and 148 GENERAL BOTANY plain regions inhabited and cultivated by man (Fig. 70). These so-called mesophytes thus furnish the principal plant environ- ment of the civilized races of mankind, from which have been derived the main food, forage, and fuel plants which minister to man's comfort and progress. The form, structure, and physiol- ogy characteristic of the typical mesophytes have been sufficiently outlined in the preceding account of the structure and function of the root, stem, and leaf, and need not be elaborated here. FIG. 71. Xerophytic vegetation Desert vegetation made up principally of " ornamental cacti." Photograph furnished by the United States Department of Agriculture Xerophytes. The traveler in desert regions of the United States, or in sandy areas like the peninsula of Florida, is at once impressed with the unusual forms assumed by the characteristic native plants of these regions. In the desert conspicuous forms of plant life are mostly of the contracted, cactuslike type, which is in strong contrast to the expanded, broad-leaved types which clothe the more productive mesophytic areas inhabited by man. Plants of this type are called xerophytes (Fig 71). The reason for the difference is obvious if we consider for a moment the statement made at the outset of this topic, namely, that the water "supply available to the plant is dependent upon the avail- able soil water and upon the conditions controlling evaporation in any given plant habitat. In the American deserts during the THE RELATION OF PLANTS TO WATER 149 dry season the amount of available water in the sandy or alkaline soil is very small, and the roots, which often extend to great depths, are wholly unable to provide a large volume of water for the aerial stem and leaves. The atmosphere also, in these regions, is dry and hot during the long, dry season, and the plant is thus in danger of losing the small amount of water available from the roots. Under these extreme conditions all plants of the expanded mesophytic type are likely to be destroyed, and only FIG. 72. Leaves of xerophytes protected by hairs from excessive loss of water a, hairs of wormwood ; b, of Convolvulus; c, of Elaeagnus. After Korner those contracted xerophytic forms survive which are adapted to the peculiar conditions existing in the desert. These xerophytic desert plants are characterized not only by their contracted form and restricted leaf surface but by peculiarities in their structure as well. The protective epidermis in such plants is wont to be coated with wax or cutin, or is supplied with abundant hairs to protect the plant from excessive surface evaporation. The inter- nal tissues are also more compact, with fewer and smaller inter- cellular spaces into which water vapor can pass from the living cells and then into the external atmosphere. Many plants of this character are known also to have dense cell sap, which enables them to hold water vapor and thus prevent its loss by evapora- tion. These and various other modifications looking toward the 150 GENERAL BOTANY conservation of water characterize the typical dry-land xero- phytes of desert regions. In regions like the Florida peninsula and the coastal regions of the American continent sandy soil and brackish salt water often bring about a condition approxi- mating that of the desert, since the roots of plants in these regions are unable to secure a large amount of water from the dense soil solution. Similar conditions exist in undrained fresh- water bogs and marshes. Tropophytes. The student must not entertain the idea, from the above account of typical xerophytes, that xerophytes and mesophytes are sharply marked off from each other in all regions. Dry conditions obtain in most mesophytic areas at certain peri- ods of the year, and the xerophytic and mesophytic areas often graduate insensibly into each other. Thus, our common broad- leaved trees are typical mesophytes during those parts of the year when they are supplied with abundant moisture and when temperature conditions are suitable for the development of the more delicate leaves, flowers, and fruit. When winter comes on, however, the frozen soil restricts the absorption of water by the roots, and the frost makes it difficult for leaves and flowers to survive. These tree mesophytes then become practically winter xerophytes, in which evaporation is restricted and temperature changes are modified by a thick coating of bark on trunks and branches (Fig. 25). The buds of trees, likewise, in temperate and arctic regions, are adapted to the xerophytic conditions of winter and are protected by the highly modified (indurated and hairy or resinous) bud scales common .in oaks, poplars, and evergreens. Herbaceous plants likewise hibernate largely underground in the form of xerophytic storage roots or stems, while the more delicate mesophytic aerial parts die down with the advent of frost and are reproduced each year at the beginning of the warm season. It will thus be seen that some plants have become typical dry-land plants and retain permanently a xero- phytic habit and structure. Others, which are called tropo- phytes, adapt themselves to the changing seasons and take on alternately a typically mesophytic or xerophytic form which is adapted to the water supply at a given seasonal period. THE RELATION OF PLANTS TO WATER 151 The graduation of a mesophytic vegetation into a xerophytic one is epitomized in many mountainous regions as one ascends from the base to the summit of a high mountain. At the base and along the watercourses of the mountain streams in such regions typically mesophytic conditions often prevail, in which broad-leaved plants are the dominant type. As one ascends to FIG. 73. The effect of exposure, slope, and moisture on vegetation " North slope covered with dense forest of fir (Pseitdotsuga) ; exposed south slope, with oak scruh and pine." After Clements higher altitudes the sterility of the soil, combined with the lack of water and the drying effect of winds, produces a distinct type of xerophytic plants, which differ markedly from true desert xerophytes. The high-altitude xerophytes of mountain- ous regions are usually either herbs with thick, leathery leaves or low, straggling shrubs and dwarfed trees. In the typical pine forests which frequently inhabit mountain slopes (Fig. 73) one notices that the more resistant and hardy species are found high up, near the tree lines, while less hardy forms clothe the 152 GENERAL BOTANY lower slopes and the borders of streams. It is thus often pos- sible to see on a single mountain slope all gradations between mesophytes and xerophytes, and to gain a more definite idea of the factors which control the great mesophytic and xero- phytic plant associations of the earth's surface. Hydrophytes. Hydrophytes are plants which are wholly or partially submerged in ponds, lakes, and streams and are thus FIG. 74. Hydrophytic vegetation Pond lilies, with mesophytic vegetation in the background. From Bergen and Caldwell's " Practical Botany " subjected to conditions very different, as regards water supply, from those on land. Like the typical xerophytes, they are sub- jected to extreme conditions, which profoundly affect their form and internal organization. In a typical submerged hydrophyte, like the Elodea or the pond weed (Potamogetori), the stem and leaves are of a very delicate nature, since the plant is protected by its surrounding water medium. Partially submerged water plants, including water lilies, pickerel weed, and some grasses and sedges, partake more nearly THE RELATION OF PLANTS TO WATER 153 of the character of mesophytes. There are, therefore, as in the case of xerophytes, all gradations between true submerged hydrophytes and mesophytes. This is readily observed along the borders of streams and lakes, where the vegetation is often divided into more or less clearly defined zones (Fig. 74). In the water will be found both floating and attached hydro- phytes of a typical character, while along the shore line a zone of amphibious plants may graduate into the typical grasses, sedges, and cat-tails of a marshy area. Farther back from the stream or lake typical lowland mesophytes often pass insensibly into grass and tree associations, inhabiting the drier hills or uplands bordering the water and marsh areas. The limitations of the text will not admit of a more extended treatment of the wonderful and interesting adaptations in the form and structure of plants to the water conditions of the soil and the atmosphere. Enough has been said, however, to indicate the profound effect of water as a factor in producing the types of vegetation which inhabit the various climatic regions of the earth's surface. The problem of the way in which water as a causal agent has been able to mold plant life is one for the students of variation, adapta- tion, and evolution to solve. We know simply that the fittest plants for each particular water environment in any given habitat have been selected for survival in the struggle for existence. SECTION IV. REPRODUCTION CHAPTER IX VEGETATIVE AND SEXUAL REPRODUCTION Reproduction is a general term used to designate the various processes by which a parent organism gives rise to new organ- isms, called offspring or children. Reproduction is fundamen- tally a cellular process and consists in all cases in the separation of single cells or cell masses from a parent or parents, which possess the power of growth and differentiation to form new individuals. Two distinct kinds of reproduction occur in the higher plants, which are designated as vegetative and sexual re- production. In vegetative reproduction the cell masses which give rise to new organisms are usually much less highly special- ized than in single sex cells which unite in sexual reproduction. The results of the two processes are also quite different in their nature and importance to man, as we shall observe in the discussion which follows. VEGETATIVE REPRODUCTION Vegetative reproduction in the higher plants takes place exclusively by means of vegetative structures. These may be parts of ordinary roots, stems, and leaves. or they may be highly modified parts of the plant body, represented by bulbs, tubers, corms, etc. STEMS One of the simplest forms of vegetative reproduction is that of budding and layering, in which ordinary buds and shoots form the starting points for the production of new individuals. In the black raspberry (Fig. 75) and the strawberry (Fig. 76), layering 154 VEGETATIVE AND SEXUAL REPRODUCTION 155 is a natural process, since ordinary brandies in the black raspberry and specialized runners in the strawberry take root and form new Hooting branch arcnt plant 0/spring FIG. 75. Vegetative reproduction in the black raspberry The tip of a branch, or cane, has taken root and formed a new plant plants. This natural process is imitated artificially by man in propagating many plants, such as the gooseberry, grape, etc. FIG. 76. Vegetative reproduction by runners in the strawberry (Fragaria) 1, mother plant ; 2, 3, daughter plants ; r, runner ; b, bud on the runner Multiplying branches are often formed, as in common wild plantain (Fig. 77). In such instances the branches, which are at first connected by the mother stem axis, may become separated 156 GENERAL BOTANY by mechanical injury or by decay, leaving two separated plants instead of one. Such vegetative methods result in clusters of new plants, often seen in the case of dandelions in lawns and gardens. One of the most familiar ex- amples of vegetative reproduc- tion by highly modified stems is the tuber of the common potato (Fig. 78). The potato, as the figure shows, is merely a greatly swollen portion of an underground stem, in which the buds, or eyes, retain the power of growth into new plants when placed under proper con- ditions. Since each eye can form a new plant if it remains in contact with some of the stored food within the cells of the tuber, it is readily seen that a very rapid multiplication may result by this means. In cultivation it is the practice to cut the tuber into several pieces, each bearing one or more buds, thereby increasing the output of plants from 1 1 Root single tubers. In the bulb (Fig. 79) and the corm (Fig. 80) the stem is greatly shortened and the leaves are highly modified scales. Bulbs and corm thus resemble ordinary buds in which the stem axis ceases to grow in length p IG . 73. Production of tubers in the potato FIG. 77. Multiple stems in plantain VEGETATIVE AND SEXUAL EEPKODUCTION 157 spring and either becomes distended with reserve food or serves as an attachment for scalelike leaves and roots. Buds spring from the axils of the scale leaves as they do from the leaves of an ordi- nary branch, and these grow into new bulbs or new Parent buib.,,^^ \\ . /AX. W areH . 1 . corms. Tubers, corms, and bulbs are favorite repro- ductive structures of plants in dry regions or in cli- mates where a dry season prevails for a portion of the year. When the ^7 SeaSOll COmeS oil the aerial 2Teen , . & . part of the plant dies down, and the underground bulb, tuber, or corm is able to live without perceptible injury from drought. These fleshy stems, with great stores of food, have been changed and improved for man's use by high cultivation and selection. In nature their production illustrates the abil- ity of plants to adapt them- J (\^Parent corm- selves to various environmental conditions by variation and selection. FIG. 79. Bulbs of the garden tulip a, surface view of a large bulb, showing the origin of smaller bulbs in the axils of bulb scales ; b, sectional view of the same bulb, showing stem, roots, scales, and a lateral bulb (offspring) a, surface view of a large corm with smaller corms; 6, sectional view of the same corm TT A "\7TTQ FIG. 80. Corm of gladiolus Ordinary leaves, such as those of the cultivated begonia of the greenhouses, may repro- duce new plants vegetatively when properly treated. The leaves are usually cut and placed in moist sand, when new plants spring from the cut surfaces of the veins by budding. The buds take 158 GENERAL BOTANY root and in a few weeks may be severed from the propagating leaf as new, independent plants. In the walking fern (Camptoso- rus rJdzophyllus) (Fig. 81) new plants are formed from the ends of leaves, which bend downward, touch the soil at O/spring FIG. 81. The walking fern (Camptosorus) their tips, take root. ROOTS and The figure shows how new fern plants take their origin from the tips of leaves Many ordinary roots may be made to repro- duce vegetatively in a manner quite similar to that outlined above for the begonia leaf. On the other hand, roots, like stems, may become highly modified for vegetative reproduction. Com- mon examples of this are the roots of the dahlia and of the sweet potato (Fig. 82), which, like the bulb and the tuber, are storehouses of reserve food for the growth of the young plants which spring from them vegetatively. In all these cases of vegetar tive reproduction the offspring resemble the parent very closely, since the cells which produce the new plants by growth are all FlG ' 82 ' Enlar ed edl ' ble roots of the . sweet potato (Ipomoea) derived irom a single parent. This is often a very distinct advantage to man, since it enables him to perpetuate a favorable set of characters in a new fruit or vegetable much more easily than could be done by sexual VEGETATIVE AND SEXUAL REPRODUCTION 159 reproduction. It is well known that most of the finest fruits are now propagated vegetatively by budding or grafting, and seedless fruits are necessarily perpetuated in this manner. Such plants often lose the power of sexual reproduction after long cultivation and propagation by vegetative means. It is evi- dent from the few examples of vegetative reproduction indicated above that the higher plants, much more largely than the higher animals, retain the power of reproduction by the cells in all parts and organs of the body. This is probably due to the fact that plants, quite unlike animals, have a long period of growth and are able to renew each year the leaves and flowers of the season and to increase the length of roots and stems throughout life. SEXUAL REPRODUCTION Sexual reproduction differs from vegetative reproduction in the following important particulars. The reproductive cells in sexual reproduction are not the ordinary unmodified cells of the plant body which serve for vegetative reproduction, but are rather highly specialized naked cells termed gametes. These gamete cells fuse to form a new double cell, the zygote, and the zygote produces a new plant by cell division. When the zygote cell is formed by the union of the two gamete cells, male and female, derived from different parents, the new organism which grows from the zygote is quite certain to be unlike either parent, since it inherits through the male and female gametes two sets of hered- itary characters. The sexual process, therefore, instead of produc- ing offspring like a given parent, as in vegetative reproduction, is quite certain to produce a variety in offspring. In nature the production of new kinds of organisms is apparently advantageous to any given species, or kind, of plant in meeting the require- ments of a changing environment and the struggle for existence to which all organisms are subjected. Some of the new kinds of offspring resulting from sexual union are quite certain to have new and advantageous combinations of characters which will enable their possessors to win out in the battle of plants for food and light Man has taken advantage of this tendency in 160 GENERAL BOTANY the offspring resulting from the sexual process to produce new plants which are either pleasing on account of their beauty or are useful for food, forage, or fuel. Other advantages have been attributed to the sexual process besides those which accrue to a species by the production of a varied offspring, but these are as yet unproved. In the higher plants the male gametes are produced in the so- called pollen tube, which is an outgrowth from the pollen grain. They are small, naked cells with a conspicuous nucleus and a Egg cell Seedling plant FIG. 83. Sexual reproduction and doubling of the chromosomes in fertilization very thin sheath of cytoplasm. The end of the pollen tube rup- tures when it reaches the vicinity of the egg in the ovule, and frees the male gametes. The female gamete is larger than the male gamete and is furnished with a more conspicuous nucleus and a larger amount of cytoplasm. When the two naked gamete cells come together, the male and female nuclei, called pronuclei, approach each other and finally unite to form a new double nucleus, the conjugate nucleus, or fusion nucleus. This process of fusion of male and female sex cells is called fertilization (Fig. 83). The zygote which results is a double cell structurally and functionally ; hence the embryo and the young plantlet in the seed must possess all of the characters of the parents which entered into the zygote cell through the gametes. Since the male gametes are produced in the pollen grains and the female gamete is deeply buried in the ovule of a higher plant, a complicated VEGETATIVE AND SEXUAL REPRODUCTION 161 apparatus is necessary in order to make sure that the male gametes will reach the female gametes and fertilize them. To insure this union of the sex cells the flower has been evolved, with its complicated apparatus for attracting insects and for holding the pollen brought to the stigma by wind or insects. THE FLOWER AND ITS PARTS The flower is the reproductive apparatus of the higher plants, designed to insure sexual union of the gametes and the produc- tion of embryos in the seed. The early history of the flower shows that it is a modified shoot or bud in which the parts have been changed to meet the needs of a highly organized repro- ductive apparatus. No attempt will be made here to trace the steps in the development of the flower or to give its manifold variations in the different orders of plants. We shall rather study the parts of a typical flower and then endeavor to trace the processes of pollination and fertilization. The simple flower of the mandrake (Fig. 84) will be used to illustrate the following general discussion of the parts of a typical flower and its fruit. See also the flowers of the marigold and the buttercup in Part III. Peduncle and receptacle. Most flowers are borne on a slender stalk, or peduncle, w r hich is enlarged at its apex to form the receptacle. The floral parts have their origin on this receptacle, which corresponds to the apex of the floral branch. The perianth. The perianth is usually composed of two dis- tinct parts : the calyx, composed of separate leaflike parts called sepals, and the corolla, composed of individual parts called petals. The petals are frequently highly colored and constitute the showy part of common cultivated and wild flowers. The calyx springs from the receptacle just below the corolla ; in the man- drake it is composed of six sepals, which fall off as soon as the flower opens from the. bud. In addition to its function as a flag apparatus to attract insects the perianth serves as a protective envelope for the essential organs of the flower, the stamens and the pistil. In the bud stage these organs are completely inclosed 162 GENERAL BOTANY in. the perianth, and many flowers retain for some time the power of opening and closing the calyx and corolla in response to light, temperature, and moisture. They are thus able to serve as a daily protection to the essential organs during the entire flowering period. Essential organs. The essential organs of the flower (so named for the reason that they bear the pollen and the ovules, which are isccnce line \-Anther sac Funiculus^ spores Boots a FIG. 84. Habit of the mandrake (Podophyllum), with flowers and floral parts a, arnandrake plant with a flower; 6, a pistil in section, showing the origin of the ovules on the placenta; c, an ovule, highly magnified to show its parts; d, a stamen, with anther, showing the lines of dehiscence and the pollen ; e, a transverse section of the anther necessary to the production of seed) are the stamens and the pistil. The stamens in the simple types of flowers arise above the petals, constituting one or more whorls, the number varying in different kinds of flowers. Each stamen is composed of a delicate stalk, or filament, which bears at its apex the anther, composed of two pollen sacs. The pollen grains or spores are developed within the pollen sacs. When the pollen grains are ripe, each anther splits along two lines, called the lines of dehiscence, and the VEGETATIVE AND SEXUAL REPRODUCTION 163 pollen sacs gape open, thus enabling the pollen to escape. The pollen is then free to sift out and to be deposited by the wind or by insects upon the stigma of the same or a different flower. The deposit of pollen on the stigma is termed pollination and is essential to fertilization and the setting of seed. The pistil may be borne singly on the receptacle, as in the mandrake, or there may be a cluster of separate pistils in a single flower, as in the buttercup. The pistil is composed of a sac, or flask-shaped lower portion, called the ovary, and of a terminal portion, called the stigma. The stigma is usually roughened, irregular, or furnished with hairs or a sticky fluid for the retention of pollen brought to it during pollination. In many cases the stigma is joined to the ovary by a narrow neck called the style. The style is sometimes lacking, and then the stigma is said to be sessile. The ovary bears the ovules on a cellular out- growth, or ridge, called the placenta. The ovules (Fig. 84, , the seed with an embryo (light) and endosperm (dotted). All diagrammatic reaches the ovary cavity, it is apparently attracted by some chemical substance secreted from the micropyle of the ovule, since it turns sharply and makes its way into this pore between the ovule coats. When it reaches the embryo sac, it comes in contact with the egg apparatus and the egg cell, which always lies at the base of the micropylar canal. The pollen tube then ruptures at its thin end, and the tube nucleus and male cells enter the embryo sac in the immediate vicinity of the egg, as illustrated in Fig. 86, c. Fertilization. One of the male gametes, or its nucleus, then unites with the female gamete to form the zygote, or fertilized egg 166 GENERAL BOTANY - Roots - Fio. 87. Seedlings of the mandrake Redrawn from Holm Cotyledon -Hypocotyl cell. This union of the male and female gametes constitutes the real act of fertilization. The nucleus of the second male gamete unites with one or both polar nuclei ; at least, this is what hap- pens in a large number of instances which have been investigated. This second union of a male nucleus with the polar nuclei results in the formation of the food-reserve material, called the endosperm, which is developed for the purpose of nourishing the young embryo plant until it becomes self-supporting. Embryo. The development of the zygote into the embryo takes place immediately after fertilization. The de- tails of this process differ in different species of plants and cannot be dis- cussed here for the mandrake. The embryo in the seed (Fig. 86, d) con- sists of a stem, or hypocotyl, two cotyledons, and a plumule. The seed. The seed consists of the embryo, the endosperm, and the integu- ments, which become transformed into the hard seed coats of the ripe seed. The seed- lings, which result from the germination of the seed, are illustrated in Fig. 87. The fruit. The fruit is the ripened ovary, in which the walls and the pla- centa become fleshy and constitute the edible fruit of the mandrake (Fig. 88). The seeds are finally liberated by the decay of the fruit and lie dormant until conditions favorable for germination occur. POLLINATION Pollination is the term used to designate the transfer of pollen from the anther of a flower to the stigma. In this text, in the dis- cussion pertaining to pollination, the following terms will be used to discriminate between different kinds, or degrees, of pollination. Old stigma Old ovary fedicd of flower FIG. 88. Fruit of the mandrake VEGETATIVE AND SEXUAL REPRODUCTION 167 Kinds of pollination. The term self-pollination will be used to indicate the transfer of pollen from the anthers of a given flower to the stigma of the same flower. Close-pollination will be interpreted as the transfer of pollen from the anthers of one flower to the stigma of another flower or flowers on the same plant. Close-pollination thus defined is often designated as cross-pollination ; but since the practical effects of close- pollination in plant breeding are usually different from those of cross-pollination as defined below, it is thought best to retain the above definition of close-pollination. Cross-pollination will be used to designate all cases in which the pollen from flowers on one plant is transformed to the stigma or stigmas of flowers on another plant. Since pollination is essential in the higher plants before fer- tilization can take place, it is necessary for the perpetuation of any given race or species of plants which is not adapted to maintaining itself by vegetative reproduction. The researches of Darwin also established the fact that cross-pollination is of distinct advantage to many species in producing stronger and better offspring. It is not surprising, therefore, that nature has evolved a great variety of novel and interesting devices for insur- ing both self-pollination and cross-pollination in flowers. In the following section the papilionaceous flowers of the pea family have been selected to illustrate some devices for insuring self- pollination and cross-pollination. Inflorescence and pollination. In some species the flowers are borne singly from the axils of ordinary leaves, but in a large number of plant species the flowers are clustered, and such flower clusters are termed inflorescences (Fig. 90, c?). This flower clus- ter is evidently a modified branch system, in which the central axis, termed the axis of inflorescence, corresponds to the central stem of a shoot. The leaves have been reduced to small bracts, and the flowers replace branches which ordinarily spring from the axils of the leaves. This agrees with the statement made above that flowers are really modified branch buds. The dis- tinct advantage of such an inflorescence as that of the locust (Fig. 90, d) in securing pollination is easily understood if one 168 GENERAL BOTANY watches a bee seeking for nectar, or pollen, in its flowers. The bee will be seen to go rapidly from one flower to another on the inflorescence, probing for nectar at the base of each flower and so dusting its body abundantly with pollen. When one inflor- escence is exhausted, the bee moves to another and repeats the process. It is quite evident that abundant close-pollination will thus be effected by such a bee between flowers of the same plant, and that cross-pollination will be effected if the bee visits succes- sively inflorescences borne on different plants. Moreover, many more pollinations will occur than could possibly be secured if the flowers were borne separately from the axis of the ordinary leaves of the plant. In discussing devices for insuring abundant pollination the inflorescence is therefore of prime importance as an aid in securing frequent close-pollination and cross-pollination. The head of the common white and red clovers and the large flower clusters of the sweet pea and bean are other familiar in- stances of inflorescences in the pea family which are of advantage in securing cross-pollination of the flowers of these species. Pollination devices in papilionaceous flowers. Structure of the flower. The flowers of the pea family are very highly specialized, and some are adapted to self-pollination and some to cross- pollination. They are usually called papilionaceous flowers, from their fancied resemblance to butterflies of the genus Papilio. The general relations of the floral parts as they appear in the garden pea are illustrated in Fig. 89. The perianth is com- posed of both calyx and corolla, each having five parts desig- nated respectively as sepals and petals. The calyx is nearly regular, but the corolla is very highly modified and irregular. The largest petal is called the standard, since it projects promi- nently, like a standard, from the rest of the flower. The standard petal overlaps two lateral petals, or wing petals, and these inclose two united keel petals, which together form the boat-shaped keel. In the normal condition of the flower the stamens and pistil are inclosed and concealed from view by the keel. In Fig. 89, c, a flower is shown in which the petals on one side, including one half of the keel, have been removed so as to expose the stamens and the pistil in their natural position. VEGETATIVE AND SEXUAL REPRODUCTION 169 The stamens are united by the lower part of the filaments, which form a membranous sheath, or stamen tube (d), enveloping the ovary like a sac. Nine stamens are usually thus united, leaving the tenth stamen free. The pistil resembles closely the familiar pod of the garden pea and is composed of the inflated ovary, the style, and the stigma. The ovary forms the pod or fruit, and the slender Fruit Wing petal Fmieubu Ovary Keel petals Stigma $ f 'Style Pistil _ % h Cotyledon FIG. 89. Structure of the papilionaceous flower of the pea (Pisum) a, flower ; 6, irregular petals ; c, stamens and pistil exposed ; d, relation of stamens and stigma ; e, pistil ; /, fruit ; g, seed ; h, embryo and cotyledons style bends sharply at its junction with the ovary, thus taking up a position at -right angles to the latter structure. The style is terminated by the somewhat enlarged and roughened stigmas. Mechanism of pollination. The flowers of a large number of species belonging to the pea family are definitely adapted to securing either close-pollination or cross-pOllination through the agency of insects which visit these flowers for nectar or pollen. In some instances, however, the flowers, like those of the garden pea and the sweet pea, are so constructed that self-pollination may habitually occur. In these species the pollen ripens in 170 GENEKAL BOTANY conjunction with the maturing of the stigma, and the anthers are so placed that the pollen is dusted onto the hairs of the stigmatic surface when the anthers dehisce. Self-pollination is thus in- sured, even though insect visitors are artificially excluded. On the other hand, these flowers are admirably adapted, in many respects, to close-pollination or cross-pollination. The conspicuous standard serves as a flag apparatus, and the nectar is so located at the base of the flower that visiting insects are tempted to probe into the flower for it. The wing petals serve to support the visiting insect ; and since they are attached to the keel, this structure is certain to be depressed by the weight of the insect's body. The depression of the keel petals results in the exposure of the anthers and the hairs on the style, which snap up against the insect's body through the opening between the keel petals. The hairy abdomen of the visiting insect is thus covered with pollen, which may be borne to other flowers on the same plant, thus effecting close-pollination, or to flowers on different plants, effecting cross-pollination. The structure of the pea flower, like that of species which are wholly .unable to secure self- pollination, thus manifests remarkable adaptations for securing close-pollination or cross-pollination. In the two examples which follow, selected from the pea family, either close-pollination or cross-pollination is assured, and self-pollination is prevented by the structure of the flower and by the relative positions of the anthers, the style, and the stigma. The locust and red clover. The flower of the common locust may be used, in contrast with that of the pea, as a concrete example of an elaborate adaptive mechanism in a flower of the pea family, designed to insure either close-pollination or cross- pollination. The general arrangement and shape of the floral parts in the locust flower are so similar to those of the pea blossom just described that no additional description is necessary (Fig. 90, !, same views of kernels with small embryos having low oil content. From Bulletin 87, Uni- versity of Illinois Agricultural Experiment Station A FIG. 106. Kernels of corn with high percentage of protein A, high-protein kernel; />, low-protein kernel; e, embryo; s, starchy area; p, horny, or protein, layer.- From Bulletin 87, University of Illinois Agricultural Experiment Station 196 GENERAL BOTANY variations is followed in the improvement of other kinds of fruit, forage, and food-producing plants. The following extract from " Plant Breeding," by Professor Bailey, illustrates some of the detail and care necessary in such a selecting process when applied to the cultivation of beans. He [the breeder] starts with one plant. The next year he may have only two. If he has ten or twenty good ones, then the task is easy, for the variety has elements of permanence, that is, of heredi- tability, in it. As soon as seeds can be secured in considerable amount from a strain of beans, selected as indicated above, the grower can plant a large plot and obtain seeds for sale or for dis- tribution to other breeders. He must exercise judgment and skill each year, however, in selecting seed, even from such a carefully improved race, if he is to prevent its running out or reverting to the original type. This is due to the fact that his fields and experi- mental plots will always contain reversions, or " rogues," from which seed must not be taken if his improved strain is to be kept true to type. In the common dwarf, or bush, beans of the gardens, for instance, there is always a tendency to revert to the ancestral climb- ing variety. These rogues with a tendency to climb must, there- fore, be eliminated if the bush habit is to be perpetuated. Mr. Palmer's dwarf lima originated in 1883, when his entire crop of large white (pole) limas was destroyed by cutworms. He went over his field to remove the poles before fitting the land for other uses, but he found one little plant, about ten inches high, which had been cut off about an inch above the ground, but which had rerooted. It bore three pods, each containing one seed. These three seeds were planted in 1884, and two of the plants were dwarf like the parent. By discarding all plants which had a tendency to climb, in succeeding crops, the Burpee bush lima, as we now have it, was developed. Difficulties. Two main difficulties are experienced by breeders in their attempt to improve plants by the selection of variations. One of these difficulties has already been referred to, namely, the tendency of varieties and races thus produced to revert to the condition of ancestors of the less desirable type, a phenomenon expressed by the term running out. A second point of impor- tance is that races built up by the continued selection and PLANT BREEDING AND EVOLUTION 197 accumulation of variations are limited in the degree to which they can be improved. Thus, the sugar beet has been cultivated and selected for a hundred years or more, but it has not been found possible to increase the average sugar production of a selected field of beets beyond from 16 to 18 per cent. The Illi- nois experiment station has had a similar experience in attempt- ing to raise the percentage of oil and protein in corn beyond the high percentages mentioned above. A similar condition seems to prevail among all cultivated varieties, where ordinary variations are used as a basis for selection. Two KINDS OF VARIATIONS Two kinds of variations are now recognized by scientists and plant breeders, known respectively as fluctuating (or con- tinuous) variations, and mutations (or discontinuous variations). .Fluctuating variations are the ordinary dif- ferences observed in plants of the same kind in a field or a garden. They are due to the environment, such as rich or poor soil, abundance or lack of water, sun ex- posure or shade, and many other environ- mental influences to which plants growing in the open are sub- jected. They are fur- thermore inconstant, not tending to be perpetuated in the offspring of plants mani- festing them. We all know, for instance, that seeds from a large, healthy tomato plant, produced by rich soil and careful culture in a garden, will not bear similar offspring unless they FIG. 107. Connecticut broad-leaf tobacco, unselected Showing variation in tobacco due to change of soil and climate. Photograph furnished by Connecticut Agricultural Experiment Station 198 GENEKAL BOTANY grow under like conditions. It seems probable, therefore, that the running out of selected races of corn, beans, and other cultivated plants mentioned in the previous pages is due to the selection, on the part of cultivators and breeders, of plants with these fluctuating variations. Fluctuating variations can be accumulated to a certain extent, and a better race may thus be produced ; but such a race will always re- main inconstant and limited in range or in the degree to which it can be improved. Thus, in the tobacco plant (Fig. 107) great variation has been in- duced in certain varie- ties by planting in the , soil and climate of Connecticut and other Northern states seed obtained in Sumatra and Florida. Some of these variations are of the fluctuating variety, Showing uniformity of type in tobacco due to selec- tion of seed. Photograph by the United States Depart- ment of Agriculture FIG. 108. Uncle Sam Sumatra tobacco, selected and so will not form the basis for a stable improved race pro- duced by selection. In other instances the variations induced in tobacco by the change in climate and soil are apparently stable from the outset, and form the basis for new constant races. The Uncle Sam Sumatra tobacco (Fig. 108) is supposed to have had such an origin by mutation. The Florida-Sumatra tobacco was grown from seed secured from the island of Sumatra. When the Florida-grown seed was taken to Connecticut, the plants grown from it varied in a marked degree, and several new types developed that did not exist in Florida and, accord- ing to the best information obtainable, did not exist in Sumatra. PLANT BKEEDING AND EVOLUTION 199 I The Uncle Sam type appeared among these variations and has proved to be constant when grown from seed. In tobacco, there- fore, as in many other plants, two kinds of variations frequently occur which are of very different value and importance in the improvement of these plants by breeding. The inconstant, or fluctuating, variations are not of great value for selection in the making of an improved race, while the stable varia- tions called mutations are of the greatest interest and importance to breeders of new kinds of plants and animals. THE MUTATION THEORY Certain varieties of plants, like the Uncle Sam Sumatra tobacco, have long been known to have arisen sud- denly as a result of varia- tions affecting one or more parts of the plant body. The plants manifesting these new variations have usually been called sports in agri- culture and horticulture, but are now called mutants. The variations which distinguish them from the parent species are called mutations. Among the best known of such sports, or mutants, are the following: the moss rose and the nectarine, which are bud sports from a cultivated rose and from the peach ; the various cut-leaved varieties of the willow, maple, and birch ; many white-flowered varieties springing from plants with colored flowers ; and probably many varieties of vegetables, forage plants, cereals, and fruits whose history, when traced back, indicates a sudden origin from wild species. The muta- tion theory for the origin of new varieties by sudden constant FIG. 109. (Enothera lamarckiana This is the mother plant of mutants dis- covered and produced by De Vries. From Babcock and Clausen's " Genetics in Rela- tion to Agriculture." After De Vries 200 GENEKAL BOTANY variations has been brought into prominence in recent years through the publications of the great Dutch botanist, Hugo de Vries. De Vries first noticed some mutants of the evening primrose known as Lamarck's evening primrose {(Enothera lamarckiana) (Fig. 109) growing in a waste field near Hilver- sum in Holland. In 1886 he collected seeds from the mother plant and from the two new varieties pro- duced from it by muta- tion, and sowed them in the botanic gardens at Amsterdam. He has since carried on extensive cul- ture experiments with the primroses and their off- spring, and has succeeded in producing several new varieties which differ from the original mother plant in all of the various or- gans of the plant body. One is a dwarf species (Fig. 110), while another is a giant form with greater vegetative vigor and larger flowers than the mother plant. Others vary in the form and color of the leaves, in the character of the seedlings (Fig. Ill), and in the nature of the reproductive organs, including both the flower and the fruit. Since all of these new primroses came suddenly, by one large variation or mutation, and have bred true to seed, De Vries believes that they indicate the way in which other cultivated and wild varieties have arisen whose history indicates a similar method of production. This sudden origin of cultivated forms is well illustrated by the FIG. 110. CEnothera lamarckiana and two of its mutants, (E. lata and (E. nanella (E. lamarckiana in the middle; lata at the left; and nanella at the right. From Babcock and Clausen's " Genetics in Relation to Agriculture." After De Vries HUGO DE VRIES Hugo de Vries is director of the Botanic Garden in Amsterdam, Holland. He is the greatest living exponent of the origin of species by sudden constant variations, or mutations. His early investigations on variation, which convinced him of the truth of the mutation theory, extended over a period of nearly twenty-six years and involved experiments and breeding tests with some hundred species of wild plants. His experi- ments and theories are set forth in his greatest work, "Die Mutations Theorie,"and in numerous succeeding books and articles. In his method of work and in the im- portance of his results De Vries is perhaps more nearly comparable to the great Darwin than any other living investigator 202 GENERAL BOTANY history of certain species of grapes recounted by Professor Bailey in his " Evolution of our Native Fruits." The Concord was a chance seedling in a Massachusetts garden, and it is supposed to have sprung from the wild fox grape of the neighborhood. The Clinton came up where a handful of grape seeds was sown at Hamilton College, Clinton, New York ; and the old vine, now nearly seventy-five years old, is still grow- ing on College Hill. The Norton's Vir- ginia was found wild in 1835 near Rich- mond, Virginia. The best Ameri- can gooseberries, the Hough ton and the Downing, are sup- posed, like the Con- cord and Clinton grapes, to have origi- nated from chance FIG. 111. Seedlings of (E. lamarckiana and three -,-,. . , ,, of its mutants, showing constant differences in m S> . rosettes and leaf characters Swedish experiment Upper row, typical lamarckiana ; second row from top, station, located at (E. gigas; third row, (E. rubrinervis : bottom row, (E. QUralof Anther idiu Male, Female ga branch FIG. 120. Sexual reproduction in Vaucheria a, portion of a filament with male and female reproductive branches; b, antheridium and oogonium, with sperms and egg ; c, liberation of sperms, and sperms swarming around the receptive spot of the egg ; d, sperms ; e, fertilized egg, with union of male and female pronuclei female, divides into a very large number of naked protoplasts, each of which becomes a motile male gamete. These minute male gametes are finally liberated in the water by the softening and rupture of the cell wall at the end of the male game- tangium (e) and are ready to effect a union with the eggs in the female gametangia. Fertilization results from the union of the cytoplasm and nucleus of one of these small male gametes with that of the large female gamete (Fig. 120, c). In order to effect this union the cell wall at the end of the beak of the female game- tangium softens and (e) finally breaks down so that a free passage is made for the motile gamete to the cytoplasm of the female 232 GENERAL BOTANY gamete. Many botanists hold that the female gamete secretes some substance, such as cane sugar or an organic acid, which attracts the male gamete to the female and thus insures fertilization by directing the free motile male gamete to the stationary egg. The zygote, or fertilized female gamete, remains in the female gametangium (Fig. 121, a) and undergoes a period of rest. Dur- ing this rest period it is protected by the old gametangium wall, which remains in its former position around the egg cytoplasm. The zygote is abundantly furnished with reserve food in the form of oil and with the chloroplasts of the original unfertilized female gamete. The germination and growth of the zygote cell (6) to form a new gamete plant is a com- paratively simple process, since the adult gamete plant is unicellular and its growth from the spherical zygote cell consists mainly in a great elongation of the latter, with accompanying branching of the filament thus produced. During this elongat- ing and branching process new nuclei and chloroplasts are being formed by repeated divisions of the chloroplasts and con- jugate nucleus of the zygote. The resulting gamete plant re- sembles exactly the original mother plant, and in time, after a period of vegetative activity, reverts to reproduction. Asexual reproduction. In Vaucheria sessilis another mode of reproduction occurs which is asexual in its nature. In this process the end of a filament swells up and becomes separated into a distinct cell by a transverse cell wall (Fig. 122, a). The cytoplasmic contents of the terminal cell then rounds up and sends out minute cytoplasmic projections called cilia, thus forming a large motile asexual naked cell, or zoospore (5). By the rupture of the end of the enveloping wall this zoospore becomes a free-swimming cell (le in the life history of the organism (Fig. 153). The spring crop of spores, called ceciospores or ceeidiovpore*, are borne in open cups (F) on the leaves of the common barberry. These barberry spores are supported by a mycelium which pene- trates the barberry leaf as a parasite and absorbs food for itself and the spore-bearing hyphse produced within the barberry cups. These spring spores become mature at about the time when the young wheat plants are springing up in the fields, and when they are blown by the wind and fall upon a young wheat plant they germinate and produce a dense mycelium within the tissues of its growing leaves and stem. This inner parasitic mycelium then forms red-rust spores, in groups called sori (-5), at certain points along the surface of the leaves and the stem. These masses of spores break through the epidermis and form the long lines of spores (J[) familiarly known as red rust. These summer spores, called urediniospores or uredospores, are single-celled and are borne on a short stalk. They are quickly disseminated, and since they germinate at once under favorable conditions, they serve to spread the rust very widely in the fields of grain in early summer. In warm regions the red-rust spores often survive the winter and start the rust in the spring. FIG. 153. Wheat rust (Puccinia graminis) A, portion of stem of wheat with rust spores in groups, or sori; B, sectional view of a sorus, with young uredospores (?/?/) and mature uredospores (u), stalks (st),&nd my- celium (m) within the stem tissues; C, red-rust spore (uredospore) with stalk (st) sending out infection hyphae (m) ; D, two-celled black-rust spore (teleutospore, t) with greatly thickened cell wall and stalk (st) ; E, germination of the upper spore cell of the teleutospore (t) to form the promycelium (p), hearing disseminating spores (sporidia, s) F, section of barberry leaf showing the aecidium cup with spores (s), the wall of the cup, or peridium (p), and the upper and lower epidermis of the leaf (e and e 2 ). Adapted from Duggar's " Fungous Diseases of Plants " 276 GENERAL BOTANY The black-rust spores, called teliospores or teleutospores, are formed in the same sori and from the same mycelium as the red rust, but are produced later in the season than the latter. The black-rust spores are two-celled, thick-walled wintering spores (Z>) which live through the winter on the straw or stubble and germinate the next spring after their production. Their germination (JS) results in small spores, usually called sporidia (s), which are blown to the young leaves of the barberry and start a mycelium for the production of a fresh crop of aecidium cups and spores on the barberry leaves. This completes the remarkable cycle of these peculiar plants, which have become so highly adapted in their spore forms to the seasonal life of the organism. This seasonal life, as the above account indicates, comprises two distinct feeding mycelial plants, which grow on two different species of flowering plants. Four kinds of spores are also produced, which serve for dissemi- nating the plants in spring and summer and for wintering. Other forms of rusts occur on various cereal grains and on the grasses, to which the cereals are closely related. Not all of these rusts, however, have so complicated a life history as that of Puecinia graminis, although most of them have at least two forms of spores and many species avail themselves of two hosts in the production of these spores. The rusts are among the most destructive of the fungi and cause the loss of many millions of dollars each year by the damage to cereals of various kinds. SMUTS The most familiar smut is that of Indian corn ( Ustilago zeae) (Fig. 154), which causes the distortion of the kernels of corn on the cob and produces the large black masses of smut spores seen in fields of corn in the autumn. If the smutted ears of corn are examined early in the season, it will be found that the fungus gradually replaces the kernels as it absorbs the food stored in them. Later the infected kernels grow to many times the size of the original kernels and produce great masses of spores in cavities which resemble somewhat those already described in THE FUNGI 277 the puffball. Finally all of the tissues of the original kernel are absorbed and the hyphse die, leaving the spores surrounded by a thin hyphal membrane. The rupture of this membrane allows the spores to escape and to be widely disseminated in the fields. Like the black spores of the wheat rust these smut spores carry the plant over the winter and germinate in the spring, producing the smaller spores, or sporidia, which are blown to young corn plants and start the fungus for another season. Other smuts on onions, wheat, oats, etc. have a history similar to that of the corn smut. The smuts are very destruc- tive where they occur in abun- dance, since they attack the ker- nels of the grains affected and thus cause the com- plete destruction of the seed. FIG. 154. Kernels of corn infected with corn smut (Ustilago zeae) Observe the outline of the ker- nels still visible in the upper, greatly swollen mass. After Duggar FIG. 155. Spore germi- nation in the spores of corn smut ( Ustilago zeae) Note the conidia, or dis- seminating and infecting spores, being produced from the sides of the sin- gle germ tube, or hypha, springing from the spore. After Brefeld LICHENS Structure and habit. Lichens occur on the bark of trees, on old fences, and even on earth and stones in certain localities. They are grayish green in color and may occur as flat, leaflike expansions (foliaceous lichens) (Fig. 156), as powdery crusts (crustaceous lichens), or as shrubby, erect outgrowths (fruticose lichens). The fruticose lichens often form conspicuous coverings FIG. 156. Habit and structure of a common tree lichen, Physcia stellaris A, habit of plants bearing reproductive cups, or apothecia; B, section of a reproduc- tive cup, showing the dark spore-bearing layer (hymenium) lining the cup ; C, struc- ture of upper portion of plant body, including the green algal cells ; D, portion of the hymenial layer, greatly magnified, showing spore sacs (asci), with dark-colored spores, and the lighter paraphyses. From Bergen and Davis's " Principles of Botany " THE FUNGI 279 of branches in warm, moist climates, where they are popularly known as gray moss, since they resemble mosses very closely. The grayish-green color of lichens is due to the fact that the plant body is composed of two distinct plants : namely, a green alga resembling Protococcus and a colorless fungus like the molds. These two plants are associated as partners in the same plant body, the alga furnishing the sugar made by photosynthesis in its green cells, and the fungus ab- sorbing the water and soil salts neces- sary for the growth of each partner. The physical rela- tions of the two plants are such as to enable them to maintain their part- nership to the best advantage. The flat, leaflike body of a common foliaceous lichen is composed largely of molcllike fungus hyphse, not unlike a very dense and regularly formed mycelium. The hyphse on the upper and lower surfaces of the flat expansion are short and closely ad- herent (Fig. 156, (7), thus forming the protective outer layers termed respectively the upper and lower cortex. Between these two layers the hyphse form a looser structure termed the medulla, or central part of the lichen body. The algae are commonly dis- tributed in a layer just beneath the upper cortex, where they are most advantageously exposed to light. The fungus hyphse either penetrate the algal cells by means of short branches which absorb food from the vacuole, or they adhere closely to the algse and absorb nutriment through their cell wall. FIG. 157. A common lichen, Parmelia, on the bark of a hickory tree The spore-bearing cups are clearly shown 280 GENERAL BOTANY The relation of the fungus to the algae is therefore that of a parasite on a green plant, while the algae probably derive some benefit from the fungus in raw food materials and protection. Such a relationship is called symbiosis, which means a partnership with mutual advantage to each partner. This physical relation of the algae and the fungus of the lichen body is easily demon- strated by making sections or by teasing out small portions of an ordinary lichen in a drop of water. The striking contrast between the bright-green algal cells and the colorless hyphae of the fungus enables one to determine without difficulty in such preparations the relation and the nature of the two plant partners. Reproduction. Almost any part of the plant body of a lichen, if removed from the mother plant and placed under favorable conditions, can reproduce the plant vegetatively. The usual method of reproduction, however, is by means of asexual spores, which are formed in special club-shaped cells termed asci. These spore-bearing cells occur in great numbers in the so-called lichen cups, which are easily seen covering the upper surface of lichens in fruit. The inner surface of these cups, or apothecia, is usually dark brown or gray in color, but in some instances it may be orange or brick red. A section through the cup (Fig. 156, #) shows that its entire inner surface is lined with the club-shaped spore-bearing cells, or asci, alternating with greatly elongated sterile hyphae called paraphyses. Each club-shaped spore-bearing cell contains from two to eight spores, which are expelled with some force from the ascus when they are ripe. These, spores ger- minate like mold spores, and if the hyphae thus formed come in contact with the proper algae, they attach themselves to the algal cells and gradually form a new lichen plant. It is thus seen that the production of a new lichen is entirely dependent upon spore reproduction by the fungus. In this manner lichens arise in nature wherever rough, moist surfaces offer the proper conditions for their growth. On account of their great hardiness and their ability to make their own food the lichens are among the most widely distributed plants in nature. THE FUNGI 281 FUNGI AND PLANT DISEASE Nature and importance. The diseases of plants caused by fungi are scarcely less important than the diseases of animals and man, on account of their intimate relation to the world's food and lumber supply. These facts were abundantly empha- sized to the public during the Great War, when government and state experts demonstrated the immense importance of fungous pests to our national food supply. We have only to recall the damage done by such fungi as the grain rusts and smuts, the potato rot, the apple scab, the grape mildews, and the tree-killing fungi to realize the national importance of fungi and fungous diseases. Some plant diseases are produced by bacteria, as in man and animals, but the greater number are caused by filamentary fungi similar to the molds, rusts, and smuts studied in previous pages. Spread of plant diseases. The rapid spread of plant diseases is due in large measure to the very unusual number of asexual spores produced by the fungi, of which we have had illustrations in the molds, smuts, and mushrooms. Indeed, many fungi, like the mushrooms and smuts, have apparently abandoned the sexual method of reproduction entirely for the more rapid method of producing abundant asexual spores. These spores are light and so, like the bacteria, are constantly blown about and carried long distances by air currents. As a consequence some fungous diseases spread with startling rapidity, resulting in widespread injury. The recent spread of the disease known as chestnut blight, caused by the fungus Endothia parasitica, is a well-known illus- tration of the rapid spread of such a destructive disease by means of spores. The chestnut blight is supposed to have been imported from China and to have spread from New York City, as a center, about 1904. It has now almost wholly destroyed the native chest- nut trees over large areas in the forests and cities of the eastern United States. The pine-tree blister rust has had a similar his- tory and threatens to destroy many thousand feet of valuable pine timber unless its ravages can be checked. 282 GENERAL BOTANY The dissemination of fungi by spores lodged on seeds or by means of mycelium within the seeds is another method of spread- ing fungous diseases. The rusts and smuts are often thus dis- seminated by spores lodged on the kernels of the grain when the seeds are sown. These spores then germinate with the seeds and attack the young seedlings while their tissues are tender and allow the fungus hyphse to penetrate them. Beans are infested with a fungous disease known as anthracnose, which often does great damage to the crop. The mycelium of the fungus causing the disease grows into the ovule during its active vegetative life and hibernates there during the resting period of the seed. The next spring, when the beans are planted, these hibernating hyphse also grow and form a mycelium throughout the tissues of the bean plant. If conditions are right for vigo- rous fungus growth, the mycelium attacks the pods and develop- ing seeds and destroys the crop. In this manner a dangerous disease may readily be spread through the exchange or sale of seed, and the infection of a pre- vious year may cause the real damage to the crop in the year following infection. Local spreading of fungous diseases is often caused in forests by certain fungi which attack the roots of trees. These fungi form long stolons, composed of fungus hyphse, which grow for considerable distances and penetrate the roots of adjacent trees, infecting them with the disease. In addition, dissemination of fungi is effected by water, as in the water molds (Saprolegnici), .by animals (including insects and fish), and by man in the shipping and transportation of plants and plant parts from state to state and from one country to another. The spread of disease by this method has been greatly increased in recent years by the shipping of seeds and entire plants from state to state and from European countries into the United States. Infection. Plants have few openings into the inner tissues of the body, corresponding to the nose, mouth, and ears of man and other animals, through which germs can enter and cause disease. Infection in plants, therefore, is more likely to take place through wounds or through the stomata. THE FUNGI 283 Infection of forest trees (Fig. 151), for instance, usually takes place at points where wounds have been made by prun- ing or where the bark has been injured by animals. In a similar manner the infection of fruits is usually caused by bruising, which breaks the epidermis and thus offers an opening to fungi, whose spores germinate in the wound. It is for this reason that shippers of apples and other fruits exercise the greatest care in preparing the fruit for shipment. This is done by drying and rubbing the surface of the fruit, by discarding infected speci- mens, and by wrapping each fruit in paper to prevent contact and infection from adjacent specimens. Many of these fruits are -infected and rotted in part by molds like Penicillium (Fig. 143), the spores of which are on the skin of the fruit when it is packed. In other instances fungi infect host plants by boring through epidermal walls through the agency of fer- ments, which destroy the tissues ahead of the entering germ tube. Infection by a filamentary fungus is often accompanied by infections on the part of bacteria also, which complete the processes of decay inaugurated by the higher fungous parasite. Invasion and disease production. The fungus hyphae, when they have once penetrated into the interior tissues of a plant, spread and destroy the tissues by the growth of a mycelium, exactly as in the case of mold on bread (Fig. 140) or like a tree parasite (Fig. 152). The invasion and breaking down of the tissues is effected through the secretion of ferments by the hyphse of the invading mycelium, just as the hyphae of Rhizopus digest the starch in a slice of bread by the secretion of digestive ferments. In this manner the mycelium of a parasitic fungus may penetrate the hardest wood of trees, and then dissolve out the wall substance and the stored food by a digestive process which results in decay and death. Smuts and similar fungi, which start to grow with the seed, keep pace in the above manner with the growth of the seedling, following along the paths of the least resistant tissues to places where the spores are to be formed. This invading of the plant body, and the production of disease, is, however, a much slower process in plants than the distribution 284 GENERAL BOTANY of bacteria and their poisonous toxins by means of the blood stream in animals. As a consequence, the period of disease production is greatly prolonged in plants as compared with animals, and may continue in perennials, like trees, for many years without causing death. The exact manner in which fungi destroy plant structures and materials which are commercially important to man varies with the fungus and the host. In the case of forest trees the damage comes from the breaking down of the tissues of the tree, so that the wood is injured, as well as from the checking of the healthy growth and development of the tree. In herbaceous plants the fungous disease usually weakens the plant to such an extent that its productiveness is greatly lessened, or it may destroy the plant entirely. In other cases, as in the attack of grain by smuts, the spores are formed in such numbers in the ovules that the grain itself is almost entirely destroyed. In soft plant structures, like fruits and potatoes, the mold or other parasite simply starts the process of decay, which is then carried on by bacteria until the entire structure is destroyed. Remedies. The remedy for the great losses in food, lumber, and nursery stock, which result from fungous diseases, lies in a better knowledge of the nature of these diseases and in a more strict oversight, on the part of individuals and of state and national governments, of their control and dissemination. The United States government now employs expert plant pathologists, who study the origin and spread of plant diseases throughout the country. They are also cooperating with state governments, through the state experiment stations, in an endeavor to check the progress of dangerous fungous pests. Expert pathologists are also maintained by the United States government to inspect imported seeds and plants, so as to prevent the introduction of plant diseases from foreign countries. Entire shipments of fruits, vegetables, and nursery stock are often condemned by these government officials and denied entrance into our ports. Much insight is also being gained into the various methods of control- ling various diseases by sterilizing seeds before planting and by the destruction of certain fungous hosts which harbor dangerous THE FUNGI 285 parasites. The order of the government to destroy the bar- berry bushes during the late war is an instance in point. These barberries were known to be infected by one phase of the wheat rust, which winters on wheat straw or stubble. By destroying the barberry the government hoped to eradicate this dangerous pest. Recent orders of the government, looking toward the con- trol of shipments of bulbs and nursery stock from Holland and INCOME I. FROM SUN ENERGVv ' 2. FROM AIR V CO* \. OUTGO NON-GREEN ORGANISMS GREEN PLANT FIG. 158. The organic food cycle other European countries, are interesting indications of the awakening of the country to the great danger and commercial importance of fungous diseases to our ornamental and food plants. THE ORGANIC FOOD CYCLE From what has been learned concerning the life of the fungi the student is prepared to understand more clearly the relative position of the fungi and the green plants as regards the food supply of all organic life (Fig. 158). We have seen that the 286 GENERAL BOTANY green plants, on account of their chloroplasts, are able to con- vert the simple elements of the air and the soil into organic food. For this purpose they are able to draw on the energy of the sun through the agency of their chlorophyll pigment. Such plants are thus able to secure an unlimited supply of outside energy, which makes them the great food makers and accumu- lators for the rest of organic nature. The colorless fungi and the animals are dependent upon the green plants for organic food and are therefore food destroyers and energy producers in the organic world. Many of the green plants also serve directly as food for animals and for parasitic fungi of various sorts, but other green plants are not edible, or they die before they are devoured by animals or by plant parasites. The food stored in the cell walls and in the special storage organs of such plants would be lost to the living organic world if it were not for the saprophytic fungi. These saprophytes, as we have learned, are able to secrete ferments which decompose by fermentation or digestion the tissues of all lifeless organisms, and the result of such decomposition is that all lifeless organic matter is ulti- mately reconverted into the gases of the air and the salts of the soil from which green plants obtain their raw food materials. There is, therefore, a continuous cycle taking place in the world's food supply, in which the green plants, the fungi, and the animals are mutually interdependent and necessary factors. In this cycle green plants are the great food producers and energy storers ; animals and parasitic fungi are food users and energy pro- ducers; and the saprophytic fungi are the great scavengers and reconverters of lifeless organic matter into new compounds, which can be used over again by the food-building green plants. It is difficult to see how any one of the members of this triple alliance of organic forms could exist for long without the pres- ence of the other two members of the organic world. When we take this larger view of nature, therefore, we see that the fungi are, on the whole, useful plants. CHAPTER XIY BRYOPHYTES (LIVERWORTS AND MOSSES) The Bryophyta are higher types of plants than the Thallophyta, approaching more nearly to the conditions found in the higher spore and seed plants in the structure of the plant body and in the character of the reproductive organs. In the liverworts, or Hepaticae, the plant body is termed a thallus, since they resemble the thallop'hytes (algse and fungi) in not possessing true roots, stems, and leaves. The mosses (musci) are more highly organized than the liverworts and have stem and leaves resembling some- what those of the higher plants, but this leafy-stemmed plant is a gametophyte plant, as in the algse, instead of a sporophyte plant, like the leafy plants of all plants above the bryophytes. The liverworts and mosses, while differing in the form and structure of the plant body, are nevertheless closely related by the great similarity of their reproductive organs and the stages of their life histories. HEPATICAE (LIVERWORTS) The simplest members of the Hepaticae are amphibious plants, which occupy wet banks and overhanging rocks on the borders of streams and lakes. They are named Hepaticae from their fancied resemblance to the lobes of the human liver. As might be expected, these simplest land plants exhibit transition stages between the aquatic algae and the highest spore plants (repre- sented by the mosses and ferns). The simplest forms are mere ribbons of green cells, resembling in their structure the leaf of Elodea or of a moss. They are attached to the mud or wet rocks over which they grow by fine, hairlike roots called rhizoids, which closely resemble the root hairs of the higher plants. 287 288 GENERAL BOTANY Among the higher liverworts a more bulky plant body enables these plants to live in less moist situations than the lower types and to approximate more nearly in their form and structure to true land plants. These higher forms closely resemble in struc- ture a leaf of the higher plants and are attached, like the simpler species, by means of rhizoid roots. The production of new plants and the wider distribution of the species take place by means of nonmotile spores borne in special spore cases, or sporangia, which are new structures in the life history of plants thus far studied. RICCIOCARPUS Habit and habitat. Ricciocarpus (Fig. 159) is a hydrophytic liverwort which floats on the surface of ponds and lakes or grows on mud along the shore where the water has receded. The general structure of the plant body is similar to a green leaf in its organization, with green chlorophyll tissue on its upper exposed surface adapted for photosynthesis. The lower surface is furnished with rhizoids, arid with platelike structures in the form of scale leaves, which serve in part to keep the plant upright .on the surface of the water. In the case of plants growing on mud the rhizoids function as roots in the absorption of water and soil salts. Gametophyte. The plant body of Ricciocarpus is the gameto- phyte, which bears the reproductive organs in furrows on the upper surface, back of the growing points. The reproductive organs, which we have called gametangia in the algae, are here more highly organized and hence have other names applied to them. The male reproductive organ is called the antheridium and is composed of an outer layer of cells, called wall cells, which inclose and protect the inner mass of mother cells of the sperms, or male gametes (Fig. 159, #). The protoplasts of these mother cells differentiate to form the male gametes, and when the gametes are fully formed, the walls separating the mother cells are absorbed and the gametes are liberated by the rupture of the antheridium at its apex. BRYOPHYTES 289 The female reproductive organ is also more highly organized than similar organs in the algae and is called an archegonium (Fig. 159, c, c?). This archegonium is flask-shaped, with an elon- gated neck and an enlarged venter. The outer cells of the neck and venter form a protective wall layer like that of the an- theridium, inclosing a central column of cells more immediately Neck Yen Neck-canal cells ---Wall cells Ventral- canal cell /-^^v* " ^ a \ Archegonial furrow Antheridial furrow Neck Archegoni Venter !-- $ Pronuclcus IPronucleus d Sperms Wall cells Sperm mother cells FIG. 159. Habit and sexual reproductive organs of Eicciocarpus a, habit of a single plant of Ricciocarpiis ; 6, section of the plant represented in a cut through two reproductive furrows containing archegonia and antheridia; c, section of young archegonium ; d, section of archegtraium with fertilized egg cell containing male (d) and female (?) pronuclei; e, young antheridium; /, sperms with flagella; g, section of mature antheridium concerned with fertilization and the growth of an embryo. This central column of cells includes the neck-canal cells, the ventral- canal cell, and the female gamete. Fertilization takes place when the plants are wet with rain, dew, or spray, since this is the proper condition for liberating the gametes and for the locomotion of the motile male gametes. When the female gamete is ready for fertilization, the neck-canal and ventral-canal cells disorganize and form a mucilaginous substance which absorbs water, ruptures the archegonium at its apex, and exudes in the form of a viscid drop in which the 290 GENERAL BOTANY liberated male gametes become entangled. The male . gametes then swim down the canal of the neck, made by the disorgani- zation of the canal cells, and one successful gamete enters the egg and fertilizes it. The unsuccessful gametes, as in the algge and fungi, are unable to penetrate the fertilized egg cell and die in the neck canal or the venter. Sporophyte. The zygote germinates at once without passing through a resting period as in the algse. The growth of the zygote (Fig. 160) is accompanied by cell division and differen- tiation as in the zygote of the mandrake or the bean. The embryo \ Venter cell Wall cells Spore mother eel FIG. 160. Development of spores (sporogenesis) in Eicciocarpus a, early spore mother-cell stage, with mother cells forming a cellular tissue ; 6, the mother cells, after becoming free, have each divided twice and formed four cells, called a tetrad ; c, the four cells of each tetrad in 6 separating to form four spores which results is, however, very simple in Ricciocarpus as com- pared with the embryos of the higher plants just referred to. It is composed of an outer layer of wall cells which incloses and protects a mass of cells which ultimately become the mother cells of spores. With the enlargement of the embryo the spore mother cells become free from each other, and each mother cell divides twice to form groups of cells in fours, called tetrads. Each cell of a tetrad gives rise to a spore. By this process the embryo is converted into a spore case, or sporangium, filled with spores. The rupture of the wall of this spore case -liberates the spores, which then germinate at once to form new plants in a proper habitat. Such a plant body as that described above, which results from cell division and differentiation of the zygote, is called a sporophyte, since its main function is the bearing of spores. BRYOPHYTES 291 We shall learn, as indicated above, that this simple sporophyte of the liverwort is the forerunner of the leafy green plant body of the ferns and the seed plants like the mandrake and the bean. The sporophyte of Ricciocarpus is therefore a simple kind of sporophyte plant, produced from the zygote, which grows as a parasite on the mother plant, or gametophyte, and is devoted wholly to the production of spores. Life history. In Fig. 161 the main stages in the life history of Ricciocarpus and Vaucheria are represented by diagrammatic figures to bring out the essential points of contrast between them. Sex Gametes Sporo- Gametopliyte organs \ Zygote phyte Spore Gametophyte Gametophyte Sex "~T organs Gametes Gametophyte FIG. 161. Life history of Ricciocarpus and of Vaucheria, represented graphically a, stages in the life history of Ricciocarpus ; b, stages in the life history of Vaucheria In the liverwort the gametophyte is bulkier and more highly differentiated than in the alga, and is thus better able to meet the aquatic or amphibious conditions under which it has to live. The reproductive organs, instead of being, as in the alga, single-celled structures adapted to form the gametes, are com- plex cellular organs, with an outer protective cell layer inclosing the more delicate cells which form the gametes and aid in fertilization. The zygote is formed in both the liverwort and the alga by the union of the male and female gametes in fertilization, and consists in each case of a single cell with a protective cell wall, cytoplasm, and nucleus, like the zygote of Spirogyra. 292 GENERAL BOTANY In the alga this cell usually passes through a resting period, after which it germinates to form a new filamentary plant, or gametophyte. In Ricciocarpus the zygote becomes an embryo plantlet, or sporophyte, through cell division and cell differentiation, as in the higher plants already studied, but in the embryo of Ricciocarpus cell di- vision and differen- tiation are limited, and so the resulting sporophyte plant is relatively simple, as we have seen. The sporophyte of Ricciocarpus evi- dently corresponds in the life history to the embryo and the adult plant of the bean or the man- drake, since they are all derived by simi- lar processes from an initial fertilized egg cell, or zygote. Plants like Riccio- carpus and the man- drake, which have a sporophyte plant alternating in the life history with the gametophyte plant, are said by botanists to have an alternation of generations in the life cycle. FIG. 162. Habit of a moss, showing the leafy game- tophytes and sporophytes. After Sachs BRYOPHYTES 293 MUSCI (MOSSES) Habit. The mosses are leafy-stemmed plants and are hence much more highly organized than the liverworts, which are their nearest relatives. The stem of such a moss as Funaria is a delicate structure with the leaves arranged spirally upon it. The leaves are also very simple, being composed of a single layer of chlorophyll-bearing cells, except in the central conduct- ing strand, corresponding to a midrib, where the cells are elongated and are two or more layers thick. The plants are anchored in the soil by delicate rhizoids, like those of liverworts, which serve a similar function. Funaria, like most mosses and liverworts, grows in clusters, a habit which is an advantage in conserving moisture and in insuring fertilization, since the male and female organs are borne on separate plants and the clustering habit is necessary in order to insure the proximity of male and female organs (Fig. 163). FUNARIA Gametophyte. The leafy moss plant just described is the game- tophyte corresponding to the 'flat, leaflike plant body of Riccio- carpus and the other liverworts. In Funaria the reproductive organs (archegonia and antheridia) are borne in terminal repro- ductive buds. The antheridia are borne in the form of open disks (Fig. 163, , a. GAMETOPHYTES, EMBRYO, AND SEED The microspores and megaspores both form gametophytes within the spores by germination, as in Selaginella, and the female gametophyte and archegonia are permanently retained within the sporangium. The megasporangium is also furnished with an integument, which marks a new departure in the plants studied thus far in our consideration of the great plant groups. GYMNOSPERMS 323 The female gametophyte (Fig. 187, B) is forme.d within the megaspore, which enlarges as the gametophyte grows, until it comes to occupy most of the space within the megaspore wall, only a remnant of the sporangial tissue being left at the micro- pylar end of the sporangium. At the micropylar end of the game- tophyte from three to five archegonia are formed. The male gametophyte at first consists of a single gametophyte cell and of Miorosporcmffia Microsporangvutm -Microspore 'ametophyte Generative cell Tube nucleus Gametophyte Stalk cell. Sperm mother cells Pollen tube FIG. 187. Habit and reproductive structures of a Florida cycad, Zamia A, plant of Zamia bearing a female strobilus; 13, a, megasporophyll, sporangia, ard spores; &, megasporangium (ovule) with gametophyte, archegonia, pollen grains, and pollen tubes ; (7, a, microsporophyll and sporangia ; d, e, two stages in the germination of the microspore and in the production of the male gametophyte Fleshy layer -^Integument Pollen chamber Micropyle an antheridial cell, called the generative cell (Fig. 187, (7, d). The generative cell then divides and there are ultimately formed a stalk cell and two sperm mother cells, each of which then de- velops a motile sperm (Fig. 187, (7, , /). Pollination takes place when the sporophylls of the female strobili separate and the microspores are borne to them by air currents. The microspores are drawn into the micropyle by a secretion which contracts with drying and carries the spores into a chamber (called the pollen chamber) developed in the sporangial tissue at the base of the micropyle (Fig. 187, B). The pollen tubes developed by the 324 GENERAL BOTANY microspores bore into this sporangial tissue and absorb nourish- ment for the developing gametes. When the eggs are ripe, the sporangial tissue between the pollen chamber and the female gametophyte breaks down and the motile male gametes are cast out into a depression in the gametophyte, called the archegonial chamber, into which the necks of the archegonia open. Fer- tilization takes place when a motile male gamete bores through the neck of the archegonium and fertilizes the egg. Embryo and seed. The embryo spo- rophyte bores its way into the gameto- phyte tissue, much as in Selaginella, by means of a suspensor. The mature embryo in the seed consists of two cotyledons, a hypocotyl, and a plumule (Fig. 188). The seed is therefore much like that of the higher plants studied in Part I and consists of the seed coat, or integument, a mere remnant of the megasporan- gium, the gametophyte, and the embryo sporophyte. Zamia Micrvpyle FIG. 188. Seed of Zamia with embryo Sporophyte plant Gametophyte organs Gametes Gametophyte Sporophyte FIG. 189. Life history of Zamia represented graphically is thus the first plant among the plant groups now under con- sideration which forms a true seed. Germination of the seed results in a seedling sporophyte which develops into the mature Zamia plant, similar in habit to Diom (Fig. 186). GYMNOSPEEMS 325 Life history. It will be seen from the above brief account of the cycad that it has the same general stages in its life history as Selaginella (Fig. 185). The new features are concerned prin- cipally with the formation of a pollen tube, the changed rela- tions of the megasporangium and megaspore, and the formation of seeds. The retention of the megaspore in the sporangium, which remains longer on the mother plant than in Selaginella, is accompanied in the cycad by pollination and the formation of a pollen tube to serve as an anchoring and absorbing structure during the development of the motile male gametes. The per- manent retention of the megaspore has also resulted in the formation of a true seed composed of the megasporangium, the garnet ophyte, the megaspore, and the embryo sporophyte. When the seed germinates, the sporophyte resumes its growth and gives rise to a new adult cycad plant, or sporophyte. CONIFERALES THE SPRUCE (PICEA) SPOROPHYTE Habitat and habit. The spruce tree, which is the spore-bearing plant, or sporophyte, has the same general form and mode of growth as the pine tree described earlier in the text. Like the pine the spruce is an erect tree type with an excurrent trunk and pyramidal crown, which results from its mode of growth and the spiral arrangement of its branches. The spruce differs from the pine in that its needlelike leaves are borne directly and singly on the main shoot instead of in pairs or clusters on the end of minute dwarf shoots. Like the pines and their allies the spruces also inhabit mainly northern or mountainous regions and are typically xerophytic in habit and structure, although they adapt themselves readily to cultivation and to mesophytic conditions. In stem structure the spruces are intermediate between the pteridophytes and the woody-stemmed flowering plants, as the following account of the structure of the spruce will indicate. 326 GENERAL BOTANY Structure. The broad outlines of structure in the spruce stem are identical with those of the woody stems of the alder, ash, and other trees and shrubs belonging to the flowering plants already studied (Figs. 191 and 192). The bark is com- posed of thick layers of cork, which scale off in flakes on the main trunk and the older branches of the tree. This corky bark, like that of other trees, is formed by a special cork cambium which arises in a layer of cortical cells beneath the epidermis dur- ing the first year's growth of the main stem or its branches. The epidermis is soon cut off from its water supply by this new growth of a corky bark, and is then sloughed off. The cortex forms a wide zone of storage tissue in the young stem, but is destroyed later by being crushed between the corky bark and the expanding central cylinder of phloem and xylem. A distinctive feature of the cortex in the spruces and pines is the formation of the large resin canals, which contain the resin common in coniferous trees. The central cylinder presents the same general features as that of other trees. The cambium is flanked on either side by the phloem and xylem rings, and the annual wood rings are also sharply marked by the difference in structure of the spring and summer wood. FIG. 190. Virgin forest of red spruce (Picea rubra) in the Adirondack Mountains Photograph furnished by the United States Divi- sion of Forestry GYMNOSPERMS 327 Cortex Phloem Xylem Leaf gap Leaf base FIG. 191. Cross section of spruce stem two years old The xylem is distinctive in Picea ancLthe other Conifer- ales (including the common pines, hemlocks, and cedars) by being made up almost exclusively of single-celled water-conducting elements called tracheids (Fig. 193). These single-celled trache- ids resemble those of the ferns and do the work of the long vessels, or ducts, in woody and herbaceous stems of the flowering plants. The tracheids of Coniferales are furnished with peculiar bordered pits representing thin places in the cell wall, which later become partially roofed over by the extension of the adjacent thicker portions of the tracheid wall. The wood rays have also certain dis- tinctive features in the spruce and its allies, but their general struc- ture and function is the same as that of the rays of the higher flow- ering plants. The pith is small and is more or less irregular in outline on account of the leaf gaps, which, as in the ferns, cause breaks, or gaps, in the otherwise solid vascular cylinder FlG 192 Crogs gection of gpmce twig of the first year The radiate form of the pith is .due to the numerous leaf gaps at the bases of leaf traces, or vascular bundles, going out to the leaves. The irregularities --Leaf base of a young stem. The general relation of the leaf traces and leaf gaps to the vascular in the outer cortex are due to the effect of leaf bases 328 GENERAL BOTANY cylinder in a two-year-old stem of a spruce are diagrammati- cally shown in Fig. 194. In the lower part of the figure the cortex and one annual ring of wood are represented as having been removed so as to expose the outer surface of the wood of the first year. On this exposed surface the leaf gaps appear as in the vascular cylinder of the maidenhair fern (Fig. 170). In the upper portion of Fig. 194 the pith is removed and the long leaf gaps are visible on the inside of the wood cylinder. In the Tracheids . Wood ray a b '"* parenchyma; FIG. 193. Structure of the wood of the spruce (Picea) a, transverse section ; 6, long section. Copied from Jeffrey's " Anatomy of Woody Plants " partial cross-section view at the junction of the upper and lower portions of the figure the pith is seen to continue into the leaf gaps and in a living stem would be continuous with the cortex through the gap during the first year's growth. This struc- tural feature is also shown in the first annual ring of wood as it appears in actual sections of one- and two-year-old stems of the spruce (Figs. 191 and 192). The young wood cylinder of the spruce during the first year is therefore similar to that of a fern like the maidenhair in having leaf gaps where portions of the wood and phloem cylinder pass out to form a leaf trace. This persistence of a fern characteristic in the stem structure of GYMNOSPERMS 329 a seed plant indicates, according to the teachings of modern anatomy, that the spruces have been derived from plants with a fern ancestry. In older spruce stems the leaf gaps are covered over by the later-formed annual rings of wood, but they are still evident as radial projections of the pith. The leaf traces are, however, persistent throughout the life of the evergreen leaves and may often be Leaf petiole seen to connect with rr. -Leaf trace the leaf gaps, as in Fig. 194. These leaf traces serve to connect the phloem and xylem of the vascular cylin- der of the branches with the green tissues of the needle leaves. The summary on the following page gives the important points of similarity between the anatomy of the stem in the maidenhair fern and in the spruce, and also the general advances in structure made by the Oonifer- bium Leaf gaps FIG. 194. Gross anatomy of the stem of a spruce branch two years old The surface of the wood cylinder is exposed in the lower half of the figure by the removal of the cortex. The inner portion of the wood cylinder is shown above by the removal of the pith. Compare with the similar ales as Compared with fi S ure of the fern rhizome (Fig. 170) . Note,the breaks, or leaf gaps, in the wood cylinder, as in the fern the ptendophytes. The leaves of the spruce are strictly xerophytic in structure, as is shown by their small size and by the thick-walled outer layers of cells, which include both the epidermis and one or more layers of cells beneath it. Under this hard outer cov- ering of cells the green mesophyll forms a wide, cortexlike layer containing chloroplastids. The central cylinder of the leaf is occupied at the base by two bundles which join into one in its upper portion. This vascular system of the leaf, as already indicated, is a continuation of the leaf trace connecting 330 GENERAL BOTANY the living mesophyll cells of the leaf with the water-conducting tissue of the main stem and its lateral branches. The root of the spruce and its relatives does not present any new features that need be discussed in an elementary textbook. SUMMARY 1. Two cambium layers are developed in the spruce, which enable it to increase its stem in thickness and to form a protective outer cork jacket which insures against too rapid changes in temperature, loss of water, and the attacks of insects and fungi. In ferns this cork jacket is unnecessary, since the stem is usually underground and the outer skeletal layer, once formed, is in no danger of being destroyed by the annual growth of the stem in diameter. 2. The growth of the cambium forms a wide wood and phloem cylinder for conducting the larger quantities of foods and water made necessary by the greatly increased leaf exposure of the spruce trees and their allies. 3. Food storage is provided for in the wood rays of the central cylinder instead of in the pith and cortex, as in the ferns. This provision was made necessary in the higher plants when the wide pith and cortex of the ferns was gradually eliminated as a result of the secondary production of wood and phloem by the cambium. 4. The leaf traces and gaps are present in the spruce, but they become buried by the secondary products of the cambium. Their presence in the spruce stem is an indication of the relationship of two groups of plants which in other respects are widely separated. Asexual reproduction. The spruces are monoecious, bearing both staminate and ovulate strobili on the same tree. Each stro- bilus is a modified shoot, like the strobili of the lycopods and cycads, with a central axis and lateral sporophylls arranged in a spiral form. The staminate strobili terminate lateral shoots at the ends of the main branches (Fig. 195, a), where they live through the winter in the bud stage and first make their appear- ance, in temperate climates, early in May. The microsporophylls are scalelike, and each microsporophyll bears two microspo- rangia on its lower abaxial surface (Fig. 197, e,/). A micro- sporophyll with its two sporangia is commonly called a stamen, GYMNOSPEEMS 331 as in the mandrake, although it resembles the sporophylls and sporangia of the lycopods and cycads quite as closely as it does the stamens of ordinary flowering plants. Each microsporangium produces a large number of microspores by tetrad division of microspore mother cells, exactly as in the ferns, Selaginella, and cycads, so that the spore-forming processes in the spruce micro- sporangia are identical with those of the sporangia of the lower vascular plants already studied* The microspores of the spruce are therefore true spores, exactly comparable to the microspores FIG. 195. Spruce twigs with staminate and ovulate strobili in May a s male strobili ; b, female strobili. Note the erect position of the female cones ready to receive pollen of Selaginella. Each microspore, or pollen grain, when mature, is furnished with two expanded sacs, or wings (Fig. 198, c), formed by the inflation of the outer coat of the microspore. When the microspores are ripe, the microsporangia split down the center of each sporangium, or anther sac, and the light-winged spores are widely scattered, thus effecting pollination. The ovulate strobili grow at the ends of last year's twigs, where they remain in the bud stage, like the staminate strobili, through the first winter. They make their appearance, in tem- perate regions, from the first to the fifteenth of May, occurring as beautiful red erect strobili (Fig. 195, 5). They retain this erect position for two or three weeks, until pollination is effected 332 GENEBAL BOTANY by the microspores' falling into the space between the open megasporophylls and the axis of the strobilus. After pollination the megasporophylls close by excessive growth on the abaxial surfaces, and the cones gradually change their position, owing to carpotropic movements, finally assuming the pendulous posi- tion shown in Fig. 196. These ovulate strobili of the spruce are more complex in structure than the staminate strobili, since each ovulate strobilus bears on its axis two kinds of scales, or modified leaf structures, instead of one, as in the staminate strobili. The large scales which con- stitute the conspicuous part of the mature cone, or strobilus, are the ovuliferous scales, each of which bears two megasporangia, or ovules, at its base. These large ovuliferous scales really arise as adaxial outgrowths from very small scales which are only evident in the early stages of the strobilus, before the ovuliferous scales have outstripped them in growth (Fig. 197, c). The large ovuliferous FIG. 196. Spruce cones in June after pollination These cones were photographed scales probably represent two sporo- about a month later than those phylls of a reproductive branch, which represented in Fig. 195. Note J . r . the change in size and position of grew in the axils of leaves correspond- the cones at the time of poiiina- tion and during seed formation m g to t h e minute scales of the young cones. For our purposes we may prop- erly term the ovuliferous scales megasporophylls, and consider the ovulate strobilus a compound strobilus with both bracts and sporophylls, instead of a simple strobilus, like that of the staminate cones. The megasporangia, or ovules, of the spruce are similar to those of the cycad, with a single integument surrounding the sporangium tissue proper. Each megasporangium produces a single large megaspore (Fig. 197, 6), which finally occupies a considerable portion of the sporangium. The history of its GYMNOSPEEMS 333 development shows that each megaspore is produced by a single mother cell, which lies deeply buried in the tissue of the young sporangium. This mother cell divides by tetrad division, and one of the cells of the tetrad forms the single successful megaspore. The ovules of the spruce, like those of the cycad, are therefore true megasporangia, in which a single megaspore, produced by the usual processes of spore formation, is formed and permanently tegasporophylli Microsporophyll Megasporangi Strobilus Microspores FIG. 197. Megasporangia and microsporangia with spores a, megasporophyll (adaxial view) with two ovules (megasporangia); 6, mega- sporangium in median section, showing the single megaspore ; c, portion of a stro- bilus in section, showing megasporophylls, bracts, megasporangia, and spores; d, male strobilus ; e, microsporophylls and sporangia ; /, microsporophylls, spores, and sporangia (sectional view) retained within the sporangium, instead of being shed, as in Selag- inella. As the megaspore enlarges, .it germinates and produces a true cellular gametophyte (Fig. 198, a). After fertilization the megasporangium becomes the seed, furnished with a hard seed coat, or integument, and an embryo spore plant produced by the fertilized egg. GAMETOPHYTES AND EMBRYO The male gametophyte in the spruce is similar to that of the cycads and is formed within the microspore (Fig. 198, d) as a result of germination. It consists at first of two cells (the 334 GENERAL BOTANY gametophyte proper) and of an antheridial cell called the gener- ative cell. The generative cell then divides to form two cells, a stalk cell and a body cell. When the pollen tube forms, the stalk cell disorganizes and frees the body cell, which then divides in the tube to form two male cells (Fig. 198, e). These nonmotile male cells correspond to the motile sperms of the cycads and ferns. The female gametophyte is formed within the megaspore by the process of germination, resulting in a- -cellular gametophyte (Fig. 198, a). From three to five archegania are formed on this Tube nucleus -Stalk cell nucleus \;rMale cells ^? Pollen tube-'' -Pollen grain if Micropyle Gametophyte FIG. 198. Gametophytes and fertilization in the spruce o, ovule at the time of fertilization ; b, archegonium with a fertilized egg and male (cf) and female (9) pronuclei ; c, d, microspores before and after germination to form the gametophyte ; e, pollen tube and male cells, or sperms gametophyte at its micropylar end, each archegonium (6) being composed of a large egg cell, a layer of cells called the jacket cells, and the neck cells. Pollination is effected by means of the wind when the young female cones are erect on the ends of the branches. The cone scales are then^open (Fig. 195, 6), and the pollen sifts down "between them and comes to rest in con- tact with the micropyles of the ovules. A sticky secretion is * exuded by the micropyle, as in the cycads, which draws the microspore into the micropyle until it rests on the surface of the megasporangium. No distinct pollen chamber is formed in the spruce, like that in the-cycad megasporangium. The pollen tube begins to grow down into the megasporangium early in May, soon after pollination, and reaches the archegonia late GYMNOSPERMS 335 in June (Fig. 198, a). Just before fertilization the end of the pollen tube penetrates the neck of an archegonium and then ruptures, liberating the male cells in contact with the egg. The union of one of these male cells with the egg cell com- pletes ttye process of fertilization and initiates the formation of the embryo. Embryo. As soon as fertilization has taken place, the con- jugate nucleus, formed by the union of the male and female i Cotyledons Hypocotyl a Mlcropyk Micropyle^ FIG; 199. The ovule, seed, and seedling of the spruce a, ovule at the time of fertilization'; b, two embryos developing as a result of fertili- zation ; c, seed developed from a with only one'embryo ; d, young seedling developed from a seed by germination and growth pronuclei, divides to form eight nuclei, which then pass to the bottom of the egg. Around these nuclei eight cells are ulti- mately formed, which constitute the beginning of the pro- embryo. This proembryx) soon differentiates into a suspensor, composed of four greatly elongated cells, and the embryonic cells which are to form the embryo proper (Fig. 199, J). The embryonic cells finally produce the embryo within the seed. This embryo is composed of the 'hypocotyl, or stem, the root, and numerous first leaves, or cotyledons, surrounding the ter- minal plumule, or bud. These structures are shown more plainly in Fig. 199, 6?, which represents a seedling sporophyte of the spruce produced by the germination of the seed. The seed 336 GENEBAL BOTANY is thus composed of the seed coat, or integument (which forms a part of the mother sporophyte plant), of the gametophyte, and of the young sporophyte, or embryo, which represents a new sporophyte generation (Fig. 199, c). Life history. The life history of the spruce is similar in all essential respects to that of the cycads, represented by Zamia. In both instances the megaspore is permanently retained in the megasporangium. In the spruce and its relatives the male gametes have lost their motile organs, and the pollen tube is consequently used to convey them to the eggs. Correlated with this change we find that the spruce has no archegonial chamber, since the pollen tubes enter the archegonial necks and intro- duce the male gametes directly to the eggs. The results of ferti- lization are the development of the embryo and the formation of a seed. In order to have these points of difference between the cycads and the spruce clearly in mind the student should construct a graphical history of the spruce similar to that of Zamia (Fig. 18.9> CHAPTER XVII ANGIOSPERMS DICOTYLEDONS SPOROPHYTES The sporophytes of the angiosperms include the common herbaceous and woody-stemmed plants, such as the mandrake, clovers, and elms, with which we became familiar in the first part of the text. On account of the large amount of time already devoted to the vegetative and reproductive structures of this important group of plants it will only be necessary at this point to review the knowledge already gained and to relate the life history of angiosperms to the higher spore and seed plants which we have recently considered. In this discussion the angiosperms with two cotyledons in the embryo, namely, the dicotyledons, have been chosen to represent the group, while the monocoty- ledons will be reserved for a separate and special treatment. Structure. In connection with the following brief summary of the important advances in anatomy made by the dicotyle- dons the student should consult the figures and review the text relating to the structure of woody and herbaceous stems in Part I, and also the structure of Adiantum and the spruce in Part II. The advances in structure relate mainly to the stem tissues, since the leaves of dicotyledons are not much more highly organized than those of ferns and cycads. The general arrangement of the stem tissues in the woody types of dicotyledons is very similar to that of the spruce, and the advances made by the spruce in this respect, as compared with the pteridophytes, apply to the trees and shrubs among dicotyledons. 337 338 GENEBAL BOTANY The herbaceous dicotyledons resemble the herbaceous ferns in having a wide storing cortex, a large pith, and a narrow vas- cular cylinder. They differ from the ferns and are like the woody dicotyledons in the collateral structure of the phloem and xylem and in the nature of the tissue elements, which are essentially the same in all dicotyledonous stems. The storage system of cells in the dicotyledons, particularly in trees and shrubs, is for the first time amply provided for by large and small wood rays and by wood parenchyma abundantly distributed throughout the primary and secondary wood. In living pteridophytes the wood rays are lacking, and gymnosperms have neither the rays nor the wood parenchyma so largely de- veloped as in the woody dicotyledons. (Compare the figures illustrating the anatomy of Adiantum, alder, and spruce.) This highly developed storage system of the woody dicotyledons com- pensates for the small size of the pith and cortex, which serve the storage function for a short time only in these plants, since the death of the cortex and pith in old dicotyledonous stems relegates the entire storage function to the wood rays and the parenchyma of the vascular cylinder. The conducting cells are the familiar ducts which constitute long tubes for the rapid transfer of water necessitated by the immense leafage of the broad-leaved dicotyledons. The average length of the tracheids which compose the water- conducting elements of gymnosperms is from two to four milli- meters, while that of the ducts in dicotyledons ranges from a few centimeters to several feet in length. The ducts, therefore, offer much less resistance to the rapid flow of water up the tree trunk in a dicotyledon than the tracheids do in a spruce or other gymnosperm (compare Figs. 53 and 193). The great differentiation in kind and arrangement of tissues in the stems of dicotyledons is also a distinctive feature in these plants, since ducts of various kinds and sizes, strengthening fibers and tracheids, wood rays and storage parenchyma, are all adapted, in them as in no other plants, to the proper performance of their respective functions. This elaborate differentiation of tissues culminates in the woody-stemmed trees and shrubs. AKGIOSPERMS 339 The flower. The flower in the angiosperms presents some new and distinctive features which are common to both dicotyledons and monocotyledons. The important advances made by the flower, as compared with the strobilus of plants below the angio- sperms, relate to the development of a showy perianth and of a closed megasporophyll, or pistil, and to certain modifications in the relation and number of sporophylls borne on the floral axis, or receptacle. These new features can be most easily presented FIG. 200. Diagram designed to illustrate the corresponding parts of the spruce strobili and the flower of the marsh marigold (Caltha palustris) a, flower of the marigold; 6, section of the flower; c, median long section of the staminate strobilus of the spruce ; d, similar section of the ovulate strobilus by instituting a comparison between a simple flower like that of the marsh marigold (Caliha palustris) and the strobili of a gymnosperm like the spruce (Fig. 200). In the marigold flower the perianth and the numerous stamens and pistils are arranged in a spiral form on a dome-shaped receptacle like the sporophylls on the axis of a spruce cone. Such flowers with spirally arranged parts evidently correspond more nearly to the strobilus of the plants below them than the cyclic flowers of the mandrake and locust, in which the separate sets oL floral organs are arranged in cycles on a flattened receptacle. If a median longitudinal section of a marigold flower is compared with sim- ilar sections of the male and female strobili of the spruce, the 340 GENERAL BOTANY corresponding parts of the flower of the angiosperms and the strobili of the gymnosperms are at once made apparent. The receptacle of the flower evidently corresponds to the axis of a strobilus, although it is greatly shortened and somewhat flattened at its apex. The stamens correspond to the microsporo- phylls and microsporangia of the spruce strobilus, the filament representing a highly modified, slender microsporophyll, and the anther sacs representing microsporangia borne at the apex of the sporophyll. A single pistil of the marsh marigold flower corre- sponds to a single megasporophyll on the female strobilus of the spruce, in which the edges have folded in and united so as to inclose the megasporangia, or ovules, in a cavity called in the angiosperm flower the ovary cavity. The perianth is evidently a new structure which functions to protect the essential organs, the stamens and pistils, during their development. We have learned that in highly organized flowers like the locust and the bean the perianth may also serve an important function in securing cross-pollination by insects. The perianth in some flowers undoubtedly represents transformed sporophylls at the base of a strobiluslike flower, while in other instances it is apparently formed from ordinary green leaves below the sporophylls. We may conclude, therefore, that the angiosperm flower, represented by the flower of the marsh mari- gold, is a highly modified strobilus, in which many changes have taken place during its long course of evolution, including the shortening of the axis, or receptacle, and the transformation of simple sporophylls and sporangia into stamens and pistils and of certain sporophylls, or green leaves, into the parts of the perianth, namely, the calyx and corolla. The evolution of the sporophylls and sporangia of the angio- sperm flower will be more fully understood if a further com- parison is made between these structures and the corresponding structures in the other vascular plants already studied. In Fig. 201 the microsporophylls and sporangia of Selaginella^ spruce, and marsh marigold are compared with a portion of the sporo- phyll, or leaf, of Adiantum. From this figure it will be seen that the sporophyll has become gradually reduced in size from ANGIOSPEKMS 341 the fern to the angiosperm, until it has reached its limit in the slender filament of the angiosperm stamen. With this grad- ual reduction in size its original chlorophyll tissue has been lost, together with its power of making starch, so that the microsporo- phyll now serves a single function, namely, that of producing Microsporophylls Microsporophyl Micro- sporangici Micro - sporangia Megasporophylls _Mega- '/sporancjid Fig. 201. Diagram illustrating the homologous, or corresponding, parts of the sporophylls and sporangia of the higher spore and seed plants , pinnule (sporophyll) and sporangium of the maidenhair fern ; 6, microsporophyll and microsporangium of Selaginella ; c, d, corresponding parts of the anthers of the spruce and of the marsh marigold ; e, pinnule (sporophyll) and sporangium of the maidenhair fern; /, megasporophyll and megasporangium of Selaginella; g, ovu- lif erous scale and ovules of the spruce ; h , ovary and ovules of the marsh marigold microspores, instead of the double function of spore production and photosynthesis, as in the ferns. The sporangia have also become transformed from the simple, distinct sporangia of the ferns .into the four united microsporangia of the angiosperms, borne .on a single sporophyll. The pistil, which may be composed of a single megasporophyll, as in the, marsh marigold, or of several megasporophylls, as in the lily or the apple, represents a still greater transformation in sporophyll 342 GENERAL BOTANY structure than that outlined above for the microsporophyll. A single pistil of the marigold evidently corresponds to one mega- sporophyll of the spruce, with its edges turned in and united to form the ovary cavity. At the point of union of the edges of the sporophyll, which form the placenta, the megasporangia, or ovules, bud out and develop the integuments and sporangium proper, which are characteristic of the megasporangia of the seed plants. At the apex of the megasporophyll the stigma is developed, which in the marigold, as in many other flowers, is furnished with hair- like outgrowths for the retention of pollen. This highly modified megasporophyll, or pistil, is the most universally characteristic and important feature of the angiosperm flower. Such a pistil as that of the marigold, which is composed of a single mega- sporophyll, is called a simple pistil, to distinguish it from the com- pound pistils like that of the tulip or the apple, in which more than one megasporophyll enters into the composition of the ovary. The compound pistil is therefore a union of several simple pistils into one structure. It is an interesting fact that the development of a pistil in the young flower of a marigold or a buttercup corroborates the above interpretation of the probable origin of the closed angio- sperm pistil from an open megasporophyll similar to that of gymnosperms. The young sporophyll which buds out on the receptacle of a developing flower of a marigold or a buttercup is at first an open sporophyll resembling a rudimentary spruce sporophyll but with a concavity toward the axis of the flower. The edges of this concave sporophyll gradually approach each other by growth and finally unite to form the ovary cavity. Meanwhile the megasporangia, or ovules, bud out upon the uniting edges where the placenta is to be formed, and the stigma develops at the apex of the leaflike sporophyll. By further growth the mature closed pistil of the marigold is formed. Asexual reproduction. The anther of the angiosperms is usu- ally composed of four lobes, visible from the outside, which represent four microsporangia. These four microsporangia are shown in Fig. 202, A, as they appear in a transverse section of a mature anther of a lily. In a younger anther than that ANGIOSPEEMS 343 represented in the figure the pollen grains would be replaced by mother cells, which form the pollen, or microspores, by tetrad division, as in ferns, cycads, and spruces. The mother cell first FIG. 202. Anther and pollen formation in a lily A, mature anther with four microsporangia containing pollen grains ; B, the process of forming microspores by tetrad division ; (7, gametophyte (g) and tube nucleus (t) in a germinated microspore. From Bergen and Davis 's " Principles of Botany " divides into two cells (5), with a reduction in the number of chromosomes (consult Fig. 43, 5). These two cells then divide again and form the four cells of the tetrad. Each cell of the 344 GENERAL BOTANY tetrad then develops into a microspore, or pollen grain. When the pollen grains are ready to be shed, the cellular partition separating the two microsporangia on each side of the anther breaks down, and the two anther sacs are thus formed. The anther wall then ruptures along the line of dehiscence and sheds the microspores (consult Fig. 202, A, x). The young megaxporangia, or ovules, arise in the shepherd's purse (Capsella) from two placentae, formed at the junction of the two sporophylls in the ovary. Each megasporangium, when young (Fig. 203, A), consists of a sporangium proper, called the nucellus, and of two cellular outgrowths at the base of the sporangium, which are the beginnings of the outer integument and the inner integument. The funiculus is not perfectly developed in the young ovule, but as the megasporangium increases in size it grows more rapidly on one side than on the other, which gives it a curved form (Fig. 203, j5). Coincident with these changes in form a single cell of the sporangium enlarges and becomes the mother cell of the future megaspore. In Capsella this cell divides into a row of three cells which are potential megaspores, and the lowest of the three then enlarges ind forms a large megaspore, such as is shown in B, es. In many angiosperms four cells arise from the megaspore mother cell instead of three, as in Capsella, which indicates that these cells constitute a spore tetrad. This process relates the formation of the megaspore in the angiosperms to the usual process of sporogenesis as it occurs in Selaginella and in the microspores of angiosperms. The megaspore, when it has reached the size shown in Fig. 203, B, germinates at once and forms a female gametophyte (<7) exactly like that already described in the mandrake. This germinated megaspore is called the embryo sac. GAMETOPHYTES AND FERTILIZATION The female gametophyte of Capsella (Fig. 203, C) corresponds exactly to that of the mandrake (Fig. 86, 5) and the iris (Fig. 204). It consists of the egg cell, or female gamete, the synergidae (which are closely associated with the egg), the polar ANGIOSPEBMS c 345 FIG. 203. Development of the ovule and embryo of shepherd's purse (Capsella) A-C, stages in the development of the ovule ; n, nucellus of megasporangium ; ii and ol, inner and outer integuments ; m, micropyle ; es, embryo sac ; //, mature ovule (megasporangium) with emhryo sac, embryo (em), and endosperm nuclei (e); D~G, stages in the development of the embryo ; s, suspensor ; r, root ; c, cotyledons nuclei, and the antipodal cells. The student will note at once the great difference between this reduced gametophyte of the an- giosperms and that of the cycads and spruce. In the angiosperm the female gametophyte consists of six cells and two nuclei, 346 GENERAL BOTANY while in the gymnosperms (cycad, spruce, and pine) the gameto- phyte is a definite cellular structure which bears true archegonia. The male gametopliyte is also greatly reduced in Capsella, as in the mandrake (Fig. 85, 6) and the iris (Fig. 204, a). Pollina- tion and fertilization take place in the manner already described for the mandrake and the bean. Double fertilization probably occurs in Capsella, although this has not been definitely investi- gated. In this process, as shown in Fig. 204, 6, the nucleus of one of the male cells unites with the egg, while that of the other combines with the polar nuclei to form the endosperm nucleus. The fertilized egg cell develops into the embryo, while the endo- sperm nucleus initiates the formation of the endosperm. THE EMBRYO, ENDOSPERM, AND SEED The embryo. After fertilization the egg secretes a cellulose wall and then divides by repeated mitoses, forming an elon- gated, rodlike proembryo (Fig. 203, D). The upper cell of this proembryo forms the embryo proper by cell division and differ- entiation, while the remaining cells form the suspensor (E, F) which supplies the embryo with nutriment and anchors it within the megaspore, or embryo sac. The two cotyledons bud out from the upper half of the embryo, while the root grows from the lower portion (6r). In If the proembryo is shown in posi- tion in the embryo sac, with the lower cell of the suspensor occupying an enlarged portion of the sac next to the micropyle. The endosperm. In Capsella, as in other angiosperms, the endosperm begins to form by repeated divisions of the endosperm nucleus, formed, as indicated above, by the union of one male nucleus and the two polar nuclei. The numerous endosperm nuclei thus formed accumulate in the peripheral layer of cytoplasm which surrounds the large central vacuole of the embryo sac, where they form a layer of free nuclei (Fig. 203, H, e). These free nuclei never form a per- manent cellular endosperm in Capsella, since they are gradually absorbed by the growing embryo, which in a later stage of its development fills the embryo sac. ANGIOSPERMS 347 The seed. In the ripened seed of Capsella the embryo fills the entire cavity of the embryo sac ; the food reserve necessary for its growth during seed germination is stored in the cotyledons, as in the pea and bean. The embryo, as in the latter seeds, has two cotyledons, a plumule (or first terminal bud), a hypocotyl, and the root meristem at the tip of the hypocotyl. The seed coats are formed of cells with greatly thickened walls, which ale cell fc- ,( sperm) Tube nucleus Pollen tube''' FIG. 204. Ovule, pollen tube, and fertilization in Iris and a lily a, ovule of Iris with embryo sac and female gametopliyte at the time of fertiliza- tion ; 6, double fertilization in a lily ; c, pollen tube of Iris, b, after Gu'inguard ; a and c, from original drawings by M. Louise Sawyer effectually protect the embryo during its period of rest. The germination of the seed and the adjustments of the seedling to the environment are essentially the same as in the bean. Life history. It is evident from the above discussion that the angiosperms have the same general stages in their life history as the spruce. The new features relate to details of structure and reproduction already discussed and hence need only a brief treatment in the form of a summary at this point. The flower of the angiosperms is a Highly modified strobilus in which the microsporophylls have been transformed into stamens and the megasporophylls into one or many pistils. The closed pistil, with the stigma differentiated for the reception of pollen, is the most distinctive feature of the angiosperms, although the perianth, when developed, is an important characteristic. The 348 GENERAL BOTANY perianth is usually composed of modified sporophylls, but in some flowers the leaves immediately below the sporophylls have been transformed into sepals and petals. Pollination devices are more highly developed in the angio- sperms than in any of the gymnosperms, and the pollen tube traverses the tissues of the style, stigma, and ovary cavity before coming in contact with the micropyle. The development of a closed pistil with a receptive stigmatic surface has therefore greatly modified both pollination and the growth of the pollen tube. The gametophytes are greatly reduced in size and in cellular differentiation. The male gametophyte is represented by the single generative cell, which gives rise to the two male cells Selaginella Spruce Mandrake Nuclei r *~ Cytoplasm 'I Gametophyte,^ FIG. 205. Diagram illustrating the homologous, or corresponding, structures of microspores and male gametophytes in Selaginella, spruce, and mandrake a-c, microspores ; d-f, germinated microspores and gametophytes within the germinated pollen grain, or microspore. The female gametophyte is reduced to the egg apparatus, polar nuclei, and antipodals in the embryo sac. The embryo develops from the fertilized egg cell within the embryo sac and passes into a rest- ing stage within the seed. The full development of the sporo- phyte begins with seed germination, when the embryo resumes its growth, fed by the endosperm stored up around the embryo or within its cotyledons. SUMMARY AND COMPARISONS A brief summary and comparison of the heterosporous plants from Selaginella to the angiosperm will suffice to indicate the strik- ing advances made by the highest spore-bearing and seed plants in ANGIOSPEEMS 349 the evolution of the seed and the seed habit. The more important of these changes are represented graphically in Figs. 205 and 206. In these figures it may be seen that the microspores have remained quite similar in form, size, and structure, since their function has not changed. In the angiosperm, as in Selaginella, the microspores still serve the function of bearing the male gametes to the female gametes, and are hence small, light, and highly protected cells, pro- duced in great numbers by the microsporangia to guard against waste in distribution oelagtnelut spruce by wind or insects. The male gameto- phyte has been grad- ually reduced, as was explained in a previous paragraph, until it is represented in the an- giosperm by a single generative cell. The megaspores (Fig. 206) have become reduced to a single megaspore in each megasporan- gium, both in the gym- nosperms, represented by the spruce, and in the angiosperms, rep- FIG. 206. Diagram showing the homologous, or resented by the man- corresponding, parts of megaspores, megasporan- g^metophytes, and sporophytes of Selaginella, spruce, and mandrake a-c, megaspores; d-f, gametophytes ; g-i, gameto- ph ytes and sporophytes. A and i are seeds drake. The important advance made by the -, , P-, latter groups of plants over Selaginella and similar heterosporous Pteridophyta is that of retaining the mega- spore permanently in the sporangium. The megatporanpium thus becomes indehiscent and is shed from the mother plant and dis- seminated with the contained megaspore and embryo. The female gametophytes remain cellular structures produced by the germina- tion of the megaspore in both Selaginella and the spruce. In Selagi- nella, however, the gametophyte and archegonia are still exposed by the opening of the megasporangium and spore. In the indehis- cent megasporangium of the spruce the cellular gametophyte is retained permanentlv in the spore within the megasporangium, but 350 GENERAL BOTANY it still bears definite archegonia with neck cells and a ventral-canal cell nucleus inherited from its fernlike ancestors of the coal period. In the angiosperm no cellular gametophyte is formed before fer- tilization, and the gametophyte is reduced to the egg apparatus, polar nuclei, and antipodals. Pollination and fertilization methods are also greatly modified, in both the higher gymnosperms and angiosperms, by the permanent inclosure of the gametophyte in the megasporangium. In Selaginella the motile sperms reach the eggs in the exposed archegonia by swimming in films of water. In cycads the sperms are still motile, with cilia, but they are liberated in a special chamber, called the archegonial chamber, into which the mouths of the archegonia open. In the higher gymnosperms and angiosperms, however, fertilization depends upon the growth of the pollen tube through the micropyle down to the egg ; by this growth a canal is formed, down which the nonmotile male gametes reach and fertilize the eggs. This is true siphonogamy, or the fertilization of the egg through the intermediary of a pollen tube. In the angio- sperms the development of a pistil, with stigma and style, renders pollination and fertilization even more difficult and is correlated with the elaborate devices for securing pollination observed in many angiosperm flowers. The seed is a complicated structure composed of the megasporan- gium of the mother plant and the embryo. In the angiosperms the true gametophyte of the gymnosperms is replaced in many species by the endosperm, which, as we have learned, is a nutritive tissue formed as a result of fertilization. In 'other respects the seeds of angiosperms and gymnosperms are quite similar in structure. The most striking advances and changes leading to the evolution of seeds are, therefore, the reduction of both male^and female gameto- phytes, the reduction in the number of megaspores produced by the megasporangia, the retention of the megaspores and female gameto- phy tes permanently within the sporangia, and the changes in methods of pollination and fertilization correlated with this retention. PART III. REPRESENTATIVE FAMILIES AND SPECIES OF THE SPRING FLORA CHAPTER XVIII DESCRIPTIVE TERMS For the study of plants in the field the student will need cer- tain descriptive terms which have not been given in the preceding pages. In the following brief discussion, therefore, we shall define the more important descriptive terms which the student will need in studying the trees, shrubs, and herbaceous plants of the spring flora. VEGETATIVE PARTS OF PLANTS Habitat. The term habitat is used to indicate the nature of the environment in which individual plants or plant groups live. The most common classification of habitats is that already described, based upon the conditions of moisture, soil, and light which constitute the environment. Habitats may therefore be designated as metophytie, xerophytic, hydrophytic, and tropophytic, according as the plants inhabiting these areas are mesophytes, xerophytes, hydrophytes, or tropophytes in habit. Habit. The term habit includes the form and general appear- ance of plants, based upon stem, branch, and leaf characters. Thus, trees like the pines and spruces, with a main excurrent trunk, are said to be erect in habit as compared with trees like the elm, apple, and oak, in which the equal growth of several branches produces a spreading habit. Plants are also said to be caulescent when possessed of a definite aerial stem, to distinguish them from plants like dandelions and strawberries, which are designated as acaulescent, or without a visible aerial stem. Stems. Stems are either aerial, growing aboveground, or sub- terranean, growing largely or wholly underground. The main types of underground stems may be defined as follows : Rhizomes, like those of ferns or Solomon's seal (Fig. 207), are horizontal underground stems furnished with buds and scalelike 353 354 GENERAL BOTANY leaves which serve both for storage and for the support of aerial parts growing from them. On account of their underground habit they often become highly modified in both form and structure. Runners and stolons are horizontal stems much like rhizomes except that they run over the surface of the ground, in which they frequently take root and give rise to new plants, as in the strawberry (Fig. 76). Prostrate stems are like runners except that they rarely take root in the soil over which they trail. Bulbs like the onion and tulip (Fig. 79) are short, erect stems which bear scalelike leaves filled with reserve foods sur- rounding a terminal bud. Corms are short, erect, FIG. 207. Rootstock of Solomon's seal . ,. OAN fleshy stems (Fig. 80) &, &, buds; r, roots; s, flowering stem . , . . , with inconspicuous scale leaves. They usually have a prominent terminal bud and less conspicuous lateral buds in the axils of the scale leaves. Tubers, like the potato (Fig. 78), are essentially greatly shortened rhizomes with scale leaves and lateral buds. They are usually filled with stored reserve food. Leaves. Fig. 208 illustrates certain characters of leaves, per- taining to their form, margin, leaf tips, and venation, which are of importance in characterizing and identifying plants. The figures are self-explanatory, since the proper terms descriptive of the leaves are used in connection with the figures. The terms pinnate, palmate, and parallel, used in connection with the leaf shapes, indicate the type of venation characteristic of each leaf. Parallel venation is characteristic of such leaves as those of the linear type, found in grasses and members of the lily family, where the secondary veins run lengthwise of the leaf parallel to the midvein. Pinnate and palmate venation are found in the broad-leaved herbs and trees which belong to the dicotyledons. FIG. 208. Diagrams illustrating differences in the form, venation, margin, and apex of leaves a-k, form and venation of leaves: a, lanceolate pinnate; 6, ovate pinnate; c, heart- shaped palmate ; d, halberd-shaped palmate ; e, linear parallel ; /, oblong pinnate ; g, oval pinnate; h, orbicular pinnate; i, oblanceolate pinnate; .;, spatulate pinnate; k, obovate pinnate, l-r, different kinds of leaf margins : I, serrate ; m, double serrate ; n, dentate ; o, crenate ; p, undulate ; Stamen \- Pistil FIG. 215. Complete flower of the alpine azalea (Loiseleuria) A, exterior view ; B, sectional view. After H. Miiller 360 GENERAL BOTANY is roofed over by the sporophyll or sporophylls of the pistil proper. The remaining parts of the epigynous flower seem to arise from the upper margin of the ovary, surrounding the style and stigma. Flowers are also variously classified on the basis of the pres- ence or absence of certain floral parts and of the form of floral parts, and also on the basis of certain arrangements for insuring pollination by wind or insects. The following are the main classes which the student may be expected to meet in an elementary course. Complete flowers have all four sets of floral organs (calyx, corolla, stamens, and pistil) represented in one flower, while incomplete flowers have one or more sets lacking. Perfect flowers, like the mandrake and marigold, a willow (Salix alba) have both sets of essential a, staminate catkin; 6, pistillate catkin ; c,stam- Organs (stamens and pis- inate flower ; d, pistillate flower. From Bergen tQ s ^ present, while oper- and Cald well's " Practical Botany " *' i . j feet flowers lack one kind of essential organs and are thus either wholly staminate or wholly pistillate. Plants which bear imperfect flowers are said to be either monoecious or dioecious, according as they bear imper- fect flowers of both kinds on the same or on different plants. Thus, the willows (Fig. 216) are imperfect and dioecious, since they have only staminate flowers on one tree and only pistillate flowers on another tree of the same species. The oak, on the contrary, is imperfect and monoecious, with both staminate and pistillate flowers on the same tree (Fig. 238). Regular flowers have all of the parts of one set of organs alike in form, as in the azalea, while irregular flowers have the FIG. 216. Catkins and dioecious flowers of DESCRIPTIVE TEEMS 361 parts of one or more sets of organs irregular in form, as in the flowers of the pea, bean, and locust (Fig. 89). Floral plan. In a floral plan (Fig. 217>the parts of the flower are represented in transverse section as though reduced to a common plane. In dicotyledons the parts are usually in fours or fives, while in monocotyledons they are on the plan of three. It will be noticed also that the parts of each set of floral organs alternate with those adjacent to them, like the leaves on a leafy shoot. This alternate arrangement is another evidence that flowers are modified shoots. Pollination features. The special structural and physiological phenomena concerned with the pollination of flowers relate for b c FIG. 217. Floral diagrams a, lily family; b, heath family; c, madder family; d, composite family. The dot above b and d indicate the stem axis ; the sepals are represented with midribs ; the lighter stamens in b represent an alternate whorl of stamens. After Sachs the most part to devices for securing cross-pollination and close-pollination. The more important of these structural and physiological phenomena are the following: Anemophilous and entomophilous. The principal agents by which flowers are close-pollinated or cross-pollinated are the wind and insects. Flowers in which the pollen is carried to the stigma by the wind, as in corn, poplar, and oaks, are said to be ane- mophilous, or ^ wind-loving," while insect-pollinated flowers, like the locust and bean, are said to be entomophilous, or " insect- loving." Anemophilous flowers are usually characterized by in- conspicuous color, abundance of light pollen, and lack of odor. Dichogamy. In many perfect flowers the stamens and stigma mature at different dates in the same flower, a condition defined by the term dichogamy (Fig. 218). If the stamens ripen earlier than the stigmas in such flowers, the flowers are said to 362 GENERAL BOTANY be protandrom, while flowers in which the stigmas ripen before the anthers are said to be protogynous. It is evident that protan- drous and protogynous flowers are necessarily either close-pollinating or cross-pollinating. Homogamous flowers are flowers in which the sta- mens and pistils ripen together, thus making self-pollination possible. It is evident that where flowers are imperfect either close-pollination or cross-pollination is insured, since self-pollination would be impossi- ble in such flowers. Heterostylous flowers. Flowers in which the stamens and styles are of different lengths are said to be heterostylous. Heterostylous flowers may be either dimorphic (Fig. 219) (with two lengths of stamens and pistils) or trimorpJiic (with three lengths of stamens and pistils). In either 'case each set of stamens matches one length of pistils, so that insects carry pollen from the anthers of one flower to stigmas of the same height in other flowers on the same or on a different plant, thus effecting either a close-pollination or a cross-pollination. Odor, nectar, color, and movements. In flowers adapted to insect pollination, or entomophily, the insects are undoubtedly attracted to many flowers by their odor, nectar, or color, a fact which insures close-pollination and cross-pollination. The odor is due to the FIG. 218. Dichogamy in flowers of Clerodendron a, the pistil is hent to one side away from the ripe stamens in the young flower; 6, in older flowers the sta- mens wither and the stigmas are exposed for the reception of pollen a A B FIG. 219. Dimorphic stamens and pistils in bluets A, form with long style ; 13, form with short style ; a, anthers; s, stamens DESCKIPTIVE TEEMS 363 secretion of volatile oils by the petals or other floral parts, while the nectar is secreted by nectar glands, usually located at the base of the pistils on the receptacle. In addition many flowers possess the power of movement in the stamens and pistils by which the anthers and stigmas are either separated or approximated when ripe, thus insuring either close-pollination, cross-pollination, or self-pollination. Pistils, seeds, and fruits. Pistils are either simple or com- pound, according as they are composed of one or more spo- rophylls, or carpels. Simple pistils are composed of one carpel, 'Stigma" Carpel^ Placenta Ovary FIG. 220. Simple and compound pistils A, simple pistil with one carpel; B, compound pistil with two carpels and central placenta ; C, compound pistils (a, with parietal placentae ; b, c, with central placenta) or megasporophyll, as in the mandrake, bean, and locust (Fig. 220, A). Compound pistils are composed of two or more carpels, or sporophylls, so united as to inclose one or more seed cavities, or locules (B). The placentae, or lines of attachment of the ovules, may be either central or parietal (.C'). Ovules are of three main types, according to their form and the relation of the ovule proper to the funiculus. Orthotropous, or straight, ovules grow straight, without curvature, from the funiculus, or stalk. Campylotropous ovules are curved, owing to the greater growth of one side of the ovule during its development, as in Capsella (Fig. 203, (7). Anatropous ovules are the most common type, in which the ovule becomes com- pletely inverted during its early development and adheres to 364 GENERAL BOTANY the funiculus throughout its entire length. The ridgelike junc- tion of the ovule and the funiculus is called the raphe. Fruits are usually formed as a result of fertilization, and consist of the ripened ovary or of the ovary and the receptacle, .\ Follicle .^ (one carpel) Capsule (three carpels) Silique Capsule (two carpels) (four carpels) Legume larpel (one carpel) Achenes Single achene (buttercup) Samara, or key fruit (elm) Samara, or key fruit (maple) Berry (Smilacina) Aggregate (mulberry) Pome (apple) Drupe, or stone fruit (peach) FIG. 221. Different kinds of fruits Upper row, dry dehiscent fruits ; middle row, dry indehiscent fruits ; lower row, fleshy fruits as in the pome, drupe, and aggregate fruits (Fig. 221). They are variously classified on the basis of their form and structure. In the following classification the terms dry and fleshy indicate whether the ovary (or the ovary and receptacle combined) DESCRIPTIVE TERMS 365 becomes dry and hard in ripening, like the fruit of the pea and buttercup, or soft and fleshy, as in the apple, pear, etc. Dehiscent fruits, upon ripening, split open along the junction of the carpel or carpels, namely, along the placenta (septicidal dehiscence) or along the back of the carpels between the pla- cental junction line (loculicidal dehiscence). Indehiscent fruits remain closed, as in the cereal grains and strawberry, where the entire ovary and inclosed ovule is shed and disseminated together. The following classification and the accompanying illustra- tions will enable the student to classify most of the common fruits with which he comes in contact in the field. Dry dehiscent fruits : The follicle is a simple fruit which dehisces along one side. The legume, or pod, is a simple fruit which dehisces along two sides, as in the bean and pea. The silique is a fruit, like that of mustard, composed of two spo- rophylls, or carpels, which separate from the central partition. The capsule is a fruit formed from a compound ovary which opens at the junction of the sporophylls or between these junction points. Dry indehiscent fruits : The achene is a simple dry fruit in which the single seed is free from the ovary wall, as in the buttercup. The caryopsis, or grain, is the fruit of the grasses and cereals in which the ovary wall adheres to the seed. The nut is a fruit in which the ovary wall becomes the indurated resistant wall of the fruit. The samara, or key fruit, like that of the maple and ash, is a fruit furnished with a winglike outgrowth of the ovary wall. Fleshy fruits, simple or compound : In the berry the ovary wall becomes fleshy and incloses one or more seeds. In the pome the ovary wall is fleshy but with an indurated central part inclosing the seeds, as in the core of the apple. Drupes are stone fruits, like the cherry and plum. Aggregate fruits, like the blackberry, have several simple stone fruits aggregated or massed together on one receptacle. CHAPTER XIX TREES, SHRUBS, AND FORESTS IMPORTANCE AND USE Ornament and protection. The ornamental and protective function of trees is so well known that very little can be said to emphasize this aspect of their importance to man. The shade trees of our cities and towns, the great beauty of trees and shrubs on private lawns and in public parks, the pictur- esqueness of the mountains and of the open country with wooded hills and streams, all attest to the value of trees and shrubs and to the need of an adequate knowledge of their habits and uses. These facts are more evident if one travels from the wooded regions of the East or the Far West across the Western prairies, where little protection is offered against wind, sun, and storm except where early settlers have established wind- breaks by setting out trees or where streams are bordered by protective stands of timber. While isolated trees, or trees in small groups, are thus contributory to man's pleasure and com- fort, it is to trees aggregated in forests that one must turn in order to understand the great importance of tree life to the industrial life of men and to the progress of civilization. The national forests. The national forests of the United States formerly occupied an area of 850,000,000 acres, which has been reduced at the present time to about 545,000,000 acres. This vast forest domain includes the northern coniferous forests of the Great Lakes and the mixed coniferous and hardwood for- ests of the New England States ; the great southern forests, composed largely of pines ; the central, sparsely covered forest of hard woods; and the Rocky Mountain and Coast Range forests of the extreme western states (Fig. 223). 366 TBEES, SHRUBS, AND FOEESTS 367 The great value of trees in this forest domain is enhanced by the fact that a large part of it is in the mountains and in regions like the pine barrens of the Southern states, where the land is not of value for agriculture on account of the un- productiveness of the soil in those regions. The trees thus render an otherwise unfruitful region productive, and serve at the same time as a protection against floods, erosion, and drought by their control of rainfall and other climatic factors. FIG. 222. The ornamental function of trees and shrubs Photograph furnished, by the United States Forest Service Climate and water supply. The factors of climate which are controlled in any measure by forests are concerned largely with temperature, air movements, and water control. The effect on tem- perature is one which is felt only in the immediate vicinity of the forests themselves, and not over the country at large. It is a well-known fact that the leaves of trees in a forest absorb a large part of the heat which falls upon them, and that they utilize this heat in warming the leaves, in making sugar and starch, and in the evaporation of water vapor. The rich covering of humus on the forest floor also absorbs heat and protects the soil beneath from absorbing and radiating it as the soil in. naked exposed regions would. As a consequence 368 GENERAL BOTANY the forest has a general cooling effect on the air in its vicinity and so protects 'the soil from drying up. Air movements in the form of winds and storms are also restricted by forests, which therefore serve as effective windbreaks and at the same time affect the temperature of a region in both winter and summer. The early settlers in the West soon learned this advantage of trees as windbreaks and planted cottonwoods and other quick- growing trees on the north and west sides of their holdings. 1 Eastern Region \. % 2 Central Treeless Region 3 Western Region TROPICAL FOREST FIG. 223. General map of the forest areas of the United States In addition to the effect on the relative humidity and tem- perature of a region the forests have an important function in the control of water falling in the form of rain or snow. Forest control of rainfall and floods. Rainfall is supposed by some to be increased by the presence of great forests, and the investigations of European foresters would seem to bear out this assumption. Other data, however, gathered with equal care by experienced scientific foresters, yield opposite results, and it is doubtful whether the forests have any marked effect on precipitation. Floods and erosion, or the wearing away of soil by water, are so largely controlled by forests that this control is now TREES, SHKUBS, AND FORESTS 369 reckoned among the most beneficent and important effects of the forest cover. When rain falls over a dense forest, from one tenth to one fourth of it is caught by the crowns of the trees, while the forest floor of humus soil and roots holds the re- mainder. It is estimated that a forest floor " can hold for a while a rain- fall of five inches." This water is then gradually evaporated or is slowly drained off into streams, lakes, and the sources of springs. Mountain streams, which irrigate fertile valleys, are thus fed and sustained at their source. In like manner the supply of water for the great irrigation sys- tems of the West, and for the water supplies of large cities like Denver, Los Angeles, and San Francisco, comes from mountain springs, lakes, and streams, which are protected at their source FIG. 224. Pacific-coast forest of Douglas fir and western red cedar, Tacoma, Washington Photograph furnished hy the United States Forest Service by forests. Rain which falls upon unf orested soil has a very different effect from that outlined above, especially in mountainous and hilly regions, such as those occupied by most of our national forests. In a region denuded of forests the rain falls directly upon the soil, 370 GENERAL BOTANY which is apt to be beaten into a hard surface layer or, if soft and porous, to become quickly saturated and give way. The result is almost certain to be a disastrous flood and often immense damage caused by greatly swollen streams. In the Adirondacks and in California great damage has already been done in this way where the forests have been wholly or partially cut off or injured by grazing. It is estimated that " upward of two hundred square miles in the United States is annually laid waste by erosion " and that much of this great waste could be prevented by protecting or replanting the forests in the eroded regions. When swollen mountain streams reach the valleys at the foot of the mountains, they flood them and at the same time deposit sand, gravel, and even large bowlders on once fertile and pro- ductive soil. Not only must the cutting of forests on mountain slopes be carefully regulated, therefore, but denuded areas need to be systematically reforested by governments, either state or national, which possess resources adequate for such great tasks. Forest products. A great variety of forest products are derived from the national forests, including turpentine, tar, formalde- hyde, and rosin, in addition to the more important wood pulp, timber, and lumber supplies. The lumber and timber are used for various purposes in the industries and the home. These uses include firewood, lumber for construction and building, cooperage, and veneers, and timber for the making of excelsior and wood pulp, railroad ties, and telephone and telegraph poles. For these various purposes it is estimated that " we take from our forests yearly, including waste in logging and manu- facture, more than 22,000,000,000 cubic feet of wood, valued at $1,375,000,000." It is also estimated that almost half of the original lumber supply of the United States has already been used and that "the present rate of cutting for all purposes exceeds the annual growth of the forests." The remedy. The obvious remedy for this condition is the scientific control of timber cutting and the replanting of the forests, already in process of depletion, by the state and national governments. It is therefore of the greatest importance that the United States government has adopted the policy of caring for TREES, SHRUBS, AND FORESTS 371 and extending its control over an ever-increasing forest area. The extent to which this policy is being carried out by our national government is indicated by the following data. " On June 30, 1917, there were 147 national forests with a total of 155,166,619 acres," yielding an annual income of $3,500,000. On the above date the government employed in this work between three and four thousand men, including forest supervisors and rangers, lumbermen, sealers, planters, and clerks. These various officers are distributed to the different national forests in the pro- portion necessitated by the labor to be performed. They have a great variety of work, including the prevention and control of forest fires, the scientific cutting and marketing of timber, the control of grazing privileges, and the replanting of depleted forests. This mere enumeration of the extent, use, and control of the national forests of the United States is all that can be attempted in an elementary textbook of botany, but every student should acquaint himself with this great industry of our national government, which means so much to the present and future prosperity of our country. REPRESENTATIVE GROUPS OF FOREST TREES The trees which comprise the forests of the United States belong to the gymnosperms, or naked-seeded plants, represented by the pines and the spruces, and to the angiosperms with a closed pistil, represented by the elm, oak, and maple. In the following species, selected from these two great tree groups, both the economic and the biological features will be considered as concrete illustrations of the importance and interest attached to forest and ornamental trees. GYMNOSPERMS (EVERGREENS) THE SPRUCES (PiCEA) Habitat and habit. The spruces form an important part of the great coniferous (cone-bearing) and mixed forests of the north- eastern portion of the United States, the Appalachian region, the 372 GENERAL BOTANY Rockies, and the North Pacific coast. The red, white, and black spruces are found mainly in the northeastern and Appalachian forests ; the Engelmann spruce has its home in the Rocky Moun- tains; while the Sitka spruce, so important in the construction of aeroplanes in the World War, is found exclusively on the Pacific coast. The hab- itat of the spruces is thus confined largely to well-drained up- lands or to mountain slopes. Like many other plants, spruces are often found to be occupying situations to which they are not perfectly adapted, in- cluding marshes and swamps, on account of the lack of compe- tition in these habitats with the more highly organized hard woods, such as the oak and maple. Tolerance. An im- portant factor in the distribution and suc- cess of the spruces is due to the light requirement of the different species, especially in the younger stages of growth. They belong to the so-called tolerant trees, which have the power to grow, while young, in the shade of other trees (Fig. 225). " Having once gained a foothold in a selection forest, the young spruce grips life tenaciously, strug- gles along for many years under the shade of the forest, and gradually forces its way upward as natural thinning reduces the number of its overtopping competitors." Balsam is often found FIG. 225. Virgin stand of red spruce with repro- duction of tolerant spruce and fir in the White Mountains, New Hampshire Photograph by the United States Forest Service TREES, SHRUBS, AND FORESTS 373 with spruce on the forest floor (Fig. 226), since it too is a tolerant species, growing in the shade of the other forest trees. Balsam is, however, the stronger competitor of the two in such situa- tions, on account of its more plentiful seeds and rapid growth. Seed production. The distribution of forest trees, and their power to reproduce a forest once destroyed, is determined largely by the number of seeds produced and by the viability of the seeds, or their power to germinate and grow under the conditions presented in a given habitat. Spruces, like other cone-bearing gymno- sperms, begin to pro- duce large quantities of winged seeds when the trees have reached the proper age. The seed-producing stage has been found to ^ FIG. 226. Balsam, a tolerant tree, growing beneath with the conditions a virgin stand of red spruce in the White Mountains Under which it lives. Photograph by the United States Forest Service In the forest it begins to bear when the crown succeeds in reaching the light, which may be at the age of twenty or thirty years or may be delayed until the tree is one hundred years old. "In the open, and under favorable soil conditions, seed production begins as early as the fifteenth or twentieth year, and heavy crops follow by the thirtieth or thirty-fifth year." The seeds mature in late September and germinate in the same fall or the next spring, producing in good soil a new stand of spruce. 374 GENERAL BOTANY Maintaining the supply. Maintaining the supply of spruce for wood-pulp production and other commercial purposes is closely connected with the amount and nature of the seed production, since spruce forests are recreated or regenerated largely by means of seeds. In the case of hardwood trees, to be discussed below, vegetative reproduction by means of sprouts from the stumps is often used hi the regeneration of a forest destroyed by cutting or by other agencies. In the case of most of the coniferous trees the sprout method is not possible, since, with few exceptions, these trees do not reproduce vege- tatively in this manner. The production of new stands of spruce by means of seeds may be either by the natural method, where the growth of seed- lings occurs in a spruce or a mixed forest (Fig. 229), or by artificial sowing of seed. Where seedlings are to be grown by the natural method, care must be taken to cut out enough of the standing timber to facilitate the growth of the spruce at each stage of its development. In time, most or all of the larger trees of such an area will need to be cut, to allow the new spruce forest to develop normally, with plenty of soil space and light exposure. In other instances clear spaces are cut in the forest, with bordering mother spruce trees, from which the seed will be distributed and sown naturally over the cleared ground. In such cases the surrounding trees, if the clearing is not too large, pro- tect the ground from drying and furnish partial protection to the growing seedlings. These sheltering trees must be allowed to stand until the young growth can bear direct exposure. FIG. 227. Ripe cones of big-cone spruce in the Cleveland National Forest, California Photograph by the United States Forest Service TREES, SHRUBS, AND FORESTS 375 Seeding with spruce seeds may also be done artificially by scattering seeds on soil denuded of forest trees or by sowing the seeds in prepared seed beds (Figs. 230 and 231). In this case the seedlings, when they have reached the desired age, must be transplanted to the forest area where the new forest is to be grown. This method is, on the whole, the best and will prob- ably be more largely employed in the future than in the past by the state and national governments. Valu- able species for this purpose are the white spruce (Picea canaden- sis) and the Norway spruce (Picea abies), while the red spruce (Picea rubra) is more difficult to manage on account of its slow growth in early life. Commercial impor- tance. The wood of the spruce, like that of the pines and of the other cone-bearing trees, belongs to the class called softwood, to distinguish it from that of the broad-leaved hardwood trees like the oak, maple, hickory, and poplar. The term is a purely conventional one, since many kinds of soft woods are harder and more durable than some of the so-called hard woods, like poplar, basswood, and willow. The real characteristic of spruce and other coniferous woods which gives them their value and distinguishes them from the wood of broad-leaved species is the FIG. 228. Reproduction of spruce (second growth) in New Hampshire Photograph hy the United States Forest Service 376 GENERAL BOTANY character of the wood elements which make up the bulk of the wood. The student will recall that in spruce wood (Fig. 193) the water-carrying elements were the one-celled, thick-walled tracheids instead of the wide ducts of the alder and other hard woods. The small diameter of these tracheids, their thick walls, and their uniform size throughout the tree trunk make the even, fine-grained wood of the spruces and other conifers, like the white pine, extremely valuable in the industries. FIG. 229. Regrowth of aspen and spruce on a burned area in the San Francisco Mountains, Arizona Photograph furnished by the United States Forest Service The long, fibrous character of these tracheids and their close union with each other also contribute to their great value in the wood-pulp industry and in the making of aeroplanes from the now famous Sitka spruce. " Spruce is an aristocrat among woods. Its outstanding characteristics are combined elasticity and the ability to withstand sudden strain and shock." The most extensive use of spruce at the present time is in the making of wood pulp for newspaper stock. Something FIG. 230. State nursery at Saranac, Adirondack Mountains, New York Two-year-old seedlings of yellow pine in the foreground. Photograph furnished by the United States Forest Service FIG. 231. Nursery of Austrian and yellow pine in the Kansas National Forest Austrian pine on the right, yellow pine on the left. Photograph furnished by the United States Forest Service 377 378 GENERAL BOTANY over four million cords of wood is used for this purpose annually, of which about 60 per cent has been spruce wood. Red spruce has been the principal contributor to this great enterprise, but other woods are now being used on account of the depletion of the American and Canadian forests in the trees of this species. Spruce wood is also widely used in slack cooper- age and in building and interior finishing. The great value of spruce wood in the industries has stim- ulated the govern- ment to investigate new methods for its preservation and regeneration in the forests, which will undoubtedly result in preserving these valuable trees to future generations. THE PINES Habitat and habit. FiG.232. Transversesectionof tree trunk of long-leaf Th . originally pine, showing annual rings, heartwood, and sapwood . J Photograph furnished by the United States Forest Service occurrecl mixed and pure for- ests in the northern, southern, and western national forests. The great stand of pure pine in the northern forests in the Great Lakes region has been almost wholly depleted, however, so that the southern forest of long-leaf and short-leaf pine, loblolly pine, and cypress is one of the principal sources of pine lumber to-day. This is also the great seat of the turpentine industry, which has exacted a. heavy toll on southern pines under the old wasteful system of tapping the trees for turpentine. The Western Rocky Mountain and Pacific Coast forests also supply pine lumber in large quantities from the western yellow, lodge- pole, sugar, and white pines, which find their natural home in TREES, SHRUBS, AND FORESTS 381 oaks, poplars, hickories, ash, willows, and other well-known species of broad-leaved trees. This hardwood forest of the central region differs from that of the coniferous forests in that it is not so continuous, being composed of smaller local forest stands or of groups of trees on farms, known as the farm wood lot. Reproduction. The hard woods belong to the angiosperms, or true flowering plants, and are hence sharply distinguished from the cone-bearing gymnosperms. The flowers of the fruit bearers, such as the apples and plums, have already been discussed and are familiar features of these trees in the spring on account of their great beauty and fragrance. Many of the shade and timber trees, however, reproduce by means of very simple flowers which are rarely known to any- one except the student of botany. In some cases, as in the oaks (Fig. 238), these simple flowers are thought by many botanists to indicate a very early ancestry, even antedating that of herba- ceous species, while others regard the simplicity of the flowers as indicative of a reduced condition. In most of these cases polli- nation is anemophilous (by the wind) and the trees are either monoecious or dioecious. Many trees produce winged fruits, which greatly facilitate their dissemination, as in the poplars, which so frequently re- forest burned-over areas on mountain slopes. Other species, like the oaks and hickories, produce heavier nut fruits, which are not easily distributed and hence limit the range of these species. Commercial importance. The commercial importance of the hardwood trees is determined by the character of the fruit and the wood. In the wild state the wood is the most important FIG. 234. Transverse section of the wood of sassafras, showing its ring- porous character Photomicrograph by R. B. Hough 382 GENERAL BOTANY commercial product of these trees, and there is much greater structural variation in the hard woods than in the soft-wooded gymnosperms on account of the ducts and fibers in hard wood. The hard woods are all characterized by the possession of large water ducts, which render the wood more porous and make it less uniform in texture than is the case in spruce and pine. Between the pores, as we have already learned, the wood is com- posed of strong strengthening fibers and living cells, which are FIG. 235. Diffuse-porous woods of the sycamore and Lolly Photomicrograph by R. B. Hough usually more abundant in the summer wood than in the spring wood (Figs. 47 and 55). The large number of species of hardwood trees and the great variety in the character of the wood make this group of importance in supplying lumber and timbers for almost every commercial purpose. Two varieties of hard wood are recognized in the industries, namely, ring-porous wood and diffuse- porous wood. Ring-porous wood, like that of the sassafras (Fig. 234), has the spring ducts in the early spring wood, while in diffuse-porous woods (Fig. 235) the ducts, or pores, are scat- tered throughout the entire wood ring. In ring-porous woods the annual rings are more distinct than in diffuse-porous woods. TREES, SHRUBS, AND FORESTS 383 THE WHITE OAK (QUERCUS ALBA) Habitat and habit. The white oak may be taken as a typical example of a hardwood forest tree as compared with the soft- wood coniferous trees represented by the spruces and pines. It FIG. 236. Spreading habit of the white oak (Quercus alba) Photograph furnished by the United States Forest Service forms an important constituent of the great central hardwood forest, where it attains its best development ) Flower' Floral plan. FloraTplan of three O ffi ve 'Seed Cotyledon-/.] 'Stem apex. .1 I Hypocotyl-\ I Embryo (monocotyledon} | Cotyledon- Stem apex^ -Hypocotyl- Enwryo' (dicotyledon) Seed FIG. 263. Comparison of monocotyledons and dicotyledons a, vegetative and reproductive structures of a monocotyledon ; 6, similar structures of a dicotyledon embryos of dicotyledons had two cotyledons, as in the common beans and peas. The mature plants of the two groups also manifest quite marked distinctions, which apply to the form and venation of the leaves, the structure of the stem, and the numerical plan of the flowers (Fig. 263). The leaves of monocotyledons are usually linear with parallel venation, like the leaves of lilies and grasses. The venation is HERBACEOUS AND WOODY DICOTYLEDONS 415 also designated as closed venation, since the veins do not termi- nate in the margin of the leaves, which therefore remain smooth. In dicotyledons the leaves are usually netted-veined, and the veins end free in a rough margin. The stems of monocotyledons also have scattered vascular bundles without a cambium layer, while those of dicotyledons form a cylinder in which the cambium adds new tissue to the phloem and xylem layers annually. The flowers of the monocotyledons are habitually on the plan of three, while dicotyledonous flowers are more frequently on the plan of four or five parts in a whorl for each set of floral organs. These distinctions between dicotyledons and monocotyledons are graphically illustrated in Fig. 263 and are concisely stated in the summary below. SUMMARY The embryo of monocotyledons has but one cotyledon and a lat- eral stem tip, while dicotyledons have two cotyledons and a central stem tip. The monocotyledons usually have leaves with parallel" veins, and the veinlets do not end free in the margin of the leaf, as in dicoty- ledons, in which netted venation prevails. In the stems of monocotyledons the vascular bundles are scattered. In dicotyledons the xylem and phloem form a cylinder inclosing the central pith. The flowers of monocotyledons are in threes, but those of dicoty- ledons are in fours or fives. CHAPTER XXI MONOCOTYLEDONS Habitat and habit. The monocotyledons include over forty known families and about twenty-five thousand species of plants, the greater number of which are water-loving, being either true hydrophytes, like the pondweeds (Pontederia and Potamogetori), or semiaquatics, like the marsh grasses and sedges. A few forms, such as the yuccas and desert grasses, are xerophytic, while a large number should be classed as tropophytes, adapted to alter- nating seasons of moisture and drought. Among these plants are many of the wild and cultivated species in which the underground stem takes the form of a rhizome, bulb, or corm, which enables the plant to live securely underground during an inclement season. The favorable season with such plants is used for the growth of aerial stems bearing leaves which make food, and for the production of flowers, fruit, and seeds. The majority of these species have the characters already ascribed to monocotyledons, namely, narrow, parallel-veined leaves, aerial stems with scattered bundles, and flowers on the plan of threes. Reproduction and seasonal life. The monocotyledons with underground stems reproduce sexually by means of flowers and vegetatively by means of corms, bulbs, runners, or tubers formed as offshoots from the mother plants. Vegetative reproduction facilitates local increase in the immediate vicinity of the mother plant, while seeds, formed as a result of sexual reproduction, facilitate the distribution of the species over wide areas. The seasonal life is therefore much the same as that of the white sweet clover and of the perennials outlined in the summary of the seasonal life of plants in Part I. The commercial importance of the monocotyledons is due to the great beauty of many species, such as the lilies, tulips, and hyacinths, and to the value of other species for food and forage. 416 MONOCOTYLEDONS 417 COMMELINACEAE (SPIDERWORT FAMILY) TRADESCANTIA (SPIDERWORT) Habitat and habit. The spiderworts are among the commonest wild and cultivated plants of the spring and summer flora. They usually inhabit gravelly, sandy, or alluvial soils in woods or along railroads and river banks. In Trades- cantia virginiana (Fig. 265) the stem is both aerial and subterranean. The aerial stem bears the long, parallel- veined leaves and blue flowers char- acteristic of the species. The un- derground stem is tuberous and gives rise to one or more lateral flowering shoots by means of lateral buds. It also enables the plant to live over the late summer, autumn, and Vascular bwndle A, plant in flower; B, reproductive structures and stem section (a, front view of flower; b, ground plan; c, dehis- cent fruit ; d, section of stem) ; C, leaf and parallel venation FIG. 264. Habit and reproduction of the common spiderwort ( Tradescantia) win- ter, furnished with buds and reserve food for the early spring growth. Tradescantia is thus a semi- xerophyte or semitropophyte, like many of the cultivated bulbous monocotyledons. Reproduction. The blue or purple flowers are ephemeral, that is, lasting for a day only, and have their parts arranged in threes (Fig. 264, J5) like other typical monocotyledons, 418 GENERAL BOTANY Pollination is by means of insects, which visit the flowers in great numbers and pollinate the stigma, which protrudes beyond the anthers and hairy filaments. The fruit is dry and opens by three valves, repre- senting the three car- pels of the ovary, and liberates the smooth black seeds for dis- semination. Thus the plant reproduces in two ways, sexually by means of flowers, fruits, and seeds, and vegetatively by means of tubers with lateral buds, which very often produce underground branches of consider- f ---f$ able length. LlLIACEAE (LlLY FAMILY) The Liliaceae are of principal interest on account of the large number of or- namental plants in- cluded among them, although food plants, such as the onions and asparagus, are also members of this important family. The ornamental plants include the common cultivated lilies, the spring tulips, hyacinths, narcissus, and lily of the valley. Among the wild species of greatest beauty are the dogtooth violet (Erythronium), the bell wort ( Uvularia), FIG. 265. Lengthwise section of hyacinth plant st, stem; 6, young bulb for next year's growth sc, bulb scales ; fs, flower stalk. Reduced MONOCOTYLEDONS 419 Trillium, false Solomon's seal {SmilacincC), and Ornithogalum (Fig. 266). Most of these plants spring from bulbs, tubers, or rhizomes, and so are adapted to a tropophytic existence and a Courtesy of the American Magazine of Forestry, Washington, D. C. FIG. 266. Ornithogalum, star of Bethlehem Natural habitat and habit of a flower in bloom. Photograph by Dr. R. W. Shufeldt corresponding seasonal life. Many of the cultivated species origi- nated in arid regions, where the short rainy season is followed by a long dry period, as in California or the Mediterranean region. In such habitats the underground stem enables the plant to live securely during the dry season, while the great store of food in 420 GENERAL BOTANY the bulbs, tubers, or rhizomes facilitates the rapid growth of aerial shoots and flowers during the short rainy season. In tem- perate regions the wild species often inhabit hillsides or banks where dry conditions are imposed during the summer months. Commercial importance. This habit of producing underground stems facilitates the cultivation of the Liliaceae for commercial purposes, since the bulbs, corms, and rootstocks thus produced allow of shipments and long storage, which would otherwise be impossible. Tulips and hyacinths, for example, are grown in Holland for shipment to this country. The bulbs arise as lateral buds which produce the young bulbs or offsets in considerable numbers each year from the mother bulbs (Fig. 79). Ten or twelve daughter bulbs are produced from one mother tulip bulb in a season, and as many as twenty or more from a hyacinth bulb. These offsets are grown for from three to five years, then dried and shipped to various parts of the world for the growth of flowers for ornamental and decorative purposes. The biologi- cal and commercial history of the narcissus, lilies, onions, and other ornamental and food-producing varieties of the lily family is very similar to that briefly sketched for the tulip and the hya- cinth. Most of these species have been greatly improved, as regards variation in color and size, by hybridization ; but the perpetuation of such variation must be secured by vegetative reproduction, as we learned earlier in the study of reproduction, hybridization, and breeding. SMIL AGIN A AND EEYTHEONIUM Habitat and habit. The species of Smilacina usually inhabit wooded slopes in the shade of trees, where a considerable amount of humus is present in the soil. They represent those species of the Liliaceae which have both an underground and an aerial stem, with the usual differentiation in function between the two kinds of stems. The aerial stem bears the leaves and the cluster of flowers of the summer season, while the underground stem serves for food storage, conduction, and the formation of buds for the next season's growth (Fig. 267, A). During dry periods, and in 422 GENEKAL BOTANY the winter months, the underground rhizome enables these plants to live protected from any danger of drought or destruction by freezing. In the spring the young buds and growing roots are FIG. 269. Stages in the development of Erythronium First year, germinating seed and seedling ; second and third years, first bulb ; fourth and fifth years, new bulbs being formed deeper in the soil ; sixth and seventh years, larger bulbs and plants. From Bergen and Caldwell's "Practical Botany " furnished with an abundance of food stored up during the previous season in the rhizome and now digested and circulated for use in spring growth. Erythronium (Fig. 268) occupies much the same habitat as Smilacina and has the same general habit and seasonal life. The MONOCOTYLEDONS 423 underground stem is in the form of a solid, scaly bulb from which two characteristically spotted leaves and a single flower scape grow each spring. The most distinctive feature of Erythro- nium is the peculiar method by which the bulbs of each season become more deeply buried in the soil. The new bulbs are formed at the ends of runners arising from buds in the axils of the scale leaves. The runners of each season (Fig. 269) grow downward and so bury each new daughter bulb a little deeper in the soil than the mother bulb. Six or seven years are necessary for the production of a bulb strong enough to bear flowers. The seasonal history of Erythronium and Smilacina is therefore not unlike that of the tulip and hyacinth or of a perennial woody plant in which a long period of vegetative activity is necessary before reproduction by flowers and fruit is possible. Reproduction. The flowers in Smilacina and Erythronium differ from those of Tradescantia in that both whorls of the perianth are colored like a corolla and are therefore not differentiated into a distinct calyx and corolla. This feature is also character- istic of the cultivated lilies, tulips, and hyacinths, to which Smilacina and Erythronium are closely related. Cross-pollination is effected by insects, since the stamens, when ripe, are shorter than the pistil, so that the stigma protrudes beyond the anthers. It is thus in a position to receive pollen from insects which have recently visited other flowers. The fruit in Smilacina (Fig. 267, C) is a berry, and the seeds are disseminated when the berry disintegrates, or they may be distributed by animals which eat the fruit. In Erythronium the fruit is a capsule which splits into three valves corresponding to the three carpels of the ovary, thus disseminating the seeds. IRIDACEAE AND ARACEAE (!RIS AND ARUM FAMILIES) The species of these two families are of particular interest on account of the special adaptations of the flowers for securing cross-pollination by insects. Their vegetative characteristics are also of biological importance as indicating the characteristic habit of the aerial and underground parts of the monocotyledons already emphasized in Tradescantia and Liliaceae. 424 GENERAL BOTANY IRIDACEAE (!EIS FAMILY) IRIS VERSICOLOR (COMMON BLUE FLAG) Habitat and habit. The common wild blue flag of the spring flora inhabits swampy land on the borders of streams and lakes canal. FIG. 270. Flower and reproductive structures of the iris (Iris versicolor) A, flower and its parts ; B, ovary, style, and stigma ; (7, pollen tubes and is thus adapted to a hydrophytic habitat. Like many plants of such habitats, however, its habit, represented by the form and structure of its leaves and underground stem, is that of a xero- phyte. This is probably due to the large percentage of organic matter, including organic acids, in the water of the soil which surrounds the roots of these plants in the marshes and swamps where they live. The short aerial stem bears characteristic monocotyledonous leaves and highly organized flowers. MONOCOTYLEDONS 425 The cultivated forms of Iris have a similar habit and also a similar floral structure, which has become highly modified by hybridization and selection with a view to increasing the size, color, and beauty of the flowers. Reproduction. The flower (Fig. 270, A) is epigynous, with the floral parts arranged in sets of three each. The outer lobes of the perianth are large and highly colored, while the inner ones are smaller and less conspicuous. The three stamens spring from the base of the outer perianth lobes, and each stamen is beneath a branch of the three-lobed style. The stigmas are on the upper surface of a flap which grows out from each stylar lobe. When an insect visits the flower for nectar, it comes to rest upon one of the outer lobes of the perianth and then crawls down into the flower to probe for nectar at the base of the perianth. In so doing it dusts pollen onto its back, which cannot reach the stigmatic surface as it crawls out, but is in a position to pollinate a stigma of the next flower visited. The pollen germinates quickly on the stigma of Iris, and. the pollen tubes follow down the stylar canal in the center of the style to the ovules, where fertilization takes place. The flower is thus admirably adapted to cross-pollination by insects. ARACEAE (ARUM FAMILY) AEISAEMA (JACK-IN-THE-PULPIT) Habitat and habit. The common jack-in-the-pulpit (Arisaema triphyllum) is one of the most familiar representatives of the Arum family in the spring flora. It grows naturally in moist humus soil in the shade of trees and is thus mesophytic in habit and habitat. The aerial stem bears one or two leaves and peculiar greenish flowers. The underground stem is a turnip-shaped corm which bears the annual aerial shoot from a terminal bud. The seasonal history of the jack is an interesting one and will serve as another illustration of the peculiar adaptations of some of the monocotyledons with underground stems for perpetuat- ing themselves by vegetative reproduction. The new roots arise each year from the upper part of the corm (Fig. 271, A), while 426 GENERAL BOTANY the nourishment is stored in the base of the corm for the early growth of the bud which produces the annual aerial stem. The new corm is therefore formed above the old corm each season, and the tissues of the latter disappear as the new corm is formed above on its remains. Large conns also produce lateral buds, similar to those of the gladio- lus (Fig. 80), which give rise to a circu- lar cluster of corms around the mother corm. Sexual reproduc- tion. The flowers are borne on a fleshy axis called the spadix, which is included in a bractlike structure called the spathe (Fig. 271, B, a). The entire struc- ture is often mis- taken for a flower, although it is really an inflorescence. Cross-pollination is assured, since male and female Anther FIG. 271. Habit and flower of jack-in-the-pulpit (Arisaema) A, plant with flowers and corm ; B (a, spathe, spadix, and staminate flowers ; 6, cluster of stamens; c, pistil). Copied from Curtis's " Nature and Development of Plants " flowers are usually borne on separate plants. The male plants are also smaller, as a rule, than the female plants, which is -an advantage, since the female plants must produce seed and fruit and so need the great store of reserve food contained in the larger corms. The production of pistillate flowers from the larger corms is supposed to be connected with the abundant food supply. The fruit is composed of a cluster of beautiful red berries borne on the lower, fleshy portion of the spadix. MONOCOTYLEDONS 427 ORCHIDACEAE (ORCHID FAMILY) CYPRIPEDIUM (LADY'S-SLIPPER, OR MOCCASIN FLOWER) Habitat and habit. The flowers of the lady's-slipper belong to the orchid family (the flowers of which are famous for their great Courtesy of the American Magazine of Forestry, Washington, B.C. FIG. 272. The yellow lady's-slipper (Cypripedium pubescens) Photograph by Dr. R. W. Shufeldt FIG. 273. The pink lady's-slipper (Cypripedium acaule) Photograph by Dr. R. W. Shufeldt beauty as conservatory plants) and are biologically interesting on account of their wonderful mechanisms for securing cross- pollination by means of insects. The plants inhabit moist, shady woods with soil containing plenty of humus, or, in the case of some species, low, marshy regions along streams, ponds, and lakes. Their habit is sufficiently illustrated in the text figures 428 GENERAL BOTANY of the common yellow lady's-slipper (Cypripedium pubescens) (Fig. 272) and the pink species (Cypripedium acaule) (Fig. 273). ^ Reproduction. The flowers of the lady's-slipper are simpler than those of the true orchids, but are nevertheless very highly modified in such a manner as to prevent self-pollination. One of the petals is developed into a saclike structure (the labellum, or lip) observed in the figures, with a narrow en- trance above for insects. The style is highly modi- fied and projects into this opening to the labellum, bearing the stigma and two stamens on its under- side (Fig. 274). Insects make their way into the cavity of the labellum on either side of the project- ing style. In order to get out they are obliged to rub against the stamens and o, ovary ; a, anther of a perfect stamen ; sta, im- are thus dusted with pol- perfect stamen : stig, stigma , , . , len. The stigma is situated below the stamens, so that the outgoing insect does not pollinate it. The next flower visited, however, is certain to receive on its stigma the pollen from the flower previously visited. FIG. 274. Vertical section of a flower of Cypripedium acaule ALISMACEAE AND PONTEDERIACEAE SAGITTARIA AND PONTEDERIA Habitat and habit. Sayittaria (arrowhead) and Pontederia (pickerel weed) are good examples of Jiydrophytic monocotyle- dons which inhabit shallow water on the margins of lakes or streams. In Sagittaria the flowering stem and leaves arise from stolons which are buried in the mud at the bottom of the lake or stream. In Pontederia the underground stems are rhizomes which give rise to a flower-bearing shoot and long-petioled leaves. 8 M to 'I ' ft a Ssi tM I t 430 GENERAL BOTANY Reproduction. In Sagittaria (Fig. 275) the flowers are borne in clusters of three on the inflorescence axis, while in Pontederia (Fig. 276) they occur in spikes. Cross-pollination is provided for in Sagittaria, since plants are either moncecious or dioecious. FIG. 277. A field of sugar cane at Vera Cruz After Freeman and Chandler In Pontederia the flowers are trimorphic, with three lengths of stamens and pistils, which also insures cross-pollination. G-RAMINEAE (GRASSES AND SEDGES) FORAGE AND FOOD PLANTS Habitat and habit. The members of this family, including the common grasses, bamboo, sugar cane, and cereal grains, are among the most important and widely spread of all the monocotyledons. The grasses proper are social plants, forming vast associations MONOCOTYLEDONS 431 comprising the principal vegetation of the meadows, plains, marshes, and slopes of this and other countries. They are therefore either mesophytic, hydrophytic, or xerophytic. Many of the grasses per- petuate and spread the species by means of under- ground stems in the form of either rootstocks or stolons, which enable them to form dense mats and sods wher- ever they gain a foothold (Fig. 278). The leaves are characteristic of monocoty- ledons generally, being long and -strap-shaped, with par- allel veins. The flowers are highly modified, and the fruit in the true grasses and cereal grains is an achene, known as the cary- opsis, or grain. FIG. 278. Habit of the couch grass, a weed pest Note that the aerial stems spring from nodes of the underground rhizome. When the rhizome is cut in pieces, each node can reproduce a new plant and so spread the weed Economic importance. The true grasses are of the greatest importance in furnishing pasturage and hay for animals and in providing a good turf for lawns and meadows. The cereal grains, including corn, wheat, oats, and rye, are all grasses which have 432 GENERAL BOTANY been cultivated and greatly improved by man. Corn, for instance, is supposed to have been derived from a wild grass or to be a hybrid between two grasslike an- cestors. The wild wheats of Palestine, from which the cultivated varieties are supposed to have come, are essentially grasses in which the fruit is a grain of the greatest value to man for flour. All the other cereals have originated similarly from the grasses and have been gradually improved by methods of culture de- scribed in the chapters on hybridization, selection, and evolution. In addition to the grasses and cereals the grass fam- ily includes sugar cane (Fig. 277) (used for mak- ing sugar), bamboo (used for fishing rods), and rice (used as a food plant). Reproduction. The flow- ers of the grasses are usu- ally very highly modified, and their relation to the flowers of the monocoty- ledons thus far studied is difficult to determine. In the corn plant (Fig. 102) the flowers are borne separately, the staminate flowers forming a compound inflorescence, known as FIG. 279. Structure of an ear of corn, or pistillate inflorescence A, section of a young ear, showing the cob, or axis of inflorescence (ax), and the silk, or style, and stigmas (si) ; B, ovary, showing ovule (o), and style ; C, upper portion of style (silk) and stigmas, enlarged MONOCOTYLEDONS 433 the tassel, and the pistillate flowers forming an entirely different kind of inflorescence, known as the ear (Fig. 279). Cross- pollination is almost certain in corn, although self-pollination is not impossible where the stamens and pistils ripen together. The anthers on a single corn plant are said to produce as many as Floral plan of spikelet e FIG. 280. Inflorescence and flower of the oat (Avena saliva) a, portion of an oat panicle, or inflorescence ; b, c, d, different views of the oat spikelet and its parts; e, floral plan of spikelet 50,000,000 pollen grains, so that complete wind pollination of the many stigmas constituting the silk of the ear is practically assured. Where pollination does not occur, the seeds do not develop. The flowers of the true grasses and cereal grains are usually borne in dense spikes, like those of wheat and timothy, or in compound clusters made up of separate spikelets, like the panicle of oats (Fig. 280, a). The spike, like the panicle, is composed of several spikelets, each spikelet containing one or more flowers (Fig. 280, b-e). The flower proper (Fig. 281) is greatly reduced, 434 GENERAL BOTANY being composed of a pistil with two plumose stigmas, three stamens, and two small rudimentary organs at the base of the pistil, cajled lodicules. Each flower is usually surrounded by two pairs of bracts. The outer bracts are called glumes; the inner bracts are called the palet and the lemma. Botanists are almost unanimous in regarding the flower of the grasses as a highly modified form of monocotyledonous flower, like that of Trades- cantia or the lily, in which the perianth is represented by the rudimentary lodicules. The lodicules, where they are well devel- oped, serve to open the glumes, palet, and lemma, so as to Glume Ovar Glume Flower a Floral plan Flower of flower b c FIG. 281. Flower of the oat a, &, two views of the flower and its parts ; c, ground plan of the flower expose the essential organs. Where the lodicules are not well developed, the flowers remain closed. Cross-pollination, where this occurs, is usually effected by the wind, the anthers being attached to the filaments at the middle, so as to be readily swayed to and fro for the scattering of pollen. In many of the cereal grains, like wheat and oats, the opening of the flowers and the act of pollination occur early in the day and are often completed in a relatively short time. The fruit is the caryopsis, or grain, which is formed by a union of the ovary wall with the seed coat, making a dry indehiscent fruit. CHAPTER XXII PLANT ASSOCIATIONS In the previous chapters plants have been considered as sep- arate individuals or as groups of closely related individuals known as species. In nature, however, each plant is a member of some plant community, living together with other plants which vitally affect its growth and development. The student will also find upon investigation that the vegetation of each region is not uniform in character, but is made up of smaller units, such as meadow, pond, and cultivated field and lawn, each of which is composed of plants requiring similar conditions of soil and cli- mate for their best development. It will be found that these various plant communities, known as plant associations, although they seem to the uncritical observer to be stable, are never- theless constantly changing, owing to competition among the existing members, to the advent of new species by migration, or to changes in the earth's surface caused by fires, floods, and other agencies, which have a far-reaching effect on vegetation. Kinds of plant associations. The most general classification of plant associations is that mentioned in the first part of the text, namely, the mesophytic (Fig. 282), Kydrophytic (Fig. 286), and xerophytic (Fig. 71) plant associations, based upon the avail- able water in the soil. To this general classification a fourth should be added, to include the halophytic plant associations, which inhabit salt marshes and ponds where the amount of various salts in the water is excessive. These larger associations are usually subdivided into smaller ones, which reflect more accurately the particular environmental conditions prevailing in particular habitats. Thus, the hydrophytic association comprises smaller pond, lake, stream, and swamp associations, which differ either in the kinds 435 436 GENERAL BOTANY of species composing them or in the proportion existing between the constituent species of each minor association. In a similar manner, the xerophytic associations include lesser desert, dune, cliff, bog, and saline plant associations, while mesophytic plants form distinct associations in the form of forests, meadows, prairies, and cultivated fields. These large and small plant associations FIG. 282. A new mesophytic forest association (twenty-five years old) of honey locust, white elm, and black walnut Photograph furnished hy the United States Forest Service form the units which comprise the vegetation of local areas and, finally, of the entire land surface of the globe occupied by plants. Origin of new associations. In order to understand the impor- tant phenomena connected with the origin and development of plant associations it will be necessary to consider certain dynamic aspects of plant life, including the migration of plants from one locality to another, the invasion and occupation of new territory by such migrants, and the replacing of one plant population by another in new regions. It is evident that the migration of plants by means of mobile seeds, fruits, or other reproductive parts must be a potent factor PLANT ASSOCIATIONS 437 in the formation of new plant communities in denuded areas or in repopulating old ones where space still remains for the intro- duction of new individuals or species. It is clear also that the rapidity with which any given plant will invade such areas will depend in part upon- the nature of the device which it possesses for seed dissemination. The student is already familiar with some FIG. 283. Association of plants in a forest, blackberries forming a layer associated with cedar (Thuja plicata), Cedar Mountain, Idaho After Clements of these devices in the plants previously studied in the field and laboratory. Thus, the wide distribution of willows and poplars along the borders of streams and on the shores of lakes or ponds is due to the long, silky hairs on the seeds, which enable them to migrate by means of air currents. The rapid migration and wide distribution of such composites as the dandelion and yarrow is due to the very effective parachute of hairs which serves as a flying apparatus for the fruits of these plants. In the case of fruits with wings, like the maples and pines, or of the heavy 438 GENERAL BOTANY fruits, like the nuts of the hickory and oak, the devices are less effective and serve for local rather than for wide seed distribu- tion. The seeds of edible fruits, like berries, apples, and cherries, are also very widely disseminated by birds and other animals, which cast the seeds with their excrements in regions far re- moved from the home of the mother plant. Almost innumerable FIG. 284. Invasion of a grass, Agropyron, into bare sand by groups, Mount Garfield, Pikes Peak, Colorado After Clements examples might be added of other devices by which the mobile seeds, fruits, and other reproductive parts of plants migrate and invade new regions. The immediate effect of such migrations and invasions (Figs. 284 and 285) in the formation of new plant associations is most easily observed where tracts of land occur which are devoid of vegeta- tion. Such land surfaces may exist in gardens, lawns, and fields, or they may be the result of fire, flood, or other destructive PLANT ASSOCIATIONS 439 agencies. In all such cases of denuded land areas the first inva- ders have a free field, without competition on the part of other plants, which is a very important factor in the success of invaders into old plant communities. The main restrictions on the occu- pation of these naked surfaces are those imposed by the nature of the invading plants themselves and by the environmental con- ditions obtaining in the invaded area. Thus, strictly water-loving FIG. 285. Invasion of Pinus ponderosa into plains grassland, Black Forest, Colorado After Clements plants will not thrive on an upland tract, and plants accustomed to medium, or mesophytic, conditions will not grow or thrive in a desert soil. In other words, the invading species must be more or less closely adapted to the soil and climate of the invaded region in order to survive and become a permanent member of a new plant community or association. The first invaders are usually annuals or biennials furnished with mobile seeds, fruits, or vegetative parts. These are in turn succeeded by hardy peren- nials like the grasses, which may establish a permanent plant 440 GENERAL BOTANY association. In other cases, as we shall learn later, the herbaceous plant association may be replaced in time by a shrub or a forest plant association. Many plants, also, when once established in a given locality spread by vegetative means and gradually drive out other competitors. This is notably true of plants with run- ners, stolons, rhizomes, and other underground parts, already dis- cussed under vegetative reproduction in the first pa-rt of jthe FIG. 286. The pond lily, an aquatic with floating leaves and submerged stems text. In the end a new plant association will be formed, composed of plants adapted to the conditions of soil and climate which obtain in the given region. The above sketch of the main factors involved in the making of a new plant association on a denuded tract is probably a fairly accurate picture of the manner in which the existing plant associations which constitute the earth's vege- tation have arisen. Succession. The term succession involves the idea of replac- ing the plants comprising a given association by a new plant population which invades and finally occupies the ground for- merly held by the old association. This conception can be most PLANT ASSOCIATIONS 441 easily explained by a concrete illustration of succession often exhibited in the history of a pond or lake which becomes grad- ually filled with soil and decaying vegetation. The plants comprising the first association in such a habitat are wholly hydrophytic and consist of free masses of algse and of shore plants such as flags (7m), bulrushes (Scirpus), and arrowhead : -V--X" v FIG. 287. Zonation of grass (Deschampsia), bulrushes (Scirpus), and pines (Pinus ponderosa) After Clements (Sagittaricf). As the pond or lake becomes shallower by the accumulation of vegetable remains and the iiiwash of soil the shore plants encroach more and more upon the water area, fol- lowed by grasses and sedges which convert the old shore line into a marsh or bog. This marsh or bog may form a permanent plant association for a considerable period or it may be converted by willows, alders, and similar shrubs into a thicket, and then by poplars, ash, maples, and oaks into a typical mesophytic forest. 5cm.- JSOOft, FIG. 288. Diagram illustrating zonation and succession around a pond I, pond (a, floating pipewort (Eriocaulori) b, deepest portion of pond ; c, rushes (Juncus) forming an association) ; II, bog zone ; ///, swamp thicket zone ; IV, sand pit and incomplete xerophytic zone ; V, dry meadow zone ; VI, dry woodland zone (d, birch woodland). From Bergen and Davis's " Principles of Botany " PLANT ASSOCIATIONS 443 The length of time required for one type of association to suc- ceed and supplant another in such an instance as that just described, as well as the composition and nature of the plant populations which follow one another in any given succession, will of course vary in different cases, but the general facts regarding succession will hold for all similar habitats. Another familiar illustration of plant succession is often observed in. forests where fires destroy the trees over wide tracts. The first invaders are FIG. 289. A hillside once forested, but now bare and eroded Photograph by United States Forest Service .* here, as in most other instances, herbaceous plants, including the fire weeds (Epilobium and Lrechtites), which are able to live and thrive in the burned-over area. The herbaceous plants are in time succeeded by aspens and conifers in some regions, or by forests of deciduous hardwood trees in others. Similar processes may be observed along the shores of most lakes and streams where new land is formed by changing water levels or in cultivated fields and gardens where land is plowed and allowed to lie fallow long enough for the first occupants to be routed by later competitors. 444 GENERAL BOTANY The arrangement of plants in regular zones, termed zona- tion (Figs. 287 and 288), in regions where one association is being replaced by another, is often very evident, particularly on the borders of streams, ponds, and lakes. In some such instances the zones are very distinctly marked, while in others they merge gradually into one another, creating tension lines of great competition where two kinds of vegetation struggle for supremacy. General instability of vegetation. The changes noted above in new plant associations, due to migration, invasion, and competi- tion, are far more general in their nature than is usually supposed. The great numbers of seeds formed by each plant species in nature, and the admirable devices developed for their dissemi- nation, result in a wide annual sowing of the seeds of new species in every old plant association. If the young plants which spring from these seeds are better adapted to the environment in which they chance to spring up than the existing species, the new- comers will gradually drive out the older inhabitants and become a new element in the population of the old association. Familiar instances of such changes are seen 'in the invasion of a lawn by dandelions, and of roadsides and cultivated fields by weeds. Old associations are thus constantly changing in the kinds of species constituting them. Another p'otent cause for far-reaching changes in existing vegetation is the gradual change going on in the surface features of the earth, caused by elevations and sub- sidences of the earth's crust and by water erosion. Geologists tell us that the continents are slowly but surely being leveled off by the wearing down of the hills and by the deposit of the eroded soil in the valleys by water action (Fig. 289). By these processes hills and slopes are denuded, new and barren cliffs are formed, and existing plant populations are covered and destroyed in the valleys and on flood plains. These constant readjustments initiate new plant associations, which form the first of a series of plant populations, followed by successions such as those described above. What is known as vegetation is therefore in constant flux and change, although the individual plants which comprise it are themselves immobile. PLANT ASSOCIATIONS 445 SUMMARY The vegetation of the earth's surface is not homogeneous but is composed of units of varying composition which are termed associa- tions. These units of vegetation have apparently originated, as new associations arise to-day, in regions where the land is partially or wholly deprived of its vegetation by natural or artificial means. Some of the main factors concerned in the origin and development of a new plant association are the migration of plants by means of reproductive parts, the adaptation of plants of different kinds to different environmental conditions, and competition between the individuals of a growing plant population. Plant associations have also been found to be constantly changing, owing to competition and to changes in habitats due to natural and artificial causes. The gen- eral instability of existing vegetation is the natural outcome of these various factors, which are in continual operation over large areas of the earth's surface. INDEX (References to illustrations are indicated by asterisks accompanying page numbers) Absorption, mechanism of, 137 Absorption by roots, 139 Acaulescent, 353 Accessory buds, 356 Acer saccharum, 88*, 390*, 391* Aceraceae, 390, 393 Achene, 364, 365, 412* AchiUea, flower of, 411*, 412* Achillea (yarrow), 411* Acorn, 385* Acuminate leaf apex, 355* Acute leaf apex, 355* Adiantum, embryo of, 309* Adiantum, gametophyte of, 309* Adiantum (maidenhair fern), 297*, 299*, 300*, 301* Adjustments, summary of, 44 Adjustments of American elm, 43* Adjustments of bean leaves, 28*, 29* Adjustments of caladium, 38* Adjustments of clover, 39*, 40* Adjustments of common plants, 37 Adjustments of corn roots, 32*, 33*, 34* Adjustments of dandelion, 41* Adjustments to environment, 23-45 Adjustments of garden pea, 24* Adjustments of nasturtium, 26* Adjustments of plant body, 14 Adventitious buds, 356* Adventitious roots, 357 ^Eciospores (secidiospores), 274, 275* Aerial roots, 357 Aerobic, 249 Aggregate fruit, 364*, 365, 405* Air spaces of leaf, 115 Alder, stem structure of, 91*, 92*, 93* Alfalfa, roots of, 112 Algae, 219-241 Algae, life histories of, 228, 229 Alismaceae, 428-430 Alnits, stem structure of 92*, 93* Alnus mollis, stem sections of, 91* Alternation of generations, 292*, 311* Amanita muscaria, 267 Ambrosia, 19 American crab, 408* Anaerobic, 249 Anaphase, 75*, 77 Anatomy, 213 Anatropous ovule, 363 Anatropous ovule of Ins, 347* Anemophilous, 361 Angiosperms, 337 Angiosperms, trees of, 380 Animate environment, 7 Annual bean plant, physiology of, 127* Annual bean plant, seasonal history of, 126-130, 129* Annual bean plant, summary, 130 Annual ring, 85* Annulus of fern sporangium, 306* Annulus of mushroom, 268*, 269 Anther, 162* Anther, structure of, 343* Anther and sporophylls, 341* Antheridium, 288, 289 Antheridium of Ricciocarpus, 231* Anthracnose, 282 Antipodals, 164, 165* Antitoxins, 256 Apogeotropic, 20*, 30 Apophototropic, 30 Apotropic, 30 Apple, flower and fruit of, 409 Apple tree, body plan of, 15*, 16* Aquilegia (columbine), 399* Araceae, 425, 426* Archegonium, 289* Archegonium of Ricciocarpus, 289* Arisaema, 426* Ascent of water, 144, 146 Ascent of water, path of, 141* Asci of lichen, 278*, 280 Asdepias, leaf structure of, 113*, 115* Ascophyllum, habit of, 236* Aspergillus, 266* Aspidium, fern, 307* Assimilation, 125 Assimilation of bean plant, 129 Associations, origin of, 436 Associations, plant, 435, 445 447 448 GENEKAL BOTANY Avena saliva, flower of, 433*, 434* Axil of leaf, 17* Axillary branch, 17 . Axillary bud, 356 Axis of bud, 68* Axis of strobili, 339* Azalea, flower of, 359* Bacteria, 250, 258 Bacteria, cell division in, 252* Bacteria, colonies of, 257* Bacteria, forms of, 251* Bacteria, spore formation in, 253* Bacteria, spore germination in, 254* Bark, 84, 85*, 86*, 90, 101* Bark, formation of, 102 Bark in Salvia, 105 Basidia of mushroom, 269*, 270 Bean, growth of, 67* Bean, sensitive roots of, 34, 36 Bean plant, seasonal life of, 126, 127*, 129* Bean pulvini, 15*, 16* Bean roots, sensitiveness of, 36 Bean seedlings, 4 Beer making, 250 Befry, 364*, 365, 421, 422 Bidens (Spanish needles), 413 Biennial plant, seasonal life of ,130,131* Biology, historical sketch of, 212 Biology, plant, 3 Biology, subdivisions of, 216 Bird's-eye maple, 88*, 90, 392 Black cherry, 407* Black raspberry, vegetative reproduc- tion of, 155* Blackberry, flower and fruit of, 405* Blackberry association, 437* Blackberry hybrids, 183 Body plan, summary of, 23* Body plan of apple, 15* Body plan of buckwheat, 17 Body plan of elm, 21*, 22* Body plan of herbaceous plants, 18* Body plan of lilac, 15* Body plan of pine, 20* Body plan of plants, 15*, 16*, 17* Bordered pits, 327, 328 Botany, historical sketch of, 212 Bracken fern, anatomy of, 303*, 304*, 305* Bracket fungi, 272*, 273 Brassica, flowers of, 400* Bread making, 249 Breeding, plant and animal, 216 Brown, Robert, 55 Brussels sprouts, 401 Bryophyta, 287-297 Buckwheat, body plan of, 17* Buckwheat, growth of, 60* Bud, lateral, 15* Bud growth, 68* 70*,71* Bud scales, 68* Bud structure, 68*, 70* Budding, 245 Bud-scale scars, 69* Buds, accessory, 356* Buds, adventitious, 356 Buds, flower, 356 Buds, kinds of, 356 Buds, lateral, 356 Buds, leaf, 356 Buds, mixed, 356 Buds, supernumerary, 356 Buds, terminal, 356 Bulb, hyacinth, 418* Bulb, tulip, 157* Bulbs, 354 Bulbs, culture of, 420 Bulbs, development of, 422* Bundle scars, 84* Burbank and plant breeding, 182, 183 Buttercup, 398* Cabbage, 401 Cacti, ornamental, 148* Caladium, adjustments of, 37, 38* Cattha palustris, 339*, 397* Calyptra of moss, 295* Calyx, 161, 169* Cambium in alder, 92*, 93* Cambium in lilac, 84, 85 Cambium in oak, 86 Cambium of roots, 111 Cambium in Salvia, 106* Cambium layer, 95, 96*, 98 Camptosorus, vegetative reproduction of, 158* Campylotropous ovule, 363 Campylotropous ovule of Capsella, 345 Canal cells of archegonium, 289* Capillitium of puffball, 271* Capsella (shepherd's purse), 344, 345*, 346 Capsule, 295* Capsule (compound ovary), 364*, 365 Capsule of moss, 294, 295* Carduus (thistle), 413* Carpel, 169* Carpels, 363* Carrot, root of, 356* Caryopsis, 365, 434 Catalytic agent, 248 Caulescent, 353 INDEX 449 Celery, cells of, 46* Cell, historical sketch of, 54, 59 Cell, minute structure of, 72* Cell, naming of, 54 Cell, summary of historical sketch of, 59 Cell and plant development, 57* Cell division, 73, 75*, 80 Cell parts, functions of, 51, 52 Cell parts, summary of, 52, 53 Cell plate, 79 Cell sap, 52 Cell theory, 55 Cell wall, 46*, 47*, 48, 54 Cells, parts of, 46, 51 Cells, thick-walled, of celery, 46* Cells with plastids, 50* Cells of root tips, 47* Cellular structure, 45, 53 Centgener plots, 205* Cherry, black, 407* Cherry, flower and fruit of, 409* Chlamydomonas, 222* Chlorophyll, 50 Chloroplastids, 49, 50*, 115 Chromatin, 73 Chromoplastids, 49, 50* Chromosome reduction, 311 Chromosomes, 74, 75*, 76 Chromosomes, reduction and division of, 80 Chromosomes and reproduction, 81*, 160* Cineraria, body plan of, 18* Circinate vernation, 298*, 299 Citranges, 183 Classification, 213 Claytonia, fern, 298* Climate and water supply, 367 Close-fertilization defined, 174 Close-pollination defined, 167, 174 Clover, movements of, 37, 39*, 40 Clover, pollination of, 172, 173 Clover, pulvini of, 39* Clover, seasonal life of, 130, 131* Cohesion theory, 145 Collateral vascular bundle, 301 Collecting hairs of locust, 171 Color of flowers, 362 Columbine, 399* Columella, 262* Commelinaceae, 417 Commercial relations of plants, 12 Complete flower, 360 Compositae, 410*-413 Compound pistils, 363* Concentric vascular bundle, 301* Conceptacles, 237*, 238* Conduction in stems, 99, 100 Cones of long-leaf pine, 379* Cones of spruce, 331*, 332*-, 333*, 374* Conifer ales, 327 Conjugate nucleus, 228, 335 Contrasting characters, 188, 189* Convolvulus, leaf structure of, 149* Coprinus comatus, number of spores in, 268-271 Cork bark, 85*, 86*, 101* Cork cambium, 102 Cork layer, 91, 92 Corm of gladiolus, 157* Corms, 354 Corn, breeding of, 178*, 179*, 193*, 194*, 195* Corn, hybridization of, 178*, 179* Corn, inflorescence and flower, 193*, 433* Corn, plant of, 193* Corn, root system of, 112 Corn, stem structure of, 109* Corn breeding, 193*, 194*, 195* Corn kernels, oil and protein of, 195* Corn roots, growth of, 61* Corolla, 161 Cortex in roots, 110* Cortex in Salvia, 105, 106* Cortex of stems, 92* Corymb, 358* Cotyledon of pea,- 24* Couch grass, 431* Crab apple, American, 408 Crenate leaf margin, 355* Cross-fertilization defined, 174 Crossing and hybridizing, 174 Cross-pollination, 167, 174 Cruciferae, 400*, 401 Curly grain of maple, 88*, 90 Cycad, 321, 322* Cycadales, 321, 325 Cyclic flowers, 358* Cyclic- leaf arrangement, 15*, 16* Cypripedium, 427* Cypripedium acaule, 427*, 428* Cypripedium pubescens, 427* Cytology, 59 Cytoplasm, 47*, 52* Cytoplasmic sac, 47*, 50*, 53 Czapek, experiment with roots, 36 Dandelion, flower and fruit, 412*. Dandelion, habit and flowers, 410* Dandelion, responses to stimuli, 40, 41*, 42 Darwin, Charles, 31, 207* 450 GENERAL BOTANY Darwin, experiments in crossing, 175- 180 Darwin, experiments with roots, 34* Decay, 255 Deciduous trees, 380 Dehiscence of anther, 162* Dehiscent fruits, 364*, 365 Dentate leaf margin, 355* Descriptive terms, 353 DeVries, Hugo, 201* DeVries, mutation theory of, 199 Diageotropic, 20*, 30 Diaphototropic, 30 Diatropic, 30 Dichogamy, 361, 362* Dichotomous, 300 Dichotomous branching, 235 Dichotomous venation, 300 Dicotyledons, flower morphology of, 339* Dicotyledons, herbaceous and woody, 396 Dicotyledons, megasporangia, spores, and embryo of, 349* Dicotyledons, megaspore and embryo of, 345* Dicotyledons, microspore of, 348* Dicotyledons, morphology of, 337, 349 Dicotyledons, sporophylls and sporan- gia, 341* Dicotyledons, stem structure, 105, 108 Dicotyledons, structure of Qalvia stem, 106* Dicotyledons, summary of stem struc- ture, 108 Dicotyledons and monocotyledons, 413, 414*, 415 Diffuse-porous wood, 382* Digestion, 124, 125 Digestion in the bean, 126, 127* Dimorphic flowers, 362* Dicscious, 226, 237 Dioecious moss, 294* Dioecious poplars, 389* Dioecious willows, 388* Dioon edule, cycad, 322* Disease caused by bacteria, 256 Disease caused by fungi, 281-285 Dominance, Mendelian, 184*, 185 Dotted ducts, 94 Double fertilization, 346, 347* Drupe, 364*, 365, 409* . Dry fruits, 364*, 365 Ducts, dotted, 93*, 94 Ducts, primitive, of fern, 305* Ducts, water, 85, 93* Dwarf shoots, 380 East and corn crossing, 178*, 179 Ecology, 146-153, 435-445 Ecology defined, 214 Egg, 165* Egg apparatus, 164 Elaeagnus, hairs of, 149* Elaters, 312*, 313 Elm, flowers of, 395* Elm, slippery, 395 Elm, white, 392*, 393* Elms, 394* Elodea, 49, 152 Elongation zone of root, 63*, 64, 65* Emargiriate leaf apex, 355* Embryo of Capsella, 345* Embryo of mandrake, 165*, 166* Embryo sac, 164, 165* Embryology, 57, 214 Endodermis of fern rhizome, 305* Endodermis of roots, 110* Endoenzyme, 247 Endosperm, 165,* 166 Endothia parasitica, 281 Entomophilous, 361 Environment, 3 Environment, adjustments to, 14, 23 Environment, animate, 7 Environment of animals, 11 Epicotyl of pea, 24 Epidermis of leaf, 113*, 114, 115* Epidermis of root, 110* Epidermis of Salvia, 106* Epidermis of stem, 84, 85*, 91 Epigynous flower of appl"e, 409* Epigynous flower of yarrow, 411* Epigynous flowers, 358*, 359 Equisetales, 311, 315 Equisetum, life history of, 315* Equisetum arvense, 312* Erect tree type, 19, 20* Erosion, 443* Erythronium, 421* Erythronium, bulb development of, 422* Evolution, 211, 215, 218 Exoenzyme, 247 Exoskeleton of stem, 300 Fagopyrum (buckwheat), 17* False whorls, 19, 20* Fermentation, 246 Ferments, digestive, 248 Ferments, energy-forming, 248 Fertilization, 57*, 160 Fertilization, double, 347* Fibers, central spindle, 77 Fibers, phloem, 94* INDEX 451 Fibers, traction, 77 Fibers, wood, 95*, 96 Fibrous roots, 357* Filament of anther, 162 Fiiicales, 298-311 Filicales, anatomy of, 301*, 303*, 304*, 305* Filicales, gametophyte, 308* Filicales, gametophyte and embryo, 309* Flagella, 222* Fleshy fruits, 364*, 365 Floods, forest control of, 368 Floral plans, 361* Flower, morphology of, 339* Flower, parts of, 161, 162*, 163 Flower buds, 356 Flowers, clustered, 357* Flowers, solitary, 357* Follicle, 364*, 365 Food cycle, organic, 11* Food relations of plants and animals, 9 Food storage, 98 Forces of environment, 3 Forest, central hardwood, 383 Forest control of rainfall, 368 Forest products, 370 Forest trees, groups of, 371 Forests, national, 366, 368* Form of herbaceous plants, 18* Form of plant body, 14 Form of trees, 19*, 20*, 21*, 22, 43* Fragaria (strawberry), 403*, 404* Fruit of mandrake, 166* Fruits, classified, 365, 366 Fruits, kinds of, 364*, 366 Fucus vesiculosus, 235, 241 Fucus vesiculosus, habit of, 236* Fucus vesiculosus, life history of, 240, 241 Fucus vesiculosus, reproduction, 237*, 238*, 239*, 240* Funaria, 293-297 Funaria, gametophyte and sex organs, 294* Funaria, sporophyte, 295* Fungi, 242-285 Fungi, classification of, 243 Fungi and disease, 281, 285 Funiculus of ovule, 162*, 165*, 169* Gametangium in Spirogyra, 227 Gametes, 57*, 159, 160*, 163* Gametes, female, 164 Gametes, male, 164* Gametic purity, 187 Gametogenesis, 163 Gametophyte in algas (Spirogyra), 226 Gametophyte in angiosperms, 343*, 348*, 349* Gametophyte in bryophytes, 288, 289* Gametophyte in gymnosperms, 322, 323*, 334*, 335* Gametophyte in pteridophytes, 308*, 309*, 319* Gap, leaf, of ferns, 300*, 301* Gap, leaf, of pines, 327*, 329* Garden pea, Darwin's experiments with, 177 Generative cell of cycad, 323* Generative cell of pollen, 164* Generative cell of spruce, 334* Geotropic, 30 Geum triflorum, 410 Gladiolus, corms of, 157* Gleba of puffball, 271*, 272 Glumes of oat flower, 434 Grain of wood, 87*, 88, 90 Gramineae, 430, 435 Grass, couch, 431* Grasses, 430 Gravity, root response to, 33*, 34*, 36* Gravity sense, 31 Growth, 67, 72 Growth, summary of, 69, 72 Growth of buckwheat, 60 Growth of corn root, 61* Growth of herbaceous stems, 67* Growth of leaves, 66* Growth of lilac bud. 68* Growth of root-tip, 61*, 64, 65*, 67* Growth of roots and stems, 70* Growth of root-tip cells, 63* Guard cells, 113*, 114, 115* Gymnosperms, 321-336 Gymnosperms, Coniferales, 325 Gymnosperms, Cycadales, 321 Gymnosperms, forest trees, 371-380 Habit, 353 Habitat, 353 Hairs, protective, 149* Halberd-shaped leaf, 355* Halophytic association, 435 Hanstein, 57 Hardwood trees, 380, 381*, 382* Head, 358* Heart-shaped leaf, 355* Heartwood, 86 Hepaticae, 28Y-293 Herbaceous plants, body plan and form of, 18* 452 GENERAL BOTANY Herbaceous stem, growth of, 67*, 70* Herbaceous stems, dicotyledons, 105- 108 Herbaceous stems, monocotyledons, 108, 109* Herbaceous stems, structure, 106* Herbaceous stems, summary of struc- ture, 108 Heterosporous, 314, 316, 318* Heterostylous, 362* Histology, 214 Holly, wood of, 382* Homogamous, 362 Homosporous, 313, 316 Hooke, Robert, 54 Hyacinth, plant of, 418* Hybridization of corn, 178*, 179* Hybridization and new varieties, 180 Hybridization of plums, 182* Hybridization of wheat, 180*, 181* Hydrophytes, 152*, 153 Hydrophytic association, 435, 440* Hymenium of mushroom, 269*, 270 Hypha, 258 Hypocotyl of pea, 24* Hypogynous flowers, 358*, 359* Imperfect flowers, 360* Inanimate environment, 3 Inanimate environment, forces of, 3 Inanimate environment, materialsof, 5 Inbreeding and crossing, 175 Inbreeding denned, 174 Income and outgo of animals, 9 Income and outgo of plants, 8 Incomplete flowers, 360* Indehiscent fruits, 365, 368* Industrial biology, 215 Industrial relations of plants, 12 Inflorescence, nature and kinds of, 357*, 358* Inflorescence and pollination, 107, 108 Inorganic foods, 9 Integuments, 162*, 164, 165* Internode, 4*, 15*, 16*, 17* Invasion, ecological, 438*, 439*, 463 Invasion by fungi, 283 Invertase, 248 Ipomcea (sweet potato), vegetative reproduction of, 158* Iridaceae, 423-424* Iris, ovule and pollen tube of, 347* Iris versicolor, 424* Irregular flowers, 360 Jacket cells, 334* Jack-in-the-pulpit, 426* Keel petals, 168, 169* Knight, experiment of, 32* Knight, Thomas Andrew, 31 Labellum of Cypripedium, 428* LadyVslipper, 427* Lamellse of mushroom, 268*, 269* Lanceolate leaf, 355* Lateral bud, 356 Leaf, epidermis of, 113* Leaf, section of, 115* Leaf arrangement, 15*, 16* Leaf blade, 4*, 15*, 17* Leaf gaps, 300*, 301*, 327*, 329* Leaf petiole, 4*, 15*, 17* Leaf scars, 69*, 84* Leaf structure, 113, 116 Leaf trace, 300*, 301*, 302*, 327* 329* Leaves, cyclic and spiral arrange- ments, 16* Leaves, form, venation, margins, and apex, 355* Legume, 171*, 364*, 365 Leguminosce, 39 Lemma of oat flower, 434* Leucoplastids, 49, 50* Lichens, 277, 278*, 279*, 280 Life histories of algae, 228-229 Life history of Fucus, 240, 241 Life history of GEdogonium, 235 Life history of Vaucheria, 233 Lilac, body plan of, 15*, 16* Lilac, bud growth, 68*, 71* Lilac, leaf growth, 66 Lilac, stem structure, 85* Lilac twigs, 69*, 84* Liliaceae, 418, 423 Lily, anther and pollen of, 343* Lily, double fertilization in, 347* Lily, microsporangium and spores, 343* Lily, pond association, 440 Linear leaf, 355* Lip cells of Sporangia, 306* Lobed leaf margin, 355* Locust, inflorescence and flower of, 171* Locust, pollination of, 170, 173 Locust, seasonal life of, 133* Lodicules of oat flower, 434* Lupine, Darwin's experiments with, 177 Lycopodiales, 315, 317 Lycopodium, 316* Maidenhair fern, 297*, 299* Malt, 250 Maltase, 248 INDEX 453 Mains (apple), 408 Mandrake, flower of, 161, 162* Mandrake, fruit of, 166 Mandrake, ovules and seed of, 165* Mandrake, pollen and ovary of, 164* Mandrake, seedlings and fruit of , 166* Map of forest areas, 368* Maple, 390*, 391* Maple, bird's-eye, 392 Maple, curly, 392 Maple, flowers of, 390* Marigold, marsh, 307*, 339* Mass culture, 205 Materials of environment, 5 Mechanism of movements, 28 Megasporangium, 317, 318* Megaspore, 165*, 317, 318*, 319* Megaspores, homology of, 349* Megasporophyll, 317, 318* Megasporophylls, morphology of, 341* Mendel, Gregor, 185 Mendelian ratio, 186 Mendel's laws, 184*, 188*, 189*, 190* Mendel's principles of heredity, 184* Meristem, 61, 62*, 63* Mesophyll of leaf, 114, 115* Mesophytes, 146, 147* Mesophytic association, 435, 436* Mesophytic vegetation, 147* Metaphase, 75*, 77 Metaplasmic bodies, 52 Micropyle, 162*, 164, 165* Microsporangium, 317, 318* Microspore, 317, 318*, 319* Microspores of Selaginella, spruce, and mandrake, 348 Microsporophyll, 317, 318* Microsporophylls, morphology of, 341* Midvein of leaf, 113*, 115* Migration and invasion, 438 Migration of plants, 436 Milkweed, leaf structure of, 113*-116* Mimosa (sensitive plant), 58. Mirabilis, crossing of, 186* Mitosis, 73-80 Mitosis, function of, 73 Mitosis, phases of, 75*-80 Mitosis, process of, 74 Mold, Aspergillus, 266* Mold, black, 259*, 260, 261* Mold, blue, 264*, 265* Molds, 258-267 Monocotyledons, 416 Monocotyledons, structure of, 108, 109* Monocotyledons and dicotyledons, 413, 414*, 415 Monoecious, 360, 385* Morning-glory, Darwin's experiments with, 176 Morphology, 213 Moss, 292* Mosses, 293, 296 Mosses, Funaria, 293, 294*, 295* Motor organs, pulvini, 29*, 39* Mucronate leaf point, 355* Musci, 293, 296 Mushrooms, 267*-271 Mushrooms, Amanita, 267* Mushrooms, spore formation of, 268*, 269* Mustard family, 400 Mutations, 199 Mutations in (Enothera, 200*, 202*, 203* Mycelium, 258 Nageli, 55, 56 Nasturtium, phototropic response of, 26* Neck-canal cells, 289* Nectar of flowers, 362 Node, 4*, 15*, 16*, 17* Nuclear division, 73, 75*, 80 Nucleolus, 47*, 53 Nucleus, 47*, 53, 55 Nursery of yellow pine, 377* Nut, 364*, 365, 385* Nutrition, 117 Nutrition of animal, 9* Nutrition of greerrplant, 8* Nyctitropic (sleep) position, 38, 40 Oak, sections of, 89* Oak, white, 383*, 384*, 385* Oak stem, structure of, 86* Oat, inflorescence and flowers of. 433*, 434* Oblanceolate leaf, 355* Oblong leaf, 355* Obovate leaf, 355* Obtuse leaf apex, 355* Odor of flowers, 362 (Edogonium, 233, 235 (Edogonium, asexual reproduction, 235* (Edogonium, sexual reproduction, 234* (Enothera, seedlings of, 202 (Enothera gigas, 203* (Enothera lamarckiana, 199*, 200*, 203* (Enothera, lata and nanella, 200 Oogonium, 231* Oogonium in Vaucheria, 230 Operculum of moss, 295* 454 GENERAL BOTANY Orbicular leaf, 355* Orchidaceae, 427*, 428 Organic food cycle, 11*, 285* Organic foods, 9 Ornithogalum, 419* Orthotropous ovule, 363 Osmosis, 137, 139* Osmosis, experiment in, 138* Osmotic pressure, 138 Outgo of animal, 9* Outgo of plant, 8* Oval leaf, 355* Ovary, simple and compound, 363* Oviate leaf, 355* Ovules, 164, 165* Ovules, campylotropous, 363 Ovules, kinds of, 363 Ovules, orthotropous, 363 Palet of oat flower, 434* Palisade tissue of leaf, 113*, 115* Palmate venation, 354, 355* Panicle, 358* Panicle of oat, 433* Papilionaceous flowers, pollination of, 168 Pappus of dandelion, 412* Parallel venation, 354, 355 Paraphyses of moss, 293, 294* Paraphyses of mushroom, 269*, 270 Parasites, 242 Parenchyma, palisade, 113*, 115* Parenchyma, phloem, 92*, 93* Parenchyma, spongy, of leaf, 115* Parenchyma, wood, 93*, 94 Parenchyma of cortex, 92 Parmelia, 279* Pasteurization, 255 Path of water, 141* Pathology, plant and animal, 215 Pea, crossing of, 1 78 Pea, flower of, 169* Pea, tropisms of, 24* Peas and Mendel's laws. 184* 188*, 189*, 190* Pedigree culture, 205 Peduncle, 161, 357* Penicillium, 264*, 265* Perennial, seasonal life of, 133* Perfect flowers, 360* Perianth of flower, 161 Pericycle, 111 Pericycle of fern rhizome, 305* Pericycle of roots, 111 Peridium of puffball, 271 Perigynous flowers, 358*, 409* Peristome of moss, 295* Permanent zone of root, 62, 63* Petiole, 15*, 17* Phloem, 84, 85*, 92* Phloem, elements of, 96*, 97 Phloem, fibers of, 92* Phloem, parenchyma of, 92* Photosynthesis, 117-120, 127* Photosynthesis and respiration, 123 Phototropic response, 30 Phototropism of nasturtium, 26* Physcia stellaris, 278* Physiology, 58, 214 Physiology of bean plant, 127* Picea (spruce), forest of, 325, 326* Picea (spruce), gametophytes of, 334* Picea (spruce), ovule and seed of, 335 Picea (spruce), sporangia and spores of, 333 Picea (spruce), strobili of, 331*, 332* Picea (spruce), wood structure of, 327*, 328* 329* Pileus of mushroom, 268*, 269* Finales, 325, 327 Pine, cones of long-leaf, 379* Pine, section of, 378* Pine nursery, 377* Pines, erect type, 19, 20* Pines, forest trees, 378-380 Pinnate venation, 300, 354, 355 Pinnule, 300 Pinus palustris, cones of, 379*, 380 Pistil, 162*, 169* Pistils, compound, 363* Pistils, kinds of, 363* Pistils, simple, 363* Pisum sativum, crossed and self-fer- tilized plants of, 178 Pith, 85 Pits, bordered, 327*, 328* Placenta, 162*, 163 Placentae, kinds of, 363* Plankton, 219 Plant associations, 435, 445 Plant associations, kinds of, 435 Plant associations, origin of, 436 Plant body, plan of, 14 Plant disease, 281 Plantain, vegetative reproduction, 156* Plants, industrial and commercial re- lations of, 12 Plants, relations to animals, 8, 9 Plastids, 49, 50*, 51 Pleurotus ulmarius, 272 Plums, hybridization of, 182* Plumule of pea, 24* Polar nuclei, 164, 165*, 347* Pollen, 163, 164* INDEX 455 Pollen chamber of Zamia, 323* Pollen formation, 343* Pollen germination, 343* Pollen tube, 164*, 323*, 325, 334*, 347* Pollen tube of Iris, 424* Pollination, 166-173 Pollination, kinds of, 167 Pollination and inflorescence, 167 Pollination in locust, 170, 171* Pollination in papilionaceous flowers, 168, 169* Pollination in red clover, 172, 173 Polyporus squamosus, 273* Pome, 364*, 365, 409* Pomelo, 183 Pond lilies, 152* Pond-lily association, 440* Pontederia, 429* Pontederiaceae, 428 Poplars, 389* Populus deltoides, 389 Potamogeton, 152 Potato, vegetative reproduction of, 150*, 156 Potato (sweet), vegetative reproduc- tion of, 158* Primary root, 356 Primrose, Lamarck's, 199* Proembryo, 335 Progeotropic, 20*, 30 Pronuclei, 160, 228 Prop roots, 193*, 357 Prophase, 74, 75* Prophototropic, 30 Protandrons flowers, 362 Protococus, 220, 221* Protogynous, 362, 404* Protonema of moss, 296* Protoplasm, 47* Protoplasm, naming of, 55 Protoplast, 47,* 52 Protropic, 30 Prunus (cherry), 407 Prunus serotina, 407* Pteris aquilina, 302-305 Pteris aquilina, anatomy of, 303*, 304*, 305* Ptomaine poisoning, 258 Ptomaines, 257 Puccinia graminis, 274, 275* Puffballs, 270*, 271*, 272 Pulvini, 28*, 29*, 37, 39* Pulvini of bean leaf, 28*, 29* Pulvini of clover, 39* Ptirkinge, 55 Putrefaction, 255 Pyrenoids of Spirogyra, 224* Pyrus coronaria, 408* Quarter-sawed oak, 87*, 88*, 89* Quercus alba, 383*-386 Quercus alba, flowers and fruit of, 385* Quercus rubra, wood of, 89* Raceme, 357* Rachis of fern leaf, 299* Radish, 401 Rainfall, forest control of, 368 R amentum, 299 Ranunculaceae, 397*, 399 Raphe, 364 Raspberries, 405 Rays, wood, 86*, 87*, 92*, 93*, 94 Receptacle of flower, 161, 339*, 340 Receptacle of Fucus, 237* Red clover, pollination of, 172, 173 Red clover, root system of, 111 Red oak, cuts of, 89* Reduction division, 80*, 81*, 82 Regular flowers, 360 Reproduction, sexual, 159, 160*, 161 Reproduction, vegetative, 154 Resin canals, 326 Respiration, 120-124, 127* Respiration and photosynthesis, 123 Rhizoids of Ricciocarpus, 287, 289* Rhizome, 299,* 353, 354* Rhizophore, 317* Rhizopus, 259*, 240*, 261*, 262*, 263* Ricciocarpus, 288-293 Ricciocarpus, life history of, 291* Ricciocarpus, reproductive organs 'of, 289* Ricciocarpus, sporophyte and spores of, 290* Ring-porous wood, 381* Root, functions of, 112 Root, hairs of, 110* Root cap, 61*-62*, 63* Root hairs and absorption, 140*, 141* Root pressure, 138* Root system of clover, 111* Root system of corn, 112*, 193*, 357* Root tip, cell structure of, 62*, 63* Roots, anatomy of, 110* Roots, arrangement of, 18, 19 Roots, fleshy, fibrous, 357 Roots, growth of, 61*, 63*, 65*, 70* Roots, primary and secondary, 4*, 17*, 193* Roots, prop, of corn, 193*, 357 Roots, sensitive zone of, 34*, 36* Roots, soil, water, aerial, 357 456 GENERAL BOTANY Roots, tap, 4*, 356* Kootstock of Solomon's seal, 354 Rosa (rose), 406 Rosaceae, 403-410 Rose, flower structure of, 405* Rubus (raspberries and blackberries), 405 Runners, 354 Runners of Rhizopus, 261 Runners of strawberry, 155 Rusts, 274-275* Sachs, experiment by, 33* Sachs, Julius von, 31, 59 Sagittaria, 429* Salicaceae, 387-390 Salix discolor, 388 Salma, bark of, 105 Salvia, cortex of, 105 Salma, stem structure of, 105, 106*, 107 Salma, summary of structure of, 108 Salma, vascular cylinder of, 105, 106* Samara, 364*, 365, 390* Saprolegnia, 282 Saprophytes, 242 Sapwood, 86 Sassafras, wood of, 381* Scale, ovuliferous, 332 Scales of bud, 68* Scape of dandelion, 40, 41*, 42 Scars, bud-scale, 69*, 84* Schizomycetes, 250 Schleiden, M. J., 55, 56 Schultze, Max, 56 Schwann, 55 Seasonal life, 125 Seasonal life, summary of, 135 Seasonal life of Adiantum, 300 Seasonal life of annual, 126-130 Seasonal life of bean, 129* Seasonal life of biennial, 130, 131*, 132 Seasonal life of perennial, 133*, 134 Secondary roots, 356* Sedges, 430 Seed, 165*, 166, 169* Seed of mandrake, 165 Seed of mandrake and spruce, 349 Seed of pea, 24* Seed of spruce, 335* Seed production in forest, 373 Seedling of bean, 4*, 129* Seedling of mandrake, 166 Seedling of pea, 24* Seedling of spruce, 335* Selaginella, 317*-320 Selaginella, gametophytes and embryo of, 319* Selaginella, life history of, 320* Selaginella, sporangia and spores of, 318* Selection, plant improvement by, 192 Selection in corn, 194* Selection of fluctuating variations, 208-210 Selection of mutations, 210 Selection in tobacco, 198* Self-fertilization, 174 Self-pollination, 167 Sepals, 161 Serrate leaf margin, 355* Seta of moss, 293, 295* Sexual reproduction, 159 Shasta daisy, 182, 183 Shull, George H., and corn crossing, 179 Shull and seedlings of CEnothera la- marckiana, 202* Sieve plates, 92, 93*, 305* Sieve tubes, 92, 93*, 305* Silique, 363*, 365, 400* Silver grain of wood, 88-90 Simple pistils, 363* Sinuate leaf margin, 355* 'Skeletal tissue, 86*, 301* Skeleton of plants, 49 Smilacina, 420 Smilacina, root structure, 62 Smilacina stellata, 421* Smuts, 276, 277* Soil, composition of, 141* Soil and root hairs, 140*, 141* Solitary flowers, 357* Solute, 137* Solvent, 138 Sori, 274, 275*, 306*, 307* Sorus of fern, 306, 307* Spadix, 426* Spanish needles, 413 Spathe, 426* Spatulate leaf, 355* Spiderwort (Tradescantid), 417* Spike, 358* Spikelet, 433* Spindle, 75*, 76 Spiral, leaf arrangement, 15*, 16* Spiral flowers, 358 Spirogyra, 223-229 Spirogyra, 'fertilization discovered in, 57 Spirogyra, life history of, 229 Spirogyra, reproduction in, 227* Spirogyra, structure of, 224*, 225* Spongy tissue of leaf, 115* Sporangiophore of Equisetum, 312*. 313* INDEX 457 Sporangiophore of molds, 261* Sporangium of fern, 306*, 307* Sporangium of mold, 261*, 262* Spore chambers of puffball, 271 Spore tetrads, 290*, 314 Sporidia of rust, 275* Sporidia of smuts, 277 Sporophore of puffball, 271* Sporophyte in angiosperms, 337 Sporophyte in bryophytes, 290*, 295* Sporophyte in gymnosperms, 321, 325 Sporophyte inpteridophytes, 299, 309*, Spreading tree type, 21*, 22* Spring wood, 85*, 86*, 87* Spruce, 325-337, 326*, 371-378 Spruce, anatomy of, 327*, 328*, 329* 330 Spruce, commercial importance of, 375 Spruce, embryo and seed of, 335* Spruce, gametophytes in, 334*, 335* Spruce, life history of, 336 Spruce, regrowth of, 376*, 378 Spruce, second growth of, 375* Spruce, seed production in, 373, 374* Spruce, strobili of, 331*, 332*, 333* Spruce, tolerance of, 372* Spruce and balsam, 373* Spur shoots of apple, 409 Stalk cell, 323* Stamens, 162* Standard of pea flower, 168, 169* Starch grains, 50* Starch sheath, 224* Stem structure of alder, 91* Stem structure of corn, 109* Stem structure of dicotyledons, 105, 106* Stem structure of lilac, 85* Stem structure of monocotyledons, 108, 109* Stem structure of oak, 86* Stem structure of Salvia, 106* Stems, function of tissues, 98 Stems, functions of, 83 Stems, growth of, 67*, 70*, 71* Stems, kinds of, 353, 354 Stems, multiple, of plantain, 156* Stems, vegetative reproduction of, 154-157 Sterigmata of mushroom, 269*, 270 Sterilization, 254 Stigma, 163, 164*, 169* Stimuli, internal, 29 Stimuli, kinds of, 30 Stimulus, 27 Stipe of mushroom, 268* Stolon, 354 Stolon of Rhizopus, 261* Stoma of fern sporangium, 306* Stomata, 113*, 114, 115* Storage in stems, 98, 99 Strasburger, Eduard, 56, 59 Strawberry, flower and fruit of, 404* Strawberry, vegetative reproduction of, 155* Strawberry plants, 403* Strobili, 313 Structure of stems, roots, leaves, 83, 117 Stylar brush, 412* Style, 163, 169 Subhymenium of mushroom, 269* Succession, 440, 442* Sugar cane, 430* Sugar maples, grove of, 391* Summaries : Relations of plants to environ- ment, 13 Body plan, 23 Adjustments by tropisms, 44 The parts of the cell, 52, 53 The cell and the cell theory, 59 Growth and cell division, 69-72 Structure and physiology of trees, 100-103 Structure of herbaceous stems, 108 Seasonal life of bean, 130 Seasonal life, 135, 136 Variation antt selection, 206-208 Evolution, 211 Biological sciences, 216 Bacteria, 258 Fern anatomy, 305 Spruce anatomy, 330 Angiosperms, 348, 349*, 350 Plant associations, 445 Summer wood, 85*, 86* Supernumerary buds, 356 Suspensor, 319*, 335* Sweet potato, vegetative reproduc- tion in, 158* Sycamore, wood of, 382 Symbiosis of lichen, 280* Synergidse, 164, 347* Tangelo, 183 Tangerine, 183 Taproot, 4*, 17*, 356* Taraxacum (dandelion), 410* Taxonomy, 212 Teliospores (teleutospores), 275*, 276 Telophase, 75*, 78 Terminal bud, 356 458 GENERAL BOTANY Tetrads, 81, 290* Tetrads in lily, 343* Tetrads in Ricciocarpus, 290* Thallophyta, 287 Thallus, 287 Thistle, 413 Timothy, variation in, 204* Tobacco, Connecticut broad-leaf, 197* Tobacco, Uncle Sam Sumatra, 198* Tolerance in forest trees, 372, 373* Tracheae, 93 Tracheids, 304, 327, 328* Tradescantia (spiderwort), 417* Trama, 270 Transpiration, 127*, 142-146 Transpiration, control of, 143 Trees, erect type, 19, 20* Trees, form and development of, 19 Trees, growth and bark formation, 102 Trees, hardwood, 380 Trees, importance and use of, 366 Trees, long and transverse sections of, 101* Trees, longevity of, 103 Trees, physiology of, 104 Trees, poplar, 389* Trees, responses to stimuli, 43* Trees, shrubs, and forests, 366 Trees, spreading type, 21*, 22, 43* Trees, structure and physiology of, 100-105 Trees, sugar maple, 390*, 391* Trees, white elm, 393*-395 Trees, white oak, 383*, 387 Trees, willow, 387*-389 Trees and forests, 366-380 Trifolium pratense, 37, 39* Trifolium pratense, pollination of, 172 173 Trimorphic flowers, 362 Tropisms, 23, 27 Tropophytes, 150 Tube cell, 334* Tube nucleus, 164*, 347* Tubers, 354 Tubers of potato, 156*, 334 Turnip, 401 Twig, lilac, 69*, 84* Ulmaceae, 393-396 Ulmus Americana, 392*, 395* Ulmusfulva, 393, 335* Ulmus racemosa, 393 Umbel, 358* Undulate leaf margin, 355* Urediniospores (uredospores), 274, 275* Ustilago zeae, 276, 277* Vacuoles, 47*, 52 Variation, fluctuating, 197 Variation, summary, 206 Variation in apples, 192* Variation in corn, 194* Variation and selection, 192 Variation in timothy, 204 Variation in tobacco, 197 Vascular cylinder in Salvia, 105, 106* Vascular plants, 299* Vascular system of fern, 301*, 302 Vaucheria, 230-233 Vaucheria, asexual reproduction in, 233* Vaucheria, sexual reproduction in, 230, 231*, 232* Vegetation, instability of, 444 Vegetative reproduction, 154-159 Vegetative reproduction in fern, 158* Vegetative reproduction in gladiolus, 157* Vegetative reproduction by leaves, 157, 158* Vegetative reproduction in plantain, 156* Vegetative reproduction in potato, 156* Vegetative reproduction in raspberry, 155* Vegetative reproduction by roots, 158* Vegetative reproduction by stems, 154, 155*, 156*, 157* Vegetative reproduction in straw- berry, 155* Vegetative reproduction in sweet potato, 158* Vegetative reproduction in tulip, 157* Veil, 268 Veins of leaf, 113*, 115*, 116 Venation of leaves, 355* Venter of archegonium, 289* Ventral-canal cells, 289* Viciafaba, 34 Violapinnata, flower structure of, 402* Violaceae, 401*-403 Violet flowers, 401*, 402* VonMohl, 55, 56 Walking leaf fern, 158 Water, ecological relations of plants to, 146-153 Water, relation of plants to, 137-153 INDEX 459 Water ascent, 144-146 Water ascent, path of, 141* Water ascent, rate of, 145 Water ducts, 93* Wheat, rust of, 274, 275* White oak, 383*~386 White oak, inflorescence and flowers of, 385* White oak timber, 384* White sweet clover, seasonal life of, 130, 131* Willow, 387, 388* Willow, reproduction of, 388* Wing petals, 168, 169* Wood, destruction by fungi, 273* Wood, diffuse-porous, 382* Wood, ring-porous, 381* Wood, sections of, 87*, 88*, 89* Wood, spring and summer, 85*, 86 Wood of alder, 91*, 92*, 93* Wood of lilac, 85* Wood of maple, 88* Wood of oak, 86*, 101* Wood of red oak, 89* Wood cylinder, 84 Wood fibers, 93*, 94 Wood parenchyma, 92*, 93*, 94 Wood rays, 87*, 94 Xerophytes, 148* Xerophytes, leaf structures of, 149* Xerophytic association, 435, 438* Xylem, 92*, 93* Xylem elements, 96*, 97, 98 Yarrow, inflorescence and flowers of, 411*, 412* Yeast, spore formation in, 247 Yeast cells, 245* Yeasts, 234-250 Yeasts, budding of, 245*, 246* Yeasts, top and bottom, 244* Yeasts, wild and cultivated, 244* Yeasts, wine and beer, 244* Zamia, 321-325 Zamia, life history of, 324* Zamia, strobilus, sporangia, and game- tophytes, 323 Zea mays, root system of, 112*, 193* Zea mays, stem structure of, 109* Zonation, 441*, 442*, 444 Zoosporangium of VaucJieria, 233* Zoospore of CEdogonium, 235* Zoospore of Vaucheria, 232, 233* Zoospores of Chlamydomonas, 222 Zoospores of OEdogonium, 235 Zoospores of Vaucheria, 233 Zygote, 57, 159, 160* Zymase, 246 ANNOUNCEMENTS TEXTBOOKS IN BIOLOGY FOR HIGH SCHOOLS AND COLLEGES BOTANY Bergen : Botanies (For list see High-School and College Catalogue) Bergen and Caldwell : Introduction to Botany With or without Key and Flora Bergen and Caldwell : Practical Botany Clute : Agronomy Clute : Laboratory Botany Clute : Laboratory Manual and Notebook in Botany Densmore : General Botany Duggar : Fungous Diseases of Plants Eikenberry : Problems in Botany Frye and Rigg : Laboratory Exercises in Elementary Botany Gruenberg : Elementary Biology Hodge and Dawson : Civic Biology Meier: Herbarium and Plant Description Meier : Plant Study (Revised Edition) Penhallow : Manual of North American Gymnosperms Roth: First Book of Forestry BACTERIOLOGY Conn : Bacteria, Yeasts, and Molds in the Home (Rev. Ed.) Moore : Laboratory Directions for Beginners in Bacteriology Reed : Manual of Bacteriology Russell and Hastings : Experimental Dairy Bacteriology ZOOLOGY Linville and Kelly : Laboratory and Field Work in Zoology Linville and Kelly : Textbook in General Zoology Meier: Animal Study Pratt: Course in Invertebrate Zoology (Rev. Ed.) Pratt : Course in Vertebrate Zoology Sanderson and Jackson : Elementary Entomology PHYSIOLOGY Blaisdell : Life and Health Blaisdell: Practical Physiology Brown : Physiology for the Laboratory Bussey : A Manual of Personal Hygiene Hough and Sedgwick : The Human Mechanism (Rev. Ed.) Jewett : The Next Generation 1463. GINN AND COMPANY PUBLISHERS PRINCIPLES OF RURAL ECONOMICS By THOMAS NIXON CARVER, Harvard University xx + 386 pages " Principles of Rural Economics " takes up such questions as farm management, cooperative buying and selling, and the place of agriculture. in national economy. What is good agriculture in its national significance ; why rural migrations are from densely to sparsely populated areas ; why agriculture is necessarily an industry of small units ; why rural people are more generally self-employed than urban people ; why, and under what conditions, agricultural cooperation is de- sirable and possible these and a number of other questions of tremendous practical importance in rural life are carefully worked out in the text. Chapters are included on the history of modern agriculture, the factors of agricultural production, man- agement, the distribution of the agricultural income, and prob- lems of the rural social life. SELECTED READINGS IN RURAL ECONOMICS Edited by THOMAS NIXON CARVER, Harvard University 974 P a g e s .THIS volume makes available a great deal of excellent sup- plementary material otherwise inconvenient of access. Consid- erable space is devoted to the historical matter necessary for a thorough understanding of the subject, and such questions as agricultural labor, rural marketing, and the farmer's business are fully discussed. The book contains in all forty-two selec- tions by recognized authorities. 165 a GINN AND COMPANY PUBLISHERS BACTERIA, YEASTS, AND MOLDS IN THE HOME (SECOND REVISED EDITION) By H. W. CONN, late Professor of Biology in Wesleyan University I2mo, cloth, 293 pages, illustrated THE book contains an important summary of the facts which have rapidly accumulated in recent years concerning the relation of microorganisms to all matters connected with the home. The work is a popular and not a scientific discussion, free from many technical terms, and admirably adapted to the needs of the housewife, the student of domestic science, and all others interested in home economics. Molds, which are the cause of mildew, the spoiling of many foods, and the decay of fruits ; yeasts, which are the foundation of fermentation in the raising of bread ; and bacteria, which cause food to spoil, meat to decay, and contagious diseases to spread, all these phenomena which are of the most vital- importance are presented in an interesting and helpful manner. The author explains the various actions of bacteria, and points out the sources of trouble and the principles which underlie the methods to be adopted for avoiding their effects. Special attention is paid to the problems of food preservation and to the practical methods which can be used in the home for preventing the distribution of contagious diseases. To render the work more useful for classes in domestic science there is added an appendix containing directions for a series of simple experiments which will give to the student a practical knowledge of the most important properties of microorganisms. 167 GINN AND COMPANY PUBLISHERS COUNTRY LIFE EDUCATION SERIES A SERIES of practical texts for the student and the professional farmer, written by experts in their respective lines. These books aim to give a thorough exposition of both the theory and the practice of the various branches of farming and breeding. TYPES AND BREEDS OF FARM ANIMALS (Revised) By CHARLES S. PLUMB, Professor of Animal Husbandry in the College of Agriculture, Ohio State University. 820 pages, illustrated PRINCIPLES OF BREEDING By EUGENE DAVENPORT, Dean of the College of Agriculture, Director of the Agricultural Experiment Station, and Professor of Thremmatology in the University of Illinois. 727 pages, illustrated FUNGOUS DISEASES OF PLANTS By BENJAMIN MINGE DUGGAR, Professor of Plant Physiology, Washington University, St. Louis. 508 pages, illustrated SOIL FERTILITY AND PERMANENT AGRICULTURE By CYRIL GEORGE HOPKINS, late Professor of Agronomy in the University of Illinois, xxiii +653 pages PRINCIPLES AND PRACTICE OF POULTRY CULTURE By JOHN H. ROBINSON, xvi + 611 pages, illustrated GARDEN FARMING By LEE CLEVELAND CORBETT, Horticulturist in the Bureau of Plant In- dustry, United States Department of Agriculture. 462 pages, illustrated THE APPLE By ALBERT E. WILKINSON, formerly of the Department of Horticulture in Cornell University. 492 pages, illustrated EQUIPMENT FOR THE FARM AND THE FARMSTEAD AMSOWER, Director of Agricultui :rsity. 523 pages, illustrated Other volumes in preparation By HARRY C. RAMSOWER, Director of Agricultural Extension Service, Ohio State University. 523 pages, illustrated fti GINN AND COMPANY PUBLISHERS THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO 5O CENTS ON THE FOURTH DAY AND TO $1.OO ON THE SEVENTH DAY OCT 26 1932 MAR 9 1933 26 1939 OCT H 1934 (JCT271940 WAR 6 - 1970 4 i i". NOV 30 DEC 28 1936 SEP 2 9 193 NOV 23 1938 U.C. BERKELEY LIBRARIES / 498431 UNIVERSITY OF CALIFORNIA LIBRARY