A TEXT-BOOK 
 
 OF 
 
 PLANT PHYSIOLOGY 
 
 BY 
 
 GEORGE JAMES PEIRCE, Ph.D. 
 
 Associate Professor of Plant Physiology 
 Leland Stanford Junior University 
 
 NEW YORK 
 
 HENRY HOLT AND COMPANY 
 1903 
 
Copyright, 1903 
 
 BY 
 HENRY HOLT & Co. 
 
~ o- THE 
 UNIVERSITY 
 
 ^ 
 
 PREFACE 
 
 The course in Plant Physiology which I have given for the 
 last five years at Stanford University has impressed upon 
 me the need of a text-book which treats the subject less ex- 
 haustively than Pfeffer's Handbuch and more fully than 
 Noll's section of the Bonn text-book.* As I did not know 
 of such a book in any language, I began to write my lec- 
 tures. These lectures, after repeated working over, have 
 finally taken this definite form. 
 
 My intention has been to present the main facts of plant- 
 physiology and the saner hypotheses regarding them, striv- 
 ing to express safe views rather than to echo the most re- 
 cent, attempting here and there to suggest definite problems 
 for investigation, and everywhere trying to avoid giving the 
 impression that the science or any part of it has reached 
 ultimate knowledge and final conclusions. 
 
 I have purposely made no attempt to give directions for 
 experiments, believing that a laboratory manual and a text- 
 book must meet such different needs that the style of the 
 one is impossible for the other. . This book must, however, 
 be supplemented by actual laboratory work by the reader, 
 under the guidance of a teacher or of some of the laboratory 
 manuals mentioned on page 27, or both. 
 
 No one can work on the physiology of plants nowadays 
 without being conscious of his indebtedness to Pfeffer. If 
 Pfeffer had done nothing else, the preparation of the two 
 editions of his Handbuch, in which the literature of the sub- 
 ject is brought together, and the present status of the sci- 
 ence is well set forth, would secure him an honored place. I 
 cannot refrain from acknowledging my personal as well as 
 
 *Lehrbuch der Botanik. Strasburger, Noll, Schenck, Schimper. Five 
 editions. English translation by Porter. 
 
iv PREFACE 
 
 professional obligation to him, and I do so with the utmost 
 satisfaction. 
 
 It is also a great pleasure to express my grateful apprecia- 
 tion of the help I have received from my associates in this 
 University, Dr. Anstruther A. Lawson (Botany), Professors 
 Herman De C. Stearns (Physics), Frank M. McFarland 
 (Histology), William A. Cooper (German), and especially 
 from Professor Douglas H. Campbell. I am indebted also 
 in various ways to my friends Professor William F. Ganong 
 of Smith College, and Dr. Hermann von Schrenck of Wash- 
 ington University, St. Louis. Professor Walter R. Shaw of 
 the University of Oklahoma generously allowed me to use 
 his excellent photograph of Postelsias reproduced on page 
 190. Dr. Daniel T. MacDougal of the New York Botanical 
 Garden and his publishers, Messrs. Longmans, Green & Co., 
 have given me permission to use the figures of Mimosa on 
 pages 248-9. Professor Goebel of Munich kindly sanctions 
 my use of his figure of Opuntia shown on page 210. To these 
 and all others who have assisted me I wish most heartily to 
 express my sincerest thanks. 
 
 G. J. P. 
 
 Leland Stanford Junior University, 
 California, December, 1902. 
 
CONTENTS. 
 
 CHAPTER I. INTRODUCTION. 
 
 Introduction 1 
 
 The Conditions Essential to Life 6 
 
 The Living Matter and the Actively Living Structure 7 
 
 CHAPTER II. RESPIRATION. 
 
 Respiration 12 
 
 Intramolecular Respiration 27 
 
 Fermentations 30 
 
 CHAPTER III. NUTRITION. 
 
 Nutrition 40 
 
 The Food-materials 42 
 
 Carbon 43 
 
 Chlorophyll 51 
 
 Photosynthesis 58 
 
 Nitrogen 66 
 
 Root-tubercle Plants 72 
 
 Humus Plants 78 
 
 Carnivorous Plants 81 
 
 Parasites 85 
 
 Ash-constituents 92 
 
 CHAPTER IV. ABSORPTION AND MOVEMENT OP WATER. FOOD DIS- 
 TRIBUTION. 
 
 Absorption ' . . . 103 
 
 Diffusion and Osmosis 108 
 
 Means of Absorption of Nutrient Solutions 113 
 
 Means of Transfer of Nutrient Solutions 116 
 
 Secretion 125 
 
 Sap-pressure and Bleeding 130 
 
 Transpiration 136 
 
 Stomata and the Aerating System 142 
 
 Gases and Movements of Gases 151 
 
 Translocation of Foods 155 
 
 CHAPTER V. GROWTH. 
 
 Growth 162 
 
 Periodicity of Growth 169 
 
 Relations of Growth and Turgor 172 
 
 Growth Pressures 174 
 
 Rate of Growth 176 
 
 Limit of Growth 179 
 
 V 
 
VI CONTENTS. 
 
 PAGE 
 
 CHAPTER VI. IRRITABILITY. 
 
 Irritability 183 
 
 Physical Basis of Irritability 184 
 
 Irritability and the Amount and Kind of Growth 186 
 
 Influence of Gravitation 196 
 
 Influence of Light 208 
 
 Influence of Heat 219 
 
 Influence of Water 222 
 
 Influence of Other Substances 226 
 
 Influence of Electricity 237 
 
 Influence of Contact 239 
 
 Conclusion 251 
 
 CHAPTER VII. REPRODUCTION. 
 
 Reproduction. 254 
 
 Heredity 279 
 
 INDEX.. . 285 
 
f 
 
 
 PLANT PHYSIOLOGY 
 
 CHAPTER I 
 INTRODUCTION 
 
 ALL living beings are alike in kind, differing from one an- 
 other only in degree. To this conclusion scientific men have 
 been gradually driven back by the failure of every attempt 
 to discover and to define any fundamental difference between 
 animals and plants. The differences between the higher rep- 
 resentatives of the two so-called kingdoms animal and veg- 
 etable are so striking that no one can fail to see them. 
 Between the higher and the lower animal forms are differ- 
 ences as striking as those between higher animals and higher 
 plants. Between the higher and lower plant forms there are 
 similar differences. Likenesses always accompany these dif- 
 ferences. It is these likenesses which enable us to call a 
 given organism an animal or a plant. Between the higher 
 and lower members of the same kingdom, and between the 
 higher members of the two kingdoms, the differences are so 
 striking that the attention is almost wholly occupied with 
 them. It is natural that we should look more for differences 
 than for likenesses : for our ability to distinguish our friends 
 from our enemies, our own from our neighbor's possessions, 
 the Bird-foot Violet from the Swamp Violet, the dog from 
 the tree, is dependent upon our perception of differences 
 between them. It is necessary that we see differences : the 
 more intelligent the man, the keener is his perception of 
 differences; the higher the organism, the greater is its 
 difference from others. 
 
 The study of low organisms reveals few differences. Suc- 
 cessfully to study such low organisms as the bacteria de- 
 
2 PLANT PHYSIOLOGY 
 
 mands unusual ability to detect the differences between 
 them. The differences disappear as we descend the scale of 
 development, and the likenesses become more and more evi- 
 dent. The bacteria and diatoms, which have been repeat- 
 edly regarded first as animals, then as plants, and the 
 Myxomycetes (slime-moulds), which are still believed by 
 some to be animals ( Mycetozoa ) , illustrate the difficulty 
 of determining to which " kingdom" these organisms belong. 
 It is important to the students of systematic botany and 
 zoology to know where to place these organisms in their 
 systems of classification. This enables other naturalists to 
 go further. To the physiologist it is a convenience rather 
 than an essential to know whether the organism which he 
 is studying is called an animal or a plant, but the results 
 of his work have often been useful to the systematists in 
 confirming or correcting their classifications. From the 
 classifier's point of view the differences which enable him 
 to make an orderly arrangement of the objects of his study 
 are of the utmost importance; from the physiologist's 
 standpoint the likenesses are most important. This will be- 
 come plainer when we state the aim of physiology. 
 
 THE AIM OF PHYSIOLOGY 
 
 According to Pfeffer,* "the aim of physiology is to study 
 the nature of all vital phenomena in such a manner that, 
 by referring them to their immediate causes, and subse- 
 quently tracing them to their ultimate origin, we may ar- 
 rive at a complete knowledge of their importance in the life 
 of the organism." Physiology is a study not merely of 
 structure, though to its successful pursuit a knowledge of 
 structure is indispensable ; nor of organized bodies, though 
 a knowledge of the laws which govern their organization 
 ( structure and form ) is important. It is the study of the 
 living organism. Crystals have definite and characteristic 
 structures and forms, they increase in size and in number in 
 accordance with a few laws common to all and a few laws 
 
 * Handbuch der Pflanzenphysiologie, 2te Auflage, Bd. I., p. 7, 1897. 
 English translation by Ewart, Physiology of Plants, vol. I., p. 8, 1900. 
 
INTRODUCTION 3 
 
 peculiar to each kind. The carbon compounds have definite 
 structures, as Kekule's demonstration of the benzole-ring 
 proved; they form under certain conditions and their be- 
 havior is characteristic of the kind. A machine has defi- 
 nite structure, it operates in a fashion characteristic of its 
 kind. Physiology has long been conceived to be the study 
 of the structure and operations of peculiar machines, the 
 study of functions as based upon a knowledge of anatomy. 
 The physiologist is now striving not only to know the func- 
 tions which are the manifestations of the life possessed by 
 complicated living structures or organisms, but also to 
 determine the causes both of structure and of functions. In 
 an engine we have a structure which, under certain know- 
 able conditions, does certain kinds of work. The materials 
 and the construction of the engine are lifeless; the engine 
 is the result of human intelligence acting in harmony 
 with physical laws upon lifeless material; the working of 
 the engine is the result of energy (physical force) applied 
 through human intelligence to it ; the structure itself and its 
 working are the result of physical forces acting upon inert 
 materials in harmony with physical laws comprehended by 
 the designer and driver of the engine. These are all exter- 
 nal to the structure and are consciously taken advantage of 
 and applied to and through the engine by the living organ- 
 isms concerned in its construction and operation. 
 
 A living organism is a structure existing in harmony with 
 physical forces and laws and because of them. Few living 
 organisms strive to ascertain, and none fully knows, what 
 these forces are. The materials employed in the construc- 
 tion of a living organism are inert, lifeless; they are ar- 
 ranged in harmony with, and through the operation of, 
 physical laws and forces ; but the inert materials are acted 
 upon and the physical forces employed by the organism 
 itself, and for the most part unconsciously. 
 
 Between the engine and the living organism there is then 
 a radical difference; the engine and the organism, though 
 both machines, differ in that life, external though applied 
 to the one, is internal and possessed by the other. The 
 difference thus stated is one plainly felt but inadequately 
 
4 PLANT PHYSIOLOGY 
 
 expressed, for who can tell what life is? Just as we say 
 that walking consists in the rapid restoration of the body 
 to a position of equilibrium after falling, so we may say that 
 living consists in the maintenance of the equilibrium between 
 constructive and destructive influences and processes. But 
 living is the evidence of life, just as thinking is the evidence 
 of brain ; living is not life itself. However, in studying the 
 means by which the equilibrium between constructive and 
 destructive influences is maintained the means by which 
 living is attained we are approaching the eternal question : 
 What is life itself? 
 
 In determining that the means of maintaining the equi- 
 librium between constructive and destructive influences are 
 physical and chemical, and that the influences themselves 
 are physical and chemical, not peculiar " vital" forces, not 
 occult or supernatural, one question regarding life is an- 
 swered. Whatever may be our views regarding the origin of 
 life, there is no scientific or other heresy in accepting the 
 present evidence that life maintains itself, and is maintained, 
 by physical and chemical means only. In the following 
 pages these means will be examined and discussed. 
 
 Pfeffer's statement, quoted above, of the aim of physiology 
 does not limit the study to the manifestations of life in 
 either "kingdom" of organisms. One evidence of the wis- 
 dom of such a broad view is the value which Pfeffer's own 
 investigations, conducted mainly on plants particularly 
 perhaps those on osmosis and on irritability possess for 
 the animal physiologist. When physiologists, not satisfied 
 with the observation and description of the phenomena dis- 
 played by the different kinds of living organisms, began to 
 seek for the means by which these phenomena are executed 
 and for their causes, comparison revealed that the causes, 
 the means, and even the phenomena themselves, are alike 
 in all organisms. Physiology is now, therefore, a much 
 broader as well as deeper science than it was formerly con- 
 ceived to be, and though there are now and always must be 
 animal and plant physiologists, they are studying common 
 problems and contributing to the common mass of human 
 knowledge of living organisms. 
 
INTRODUCTION 5 
 
 It is often a matter of chance or of convenience which 
 determines whether a man shall study one organism or an- 
 other. Sachs,* who may be called the founder of modern 
 plant-physiology, has said that although plant-physiology 
 owes much to animal-physiology, yet animal-physiology is 
 being enriched by the results attained by plant-physiolo- 
 gists. The appreciation of the fundamental unity of the 
 aims and the results of animal and plant physiologists has 
 recently led to the publication of two books on general 
 physiology, f In such books the manifestations of life are 
 described and discussed in a broad way; for details one 
 must turn to the special treatises on the physiology of 
 animals and plants. 
 
 Though the phenomena of life are the same in all organ- 
 isms, life manifests itself in special ways in different or- 
 ganisms, making some more favorable for the study of 
 special phenomena than others. For example, the manu- 
 facture of starch from carbon-dioxide and water, and the 
 formation of tannin and certain other by-products in nutri- 
 tion, are subjects in plant-physiology only, just as other 
 special functions, such as those of nerves and muscles, car- 
 ried out by extremely differentiated organs, and the circu- 
 lation in vessels, are subjects in animal-physiology only. 
 Again, it is more convenient to study on low, small, aquatic 
 plants and animals some of the effects of light than on 
 higher terrestrial organisms, but the comparison of higher 
 and more complex forms with the lower shows that the 
 effects are identical in kind if not in degree. For this rea- 
 son, and because no "one man can know all the parts of the 
 whole subject of physiology equally well, it must still be 
 divided. While the following pages will be devoted mainly 
 to the study of plants, the reader should bear constantly in 
 mind that, at the same time that there are special vital 
 
 * Sachs, J. von. Lectures on the physiology of plants, English transla- 
 tion, by H. M. Ward, p. 650. Oxford, 1887. 
 
 t Verworn, M. Allgemeine Physiologic, two editions. English transla- 
 tion, by Lee. General Physiology, New York, 1899. Davenport, C. B. 
 Experimental. Morphology. Parts I. and II., New York, 1897, 1899. Others 
 to follow. 
 
6 PLANT PHYSIOLOGY 
 
 activities special manifestations of life according to species 
 and individuals as well as "kingdoms/' there are also 
 general vital activities common to all living things. Before 
 passing to any consideration of these in detail, it will be 
 well to enumerate the conditions essential to life, to con- 
 sider the material and the structure in which life is resident 
 and which manifest it, and to realize that these conditions, 
 this material and structure, are the same for all living* 
 things. 
 
 I. THE CONDITIONS ESSENTIAL TO LIFE 
 
 These may be comprehended under five headings: 
 
 1. Proper Food (a) the source of the materials of which 
 
 the body is built, and 
 
 (b) of the energy by which the body is built and 
 operated. 
 
 2. Water (a) the vehicle of the food-materials and of the 
 
 foods absorbed into the body and transferred from 
 part to part, and also 
 
 (b) an indispensable component of actively living 
 protoplasm. 
 
 3. Proper Temperature which makes possible the vital, i.e. 
 
 the chemical and physical, changes which must go 
 on within the body, and in all of its parts, lest in- 
 action and death ensue. 
 
 4. Proper Illumination which furnishes the organism with 
 
 the forms of energy physical and chemical thermal, 
 luminous, and actinic of which it is directly or in- 
 directly in need. 
 
 5. Proper Freedom freedom from mechanical and other 
 
 disturbances which would interfere with its supply of 
 food, water, warmth, and light, and prevent it from 
 carrying on its natural functions. 
 (To this list some might wish to add Oxygen, but 
 this is included under the first heading. ) 
 
 The many forces and matters included in this brief sum- 
 mary are not merely passively essential to life ; they actively 
 
INTRODUCTION 7 
 
 stimulate all living organisms. The organisms are sensitive, 
 and respond to these stimuli. Supplying food to animal or 
 plant is applying a stimulus, as well as providing the means 
 to further and continued action. The response to the stimu- 
 lus may vary with the organism, be immediate or delayed, 
 be external and visible to the eye, or merely internal; but 
 every organism lives, both because the conditions make liv- 
 ing possible, and also in accordance with and in response to 
 the many and diverse stimuli exerted upon it by these con- 
 ditions. The different reactions of different organisms to 
 the same stimulus do not imply any special or peculiar 
 vital force ; on the contrary, they imply the special or pecul- 
 iar structure of these different organisms. The force of 
 gravitation pulls some things down, but it is the same 
 force which keeps other things up. The Eiffel Tower, con- 
 structed in opposition to the force of gravitation, stands 
 now because of it. So all living organisms, subjected to 
 like forces and supplied with like materials, behave accord- 
 ing to the characteristic habits of each species and the 
 peculiar habits of each individual. 
 
 II. THE LIVING MATTER AND THE ACTIVELY LIVING 
 STRUCTURE 
 
 As Hertwig has so strongly emphasized,* the living and 
 active protoplasm is to be regarded not as a chemical 
 compound or an association of chemical compounds, but 
 rather as an orderly arrangement of these into a definite 
 structure, of which water is an indispensable constituent. 
 Some of the water contained within the cell should be con- 
 sidered to be as much a constructive constituent of the liv- 
 ing protoplast as the water is of the crystal of copper 
 sulphate. As, without a certain amount of water, one can 
 never have crystals, no matter how much copper sulphate 
 may be present, so also, without the necessary amount of 
 water we can never have active protoplasm. When the 
 water of constitution is withdrawn, all the activities of the 
 
 * Hertwig, Oscar. Die Zelle und die Gewebe, Bd. I., p. 15. The Cell 
 translation by Campbell, vol. I. 
 
8 PLANT PHYSIOLOGY 
 
 cell cease with the demolition of its structure. Dehydration 
 is fatal to nearly all plants and their parts, rapid dehydra- 
 tion is fatal to all; but ripe seeds, in which all the vital 
 activities are greatly reduced as the embryo attains the 
 stage of development characteristic of the species, can with- 
 out injury slowly give up nearly all the water which they 
 contain. By this means the protoplasmic structure, the 
 arrangement of the protoplasmic particles, necessary to 
 active life, is sufficiently altered to cause an entire suspen- 
 sion of all activities. In a climate of average humidity 
 water will still remain in air-dry and wholly dormant 
 seeds, as may be shown by weighing them air-dry and re- 
 weighing after they have been dried for an hour in an oven 
 at a temperature of 70 C., or for a longer time at room 
 temperature in a desiccator. The following table, quoted 
 from Schroder,* shows the percentage cf water contained 
 in ripe and air-dry grass " seeds," thus 
 
 Hordeum vulgare, 14.65% 
 Triticum durum, 14.63% 
 Triticum spelta, 14.40% 
 
 This amount, though considerable, is less than the con- 
 stitutional water of the active protoplasmic structure, and 
 hence all vital activity is extremely slight. With the restora- 
 tion of the water of constitution, and after the lapse of a 
 part if not all of the usual "resting period' 7 of the seeds, 
 the vital functions will be resumed. 
 
 The spores of many of the lower plants are also able to 
 bear the withdrawal of the water of constitution without 
 permanent injury, and like seeds, they can withstand during 
 this period of inaction degrees of heat and cold, amounts 
 of poisonous gases, and other influences which at other 
 times would be fatal to them. The dry seeds of peas and 
 beans, the grains, etc., and the dry spores of various fungi, 
 can be subjected for hours to a temperature of about 100 
 C. without destroying their germinating power, although 
 about half that temperature would in fifteen minutes be 
 
 * Untersuchungen aus dem Botan. Institut zu Tubingen, Bd. II., p. 10, 
 
 1886. 
 
INTRODUCTION 9 
 
 fatal to them after germination. The active cells of the 
 great majority of bacteria will be killed in ten minutes 
 by a temperature of 50 to 60 C., or in five minutes by a 
 temperature of 70 C., although the dry spores of Bacillus 
 anthracis succumb only after heating for three hours at 
 140 C.* 
 
 During the period of nearly complete dryness seeds and 
 spores are still alive, but the evidences of life are extremely 
 difficult to detect. Respiration goes on very feebly indeed 
 in air-dry seeds, yet that these seeds do respire is claimed 
 by Kolkwitz f as the result of refined methods of collecting 
 and measuring small quantities of carbon-dioxide; but 
 seeds containing still less water respire even less, and with 
 no constitutional water all respiration ceases. 
 
 Since water is in itself lifeless, its presence or absence is 
 merely a condition which makes life possible or the reverse. 
 Life may be resident in some other of the chemical com- 
 pounds composing protoplasm, but it will manifest itself 
 only when w r ater is present in sufficient amount. Although 
 the ability to form crystals of the characteristic size, form, 
 and color resides in molecules of copper sulphate and not in 
 molecules of water, no copper sulphate crystals will form 
 until CuS0 4 molecules are accompanied by a sufficient num- 
 ber of water molecules to make the crystalline structure a 
 physical possibility. Another comparison, if not pressed 
 too far, may also assist in emphasizing and elucidating 
 this matter. The living protoplasm deprived of water may 
 be likened to the disconnected parts of a machine; it may 
 be heated or chilled or subjected to other kinds of harsh 
 treatment without greatly, if at all, affecting it; but set 
 up the machine, furnish it with energy, and it will work- 
 give an abundance of water to the protoplasm so that 
 it may set itself up into a machine, allow it to furnish 
 itself with energy, and it will work. But though water 
 is indispensable to the carrying on of the vital func- 
 
 * Fischer. Vorlesungen uber Bakterein, Jena, -1897, p. 72. English trans, 
 by Jones, Structure and functions of bacteria. Oxford, 1900, p. 76. 
 
 t Kolkwitz, R. f'ber die Athmung ruhender Samen. Ber. d. D. Bot* 
 Ges., Bd. XIX., 1901. 
 
10 PLANT PHYSIOLOGY 
 
 tions, it is not in all cases essential to the preservation 
 of life. 
 
 The vital functions may be suspended, without deterio- 
 ration of the germinating power of the resting body, for 
 periods varying with the species and in some instances 
 astonishingly long. As a matter of safety, the dry seed or 
 spore should regularly retain its vitality at least for some- 
 what longer than the usual period during which, by reason 
 of heat, or cold, or drought, active life is impossible. Some 
 seeds may retain their germinating power for much longer 
 times, during which all activities are practically suspended. 
 The majority of authentic cases of suspended activity indi- 
 cate that beyond thirty years exceedingly few seeds remain 
 alive. It is probable that during these periods of appar- 
 ently suspended activity, some of the vital processes still go 
 on, so slowly as to be unnoticeable, but resulting ultimately 
 in the consumption or destruction of essential constituents 
 of actively living protoplasm. Seeds which have lain dor- 
 mant too long have, therefore, lost essential constituents; 
 their actively living protoplasm cannot be reconstructed 
 when water is added ; they have lost their power of germi- 
 nation. 
 
 The survival of successive periods of drought by certain 
 species of mosses, liverworts, lichens, algae, and bacteria in 
 their vegetative instead of spore forms is even more remark- 
 able. The spores and seeds which survive periods of inac- 
 tion have at all times few functions to perform. The vege- 
 tative forms of mosses, liverworts, lichens, algse, and bac- 
 teria have to perform, or to prepare for, all the functions 
 of these organisms. It is all the more remarkable, there- 
 fore, when they are regularly able to survive, though all 
 their vital functions may be suspended. This implies a con- 
 siderably greater power of endurance than is possessed by 
 the vegetative parts of more highly organized plants. Yet 
 in California, and in other parts of the world, where there 
 are several months in each year when no rain falls, though 
 in other months it falls abundantly, plants growing in un- 
 cultivated places must be able to adjust themselves to the 
 periods of enforced inaction. So far as our Pacific Coast 
 
INTRODUCTION 11 
 
 plants are concerned, this is, however, an uninvestigated 
 subject, one which offers many attractions to the physiolo- 
 gist.* 
 
 * In the year book of the U. S. Dept. of Agriculture, 1897, Whitney re- 
 ports (p. 129) a comparison of the soils and subsoils of the eastern and 
 western States. From this he draws the conclusion that the absence of a 
 heavy subsoil, and the spontaneous formation of a "mulch" of dust on 
 the surface, cause the plants of arid regions, and those living where there 
 is a long dry season, to secure for their needs a much larger percentage of 
 the total annual rainfall than is available for eastern plants. In the East, 
 the heavy subsoil drains off about half of the total rainfall, and the re- 
 mainder is still further reduced by evaporation. But though all this is un- 
 doubtedly true, yet the perennial plants themselves must also show adap- 
 tations to the climatic conditions of the far West and of California, and 
 these deserve study by physiologists. 
 
CHAPTER II 
 
 RESPIRATION 
 
 ENERGY is necessary to the operation of every machine, 
 whether it be an engine or a living organism. The engine 
 is supplied with energy directly or indirectly through the 
 intelligent action of a living organism ; the living organism 
 supplies itself with energy. In both cases the energy is ordi- 
 narily supplied by the same means, mainlyby 
 
 _ 
 or oxidation, and the energy thus liberated is applied either 
 
 v ~r}ffectly~orindirectly in the same form. In the case of the 
 steam-engine the energy or power is applied indirectly, the 
 heat resulting from the combustion of fuel being utilized in 
 the conversion of inelastic water into perfectly elastic, com- 
 pressed, and hence active, steam. In the living organism 
 the energy resulting from combustion is applied directly in 
 the various kinds of work done by the living organism 
 either within or outside of its own body. 
 
 Energy must be supplied to the engine for two purposes : 
 first, to overcome the resistance (friction, inertia, etc.) of 
 its own parts; and second, to enable it to overcome the 
 resistance offered by the materials upon and in which it 
 works. ? Qr ; two^^urpQSs, also energy must be supplied by 
 
 /the living organism for its own use : first, to continue liv- 
 ing; and second, to enable it to overcome the resistance 
 
 Xoffered by the materials upon and in which it works. Let 
 us think first of the need of energy to continue living. Liv- 
 ing consists in maintaining the equilibrium between con- 
 structive and destructive influences. This implies work, in 
 the physicist's sense. So long as the constructive influences 
 overbalance the destructive the organism gains in some 
 way grows, or increases in weight, or moves, or reproduces. 
 To gain thus, to do any of these things, the organism 
 needs energy. When the destructive influences overbalance 
 
RESPIRATION 13 
 
 the constructive the organism loses in some way it ceases 
 to grow, or decreases in weight, or moves less, or fails to 
 reproduce. The destructive influences result in the libera- 
 tion of energy, the constructive in the storing of energy/ 
 The liberation of energy implies previous construction, for 
 the generation of energy from nothing is inconceivable. 
 The chief source of energy in the organism is combustion, 
 the destruction by oxidation of not merely already exist- 
 ing, but really of previously formed substances. The direct 
 result of oxidation, is the evolution of heat. \^ 
 - To the oxidation which goes on within the living organ- 
 ism is given the name RESPIRATION. The interchange of 
 gases between the blood of higher animals and the air, 
 which takes place at the lungs or gills, is but a part of the 
 process of respiration. The oxygen taken up by the blooc 
 at those surfaces exposed to the air is transferred by the 
 circulation of the blood to the tissues and cells by which it 
 is used. The oxygen is used by combining it with other 
 substances/ by oxidizing these substances. Respiration is 
 physiological oxidation, or combustion, as distinguishec 
 from such oxidations as take place under ordinary condi- 
 tions spontaneously. The element sodium, when exposed to 
 the air, will unite with the oxygen without heat, light, or 
 any other known form of energy being applied to encourage 
 the union. Animal and vegetable substances will not so 
 unite, the oxidation must be encouraged, and it will take 
 place outside the living body only at a temperature con- 
 siderably higher than that developed within the body of any 
 living organism. In respiration we have, then, a process 
 which differs from ordinary oxidation in the conditions 
 under which it takes place. The results, however, are the 
 same. ^In respiration we have to do not only with the 
 affinity of oxygen for certain elements and compounds, but 
 with the need of the living organism_fQiL_enegy. The or- 
 ganism must be actively living in order that the oxygen of 
 the air may be made to unite with those substances in the 
 body from which energy is to be liberated. Respiration may 
 be artificially continued, with the liberation of energy, only 
 for a short time after the death of the organism as a 
 
14 PLANT PHYSIOLOGY 
 
 whole, only until the death of the cells composing the body. 
 The cells need free oxygen during life; they remove it by 
 combining it with complex combustible compounds. So 
 long as the removal of free oxygen by combining it with 
 other substances takes place, so long will oxygen continue, 
 in obedience to the laws governing the diffusion of gases, to 
 enter the cells. The combination of oxygen with combusti- 
 ble substances at temperatures below those at which these 
 substances would spontaneously oxidize, takes place in liv- 
 ing cells and is accomplished only by them. 
 
 Though the living cell is supplied with oxygen by purely 
 physical means, by diffusion, the continued supply of new 
 molecules of oxygen is contingent upon its continued con- 
 sumption by the cell. The supply must make good the lack 
 produced by oxidation, the rate of oxidation must equal the 
 demand for energy, and the amount of work done in the 
 cell will always be directly proportioned to the amount of 
 energy it can liberate. The initiative, however, will always 
 come from the living cell, because oxidation by respiration 
 is not dependent solely upon the mutual affinities of oxygen 
 and the substances to be oxidized. The activity of the cell 
 controls the activity of respiration ( not vice versa ) , and the 
 supply of oxygen, normally exactly equal to the consump- 
 tion, is also controlled by the cell. An excessive supply of 
 oxygen, which for experimental purposes may be artificially 
 furnished, will affect the respiratory only as it, at the same 
 time affects the other activities of the cell. Each kind of 
 cell and each kind of organism will have its own optimum, 
 maximum, and minimum, any departures from which will 
 characteristically affect the cell and the organism. The 
 percentage of oxygen in atmospheric air (about 20%) is 
 approximately the optimum for the majority of land organ- 
 isms, though for a small number this amount, and even 
 much less than this, is fatal. The amount of available free 
 oxygen necessarily varies with the habitat, plants living on 
 mountain-tops, and in water, having less than those living on 
 the land at ordinary elevations. But the successful existence 
 of plants at different elevations and depths shows that they 
 are capable of supplying themselves with what they need. 
 
RESPIRATION 15 
 
 Within the limits in which normal respiration is possible, 
 plants, and hence their component cells, will supply them- 
 selves with adequate amounts of oxygen, an excessive pro- 
 portion suddenly supplied causing, in some plants, merely a 
 greater accumulation of oxygen, unaccompanied by greater 
 respiration, * in the cells ; in others, more rapid respiration 
 as indicated by higher temperature, etc. On the other hand, 
 sudden reduction of the amount of oxygen may cause a 
 diminution of the respiratory activity. 
 
 So long as the general conditions for life continue in ade- 
 quate degree, there is no cessation of respiration in plants 
 or in animals. This is true of the resting forms buds, 
 bulbs, tubers although in them the rate of respiration is 
 much lower than in active forms. It is possible to force 
 these resting forms by various means into activity, but it 
 is not possible, after the}' mature, to continue the rate of 
 respiration which they possessed during their development. 
 The forming bud, bulb, or tuber is composed of cells ac- 
 tively working, needing much energy with which to work, 
 and respiring rapidly in order to secure this. With the ap- 
 proach of maturity of the parts, the work to be done, the 
 need of energy, and the rate of respiration, diminish, no 
 matter how favorable to continued activity the external 
 conditions may be. This decrease in respiration, and in the 
 other functions, is largely due to the influences of its envi- 
 ronment upon all the functions of the organism as a whole. 
 The organism prepares itself for the regularly recurring 
 periods of drought, heat, or cold ; one activity after another 
 is suspended accordingly. After a period of rest, it is possi- 
 ble to force these forms into activity even after the lapse of 
 much less than the usual time. Lilac branches, cut from the 
 bushes late in autumn, can be forced by Christmas time to 
 develop the flowers and leaves already formed. This forcing 
 is accomplished by placing the ends of the cut branches in 
 jars of water and keeping them in a warm, damp, not too 
 brightly lighted place. This experiment, familiar enough to 
 the florist, results in the resumption of active respiration 
 
 * Pfeffer ? 8 Handbuch der Pflanzenphysiologie, 2te Auflage, Bd. I., p. 
 547, 1897. Eng. tranel. by Ewart, vol. I., p. 539, 1900. 
 
16 PLANT PHYSIOLOGY 
 
 after a period of tardy respiration shorter than usual; it 
 does not continue the respiratory and other activities at the 
 rate prevailing during the formation of the parts. 
 
 Respiration is a process conducted and regulated by the 
 living protoplasm of the cells. It will not go on indefinitely 
 and independently however favorable the physical conditions 
 may be. It can be artificially increased only by stimulat- 
 ing the protoplasm to greater activity ; it will be increased 
 whenever the protoplasm becomes more active. The respira- 
 tion of plants and plant-cells can be artificially reduced only 
 by reducing the general activity of the living protoplasm. 
 This may be accomplished by the same means as the animal 
 physiologist employs by applying a local or general anaes- 
 thetic, by lowering the temperature, by preventing move- 
 ment, etc. In most plants and plant parts the forced cessa- 
 tion of normal respiration immediately precedes, and is 
 itself the cause of, death. In certain plants for example, 
 germinating peas intramolecular respiration* may tem- 
 porarily take its place; in certain others for example, 
 anaerobic bacteriat intramolecular respiration is the nor- 
 mal mode. 
 
 Normal respiration cannot, however, be entirely suspended 
 in plants or plant parts without profound changes taking 
 place which sooner or later will result in death. The reduc- 
 tion of normal respiration to an extremely low rate, if not 
 its entire suspension, unaccompanied by any other means of 
 obtaining energy, regularly takes place in the ripe seed, but 
 only when the cells composing the seed lose the water which 
 is an essential constructive constituent of living proto- 
 plasm. 
 
 In warm-blooded animals the object of respiration is two- 
 fold the maintenance of a certain (normal) body-tempera- 
 ture and the production of energy for doing work. In cold- 
 blooded animals and in plants the object of respiration is 
 solely the latter. ;The average body-temperature of plants 
 is in general nearly the mean daily temperature of their 
 environment, and it will vary within certain limits accord- 
 ingly. The variation in the body-temperature of plants will 
 * See page 27. t See page 26. 
 
RESPIRATION 17 
 
 be large or small according to the environment. Submerged 
 aquatics will vary least, floating aquatics more, and ter- 
 restrial plants most ; but as the temperature of small, still 
 bodies of water (pools, etc.) varies considerably, so the 
 body-temperature of the organisms living therein will vary, 
 warmed by the sun and cooled during the night. The body- 
 temperature of the larger terrestrial plants is likely to be 
 higher at night (except hi the exposed surfaces) and lower 
 in the day, than that of the surrounding air. Owing to the 
 very great external surface of the larger plants in propor- 
 tion to their mass, radiation from them is rapid, and a 
 body-temperature independent of their environment could be 
 maintained only at great expense of material laboriously 
 collected and elaborated. Plants work economically, must 
 do so, and such extravagance is avoided. 
 
 Heat is the form in which the energy set free by respira- 
 tion usually makes itself evident, but it does not necessarily 
 follow that only so much energy is liberated as is recogniz- 
 able as heat, or that this is the only form in which energy 
 is liberated. Only that energy becomes evident as such 
 which is not at once used. To determine the amount of 
 energy liberated in respiration, it is necessary to know and 
 to measure the material products of respiration. 
 
 The substances ordinarily engaged in the process of physi- 
 ological oxidation are the highly complex nitrogenous and 
 non-nitrogenous compounds elaborated by the organism. 
 The ordinary products are carbon-dioxide, water, and 
 various small amounts of several other substances, e. g. 
 oxalic acid. Since the production of energy rather than of 
 any particular compounds is what is striven for in respira- 
 tion, and since the substances acted upon by free oxygen 
 are different in different plants and cells, the products will 
 differ accordingly. 
 
 Although the oxidation of nitrogenous matters also takes 
 place, it is mainly the non-nitrogenous contents of the living 
 cell which are involved in physiological oxidation. In the 
 animal body, the oxidation of organic nitrogenous com- 
 pounds ( proteids ) results in the production of urea and of 
 other similar substances no longer usable and presently 
 2 
 
18 PLANT PHYSIOLOGY 
 
 cast off from the body. In plants, the elimination of 
 these products is more economically accomplished, for they 
 furnish the foundations for the re-synthesis of albumi- 
 nous compounds, as will be discussed under the subject 
 of nutrition (see page 71). These waste substances are 
 removed by transforming them synthetically into useful 
 compounds. 
 
 The non-nitrogenous substances which become oxidized 
 are the fats and oils, the starches and sugars. The oxida- 
 tion may first convert the hydrocarbons into carbo-hy- 
 drates, with the liberation of energy and the formation of 
 by-products, carbo-hydrates and by-products then becoming 
 still further oxidized with the liberation of still more energy. 
 While respiration is going on, the other functions in opera- 
 tion also may involve the use, by chemical change, of some 
 of each substance produced in respiration and the formation 
 in the cell of other substances not the products of. respira- 
 tion at all. It is therefore evident that to ascertain the 
 material products of respiration is hardly less difficult than 
 to determine the amount of energy liberated. To isolate 
 any physiological process for purposes of study is impossi- 
 ble, for each process is normal only when accompanied by 
 all the processes normally going on at the same time. The 
 products of one set of chemical activities in the living body 
 may enter wholly or partially, simultaneously or succes- 
 sively, into other chemical activities. The end products can 
 be recognized and measured with comparative ease, but to 
 tell exactly where or how they are formed is much more 
 difficult and not now entirely possible. 
 
 Water and carbon-dioxide gas are the chief products of the 
 physiological as also of other forms of combustion of car- 
 bon-containing bodies. They are formed whenever a suffi- 
 cient amount of oxygen is united with the higher carbon 
 compounds. In organisms living under such conditions that 
 the air can penetrate to all their parts, enough oxygen will 
 always be present for such complete decomposition. Under 
 ordinary conditions oxygen does not unite of itself with the 
 combustible compound, and if active (nascent) oxygen is 
 present at all in the cell it is only in amounts insufficient to 
 
RESPIRATION 19 
 
 accomplish the whole result. * The union of ox ygen and the 
 substances to be oxidized is accomplished bythe living cell. 
 HowThts is done is not known, though conjectures are not 
 lacking. Whether the combustible substances and the oxy- 
 gen are divided into such small particles and so intimately 
 associated in the living protoplasm that union takes place 
 spontaneously, or whether the oxidation is accomplished by 
 enzyms, or whether more readily or spontaneously oxidiza- 
 ble substances are first formed from sugar ( perhaps by the 
 action of an enzym), is not known, though there is a cer- 
 tain amount of evidence in favor of each hypothesis. All 
 that is known is that sugar, or some similar substance, and 
 oxygen, unite, forming as end-products mainly carbon- 
 dioxide and water. The following reaction, without indicat- 
 ing what intermediate stages there may be, if there are any, 
 shows the material results : 
 
 C. H, s O. + 6 O t ( + Aq ) = 6 CO, + 6 H, O ( +Aq ) 
 
 (\([. ivjnvsHiits the water in which the su<i'ur is dissolved 
 in the cell : it does not enter the reaction. The 6 H 2 O may 
 unite with this or these molecules may pass off as vapor, as 
 in ordinary combustion, being kept in a state of vapor by 
 the heat liberated.) 
 
 Since other substances than sugar are also oxidized physi- 
 ologically in the cell, there are also other products of sorts 
 and in quantities varying with these. The commoner of 
 these minor products are oxalic, malic, and citric acids, 
 which accumulate in considerable quantities in certain 
 plants (e. g. in the leaves of Oxalis acetocella and in the 
 Crassulacese, in apples, etc., and in the citrous fruits 
 lemons, limes, oranges, etc. ) , or are converted into salts 
 ( e. g. calcic oxalate, crystallizing out of the solutions in 
 which it is formed in the cell), or undergo other changes 
 ( e. g. further oxidation ) . 
 
 In all organisms the oxidation of nitrogenous as well as 
 non-nitrogenous compounds occurs in normal respiration. 
 The proportional amounts of these two groups of com- 
 pounds physiologically oxidized varies with different organ- 
 
 * Pfeffer,W. Oxydationsvorgange. Physiol. of Plants, Vol. I., pp. 54-56. 
 
20 PLANT PHYSIOLOGY 
 
 isms. In the majority only organic and highly complex 
 compounds are made to yield the needed energy, but in 
 some organisms much simpler inorganic compounds suf- 
 fice, and in a few now known (more may later be dis- 
 covered) the carbon-containing compounds are not used 
 at all. 
 
 The nitro-bacteria, as shown first by Winogradsky, * oxi- 
 dize simpler nitrogen compounds in order to liberate the 
 energy which they need, employing carbon-compounds only 
 in the synthesis of food to be used in the construction of 
 their living bodies. One set of nitro-bacteria oxidize am- 
 monia, or compounds of ammonia, to nitrous acid, the first 
 and last steps of the process being indicated by the reac- 
 tion 
 
 2 NH 4 OH + 3 O 2 = 2 HNO. + 4 H 2 
 
 Another set oxidize the nitrous acid, or its salts, to 
 nitric acid thus 
 
 2 HN0 2 -f 2 = 2 HN0 3 
 
 The sulphur-bacteria ( Beggiatoa, etc. ) obtain most if not 
 all of their kinetic energy by oxidizing sulphur compounds. 
 Thus they precipitate sulphur in their own bodies by oxidiz- 
 ing the sulphuretted hydrogen ( H., S ) present in the waters 
 in which they live.f If the supply of the gas remain suffi- 
 cient, the sulphur will accumulate as a reserve supply in the 
 cells ; if it decrease, the reserve sulphur will be oxidized and, 
 uniting with water, will form sulphuric acid, or its salts, 
 thus 
 
 S + 2 ( + Aq)=S0 2 (+Aq) 
 2S0 2 + 2 (+Aq)=2S0 3 ( + Aq) 
 S0 3 + H 2 (+ Aq) = H 2 S0 4 ( + Aq) 
 
 Those bacteria (e.g. Crenothrix) which, living in water 
 rich in iron, deposit iron in some form in or upon their own 
 bodies, may obtain their kinetic energy by physiologically 
 
 * Winogradsky, S. Recherches sur les organismes de la nitrification. An- 
 nales de 1'Inst. Pasteur, IV., V., 1889-91 and other papers. 
 
 t Winogradsky, S. Uber Schwefelbakterien. Botanische Zeitung, 1887. 
 Beitrage zur Morphologic u. Physiologic der Bakterien. Leipzig, 1888. 
 
RESPIRATION 21 
 
 oxidizing ferrous compounds, presumably ferrous oxide to 
 ferric oxide.* 
 
 Other bacteria may be discovered which, needing carbon 
 and nitrogen compounds only to supply the constructive 
 elements of protoplasm, obtain energy by oxidizing other 
 substances present in solution in the waters in which they 
 live, and therefrom absorbed into their own bodies. 
 
 The^essential product of respiration, the one which dis- 
 tinguishes ilTTrom all the other functions of the living or- 
 ganism, jsjdyaetie energy . The material products vary in 
 kind and in quantity according to the nature of the organ- 
 ism and the substances which can be affected. Ordinarily 
 these substances are complex compounds elaborated within 
 the body of the respiring plant. That this is not essen- 
 tial is shown by the successful existence of bacteria which 
 oxidize nitrogen, sulphur, iron, and possibly compounds of 
 other elements. These, though necessarily absorbed before 
 they can be acted upon, are not first elaborated by the cell. 
 
 Free oxygen is not necessary to all organisms or to all 
 cells. The haemoglobin of the blood is a complex com- 
 pound from which some of the oxygen, only loosely held, 
 can be readily given off where oxidation for the supply of 
 energy is needed. Similarly the color products of certain 
 bacteria (e.g., Bacillus brunneus) are reserves of oxygen 
 which become used when there is no longer an adequate 
 supply of free oxygen. f From colorless compounds also, 
 the cells at depths in the tissues of animals (perhaps also 
 of plants^), to which free oxygen penetrates only in in- 
 sufficient amounts if at all, obtain by decomposition the 
 energy needed. These decompositions are not necessarily 
 
 * Winogradsky, S. Uber Eisenbakterien. Bot. Zeitung, 1888. Molisch, 
 H. Die Pflanze in ihren Beziehungen zum Eisen, Jena, 1892. See also 
 Miyoski, M. Studien iiber die Schwefelrasenbildung und die Schwefel- 
 bacterien der Thermen von Yumoto bei Nikko and Uber das massenhafte 
 Vorkommen von Eisenbacterien in den Thermen von Ikao. Journal Coll. 
 Science, Imp. Univ.. Tokyo, vol. X., pt. II., 1897. 
 
 \ Ewart, A. J. On the evolution of oxygen from colored bacteria. Jour- 
 nal of the Linnean Society, XXXIII., p. 123, 1897. 
 
 t Pfeffer, W. Berichte der Math-phys. Classe der Konigl. Sachs. 
 der Wissensch. zu Leipzig, 27 Juli, 1896, p. 383. 
 
22 PLANT PHYSIOLOGY 
 
 effected to secure oxygen for oxidation of other substances, 
 but the decompositions themselves release as kinetic the 
 potential energy which was needed to hold the complex 
 substances together. 
 
 The mutual attraction of one atom of carbon and two of 
 oxygen is so great that the molecule of carbon-dioxide 
 is very stable as well as very simple, for the "affinities" 
 of the carbon and oxygen are "satisfied." In such complex 
 compounds of carbon, hydrogen, and oxygen as the 
 starches, sugars, etc., the affinities of the component ele- 
 ments are not satisfied ; the compounds are much less stable, 
 as is shown by their ability to take up more oxygen. At 
 ordinary temperatures, however, and under ordinary condi- 
 tions, these compounds are stable. They owe their stability 
 to the mutual affinities of their component atoms, which 
 exert an attraction upon one another sufficiently powerful 
 to hold them together in definite form. When the atoms 
 are separated from one another and arrange themselves 
 closer together in simpler forms in space, their "bonds" or 
 "affinities" are more completely "satisfied," they unite more 
 perfectly, oxidation takes place in the rearrangement, and 
 energy is accordingly liberated and made available for other 
 purposes. Energy is "stored" in the starch, or sugar, or 
 oil molecule; the kinetic energy (solar or other) employed 
 in the construction of the molecule remains in it as poten- 
 tial energy, holding the atoms together. The destruction of 
 the molecule results in the liberation of so much kinetid 
 energy as was employed in constructing it from the simple 
 compounds worked upon. 
 
 The complete oxidation or combustion of a gram of 
 dextrose (sugar), resulting in the formation of carbon- 
 dioxide and water as represented in the following reaction 
 
 C 6 H,, O fl + 6 O a = 6 C0 2 + 6 H, O 
 
 liberates 3,939 small or ordinary calories, or mechanical 
 units of energy in the form of heat. * ( A calorie ( c ) is the 
 
 * Rechenberg, C. von. Uber die Verbrennungswarme organischer Ver- 
 bindungen. Inaug. Diss., Leipzig, 1880. See also Pembry, M. on 
 Animal Heat in Schafer's Text-book of Physiology, vol. I., Edinburg, 
 London, New York, 1898. Here the literature is fully given. 
 
RESPIRATION 23 
 
 heat required to raise 1 gram of water 1 C. in temperature, 
 a great calorie (C) is the heat required to raise 1,000 gr. 
 (1 Kilo) of water 1 C.*). But since a gram of dextrose 
 may contain a very different number of molecules from a 
 gram of any other substance, and since the liberation of 
 energy is due to the oxidation of molecules rather than of 
 mere weights of a substance, it is well to convert these 
 figures into a form which will enable one to compare the 
 yield in energy when the substances are similar in quanti- 
 ties. Thus, by using the molecular weights of the com- 
 pounds to indicate the number of grams, we have a com- 
 mon basis for the comparison of the heats liberated, namely, 
 the gram-molecule. Thus the molecular weight of dextrose 
 is 180, obtained thus- 
 atomic weight of C = 12 C 6 = 72 
 " " H = 1 H 12 = 12 
 " " O = 16 0. = 96 
 
 180 = molecular 
 weight of dextrose (C 6 H 12 6 ) 
 
 3,939 calories x 180 = 709,020 calories = 709. 02 Calories. 
 The heat of combustion (complete oxidation) of 1 gram- 
 molecule, i. P. of 180 grams of dextrose, is then 709.00 
 Great Calories (C.) 
 
 This reaction and the production of this amount of heat 
 take place only in the presence of sufficient quantities of free 
 oxygen. Molecules more complex than those of carbon- 
 dioxide and water, though simpler than sugar, may be 
 formed from sugar without free oxygen or with free oxygen 
 in smaller proportions than 6 :1. Complete oxidation (nor- 
 mal respiration) yields the largest amount of energy, less 
 profound changes yield less energy. Thus the decomposi- 
 tion of sugar by yeasts, according to the following reaction, 
 
 * It may be interesting to compare the amounts of heat liberated by 
 burning 1 gram of different substances, thus : 
 1 gram bread crumbs (contain some proteid) 3984 calories. 
 1 " butter 7264 " 
 
 1 " wood-charcoal (mainly carbon) 8080 " 
 
 1 " hydrogen 33881 " 
 
24 PLANT PHYSIOLOGY 
 
 which represents only in simplest terms the nature of the 
 chemical changes 
 
 C. H,, 6 = 2 C, H 6 + 2 CO, 
 
 (alcohol) 
 
 forming without oxygen two molecules of alcohol and two 
 of carbon-dioxide from one molecule of dextrose yields only 
 67 calories per gram-molecule.* 
 
 The decomposition of one molecule of dextrose into one 
 molecule of butyric acid, two of carbon-dioxide, two of 
 hydrogen, which is accomplished by a considerable number 
 of bacteria and may be represented by the following reac- 
 tion 
 
 C 6 H 12 6 = C 4 H e 2 + 2 C0 2 + 2 H 2 
 (butyric acid) 
 
 yields about 75 calories per gram-molecule, f 
 
 Bacteria forming acetic acid, acting on dilute solutions of 
 ethyl alcohol in the presence of free oxygen, partially oxidize 
 the alcohol and decompose it into acetic acid and water, 
 thus 
 
 C, H.O.+ 0, = C, H 4 0, + H.O 
 
 liberating 125 calories^ ; but if the alcohol were completely 
 oxidized, as in ordinary combustion, the reaction would be 
 
 C 2 H 6 + 3 2 = 2 C0 2 + 3 H a O 
 
 and the heat liberated would be nearly three times as much, 
 about 325 calories per gram-molecule. 
 
 In these figures we have indices of the relative values of 
 complete and incomplete oxidations, and of oxidations and 
 decompositions, as sources of energy in the form of heat. 
 These figures are indices, to be trusted only so far as rela- 
 tive, not exactly proportional, values are concerned ; for the 
 chemist can control all the conditions under which he makes 
 a combustion in his laboratory and determines the number 
 of heat-units liberated ; he can so regulate the process that 
 there shall be no by-products and that no other compounds 
 are included in the reaction than those upon which he has 
 
 * Rechenberg, 1. c., p. 66. t Ibid., p. 67. 
 
 \ Quoted from Berthelot in Biedermann's Chemiker-Kalender for 1897, 
 p. 193 of the Beitrage. 
 
RESPIRATION 25 
 
 determined to experiment. In the plant, on the contrary, . 
 other substances than dextrose may become oxidized, or the 
 oxidation of dextrose may be incomplete. In the laboratory 
 one can deal with definite quantities of isolated substances ; 
 in the living organism indefinitely known quantities of 
 many substances together are acted upon. Animal physi- 
 ologists have done much more in this direction than have 
 plant physiologists, and the high organisms which they 
 study are better suited for the purpose than are plants. 
 The relatively high body-temperatures of warm-blooded ani- 
 mals permit direct temperature determinations from weighed 
 quantities of known foods eaten, as well as calculations 
 from the amounts of oxygen needed to effect combustions 
 or decompositions. Thus the animal physiologist can check 
 the results obtained by one method with those obtained by 
 other methods. The results of animal physiologists indicate 
 that only about 95% of the calculated yield of energy from 
 oxidation* appears as heat. So then we must regard these 
 figures as somewhat too high, but their suggestive value is 
 great whatever must be admitted as to their exact numeri- 
 cal value. 
 
 The larger organisms demand for the normal execution of 
 their functions more energy than can be supplied by the re- 
 arrangement of the component atoms of already furnished 
 molecules ; they must oxidize these molecules, and the more 
 complete the oxidation, the greater the amount of energy 
 liberated. Some of the smaller organisms supply them- 
 selves with adequate amounts of energy by the destruction 
 of complex compounds within their own living cells. Proba- 
 bly some of the cells of all organisms have recourse, at 
 times at least, to the same means of securing needed energy, 
 and when free oxygen is not obtainable the majority of 
 organisms can continue living for a time by so doing. 
 From this the general inference may safely be drawn, that 
 the ability to obtain needed energy by the destruction of 
 complex substances in the cells is inherent in all organisms ; 
 that in the majority of organisms and of their component 
 
 * See Pembry in Schafer's Physiology, vol. I., pp. 836-7. 
 
26 PLANT PHYSIOLOGY 
 
 cells this power is little needed and hence is practically un 
 developed ; but that, owing to the position of some cells 
 deep in the tissues of many of the larger organisms, and tc 
 the peculiar habits of some of the lowest organisms, thes< 
 are obliged to obtain needed energy in this way and hav< 
 developed their inherent powers to a high degree. 
 
 INTRAMOLECULAR RESPIRATION is the name given to this 
 mode of respiration, a term not entirely satisfactory, for it it 
 not explicitly descriptive. The German term SpaltungsatL 
 mung is in this regard much more satisfactory, but it is nol 
 concisely translatable. Ordinary respiration, physiologica 
 oxidation or physiological combustion, is aerobic respira 
 tion respiration which is dependent upon free oxygen, anc 
 which yields the needed kinetic energy only by the union o 
 free oxygen with combustible substances. Intramoleculai 
 respiration, physiological simplification of complex sub 
 stances, physiological rearrangement of atoms, is anaerobic 
 respiration respiration which takes place only when fre< 
 oxygen is present in insufficient quantities or is altogethei 
 absent. The results of the two processes are the same ir 
 kind the liberation of the kinetic energy necessary to con 
 tinue living but not the same in degree, as the figures 
 above quoted show. 
 
 It is now about one hundred years since intramoleculai 
 respiration was first observed,* but only within the lasl 
 few years has the connection between the two means o 
 securing energy been demonstrated. In addition to the ani 
 mal and plant physiologists, our present knowledge is du< 
 also to Pasteur and other bacteriologists, for they hav< 
 shown the peculiar habit of a large number of micro-organ 
 isms in actively living only where free oxygen is absent. W< 
 have a chain of allied processes : first, physiological oxida 
 tion, or what may be called inter-molecular respiration, th( 
 normal respiration of most organisms; second, physiologi- 
 cal rearrangement of atoms into simpler molecules, intra 
 molecular respiration, the mode of respiration which man} 
 cells and a good many organisms have recourse to undei 
 
 * Hollo. Annales de Chimie, t. 25, 1798. 
 
RESPIRATION 27 
 
 stress of circumstances ; third, physiological rearrangement 
 of atoms into simpler molecules, also really intramolecular 
 respiration, the anaerobic normal respiration of a compara- 
 tively small number of invariably low organisms. 
 
 INTRAMOLECULAR RESPIRATION 
 
 Having examined the first of these allied processes as 
 thoroughly as our limits allow, let us pass to the second. 
 From experiments hitherto conducted, it would seem that 
 the germinating seeds of higher plants are better able to 
 survive without a copious supply of oxygen than are the 
 other parts. This is what might be expected, for the em- 
 bryo in the seed, when it becomes active in germination, is 
 a very vigorous organism, usually well supplied with just 
 such foods as may be readily broken down into simpler 
 compounds. The seeds of pea, for example, stimulated to 
 germinate by being soaked in water at room-temperature 
 for twelve or fifteen hours, will continue to respire actively 
 for forty-eight hours or longer, even in a vacuum, producing 
 carbon-dioxide in nearly the same quantity as under the 
 same conditions of temperature, etc., in ordinary air. Of 
 course some air containing free oxygen will be carried into 
 the vacuum by the peas, but this will very soon be entirely 
 consumed in normal respiration, and the continued supply 
 of energy must be obtained by intramolecular respiration. * 
 Comparative investigations have shown that different 
 plants and different organs vary considerably in their abil- 
 ity under stress to substitute intramolecular for normal 
 respiration, and that in very few of the higher plants is 
 intramolecular respiration, as measured by the yield in 
 carbon-dioxide, so effective as normal respiration. 
 
 .Exir all of the higher plants prolonged intramolecular 
 respiration is jmpossible. To what this is due is not wholly 
 
 * For directions for laboratory and demonstration experiments on this 
 subject consult Darwin & Acton's Practical Physiology of Plants ; Moor's 
 translation, under similar title, of Detmer's Pflanzenphysiologisches Prak- 
 tikum, or the special papers on the subject cited in Pfeffer's Handbuch der 
 Pflanzenphysiologie, and its English translation, and also in Ganong's and 
 MacDougal's manuals. 
 
28 PLANT PHYSIOLOGY 
 
 clear. The substances first broken up in intramolecular 
 respiration are the same as in normal respiration, namely, 
 the sugars, starches ( after conversion into sugars ) , and the 
 fats and oils; but later the proteid substances enclosed in 
 the cell, and finally the living substance itself, are decom- 
 posed to supply needed energy. Whether the cessation of 
 intramolecular respiration in experiments upon higher 
 plants, and the consequent death of the organism, are due 
 to the destruction of a part of the living substance, or 
 whether they are due to the production in the cells of 
 poisonous substances, cannot now be positively asserted. 
 Certain it is that for higher organisms intramolecular 
 respiration is a function very limited in importance, taking 
 place only under the stress of continued need of energy, in 
 the absence of an adequate supply of free oxygen, and 
 capable of being maintained for comparatively brief periods 
 only. Like normal respiration, it is carried on solely by the 
 living protoplasm, more or less actively according to the 
 greater or lesser activity of the protoplasm ; the substances 
 decomposed are like those oxidized in normal respiration 
 and differ in different species of plants; tjie products differ 
 according to the plant, the conditions unfter which it acts, 
 and the substances acted upon. 
 
 ; Besides carbon-dioxide, alcohol may be produced in con- 
 siderable amount suggesting the connection between fer- 
 mentation and intramolecular respiration and also organic, 
 acids, together with small amounts of many other com- 
 pounds. In germinating peas, the alcohol produced may 
 equal as much as 5% the weight of the moist seeds,* enough 
 to give some support to the hypothesis suggested above 
 that it may be the accumulation of the poisonous pro- 
 ducts of intramolecular respiration which, as we shall see is 
 the case in fermentation (pp. 30-37), brings about the 
 cessation of respiration and the death of the organism. 
 
 Between those plants for which aerobic respiration is in- 
 dispensable to normally active life, and for which anaerobic 
 respiration is only a means of maintaining life over un- 
 
 * Godlewski. In the Anzeiger der Krakauer Akademie, Juli, 1897. Re- 
 viewed in Botanische Zeitung, Sept. 16, 1897. 
 
RESPIRATION 29 
 
 favorable periods, and those for which anaerobic respiration 
 is similarly and equally indispensable, there are all connect- 
 ing stages. These are found among the lower plants, es- 
 pecially among the fungi ; but, as before stated, in all large 
 multicellular organisms, especially among animals, there are 
 probably cells, lying deep in the tissues, which are forced 
 by the positions they occupy to supply themselves with 
 needed kinetic energy by the same means as the anaerobic 
 organisms, namely, by decomposing the complex compounds 
 which they contain. There are, then, cells as well as or- 
 ganisms which are obligate aerobic, facultative aerobic, and 
 obligate anaerobic. The obligate anaerobic cells and or- 
 ganisms live where the access of free oxygen is impossible or 
 difficult : for instance, deep in living tissues, either as com- 
 ponent parts of these tissues, or as parasites and sapro- 
 phytes therein ; in the deeper layers of compact soils, in the 
 mud of swamps and marshes, and in the ooze below bodies 
 of comparatively still water, fresh and salt ; indeed, wherever 
 there are proper food, proper temperature, and proper 
 freedom. 
 
 As in aerobic so also in anaerobic respiration, other 
 processes take place simultaneously with it. These, if not 
 directly caused by respiration, are at all events main- 
 tained by the energy liberated in it, and are so closely 
 connected with it that to distinguish betw r een the chemical 
 products of respiration and those of the processes accom- 
 panying it, is a matter exceedingly difficult and not yet 
 fully accomplished. Fermentation, decay, and disease at 
 least accompany, if they are not actually a part of, the 
 respiratory processes of certain low plants. Anaerobic respi- 
 ration, as well as aerobic, is a function of the living pro- 
 toplasm, which acts upon substances enclosed within its own 
 body, producing simpler substances, of which some remain 
 in the respiring cell while others diffuse out of it. Some of 
 the latter are entirely inactive chemically, like carbon-diox- 
 ide and alcohol; others may act upon the substances out- 
 side of the cell. In the higher animals and plants the enzyms 
 ( like pepsin, diastase, etc. ) are produced in connection with 
 the process of nutrition, converting the substances upon 
 
30 PLANT PHYSIOLOGY 
 
 which they act into available food compounds ; but it is also 
 certain that, among the enzyms produced, there are some 
 which bring about such changes in the surrounding sub- 
 stances that these become available as sources of kinetic 
 energy. For example, the diastase formed in the germinat- 
 ing seed, which dissolves the starch deposited as a reserve 
 food in the seed, converting it into sugar, makes the reserve 
 food available for at least three purposes ; first, for the con- 
 struction of nitrogenous compounds ( amides and proteids ) ; 
 second, for the formation of cell-wall (cellulose) ; and third, 
 for the liberation of energy by aerobic respiration. The pro- 
 duction and action of this enzym furnishes material for 
 respiration as well as for nutrition. The enzyms formed by 
 the lower plants are also useful in more than one way, and 
 in many cases one of these uses is undoubtedly the con- 
 version of irrespirable into respirable substances. 
 
 FERMENTATIONS 
 
 In the group of processes which are commonly called fer- 
 mentations ( alcoholic, lactic, butyric, etc. ) , the physiologist 
 must study both respiration and nutrition. A thorough 
 understanding of the physiology of fermentation* is impos- 
 sible without previous thorough knowledge of the chemistry 
 of fermentation. As this chemical knowledge is still incom- 
 plete, it is possible to form only general ideas concerning 
 fermentation and the allied processes of decay and disease. 
 The distinction usually made between fermentation and 
 decay is this, that in fermentation there is evolution of gas, 
 whereas, in decay, there is little or none an artificial and 
 even misleading distinction. Even the attempted distinc- 
 tion between disease on the one hand, and fermentation and 
 decay on the other, is anything but fundamental; for dis- 
 ease, so far as it is the result of the activity of foreign 
 organisms, is nothing more than decay or fermentation car- 
 ried on by those foreign organisms upon or in the living 
 body of the host. 
 
 * On the subject of fermentation the student may well consult J. Rey- 
 nolds Green's "The Soluble Ferments and Fermentation." Cambridge, 
 England, 1901. In this book there is a very useful bibliography. 
 
RESPIRATION 31 
 
 Although at present unable to determine the respective 
 parts taken by respiration and nutrition in fermentation, 
 decay, and disease, let us briefly consider some examples. 
 The fermentation most studied because the most useful is 
 alcoholic fermentation, by which is understood the produc- 
 tion of the ethyl alcohol of commerce. The organisms which 
 produce most alcohol from a given quantity of sugar are 
 certain species of yeast (Saccharomyces cerevisise, S. eltip- 
 soidenfi ) , but certain species of Mucor* ( M. erectus, M. race- 
 niosus, even M. mucedo) and bacteria also form ethyl alco- 
 hol in considerable quantities. We have already seen that 
 in the forced anaerobic respiration of higher plants (fruits 
 and germinating seeds) alcohol is one of the products. It 
 may therefore be inferred that, like anaerobic respiration, 
 the production of alcohol in fermentation is not peculiar to 
 the plants most used for the purpose, but that the power to 
 produce it is possessed by most, if not all, plants. In the 
 majority, however, this power is undeveloped. 
 
 Not only the amount of alcohol but also the nature and 
 the amounts of the other substances produced, determine 
 the availability of a plant for the commercial production of 
 ethyl alcohol. The best yeasts, cultivated under the most 
 favorable conditions, convert about 6% of the sugar used 
 into other compounds than alcohol, e. g\ glycerine, succinic 
 acid, higher alcohols, ethereal compounds (to which the 
 ''bouquet" of wines is chiefly due), etc. Under less favor- 
 able conditions, with poorer yeasts, and especially with 
 mixtures of yeasts and bacteria, the amounts and kinds of 
 undesirable by-products increase. The amount of alcohol 
 produced by the bacteria commonly associated with beer, 
 bread, wine, and other yeasts used in the arts in this coun- 
 try, is slight in comparison with their other products ; and 
 it is quite as much the kinds and amounts of these other 
 
 * See E. Chr. Hansen. Meddelelsor fra Carlsberg Laboratoriet, Bd. I., 
 etc. Untersuchungen a. d. Praxis der Gahrungsmdustrie. Jorgensen. Die 
 mikroorganismen der Gahnmgsmdustrie, 3te Aufl., Berlin, 1892. E. Got- 
 schlich. Gahrungserregung ; in Fliigge's Mikroorganiemen, 3te Aufl., Leip- 
 zig, 1896, Bd. I., pp. 219-270. Lalar. Technical Mycology, tranel. by 
 Salter. London and Philadelphia, 1898. 
 
32 PLANT PHYSIOLOGY 
 
 products, as the quality of the fermentable mixture into 
 which they are introduced, which give the peculiar color, 
 texture, flavor, and odor of the different brands. Unless the 
 mixture to be fermented is first sterilized and then inocu- 
 lated with pure yeast exclusively, every bottle of wine, beer, 
 or other liquor, and every loaf of bread, represents the com- 
 bined action of yeasts and bacteria. It contains the pro- 
 ducts, alcoholic and other, so far as they are not driven 
 off or decomposed in the process of manufacture, of all 
 the organisms in the fermenting substance. Preliminary 
 sterilization, the inoculation with pure cultures of yeasts, 
 and fermenting under uniform conditions, are the se- 
 cret of the uniformly good quality of the well-known Ger- 
 man beers and wines. In America such precautions are 
 unusual. 
 
 The substances acted upon bear a very definite relation to 
 the active organisms. Not all substances are fermentable, 
 not even all substances of the same proportional composi- 
 tion. Though there may be in one substance the same, and 
 the same number of, atoms, these may be so differently ar- 
 ranged that the fermenting organism can act upon the one 
 but cannot upon the other. The yeasts are able to ferment 
 only a limited number of sugars, which are in the main 
 characteristic of the different species.* Some of the yeasts 
 secrete enzymsf which convert starch and cellulose into 
 sugar. Thus ordinary bread yeast, unable to ferment the 
 starch with which it may be brought into contact, secretes 
 an enzym which converts the starch into sugar. Further- 
 more, cane-sugar cannot be directly fermented, and when 
 present in dough, etc., it must first be acted upon by an 
 'enzym, "inverted." The resulting sugar, differing only in 
 the arrangement, not in the number or kind, of atoms, is 
 directly fermentable. These chemical changes can be sug- 
 gested, though, on account of the by-products already 
 
 * For tables of these see Fliigge's Mikroorganismen, 3te Auflage, Bd. I., 
 p. 204, and the papers by Emil ' Fischer and collaborators in Ber. d. 
 Deutsch. Chem. Gesellschaft for the last few years. 
 
 t Effront, J. Enzyms and their applications. English translation by 
 Prescott, S. C. 1902. 
 
RESPIRATION 33 
 
 alluded to, not accurately represented, by the following 
 reactions : 
 
 C 8 H 10 5 + Enzym + Aq = C 6 H lt O 6 
 
 (Starch) (Diastase?) (Dextrose, etc.) 
 
 C 6 H 12 O 6 + Yeast + Aq = 2 CO 2 + 2 C 2 H 6 O 
 
 By what has already been said we are led to infer that 
 organisms capable of such decompositions as the above sup- 
 ply themselves by this means with kinetic energy, and can 
 flourish although little or no energy may come from other 
 sources. In fact, the yeasts are facultative anaerobic organ- 
 isms, demanding free oxygen only when inadequately sup- 
 plied with the appropriate sugar. The actual fermentation 
 is often only one of a series of processes, first made possible 
 by the inverting action of an enzym upon some not directly 
 fermentable sugar, and followed by other processes for which 
 the fermentation supplies the necessary energy. Buchner 
 and his collaborators* have demonstrated within the last 
 few years that the alcoholic fermentation by yeast is ac- 
 complished by an enzym (zymase) which splits the sugar 
 into carbon-dioxide and alcohol, releasing energy which the 
 yeast cells use. This discovery furnishes the ground for the 
 hypothesis that all respiration, not merely alcoholic fer- 
 mentation, is carried on by the living cell through the 
 agency of enzyms (see p. 19). 
 
 The yeasts are more profitable for the commercial produc- 
 tion of alcohol, not only because of the larger amount of 
 alcohol which they will produce from a given quantity of 
 sugar, but also because they can continue their rate of pro- 
 duction in the presence of a larger amount of alcohol than 
 most organisms can survive. The species of Mucor studied 
 by Hansenf produce only from 3% to 8% of alcohol, whereas 
 the most active species of yeast can produce 14%, although 
 their activity decreases with 12%. It is obviously impossible, 
 therefore, to produce by means of yeasts alone any liquor 
 containing more than 14% of alcohol. Those wines which 
 
 * Buchner, E. Alcoholische Gahning ohne Hefezellen. Ber. d. Deutsch. 
 Chem. Gesellsch., 1897 and years following, 
 t Loc. cit. Bd. II. p. 160. 
 3 
 
34 PLANT PHYSIOLOGY 
 
 contain more (like port, with 17% or more) must be "forti- 
 fied," tha^BL alcohol must be added to them; while whis- 
 key, brandy, \tc., are still more artificial, being distilled 
 liquors. 
 
 The souring of milk, the so-called lactic-acid fermentation, 
 is due to jfa activity of a great number of aerobic and 
 anaerobic fflranisms, especially bacteria. These ferment the 
 sugars founoin solution in milk, first inverting the sugar if 
 necessary, then splitting it mainly into lactic acid, thus 
 
 C 6 H w 0. - 2 C, H. 3 
 
 but producing also, in proportions characteristic of the spe- 
 cies, acetic, formic, and other organic acids, and carbon- 
 dioxide. Since these organisms can be active in the presence 
 of only a comparatively small amount of free acid, only a 
 small part of the available sugar is, under natural condi- 
 tions, fermented by them. If, however, the free acid be 
 neutralized by the addition of calcic-carbonate to the milk, 
 fermentation will be resumed and continued until again an 
 excess of free acid accumulates. The addition of lime-water 
 (a dilute solution of calcic hydrate) to the milk given to 
 infants is advantageous because it neutralizes the traces of 
 free acid which may have been formed in the milk. 
 
 Butter becomes rancid when the obligate aerobic and the 
 facultative or obligate anaerobic bacteria invariably present 
 in butter succeed in decomposing the dextrose and other 
 non-nitrogenous substances, possibly also some of the pro- 
 teids, into a number of simpler compounds of which butyric 
 acid is the most abundant and characteristic. The by- 
 products, gaseous and other, odorous or not, vary with the 
 organisms accomplishing the fermentation, while the sub- 
 stances acted upon, the course of the reactions, and the 
 activity of the fermentation vary also with the conditions. 
 
 It must not be inferred that all so-called fermentations are 
 the result of splitting the compounds acted upon into sim- 
 pler ones. The acetic acid fermentation, for example, is an 
 oxidation carried on by aerobic bacteria (two species, ac- 
 cording to Hansen,* Bacterium aceti, B. Posteu rianu m), 
 
 * Hansen. Loc. cit. 
 
RESPIRATION 35 
 
 although, as we have seen, acetic acid is a common by- 
 product in anaerobic fermentations. Acetic acid is produced 
 on a commercial scale either by destructive distillation of 
 wood, or by the slow oxidation of dilute solutions of alco- 
 hol in the presence of free oxygen (L e. in the air) by living 
 organisms. The optimum temperature for these (20-30 C.) 
 is lower than the optimum for alcoholic (25-30 C.), butyric 
 (35 C.), and lactic (45-50 C.) fermentations, and they 
 can withstand a decidedly higher amount (8-14%) of free 
 acid than most other organisms.* So long as the acetic 
 bacteria are adequately supplied with alcohol for respira- 
 tion, they continue to form acetic acid to the amount above 
 indicated ; but if the supply of alcohol is insufficient, they 
 oxidize some of the acetic acid, setting free carbon-dioxide, 
 which is not produced under favorable conditions. 
 
 Besides these fermentations, which are carried on in solu- 
 tions by specific organisms, there are similar processes car- 
 ried on in the living body by specific parasitic organisms, 
 mainly bacteria, which liberate the energy and supply 
 themselves with the food which they need by attacking 
 either the living substance of the body itself or the lifeless 
 food-substances enclosed within its cells. The main and the 
 by-products of these parasitic organisms cause the system- 
 atic disturbances of the functions of the host organism, 
 which are characteristic of the different forms of disease. 
 So, for instance, the bacillus of diphtheria (Bacillus diph- 
 therite, K-L.) causes decompositions in the limited area of 
 the mucous membrane of the throat, which it commonly 
 attacks, and the products of these decompositions, poisons, 
 diffusing through the body of the host, are the direct causes 
 of the disease rather than the presence, merely locally irri- 
 tating, of the parasitic organisms. This discovery, that the 
 poisonous products of the specific bacteria of disease, and 
 not the bacteria themselves, are the direct causes of disease, 
 has opened a new chapter in the science of medicine ; but so 
 long as our knowledge of the chemistry of these products, 
 
 * Citromyces, according to Wehmer (Beitrage z. Kenntnis einheimischer 
 Pilze. I), can withstand successfully at least 20% of the citric acid which 
 it forms from sugars. 
 
36 PLANT PHYSIOLOGY 
 
 and of the processes which result in their formation, lags 
 behind our knowledge of the organisms themselves, the 
 treatment of the germ-diseases must continue to be largely 
 a groping in the dark. 
 
 In addition to those fermentations or decompositions ac- 
 complished in the living body or in lifeless substances by 
 distinct species of organisms, there are other equally pro- 
 found chemical disturbances which are due to the combined 
 activities of organisms of more than one species. In the 
 comparatively simple case of the so-called nitrogen bacteria 
 one species decomposes and converts ammonia salts into 
 those of nitrous acid, and another these nitrous salts into 
 nitric. Many other decompositions taking place in nature 
 are the result of the co-operation of several or many species 
 of organisms, some anaerobic, others aerobic, the former 
 preparing the way for the latter. The final products and all 
 the intermediate ones will vary according to the organisms 
 co-operating. The simplest, though not necessarily the 
 mildest, diseases are due to the activity of single species oi 
 organisms. The simplest fermentations are the same. Both 
 are decompositions, with or without oxygen. The more 
 complicated diseases are those due to mixed infections, 
 in which two or more species act together. The one species 
 makes life easier for the other, the products of the one fur- 
 nish food or the sources of energy for the other, the main 
 or the by-products of both supplement each other by weak- 
 ening the host. Typhoid-malaria, and even the mild pro- 
 tective disease which is the result of vaccination, are mixed 
 infections due to the co-operation of two or more ( probably 
 three or four ) species, no one of which alone is able to pro- 
 duce the disease. 
 
 We have then in nature a series of chemical decompositions 
 accomplished by living organisms, animals and plants, for 
 the liberation of needed kinetic energy, and resulting in suc- 
 cessive simplifications from the most complex compounds to 
 the simplest carbon-dioxide, ammonia, and water. The 
 normal or aerobic respiration of all higher and most lower 
 organisms results in the direct destruction by oxidation of 
 the most complex compounds. Anaerobic or intramolecular 
 
RESPIRATION 37 
 
 respiration, fermentation, decay, and disease, attacking the 
 same complex compounds first, result in the formation of 
 the same simple ones finally. This is done only slowly, 
 through many stages, with the same total of released 
 energy, but with far less energy available for the individual 
 organisms in the long chain. Thus the energy stored in the 
 compounds elaborated by living organisms is set free again 
 before and after their death, either by their own activity or 
 by the activity of others. This energy becomes available 
 for the construction of new energy-storing compounds, for 
 work of the utmost physiological variety. 
 
 From the foregoing discussion it is evident that respira- 
 tion, whether aerobic or anaerobic, is a process most inti- 
 mately connected with the other functions of the living 
 organism; a process dependent in the first place upon a 
 supply of respirable (oxidizable or otherwise decomposable) 
 material; a process which goes on, when once respirable 
 material is adequately supplied, actively or tardily, accord- 
 ing to the activity of the other functions, in other words, 
 according to the demand for energy to do work. In the 
 healthy organism, under natural conditions, the rate of 
 respiration will equal the demand for energy; it will not 
 exceed the demand, for stimulating respiration results in 
 stimulating the other activities ; it will not be less than the 
 demand, for decreasing respiration decreases the amount of 
 energy available for the other activities. The rate of res- 
 piration is controlled by the living cell, and whatever in- 
 fluences affect the total amount of energy demanded by it 
 will also affect the rate of respiration, increasing or de- 
 pressing the rate accordingly. 
 
 Respiration lias no optimum temperature, no optimum 
 illumination, of its own: it will increase or decrease with 
 changes in temperature and illumination only as these affect 
 the demand for energy on the part of the organism. Nutri- 
 ent and poisonous substances and the amount of water will 
 affect respiration only as they affect the organism. Injuries* 
 cause a change in the rate of respiration, but they do so 
 
 * Richards, H. M. The respiration of wounded plants. Annals of Bot- 
 any, Vol. X., 1896. 
 
38 PLANT PHYSIOLOGY 
 
 only by changing the demand for energy. To repair an 
 injury or to close a wound new cell-walls must be deposited, 
 new cells must be formed, more food must be manufactured, 
 or stored food must be dissolved, the food must be carried 
 to the seat of the injury and used there, and other kinds of 
 work must be done. For all this extra work extra energy 
 must be supplied, and hence the rate of respiration must be 
 increased. This increased rate shows itself in plants as in 
 animals by the increased production of carbon-dioxide and 
 by a rise in temperature ( wound-fever ) . In parts and in or- 
 ganisms most rapidly growing the rate of respiration will be 
 high, but growth is only one function and when it decreases 
 other functions may become more active, keeping the de- 
 mand for energy uniform, and hence the rate of respiration 
 will not necessarily decrease with a decreasing growth- 
 rate. 
 
 The rate of respiration varies with the total demand for 
 energy, not with any one function. For instance, the rate 
 of respiration and consequently the liberation of energy 
 (heat) reach their maximum in the spathes of Aroids* after 
 the parts have passed the period of most rapid growth ; for, 
 though the growth-rate diminishes, the various processes 
 concerned in and stimulated by fertilization demand an 
 equal or even greater amount of energy. 
 
 In normal respiration the volumes of oxygen fixed and 
 of carbon-dioxide evolved are approximately equal. When 
 this ratio is not obtained in an experiment, the difference is 
 to be accounted for in one or more of the following ways. 
 First, the ratio will vary with the amount of oxygen en- 
 closed in the body of the organism at the beginning of the 
 experiment. This oxygen will be used before more can be 
 absorbed. Second, only so much of the carbon-dioxide 
 evolved (exhaled) can be measured as is not absorbed, or 
 does not remain enclosed, by the tissues, or, in the case of 
 water-plants, is not dissolved in the water. Third, other 
 substances than carbon-dioxide and water may be formed 
 in respiration ( e. g. organic acids, alcohol, etc. ) and there- 
 fore the different amounts of these other products in differ- 
 
 * See Pfeffer, Pflanzenphysiologie, Bd. II., p. 404, of the first edition. 
 
RESPIRATION 39 
 
 ent organisms will evidently affect the ratio between oxygen 
 absorbed and carbon-dioxide given off. The ratio will be 
 changed either by the greater absorption of oxygen for the 
 oxidation of organic acids or by the smaller amount of 
 carbon-dioxide produced when such incompletely oxidized 
 by-products as alcohol are formed. Finally, every factor in 
 the environment of the organism, its food, the influence of 
 its neighbors, the weather, etc., will affect the ratio more or 
 less by affecting the organism itself. 
 
 For the reasons made clear in the foregoing pages, the 
 amount of carbon-dioxide exhaled is by no means an ac- 
 curate index of the rate of respiration and of the amount 
 of energy liberated, but taking this substance only, we may 
 still make some interesting comparisons. For instance * 
 nan exhales in 24 hours CO 2 , equal to 1.2% of his body- 
 
 eight; moulds exhale in 24 hours CO 2 , equal to 6.0% of 
 their body- weight ; active bacteria exhale in 24 hours CO., 
 equal to 200.0% of their body- weight. Organisms which, 
 like man, have comparatively feeble respiratory power, 
 must either, as he does, supply themselves with energy by 
 external means or limit their activities to what they can 
 themselves produce. ~lvjng birds produce much larger 
 amounts of carbon-dioxide and hence of energy than man, 
 and upon the latter depends their abilit}^ to fly. The very 
 important part taken by the moulds and bacteria in 
 Nature is accounted for by their being able to supply 
 themselves with such tremendous amounts of energy. 
 
 SUMMARY 
 
 Respiration, controlled and accomplished by living organ- 
 isms, produces disturbances of equilibrium in the living 
 substances of the organism ; these disturbances give im- 
 pulses to further molecular and atomic movements and to 
 further vital acti vit ies-rrespir ation traqffif flTin latent (po- 
 tential) to active (kinetic) energy, and reduces the amounts 
 of carbohydrates and fats, to a less degree also proto- 
 plasmic substances, in the body. 
 
 * Pfeffer. Pflanzenphysiologie, Bd. I., p. 526. Engl. transl.,vol. I., p. 522. 
 
CHAPTER III 
 
 NUTRITION 
 
 FROM the preceding chapter we have learned that respira- 
 tion is a destructive process consisting either in the break- 
 ing up of complex compounds into simpler ones, or in the 
 physiological oxidation of various combustible substances. 
 With the important exception of the nitrogen, sulphur, and 
 iron bacteria, which derive their energy from simple com- 
 pounds, all living organisms depend for their chief supply of 
 energy upon complex compounds existing in nature only as 
 the result of the constructive activities of living organisms. 
 In the last chapter we assumed the presence of these com- 
 plex compounds, examining only the means of deriving 
 energy from them. 
 
 Energy is needed for construction, to do work. Only so 
 much energy can be liberated by complete combustion or 
 complete decomposition as was employed in construction. 
 Theoretically just as much energy should be liberated in the 
 combustion of a starch grain as was needed to make it, but 
 the cell is not a perfect machine; not all the energy or 
 power used goes into the finished product, some is expended 
 in overcoming the internal resistance of the machine, some 
 is radiated, or "lost" in other ways. There is waste of 
 energy and of material in every machine, the product does 
 not represent the total expenditure of material and energy. 
 As it costs a certain amount of energy to keep an engine 
 going without its doing any other work, and a larger 
 amount to make it do work as well as go, so it costs a 
 certain amount of energy to keep an organism alive and 
 more to make it do anything. The sum of the energy ex- 
 pended in making the engine go, plus the amount expended 
 in making it do work, equals the amount of energy which 
 must be developed to run it, if there is no loss by radia- 
 
NUTRITION 41 
 
 tion, etc. So also the sum of the energy necessary to living, 
 plus the energy necessary to action, equals the amount de- 
 veloped, provided there is no loss by radiation, etc. In the 
 living organism this loss is less than in the machine; the 
 organism is economical to a high degree. 
 
 With the exception of the bacteria mentioned (p. 40 ), all 
 living organisms must have complex compounds, elaborated 
 either by themselves or by other living organisms, from 
 which to derive the energy which they need to continue 
 living and to carry on their various forms of activity. 
 These complex compounds are elaborated from simple ones. 
 Owing to the discrepancy existing, even in the most eco- 
 nomical organism, between the amount of kinetic energy 
 required to elaborate a complex compound and the amount 
 of kinetic energy which can be liberated from it, the organ- 
 ism cannot supply itself by respiration alone with sufficient 
 energy to elaborate all the compounds it needs. It must 
 acquire energy from outside of itself. This the majority of 
 organisms do by taking into their bodies more food than 
 they incorporate. Some of this food is destroyed to furnish 
 energy for the elaboration, assimilation, and incorporation 
 of the remainder. 
 
 Obviously this system cannot prevail throughout the 
 whole series of living organisms. Some must obtain energy 
 from a source entirely independent of organic life. This 
 source is the sun, from which energy comes to the earth in 
 the form of light. As will be shown more plainly later ( pp. 
 55, 56), the sunlight is composed of radiant energy of at 
 least three distinct sorts, of so-called heat, light, and chemi- 
 cal rays. Of these the light rays are the ones most used in 
 making good the losses of energy due to living organisms 
 not working with absolute economy. These rays are used 
 to combine two of the food-materials concerned in the 
 process of nutrition. 
 
 Nutrition consists in, 1st, the absolution, and 2d, the 
 combination of food-materials, and 3d, the assimilation, 
 and 4th, the incorporation of foods. Nutrition furnishes 
 the materials for (a) supplying energy. (b) the construc- 
 tion of new parts, (c) the repair of worn parts. Every 
 
42 PLANT PHYSIOLOGY 
 
 individual must sooner or later nourish itself, because food 
 enough for continued living is not given to it by its parents 
 in the seed, spore, or egg, or later. 
 
 THE FOOD-MATERIALS 
 
 The food-materials of all organisms are fundamentally 
 alike. This conclusion may be formed from the observation 
 that all animals are directly or indirectly dependent upon 
 plants for food the Carnivora preying upon the Herbivora, 
 the Herbivora devouring plants. It might be suspected that 
 the nitrogen, sulphur, and iron bacteria, peculiar in their 
 sources of energy, might be peculiar in the foods used for 
 the construction of their bodies. Chemical analysis of the 
 bodies of all organisms, especially of the protoplasmic parts, 
 shows that there are no exceptions. The conclusion above 
 stated is further confirmed by culture experiments in which 
 food-materials and foods of known composition are the only 
 ones employed. 
 
 Analyses and cultures have shown that of the seventy or 
 so chemical elements only ten are absolutely indispensable 
 as food constituents, namely, carbon, hydrogen, oxygen, 
 nitrogen, sulphur, phosphorus, potassium, calcium, magne- 
 sium, and iron. Two other elements are invariably found in 
 analyses of the bodies of animals and plants, namely, sodium 
 and chlorine, which are of universal occurrence combined as 
 common salt. Only with the utmost difficulty is this com- 
 pound excluded from cultures, and there may be doubt 
 of its ever having been done. Owing to the universal 
 presence of common salt, rather than because of its useful- 
 ness, it occurs in all organisms. Besides these twelve ele- 
 ments, others occur in more or less soluble compounds in 
 the bodies of organisms. Some of these may be very useful 
 though not indispensable. The composition of the soil or 
 water in which plants live, will directly affect the composi- 
 tion of their bodies. Plants living in soils containing large 
 quantities of copper, zinc, arsenic, aluminum, or silica will 
 necessarily absorb larger amounts of the salts of these ele- 
 ments than plants living elsewhere. The plants containing 
 most silica the Equisetuws, grasses, and sedges employ 
 
NUTRITION 43 
 
 this in stiffening their bodies; but they can successfully be 
 brought to maturity without silica, although they will then 
 be comparatively soft, mechanically weak, though physio- 
 logically vigorous. 
 
 From the few elements composing their food-materials 
 plants manufacture foods of the utmost variety. These are 
 present in the cells, either in solution or deposited in solid 
 form. The foods may be divided into two groups according 
 to the presence or absence of nitrogen as a component ele- 
 ment. The non-nitrogenous foods are the ones first formed 
 and the simpler the sugars, starches, and oils; the nitro- 
 genous foods are elaborated from the former and are more 
 complex the amides, proteids, nucleines, etc. Carbon, hy- 
 drogen, and oxygen are constituent elements of all the foods 
 in both groups, and carbon must be considered the charac- 
 teristic component element both of living organisms and of 
 their food. 
 
 CARBON 
 
 The source of carbon for all organisms except the nitrogen 
 bacteria and plants containing chlorophyll or its apparent 
 equivalent physiologically, bacterio-purpurin, is, directly or 
 indirectly, these color-containing plants. Since, however, the 
 purple-bacteria play a comparatively unimportant role as 
 food-producers, and their relations to carbon are like those 
 of green plants, we need not consider them. The nitrogen 
 bacteria will be discussed on page 68. 
 
 The direct source of carbon for green plants, the ultimate 
 source for all organisms, is the carbon-dioxide of the air. 
 This is proved by the following facts : * 1. Green plants 
 cultivated in an atmosphere of normal composition, except 
 that every trace of carbon-dioxide has been removed, soon 
 cease to grow and finally die. 2. Green plants cultivated in 
 a soil of otherwise normal composition, but containing no 
 
 * For details as to experiments demonstrating these facts consult Dar- 
 win and Acton's Practical Physiology of Plants, Moor's translation of 
 Detmer's, and the more recent "Laboratory Course in Plant Physiology" 
 by Ganong, and " Practical Text-book of Plant Physiology" by Mac- 
 Dougal. 
 
44 PLANT PHYSIOLOGY 
 
 trace of carbon-compounds, thrive perfectly. 3. Green 
 plants cultivated in water holding in solution the ordinary 
 soil constituents except the carbon compounds, also thrive 
 perfectly. 4. Submersed water plants cultivated in water 
 containing all the usual salts, but no trace of carbon-diox- 
 ide, soon cease to grow and presently die. The second and 
 third of these experiments demonstrate the error in the old 
 theory and in the present popular belief that leaf-mould, 
 loam, manures, and fertilizers, natural and artificial, are 
 valuable because of their carbon content. Besides the 
 phosphates which they contain, it is probable that the only 
 valuable constituents are the nitrates. 
 
 Carbon-dioxide forms about five-hundredths of one per 
 cent, of normally pure air (.05%). From this we can calcu- 
 late the percentage of carbon thus : 
 
 Atomic weight of carbon = 12 C = 12 
 " " " oxygen = 16 2 = 32 
 
 C0 2 = 44 
 C 12 3 
 ~ r 44 := 11 
 
 CO, = .05% of air. 
 
 C ==.05 + fV = .0135+ 
 .-. C == .01% of air. 
 
 From figures given by Noll* we can calculate the volume of 
 air necessary to furnish the carbon contained in a tree 
 weighing 11,000 pounds dry and 50% of the dry weight of 
 which (5,500 Ibs. =2,500 Kilos) is carbon. Thus: 
 10,000 litres of air contain 5 litres of C0 2 
 5 " " CO, weigh 10 gr. 
 of this C weighs $=2 gr. 
 .*. 5,000 litres of air contain 1 gr. of C. 
 
 "2,500 Kilos = 2,500,000 grs. 
 .-. 2,500,000 x 5,000 = 12,500,000,000 litres 
 
 = 12,500,000 cubic metres 
 = 16,125,000 " yards 
 Hence 16,125,000 cubic yards of air must have their car- 
 
 * Text-book of Botany, by Strasburger, Noll, Schenck, Schimper, trans- 
 lated by Porter, p. 196. New York, 1898. 
 
NUTRITION 45 
 
 bon-dioxide wholly extracted to supply the tree. Does a 
 line drawn through the periphery of a tree weighing 11,000 
 pounds when dry enclose this volume of air? Certainly not, 
 but this is immaterial, for the air is constantly in motion, 
 agitated by mechanical means and by the winds. Further- 
 more, the air, a mixture of gases, is subject to the laws 
 of diffusion of gases. The absorption or decomposition of 
 one volume of any gas in a mixture will produce a diffusion 
 current of that gas to make good the loss. Hence the tree, 
 whenever it is absorbing carbon-dioxide, is the centre to- 
 ward which numberless diffusion currents of carbon-dioxide 
 molecules are moving. The rapidity of their movement is 
 directly proportional to the rapidity of the absorption. 
 
 Noll's figures, giving the amount of carbon contained in a 
 tree and the volume of air required to furnish this, may well 
 be supplemented by Brown's figures of the hourly absorp- 
 tion of carbon-dioxide.* According to Brown's determina- 
 tions there is an increase of one gram of dry substance 
 (mainly carbohydrates) to every square meter of leaf sur- 
 face of Csbtalpa, bignonioides per daylight hour. To form 
 
 1 gr. starch 1.628 gr. CO 2 must be absorbed 
 
 1 " glucose 1.466 " " ' " " 
 
 1 " saccharose 1.543 " " " " 
 From these data 1.545 gr. of carbon-dioxide may be taken 
 as the mean amount needed to form one gram of carbo- 
 hydrate. This at normal temperature and pressure equals 
 784 cubic centimetres of carbon-dioxide, the volume of 
 carbon-dioxide absorbed per hour by each square meter of 
 leaf surface. If we take into consideration the fact that 
 stomata occur only on the under side of Catalpa leaves, 
 and that they occupy only 1% of the total leaf surface, we 
 see that the absorbent power of the leaf is very great. 
 
 The daily absorption of such large volumes of carbon- 
 dioxide from the air suggests the query as to the means of 
 maintaining the supply. We have already seen that the 
 
 * Brown, H. T. The fixation of carbon by plants. Address to Chemical 
 Section, Brit. Assoc. Adv. Science, 1899. Published in Nature, Sept. 14, 
 1899. 
 
46 PLANT PHYSIOLOGY 
 
 respiration of plants and animals returns to the air, in the 
 form of carbon-dioxide, all of the carbon contained in the 
 compounds physiologically oxidized. In ordinary combus- 
 tion also the carbon of the material burned returns to the 
 air as carbon-dioxide. The subterranean fires that vent 
 themselves in volcanic activity pour into the atmosphere 
 very considerable quantities of carbon-dioxide. In these 
 three ways the present proportion of caB^on-dioxide in the 
 atmosphere is maintained. 
 
 The enormous deposits of coal and the accumulations of 
 petroleum and natural gas suggest the hypothesis that 
 in past geologic times the earth's atmosphere contained 
 a higher percentage of carbon than now. It is objected to 
 this hypothesis that the animals of the Carboniferous 
 Period, the time of greatest coal deposition, could not live 
 in air so poisonous as to furnish carbon enough for these 
 deposits. This objection does not appeal with especial force 
 to physiologists. It by no means necessarily follows from 
 the inability of the living relatives of the animals of the 
 Carboniferous Period to withstand a high percentage of 
 carbon-dioxide in the air, that the earlier animals were 
 equally sensit ye. Probably the animals of the Carbonifer- 
 ous Period could no more exist under the conditions now 
 prevailing, than their nearest living relatives could survive 
 the conditions of the Carboniferous, no matter how like or 
 how different might be the percentages of carbon in the air 
 then and now. Furthermore, there is at least a suggestion, 
 though not evidence, of a difference between the respiratory 
 and other functions of the organisms of the Carboniferous 
 and those of to-day in the fact that the nearest living rela- 
 tives of the plants of the coal measures are the ones best 
 able to survive high percentages of carbon-dioxide. By in- 
 creasing the intensity of the light as well as the percentage 
 of carbon-dioxide, ferns may be made to grow more rapidly 
 than under the normal conditions of the present geologic 
 age.* 
 
 For perfect understanding of the movements of gases we 
 
 * See Pfeffer. Pflanzenphysiologie, Bd. I., pp. 310-15, Eng. Transl. I., 
 p. 332. 
 
NUTRITION 47 
 
 must engage in the study of physical chemistry. For us, 
 however, who are studying only the elements of plant- 
 physiology, the mention of a few of the simplest principles 
 concerned, will sufficiently illuminate the subject. First, a 
 vacuum cannot be produced adjacent to an unconfined 
 volume of gas; the gas will flow, will be drawn, into the 
 space whence gas is being removed in the attempt to form a 
 vacuum. Second, the molecules of adjacent and unconfined 
 volumes of gases of different compositions will move spon- 
 taneously until the gases are perfectly mixed. Third, the 
 same molecular movement, which is called diffusion, will 
 take place when gases of different composition are separated 
 from one another by permeable substances. Most animal 
 and vegetable membranes are permeable. If one gas consist- 
 ing of oxygen and carbon-dioxide is separated only by a 
 permeable membrane from another gas consisting of oxygen 
 alone, carbon-dioxide will pass from the first through the 
 membrane into the second until there are equal proportions 
 of this gas on the two sides of the partition, and oxygen 
 will pass from the second into the first until the proportion 
 of oxygen to carbon-dioxide is the same in the two volumes. 
 If for any reason ( because of higher or lower temperature, 
 for instance) the pressures of the two gases are unequal, 
 diffusion of both gases will continue until the pressure be- 
 comes the same on both sides of the separating membrane. 
 Diffusion tends, then, to equalize the pressures and to pro- 
 duce uniform composition in adjacent volumes of gases either 
 unconfined or separated only by permeable substances. 
 
 Turning now to plants, we find that the simplest plants, 
 consisting either of single cells or of cells in filaments or 
 small masses, enclose gases by the permeable membranes 
 bounding their cells. In higher plants, which consist of a 
 larger number of cells in larger masses, the gases are not 
 only enclosed within the cells by permeable membranes, but 
 the spaces between the cells also contain gases. These spaces, 
 and hence the volumes of gas which fill them, are continu- 
 ous with the unconfined mixture of gases which forms the 
 atmosphere. Any difference, therefore, between the compo- 
 sition of the mixture of gases filling the intercellular spaces 
 
48 PLANT PHYSIOLOGY 
 
 and enclosed within the permeable membranes which bound 
 the cells of plants and the atmospheric air outside will 
 cause diffusion. This will tend to make the composition the 
 same within and without the plant. But since every living 
 active cell is constantly respiring, taking in oxygen and 
 giving out carbon-dioxide, a difference in composition be- 
 tween the mixture of gases contained in the plant-body and 
 the atmosphere is constantly brought about. As constant 
 will be the diffusion currents, one inward consisting of oxy- 
 gen molecules, one outward consisting of carbon-dioxide 
 molecules. These currents, tending to restore the uniformity 
 in composition, but never accomplishing this so long as the 
 plant respires, prevent the undue accumulation of carbon- 
 dioxide and maintain an adequate supply of oxygen.- 
 
 Only in green plants, and in the green cells of these, is 
 there any even apparent exception to this simple physical 
 law. If, however, any cells which liberate carbon-dioxide in 
 respiration also absorb and combine it again in food-manu- 
 facture, the diffusion currents to and from these cells will 
 be made up of different molecules from those composing the 
 diffusion currents maintained by other cells. In the green 
 cells the process and the products of food-manufacture are 
 the reverse of respiration, and the diffusion currents neces- 
 sarily correspond. The green cells respire as constantly and 
 at least as actively as other cells containing no chloroplryll ; 
 but while they are manufacturing food, they absorb and use 
 more than all the carbon-dioxide which they exhale. Hence, 
 in experimenting upon the respiration of green plants or of 
 parts containing chlorophyll, it is necessary to stop the 
 opposite and constructive process. 
 
 The absorption of carbon-dioxide gas is accomplished by 
 means of the diffusion currents of this gas set and kept in 
 motion by the combination of carbon-dioxide with water 
 in the production of food. The absorption takes place 
 through the walls of all the chlorophyll-containing cells of 
 the lower plants. These are all small and for the most 
 part aquatics. In Fucus and other large algae periodically 
 exposed to the air, there is a difference in the absorption to 
 correspond with the division of labor among the tissues. 
 
NUTRITION 49 
 
 Cell-walls which are gelatinized or cutinized and infiltrated 
 with waxy matters, as are those of the epidermal cells of 
 Fucus, are less permeable to gases than are cellulose walls. 
 Through the cryptostomata of Fucus, therefore, as well as 
 through the stomata of higher land plants, the inward 
 diffusion of carbon-dioxide is more rapid than through the 
 walls of the epidermal cells. 
 
 For land-plants the value of the stomata as furnishing the 
 gates through which the exchange of gases takes place, has 
 been strikingly demonstrated by Stahl. * He shows that by 
 smearing the stomata-bearing surface of leaves having 
 stomata on one side only (usually the under side) with 
 cocoa-butter or vaseline (neither of which is irritating), the 
 absorption of carbon-dioxide, as indicated by the amount 
 of starch formed, will be much less than when the gas can 
 diffuse through both the open stomata and the more perme- 
 able walls of the epidermal cells of the stomata-bearing sur- 
 face. If the epidermal cells of the upper surface of a leaf 
 which has stomata only on the under and smeared side are 
 injured by a cut or scratch just deep enough to penetrate 
 the wall, the much more rapid diffusion of carbon-dioxide 
 through the cut into the leaf than through the walls 
 of uninjured epidermal cells will be shown by the greater 
 quantity of starch formed in the chlorophyll-containing 
 cells adjacent to the cut. In green land-plants the ab- 
 sorption of carbon-dioxide into the intercellular spaces is 
 accomplished by the free inward diffusion of the gas, a 
 diffusion which is controlled by the stomata, which can 
 open or close as occasion demands. From these intercell- 
 ular spaces the green cells absorb what carbon-dioxide 
 they need through their permeable cellulose walls, just as 
 the luwer algae absorb all their food-materials through 
 their permeable cell-walls. 
 
 As we have already seen (p. 45), the volume of carbon- 
 dioxide absorbed implies the very considerable absorbent 
 power of the leaf, but when we recall how small a propor- 
 tion of the leaf surface (e. g. \% in Catalpa bignonioides) 
 
 * Stahl, E. Einige Versuche iiber Transpiration und Assimilation. 
 Botanische Zeitung, Bd. 52, 1894. 
 4. 
 
50 PLANT PHYSIOLOGY 
 
 can be opened for the free diffusion of gases, we see that the 
 absorbent power of leaves is still greater. According to 
 Brown's measurements it is fifty times that of a strong 
 potassic hydrate solution of equal surface.* That the dif- 
 fusion will keep pace with this absorption is conceivable 
 when it is known that the rate of diffusion through an 
 opening one millimeter in diameter is forty times as fast as 
 through an opening ninety millimeters in diameter. The 
 smaller the opening, within certain limits, the more rapid 
 the diffusion through it. Hence the movement of carbon- 
 dioxide molecules through the stomata of an insolated leaf 
 must be very rapid. 
 
 The absorption of needed carbon-dioxide by green plants 
 is accomplished, then, by purely physical means, by diffu- 
 sion. The continued absorption of carbon-dioxide, and of 
 all other food-materials also, is accomplished by the same 
 means ; but in order that there should be continued absorp- 
 tion, continued inward diffusion, there must be continued 
 removal of what is absorbed. If this were not so, there 
 would presently result from the diffusion equal proportions 
 of the food-materials within and without the plant, and the 
 diffusion would then cease. The construction of foods out 
 of the food-materials absorbed removes the food-materials, 
 prevents their accumulating, and continues the absorption 
 by maintaining the conditions "for diffusion. 
 
 If we now examine the conditions and the means necessary 
 for the elaboration of carbon-dioxide, itself innutritious 
 though a food-material, into nutritious matters or foods 
 proper, we shall at the same time come more clearly to 
 understand how it continues to be absorbed. The condi- 
 tions and the means, besides the presence of carbon-dioxide, 
 are warmth, water, light of definite composition, chloro- 
 phyll, and living protoplasm. Warmth is necessary mainly 
 as one of the general conditions for life, for at any tempera- 
 ture at which protoplasmic activity is possible diffusion and 
 chemical combination are also possible. Water is necessary, 
 for its component atoms become chemically combined with 
 
 * Brown, H. T. The fixation of carbon by plants. Nature, Sept. 14. 
 1899. 
 
NUTRITION 51 
 
 the carbon atoms of the carbon-dioxide. Light rays of 
 definite wave-lengths are the force employed by the living- 
 protoplasm to separate the carbon, hydrogen, and oxygen 
 atoms and to reunite them into another and more complex 
 molecule than either carbon-dioxide or water. Chlorophyll 
 is the pigment which absorbs these light rays. Living pro- 
 toplasm is the only agent yet known which can combine 
 these tw r o universally occurring and extremely stable com- 
 pounds, carbon-dioxide and water, into food for living 
 beings. 
 
 CHLOROPHYLL 
 
 Chlorophyll is a pigment, more properly a mixture of two 
 or more pigments, of unknown chemical composition, proba- 
 bly nitrogenous, extremely unstable, lifeless, but enclosed in 
 the living protoplasm, usually in definite living organs of 
 the protoplasm called plastids or chromatophores. In the 
 cells of the lowest algae, the Schizophyceae, the peripheral 
 layers of protoplasm contain a mixture of blue and green 
 pigments which give to them their peculiar blue-green or 
 olive-green hue. Whatever may be the function of the blue 
 pigment, soluble in cold water, it is accompanied by a green 
 pigment, or a mixture of pigments which is green, of similar 
 composition and the same functions as chlorophyll. Higher 
 algae consist of cells more differentiated in that the chloro- 
 phyll is confined to definite parts of the peripheral layers of 
 protoplasm, in bodies possessed of denser structure than the 
 rest of the protoplasm, apparently of a living framework 
 between the parts of which the chlorophyll, as a dense and 
 oily liquid, is enclosed. With further division of labor in 
 the cell the size of the chromatophores is decreased, their 
 numbers are increased, and the enclosed pigments become 
 denser. By these means the effectiveness of the chromato- 
 phores themselves is increased, their smaller size admits of 
 their moving about in the cell, thus enabling them to oc- 
 cupy different positions at different times, but at all times 
 the most advantageous ones; at the same time, a smaller 
 proportion of the cytoplasm is limited to the work of food- 
 manufacture. 
 
52 PLANT PHYSIOLOGY 
 
 The chief physical characteristics of chlorophyll, which is 
 probably made as well as contained in the chromatophores, 
 are its complete insolubility in water, its ready solubility 
 in alcohol, its fluorescence, and the absorption bands of its 
 spectrum. By reason of its insolubility in water, it is pos- 
 sible to separate chlorophyll from the other pigments some- 
 times masking its presence in the cell the blue of the Sclri- 
 zophycetP, the brown of the PhseopliyceiP, the red of the 
 Carpophycete, and the red and purple coloring-matters 
 often present in the cell-sap of higher plants. * Having ex- 
 tracted these pigments in cold water, alcohol may be used 
 to dissolve the chlorophyll, which will certainly go into 
 solution more rapidly, and possibly also with less chemical 
 change, if the alcohol be applied hot. Besides the chloro- 
 phyll, this extract will contain other cell-contents soluble in 
 alcohol and not removed by the water, notably the fats and 
 oils. The alcoholic solution is ordinarily very unstable, for 
 though it will retain its color for a few days if kept in 
 the dark, the change in color, which begins at once, continu- 
 ing only less rapidly than in the light, is evidence of chem- 
 ical changes finally resulting in its complete destruction. 
 From a freshly made alcoholic extract of chlorophyll at 
 least two colored substances may be separated in solution. 
 Benzine or gasoline shaken with an equal volume of the 
 alcoholic extract will take up the chlorophyll-green proper, 
 leaving a yellow pigment or pigments in solution in the 
 alcohol. This will not secure a complete separation of the 
 two, however, for some of the yellow pigment will remain in 
 the benzine. This yellow pigment is usually called carotin. 
 To separate chlorophyll-green in anything approaching 
 purity is a difficult task. The extreme instability of the pig- 
 ments enclosed in the chromatophores makes it almost cer- 
 tain that they will be acted on by the organic acids present 
 in the cells and extracted with them. Furthermore, the 
 solvents of chlorophyll (alcohol, ether, etc.) extract the 
 fats and oils also. Although many attempts have been 
 
 * Molisch, H. Dae Phycoerythrin, seine Krystallieirbarkeit und chem- 
 ische Natur. Bot. Zeitung Bd. 52 1894. Das Phycocyan, ein krystal- 
 lisirbarer Eiweisskorper. Ibid., Bd. 54, 1895. 
 
NUTRITION 53 
 
 made to isolate chlorophyll none has yet been really suc- 
 cessful. * 
 
 The fluorescent quality of chlorophyll, more evident in 
 its solutions than in the leaf, is revealed by the blood-red 
 color visible when light is reflected from it. However, the 
 physiological significance of this fluorescence is not under- 
 stood. 
 
 The spectrum of a fresh alcoholic solution of chlorophyll 
 is essentially the same as that of chlorophyll in the unin- 
 jured leaf. The spectra vary in any case with the density 
 of the solution, the thickness of the layer through which the 
 light passes, whether of solution or of uninjured tissue, and 
 also with the plants examined. The different shades of 
 green leaves similarly exposed to light, the differences in 
 their spectra, and in the spectra of solutions derived from 
 them, and the different substances obtained in the chemical 
 examination of chlorophyll solutions from different species 
 of plants, all point to there being at least different propor- 
 tions of the component pigments of the mixture called 
 chlorophyll, if not different pigments in the chlorophyll of 
 different plants. Though the function of chlorophyll the 
 absorption of energy in the form of light rays is the same 
 in all green plants, it by no means follows that the chloro- 
 phyll of all plants is equally efficient in absorbing light rays 
 or that exactly the same rays are used in all plants. Slight 
 differences in the composition of chlorophyll would cause 
 corresponding differences in the quality and in the quantity 
 of the energy absorbed by it. 
 
 The spectra obtained from light which has passed through 
 a leaf, and of alcoholic solutions of chlorophyll, are char- 
 acterized by dark bands called " absorption bands," the 
 broadest and darkest of which are between the lines B and 
 C, and E and F, of the normal solar spectrum, that is, in 
 
 * The literature of the subject is voluminous. References to the older 
 literature may be found in Monteverde's papers ( Acta Horti Petropolitani, 
 t. XIII. . 1893) ; Kohl's (Bot. Centralbl., Bd. 73 et seq., 1898) ; Bode's 
 (ibid., Bd. 77, 1899), and in an extended paper promised by Tswett, in 
 his preliminary announcement in Bot. Centralbl. Bd. 81. 1900, also in a 
 collective review by Czapek in Bot. Zeitung. II. Abth.. May, 1900. 
 
54 
 
 PLANT PHYSIOLOGY 
 
 the red, orange, and yellow, and in the green and blue parts 
 respectively. In the normal solar spectrum are certain dark 
 narrow lines, named after their discoverer, Fraunhofer's 
 lines. These indicate parts of the spectrum in which there 
 is no light or other known form of energy. When a chloro- 
 phyll screen is interposed in the path of a beam of sunlight 
 the spectrum differs from that of normal light in that be- 
 tween the Fraunhofer lines B and C, and E and F, energy 
 in the form of light has been absorbed by the chlorophyll. 
 This energy, absorbed by the lifeless chlorophyll pigment, is 
 what is employed by the living chloroplastid in elaborating 
 a nutritious compound from carbon-dioxide and water. The 
 
 Blue 
 
 2 Leaves 
 
 B C D E b 
 
 FIG. 1. CHLOROPHYLL SPECTRA. (AFTER REINKE.) 
 
 accompanying figure indicates the absorption, as reported 
 by Reinke,* of an alcoholic extract of green leaves, and of 
 two leaves and of seven leaves of Iinpatiens parvitiorn 
 interposed in the path of a beam of light. 
 
 The figure shows plainly that light which has passed 
 through one layer of chloroplastids is essentially and obvi- 
 ously poorer in energy than that which falls upon the first 
 layer. The chromatophores of a single-celled green plant, 
 and of plants consisting of filaments or single layered films 
 of cells, receive beams of light containing the maximum of 
 energy assuming that none is absorbed by the medium in 
 which they live. In plants composed of masses of cells, 
 only the outermost layer of chlorophyll-containing cells re- 
 ceives the maximum amount of light and energy. The 
 deeper layers evidently receive less and less. A moment's 
 thought will lead one to conclude that only the peripheral 
 la.yers of cells in a massive plant or organ will receive much 
 
 * Reinke. J. Untersuchungen iiber die Einwirkung des Lichtes auf die 
 Sauerstoffausscheidung der Pflanzen. 2te Mittheilung: Wirkung der 
 einzelnen Strahlengattungen des Sonnenlichtee. Bot. Zeitung. 1884. 
 
NUTRITION 55 
 
 utilizable light, and that flat organs like leaves cannot 
 profitably consist of many layers of chlorophyll-containing 
 cells. Examination of plant parts and organs shows that 
 chlorophyll is developed in greatest amount in the outer- 
 most layers of the parts exposed to light, 7. e. in the outer- 
 most of the cortical parenchyma cells and in the uppermost 
 of the mesophyll layers of the leaf. Ferns and other shade 
 plants develop chloroplastids in the epidermal cells, but in 
 plants growing in situations abundantly lighted, it is not 
 necessary to dispose the chlorophyll in the cells of the ex- 
 posed surfaces. 
 
 The effectiveness of chlorophyll as an absorber of needed 
 energy is proved by the excessively small quantity which is 
 adequate to the needs of the plant. According to the fre- 
 quently quoted estimates made by Tschirch,* there are be- 
 tween 0.2 and 1.00 of a gram per square meter of leaf- 
 surface, or, in Ricinus, about 0.1% of the chloroplastids is 
 chlorophyll pigment, and in leaves 2-4% of the ash-free dry- 
 substance, 1.75-3.50% of the total dry-substance, is chloro- 
 phyll. As might be expected from the small amount of 
 chlorophyll, the amount of energy absorbed by the leaf 
 bears only a small proportion to the total amount falling 
 upon it as sunshine on a bright day. Brownt estimates 
 that 600,000 calories per square meter fall on a sunflower 
 leaf per hour on a bright August day in England certainly 
 more than this in many other parts of the world. Sun- 
 flower leaves under these conditions form 0.8 gram of 
 carbohydrate per square meter per hour, which requires 
 3,200 calories or 0.5% of the available energy. In diffuse 
 daylight the percentage of energy absorbed and used is 
 more in proportion to the amount received. It would seem, 
 then, that the chlorophyll apparatus is so adjusted as to 
 make the most of the amount of light daily available rather 
 than being most efficient only when the light is unusually 
 strong. 
 
 The relative values, as sources of energy, of the rays com- 
 posing sunlight are indicated by the proportions of the 
 
 * Tschirch. A. Angewandte Pflanzenanatomie, p. 57 et seq., 1889. 
 
 t Brown, H. T. The fixation of carbon by plants. Nature. Sept., 1899. 
 
66 PLANT PHYSIOLOGY 
 
 absorption bands of the chlorophyll spectrum, the largest 
 and darkest being in the most luminous part of the spec- 
 trum, the next toward the actinic violet end. Sachs's 
 classical experiment of cultivating plants behind colored 
 screens the one consisting of a layer 1.2-1.5 cm. thick of a 
 saturated solution of potassic dichromate, which absorbs 
 the blue and violet rays wholly, but allows the others to 
 pass; and the other of a layer 1 cm. thick of a dark 
 solution of cuprammonia, which absorbs only the red, 
 orange, and yellow rays shows that the luminous rays 
 absorbed are used to elaborate about 90% (starch serving 
 as the indicator) of the food first formed by the plant, 
 while the more actinic rays yield only 5-7%. Thus the en- 
 ergy absorbed by chlorophyll as indicated by the two main 
 absorption bands in the spectrum is made to produce 95- 
 \ 97% of all the carbon food compounds first formed in the 
 plant. The remaining 5-3% are formed by the other visi- 
 ble rays absorbed. The thermal ultra-red rays and the 
 actinic ultra-violet rays, although absorbed by the plant, 
 are not used in manufacturing non-nitrogenous foods. 
 .Engelmann's bacteria method, f now almost as famous as 
 Sachs's colored screens, though applicable only to small 
 plants, affords much more delicate and exact evidence of 
 the statements just made. The method consists in the 
 employment of motile aerobic bacteria in conjunction with 
 small green aquatic plants. Beneath the stage of the 
 microscope a prism is so placed as to throw a short spec- 
 trum upon the object on the slide. Upon the slide, in a 
 drop of water containing motile bacteria, is laid a filament 
 of (Edogonium, Spirogyra, or some similar plant. Around 
 those cells illuminated by the red, orange, and yellow, rays, 
 the bacteria will congregate in greatest numbers, there 
 moving with the utmost activity. Around other illumi- 
 nated cells also the bacteria will collect, but in much smaller 
 
 * Sachs, J. von. Wirkungen farbigen Lichte auf Pflanzen. Gesammelte 
 Abhandhmgen, Bd. I., p. 261 et seq. 
 
 t Engelmann, Th. W. Botanische Zeitung, 1882, 1883, 1884, 1886, 
 1887. Also Die Erscheinungsweise der Sauerstoffausscheidung chlorophyll- 
 haltiger Zellen. Verhandl. d. Amsterdamer Academic, 1894. 
 
NUTRITION 57 
 
 numbers and in numbers about equal in all the colors, 
 though somewhat more in the green-blue. 
 
 That both living protoplasm and chlorophyll pigment are 
 necessary is shown, on the one hand, by the ineffectiveness 
 of plastids well developed but colorless, and, on the other, 
 by the complete inability of chlorophyll solutions either 
 themselves to liberate oxygen from carbon-dioxide in the 
 light or to enable colorless plastids to do so. The energy 
 needed must be absorbed by chlorophyll in direct contact 
 with and enclosed in living protoplasm, otherwise both 
 energy and chlorophyll are useless. Chlorophyll grains 
 mechanically isolated from living cells continue for a time to 
 absorb carbon-dioxide and to give off oxygen, as may be 
 shown by motile aerobic bacteria.* But if such isolated 
 plastids are so treated that they pass over into a state of 
 anaesthesia, they no longer absorb carbon-dioxide, and 
 liberate oxygen although they still absorb light rays. The 
 living protoplasmic body of the plastids, together with the 
 chlorophyll pigment contained in it, are the organs by 
 which plants begin the manufacture and elaboration of their 
 food. These organs are to a high degree independent of the 
 other organs of the cell and of the cytoplasm itself, but not 
 entirely so; they are still cell-organs. 
 
 For the formation of chlorophyll in cells which normally 
 contain it two conditions may be mentioned as of special 
 importance, namely, illumination and the presence of iron. 
 Most plants fail to produce chlorophyll in darkness, al- 
 though a larger number than was formerly supposed are 
 now known to produce it regardless of the illumination. 
 Among these may be mentioned the seedlings of conifers 
 and maples. The influence of light on its formation in most 
 plants must be regarded as a stimulus rather than a pre- 
 requisite, although the work of the chlorophyll pigment can 
 be done only in the light. Then only is there energy to be 
 absorbed, and the radiant energy, falling upon the cell, will 
 stimulate it to develop a more effective absorbent than wall 
 and cytoplasm afford. The action of iron also, since it is in 
 
 * Ewart, A. J. Can isolated chloroplastids continue to assimilate? 
 Bot. Centralblatt, Bd. 75. No. 2, 1898, and other papers there cited. 
 
58 PLANT PHYSIOLOGY 
 
 all probability not a constituent element of the chlorophyll 
 pigments, must be either that of a stimulant to the living 
 protoplasm to form chlorophyll, or else in some purely non- 
 vital way an assistant in the synthesis of these color-com- 
 pounds. The former supposition seems more probable, but 
 neither has any strong evidence in its favor. However, 
 neither light nor iron will bring about the production of 
 chlorophyll in cells which do not contain chromatophores as 
 living organs. The cells of animals, fungi, and certain of the 
 phanerogamic parasites and saprophytes, contain no chro- 
 matophores, no organs, therefore, capable of forming, con- 
 taining, and using the chlorophyll pigments. 
 
 PHOTOSYNTHESIS 
 
 The absorption of carbon-dioxide and the combination of 
 water with it by green plants have for years been mislead- 
 ingly termed assimilation or carbon-assimilation. The word 
 assimilation may much more correctly and significantly be 
 applied to the working up, by the living organism, of sub- 
 stances already somewhat resembling in constitution the 
 substance of the organism itself. The word really means 
 making like. No organism can make carbon-dioxide and 
 water like itself till it has combined them. The food formed 
 by combining the two innutritions substances, carbon-diox- 
 ide and water, can be used directly only as fuel, for respira- 
 tion. It must be modified, added to, elaborated, changed 
 in various ways not yet understood, until it becomes chem- 
 ically and physically like the component matters of the 
 body of the organism. Combining carbon-dioxide and wa- 
 ter is food-manufacture a synthetic process; the modifica- 
 tion of this food which makes it immediately available as 
 constructive material is assimilation not necessarily a syn- 
 thetic process. Since, as we have seen, the energy by which 
 the synthetic process is accomplished comes from the sun, 
 the name photosynthesis has been proposed for it. 
 
 Besides the substances absorbed carbon-dioxide and 
 water and the final products of the synthesis starch or 
 some equivalent nothing is known of the chemistry of pho- 
 tosynthesis. Only the relative numbers of the atoms form- 
 
NUTRITION 
 
 59 
 
 ing the starch molecule are known, and therefore it is custom- 
 ary to give either the minimum formula or to indicate by 
 n that the starch molecule should be represented by some mul- 
 tiple, probably high, though still undetermined, of the mini- 
 mum proportional formula. The proportional amounts of 
 the end substances are indicated, therefore, by this equation 
 
 n(6 CO, + 5 H 9 0)=n(C 6 H 1 .O fc + 6 O f ) 
 
 Starch, though frequently termed the first visible product of 
 photosynthesis, is not formed in all chlorophyll-containing 
 cells (e.g. those of onion leaves), and should never be re- 
 garded as the first product of photosynthesis. The com- 
 plexity of the starch molecule and the simplicity of the 
 molecules of carbon-dioxide and water, indicated by their 
 respective proportional formulae and still more plainly by 
 their structural formulae, * make it extremely doubtful if even 
 the commoner sugars are formed directly. These, because 
 they are formed and remain in solution, can be recognized 
 only by chemical means, while starch, a solid with a refrac- 
 tive index different from the other substances in the cell, is 
 
 * The structural formula of CO 2 is probably O = C 
 " " " " H 2 O " H O H 
 
 " " n (C. H 10 ) may be 
 
 As examination of this structural formula shows, 
 C fi H 10 O 5 is only an approximate proportional for- 
 mula for starch. It would be more correct to rep- 
 resent starch thus, n ( C 6 H 12 O 6 ) ( n 1 ) H 2 O. 
 Evidently, the higher the multiple represented by n, 
 the nearer this formula will come to the propor- 
 tional C 6 H 10 O 5 . These proportions were obtained 
 by analysis. The exact numbers can be learned only 
 by synthesis, and no one has yet been able to make 
 starch. 
 
 = O 
 
 n 
 
 
 H 
 
 I 
 H-C-OH 
 
 H C-OH 
 
 H-C-OH 
 
 I 
 H-C-OH 
 
 I 
 H-C-OH 
 
 H C-OH 
 H-C-OH 
 
 H-C-OH 
 
 I 
 H-C-OH 
 
 H C= O 
 
60 PLANT PHYSIOLOGY 
 
 visible as soon as formed, and it can be identified by the 
 familiar blue-black color imparted to it by iodine. 
 
 In a very considerable number of plants and plant organs, 
 starch is never formed, whether as the immediate product 
 of photosynthesis or as food in store for future use. In 
 these the food remains as sugars in solution, accumulates in 
 visible form as drops of oil, or may be stored as reserve 
 cellulose. It is in general true that in plants and in organs 
 which, for any reason, should be especially light, oil sepa- 
 rates instead of starch : e. g\ in the Diatoms, most of which 
 float either free or attached ; in many of the Phaeophyceae, 
 in the Characese (except the bulbils), and in such light 
 and mechanically weak structures as the cylindrical leaves 
 of onion, etc. The specific gravity of starch is higher, of oil 
 lower, than that of water. Their nutritive values may be, 
 probably are, about equal. Their values as sources of en- 
 ergy through respiration are not equal.* The advantage 
 of oil as reserve food, or as the immediate product of pho- 
 tosynthesis, where lightness is important, is evident. Sugar 
 never crystallizes in the cell, it remains always in solution, 
 osmotically active (though comparatively weak), readily 
 diffusible, and although occurring much more commonly as 
 food immediately available or in transit from one part of 
 the plant to another, it is sometimes the form in which 
 food is accumulated and stored. 
 
 From considerations insufficiently supported by experi- 
 
 * Complete oxidation (perfect respiration) of oil yields half again as 
 many calories per gram-molecule as starch and sugar. See Chemiker 
 Kalender where heat of combustion of vegetable oils is given, about 932 
 Gals while that of starch and sugar is about 709 Cals. 
 
 According to Stahl (Jahrb. f. wiss. Botanik, Bd. 34, p. 565, 1900), 
 constantly submersed Chlorophycea?, which run no risk of drying up 
 during a period of vegetation, form starch, while on the other hand, those 
 alg* subjected to frequent drying up form sugar. Water would evaporate 
 less and less rapidly from a sugary or other solution of increasing density. 
 The rate of evaporation would not be affected in any way by starch or 
 other substances not in solution. Vaucheria and certain other algae, 
 some species of which are subject to drying up, contain oil in quantity. 
 This cannot reduce the loss of water by evaporation. Are these algse 
 therefore less well protected against harm by drying, or is Stahl's con- 
 jecture a mistaken one? 
 
NUTRITION 61 
 
 ment upon the living plant, though based upon the relations 
 of the members of the sugar group to one another, a series 
 of equations suggesting the possible (some say probable) 
 stages of the photosynthetic process has been proposed by 
 organic chemists. * The following, proposed by von Baeyer, 
 has much in its favor : 
 
 CO, + H 2 O = HCOOH + O = HCOH + O 2 
 
 ( Formic Acid ) ( Formic Aldehyde ) 
 
 6n (HCOH) == n (C 6 H,O 6 ) == n(C 6 H 10 O 5 + H 2 O) 
 
 (Sugar) (Starch) 
 
 To this series of reactions there are evident objections : e. g. 
 formic acid and formic aldehyde are extremely poisonous, 
 they cannot be formed free in the cell unless their conversion 
 into harmless substances is Jnstantaneous. The union of 
 any of the alkaline elements with formic acid would imply 
 the decomposition of the salts in which these elements enter 
 the cell, namely, the phosphates, nitrates, sulphates, and 
 chlorides. The decomposition of these salts, assuming that 
 it could be immediately accomplished by formic acid or other- 
 wise, would set free in the cell acids which, though less pois- 
 onous than formic acid, would still be injurious. To neu- 
 tralize these another series of reactions would be necessary. 
 Pursuing this theoretical discussion further along this line is 
 unnecessary, for after all, assuming that the formic acid is 
 neutralized by sodium, potassium, or any other element in- 
 variably present in combination in the cell, and that the re- 
 duction of this to an aldehyde takes place, the alkaline element 
 must be eliminated to permit the condensation (polymeriza- 
 tion ) of formic aldehyde to sugar, and the element must be 
 united again with some inorganic acid to form a harmless 
 salt. There must, therefore, be at least one other series of 
 reactions taking place simultaneously with the others. 
 
 Loew and Bokornyf have conducted experiments on Spiro- 
 gyra, using as sole source of carbon a 0.1% solution of 
 
 * Baeyer, A. von. Uber die Wasserentziehung und ihre Bedeutung fur 
 das Pflanzenleben und die Gahrung. Berichte d. Deutch. Chemischen 
 Gesellsch., Bd. III., 1870. Fischer, E. Synthesen in der Zuckergruppe I. 
 ibid, Bd. XXIII., 1890. II. ibid., Bd. XXVII., 1894. 
 
 t Loew, O. Ernahrung von Pflanzenzellen mit Formaldehyd. Botan. 
 Centralblatt, Bd. XLIV., p. 315-f . 1890. Bokorny, Th. tJber Starke- 
 
62 PLANT PHYSIOLOGY 
 
 oxymethyl-sodium-sulphonate, which can readily be broken 
 up at a regular rate into formic aldehyde and acid sodic 
 sulphite. Spirogyra thrives on this diet, but this fact does 
 not justify the conclusion of the experimenters that formic 
 aldehyde is first formed or formed at all by green plants 
 in the photosynthesis of food; it simply proves that Spi- 
 rogyrn is able under the conditions of the experiment 
 to supply itself with carbon from other than the usual 
 source. 
 
 Emil Fischer's hypothesis regarding the formation of 
 sugar ( starch, etc. ) by green plants, * represents the mature 
 ideas of an eminent chemist; it is supported by no experi- 
 mental evidence from physiologists, yet it is not out of 
 place here. " The formation of sugar is accomplished, accord- 
 ing to the plant-physiologists, in the chlorophyll grain, itself 
 composed of optically active substances exclusively. I be- 
 lieve that the formation of a compound of carbon-dioxide or 
 formic aldehyde with these precedes the formation of sugar, 
 and that then the condensation to sugar, because of the 
 asymmetry of the whole molecule, is accomplished asymmet- 
 rically. The elaborated sugar is torn away from the whole 
 (chlorophyll) molecule and then used by the plant for the 
 preparation of its other component organic compounds." 
 
 The elaboration of carbohydrates by the more or less 
 indirect methods successfully followed by chemists, furnishes 
 little more than plausible hypotheses as to how the green 
 plant elaborates them. The chlorophyll apparatus is not 
 identical in structure, probably much less in its operation, 
 in all green plants, and it is not reasonable to suppose that 
 the different substances formed by its operation are all 
 elaborated in the same ways. Formic aldehyde may be an 
 intermediate product in the formation of sugars and starch, 
 and it may not ; it is hard to see how it can be in the for- 
 mation of fats and oils. Aided by the energy absorbed by 
 the lifeless chlorophyll pigments, the living chloroplastids 
 may form definite new molecules by uniting carbon-dioxide 
 
 bildung aus Formaldehyd. Ber. d. Deutch. Bot. Gesellsch., Bd. IX., p. 
 103+, 1891, and Biologisches Centralblatt, Bd. XVII., 1897. 
 * Fischer, Emil. Synthesen in der Zuckergruppe, II. 1. c., p. 8231. 
 
NUTRITION 63 
 
 or for mic aldehyde with the already complex molecules of the 
 pigments. By the same living agent, with or without solar 
 energy, these molecules may be split into chlorophyll and 
 sugar, but the chlorophyll pigments are so many that no 
 one reaction or series of reactions will truthfully represent 
 the process in all plants. The problem remains unsolved, 
 and suggestive as the deductions of chemists are, its solu- 
 tion is to be expected from physiologists equipped with 
 chemical knowledge rather than from chemists devoid of 
 physiological experience. ^ 
 
 In summing up this part of our study of the nutrition of 
 plants, we may say that the non-nitrogenous foods, which 
 first appear in the cells as sugars (cane and grape), mannit, 
 etc., starch, fats, and oils, are formed from carbon-dioxide 
 and water in the chromatophores (starch) or in the next 
 adjacent cytoplasm (oil) by these living organs of the cell, 
 the energy necessary for this synthetic process being ab- 
 sorbed by the lifeless chlorophyll pigments and used by the 
 living protoplasm. 
 
 These non-nitrogenous foods accumulate temporarily in 
 the plastids ( starch ) , the cytoplasm ( oil ) , and the cell-sap 
 (sugars). The manufacture of these foods can be accom- 
 plished only during the hours when the plant can secure the 
 necessary energy, 7. e. while it is suitably illuminated. The 
 removal of the manufactured product goes on independently 
 of the illumination, for whenever the food is in soluble form 
 and in solution, it will diffuse out of the cells in which it is 
 made into others containing smaller amounts of these sub- 
 stances. This diffusion goes on constantly, the rate of 
 diffusion varying only with the differences in the amounts of 
 the foods in the different cells. While the plant is strongly 
 illuminated, however, the rate of manufacture is higher than 
 the rate of removal by diffusion, and the food accumulates 
 at this time in the cells forming it. Hence, in the early 
 morning, we may find the chlorophyll-containing cells free 
 from starch, oil, and anything more than small amounts of 
 sugar, at the close of the day, full of them. If, because of 
 prolonged or too intense illumination or because of any 
 other unusual circumstance, the rate of manufacture exceeds 
 
64 PLANT PHYSIOLOGY 
 
 the rate of removal in twenty-four hours, there will be an 
 increasing excess of food in the chlorophyll-containing tis- 
 sues. If this continues for a number of days, the leaves 
 will become filled with accumulated starch, etc., will become 
 abnormally heavy, and by their unusual plumpness, differ- 
 ence in color, etc., will give evidence of their morbid condi- 
 tion. Such "fatness" of leaves is sometimes found in nature, 
 more frequently, however, in cultivation. The cure is easy: 
 shading the plants will reduce the rate of food manufacture 
 and permit the nightly emptying of the daily filled cells. 
 
 When food is deposited in indifmsible form in any cell, it 
 must be dissolved before it can be removed. Water is in- 
 variably the vehicle of food substances in the plant as in the 
 animal body, and the foods are carried from cell to cell only 
 in solution. Starch grains, deposited either in the chloro- 
 plastids, in which the carbohydrate is formed, or in the 
 adjacent cytoplasm, must be converted into sugar and 
 dissolved before they can be removed. In the chloro- 
 phyll-containing cells which manufacture food in the light 
 and accumulate the manufactured product in the form of 
 starch, diastase or some similar starch-converting enzym 
 must be present in the green tissues of leaves, and in other 
 chlorophyll-containing parts. To demonstrate its presence, 
 however evident its effects may be, is difficult, because of the 
 very small amount required to do the work in the time 
 allowed the plant. To convert a large amount of starch 
 quickly into sugar, a comparatively large amount of dias- 
 tase is needed. If the time be longer, less diastase will do 
 the same work. Its converting power is astonishingly great 
 10,000 times its own weight of starch.* The removal of 
 food goes on constantly. Its manufacture is only periodic. 
 A small amount of diastase secreted by the cell will therefore 
 accomplish, in the twenty-four hours, the removal of all the 
 starch normally deposited in the cell as the result of photosyn- 
 thetic activity. That diastase is formed in small quantities 
 in green leaves is indicated by the investigations of Vines. \ 
 
 * Schleichert, F. Das diastatische Ferment der Pflanzen. Halle, 1893. 
 t Vines, S. H. On the presence of a diastatic ferment in green leaves, 
 s Annals of Botany, V., 1891. 
 
NUTRITION 
 
 65 
 
 The products of photosynthetic activity are, as we have 
 seen, food, water, and oxygen. These, in obedience to the 
 laws of diffusion, pass out of the cells in which they are 
 formed into others which contain less, or into the air. The 
 oxygen set free may be used in part in respiration, but only 
 a small part of the considerable quantity of oxygen liber- 
 ated in active photosynthesis can be respired during that 
 time by the plant producing it. The volume of oxygen 
 given off during the life-time of a plant will equal that of 
 the carbon-dioxide absorbed, for respiration and photo- 
 synthesis balance each other. As the volume of carbon- 
 dioxide given off may be used as a quantitative index of the 
 respiratory activity, so the volume of oxygen may be used 
 as a quantitative index of the photosynthetic activity ; but 
 for reasons corresponding to those discussed under Respira- 
 tion ( pp. 38, 39 ) , these indices are not exact. Both gases are 
 at the same time concerned in other processes besides those 
 which we have so far separately examined. The accompany- 
 ing figure shows the relative activity of photosynthesis and 
 respiration as indicated by the volumes of CO, concerned. 
 
 2.3 7.5 11.3 15.8 20.6 
 
 29.3 33 37.3 11.7 46.4 C. 
 
 Figure 2. Photosynthesis and respiration of a leafy branch of Rubus 
 fruticosus (after Kreusler*). During photosynthesis the branch was in 
 air containing 0.3% CO 2 , and was illuminated with electric light approxi- 
 mately equal to the diffuse sunlight. The upper curve indicates the amount 
 of C0 2 actually consumed in photosynthesis. 
 
 C0 2 used in photosynthesis I per hour in each sq. dm. of 
 
 CO 2 produced in respiration f leaf surface. 
 
 * Copied by Pfeffer, Pflanzenphysiologie, Bd. I., p. 321. Eng. transl. I., p. 
 337, from Kreusler's paper in Landwirthschaftliches Jahrbuch, Bd. 16, 1887. 
 5 
 
66 PLANT PHYSIOLOGY 
 
 The water liberated in the cell by the condensation of 
 starch from sugar passes off in the ways to be discussed 
 later in connection with the transfer of water (see 
 p. 137). 
 
 The food formed is used by the cell which forms it and by 
 the other cells of the plant, either immediately or later. 
 As we have seen, the food accumulates temporarily, hence 
 only small amounts are used immediately. What is removed 
 is soon disposed of in one or more ways; fii*st, it may be 
 used at once to build cell-walls, or to furnish energy by be- 
 ing respired ; second, it may be stored as reserve food in or- 
 gans other than those that produced it ; third, it may be still 
 further elaborated, serving as the basis for the construction 
 of nitrogenous food. This leads us from the relations of 
 carbon in the nutrition of plants to those of nitrogen. 
 
 NITROGEN 
 
 Although as essential a constituent element of the living 
 matter and therefore of the food of all organisms as car- 
 bon, nitrogen forms a much smaller percentage of the dry 
 weight of plants, and its distribution in the body is much 
 less uniform. Carbon is found in all the skeletal parts of 
 the body of a plant as well as in the living matter of the 
 cells. The cell-walls, containing no nitrogen, composed 
 mainly of a cellulose or of some derivation of a cellulose, 
 form the skeleton of the whole plant-body, of the dead as 
 well as the living parts. On the other hand, nitrogenous com- 
 pounds occur only in the living parts or as small remnants 
 in parts formerly living. They are found as reserve foods in 
 the cells and as the living constituents of the protoplasm. 
 The amounts and the distribution of nitrogen in the plant- 
 body are indicated by the following table : * 
 
 In mushrooms 7.26% N. in the dry weight 
 
 " lupin seeds 5.0% " " " " " 
 
 " pea " 3.5% " " " " " 
 
 " rye fruits 1.9% " " " " 
 
 " green leaves 3-5 % " " " " 
 
 11 potato tubers 0.3 % " " fresh condition. 
 
 * Frank, A. B. Lehrbuch der Botanik, Bd. I., p. 563, 1892. 
 
NUTRITION 67 
 
 These figures show plainly tnat where there is the greatest 
 proportion of living cells there is also the greatest amount 
 of nitrogen. Where living cells are most active, as in green 
 leaves and at growing points, and where there is the great- 
 est and most compact accumulation of stored food, as hi 
 some seeds, the larger part of the nitrogenous substance 
 will be found. In many seeds, and in such places of stor- 
 age as potato tubers, reserve food is accumulated mainly as 
 starch, to be worked up into nitrogenous compounds only 
 as needed. 
 
 Nitrogen occurs in nature in the uncombined state, form- 
 ing nearly four-fifths of the earth's atmosphere, and also 
 united into compounds of very different degrees of complex- 
 ity, solubility, and availability. Some of the compounds 
 are the results of the constructive activities of living organ- 
 isms (see p. 71), others are the products of their waste 
 or decay (see p. 36), and still others originate indepen- 
 dently of living organisms. Some of the nitrogen com- 
 pounds absorbed by plants contain carbon, others do not. 
 
 The forms in which nitrogen is obtained by the plant are, 
 therefore, much more varied than are the sources of carbon. 
 The supply of nitrogen compounds is also far less constant. 
 At times plants may obtain from the air, as well as from 
 the soil and from the water, simple compounds of nitrogen, 
 such as ammonia and other gaseous substances, formed in 
 the air only under electrical influences, or escaping into the 
 air from terrestrial sources. 
 
 With the exception of certain bacteria, either free-living or 
 associated with higher plants, no living organisms can use 
 the free nitrogen so abundantly supplied to them in the air. 
 They are absolutely dependent upon compounds of nitrogen. 
 Animals obtain all their nitrogenous as well as non-nitro- 
 genous foods from plants; most plants must obtain their 
 nitrogenous food-materials in the form of nitrates. In the 
 soil and in fresh and salt water there occur, besides vari- 
 ous nitrates, other soluble nitrogen compounds, some en- 
 tirely free from carbon, others united with it in more or less 
 complex forms. Substances of the latter sort are the con- 
 stituents or the derivatives of animal excreta or of the dead 
 
68 PLANT PHYSIOLOGY 
 
 bodies of animals and plants. These serve as the foods or 
 as the food-materials of plants of various degrees of depen- 
 dence, e. g. certain species of orchids, the so-called sapro- 
 phytes, etc. Plants of this sort we shall consider later 
 (pp. 78-92). 
 
 The main source of nitrogen for independent plants, that 
 is, for plants which contain chlorophyll and which by means 
 of it elaborate their own non-nitrogenous foods, is the salts 
 of nitric acid, which are found in the soil and in water. 
 The origin of these compounds in the soil, once supposed to 
 be purely chemical, has only recently been shown, by the 
 brilliant researches of Winogradsky, * to be the result of 
 the activity of micro-organisms universally present in soil 
 These organisms, bacteria of only a few species and of ex- 
 ceedingly small size, have been isolated and cultivated by 
 Winogradsky. He found them to be of two sorts, strikingly 
 different in their physiological activity. The one sort to 
 which Winogradsky gave the generic names Nitrosococcus 
 and Nitrosompnas oxidize ammonia compounds (e. g. 
 ammonic sulphate (NH 4 ) 2 S0 4 ) to nitrites, salts of nitrous 
 acid. The other sort belonging to a single genus, called by 
 Winogradsky, Nitrobacter oxidize these to nitrates, salts of 
 nitric acid. In the soil there is no accumulation of nitrous 
 acid or its salts, for they are as rapidly oxidized by Nitro- 
 bacter as they are formed by Nitrosococcus and Nitrosonio- 
 nas. Furthermore, there accumulates no free nitric acid, for 
 this is neutralized by the carbonates commonly present. 
 These reactions may be represented in simplest form thus : 
 (NH 4 ) S0 4 + 3 2 = 2 HN0 + H 2 SO 4 + 2 H 2 O 
 
 2 HNO 2 + 2 = 2 HN0 3 
 2 HNO 3 + K 2 C0 3 = 2 KN0 3 + C0 2 + H 2 O 
 
 As already suggested (see p. 20), these nitrogen bacte- 
 ria are strikingly different from all other known organ- 
 isms. Though they obtain the carbon needed for food- 
 manufacture directly from the air as carbon-dioxide, they 
 elaborate this without chlorophyll or bacteriopurpurin ( see 
 
 * Winogradsky, S. Recherches sur les organismes de la nitrification. 
 Annales de 1'Institut Pasteur, IV., V., 1889-91. Zur Mikrobiologie des 
 Nitrifikationsprozessee. Centralblatt f. Bakteriologie, 2te Abth., II., 1897. 
 
NUTRITION 69 
 
 p. 43) and without the aid of light, obtaining the requi- 
 site energy by the oxidations which they accomplish as aero- 
 bic organisms. * Unlike the purple-bacteria, the nitrogen bac- 
 teria play an obviously important part in nature, preparing 
 from otherwise useless materials those compounds of nitro- 
 gen upon which the existence of all other organisms depends. 
 Like all other substances concerned in the nutrition of 
 k organisms, the nitrates are soluble and enter the body only 
 in solution in water. The nitrates, especially potassic ni- 
 trate, which seems to be the most useful of these compounds, 
 absorbed by the roots of land plants and through other 
 organs of floating aquatic plants, are transferred by diffu- 
 sion throughout the body. The elaboration of these into 
 organic compounds can probably be accomplished by all 
 living plant-cells, but in higher plants, in which division of 
 labor is carried to a high degree, it is accomplished by 
 some cells and not by all, very likely in the leaves and 
 other green parts or in parts not far distant. The manu- 
 facture of nitrogenous foods is accomplished both in dark- 
 ness and in the deeper tissues, and also in the light and in 
 superficial tissues, f It is probable that light, and even 
 certain rays of light, favor the elaboration of nitrogenous 
 foods.]; The action of light is presumably that of a stimu- 
 
 * Godlewski, E. Tiber die Nitrification des Ammoniaks und die Kohlen- 
 stoffquellen bei der Ernahrung der nitrificierenden Fermente. (Polish) 
 Reviewed in Centralblatt f. Bakteriologie, 2te Abth., II., p. 458. Well sum- 
 marized in Fischer's Vorlesungen iiber Bakterien, p. 103, Eng. trans, p. 106. 
 
 t Susuki, U. Uber die Assimilation der Nitrate in Dunkelheit. Botan. 
 Centralblatt, Bd. 75, p. 289, 1898. On the formation of proteids and 
 the assimilation of nitrates by phaenogams in the absence of light. Bul- 
 letin VHI., No. 5, Imperial University, College of Agriculture, Tokyo, 1898. 
 
 + Godlewski, E. Zur Kenntniss der Eiweissbildung aus Nitraten in der 
 Pflanze. Anzeiger der Akademie d. Wissenschaften, Krakau, March, 1897. 
 Laurent, E. Recherches experimentales sur Tassimilation de 1'azote am- 
 moniacal et de 1'azotte nitrique par les plantes superieures. Bulletin de 
 1' Academic Royale de Belgique, t. 32, 1896. Palladine, W. Influence de 
 la lumiere sur la formation des matieres proteiques actives et sur Penergie 
 de la respiration des parties vertes des vegetaux. Rev. gen. de Bot., t. XI., 
 1899. Jost, L. Die Stickstoffassimilation der grunen Pflanzen. Biol. 
 Centralblatt, Bd. 20, 1900. (Review of subject, and a bibliography, 
 to date.) 
 
70 PLAN1 PHYSIOLOGY 
 
 lus; light is probably not the direct source of the energy 
 used. The energy used may be wholly of chemical origin, 
 the elaboration of nitrogenous foods taking place by purely 
 ehemosynthetic processes; or it may be furnished by the 
 physiological oxidation (respiration) taking place in the 
 cells in which the compounds are being formed. 
 
 The nitrogenous foods are found in plants either in soluble 
 or insoluble form. The soluble compounds are the only ones 
 immediately useful. They can be transferred from cell to 
 cell, they can be directly acted upon by the various chemical 
 influences brought to bear upon them by the living cells, 
 they can be assimilated converted into substances like 
 those composing the living substance and when so elabo- 
 rated and assimilated, they become a part of the living 
 protoplasm. In soluble form they occur mainly as amides, 
 of which asparagin is the most abundant and probably the 
 most important. The amides are found in greatest abun- 
 dance in parts where the formation and growth of new 
 cells are most rapidly taking place, that is, in those 
 parts which demand most food of non-nitrogenous and 
 nitrogenous sorts for construction of new parts (cell-walls 
 and protoplasm) and for energy* From analogy with the 
 known behavior of the carbohydrates, and from experiment, 
 it may be concluded that the amides are not elaborated in 
 growing parts, but rather are transferred to them from 
 parts where they have been elaborated from sugar and in- 
 organic nitrogen compounds^ or from parts where the 
 already elaborated nitrogen compounds have been stored. 
 These compounds are stored, in most cases, in insoluble 
 forms (e. g. the proteids), corresponding in this physical 
 quality with the starches and oils, and associated with them 
 in the cells of the storing-tissues, in the pith, medullary 
 rays, the cortical and pith parenchyma of roots, tubers, and 
 other underground parts, in seeds, and in spores. 
 
 The complex nitrogen compounds are very unstable and 
 are doubtless subject in the plant to more or less constant 
 destruction and reconstruction. The destruction of complex 
 nitrogenous compounds does not result in the higher plants 
 in the excretion of waste substances, as in the animals. 
 
NUTRITION 71 
 
 The plant operates more economically. From the simpler 
 compounds formed by breaking down a highly complex or- 
 ganic nitrogen compound, the plant reconstructs, with the 
 necessary additions, complex compounds of the same or 
 similar sorts. In green plants there is nothing correspond- 
 ing with the excretion of urea by higher animals, and the 
 view long held that asparagin and other amides retained 
 within the plant-body are useless substances, like the ex- 
 crementitious matters cast off by higher animals, has been 
 shown to be false. Although the amides are undoubtedly 
 formed in plants in consequence of the breaking down of 
 proteid compounds, they must be regarded as also represent- 
 ing one stage in' the syntheses of proteids,* The greater 
 economy of plants consists in their being able to use the 
 amides, and probabty also other and simpler substances, 
 formed during the destruction of the molecules of proteids, 
 whereas animals are obliged to cast off large quantities of 
 highly elaborated nitrogenous substances, f 
 
 By means and through progressive syntheses not defi- 
 nitely known, and not necessarily the same in plants of 
 different species, the inorganic nitrogen compounds (ni- 
 trates ) occurring in soil and in water are elaborated, in the 
 body of the plant, to complex compounds containing car- 
 bon, hydrogen, oxygen, and nitrogen. Amides, of which 
 asparagin (C^HgN^O,,) is the most familiar, are gradually 
 built up from nitrates, ammonia compounds, etc., in the 
 plant-body. The amides are soluble, diffusible foods; they 
 may be moved from part to part of the plant as needed, 
 and used in the synthesis of more complex organic nitro- 
 genous compounds, the proteids, etc. Phosphorus and sul- 
 phur are subsequently added to these complex nitrogenous 
 compounds. Substances finally are formed which possess 
 the same physical and chemical structure and properties as 
 those composing the living protoplasm. 
 The incorporation of these final compounds, which are at 
 
 * Schulze, E. f her den Umsatz der Eiweisstoffe in der lebenden Pflanze. 
 Zeitschr. f. physiol. Chemie, Bd. 24. 1898. 
 
 t When I was a student in his laboratory, Strasburger once said to me : 
 " Es ist nicht wie im Thierreich. Die Pflanzen machen nie dummes Zeug I " 
 
72 PLANT PHYSIOLOGY 
 
 the summit of the series of constructive processes carried on 
 by the plant, into the living protoplasm, results in making 
 them parts of the living substance. These molecules, the 
 building up of which we have tried to trace, do not them- 
 selves necessarily become alive when they are made parts of 
 the living substance, and yet these molecules recently formed 
 and others similar to them constitute the physical basis of 
 life, the living protoplasm. The new molecules incorporated 
 into and made parts of the living protoplasm are not given 
 any new or peculiar vital force, vis naturae, or any other 
 occult power, but are so distributed in space, have such rate 
 and amplitude of movement, that under the conditions pre- 
 vailing in the cell the conditions which make life possible 
 anywhere they behave as do the old and 'are as the old. 
 
 The majority of chlorophyll-containing plants depend, 
 as we have seen, upon nitrates for their supply of nitrogen. 
 In the laboratory, ammonia salts and even ammonia 
 vapor may be made to serve as the source of nitrogen, but 
 in nature they are not the immediate source for green 
 plants. Besides the nitrates and ammonia, animals and 
 plants may obtain needed nitrogen from three sources, 
 namely, (#) in the uncombined state from the air, (/>) 
 from its compounds in excrementitious matter of animals, 
 and in dead bodies of animals and plants, ( <- ) from its com- 
 pounds in living bodies. The last two sources (b and c) 
 are drawn upon by dependent organisms animals and 
 saprophytic and parasitic plants. The first is used by bac- 
 teria, living either by themselves in the soil or associated 
 with higher plants in special outgrowths of their roots. 
 
 ROOT-TUBERCLE PLANTS 
 
 For centuries it has been the profitable practice of farmers 
 to use leguminous crops to enrich impoverished soils. A 
 worn-out field w r ill more rapidly recover its fertility if sown 
 to clover than if allowed to lie fallow. Sowing to clover and 
 plowing the crop under will evidently enrich the soil more 
 than mowing it and plowing the stubble under, but even the 
 latter is better for the soil than sowing to grass and plow- 
 
NUTRITION 73 
 
 ing the whole crop under. * Chemical analysis of soils origi- 
 nally alike show that that of a clover field gains in nitrogen 
 as well as in carbon, while that of a grass field gains only 
 in carbon. In other words, plowing a grass-crop under re- 
 turns to the soil only what the plants took from it, plus 
 ,the amount of carbon- absorbed from the air. On the other 
 hand, plowing a clover-crop under returns to the soil what 
 the plants took from it, plus the amount of carbon ab- 
 sorbed from the air, and nitrogen. Where did the nitrogen 
 come from? Obviously it could come only from the air. 
 
 If clover, pea, alfalfa, or other leguminous seed be sown 
 in sterilized soil of known composition, no increase in ni- 
 trogen will be discovered on analyzing the soil after the 
 crop has been turned under. These seeds sown in soil of 
 exactly the same composition, unsterilized, but otherwise 
 treated in the same way before and during cultivation, 
 will yield a larger crop, which will be found to have added 
 nitrogen to the soil. These seeds, sown in soil of the 
 same composition, sterilized and then inoculated, either by 
 the addition of a small quantity of unsterilized soil or of 
 clear water first sterilized and then shaken with unsterilized 
 soil, will produce a crop as large, and yield as large a gain 
 of nitrogen in the soil, as those sown in unsterilized soil. 
 
 From these experiments t the only possible inference is 
 that the micro-organisms of the soil, and not the legumi- 
 nous plants alone, are effectively concerned in the increase 
 
 * See Year Book of U. S. Dep't. Agriculture for 1897 and other publica- 
 tions of the national and state agricultural departments for statistics of 
 relative values of different crops as "green manures" in adding to the 
 available nitrogen content of soil. 
 
 t For details see Frank, A. B. Die Assimilation des freien Stickstoffs 
 durch die Pflanzenwelt. Bot. Zeitung, Bd. 51, 1893, and elsewhere, to 
 which references are given in this paper. 
 Also various papers in the Deutsche Landwirthschaftliche Presse, 
 
 " " " Landwirthschaftliches Jahrbuch, 
 " " " " " Annales Agronomiques, 
 
 " " " " " Year Book U. S. Department of Agriculture, 
 
 " " " " " Reports of " " " " 
 
 " " " " " " Agricultural Experiment Stations 
 
 of different states, etc. 
 
74 PLANT PHYSIOLOGY 
 
 in the nitrogen content of the soil. The effectiveness of the 
 micro-organisms, however, may be of one of two sorts; 
 either they may themselves absorb and elaborate the free 
 nitrogen of the air, or they may stimulate the leguminous 
 plants to do so. An acquaintance with the nature of their 
 association with the leguminous plants and with the micro- 
 organisms themselves is a necessary preliminary to an in- 
 telligent discussion of this question. 
 
 On the roots of leguminous plants* develop nodules of 
 various sizes, smooth or convoluted. The roots of plants 
 only a few weeks old begin to lose their even contour and 
 uniform diameter, swellings occur at irregular intervals, and 
 these increase in size very considerably until the plant 
 fruits. These nodules or root-tubercles, white or rose- 
 colored, are composed mainly of thin-walled parenchyma 
 cells, enclosing small intercellular spaces, and directly ad- 
 joining the vascular tissues. Sections of young tubercles 
 show large cells with dense, coarsely granular contents, 
 which, on closer examination, fall under two distinct heads, 
 the protoplasm of the cells, and slender rods which prove 
 to be living bacteria. In older tubercles many of the bac- 
 teria have enlarged and degenerated, are of irregular Y and 
 T forms. In still older ones, the bacteria are dead. In 
 other cells than those composing the tubercles no bacteria 
 are to be found.! When the plant begins to fruit, the tuber- 
 cles lose their plump and even appearance, become emptied 
 and shrivelled. Their cells then contain only fragments of 
 the deformed and dead bacteroids (as the "involution" or 
 degenerate forms are called ) and only a few intact and liv- 
 ing rods. These last, set free in the soil by the complete 
 breaking down of the walls of the tubercles, survive until 
 
 * Also on those of alder (see Hiltner in Versuchsstationen, Bd. 46, 
 1896), Eleagnus (see Nobbe in Versuchsstationen, Bd. 41, 1892, and 
 Bd. 45, 1894) and Podocarpus (see Janse in Annales du Jardin Botan. 
 de Buitenzorg, Bd. 14, 1896). 
 
 f Contrary to Frank's assertion (Uber die Pilzsymbiose der Legumi- 
 nosen. Landwirthschaftliche Jahrbiicher, Bd. 19, 1890), Zinnser (fiber das 
 Verhalten von Bakterien, insbesondere von Knollchenbakterien, in leben- 
 den pflanzlichen Geweben. Jahrb. f. wiss. Bot., Bd. 30, 1897) found bac- 
 teria in the tubercles exclusively. 
 
NUTRITION 75 
 
 the following season and then accomplish the infection of 
 the roots of the new crop. Infection takes place through 
 root-hairs attacked and entered by these bacteria.* The 
 bacteria may be grown in artificial culture media inoculated 
 from tubercles. In such cultures there is an appreciable 
 increase in nitrogen.! 
 
 Leguminous plants will grow in sterilized soil containing 
 nitrates in forms and in quantities suitable for the successful 
 cultivation of other plants, but under the conditions em- 
 ployed in experimenting they do not produce crops so good 
 as when grown in unsterilized soil. The following figures 
 will indicate the benefit they derive from association with 
 the proper bacteria : J 
 
 Per culture-jar, each holding two plants of Lupinus 
 lutews 
 
 I. WITH TUBERCLE FORMATION 
 
 Yield of dry substance Weight of N Weight of N supplied Gain or loss 
 
 in grams. therein. in seed, soil, and water. of N. 
 
 (a)38.919 0.998 0.022 + 0.975 
 
 (6)33.755 0.981 0.023 + 0.958 
 
 II. WITHOUT TUBERCLE FORMATION 
 
 Yield of dry substance Weight of N Weight of N supplied Gain or loss 
 
 in grams. therein. in seed, soil, and water. of N. 
 
 (c) 0.989 0.016 0.020 0.004 
 
 (J)0.828 0.011 0.022 0.009 
 
 (d) was watered with sterilized lupin-soil water (40 
 grams ) . 
 
 (# and b) watered with unsterilized lupin-soil w r ater (40 
 grams ) . 
 
 ( c ) was watered with sterilized ( tap ? ) water. 
 
 It must be borne in mind, however, that though the legu- 
 minous plants may appear to profit by such association 
 with bacteria, these results are derived from experiments 
 
 * Peirce, G. J. Root-tubercles of Bur Clover and of some other legumi- 
 nous plants. Proc. Cal. Acad. Sciences, Botany, vol. II., 1902. 
 
 t Maze. Fixation de 1'azotte libre par le bacille des nodosites des Legu- 
 mineuses. Annales de Tlnstitut Pasteur, t. XI., 1897. 
 
 | Hellriegel und Wilfarth. Erfolgt die Assimilation des freien Stickstoffs 
 durch die Leguminosen unter Mitwirkung niederer Organismen? Ber. d. 
 Deutsch. Bot. Gesellschaft. Bd. VII., 1889. 
 
76 PLANT PHYSIOLOGY 
 
 under conditions wholly unnatural to all the plants experi- 
 mented upon, and must therefore be taken with reserve. 
 Leguminous plants grown in glass vessels undeniably do 
 better when their roots are infected by bacteria than when 
 their roots are sterile, but it has riot been proved that 
 leguminous plants do better with their roots infected than 
 with sterile roots when they are grown where their roots 
 can be properly aerated. Infected LeguniinostB benefit the 
 soil more than do sterile ones, but what the gain to the 
 plants themselves may be, remains to be shown, for the 
 bacteria are plainly parasites.* 
 
 The bacteria found in and causing the root-tubercles of 
 the Leguminosie, Eleagnus, etc., have not }^et been isolated 
 from the soil and are known only in the tubercles and in 
 cultures inoculated from tubercles. The isolation of another 
 species of bacteria which fix free atmospheric nitrogen 
 (Clostridium Pasteurianum) has, however, been accom- 
 plished by Winogradsky.f Other species will doubtless be 
 found in the cultivated and undisturbed soils of field and 
 forest. The green and blue-green algse growing on the soil 
 were suspected of being able to fix uncombined nitrogen, 
 but it has been demonstrated that they cannot do this 
 alone. | It may be that nitrogen-fixing soil-bacteria and 
 low algPB live together in an association similar to that of 
 bacteria and leguminous plants. 
 
 The number of organisms which can use free nitrogen will 
 undoubtedly be found to be small, for in the present balance 
 of nature little more nitrogen need be added to the soil 
 than is yearly returned to it in the excrementitious matters 
 and in the dead bodies which fall upon it. In these waste 
 matters are organic nitrogen compounds upon which de- 
 
 * Peirce, G. J. Loc. cit. 
 
 t Winogradsky, S. Sur 1'assimilation de 1'azote gazeux de I'atmosphere 
 par les microbes. Comptes Rendus, t. 116. 1893 t. 118, 1894; also 
 Archives des Sciences Biologiques St. Petersburg, Bd. 3, 1895. 
 
 J Kossowitsch. P. Untersuchungen iiber die Frage ob die Algen freien 
 Stickstoff assimiliren. Botanische Zeitung, 1894. 
 
 Pfeffer, W. Pflanzenphysiologie, Bd. I., p. 386. Engl. transl. I., p. 
 396. See also Kruger und Schneiderwind in Landw. Jahrb., Bd. 29, Nos. 
 4 and 5, pp. 771, 804 1900. 
 
NUTRITION 
 
 pends the existence of a very large number of org* 
 which break down these complex substances to simple 
 nitrates, the nitrogen compounds which alone are useful to 
 the majority of green plants. 
 
 The organisms accomplishing these decompositions take 
 into their own bodies nitrogenous and non-nitrogenous car- 
 bon compounds elaborated by other and higher organisms, 
 reconstruct and assimilate some of the substance, making it 
 a living part of themselves, decomposing the rest by physio- 
 logical oxidation or by anaerobic respiration in order to 
 obtain energy. In the farmer's manure pile there are count- 
 less dependent organisms which fall into separate species, 
 I easily conceivable but most difficult to isolate. On the sur- 
 face of the pile are fungi and bacteria which respire aerobi- 
 cally and attack mainly the non-nitrogenous matters, the 
 cellulose walls in the fragments of straw, and other vege- 
 table remains. Within the pile are the anaerobic organisms, 
 the first set living on the proteids and amides contained in 
 the animal and vegetable matters, building up their own 
 body substance from some of these and decomposing others. 
 Living upon the decomposition products and upon the dead 
 bodies of the first is the second set, which similarly build up 
 and break down. A third set subsists on the products and 
 upon the remains of the second; and so on down to the 
 nitrite and nitrate bacteria w r hich oxidize ammonia (the 
 ultimate product of a great number of decompositions ) to 
 nitrites and these to nitrates respectively. 
 
 When the highly complex protoplasmic substances of ani- 
 mals and plants, upon which few organisms can live, are 
 broken down to ammonia, water, carbon-dioxide, etc., and 
 the ammonia is oxidized to nitric acid, green plants can 
 begin again the constructive processes which end in the for- 
 mation of living protoplasm from inorganic nitrogen com- 
 pounds of the simplest sort. Thus we have the cycle of 
 nitrogen in the physiology of living organisms. 
 
 A number of plants some leading an apparently inde- 
 pendent existence are forced, or at least find it advanta- 
 geous, to add to their supply of nitrogenous matters by 
 taking into their bodies organic (that is, carbon) com- 
 
78 PLANT PHYSIOLOGY 
 
 pounds of nitrogen. These plants fall into one of three 
 classes, the humus plants, the carnivorous plants, and the 
 parasites. 
 
 HUMUS PLANTS 
 
 The humus plants live in soil containing a large amount 
 of organic matter, mainly of vegetable origin, in a more or 
 less decomposed condition. Besides these organic remains, 
 living saprophytic fungi form an important constituent of 
 humus. Owing to the very diverse composition of humus 
 soils (loam, leaf-mould, etc.), it is very difficult to deter- 
 mine what substances are absorbed from them by plants, 
 but since the majority of the humus plants contain either 
 no chlorophyll or only a little, they must absorb elaborated 
 non-nitrogenous, as well as nitrogenous, carbon compounds. 
 These soils consist largely of substances insoluble or only 
 slightly soluble in water. It is very probable, therefore, 
 that there is a solvent action exerted by the underground 
 parts of humus plants. The fact that even the immediately 
 soluble constituents of humus soils diffuse only slowly, 
 strengthens the supposition that some if not all of the 
 humus plants dissolve the nutritious substances upon which 
 their life depends. Those growing in humus soils containing 
 little soluble food-material, but incapable of secreting acids 
 or other solvents, depend upon other organisms which can 
 exert a solvent action and with these they live in more or 
 less intimate association. 
 
 The plants most active in converting the insoluble nitro- 
 genous substances in lifeless remains of higher organisms 
 into soluble and hence more generally available compounds, 
 are, as we have seen, the bacteria. Almost equally impor- 
 tant are the saprophytic fungi toadstools and similar 
 plants common in all sufficiently moist humus. These fungi 
 and the bacteria, in nourishing themselves, prepare the 
 materials needed and used by other humus plants growing 
 with them. One step farther some of the higher humus 
 plants go. Instead of living merely close to the lower 
 humus organisms, upon the activities of which they are de- 
 pendent, they come into actual contact with these, their 
 
 
NUTRITION 79 
 
 roots becoming invested or even penetrated by them. These 
 associations, described by Frank* most fully, are still too 
 little understood to enable one to determine what parts are 
 played by the members, or to decide whether the associa- 
 tion is of mutual advantage or not. 
 
 The association of filamentous fungi with the roots of 
 higher plants called Mycorhiza by Frank* is not confined 
 to those poor in chlorophyll (e. g. Neottia) or devoid of it 
 ( e. g. Monotropa ) , but occurs also in a considerable number 
 of green plants (e. g. many Orchidaceae, Ericaceae, Cupuli- 
 ferse, Piuus, etc.). This, coupled with the fact that so little 
 is known of the chemistry of nutrition in these associations, 
 renders it impossible to draw any general conclusions re- 
 garding the work accomplished by the fungi. Those closely 
 investing and making felt-like sheaths over the roots of 
 certain forest trees (e.g. beech and pine) cannot be sup- 
 posed to furnish the larger member of the association with 
 non-nitrogenous carbon compounds from the soil, for these 
 it can elaborate in abundance in its own green leaves. 
 Mineral salts and appropriate nitrogen compounds the fun- 
 gus may supply, first, because of its ability, by reason of 
 its smaller size, to branch more finely and spread more 
 widely among the soil-particles than can the roots ; second, 
 because of its decomposing action upon insoluble nitroge- 
 nous remains in the soil ; and third, because it may elaborate 
 or oxidize the otherwise useless ammonia compounds. f 
 
 It is claimed that the fungi may be of additional ad- 
 vantage because roots invested by them branch more pro- 
 fusely than naked ones, and hence are intimately in contact 
 with more soil particles and have a larger absorbing sys- 
 tem. J Neither in this claim, nor in the observation that 
 root-hairs are largely absent from the roots of green as well 
 
 * Frank, A. B. Lehrbuch der Botanik, Bd. I., 1892. Also Percy Groom 
 in Annals of Botany, Vol. 9, 1895. See also Stahl, E. Der Sinn der 
 Mycorhizenbildung. Jahrb. f. wiss. Bot., Bd. 34, 1900. MacDougal, D. 
 T. Symbiotic saprophytism. Annals of Bot., XIII., 1899. 
 
 t Frank, A. B. Die Krankheiten der Pflanzen, 2te Aufl., 1895. Also 
 die Bedeutung der Mycorhiza-Pilze fiir die gemeine Kiefer. Forstwissen- 
 schaftliches Centralblatt, XVI., 1894. 
 
 t Stahl, E. Loc. cit. 
 
80 PLANT PHYSIOLOGY 
 
 as other plants .associated with Mycorhiza fungi, are there 
 any grounds for assuming that we have other than patho- 
 logical conditions due to the interference of the fungi with 
 the normal independent habits of higher plants. So far as 
 the chlorophyll-containing plants are concerned, Mycorhiza 
 seems an affliction rather than a blessing, despite the claims 
 of Frank and of Stahl. Frank says the growth of seedlings 
 of beech and pine, cultivated in sterilized humus soil, is less 
 than of other seedlings of the same sort in the same soil 
 unsterilized. Stahl shows that plants ordinarily free from 
 Mycorhiza grow better in sterilized than in unsterilized 
 humus. It remains for experiment to show whether beech, 
 pine, and other plants with fungi usually on or in their 
 roots grow better in sterilized fertile soil free from humus 
 or in humus which has not been sterilized. Some green 
 plants are strictly humus plants, refusing to grow in other 
 soils, but one cannot now conclude from this that they are 
 dependent upon the fungi, rather than upon any other con- 
 stituent of the humus. Sterilizing a humus soil causes 
 changes in the physical arid chemical conditions of many 
 nitrogenous matters in the humus. Some plants are able to 
 accommodate themselves to these changes, others are not. 
 Sterilizing the humus causes the death, not only of the 
 Mycorhiza fungi, but also of the nitrifying bacteria and of 
 those other bacteria and fungi which directly or indirectly 
 produce ammonia from the organic nitrogen compounds. 
 These changes must profoundly disturb the balance of ac- 
 tivities in the soil. It is, therefore, much easier to under- 
 stand the benefit derived by the fungi from their intimate 
 association with the roots of plants able to manufacture 
 food for themselves, than to be convinced that the indepen- 
 dent green plants greatly benefit by association with de- 
 pendent ones. However, the subject deserves further investi- 
 gation. 
 
 The association of high and low colorless plants in My- 
 corhiza is different only in degree, not in kind, from their 
 simultaneous occurrence in all places where there are highly 
 elaborated nitrogenous and non-nitrogenous materials. As 
 the nitrate bacteria can work only upon nitrites formed 
 
NUTRITION 81 
 
 from ammonia by the nitrite bacteria, and the sulphur 
 bacteria can live only where sulphuretted hydrogen is abun- 
 dantly set free either by organisms or in sulphur springs, so 
 in the humus soils some organisms live upon the simpler 
 products of their neighbors. It is easy to conceive that more 
 than neighborhood nearness might be advantageous to some, 
 and that these, gradually growing together, might form 
 associations, mutually though perhaps not equally benefi- 
 cial. The penetration of small organisms, or small parts of 
 organisms, into and even through the living cells of others, 
 is not necessarily fatal to the penetrated cells, as is shown 
 by the symbiotic association of algae enclosed in the cells 
 of certain Infusoria, and even by some parasitic associa- 
 tions. * 
 
 In all of these cases, however, we have no new physio- 
 logical principles. The nutrition is fundamentally the same, 
 the food-materials are acquired and elaborated, the foods 
 are assimilated and incorporated, in the same way in all 
 organisms, although the sources of food may be different in 
 different cases. 
 
 CARNIVOROUS PLANTS 
 
 The carnivorous plants have been exploited especially 
 by "ecologists," students of the adaptations of plants to 
 their surroundings, and their writings furnish the proper 
 sources for more general information regarding them ;t but 
 the peculiarities of their nutrition are still within the prov- 
 ince of pure physiology. These extraordinary plants may 
 be divided into two classes: first, those which forcibly 
 capture, and second, those which simply entrap, their prey. 
 These classes may be represented respectively by Dion&a, 
 
 * For example, the cases cited by De Bary (Morphology and Biology of 
 the Fungi, Mycetozoa, and Bacteria, Eng. transl., p. 392, 3) and the sur- 
 vival of chlorophyll-containing cells of the host even when penetrated 
 by cells from the haustoria of Cuscuta (Peirce, Annals of Botany, Vol. 
 vii., p. 308, 1893). 
 
 \ See Darwin's Insectivorous Plants, Goebel's Pflanzenbiologische Schil- 
 derungen (Insektivoren), Kerner and Oliver's Natural History of Plants 
 
 ((Vol. I., part I.), Cohn's Die Pflanze (Bd. II., Insektenfressende Pflanzen), 
 Ludwig's Lehrbuch der Biologic der Pflanzen, etc., etc. 
 
 
82 PLANT PHYSIOLOGY 
 
 Droseru, Pinguicula, and by Sarrace nhi, Darlingtonia, the 
 Nepenthes, and Utricularia. Of these Di'osem, Sarraceiria, 
 and Utricularia are most familiar and will sufficiently illus- 
 trate the principles concerned. 
 
 Drosvm, the " Sun-dew" of northern bogs, is a low annual, 
 with nearly horizontally expanded, round, or elongated 
 leaves, characterized by peculiar columnar outgrowths 
 from the upper surface. The chlorophyll of the leaves is 
 usually masked by the red cell-sap of the superficial cells and 
 by the slender outgrowths. These last are multicellular, 
 traversed for about half their length by a single vertical 
 vascular bundle, and covered except on the top by ordinary 
 epidermal cells with cutinized walls. The free ends of the 
 columnar structures are enlarged and globular, glandular, 
 and covered by a glistening, sticky, syrupy, more or less 
 sweet secretion. Attracted by the unusual color and the 
 glistening surface of these leaves, small insects alight upon 
 them, taste the sweet secretion, and while they feed upon it 
 are detained by its stickiness. The weight and movements 
 of the insect induce movements in the hairs adjacent but 
 untouched and in the blade of the leaf itself, as well as in 
 the hairs with which it is in contact. Unless the insect 
 is powerful enough to break away from the sticky surface, 
 it presently comes into contact with other hairs and sticks 
 to them also. Finally the blade of the leaf bends upward 
 and the prey becomes enclosed between it and the many hair- 
 like tentacles bending over upon it from all sides. The 
 continued mechanical irritation of the hairs causes a more 
 abundant secretion from the glandular ends ; but, as Darwin 
 has shown,* besides the mechanical there is also a chemical 
 irritation, and this latter induces a change in the composi- 
 tion of the secretion. Non-nitrogenous material a stone or 
 a piece of wood of similar size and weight will not induce 
 the same response as an insect, a piece of meat or of hard- 
 boiled egg, plainly showing that upon the chemical composi- 
 tion of the irritating body depends in part the nature of 
 the irritation and of the response. 
 
 The continued contact of highly elaborated organic nitro- 
 * Darwin, Charles. L. c. 
 
NUTRITION 83 
 
 gen compounds, largely proteids and insoluble, is followed 
 by the secretion from the glandular ends of the hairs of a 
 peptonizing enzym which attacks the nutritious substances 
 and dissolves them. These solutions are absorbed into the 
 plant through the cells secreting the enzym, diffuse from 
 them into other cells, and are conducted away by the vas- 
 cular bundle. After all the nutritious substances in the 
 captured insect have been digested (dissolved) and ab- 
 sorbed, the leaf unrolls, again becomes flat, the tentacles 
 loose their hold and become straight, the chitinous shell 
 and the other useless remnants are exposed, dry. and blow 
 away. The leaf is now ready to capture another insect. It 
 is reported that each leaf has a digestive capacity for two 
 or three flies. 
 
 When Drosera is able, by capturing and digesting insects, 
 to supplement the nitrogenous food which it, like other 
 plants, elaborates from the nitrates absorbed from the soil 
 and the sugars made in its own leaves, it attains a larger 
 size and produces more and better seeds ( other things being 
 equal) than when it must depend solely upon the complex 
 nitrogenous foods made by itself. 
 
 It has been claimed by Tischutkin, * and the view has been 
 somewhat generally accepted without due investigation, 
 that the peptonizing enzym is secreted mainly, if not wholly, 
 by bacteria symbiotically associated with Drosera on its 
 tentacles, and not by the gland cells of the tentacles. There 
 can be no question of the constant presence, under natural 
 conditions, of bacteria on the tentacles, and it would be re- 
 markable if there were none among these which formed a 
 peptonizing enzym. However, these bacteria, peptonizing 
 and other, are no more symbiotically associated with Dro- 
 sera and no more concerned in the digestion of its insect 
 food than the bacteria of the human mouth and digestive 
 tract are symbiotically associated with man and aid in his 
 digestive processes. In both cases we have to do with bac- 
 teria unavoidably and constantly, but also accidentally and 
 
 * Tischutkin N. A Russian paper reviewed in Botan. Centralblatt 
 Bd. 50. p. 304-j-. 1892 ; also a later paper reviewed in Botan. Central- 
 blatt Bd. 53. p. 322. 1893. 
 
 
84 PLANT PHYSIOLOGY 
 
 independently, present. They are not more intimately as- 
 sociated with the higher organism. 
 
 The same hypothesis has been extended to the digestion 
 which takes place in the pitcher-like leaves of Nepenthes, and 
 with much more justice to the decompositions in the similar 
 leaves of Sarracenia. For Nepenthes, Goebel* and Vinest 
 have proved the presence in the pitchers of an enzym capa- 
 ble of digesting proteid matters. Goebel J says that the 
 Sarracenias and Darhngtonia secrete neither an enzym nor 
 a substance which checks decay ; that is, they do not them- 
 selves digest the bodies of insects, but, on the other hand, 
 they do not prevent the decomposition of these by living 
 organisms contained in the pitchers. The Sarracenias and 
 Darlingtoma, like Drosera, inhabit northern bogs, the soils 
 of which are relatively poor in nitrogen. The leaves are 
 large, erect or inclined, pitcher-shaped, holding often con- 
 siderable volumes of water. Owing to the peculiar form, the 
 slippery inner surface, and its downward-pointing hairs, the 
 mouth of a pitcher is an alighting-place as uncertain as it 
 is natural for flying insects. They fall down into the pitch- 
 er; their escape is prevented by the shape, slippery sur- 
 face, and hairiness of the inside of the pitcher ; finally they 
 die, either by drowning, or of starvation and exhaustion 
 from their futile efforts to get out. They now decay, bac- 
 teria feeding upon their dead bodies liberating soluble or- 
 ganic nitrogen compounds which are absorbed through the 
 walls of the pitchers. If Goebel's conclusions, based on 
 investigations carried on in the Botanic Garden at Marburg, 
 are confirmed by equally reliable investigations of these 
 plants in their native homes, they may constitute an inter- 
 esting case of an association mutually beneficial; but it 
 seems hardly a sufficient reason for the formation of such 
 extremely modified leaves as these pitchers, that they 
 serve only as traps for insects, culture-tubes for bacteria, 
 
 * Goebel, K. Pflanzenbiologische Schilderungen , 2ter Theil, p. 186 et 
 
 SPf], 
 
 f Vines, S. H. The proteolytic enzyme of Nepenthes. Annals of Botany, 
 Vol. XI., 1897. Vol. XII., 1898. 
 J Goebel K. L. c. p. 170. 
 

 NUTRITION 85 
 
 and absorbers of the unused products of these micro-organ- 
 isms. It would seem much more probable that, under their 
 normal conditions, the walls of these pitchers, as of the 
 Xepenthes, secrete an enzym which digests the bodies of en- 
 trapped insects. If this be true, the bacteria inevitably 
 present do not aid the plant, they simply rob it of food which 
 it can itself digest and afterwards absorb and assimilate. 
 
 Most of the Utricularias are aquatics, the peculiar sub- 
 merged leaves of which entrap small crustaceans. It is still 
 undetermined whether the crustaceans are killed and di- 
 gested, * or whether they live on indefinitely. When they die 
 naturally their bodies become decomposed by the water 
 bacteria invariably present in the bladder-like leaves. At all 
 events, the excreta, containing considerable quantities of 
 organic nitrogenous matter, cannot fail to be directly or 
 indirectly useful to the plant which harbors the animals 
 producing them. The excreta may contain immediately 
 available substances ; the more refractory may first undergo 
 chemical transformation by water-bacteria ; finally the dead 
 bodies and the lifeless excreta, falling a prey to bacterial 
 activity, become available to the Utricularia , which profits 
 accordingly. . I 
 
 PARASITES 
 
 A considerable number of plants of the most diverse sorts, 
 from the simplest to the most highly developed, are able to 
 live actively only upon or in the living bodies of other or- 
 ganisms. They may be able to survive in the resting condi- 
 tion entirely independently, but in this regard their seeds, 
 spores, and cysts are like all others. For active vegetation 
 they need their proper hosts. The relation of the parasite 
 to the host is not in all cases simply that of the fed to the 
 living organism which nourishes it. The host may do much 
 more than supply the parasite with food: it may give it 
 mechanical support, it may stimulate it to grow in charac- 
 teristic forms, it may assist in the dissemination of its off- 
 spring, it may protect it in a variety of ways. In all of 
 these respects the parasite is benefited. 
 
 * Goebel K. /.. ,-. p. 173. 
 
86 PLANT PHYSIOLOGY 
 
 In certain instances it is claimed that parasitism is ad- 
 vantageous not only to the parasite but also to the host. 
 These cases must be examined separately. Pure parasitism 
 is beneficial only to the parasite. Whatever may be our 
 views as to the origin of parasitism, we must admit that 
 parasitic associations are entered into each season only 
 because the parasite is a dependent organism, incapable of 
 elaborating its own complex foods from simple compounds. 
 The parasite may be wholly dependent, taking from its host 
 all the food it needs, or it may be only partly dependent, 
 taking only certain kinds of food. In either case the foods 
 absorbed are worked over, assimilated, incorporated or con- 
 sumed, by the parasite itself. No food is absorbed by the 
 parasite in forms which it can use without modification for 
 building its own body. 
 
 The nutrition of parasites differs from that of other or- 
 ganisms only in certain stages of the process, and not fun- 
 damentally even in these. Instead of absorbing from water, 
 soil, and air the raw materials which must be elaborated 
 into foods, the parasite absorbs from its host matters al- 
 ready elaborated by its host. The means of absorption are 
 the same in parasites as in other organisms (see Chapter 
 IV.). 
 
 Parasitism consists essentially in the absorption from a 
 living organism of more concentrated solutions of more 
 highly elaborated food-materials or foods than can other- 
 wise be obtained. Using the division of the process of nutri- 
 tion proposed on page 41, into, first, the absorption, and 
 second, the combination of food-materials, third, the assimi- 
 lation, and fourth, the incorporation of foods, we see that 
 the parasite differs from the self-sustaining and independent 
 green plant only in omitting the second stage of the process, 
 absorbing from its host the food-materials or foods already 
 elaborated, which an independent plant would elaborate for 
 itself. To make this clearer, let us examine a few instances. 
 
 Among higher plants, perhaps the simplest, in other words, 
 the least complete form of parasitism is that of the Euro- 
 pean and American mistletoes, Viscum album and Phorn- 
 dendron villosun). The mistletoes are green perennials con- 
 
NUTRITION 87 
 
 taining in their leaves and in the cortex of the young and 
 even older branches, an abundance of chlorophyll in nor- 
 mally effective chloroplastids. They are plants which can 
 and do photosynthetically manufacture from water and 
 carbon-dioxide all the non-nitrogenous food which they 
 need. Elevated far above ground on the branches of oaks, 
 poplars, apples, etc., they must draw from their hosts the 
 water which they combine with the carbon-dioxide absorbed 
 by their own leaves. Since the water in plants is a dilute 
 solution of a large variety of matters, chiefly mineral, the 
 mistletoe absorbs from its host these needed substances 
 also. Their absorption is accomplished through peculiarly 
 modified roots called haustoria, the wood or xylem elements 
 of which connect directly with the wood elements of the 
 vascular bundles of the host. Through the xylem the water 
 absorbed from the soil and the mineral salts dissolved in 
 it are transferred to the leaves. The mistletoe, tapping the 
 water-conducting tissues of the host, establishes a water- 
 conducting system continuous with that of the host, and so 
 secures a supply of water and mineral salts as constant and 
 as abundant as that of the host. On the other hand, the 
 chief paths of transfer for elaborated foods, nitrogenous as 
 well as non-nitrogenous, are furnished in higher plants by 
 the phloem elements of the vascular bundles, but from these 
 the foods are distributed osmotically through parenchyma 
 cells to the tissues needing them. The phloem elements of 
 the haustoria of Viscum and Phoraclendron are not con- 
 tinuous with those of the host. * From this fact it has been 
 inferred that mistletoe does not absorb elaborated foods of 
 any sort from its host, that it is, therefore, only a "water 
 parasite.' 7 This inference is scarcely defensible, though in the 
 absence of direct evidence to the contrary the supposition 
 is justified that Viscum robs its host of much less elaborated 
 food than those parasites which have a direct phloem, as 
 well as xylem, connection with their hosts. 
 
 * Peirce. G. J. On the structure of the haustoria of some phanerogamic 
 parasites. Annals of Botany, vol. VII., pp. 317, 318, 1893. Cannon, W. A. 
 The anatomy of Phoradendron villosum. Nutt. Bull. Torrey Bot. Club, 
 vol. 28, 1901. 
 
88 PLANT PHYSIOLOGY 
 
 Gaston Bonnier* claims that Viscum is sometimes directly 
 beneficial to its host. In summer the host produces both 
 actually and proportionally more food than the parasite, 
 although the parasite is then photosynthetically active. At 
 times during the winter and always during the early spring, 
 when the host is leafless, the evergreen parasite may manu- 
 facture carbo-hydrates and the host cannot. While the host 
 is not able to manufacture food and the parasite is, the 
 host is alleged to draw upon the mistletoe for freshly manu- 
 factured non-nitrogenous food. It is, therefore, claimed by 
 Bonnier that the mistletoe ( Viscum ) is parasitic either not 
 at all or only very slightly, although obviously it obtains 
 all its water and mineral salts from the host. But because 
 the parenchyma tissues of the parasite are continuous 
 through the haustoria with those of the host, furnishing the 
 paths of osmotic transfer, the transfer taking place in one 
 direction at one time may be in the opposite direction at 
 another; and so it must be conceded, until proof to the 
 contrary is adduced, that the mistletoe may be more than 
 a "water parasite." Why should the host, on the warm 
 days of winter and in the early spring, have any occasion 
 to draw food from the mistletoe? A healthy apple-tree, or 
 oak, or poplar, will normally lay by in its own body during 
 the summer enough elaborated food to start it well in the 
 succeeding spring. Upon this store it will draw promptly 
 and satisfyingly when the need comes. Is there any less 
 food stored in an apple-tree upon which mistletoe grows? 
 If so, is not the mistletoe the cause, and can this lack ever 
 be wholly compensated for? It would appear, then, that 
 Viscum is a periodic rather than a partial parasite, and 
 that only further investigation can show how nearly its 
 indebtedness to its host is annually balanced. 
 
 Closer association with the host und greater dependence 
 upon it are exhibited by the various species of Cuscuta or 
 dodder, thread-like plants belonging to the Convolvulacese, 
 
 * Bonnier, G. Assimilation du Gui comparee a celle du pommier. Actes du 
 Congres de 1889 d. 1. Societe Bot. de France : Bull. Soc. Bot. de France, 
 1890. Sur 1' assimilation des plantes parasites a chlorophylle. Comptes 
 Rendus, t. 113, p. 1074-6. 
 

 NUTRITION 89 
 
 but differing strikingly from the other members of the 
 family in habit and habits. They are leafless twiners, yellow, 
 orange, or even sometimes claret-red, in color, with very 
 little if any chlorophyll. At frequent intervals their stems 
 and branches form close coils around their hosts,* and 
 from the inner surfaces of these coils haustoria grow into 
 the tissues of the hosts. The haustoria have well-developed 
 vascular bundles, the xylem and phloem of which are united, 
 cell to cell, with the xylem and phloem of the adjacent vas- 
 cular bundles in the host, thus perfectly connecting, in host 
 and parasite, those tissues which conduct aqueous solutions 
 of mineral salts and of elaborated foods respectively, t 
 When the dodder has fastened upon a suitable host, sent 
 haustoria into it, and connected its own vascular tissues 
 with the corresponding ones of its host, it draws food 
 in abundance. Chlorophyll develops only in smallest quan- 
 tity in any part of it.J The dodder absorbs already elab- 
 orated all the sugars which it needs for the construction of 
 cell-wall, for the supply of energy liberated by respiration, 
 for the synthesis of amides, proteids, etc. Not having chlo- 
 rophyll, it lacks the means by which to secure the energy 
 needed for the elaboration of non-nitrogenous food, and for 
 this food it depends wholly upon its host. Presumably it 
 takes its nitrogenous food also in the soluble forms elabo- 
 rated by its host, though it modifies, assimilates, and in- 
 corporates this for itself. When the dodder, having fastened 
 upon an unsuitable host, is inadequately fed, it may become 
 green by the formation of chlorophyll in the chromatophores 
 always present in rudimentary condition in its cortical 
 cells. It can thus add what it manufactures itself to the 
 insufficient supply of non-nitrogenous food which it draw r s 
 from its own innutritions host. The dodder make& food for 
 itself only when it is unable to secure enough ready made. 
 
 * See Chapter VI. for a discussion of the irritability of these and other 
 plants. 
 
 f Peirce G. J. L. c. 
 
 t Ibid., A contribution to the physiology of the genus Cuscuta. Annals 
 of Botany, vol. VIII., p. 91, 1894. 
 
 fbirL, L. c.. p. 83. 
 
90 PLANT PHYSIOLOGY 
 
 Its ability to develop chlorophyll in times of need suggests 
 two hypotheses : that it has only recently abandoned the in- 
 dependent habits still followed by the other members of the 
 family Convolvulacese, and that it has done so in much 
 the same way as the mistletoes, though the latter have 
 progressed by no means so far toward permanent para- 
 sitism. 
 
 It is hardly necessary to discuss whether the host is bene- 
 fited by the encircling dodder, for the dodder is an annual, 
 dying soon after ripening its fruits, into which it removes 
 the greater part of the nutrient substances contained in and 
 composing its body. Thus it leaves little or nothing for its 
 host to absorb and feed upon. When it attacks shrubby 
 perennials e. g. willows its effects are only impoverishing 
 and debilitating; when it successfully attacks annuals, it 
 exhausts and kills them. European growers of flax and 
 clover find the dodder one of the most destructive enemies 
 of their crops. The dodder is, then, a permanent parasite, 
 the parasitism of which is complete, however, only when the 
 host can supply it with all needed foods. 
 
 The most intimate associations between parasite and 
 host, and the most complete dependence of a parasite upon 
 its host, are found among the fungi and bacteria parasitic 
 upon higher organisms. In these associations the parasite 
 not only sends root-like absorbing organs into the host, 
 but in many cases it is very completely enclosed within the 
 host.* The parasitic fungi and bacteria, always wholly 
 devoid of chlorophyll, are entirely dependent upon their 
 hosts for both kinds of food, non-nitrogenous and nitro- 
 genous. These foods they work over, assimilate, incorpo- 
 rate and consume, in their own w r ays, but the elaborated 
 foods they must have. These, then, are examples of per- 
 manent and complete parasitism. About the nature of the 
 association between green plants and the fungi which cause 
 disease in them, e. g. grape-mildew, potato-rot, wheat-rust, 
 etc. there cannot be the least question : the host gains 
 
 * This last is not without parallel among phanerogamic parasites, for 
 Rafflesia, Brugmnnsia,, and our own Arceuthobium (Razoumowskia) are 
 more or less completely imbedded in the tissues of the host. 
 
NUTRITION 91 
 
 nothing from association with the fungus, the parasite 
 gains everything from association with the green plant. 
 
 Only about those remarkable structures called lichens 
 is there any difference of opinion. Lichens are composed 
 of algae and fungi living together. The algae green, blue- 
 green, or brownish contain chlorophyll, and by its aid man- 
 ufacture non-nitrogenous foods from carbon-dioxide and 
 water. Nitrates they absorb in solution. From these and 
 the sugars they elaborate amides and proteids like other 
 independent plants. The fungi, on the contrary, devoid of 
 chlorophyll, cannot elaborate non-nitrogenous foods and 
 must absorb them ready formed. In the lichen the fungus is 
 always closely applied to the algal cells, sends out short 
 branches which clasp the algal cells, and, in a considerable 
 number of already reported cases, these short branches send 
 still shorter haustoria into the algal cells.* Whether only 
 closely applied to the walls, or sending haustoria into the 
 cells, the fungus filaments are so placed that they can draw 
 food by osmosis from the alga. Because of the small size of 
 the alga, the always larger fungus cannot become entirely 
 enclosed within it ; on the contrary, the fungus surrounds the 
 alga with a more or less firm mycelium, confining the alga 
 between the parts of its body. The association of fungus 
 and alga, always intimate enough for the fungus to supply 
 itself osmotically with non-nitrogenous foods elaborated by 
 the alga, is in many cases so exhausting to the alga that 
 many of its cells become entirely emptied. In spite of this 
 evidence of the complete parasitism of the fungus, some 
 botanists claim that the alga benefits also. It is alleged 
 that the carbon-dioxide exhaled by the fungus, the mineral 
 salts dissolved and held in solution, the protection against 
 too rapid drying, too intense illumination, and too sudden 
 changes of temperature, are of sufficient value to the 
 alga to compensate it for the food taken from it and for 
 the deformities and limitations in its growth. It may be 
 
 * Peirce, G. J. The nature of the association of alga and fungus in 
 lichens. Proc. Cal. Acad. Sci., Series III., Botany, vol. I., 1899. The rela- 
 tion of fungus and alga in lichens. American Naturalist, vol. XXXTV., 
 1900. 
 
92 PLANT PHYSIOLOGY 
 
 true that all these benefits do accrue to the alga though 
 this is very far from being demonstrated but even if this be 
 true, are these benefits needed, are they not superfluous? 
 Man's association with his domesticated animals is bene- 
 ficial to them, but are these animals really any better off 
 than by themselves in nature? Even if man's association 
 with them is beneficial for the time, in the end it is fatal or, 
 at least, onerous. The absolute dependence of the fungus 
 component of the lichen upon some green plant for food, 
 and the damage and death to the alga wrought by the 
 fungus, furnish the strongest evidence that the association 
 is not equally beneficial to the two members, mutually bene- 
 ficial though it is sometimes claimed to be. 
 
 In these cases, we have examples of the stages through 
 which parasitism has advanced first, green plants, incom- 
 pletely and only periodically parasitic; second, plants nor- 
 mally not green, permanently parasitic, and completely 
 parasitic when the host is suitable; third, plants never 
 green, permanently and completely parasitic. 
 
 The bacteria living in the bodies of animals, either inter- 
 cellularly or intracellularly, absorbing already elaborated 
 foods, utilizing these by processes fundamentally like those 
 already discussed, present no new physiological principles, 
 and hence those who would know more of this, as of the 
 other special groups of organisms which we have just been 
 discussing, must turn to the treatises devoted to them. 
 
 OTHER ELEMENTS ESSENTIAL TO PLANTS 
 
 The physiological chemistry of the other elements con- 
 cerned in the nutrition of plants is still so vague that little 
 can be said about them till further investigations give 
 definite facts to deal with. These elements are obtained in 
 analyses of plants in the form of incombustible compounds 
 forming the ash resulting from combustion. For this reason 
 they are collectively termed ash-constituents, to distinguish 
 them from hydrogen, oxygen, carbon, and nitrogen, which 
 go off in drying and burning. Besides the salts containing 
 the elements absolutely essential to the normal nutrition of 
 the plant, others are invariably present in the ash because 
 
NUTRITION 93 
 
 they are present in soluble form in the soil. Though some 
 few of these may be useful, they are not necessarily es- 
 sential. 
 
 Chemical analysis reveals whatever is present in the plant- 
 body, but neither indicates the compound in which an ele- 
 ment exists in the living body, nor enables one to distin- 
 guish between necessary substances and those present in the 
 plant simply because they are present in the media in which 
 it lives in soil, air, and water. Only by the culture of 
 plants in media of known composition can the essential 
 elements and compounds be distinguished from the non- 
 essential, the useful but not essential from the absolutely 
 useless and the absolutely necessary. Analysis shows that 
 ordinarily 1.5-5% of the dry weight of plants is furnished by 
 the ash constituents of all sorts, though in some cases 
 10-30% is ash. Analysis alone cannot account for this dis- 
 crepancy. Culture in media of known composition shows 
 that the greater amount of ash is due to peculiarities of 
 the soil or to peculiarities of certain species or even families 
 of plants. For example, 18-23% of the ash of Indian Corn 
 is silicic dioxide, useful to the plant in hardening its outer 
 surfaces and making projecting parts and the edges of 
 leaves harsh and cutting, but not essential to its stiffness, 
 growth, and perfect maturity. * Diatoms and the scouring- 
 rushes (Equisetum) are much richer in silica than the 
 grasses, but it is not yet proved that it is indispensable 
 even for them. 
 
 Analysis reveals the presence of sodium and chlorine, as 
 common salt, in all plants. Because culture without these 
 elements is so difficult that it is doubtful whether it has 
 ever been accomplished, no one can say whether they are 
 absolutely essential or not. Experiment has already con- 
 clusively shown, however, that they are needed only in the 
 smallest possible quantities if at all, though in certain cases 
 larger quantities may act as favorable stimulants. This 
 last is especially evident in the bacteria, the growth of 
 which in the artificial culture media and under the unnatural 
 
 * Quoted by Pfeffer (Pflanzenphysiologie, Bd. I., p. 429, Engl. tranel. 
 p. 435) from Sachs (Flora, p. 52, 1862). 
 
94 PLANT PHYSIOLOGY 
 
 Conditions of the laboratory seems to be facilitated by the 
 addition of a small amount of common salt to the culture. 
 The ash of strand and marine plants contains a larger per- 
 centage of sodic chloride than that of inland plants. This 
 is due simply to the presence of so much salt where they 
 grow. Strand plants do not need salt, as is proved by 
 cultures.* Inland plants are unfavorably influenced by a 
 percentage of salt, in the soil or in water, which strand 
 plants bear without injury. These last have succeeded in 
 becoming adapted to conditions which preclude or minimize 
 competition from more sensitive forms. The adaptations 
 are discussed in the rapidly increasing literature on the 
 ecology of the so-called halophytes.f Although the common 
 salt in sea water is needed as food only in the smallest 
 quantities if at all by the marine algge, they will bear only 
 the most gradual transfer to fresh water. This is probably 
 due, however, to the greater density of the sea water, and 
 this depends upon the other salts dissolved in it as well as 
 upon this single one. 
 
 An interesting experimental study of strand and other 
 plants with relation to common salt and sea water has re- 
 cently been made by Coupin.J; He finds that 1.5% of com- 
 mon salt in soil or in water is poisonous to plants which 
 do not naturally grow on the sea-shore. Since sea 
 water contains about 2.5% of common salt and the soils 
 bathed by the sea contain still more than this proportion, 
 we can readily understand the sharp line which separates 
 the marine and strand floras from those of the interior. 
 oupin attributes the poisonous property of sea water for 
 inland plants mainly to its content of common salt, for the 
 two salts next to this in abundance, magnesium sulphate 
 and chloride, are present in quantities which he says are 
 below the toxic proportions. Magnesic sulphate is poison- 
 
 * See Nobbe in Versuchsetationen, Bd. 13, 1870, the literature there 
 cited, and Pfeffer, Pflanzenphysiologie, I., p. 424, etc., Engl. transl. I. 
 pp. 429, 30, etc. 
 
 f For example, Schimper, A. F. W. Pflanzengeographie auf physiolo- 
 gischer Grundlage, Jena, 1898. 
 
 t Coupin, H. Sur la toxicite du chlomre de sodium et de 1'eau de mer 
 a 1'egard dee vegetaux. Rev. gener. de Botanique, T. X., 1898. 
 
NUTRITION 95 
 
 ous at a concentration of 1%, magnesic chloride at 0.8%, 
 but they occur in sea water only to the extent of 0.75% 
 and 0.6% respectively. For strand plants the propor- 
 tions of common salt are very different, as these figures 
 indicate : 
 
 Fatal. Injurious. Harmless. 
 
 Beta maritima 4% 3% 2.6% 
 Atriplex hastata 
 
 var. maritima 5% 4% 3.5% 
 
 Cakile maritima 4% 3% 2.8% 
 
 For these three plants 3% of magnesic sulphate, 2.5% of 
 magnesic chloride, are poisonous. From these figures it is 
 obvious that strand plants are very accurately adapted to 
 the amount of common salt to which they are exposed, and 
 that they can withstand much more of the other salts than 
 they are ever exposed to.* 
 
 The soluble compounds of zinc are poisonous to all plants. 
 In quantities not excessive when first encountered, or in 
 amounts to which the special plants have become accus- 
 tomed by long habit, zinc salts seem either to be ineffective 
 or else to act as stimulants to more active growth and 
 life.t It is claimed that there is a flora characteristic of 
 soils rich in zinc. This is an exaggeration, but it cannot be 
 denied that certain plants are found on zinc soils and not 
 elsewhere (e. g. Viola calaminaria and Thlaspi calami- 
 narium ) . These, however, are varieties of other species ( viz. 
 of V. lutea and T. alpestre), the variation being induced by 
 the poison, t 
 
 Aluminum salts, though of very general occurrence, are 
 
 * Schimper claims (L. c. pp. 98 102) that plants living in soils rich in 
 freely soluble salts like sodic chloride, saltpeter, etc., present structurarand 
 other characters almost identical with those of desert plants. This he re- 
 gards as evidence that halophytes as well as xerophytes seek to reduce 
 transpiration, the latter because of the scarcity of water, the former be- 
 cause of the presence in it of poisonous compounds. 
 
 \ Richards, H. M. Beeinflussung des Wachsthums einiger Pilze durch 
 chemische Reize. Jahrb. f. w. Bot., Bd. 30, 1897. 
 
 \ Schimper, A. F. W. Pflanzengeographie auf physiologischer Grundlage, 
 Jena, 1898. 
 
96 PLANT PHYSIOLOGY 
 
 found in quantity only in the lycopods.* It is doubtful 
 whether aluminum is necessary or even useful for these 
 plants. 
 
 The essential soil ( ash ) constituents, as shown by water- 
 culture, are salts of phosphorus; sulphur, potassium, cal- 
 cium, magnesium, and iron. Calcium seems to be un- 
 necessary for fungi, though indispensable for higher 
 plants. 
 
 PHOSPHORUS is a constituent element of protoplasmic 
 matters. In the nucleins it may amount to 6%. According 
 to Wolff's analyses, t phosphoric oxide (P 2 5 ) constitutes 
 about one-third of the ash obtained from embryonic tissue. 
 This tissue is rich in protoplasmic matters. In older tissues 
 containing a smaller proportion of protoplasmic matters, 
 and in dead and emptied cells, the amount of phosphorus 
 compounds is much less. In the total dry weight of a 
 plant, the amount of phosphorus, calculated as phosphoric 
 acid, is slight. This is shown by the following figures J 
 
 in lupine seeds 1.63% 
 
 straw 0.30% 
 
 " potato tubers 0.63% 
 
 " wood of trees 0.05% 
 
 Though the percentage of phosphorus in the body of an or- 
 ganism indicates the degree to which it is used, it by no 
 means indicates the degree in which it is needed. Without 
 phosphorus,- protoplasm could not exist. 
 
 The source of phosphorus for the majority of plants is 
 the phosphates in soil and water. Other plants under 
 
 * Pfeffer, W. Pflanzenphysiologie, I., p. 432, Engl. transl. I., p. 437. 
 L. Chamsecyparissus and //. Alpinum have 22-27% aluminum in the ash, 
 while L. phlegmfiria, Selaginella, etc., contain only traces. Yoshida, H. 
 On aluminum in the ash of flowering plants. Journ. Coll. Science, Im- 
 perial University, Tokio, 1887. In analyses of rice, wheat, oats, beans, 
 etc. the Al. varies from 0.05% to 0.27% of the ash. 
 
 f Quoted from Versuchsstationen, Bd. 30, by Pfeffer in his Pflanzen- 
 physiologie, L, p. 407, Eng. transl. I., p. 414. 
 
 t Frank, A. B. Lehrbuch der Botanik, Bd. I., p. 587. Also in reports 
 of State Agricultural Experiment Stations, etc., similar figures may be 
 found. 
 
NUTRITION 97 
 
 natural conditions may at all times supply themselves with 
 phosphorus from its organic compounds in humus, in dead 
 bodies of plants and animals, and in living bodies. The 
 phosphates are less freely soluble in water than the salts of 
 the other necessary elements ; but since plants demand only 
 small quantities, and since the carbon-dioxide and possibly 
 also other acid secretions of their roots dissolve them more 
 rapidly than does pure water, plants secure under ordinary 
 conditions in nature all the phosphates needed. Manuring 
 with phosphates has to be resorted to only wiien the soil 
 has become impoverished by the removal of the crops culti- 
 vated upon it. 
 
 In the plant phosphorus occurs not only as a constituent 
 element of living protoplasm, but also in those already 
 highly elaborated substances to be used in the construction 
 of protoplasm, and in the simpler compounds formed by the 
 breaking up of living or lifeless protoplasmic matter. The 
 compounds of phosphorus occur, therefore, in the plant as 
 solids as parts of its structure and as stored material; 
 also in solution as constructive material and as the pro- 
 ducts of destructive metabolism. 
 
 SULPHUR, also a constituent element of protoplasm and 
 therefore always present in the ash of plants, is found in 
 even smaller proportions in the plant-body than phos- 
 phorus, as the following figures show * 
 
 in lupine seeds 0.36% 
 
 " potato leaves 0.43% 
 
 tubers 0.24% 
 
 " wood of trees 0.025% 
 
 The source of sulphur for most plants is the various salts 
 of sulphuric acid commonly found in the soil and dissolved 
 in ordinary waters. Dependent plants the so-called para- 
 sites and saprophytes may perhaps obtain some sulphur in 
 the form of organic compounds. The moulds can use salts 
 of sulphurous acid, if present in sufficiently dilute solution, f 
 although they are nearly as poisonous to all higher plants 
 
 * Frank, A. B. Lehrbuch, I., p. 586. 
 
 f Quoted by Pfeffer from Nageli, Bot. Mittheilungen, Bd. 3, 1881. 
 
 7 
 
98 PLANT PHYSIOLOGY 
 
 as sulphurous oxide and dioxide, gases poured forth in con- 
 siderable quantities from chimneys in which inferior sorts of 
 coal are burned.* A few species of bacteria use as food- 
 material, as well as their source of energy by respiration 
 (see page 20), the sulphuretted hydrogen and metallic 
 sulphur occurring in considerable quantities in mineral 
 springs and set free in decompositions taking place in the 
 ooze under bodies of water. 
 
 Sulphur occurs in plants in the elementary condition only 
 in well-fed sulphur bacteria. In these, as in all other plants, 
 it occurs also as a constituent of protoplasm. A few plants, 
 notably the Cruciferze, contain sulphur in mustard oil, and 
 it is a constituent of the garlic oil in the various species of 
 Allium. 
 
 Nothing whatever is known of the stages through which 
 the amides are elaborated into protoplasmic matters by the 
 addition of sulphur and phosphorus. 
 
 POTASSIUM, although not a constituent element of proto- 
 plasmic substances, is an indispensable food-constituent. 
 The compounds of sodium, frequently much more abundant, 
 fail to serve as complete substitutes, although in the pres- 
 ence of an abundance of sodium salts plants demand less 
 potassium than otherwise. In the total dry weight of plants 
 potassium occurs in much larger amount than sulphur and 
 phosphorus t 
 
 in potato tubers 2.27% 
 
 plants 2.53$ 
 
 " tobacco leaves 4.99% 
 
 " young red clover plants 3.59% 
 
 " lupine seeds 1.31$ 
 
 " wood of trees G.05%-0.15% 
 
 Potassium salts are most abundant in young and growing 
 parts, least abundant in those which have ceased to grow 
 or to be otherwise active, and from which the potassium 
 
 * The striking absence from certain cities in which one might otherwise 
 expect to find them, of lichens and other plants especially sensitive to 
 gaseous poisons, may be attributed to the poor coal. 
 
 t Frank. Lehrbuch, I., p. 589. 
 
NUTRITION 99 
 
 has been withdrawn. From this it appears that potassium 
 compounds are intimately concerned in the construction of 
 protoplasmic matters. What compounds these are is not 
 now known, though it is evident that potassium may be a 
 component of reserve foods. Nobbe's hypothesis* that po- 
 tassium salts are directly concerned in the translocation of 
 the starch formed in chlorophyll-containing cells is interest- 
 ing, because it suggests one of their possible uses in the 
 plant, but that this is their main function follows neither 
 from his experiments nor his arguments. 
 
 According to Copeland,t the potassium salts are 4 'an im- 
 portant part of the osmotically active material which keeps 
 the cell and plant turgid," and "there is no experimental 
 ground for attaching this significance to any other con- 
 stituent of the mineral food." 
 
 Potassium salts are never very abundant in soils. They 
 occur as sulphate, phosphate, and chloride, freely soluble, 
 and hence easily washed away as well as absorbed. It is 
 necessary frequently to manure soils with potassium salts 
 when tobacco and other crops demanding considerable 
 amounts of potassium are cultivated year after year on 
 the same soil. 
 
 CALCIUM is neither a constituent of protoplasm nor neces- 
 sary to all plants. For the fungi, and perhaps for certain 
 algae, I it is entirely unnecessary, magnesium successfully 
 taking the place of calcium, besides performing its own part 
 in nutrition. All other plants demand both calcium and 
 magnesium. Unlike the elements already considered, the 
 salts of calcium are found in tissues which have attained 
 their full growth and in which work of another kind is 
 especially going on, namely, in the chlorophyll-containing 
 food-making cells of the leaves and cortex. In organs in 
 which elaborated foods are stored, and in such dead parts 
 
 * Nobbe, in Landwirtschaftliche Versuchsstationen, Bd. 13, 1870. 
 
 t Copeland, E. B. Relation of nutrient salts to turgor. Botanical 
 Gazette, Vol. 24, 1897. 
 
 t Molisch, H. In Botanisches Centralblatt, Bd. 60, 1894, and Sitzungs- 
 ber. d. Wiener Akademie, Bd. 103, 104, 105, Abth. I., 1894, 1895, 1896. 
 Benecke, W-, in Botanisches Centralblatt, Bd. 60, 1894. Jahrbuch f. 
 wiss. Botanik, Bd. 28, 189, and Botanische Zeitung, 1896. 
 
100 PLANT PHYSIOLOGY 
 
 as the wood, calcium salts are less abundant. These facts 
 are indicated by the following table * 
 
 Potato leaves have 2.90% of dry weight as calcium salt. 
 
 tubers 0.10% 
 
 Pea straw 1.88% 
 
 " seeds 0.13% 
 
 Tobacco leaves 6.18% 
 
 Hop " 7.38% 
 
 Wood of trees 0.02-0.10% 
 
 From such figures, the result of gross analyses confirmed 
 by microchemical tests, it has been inferred that calcium 
 is concerned in the formation of cell-wall and in neutralizing 
 the oxalic acid set free in various chemical changes ( notably 
 respiration) taking place in plant cells. Proof that these 
 inferences are correct is still lacking, however. 
 
 Loew's hypothesis! that the framework of nucleus and 
 plastids is a double organic salt of calcium and magnesium, 
 or a complex union of calcium and magnesium compounds, 
 is a hypothesis only, and hence need only be mentioned. 
 
 The value of calcium to higher plants is beyond question, 
 but how it is used is still unknown. It is found in plants 
 usually as the oxalate (crystallized out as needles or as 
 polyhedra of more symmetrical dimensions), as carbonate 
 ( deposited in the peculiar outgrowths of cell-wall known as 
 cystoliths), much less frequently as sulphate and phos- 
 phate. Calcium salts are abundant enough in the soil to be 
 taken up by all plants, and although the sulphate and 
 phosphate are only slightly soluble, the large volumes of 
 water absorbed will still carry adequate amounts, even from 
 soils which contain little or no other calcium compounds, 
 to the cells using them. 
 
 MAGNESIUM, absolutely indispensable to all plants, $ in 
 spite of the assertions of earlier authors to the contrary, 
 
 * Frank's Lehrbuch, I., p. 590. 
 
 t Loew, 0. Uber die physiologischen Functionen der Calcium-und Mag- 
 nesiumsalze im Pflanzenorganismus. Flora, 1892. Uber das Mineralstoff- 
 bediir f. niss von Pflanzenzellen. Bot. Centralblatt, Bd. 63, 1895. 
 
 t Molisch, H. L. c. under calcium. Benecke, W. L. c. under calcium. 
 
 Nageli. Botanische Mittheilungen, Bd. 3, 1881 and others. 
 
NUTRITION 101 
 
 is even less understood in its physiological relations than 
 calcium. As phosphate it may stand in some relation to 
 the formation of protoplasmic matters, although it is not a 
 constituent element of protoplasm. Like calcium, it occurs 
 as a constituent of some of the reserve foods stored hi such 
 oily seeds as Castor Bean (Ricinus) and Brazil Nut (Ber- 
 tholetia ) ; but even here, hi these complex compounds, it is 
 not clear whether it is the magnesium ( or calcium ) itself, or 
 the acid radicle of the salt, which is valuable. 
 
 IRON. Although this element is not known to be a con- 
 stituent of protoplasm or of any compound incorporated 
 into the living protoplasm, it is indispensable to all plants. 
 The minimum needed is, however, smaller than that of any 
 other element. If more than the minimum amount of iron 
 is supplied, growth seems to be proportionally stimulated.* 
 Unless green plants receive enough iron they will not be 
 able to form chlorophyll, although iron is not a constituent 
 element of any chlorophyll pigment. Plants remaining 
 white from lack of iron are said to be chlorotic. Chlorosis 
 may be regarded either as the evidence of an abnormal 
 state of health or as the disease itself. The former seems 
 by far the more probable. 
 
 The plant ordinarily obtains all the iron it needs from 
 the salts of iron dissolved in all natural waters. Sometimes, 
 however, chlorosis occurs in spite of the abundance of iron 
 in the soil. When a shoot grows so rapidly that iron salts 
 do not reach the developing parts rapidly enough, the new 
 leaves will be white instead of green. 
 
 Some of each of the necessary elements found in the ash 
 of plants is necessary to the normal development of the 
 plant. The plant will develop normally only when it can 
 obtain an amount of mineral matter in solution more than 
 equal to the sum of the minimal amounts of each of the 
 ash constituents. In virgin soil, and in the natural forest, 
 the soil waters will contain all the necessary mineral salts 
 
 * Richards, H. M. Die Beeinflussung dee Wachsthums einiger Pilze diirch 
 chemische Reize. Jahrb. f. wise. Botanik, Bd. 30, 1897. The effect of 
 chemical irritation on the economic coefficient of sugar. Bull. Torrey Bot. 
 Club, vol. 26. 1899. 
 
102 PLANT PHYSIOLOGY 
 
 dissolved in sufficient quantities for normal growth. Where 
 man interferes by defective cultivation or mistaken selection 
 of crops, the soil must be artificially enriched. In nature 
 the constant supply of adequate amounts of all the food- 
 materials is secured by the action of those forces and or- 
 ganisms which break down the inorganic and organic mat- 
 ters on and in the upper layers of rock and soil. 
 
CHAPTER IV 
 
 ABSORPTION AND MOVEMENT OF WATER FOOD DISTRIBUTION 
 
 IN the preceding chapter we -have discussed the elements 
 and their compounds, which constitute the food-materials of 
 plants, and we have gained some idea as to when, by what 
 means, and through what stages these are elaborated into 
 foods. How foods are assimilated rendered like the living 
 protoplasm which they are to nourish is not now evident. 
 We have finally seen that these assimilated foods may be 
 incorporated into and made a part of the living protoplasm, 
 but how this is accomplished remains one of the wholly 
 unsolved problems of physiology. 
 
 We must now consider how the plant absorbs its food- 
 materials and transfers from part to part the foods which 
 it elaborates from them. These processes underlie and at 
 the same time form an essential part of nutrition, but 
 since other substances than those needed and used as food- 
 materials and foods are concerned, we may well consider 
 this subject by itself. 
 
 With the exception of water, which is both a food-material 
 and the vehicle of nutrient substances, the food-materials of 
 animals and plants are of two sorts either gases or solids. 
 The latter are available only in solution, entering the body 
 and passing from part to part only when dissolved in 
 water. * The absorption and transfer from part to part in 
 the plant-body of the gaseous food-materials carbon-diox- 
 
 * The apparent contradiction to this statement offered by the Myxomy- 
 cetes, Amoeba, etc. naked masses of protoplasm not enclosed by cell-wall 
 which may in their movements surround solid particles, both innutritions 
 and nutritious, is not a real one. Only those particles which are soluble 
 and dissolved are absorbed by the protoplasm and really enter it; the 
 others are left unaffected by it although they may remain enclosed for a 
 time. 
 
104 PLANT PHYSIOLOGY 
 
 ide, oxygen, and free nitrogen ( when the last is used at all ) 
 take place in accordance with the simple and generally 
 known laws governing the diffusion of gases. We must, 
 however, bear distinctly in mind that these gases are in 
 two states in the bodies of all higher and of many lower 
 plants. Through the stomata gases pass in the gaseous 
 condition into or from the intercellular spaces,, the air pas- 
 sages, etc. This movement through the stomata and in the 
 intercellular spaces is strictly by diffusion, except where 
 affected by mechanical forces such as compress or expand 
 the air-spaces, etc., etc. But when gases pass through the 
 cell-wall, into or out from a cell, their molecules mix with 
 the water-molecules in cell-wall, protoplasm, and vacuoles, 
 becoming dissolved in the water which the living body con- 
 tains as an essential part of its structure. The movements 
 of gases into and out from living cells, and from cell to cell, 
 are therefore the movements of solutes (dissolved sub- 
 stances ) . The absorption of solutions into living cells, and 
 their transfer from cell to cell, take place in accordance 
 with the laws governing the diffusion of liquids. We can, 
 therefore, study the movements of solids and of gases at the 
 same time, for, so far as living cells are concerned, these 
 two classes of substances behave alike. So far as the supply 
 of solids and of gases to the living cells is concerned, we 
 have to deal with different phenomena, and these we must 
 study separately. 
 
 The water in the soil, and consequently all flowing water 
 and that in pools, ponds, lakes, and in the sea, is a dilute 
 solution of nutrient and other soluble substances from the 
 air, from the mineral matters of the soil, and from the 
 mixture of organic and inorganic matters collectively termed 
 humus, which is found in all but the most sterile soil. The 
 water of streams, ponds, lakes, and of the sea, is in the 
 hydrostatic or massive state. After heavy rain, flood, or 
 the melting of snow and ice, water is in the hydrostatic 
 state in the soil also. Water in this condition can be 
 drained off, but much will remain in other conditions, the 
 amount depending upon the character of the soil. Soil 
 which has been thoroughly drained, but not dried, will feel 
 
ABSORPTION AND MOVEMENT OF WATER 105 
 
 damp to the touch because of the considerable amount of 
 water held in the capillary state between the soil particles. 
 This can be removed by applying material possessing 
 stronger capillary attraction, for instance, blotting or fil- 
 ter paper, which will quickly become damp by withdrawing 
 water from the soil capillaries.* After soil has been dried 
 as thoroughly as possible by removing the capillary water 
 through stronger capillary attraction, water will still be 
 retained in the hygroscopic state, held on the surfaces of the 
 soil particles themselves. To overcome the attraction of the 
 soil particles and to remove the last traces of water much 
 greater force must be employed. In order to make soil 
 absolutely dry it must be taken away from its natural 
 position and exposed to some powerful dehydrating influ- 
 ence, e. g. concentrated sulphuric or glacial phosphoric acid- 
 in a desiccator, or, more simply, to heat in an open vessel. 
 Another means of demonstrating the very considerable force 
 by which soil particles attract and hold water and the sub- 
 stances in solution in it, is by filtering a dilute solution of 
 some convenient copper salt or of Fuchsin through soil. 
 The last trace of copper or of color will be removed from 
 the solution, and the filtrate may be successfully used for 
 the culture of plants, though the original copper solution 
 would have been poisonous. 
 
 The amounts of water occurring in the hydrostatic, capil- 
 lary, and hygroscopic states in soils will vary with their 
 composition, with the fineness of the particles, and with their 
 compactness on the surf ace. f It is essential that plants 
 growing in coarse soils which drain rapidly, and in re- 
 gions where no rain falls during the growing season, 
 should be able to supply themselves with water even after 
 the hydrostatic and capillary waters have been entirely re- 
 moved from those layers of the soil traversed by the roots. 
 To accomplish this it is absolutely necessary that the plant 
 should be able to exert sufficient force to draw into its own 
 
 * It must, of course, be noted that the cellulose walls of the capillaries 
 in filter paper also imbibe water. 
 
 t See Year Book U. S. Dep't Agriculture, 1897, p. 129, report by M. 
 Whitney, and also other papers on Soils published by U. S. Dep't Agriculture. 
 
106 PLANT PHYSIOLOGY 
 
 body the water so strongly held on the surfaces of the soiLpar- 
 ticles. In California, the only water upon which most plants 
 not subjected to irrigation can draw during the greater part 
 of the dry season is that held hygroscopically, and that plants 
 grow at all or even survive during the dry season is positive 
 evidence that they do exert such an attractive force. What 
 are the means at hand? To answer this question we must 
 consider the physical properties of vegetable cells. 
 
 The typical vegetable cell an alga, a root-hair, a paren- 
 chyma cell is bounded by a thin cellulose membrane per- 
 meated with water holding in solution a variety of mineral 
 and other substances. This membrane is firm, strong, 
 elastic, and is not only permeated with water, 7. e. has 
 molecules of water between its molecules or groups of mole- 
 cules, but also permits the movement of molecules of water 
 and of substances in solution in water in and through itself. 
 The movement of water and of aqueous solutions through 
 a membrane is known as osmosis. Lining the cellulose 
 wall is the layer of living protoplasm which produced it. 
 The layer of protoplasm not only varies in thickness, being 
 thickest in young and thinnest in old cells, but is never 
 homogeneous. Apart from the nucleus, chromatophores, 
 and granules of food and other substances contained in it, 
 the surface of the protoplasmic layer in contact with the 
 cell-wall is differentiated into an exceedingly thin living 
 membrane, the ectoplast. Within the living protoplasm are 
 the vacuoles one or more bodies of water holding in solu- 
 tion a great variety of compounds, organic and inorganic 
 and the nucleus, also bounded, like the living protoplasm 
 against the cell-wall, by cytoplasmic membranes. The mem- 
 branes bounding the vacuoles are called tonoplasts. The 
 solution filling the vacuoles and permeating the cell, the cell- 
 sap, is the active agent in absorption, although its composi- 
 tion, and therefore its action, are controlled by the living pro- 
 toplasm, either by the substances formed by the protoplasm 
 and transferred to the cell-sap, or by the substances permit- 
 ted by the cytoplasmic membranes to enter or pass out of the 
 cell, the vacuoles, or the nucleus. That the cytoplasmic 
 membranes do exert a controlling power over substances in 
 
ABSORPTION AND MOVEMENT OF WATER 107 
 
 solution in the vacuoles and outside the cell is evident from 
 the following. Under normal conditions the protoplasm is. 
 slightly alkaline, the cell-sap slightly acid! If the cytoplas- 
 mic membrane bounding a vacuole (the Vacuolenhaut, 
 as Pfeffer calls it) were permeable to all substances, and 
 equally permeable in both directions, this difference in 
 chemical reaction could exist only momentarily ; it could 
 not be the normal condition. Furthermore, so long as the 
 cells are alive, no color will pass from clean slices of beet or 
 of red or black cherry, or from other tissues composed of 
 cells containing colored sap in the vacuoles ; but if the cells 
 are killed by immersion in hot water or by steam, the 
 color will rapidly pass out into the water. Again, although 
 harmless coloring solutions will pass into and stain the 
 walls, the majority of such solutions, no matter what their 
 concentration, will not pass into the protoplasm or stain 
 any part of it, so long as the cells are alive. Some few 
 harmless stains, if applied in sufficiently dilute solutions, 
 may be employed to stain living protoplasm* and nuclei, t 
 or may be accumulated in the vacuoles t of living cells. 
 
 From these experiments it is obvious that the living proto- 
 plasm, especially the structurally and physiologically differ- 
 entiated layers adjoining the cell- wall and bounding the vacu- 
 oles and nucleus, do exercise some control over the dissolved 
 substances adjacent. That this power is limited is shown by 
 the above experiments with staining agents, by daily experi- 
 ence in the laboratory with the various poisons employed 
 as fixing agents for histological purposes, and by the 
 sensitiveness of plants to the soluble substances by which 
 they are surrounded. If the cytoplasmic membranes could 
 exclude poisons, at the same time allowing nutritious solu- 
 tions to enter freely, the advantage would be great. 
 
 * Pfeffer, W. Uber Aufnahme von Anilinfarben in lebende Zellen. Unter- 
 suchungen aus d. bot. Institut zu Tubingen, Bd. II. 
 
 f Campbell, D. H. The staining of living nuclei. Ibid. 
 
 Pfeffer, W. L. c. 
 
 $ The living cell may, however, control the osmotic exchanges taking 
 place between itself and the solutions outside, not only by regulating the 
 composition of the cell-sap, but also by changing the permeability of the 
 cell-wall. See Nathansohn, Zur Lehre vom Stoffaustausch. Ber. d. D. 
 Bot. Gesellsch., XIX., pp. 509-13, 1901. 
 
108 PLANT PHYSIOLOGY 
 
 The living plant-cell is then a series of concentric perme- 
 able ( or partially permeable ) membranes of different compo- 
 sition, properties, and needs, surrounding and enclosing 
 one or more bodies of water holding various substances 
 in solution. Under ordinary conditions the density of this 
 aqueous solution is greater than that of the solutions out- 
 side the cell, and its composition is different. The difference 
 in density is maintained in land plants, as we shall see from 
 another section of this chapter, by the loss of water from 
 the leaves ; the difference in composition is due to the activi- 
 ties of the protoplasm. In constantly submerged aquatics 
 the means of maintaining the density of the cell-sap is less 
 obvious, for from these plants no water is lost by evapora- 
 tion, and no concentration by this means takes place. Al- 
 though the water in the cells of algae may not be lost or 
 changed, the same result as regards the density of the cell- 
 sap may be attained in another way. If one or more solu- 
 ble substances are formed by the protoplasm and trans- 
 ferred to the cell-sap faster than they can pass out into the 
 surrounding water, greater density will be maintained. It 
 is upon the density and composition of the cell-sap, whether 
 this is accumulated in larger volumes in vacuoles or is 
 uniformly distributed throughout the living protoplasm, 
 that absorption depends. 
 
 DIFFUSION AND OSMOSIS 
 
 Let us turn aside for a moment from the living cell to 
 consider some of the purely physical phenomena and princi- 
 ples underlying the physiological process we are studying. 
 If two equal volumes of liquid of exactly the same compo- 
 sition say, two volumes of pure water are brought into 
 contact with each other, there will be molecular movements 
 in and between them, but there can be no change in the 
 composition or pressure or any other quality of either. 
 Suppose five grammes of common salt to have been perfectly 
 and uniformly dissolved in one of these volumes of water 
 before the two were brought into contact. As a result of 
 bringing the two volumes of water into contact the molecu- 
 lar movements in and between the two volumes will produce 
 
ABSORPTION AND MOVEMENT OF WATER 109 
 
 a change in the composition of both. The molecules of 
 salt, being free to move not only through the one volume 
 in which they were dissolved, but also into and through the 
 second volume, will do so, and there will therefore come to 
 be finally an equal distribution of salt molecules in the two 
 volumes, each volume then containing two and one half 
 grammes of salt. To such molecular movement, unaided by 
 stirring, jarring, or other mechanical means, the name diffu- 
 sion is given.* Let us suppose now that one volume of 
 water contained five grammes of common salt and the 
 other five grammes of any other salt, say potassium nitrate. 
 The diffusion of the common salt from the first into the 
 second volume of water, and of the potassium nitrate from 
 the second into the first volume of water, would be at practi- 
 cally the same rate as into pure water. Theoretically there 
 should be a difference in rate ; actually there is no difference 
 which can be detected. There would presently be two and 
 one-half grammes of common salt and two and one-half 
 grammes of potassium nitrate in each volume of water. 
 But if we had five grammes of common salt in one volume 
 and two grammes of the same salt in the other, the diffu- 
 sion would not be so rapid, although the mixture would 
 finally be as perfect. The rate of diffusion will vary with 
 the difference in the proportions of salt in the two volumes, 
 the greater the difference at the beginning the more rapid 
 the diffusion; the nearer the proportions come to being 
 equal the slower the diffusion, till, ultimately, with equal 
 proportions, the diffusion ceases. So long as there is no 
 chemical action of one salt upon another, this rule applies 
 as well to solutions containing mixtures of salts as to 
 solutions of single salts. 
 
 If now we bring together two equal volumes of pure water, 
 only interposing a permeable membrane ( bladder, vegetable 
 parchment, cell-w T all) between them, there will be the same 
 molecular movements as in the first case above supposed, 
 but there can be no change in composition. Similarly there 
 
 * Illustrative experiments on diffusion are described in Darwin and 
 Acton's and in the other laboratory manuals of plant physiology already 
 referred to (p. 27). 
 
110 PLANT PHYSIOLOGY 
 
 will be movements of the salt as well as of the water mole- 
 cules through the membrane if, in the other cases, we sepa- 
 rate the two volumes of liquid by a permeable membrane. 
 As we have already seen (p. 106), this form of movement, 
 of diffusion, is called osmosis. The rate of osmotic transfer 
 will vary for every salt according to the difference in the 
 proportions of the salt in the two adjacent liquids. This 
 difference is known as the osmotic pressure. Furthermore, 
 the rate of movement will differ with the salt, with the 
 composition, thickness, etc., of the membrane, and with 
 other factors ( e. g: the relations of the salts to one another, 
 with the dissociation, etc. ) which find their natural place 
 for discussion in a text-book on physics.* 
 
 Turning back now to our vegetable cell an alga, a root- 
 hair, a parenchyma cell, etc. a body consisting of aqueous 
 solutions enclosed in and permeating concentric membranes 
 of different physical and chemical properties, we see that we 
 have precisely the conditions imagined in our discussion of 
 the purely physical phenomena. The cell-sap is a solution 
 greater in density than the water outside the cell and dif- 
 fering from it in composition. In consequence, molecular 
 movements will take place into and from the cell. These 
 movements will tend to reduce the density and modify the 
 composition of the cell-sap. Because of the greater density 
 of the cell-sap in other words, because of the smaller pro- 
 portion of water in the cell-sap to substances dissolved in 
 it water molecules will pass into the cell through the cellu- 
 lose wall, the cytoplasmic membranes, and the protoplasm, 
 diffusing- throughout the cell as well as in the vacuoles. 
 Thus the density of the cell-sap will be lowered, the propor- 
 tion of water to substances dissolved in it will be raised, 
 and the volume of the cell-sap will be increased. But if the 
 cellulose-wall resist any increase in the volume of the cell- 
 sap and of the cell, the cell-sap will be subjected to pressure, 
 the protoplasm will be forced by this means against the 
 cell-wall, the cell-wall itself will be stretched, the whole cell 
 will be in a state of tension, will be plump, will be turges- 
 
 * See, for example, Ostwald's Solutions, translated by M. M. P. Muir, 
 London, 1891. 
 
ABSORPTION AND MOVEMENT OF WATER 111 
 
 The pressure of the cell-sap and of the cell brought 
 about by this means is called turgor, and it is evidently a 
 very important factor in maintaining the form of the cell 
 and, therefore, comprehensively, of the organs and of the 
 individual. The turgor or turgescence of the cell tends to 
 be maintained by the continued absorption of water; but 
 unless the absorption continually exceed the loss of water, 
 by evaporation or otherwise, there will be no turgor, the 
 plant will be flabby, wilted. Absorption of water can be 
 continued only by keeping the density of the cell-sap always 
 greater than that of the water outside. 
 
 The production, control, and maintenance of this physical 
 condition is accomplished in part by the living protoplasm, 
 in part, in multicellular plants, by the osmotic absorption 
 of water from one cell by another. In the latter case, water 
 is drawn off by physical means only, and in obedience to 
 unavoidable physical law, by the cells that need it. In the 
 former case, the living protoplasm, by what it takes from 
 and gives to the cell-sap in respiration, nutrition, and excre- 
 tion, maintains the greater density of the cell-sap, that is, 
 maintains the higher proportion of dissolved matter to 
 water than prevails outside the cell. 
 
 Upon the composition of the cell-sap depends the absorp- 
 tion of the substances dissolved in the water outside the 
 cell. We have already seen that a dissolved salt will pass 
 through a permeable membrane into another volume of 
 water which contains less or none of it, and that the rate of 
 osmosis (or, in this case, of absorption) will depend upon 
 the salt, the difference in the proportions of the salt in the 
 two volumes of liquid, upon the nature of the membrane, 
 etc. The cell and by means of it, the plant will absorb by 
 osmosis those salts which occur in the soil and in water in 
 proportions larger than in the plant. For example, com- 
 mon salt will be absorbed by a root-hair or by an algal cell 
 until in the cell-sap there is the same proportion of salt to 
 water as in the solution outside the cell. When there have 
 come to be the same number of molecules of salt in equal 
 volumes of cell-sap and outside water, there will be no fur- 
 ther absorption of salt, and although the movements of 
 
112 PLANT PHYSIOLOGY 
 
 salt molecules will continue, there will be no accumulation 
 of these either within or without the cell. Common salt will 
 be absorbed by the cell, therefore, as a purely physical 
 necessity, regardless of the presence of the other salts dis- 
 solved in the cell-sap, and regardless of the fact that it is 
 needed and used by the cell only in the minutest quantity 
 if at all. On the other hand, if the common salt were used 
 in quantity by the cell, or in any other way removed from 
 the cell-sap ( by decomposition, precipitation, or otherwise ) , 
 the proportion of common salt to water in the cell-sap 
 would always be lower than outside the cell, there would 
 always be osmotic pressure inward, the molecules of salt 
 would constantly force their way into the cell in the attempt 
 to attain osmotic equality or balance, there would be con- 
 tinued absorption of common salt, and the rate of absorp- 
 tion would vary with the osmotic pressure. Such is the case 
 with salts used by the cell or otherwise removed from their 
 solution in the cell-sap. For example, the nitrates are, as 
 we have seen, the best form in which nitrogen is taken in by 
 most plants, and of these potassium nitrate is perhaps the 
 most common. Potassium nitrate will be absorbed by the 
 plant with an avidity proportioned to the need of nitrogen, 
 or to state this in physical instead of physiological terms 
 at a rate depending upon the difference in the proportions 
 of potassium nitrate within and without the cell. If the 
 nitrate be decomposed and the nitrogen used by the plant 
 as fast as it is absorbed, the osmotic pressure will remain 
 as great, the absorption will continue as rapid, as at the 
 beginning. Upon the amounts needed, used, or otherwise 
 taken out of the cell-sap by the living protoplasm will de- 
 pend the amounts of different salts absorbed by plants be- 
 yond those amounts necessary to secure uniformity of 
 composition in cell-sap and outside water if no salts were 
 consumed. In this consists the so-called "selective power 
 of roots" and of plant-parts in general. 
 
 We see, then, that the absorption by the cell of those sub- 
 stances which can pass through cell-wall, cytoplasmic mem- 
 branes, and protoplasm is a physical necessity whenever 
 there is any higher proportion of these substances outside 
 
ABSORPTION AND MOVEMENT OF WATER 113 
 
 the cell than in it. When there is no difference in propor- 
 tion, there will be no absorption or no further absorption. 
 When, by the vital needs and activities of the cell or of the 
 plant, a difference is maintained, there will always be ab- 
 sorption, proportioned in rate to the difference, propor- 
 tioned in amount to the duration of the difference. This 
 accounts for the much higher percentage of potassium than 
 of sodium in the ash of marine algae. The amount of 
 sodium salts absorbed is only such as to attain the osmotic 
 balance of sodium salts in the cell-sap and in sea water, 
 wherea the amount of potassium salts is such as to satisfy 
 the need of the plant for potassium and for the elements 
 associated with it in these salts. The accumulation of io- 
 dine in marine algae is due, not to the demand of these 
 plants for iodine, but rather for the element or elements 
 with which the iodine is combined in sea water : the iodine 
 is. therefore, removed from solution in cell-sap and accumu- 
 lates in insoluble form in the cell. 
 
 THE MEANS OF ABSORBING NUTRIENT SOLUTIONS 
 
 From the foregoing discussion of the physical principles 
 underlying the absorption of nutrient solutions, we can now 
 understand how an alga supplies itself with adequate 
 amounts of food-materials. A land plant, however, in addi- 
 tion to its demand for other food-materials, must regulate 
 its absorption according to its demand for water to make 
 good that lost by evaporation. W T e must now consider how 
 the land plant adapts itself to the conditions prevailing on 
 land and successfully employs the physical means of absorb- 
 ing the nutrient salts in the soil. 
 
 The root is generally regarded as especially the absorbing 
 organ of higher plants. The cells on the surface of the root, 
 being the only ones which are directly in contact with the 
 soil particles and with the soil water, are the only ones 
 which can absorb solutions from the soil. Only the young- 
 est of these cells have walls of such composition and thin- 
 ness that osmosis can take place rapidly. Furthermore, 
 owing to the granular nature of soils, and owing to the 
 fact that the water, at times when it is most needed, is held 
 8 
 
114 PLANT PHYSIOLOGY 
 
 longest and most strongly upon the soil particles, and not 
 between them, no smooth cylindrical organ of the size of 
 even the smallest roots will be able to bring enough of 
 those cells capable of absorption into sufficiently intimate 
 contact with a large enough number of soil particles to 
 ensure the osmotic absorption of water from the soil parti- 
 cles into the root. For osmotic transfer, as we have seen 
 before (p. 109), both of the two liquids concerned must be 
 in contact with the permeable membrane. Furthermore, as 
 is the case on soil particles, water strongly held as a thin 
 film over an irregular surface will not rapidly move from 
 part to part of that surface. To ensure the absorption of 
 much water from such a surface, there must be the most 
 extended proximity possible of the osmotically active 
 liquids. The permeable membrane must therefore cover the 
 irregular surface as widely and as closely as possible. The 
 most intimate contact of absorbing cells and liquid to be 
 absorbed will be effected when hairs of such size and length 
 that they will fit the soil particles develop on the root. 
 
 The length, thickness, and number of root-hairs will vary 
 according to the medium in which they develop, and also 
 according to the amount of water given off by the plant. 
 The root-hairs will be numerous directly in proportion to 
 the difficulty of getting enough water. This can be easily 
 demonstrated by cultivating young seedlings of corn with 
 their roots in moist air, in soil, and in water. The root- 
 hairs will be most numerous in the air, less in the soil, and 
 there will be exceedingly few if any in the water. The length 
 of the root-hairs will also differ strikingly ; they will be 
 longest in the air, shortest in the water. The diameter of 
 the hairs is necessarily limited by the size of the epidermal 
 cells of which they are branches, but within this limit the 
 hairs certainly vary according to the size of the soil parti- 
 cles among and around which they must grow. That the 
 root-hairs not only grow between the soil particles, but 
 actually apply themselves very closely to them, is abun- 
 dantly proved by the common experience of up-rooting 
 plants grown in loose soils. When such plants are pulled 
 up gently, numberless soil particles of minute size cling to 
 

 ABSORPTION AND MOVEMENT OF WATER 115 
 
 the roots, held there by the root-hairs, just as larger lumps 
 of soil are held by the root-branches. 
 
 The function of root-hairs is to absorb nutrient solutions 
 from the soil. They, and their physiological equivalents, 
 the rhizoids of lower plants, are the chief absorbing organs 
 of larger plants. The roots themselves are for the conduc- 
 tion of the solutions absorbed by the root-hairs, and also 
 for the mechanical support of the whole plant. The root- 
 hairs should not be regarded merely as structures increasing 
 the surface through which aqueous solutions are absorbed 
 by roots ; the root-hairs are the main surface through which 
 the absorption takes place. But more than this, they are 
 the very perfect means by which the only parts capable of 
 copious absorption living cells bounded by thin cellulose 
 walls and containing cell-sap of proper composition and 
 density are brought into the necessary intimate contact 
 with the nutrient solutions adhering to the irregular sur- 
 faces of the small soil particles. In other words, the root- 
 hairs are the means of bringing together, so that they are 
 separated only by a thin permeable membrane, two aqueous 
 solutions of such osmotic pressures that the one enclosed 
 will absorb the one held by surface attraction (p. 105). 
 
 It has been estimated* that the surface of a root is in- 
 creased 5 to 12 times by the production of hairs. From what 
 has just been said, and because the hairs are bounded by 
 w r alls at least thinner if not otherwise more permeable than 
 the cells between, we see that this does not necessarily 
 fairly indicate the increase in absorbing power even of the 
 part producing the hairs. This may be more than 5 to 12 
 times increased, according to circumstances. 
 
 The life of a root-hair is necessarily brief. Its delicacy, 
 and the fact that it may be torn and broken by the con- 
 tinued forward growth of the part where it is borne, favor 
 this. The root-hairs are formed by the outward branching 
 of epidermal cells on that part of the root just behind the 
 tip which has almost or quite ceased to grow in length. If 
 any considerable growth in length does occur after the for- 
 
 * Schwarz, F. Die Wurzelhaare der Pflanzen. Untersuch. aus dem botan. 
 Institut zu Tubingen, Bd. I., p. 140. 
 
116 PLANT PHYSIOLOGY 
 
 mation of the hairs, these must surely be dragged forward 
 and broken. Secondary growth in thickness of the part will 
 crush them. Each young and growing root or root branch 
 is covered for a time by a zone of root-hairs. This zone 
 will vary in breadth according to growth conditions. The 
 hairs will vary in length, diameter, and number according 
 to soil conditions. As the root grows, the work of the 
 older and less effective root-hairs is taken up by the younger 
 ones newly formed farther forward and nearer the growing 
 point. In this way new soil-particles are relieved of their 
 small stores of water, and the absorbing surface is cor- 
 related with the growth of the plant as well as with its 
 demands in the stationary condition. 
 
 THE MEANS OF TRANSFER OF NUTRIENT SOLUTIONS 
 
 Similar to the differences between cell-sap and soil- water 
 in density and in composition, which enable the root-hairs 
 to absorb water and dissolved food-materials, are the differ- 
 ences in the density and composition of the cell-sap of ad- 
 jacent cells. The cell with denser cell-sap will absorb water 
 from its neighbor with more dilute cell-sap, the cell-sap with 
 less of a needed food-material or food will absorb from one 
 with more. Such osmotic transfers are inevitable wherever 
 miscible solutions or liquids of different densities and compo- 
 sitions are on opposite sides of a permeable membrane and 
 in contact with it. By osmosis the distribution of food- 
 materials will take place through the body of a small land 
 plant, P. g. a fungus or a liverwort, and in submersed 
 aquatics. For all plants not subjected to the loss of water 
 by evaporation, and in the bodies of most land plants so 
 small and so simple as the liverworts and the fungi, the 
 rate of transfer by osmosis alone is rapid enough to ensure 
 the adequate distribution of water and of dissolved foods 
 and food-materials. Larger land plants are subjected to 
 such losses of water from their aerial parts that osmotic 
 transfer is too slow always to keep pace with evapora- 
 tion. In plants provided with special organs for excret- 
 ing water (see pp. 126-8) these organs must be copiously 
 supplied. 
 
ABSORPTION AND MOVEMENT OF WATER 117 
 
 Evaporation and excretion, taking place on the exposed 
 surfaces and also from those cells bordering on air-passages, 
 increase the density of the cell-sap of the cells directly con- 
 cerned. These draw osmotically upon their neighbors for 
 water to make good their loss. The neighboring cells in 
 their turn draw upon cells still more remote from the losing 
 surface. In this way the demand for water is developed in 
 cell after cell away from the surface. To meet the demand, 
 water is transferred from cell to cell toward the surface. 
 Ordinarily, cells are small and short, and though their 
 bounding cellulose membranes and their component proto- 
 plasm may be freely permeable, water can move more 
 rapidly in response to other than osmotic forces if only the 
 way is clear. Through living cells water can make its way 
 best by osmosis, but as water will pass more rapidly through 
 a tube in which no filtering membrane is interposed, so water 
 in the plant will pass more rapidly through elongated cells 
 than through a series of short ones, through dead and 
 empty tracheids than through living cells of the same di- 
 mensions (other things being equal), and through con- 
 tinuous ducts than through a succession of tracheids. These 
 stages in the development of conducting tissues one finds in 
 the larger erect mosses, in the Coniferae, and in the Angio- 
 sperms. The most perfect development of a vascular sys- 
 tem is found perhaps in twining plants, especially those of 
 tropical countries, * in the slender stems of which the ducts 
 are large, long, and no thicker walled than is consistent 
 with the necessary mechanical strength. 
 
 Upon the vascular tissues, throughout their whole length, 
 parenchyma cells abut directly. In the root these paren- 
 chyma cells receive more and more water so long as fhe 
 root-hairs continue to absorb any from the soil, and pres- 
 ently, not being able to expand beyond a certain point by 
 reason of the pressure of their neighbors, they are obliged to 
 get rid of the excess in some way or other. Into the vascular 
 tissues of the root, therefore, the parenchyma cells discharge 
 the water and dissolved matters, the discharge taking place 
 * Schenck, H. Beitrage zur Biologie der Lianen. Bot. Mittheilungen 
 aus den Tropen, Bd. II., 1893. 
 
118 PLANT PHYSIOLOGY 
 
 in the direction of least resistance, that is, from living and 
 turgid cells into empty tube-like tracheids and ducts. At 
 any point in the plant the adjacent parenchyma cells may 
 absorb water from the vascular tissues just as the root- 
 hairs absorb, water from the soil, and by the same physical 
 means. Whether there are continuous columns of water in 
 the ducts or not, there is a continuous body of water in the 
 walls of the ducts, and so the withdrawal of water at any 
 point will induce a movement of water toward that point 
 from parts better supplied. It is ordinarily from below that 
 water is drawn, for ordinarily the root-hairs supply the 
 needs of the whole plant, but this is not necessarily the case, 
 for through the conducting tissues water will pass up or 
 down according to circumstances. 
 
 The vascular tissues form a continuous system, often 
 much complicated in arrangement but proportionally in- 
 creased in usefulness, of water-conveying cells and vessels 
 extending from base to tip of the plant. At frequent inter- 
 vals the bundles, which run more or less distinct from one 
 another, anastomose and thus combine the vascular tissues 
 into one effective system. Branches are given off from the 
 main channels, so that buds, leaves, branches, even hairs 
 (e.g. glandular hairs otDrosera), are reached by the con- 
 ducting system. By this means all the parenchyma cells, 
 which are the actively living cells of the plant-body, are 
 supplied directly or indirectly. The amount of water and of 
 dissolved matters supplied to any part will depend upon the 
 demand, upon the amount lost and consumed. For ex- 
 ample, two adjacent vascular bundles, running to two 
 leaves, will convey different volumes of solutions if from the 
 one leaf more water evaporates than from the other, or if 
 in one leaf more water and dissolved matters are used than 
 in the other. We thus see that the amounts transferred 
 through parts, and consequently through the whole, of the 
 vascular system are dependent upon the activities of living 
 cells : first, upon those living cells which absorb nutrient 
 solutions from the soil and from which other living cells 
 osmotically absorb them, the cells abutting upon ducts 
 and tracheids discharging the excess of water into these 
 
ABSORPTION AND MOVEMENT OF WATJER 119 
 
 empty spaces; second, upon the living cells of the aerial 
 parts, in the leaves, branches, and stems, in which the solu- 
 tions are worked over and from which the water is given oft'. 
 The living parenchyma cells near the absorbing cells in the 
 roots, and the living parenchyma cells composing the food- 
 making tissues in other parts may be many metres apart. 
 The absorbing and consuming tissues of herbaceous plants 
 are usually close together ; in tall trees they are separated, * 
 but are farthest from each other in some of the "lianes." 
 How is the water raised from the IOW T levels at which it enters 
 the vascular bundles, in the region where it is absorbed from 
 the soil, to the cells needing it but far removed?! This ques- 
 tion has occupied botanists from the time when physiolog- 
 ical experiments were first undertaken until now. Despite 
 the most acute study, the question one of the most allur- 
 ing and important in botany is still unanswered. Hy- 
 potheses, deserving respectful consideration both because of 
 their reasonableness, and also because of the fame of their 
 authors, have succeeded one another in the text-books, have 
 been accepted and then discarded, according to the prevail- 
 ing fashion. As Sachs was for many years the leading 
 plant-physiologist, so his idea, laid down in his writings 
 with all his brilliant power, that the water ascended only 
 through the walls of the wood-elements,! was the only one 
 echoed by the smaller text-books. Then followed Godlew- 
 
 * From Kerner and Oliver's Natural History of Plants these "certified 
 estimates" of heights and lengths are quoted : 
 
 Eucalyptus amygdalina 140-152 metres page 722, vol. I., part 2. 
 
 Sequoia gigantea 79-142 " " " " " " " 
 
 Calamus Rotang 200 " " 677, " " " " 
 
 In his Silva of North America, Vol. X., p. 141, Sargent makes the 
 following statement regarding Sequoia sempervirens : "The Redwood, 
 which is the tallest American tree, probably occasionally attains the height 
 of four hundred feet or more. The tallest specimen I have measured was 
 three hundred and forty (340) feet high." 
 
 f While this book is in the press, Copeland is publishing in the Botanical 
 Gazette, vol. 34, 1902, "The rise of the transpiration: an historical 
 and critical discussion." 
 
 t Sachs, J. von. The Physiology of Plants, Oxford, 1887, pp. 241-242. 
 
 Godlewski, E. von. Zur Theorie der Wasserbewegung in den Pflanzen. 
 Jahrb. f. wiss. Bot., 1884. 
 
120 PLANT PHYSIOLOGY 
 
 skTs hypothesis, on the face of it much more reasonable, 
 /but not directly supported by experiment,(that living cells 
 j adjoining ducts and tracheids exert a pumping action^ 
 By dissolving poisonous substances in the water to be ab- 
 sorbed by the plants selected for examination, Strasburger* 
 attempted to demonstrate that living cells are not con- 
 cerned. Sap-pressure (see pp. 127, 131) was for a time con- 
 sidered the propelling force, but the absence of sap-pressure 
 at the times when water is most needed is sufficient evidence 
 against this notion. The varying pressure of the gases 
 within the body of the plant was supposed to be the answer 
 to the question, until it was shown that gas-pressures do 
 not vary enough and rapidly enough. Then came the era 
 of Jamin's chains, when the ducts and tracheids, found to 
 contain alternating columns of air and water, were supposed 
 to furnish the tubes through which these pass. Schwen- 
 dener,f Dixon and Joly,J and Askenasy have contributed 
 much to a knowledge of the physical qualities of such 
 chains, but no one has succeeded in proving that they have 
 much if anything to do with water-transfer. Eepeatedly 
 botanists have returned, from lack of anything better, to 
 the idea that capillarity conveys the water through the 
 vascular system ; but this notion is inadequate because the 
 vascular system of many plants is composed of tubes so 
 small and so short that, although the capillary force is 
 great, the resistances are also so great that a sufficiently 
 rapid transfer by this means alone is inconceivable. 
 
 From all the contradictory views this much may be ex- 
 tracted as proved. The water, which certainly permeates 
 the walls of the elements composing the vascular system, is 
 also contained in the cavities and passes through the cavi- 
 ties in the direction of strongest attraction, conversely of 
 least resistance. That the water does actually pass into 
 
 * Strasburger, E. Bau und Verrichtung der Leitungsbahnen, 1891. 
 tJber das Saftsteigen, 1893. 
 
 f Schwendener, S. In Sitzungsberichte der Berliner Akademie, from 1886 on. 
 
 J Dixon and Joly. On the ascent of sap. Annals of Botany, 1894. The 
 Path of the transpiration current. Ibid., 1895. 
 
 Askenasy. IJber das Saftsteigen. Verhandlung d. naturh. Vereins in 
 Heidelberg, 1895. 
 
ABSORPTION AND MOVEMENT OF WATER 121 
 
 and through the cavities of ducts and tracheids is demon- 
 strated by using a solution of gelatine, which melts at a 
 temperature so low as not to injure the plant, but which is 
 solid at ordinary temperatures. For example, gelatine 
 melted at about 30 C. will be taken into the freshly-cut 
 butt of an amputated branch and can then be hardened by 
 plunging into water at 20 C. If now the branch, with the 
 butt freed from superfluous adherent gelatine, is stood up in 
 a jar of cool water its leaves w r ill wither. They will recover, 
 however, if placed in water warm enough to melt out the 
 gelatine. This experiment is significant in two ways at 
 least. First, solid gelatine permeated with water is no bar 
 to osmotic transfer of aqueous solutions, while paraffine, or 
 any similar material which is impermeable to water but 
 might also be used to fill the cavities of the vessels, would 
 stop osmosis. We see then that osmotic transfer, even if it 
 could take place in the ducts as it does between the paren- 
 chyma cells, is too slow for the conduction of more than 
 small volumes of solutions and for short distances. Second, 
 if the solidified gelatine is properly removed from the sur- 
 face of the butt of the branch experimented upon, little or 
 no penetration of gelatine into the walls of the ducts having 
 taken place^ the permeability and conducting-power of the 
 walls will be only very slightly diminished if impaired at all. 
 
 Further than this no positive assertions can be made. 
 The water certainly ascends, mainly in the cavities of ducts 
 and tracheids, though also in the walls, and whenever the 
 physical conditions demand it, water and dissolved salts will 
 be drawn from the phloem as well as from the xylem. The 
 main path for the transfer of the solutions of food-materials 
 is the wood ; for the transfer of the solutions of elaborated 
 foods, the phloem or bast-portion of the vascular bundles. 
 The physical force needed to raise the water is still unknown. 
 
 Of the various views regarding the means of transfer to 
 which reference was made on pages 119 and 120, one is further 
 from disproof, perhaps also from proof, than the others. This 
 is the one put forth by Godlewski, * and advocated in more 
 
 * See Pfeffer, Pflanzenphysiologie, Bd. I., p. 208 ; Engl. transl., I., pp. 
 220 et seq. 
 
122 PLANT PHYSIOLOGY 
 
 or less modified form by a number of other authors. Accord- 
 ing to this view, the living cells which are always found to 
 be the close neighbors of ducts and tracheids participate 
 actively in raising water from roots to leaves. Apart from 
 the anatomical relationship of these living and lifeless ele- 
 ments, which suggests that the living cells may aid in as 
 well as influence the movement in the lifeless ducts and 
 tracheids, it is definitely proved by experiment that it is the 
 youngest wood, that is, the wood containing the most and 
 the most active living cells, which transfers most water and 
 does it most rapidly. The method of proof consists in using 
 solutions of harmless coloring-matters not fixed by living 
 cells ( e. g. Indigo-carmin, Anilin Blue, etc. ) . If amputated 
 branches are placed in such solutions the path of transfer 
 will be indicated by the staining of cell-walls, and if the ex- 
 periment is not prolonged, the stain will be found highest 
 in the youngest, that is, the best-conducting wood. The 
 "sap-wood" conducts most if not all of the water. On the 
 other hand, the " heart- wood " conducts little or none. The 
 heart-wood not only contains fewer living cells the oldest 
 heart- wood none at all but its permeability diminishes as 
 the infiltration of the walls with coloring, hardening, pre- 
 serving, and other substances progresses. In the youngest 
 wood, where there are the most living cells, the maximum 
 transfer of water takes place. 
 
 Against the idea that living cells are actively concerned in 
 raising water from root to leaf are the conclusions drawn 
 by Strasburger from his experiments. The following will 
 serve as an Illustration of one line along which he experi- 
 mented. * A specimen of Acer platanoides twenty-one metres 
 high was obliquely sawed through at the base, a strong 
 stream of water playing constantly into the cut as the saw- 
 ing progressed. The tree was then placed with the butt in 
 water. After the lapse of a half-hour it was hoisted, the 
 cut surface smoothed with a sharp knife, and then lowered 
 into a 5% solution of copper sulphate. In two weeks it took 
 up nearly 30 litres of this liquid and the presence of the 
 copper could be demonstrated up to, but not in the finest 
 * Strasburger, E. Bau und Verrichtung der Leitungsbahnen, p. 617, 1891. 
 

 ABSORPTION AND MOVEMENT OF WATER 123 
 
 and highest branchlets. The rapid ascent of so large a 
 volume of poisonous liquid is alleged to prove that living 
 cells are not necessary to the transfer. One very strong 
 objection to this conclusion is this .-although any cell into 
 which even a small amount of copper penetrates will be 
 poisoned and killed thereby, the cell next above will not 
 cease its activity until it in turn absorbs and is poisoned by 
 the copper. Furthermore, it is weU known that water, 
 which already permeates all cell-walls, will ascend faster 
 than substances dissolved in it but not permeating cell- 
 walls. The poison will ascend less rapidly than its solvent 
 because the copper-salt will be taken up by the cell- wall, and 
 will diffuse osmotically through the cell. If then, living cells 
 do take an active part in the transfer of water, the ones 
 above and not killed by the copper can still pull up the 
 solution though to then 1 own ultimate undoing, and they 
 will pull up water faster than copper salt. 
 
 Whether living cells are actively concerned in water-trans- 
 fer or not, the popular idea of the lifetime of a cell must be 
 modified somewhat. Those trees which form no " heart- 
 wood" and in which living cells may be found quite to the 
 centre (e. g. Beech and Birch), and the Palms and other 
 Monocotyledons which do not increase in thickness, offer 
 striking examples of the\age attained by living ceUs. Stras- 
 burger* reports finding living cells in large numbers almost 
 to the centre in sections of seventy-year old beech trees. 
 
 In the wood of trees growing hi regions with pronounced 
 seasonal differences there are seasonal as well as age differ- 
 ences. The " annual rings" are divisible into so-called 
 'spring- wood" and "autumn- wood," although the latter is 
 formed long before autumn. The anatomical differences 
 which distinguish these layers of the annual ring from one 
 another are accompanied by differences in their conducting 
 power. Spring-wood is formed at the time when sap-pressure 
 is greatest, when the opening of buds is followed by the ex- 
 pansion of the leaf and other surfaces from which w r ater can 
 be 'given off, when the plant resumes all at once the activi- 
 ties which have been suspended for a season, and when most 
 
 * L. c., p. 534. 
 
 
124 PLANT PHYSIOLOGY 
 
 food as well as most water must be carried to the active 
 parts. Spring- wood conducts better than autumn wood, 
 although according to Strasburger, * single rows of cells 
 formed last in the autumn-wood possess higher conducting 
 powers than those formed earlier. This he regards as con- 
 tributing to a better connection between the succeeding 
 rings, and this is especially necessary because the new bundles 
 for the forming and growing parts must be adequately sup- 
 plied with liquid while the young spring-wood, with which 
 they connect, is attaining effective dimensions. (For the 
 cause of "annual ring" formation, see pages 191-4.) 
 
 Perhaps in all plants, certainly in many plants growing in 
 desert regions and in places where there is a distinct dry 
 season, tissues are developed in which water may be stored 
 and kept for a long time, in spite of the dryness of the sur- 
 rounding air. It is quite possible that the wood-nbres, 
 present always in the xylem of the vascular bundle and 
 sometimes numerous there, serve as temporary holders of 
 water at the same time that they contribute to the me- 
 chanical strength of the plant. After collenchyma has 
 served its first purpose in strengthening the rapidly growing : 
 parts in which it differentiates so early, its thickened and ' 
 chemically modified walls, as well as the living cells which 
 formed them, retain water with considerable power, t The 
 most striking examples of water-storing tissues are to be 
 found among desert plants (.#. in the Cactacea* and 
 Euphorbiace*),! and in the leaves of Sphagnacea\ which 
 live under exactly opposite conditions. The possession by 
 swamp-plants, especially those living in undrained swamps 
 and in bogs, of characters found otherwise only in desert- 
 plants has been remarked by a number of authors. This 
 may be due to the plants trying, by reducing evaporation 
 and therefore the need of absorption, to avoid absorbing in 
 excess any of the poisonous matters (humic acid, etc.) in 
 
 * 7. c.. p. 592. 
 
 f Miiller, C. Beitrag zur Kenntniss der Formen des Collenchyms. 
 Berichte der Deutsch. Bot. Gesellschaft. 1890. 
 
 i See Goebel's Pflanzenbiologische Schilderungen find Volkens's Flora der 
 Mgyptisch-arabischen Wiiete, 1887. 
 
ABSORPTION AND MOVEMENT OF WATER 125 
 
 iswamps, or it may be due to the actual difficulty of ab- 
 sorbing water.* 
 
 SECRETION 
 
 Before leaving the subject of the osmotic phenomena in the 
 plant-body to discuss those of water vaporization and gas 
 exchange, those osmotic . processes which result in the re- 
 moval of material from the plant should be mentioned. If 
 there is absorption by means of osmotic currents set up and 
 maintained because there are smaller proportions of water 
 and of various other substances inside than outside the cell, 
 there must be excretion by the same means whenever the 
 opposite is true. Such excretion does take place ; there are 
 exosmotic as well as endosmotic currents. They are very 
 different, however, in amount, rate, and character. By the 
 root-hairs of higher plants water and dissolved substances 
 are endosmotically absorbed. Minute quantities of a number 
 of substances are exosmotically excreted also by the root- 
 hairs. Since Sachs's classic experiment in growing roots 
 in contact with polished marble plates, t it has been known 
 that roots can exert a corrosive action on such solid mat- 
 ters as they touch. Recent experiments by CzapekJ have 
 shown that the principal substances diffusing from roots 
 are mainly carbon-dioxide (passing out as carbonic acid, 
 H.,CO 3 ), phosphoric, hydrochloric, sulphuric, and phormic 
 acids and their salts, preeminently substances which would 
 aid the plant to obtain needed food-materials from the soil. 
 
 Besides the excretion from roots, the secretions in the 
 various glands and reservoirs are dependent upon exosmotic 
 
 * Cowles, H. C. The ecological relations of the vegetation of the sand 
 dunes of Lake Michigan. Botanical Gazette, vol. 27, 1899. Schimper, A. 
 F. W. Pflanzengeographie auf phisiologischer Grundlage, 1898. 
 
 t Sachs, J. von. Auflosung des Marmors durch Mais-Wurzeln. Bota- 
 nische Zeitung, 1860. Lectures on the Physiology of Plants, pp. 262, 
 263, Oxford, 1887. 
 
 J Czapek, F. Zur Lehre von den Wurzelausscheidungen. Jahrb. f. wiss. 
 Botanik, Bd, 29, 1896. See also Sistini in Atti di Soc. Tosc. di nat., 
 Proc. Verb., 1899 (reviewed in Just's Jahresbericht, Bd. 27, 2te Abth., 
 p. 194), who says roots convert feldspar into clay, working at least 
 four times as fast as the weather. 
 
126 PLANT PHYSIOLOGY 
 
 movements. Nectaries are special organs on the surface of 
 which sugar is abundantly produced. What causes the for- 
 mation and excretion of sugar by the cells of nectaries is 
 not known, but given the sugar on the surface of a nectary, 
 the excretion of water to dissolve this is inevitable. * Why 
 this sugary solution is not resorbed is also unknown. Ap- 
 parently no clear idea of the action of nectaries can be had un- 
 til the physiological chemistry of these organs is worked out. 
 
 The accumulation of resins in the intercellular resin-reser- 
 voirs of the Conifers, etc., and the incrustations of lime 
 and iron on the surfaces of various plants, are accounted 
 for in the following way. Certain substances elaborated or 
 formed as by-products by glandular cells, are excreted os- 
 motically into intercellular spaces, or upon the surface, or 
 under the cuticula (in certain hairs), there undergoing such 
 chemical change that they are no longer capable of osmotic 
 movement in water.! 
 
 The excretion of fluid water by many plants is also ac- 
 complished by purely physical means. Water vapor is uni- 
 versally given off by land plants (p. 136), but the escape 
 of fluid water is a less frequent and regular occurrence. In 
 nectaries the passage of fluid water from the cell is due to 
 osmotic pressure, the attraction of the excreted sugar. In 
 other cases, on the contrary, fluid water is excreted, not 
 because of the attraction (osmotic suction) of substances 
 outside the cell but because of the pressure (turgor, p. 110) 
 within the cell. 
 
 Turgor will develop in a cell whenever the cell-sap, be- 
 cause it contains a higher percentage of dissolved salts 
 than the water outside, can absorb water. The volume 
 of the cell-sap and of the enclosing protoplasm tends to 
 
 * Wilson, W. The cause of the excretion of water on the surface of necta- 
 ries. Untersuchungen aus dem Bot. Institut zu Tiibingen, Bd. I., 1881. 
 Schimper, A. F. W. Wechselbezienhungen zwischen Pflanzen und Ameisen, 
 1888. 
 
 i See Pfeffer, Pflanzenphysiologie, Bd. I., pp. 115, 116 501; Engl. transl., 
 pp. 129, 500. Kohl, F. G. Kalsalze und Kieselsaure in der Pflanze, 1889. 
 Giesenhagen, C. Die radialen Strange der Cystolithen von Ficus elastica. 
 Berichte der Deutsch. Bot. Gesellsch., 1891. Tschirch, A. Die Harze und 
 die Harzbehalter. Berlin, 1900. 
 
ABSORPTION AND MOVEMENT OF WATER 127 
 
 increase with the absorption of water. Such increase in 
 volume is only feebly resisted by the mechanically weak 
 protoplasm. It can be resisted only by the cell-wall, a 
 strong, elastic, permeable membrane, composed of one of the 
 celluloses. Turgor and sap-pressure result from the resist- 
 ance by the cell-wall to increase in volume. If the sap- 
 pressure in a cell becomes greater than the retaining power 
 of the wall, something will change. The absorption of water 
 may be stopped by modifying the composition of the cell- 
 sap, by exosmosis of the osmotically active salts to other 
 cells, or by chemical change of the salts in the sap ; or water 
 may pass out through the wall in a direction of less pres- 
 sure, either into adjacent cells or out upon the surface of the 
 organ. If none of these things occur and absorption con- 
 tinues, the cell-wall will break. 
 
 The cortical parenchyma cells in the root, in the region 
 where absorption through the hairs is taking place, are 
 under sap-pressure whence the misleading name of root- 
 pressure and consequently force water into the conducting 
 elements, tracheids and ducts, with which they are in con- 
 tact. The same process underlies the action of water-pores. 
 
 Certain weather conditions favor the excretion of water by 
 making it possible to develop the necessary sap-pressure. 
 When the air is warm and moist above a warm damp soil, 
 there will be copious absorption through the roots and pro- 
 portionally little loss of water from the upper parts of the 
 plant by evaporation. Sap-pressure necessarily develops, 
 and if these conditions continue, the pressure will presently 
 exceed the retaining power of some or many cells, water 
 will either filter through or break through the cell-walls. 
 If it break through, a wound is formed, and the escape of 
 liquid from it is bleeding (see pp. 130-36). Such wounds 
 are not altogether uncommon.* 
 
 The filtering of water under pressure through cell-walls 
 does not take place indiscriminately, for the permeability of 
 the walls of the cells composing the different tissues is not 
 
 * See Pfeffer, Pflanzenphysiologie, Bd. I., pp. 255 et seq.; Engl. transl., 
 I., pp. 272 et seq. Strasburger, E. Bau und Verrichtung der Leitungs- 
 bahnen, 1891. 
 
128 PLANT PHYSIOLOGY 
 
 equal. Thus, as a rule, the walls of superficial cells are so 
 water-proofed, either by chemical change or by infiltration 
 of the cellulose, that water will be pressed out of other cells 
 before it will pass from them. The so-called water-pores 
 such as occur on the garden nasturtium are merely the 
 openings on the edges of the leaves of cavities at the tips of 
 vascular bundles. The thin-walled cells bordering upon the 
 ducts and tracheids of these bundles squeeze out water into 
 them, the water makes its way toward the surface, escapes 
 into the cavity, and finally passes out through the pore. 
 The excretion of water can, however, be observed on many 
 plants not provided with such highly developed filtering 
 organs. On the leaves of grasses grown under glass in the 
 laboratory, and on the filaments or erect fruiting bodies of 
 various fungi similarly cultivated, water will collect in drops 
 whenever the substratum is so moist that checking the 
 evaporation will raise the sap-pressure to the filtering 
 point. This occurs regularly in grass-plats at night. The 
 "dew" there formed is mainly expressed water rather than 
 moisture condensed from the air. 
 
 A considerable number of plants, especially those growing 
 in damp tropical regions, rid themselves of superfluous water 
 by means of living glandular hairs on the surface, usually 
 the under surfaces of leaves. According to Haberland* these 
 glandular hairs, to which he gives the name hydathodes, 
 press out liquid only when living. 
 
 The liquid passing out of water-pores and excreted by 
 hydathodes is usually a very dilute solution, mainly of 
 mineral substances, with little or no sugar or other organic 
 compounds. These organs are therefore quite different as to 
 their products from nectaries. 
 
 Other water-excreting glands are not uncommon. The 
 accumulation of water in the pitchers of Nepenthes, Sarra- 
 cenia, Darlingtonia, the secretions on the hairs of Drosera, 
 and on the leaves of Dionea, are due to the action of water- 
 glands. The cells composing these glands change not only 
 
 * Haberland, G. In Sitzungsberichte d. Akad. d. Wissensch., Math-phys. 
 Klasse, Bd. 103, Abth. 1, Wien, 1894; ibid. Bd. 104, 1895; also Jahrb. 
 f. wiss. Botanik, Bd. 30, 1897. 
 
ABSORPTION AND MOVEMENT OF WATER 129 
 
 the rate but, as we have already seen (page 83), the kind 
 of activity in accordance with certain stimuli. On the hairs 
 of Droseraj and on the leaves of other carnivorous plants, 
 the sugary secretion which is used for attracting and cap- 
 turing prey may be more like that formed by nectaries 
 (page 126) due, so far as the water is concerned, to os- 
 motic suction rather than to active pressing out. The fill- 
 ing up of the pitchers of the pitcher-plants is much more 
 likely to result from active excretion of water. 
 
 The mechanics of the secretions commonly taking place on 
 the surface of stigmas are probably identical with those of 
 nectaries, although it must be seen that the first secretion of 
 sugar, and of the water which carries it, may in both cases 
 be accomplished by active pressing out of these substances 
 by the living cells forming the surface of the gland. When 
 sap-pressure is high in the body of the plant or in the root- 
 hairs themselves, the dissolved substances passing out 
 through root-hairs may be pressed out mechanically by the 
 vital activity of the living protoplasm, as well as by the 
 difference in the osmotic pressures within and without 
 the cell (page 125). 
 
 The substances passing out exosmotically or by other 
 pressure from the cells are seldom, if ever, such as con- 
 tribute directly to the formation of protoplasm. Proteids, 
 albuminoids, and the like, remain in the cells in spite of 
 the differences in proportional composition of the liquids 
 within and without the cells. Though some of these sub- 
 stances may be soluble, none is freely diffusible. According 
 to the differences in diffusibility, soluble substances have 
 been divided into crystalloids and colloids, respectively sub- 
 stances readily and tardily diffusing. It is now known, con- 
 trary to former supposition, that colloids may crystallize. 
 For this reason the names are inapt and misleading. The 
 hypothetical explanation of the feeble diffusibility of colloi- 
 dal substances is this : the molecules of these highly complex 
 compounds are so large (e. g. egg-albumen, which, according 
 to Lieberkuhn, may have the formula C^H^N^OJ^*) that 
 
 * Loew and Bokorny. Die chemische Kraftquelle im lebenden Proto- 
 plasma. Munich, 1882. 
 9 
 
130 PLANT PHYSIOLOGY 
 
 they cannot pass through the spaces between the molecules, 
 or groups of molecules, of other substances. On the other 
 hand, a colloidal membrane is no bar to the diffusion of 
 crystalloids, for though its molecules and groups of mole- 
 cules are large, it is supposed that the spaces between are 
 large enough for smaller molecules to pass through. Thus, 
 apart from Hertwig's conception ( p. 7 ) that the living proto- 
 plasm is a definite structure and not a substance or mixture 
 of substances merely, we have a reason why the protoplasm, 
 while permitting the free passage of w r ater and of the sub- 
 stances dissolved in it, remains enclosed within the cell-wall, 
 although by the absorption of much water, or for other rea- 
 sons, the density of the protoplasm may be greatly reduced. 
 In this sense the living cell is an apparatus that permits 
 endosmosis while preventing exosmosis. In addition to the 
 failure of protoplasmic (7. e. of colloidal) substances to 
 pass through the cell- wall because of the size of their mole- 
 cules, the living protoplasm (see p. 107) prevents by its 
 bounding membranes the exosmosis of dissolved coloring 
 and of some other substances contained in the vacuoles. 
 
 SAP-PRESSURE AND BLEEDING 
 
 The transfer of single cells containing osmotically active 
 substances in abundance for example, ripe pollen-grains 
 into pure \vater, or to an aqueous solution of too low den- 
 sity, will cause the cells to swell and finally to burst, in 
 consequence of turgor-pressure which finally ruptures the 
 cell- walls. Such cells would not swell and burst in air, or in 
 a solution nearly or quite equalling the cell-sap in density. 
 Similarly, the accumulation of osmotically active substances 
 in some of the cells of a multicellular plant which can ab- 
 sorb water in abundance, will result in developing pressure 
 turgor-pressure in those cells. These, exerting pressure 
 upon neighboring cells, will transmit the mechanical force, 
 often for a considerable distance, and it may ultimately be 
 exerted directly upon the soil or other material surrounding 
 the plant. The materials causing the development of pres- 
 sure may pass by osmosis to the cells against which pressure 
 is exerted. Thus, though the pressure in one cell or several 
 
ABSORPTION AND MOVEMENT OF WATER 131 
 
 may thereby be reduced, the means of developing turgor 
 will be extended and the total pressure of the organ or of 
 the plant may remain the same. The turgor-pressure may 
 also be reduced by the living cells which abut on the 
 lifeless ducts and tracheids, pressing out water into these 
 otherwise empty shells. Continuing this process of excre- 
 tion into the wood elements may result in pressure develop- 
 ing in them also. This pressure in lifeless cells may justly 
 be called sap-pressure in distinction from turgor-pressure, 
 which is possible only in living cells or in an apparatus 
 similarly constructed. 
 
 Perennial plants in temperate climates exhibit all of these 
 phenomena each year with the return of spring. When there 
 are again sufficient warmth and water, the cells in which 
 starch and other reserve foods were stored for the winter are 
 awakened to new life, and form enzyms needed to convert 
 the starch and other insoluble solids into soluble ones. 
 These go into solution in the cell-sap, which thereby in- 
 creases in density and in osmotic potential. The cell-sap in 
 this condition rapidly absorbs water and, tending to in- 
 crease proportionally in volume, develops a pressure equal 
 to the force needed to keep it at that volume. This pressure 
 will necessarily be exerted upon the adjacent cells, will thus 
 be extended from cell to cell, the substances dissolved and 
 the water dissolving them will pass by osmosis in the same 
 directions, 7. e. in the directions of least resistance, mechan- 
 ical or osmotic. Thus the local pressure will be reduced and 
 the danger that the cells may burst will be removed. The 
 total pressure may remain the same, or the local pressure 
 may become so distributed and equalized that in the plant 
 as a whole there will be none. This last is accomplished 
 whenever water is given off, as vapor or liquid, from the 
 surface of the plant in amount equal to that absorbed. 
 
 The ratio between water absorbed and water given off 
 indicates whether there can be any pressure of the cell-sap 
 in any living cell (turgor-pressure) or in the whole plant 
 (sap-pressure). The greater the absorption in proportion 
 to the loss of water by evaporation, transpiration, or secre- 
 tion, the greater the pressure, local and general ; conversely, 
 
 
132 PLANT PHYSIOLOGY 
 
 the greater the loss of water in proportion to the absorp- 
 tion the lower the pressure, local and general. Copious 
 absorption is dependent upon the presence in the cells of a 
 large amount of soluble and osmotically active substance 
 and upon the presence outside of a large amount of water. 
 Small loss of water is dependent upon small surface, upon 
 the impermeability of the walls of the superficial cells, and 
 upon low protoplasmic activities in them. These condi- 
 tions are met especially in spring, but also to a limited extent 
 during and immediately after rain in summer. In spring, 
 water is taken up in quantity from the moisture-laden soil 
 by the dense cell-sap of the root cells; from the still bare 
 branches and the unopened buds water is given off only in 
 very small amount ; * sap-pressure develops in consequence. 
 If a plant in this condition has been so recently trimmed 
 or pruned that the wounds are not yet closed, or if new 
 wounds are opened, we shall have the familiar phenomena 
 of " bleeding" and of sap-flow. The name "bleeding" or 
 "weeping" is given to wholly useless if not injurious ex- 
 hibitions of the phenomena, employed by the farmer in 
 northern North America when he "taps" his maple trees 
 in spring to secure the highly prized maple syrup and maple 
 sugar. What flows from the plant, whether in bleeding or 
 in the run of sap after tapping, is the water expressed into 
 the wood elements by the living cells bordering upon them. 
 This sap, flowing out under pressure, is a solution contain- 
 ing various organic compounds in maple chiefly sugars 
 and mineral salts. The presence of mineral salts in the sap, 
 and their accumulation in the evaporating pans employed in 
 sugar-making, are due to their being taken up, a little at a 
 time, by the plant in the water absorbed from the soil. The 
 amount and the kinds of salts present in the sap will vary 
 with the nature of the soil and with the kind of plant, for 
 the reasons which we have above considered (p. 112). The 
 
 * It is not a question of surface merely, however, for in evergreens, 
 although the surface is not materially less in winter and spring than in 
 summer, the amount of water given off during winter and spring is much 
 less than in summer. The lessened activities of the protoplasm in leaves 
 and branches, and the decreased evaporation at the lower temperatures, 
 account for this. 
 
ABSORPTION AND MOVEMENT OF WATER 133 
 
 amounts and the kinds of organic matters will vary not 
 only with these two factors, but also with the condition of 
 the individual, both at the time, and during the foregoing- 
 season, when organic substances were being formed and 
 stored by the plant. 
 
 Sap-pressure, which determines the rate of sap-flow and of 
 bleeding, varies at different times in the season and in the 
 day. The amount of water in the soil and of moisture in 
 the air will directly affect the physical conditions of sap- 
 pressure and of sap-flow. If the soil is dry, only small 
 amounts of water can be absorbed, the turgor-pressure in 
 the living cells of the root will be relatively low, compara- 
 tively little water will be pressed from these cells into the 
 wood-elements, and therefore the sap in the wood will in- 
 crease proportionally little in volume and pressure. If the 
 air is dry, more water will evaporate from the plant, the 
 ratio between the amounts absorbed and given off will be 
 lowered, and the volume and pressure of the sap in the wood 
 will be proportionally lowered. 
 
 Other factors, acting directly upon the protoplasm and 
 only by this means affecting the physical conditions of sap- 
 pressure, cause the pressure and rate of flow to vary from 
 time to time. Whatever stimulates the protoplasm of those 
 cells in which food is stored in solid form to dissolve the 
 food, will tend, other things being equal, to raise the sap- 
 pressure by increasing the absorption of water and the 
 turgor-pressure. The increased secretion by the protoplasm 
 of diastatic or other enzyms, by means of which more in- 
 soluble solids will be converted into osmotically active solu- 
 ble substances, increases the absorbing power of the cell-sap 
 and proportionally increases its volume and pressure. Con- 
 versely, whatever influences depress the protoplasmic activi- 
 ties also tend to reduce the sap-pressure. 
 
 Experiments by Wieler* on plants in pots, and there- 
 fore under controllable though somewhat artificial con- 
 ditions, confirm the observations made on plants in 
 
 * Wieler, A. L. Das Bluten der Pflanzen. Cohn's Beitrage zur Biologic 
 der Pflanzen, Bd. VI., 1893. Gain, E. Action de Peau du sol sur la veg- 
 etation. Revue Generate de Botanique, t. VII., 1895. 
 
134 PLANT PHYSIOLOGY 
 
 nature* that low temperatures (freezing or lower) decrease 
 the sap-pressure and the sap-flow and may stop the bleeding. 
 With a rise in temperature the pressure will rise and sap- 
 flow will be resumed. Experience shows that cold at night, 
 stopping the sap-flow, and warmth by day, causing it to be 
 renewed with vigor, are most favorable to a copious yield 
 of sap of good quality. 
 
 The means of measuring the pressure developed by the 
 absorption of water are at the best inadequate, for the 
 various forms of pressure-gauges (manometers) employed 
 cannot be made to measure the total amount of force de- 
 veloped. A manometer measures merely the net force. 
 Each individual cell which is restrained from expanding, and 
 which absorbs more water than it gives off, exerts force, 
 develops pressure. But by no means all the cells of a plant 
 develop pressure simultaneously ; some cells develop no more 
 than average pressure, and some dead parts (e. g. ducts, 
 tracheids, etc. ) cannot develop osmotic pressure under the 
 conditions ordinarily prevailing in the plant. Against these 
 less resisting cells those under pressure and seeking to ex- 
 pand, exert force. This force, being partly or wholly un- 
 resisted, expends itself, the pressing cells expand, the others 
 collapse. Again, water may be forced into tracheids and 
 ducts by adjacent cells and thus the pressure of the latter will 
 be reduced. If, however, tracheids and ducts become filled 
 with sap, as is the case in early spring in the sugar maple, 
 vine, etc., pressure will develop in these dead parts also, be- 
 cause of the force exerted by the osmotically active living cells, 
 adjacent or more or less remote. The pressure developed by 
 one group of cells may, then, expend itself wholly in some 
 other part of the body of the plant, leaving no force to be 
 exerted upon the pressure-gauge. The pressure-gauge indi- 
 cates only that amount of force due to sap-pressure which is 
 not expended in the body of the plant itself, that is, the net 
 force as it may be called, to distinguish it from the total force. 
 
 * Bibliography in Wieler's paper above and papers by Jones and Orton, 
 Sap-pressure and flow in Sugar Maple. Ann. Report Vermont Agric. 
 Exp. Station, 1898. Morse and Wood. Studies of Maple Sap. Bulletins 
 24, 25, 32, New Hampshire Coll. Agric. Exp. Station, 1895. 
 
ABSORPTION AND MOVEMENT OF WATER 135 
 
 In spite of the inadequacy of the means for measuring 
 sap-pressure, figures of very considerable magnitude are to 
 be found in the published studies on this subject. For ex- 
 ample 
 
 in Ricinus communis 6% Ibs. to sq. in. 
 Urtica dioica 9% " " " 
 
 Vitis vinifera 21% " " " 
 
 Betula alba 28 " " " 
 
 Strangely enough, since the researches of Clark* in 1873, 
 little attention has been paid in this country to the phe- 
 nomena of bleeding, hi spite of the facts that the important 
 maple-sugar industry depends upon it, and that there are 
 botanists at the Agricultural Experiment Stations of the 
 sugar-making States. 
 
 For the very natural but also very poor reason that sap- 
 pressure has often been measured on the stumps of small 
 plants cut off near the ground that is, where the sap-pres- 
 sure must develop almost wholly within the root it has 
 been commonly called root-pressure. That this is a mis- 
 nomer follows not only from the foregoing consideration of 
 the physics of sap-pressure, but also from the introductory 
 experiments of Pitra,f repeated and extended by others, 
 upon the sap-pressures which may be developed in parts 
 above ground, for example, branches cut off from the main 
 stem and thus wholly separate from the root. The sap- 
 pressure of such amputated parts may even be higher than 
 of those left attached to the root. It is true that the sap- 
 pressure develops first in the lowest parts and gradually 
 ascends the stem or branch, but this is owing to the absorb- 
 ing part being below. It is the roots, or the lower ends of 
 amputated branches, which absorb the water, and it is 
 necessarily the cells nearest the absorbing parts that first 
 develop pressure and from which, after a time and after the 
 pressure goes beyond a certain height, sap is pressed out 
 
 * Clark, W. S. Circulation of sap in plants. Lecture before Mass. State 
 Board of Agriculture, 1874. 
 
 t Pitra, A. Versuche iiber die Druckkraft der Stammorgane bei den 
 Erscheinungen des Blutens und Thranens der Pflanzen. Jahrb. f. w. Bo- 
 tanih. Bd. XI., 1877. 
 
136 PLANT PHYSIOLOGY 
 
 into the tracheids and tracheae to make room for the water 
 which continues to be absorbed. Although the sap-pressure 
 is generally greatest at the base of a stem or the butt of a 
 branch, the pressure is clearly due to a condition in some 
 group of cells, whether these are in the root or elsewhere. 
 We should, therefore, abandon the misleading name of root- 
 pressure and use only the equally self-descriptive but correct 
 term sap-pressure. 
 
 The amount of sap-flow is exceedingly different, not only 
 for different species and different individuals of % the same 
 species, but also for the same individuals in different sea- 
 sons. The reasons for this diversity we have already con- 
 sidered. The following figures will serve to indicate the 
 volumes sometimes obtained 
 
 Birch in 7 days yielded 36 liters sap (Wieler) 
 
 (12 years old) daily average 5+ 
 
 Agave 7.5 " " (Humboldt) 
 
 (decapitated flower in 4.5 mont hs 995.0 " 
 
 Sugar maple in 23 days 93095.0 grs. " (Morse) 
 
 ( cuSference?; r " daily average 4047.0 
 
 =3.6 litres " 
 
 (Sp. gr. 1.32) 
 
 TRANSPIRATION. 
 
 From all their surfaces exposed to the air, plants give off 
 water-vapor. This is a physical necessity, for water-vapor 
 will be given off from any mass, lifeless or living, which con- 
 tains water, whenever the surrounding air is not saturated 
 with moisture, or when the mass has a temperature higher 
 than that of the air, or when the mass, in relatively dry 
 air, is not enclosed in a waterproof covering. Other things 
 being equal, the amount of water-vapor given off will be 
 greater the greater the exposed surface in proportion to the 
 mass. With like conditions of humidity, temperature, sur- 
 face-composition, and surface-area, equal masses of different 
 composition will dry, 7. e. lose water by evaporation, at 
 different rates, a gelatinous or slimy mass more slowly than 
 a woody one, for example. The living plant differs from a 
 dead one of exactly the same dimensions in being able to 
 control four of these five factors, and to that degree it is 
 
ABSORPTION AND MOVEMENT OF WATER 137 
 
 able to control the rate and the amount of evaporation. 
 Because evaporation from the body of the living plant is 
 controllable within certain limits by the plant itself, and to 
 this extent is a physiological process, it has been given the 
 separate name of transpiration. With this idea of evapora- 
 tion controlled by the living organism has been coup- 
 led the notion that water is vaporized by the chlorophyll 
 grains illuminated by sunlight and manufacturing carbo- 
 hydrates, and that this water-vapor, produced by physical 
 means acting through the living organs of the plant, is 
 an important part of the total volume of water given off. 
 This process Van Tieghem* distinguishes from evaporation 
 by the name of chloro vaporization. Assuming that water is 
 set free in combining carbon-dioxide and water into carbo- 
 hydrate, it is hard to conceive that the liberation of water 
 in a water-containing cell would be in the form of escaping 
 vapor any more than in the familiar reactions carried on in 
 solutions in the laboratory. The water molecules liberated 
 at the temperatures prevailing in cells photosynthetically 
 active would mix with the water in the cell-sap both in the 
 protoplasm and in the vacuoles. Because light is absorbed 
 in chlorophyll-containing cells, the temperature of these cells 
 may be (but not necessarily will be) higher than of other 
 cells not absorbing light. If this is the case, and if these 
 cells are warmer than the air, evaporation will of course 
 take place. Transpiration is, therefore, a physical process 
 controlled but not carried on by the living plant. Accord- 
 ing to circumstances it may be more or less rapid than 
 simple evaporation. 
 
 Plants living in regions where the rain-fall is slight or is 
 very unequally distributed through the year, and where the 
 soil is not an efficient reservoir of water, are forced not only 
 to store water in their bodies (see p. 124) but also to 
 check the loss of water by every possible means. The 
 greatest economy of water is shown by the Cacti. In the 
 most condensed forms we have nearly spherical plants, the 
 
 *Van Tieghem, Ph. Traite de Botanique, t. I., p. 185, 1891. Also 
 Transpiration et chlorovaporisation. Bulletin de la Soc. Bot. de France, 
 
 1886. 
 
138 PLANT PHYSIOLOGY 
 
 outer surface of which is rendered as impermeable as possi- 
 ble to water and water-vapor by the waxy covering of the 
 outer walls of the epidermal cells, by greatly thickening and 
 cutinizing these walls, by forming more than one layer of 
 epidermis, by sinking the guard-cells of the stomata to the 
 second layer of epidermis, by greatly reducing the number 
 and size of stomata, and in some cases by forming a woolly 
 covering of dead hairs which still further insulates the tissues 
 within. From such plants the loss of water is slight in pro- 
 portion to the mass, less in proportion to the surface, least 
 in proportion to the amount of water contained in .the 
 plants. Evaporation from these plants is reduced to the 
 minimum, transpiration is lowest in rate and in volume, 
 first, by reason of the composition of the plant-body, slimy 
 and gelatinous materials holding water, and water-storing 
 tissues, forming a considerable part of the volume of the 
 plant ; second, by the area, composition and covering of the 
 surface ; third, by the small number of openings through the 
 insulating covering ; and fourth, by the body-temperature of 
 the plant being lower than that of the air when the air 
 could otherwise take up most moisture.* 
 
 The opposite extreme is represented by plants living in the 
 very humid regions of the tropics. There the air is always 
 near the point of saturation, and the almost daily showers 
 at certain seasons attest both the frequency with which the 
 air attains the point of saturation and also the great 
 amount of moisture which can be quickly precipitated. The 
 well-known copiousness and frequency of the tropical down- 
 pours indicate that great volumes of water must somehow 
 be vaporized. It has sometimes been concluded, from the 
 tardy evaporation of water from wet masses having the 
 same temperature as the surrounding air, that all tropical 
 plants must have other means than transpiration for get- 
 ting rid of the water absorbed by them. Some do have 
 other means in the hydathodes (p. 128), but not many 
 
 * For a further discussion of these interesting adaptations consult 
 Goebel, Pflanzenbiologische Schilderungen ; Volkens. Flora der agyptisch- 
 arabischen Wuste, 1887; Schimper. Pflanzengeographie auf physiolo- 
 gischer (jrundlage, 1898. 
 
ABSORPTION AND MOVEMENT OF WATER 139 
 
 plants are so equipped. Whenever the temperature of the 
 plant is higher than that of the moisture-saturated air 
 outside, transpiration will take place, the water-vapor 
 condensing on the surfaces of the plant and elsewhere. 
 Because the plant in its respiration has a means of develop- 
 ing heat, it must often be the case in the tropics that the 
 plant is warmer than the surrounding air. That transpira- 
 tion into the moisture-laden air of the tropics is sufficient 
 for getting rid of w r ater is evident when we take into con- 
 sideration the following matters. First, the plant absorbs 
 water because it needs and uses both water and the salts dis- 
 solved in it. Of the salts it needs only very small amounts, 
 as shown by culture experiments, and though analyses 
 of the mature plant may reveal the presence of much larger 
 amounts of some or all of the useful salts, it does not by 
 any means follow that these amounts are used, or, if they 
 are used, that the plant is not over-fed. Of the water it 
 needs enough to bring adequate amounts of the indispens- 
 able salts, and if the plant absorb enough water for this pur- 
 pose, it will certainly have sufficient for all other purposes 
 also, because the solutions of needed salts are so dilute. 
 In certain places one of the advantages attained by trans- 
 piration certainly consists in the lower body-temperature of 
 the plant, since vaporization is a cooling process. Second, 
 transpiration even into a very humid atmosphere will suffi- 
 ciently concentrate the cell-sap of superficial cells to ensure 
 an osmotic current into these cells, supplying them with 
 both the water and the salts which they may need. Third, 
 'no more water will be absorbed than the plant needs, for 
 the living cells control by their activities those physical 
 "conditions which make absorption possible. If the trans- 
 piration of plants living in the humid tropics is less than of 
 plants living in drier regions, the absorption of water will 
 be less. So long as water and needed salts are absorbed 
 in sufficient quantities, growth and the other activities of 
 the plant will be normal. The luxuriant vegetation of the 
 tropics impresses every one with the idea that there growth, 
 food-manufacture, etc., must be more rapid and more abun- 
 dant than in temperate regions. The correctness of this 
 
140 PLANT PHYSIOLOGY 
 
 view is questioned by Giltay,* at least so far as food- 
 manufacture is concerned. On equally fertile soil in equal 
 lengths of time, the activities of tropical and temperate 
 plants are not unlike. If this is true, there is no need of 
 more rapid absorption and transfer of food in tropical 
 plants than in those of temperate climes, and transpiration 
 may at least be no more rapid, may safely be less rapid, 
 than in dryer temperate regions. It may easily happen in 
 temperate regions that the plant takes in more water and 
 more salts than it really needs, and that while the former 
 evaporates, the latter accumulate in useless forms and 
 quantities with or without chemical change. Whether the 
 transpiration of plants adapted to the climatic conditions of 
 different parts of the world differs greatly must be regarded 
 as still unsettled', f 
 
 The result of the evaporation of water from any solution 
 is the concentration of the solution and the lowering of 
 its temperature. With the evaporation or transpiration of 
 water from the leaves of plants, the cell-sap of the cells 
 giving off water will tend to increase in density and to de- 
 crease in temperature at a rate proportional to the rate of 
 transpiration. The increase in density of the cell-sap is ac- 
 companied by an increase in osmotic pressure, and the cell- 
 
 * Giltay, E. Uber die vegetabilische Stoffbildung in den Tropen und in 
 Mitteleuropa. Annales du Jardin Botanique, Buitenzorg, t. XV., 1898. 
 
 t The discussion of the question is represented by the following papers : 
 Haberlandt, G. Anatomisch-physiolog. Untersuchungen iiber das tropische 
 Laubblatt. I. Uber die Transpiration einiger Tropenpflanzen. Sitzungs- 
 ber. d. K. K. Akad. d. Wiss. zu Wien, Bd. CL, Abth. I., 1892. Uber die 
 Grosse der Transpiration im feuchten Tropenklima. Jahrb. f. wiss. Bot- 
 anik, Bd. 31, 1898. Erwiderung zu Giltay's Abhandlung. Jahrb. f. wiss. 
 Bot., Bd. 33, 1899. Stahl, E. Einige Versuche iiber Transpiration und 
 Assimilation. Botanische Zeitung, 1894. Burgerstein, A. fiber die Trans- 
 pirationsgrosse von Pflanzen feuchter Tropengebiete. Ber. d. Deutsch. 
 Bot. Gesellsch., 1897. Materialien zu einer Monographic betreffend die 
 Erscheinungen der Transpiration der Pflanzen. Verhandl. d. K. K. Zool.- 
 Bot. Gesellsch., Wien, 1901 and earlier. Giltay, E. Vergleichende Studien 
 iiber die Starke der Transpiration in den Tropen und im mitteleuropai- 
 schen Klima. Jahnb. f. wiss. Botanik., Bd. 30, 1897. Die Transpiration 
 in den Tropen und in Mittel-Europa, II., Jahrb. f. wiss. Bot., Bd. 32, 1898 ; 
 Ibid., III., loc. cit., Bd. 34, 1900; Ibid., Bot. Centralbl., Beihefte, Bd. 9, 
 1900. 
 
ABSORPTION AND MOVEMENT OF WATER 141 
 
 sap draws water from adjacent cells with corresponding 
 force. The decrease in the water-content of the parenchyma 
 cells causes a greater draft upon the water-conducting 
 tissues not only adjacent but throughout the conducting 
 system, for a draft upon one part disturbs the balance 
 throughout the whole. Thus, assuming an adequate force 
 by which water is raised through the ducts from root to 
 leaves, we see that this force is set in motion, and its action 
 is regulated, by the amount and the rate of transpiration. 
 Transpiration must then also affect the root-hairs, regulat- 
 ing the amount of water which they absorb. A current for 
 parts of its course osmotic, for the remainder of much 
 larger dimensions is set up, maintained and controlled by 
 transpiration. Transpiration is, however, only one means 
 of doing this, water-pores (p. 128) and hydathodes (p. 128) 
 being the others, and perhaps equally important for the 
 plants which possess them. 
 
 The ordinary means by which evaporation is controlled by 
 higher plants are two: fiist, the stomata (p. 142), which 
 control the exchange of gases as well as of water-vapor be- 
 tween the plant and the air ; second, the character of the 
 epidermal and other cells (cork, etc.) covering the plant.* 
 Special means of facilitating or checking transpiration are 
 possessed by plants inhabiting especially damp or especially 
 dry regions, f 
 
 * Consult De Bary, A. Comparative anatomy of the vegetative organs 
 of phanerogams and ferns. Oxford, 1884. 
 
 t These topics need not be discussed here. They illustrate no new princi- 
 ples in plant physiology. The following papers may be read by the inter- 
 ested student. Other papers and books have been referred to in the pre- 
 ceding pages. Stahl, E. Regenfall und Blattgestalt. Ann. du Jardin Bot. 
 de Buitenzorg, Bd. XI., 1893. Uber bunte Laubblatter. Ibid., Bd. XIII., 
 1896. Kny, L. Zur physiologischen Bedeutung des Anthocyans. Atti del 
 Congresso Botanico Internationale, 1892. (Older literature here cited.) 
 Keeble, F. W. The hanging foliage of certain tropical trees. Annals of 
 Botany, vol. IX., 1895. Darwin, Charles and Francis. The Power of Move- 
 ment in Plants, 1880. Stahl, E. Uber den Pflanzenschlaf und verwandte 
 Erscheinungen. Botanische Zeitung, I. Abth., Heft. V.,VI., 1897. Wilson, 
 W. P. and Greenman, J. M. Preliminary observations on the movements 
 of the leaves of Afelilotus alba L. and other plants. Contrib. Botan. Lab- 
 oratory, Univ. of Pennsylvania, 1892. 
 
142 PLANT PHYSIOLOGY 
 
 STOMATA AND THE AERATING SYSTEM 
 
 The preceding paragraph leads us to a consideration of 
 those special epidermal structures, the stomata, which are 
 the chief means by which the plant controls the transpira- 
 tion of water-vapor and the exchange of oxygen and carbon- 
 dioxide. The stomata are the guarded openings, on the 
 surface of the plant, of those intercellular spaces which form 
 throughout its body a system of continuous passages 
 through which gases, passing diosmotically into and out 
 of the adjacent living cells, make their way from and to 
 the outside air. 
 
 Through open stomata and intercellular passages gases 
 can diffuse more rapidly than they can pass by osmosis 
 through cell-walls soaked with water. Besides the uninter- 
 rupted diffusion which these passages and openings make 
 possible, still more rapid movement of enclosed gases and 
 vapors must take place whenever the plant is agitated, 
 swayed by the wind, or by passing animals. Even changes 
 in the positions of the organs of the plant, resulting in 
 changes in the diameter of the intercellular spaces in one 
 region and a consequent change in the equilibrium of the 
 enclosed gases and vapors, will facilitate the movement of 
 gases throughout the whole aerating system. Thus the 
 constant trembling of the leaves of the aspen (Populus 
 tremula, and P. tremuloides), and often of other plants 
 also, in the slightest breeze, and the autonomic movements 
 of the leaflets of Desmodium gyrans are claimed by Stahl* 
 to increase transpiration. If they do this, they must also 
 accelerate the interchange of gases. 
 
 The intercellular spaces which, uniting together, form with 
 the stomata the aerating system of larger massive plants, 
 .are of very different sizes according to their position and 
 according to the needs of the cells enclosing them. Between 
 the chlorophyll-containing, food-making cells of the leaf the 
 intercellular spaces are comparatively large. In the leaves 
 of plants growing in damp places they are even larger than 
 
 * Stahl, E. f T ber den Pflanzenschlaf und verwandte Erseheinungen. Bot. { 
 Zeitung, 1897. 
 
ABSORPTION AND MOVEMENT OF WATER 143 
 
 in other leaves. * The large size is due to the need of get- 
 ting rid of water-vapor as rapidly as possible, and of ob- 
 taining sufficient quantities of the carbon-dioxide contained 
 in such minute proportions in the air. There must, there- 
 fore, be a rapid passage of comparatively large volumes of 
 air past these mesophyll cells. On the other hand, the in- 
 tercellular spaces in the deeper tissues, where the cells de- 
 mand mainly, if not exclusively, the much more abundant 
 oxygen, are relatively very small. Merismatic tissues for 
 example, cambium enclose no intercellular spaces, and 
 though aerated only osmotically, they obtain oxygen in suf- 
 cient quantities from the adjacent tissues which do enclose 
 air passages. Hollow stems such as those of the grasses 
 and dead cells are filled with air, but these are not to be 
 regarded as forming part of the aerating system. On the 
 contrary, they merely contribute, like the air chambers, blad- 
 ders, etc., of water-plants, to the lightness of the organism. 
 The cells enclosing the air spaces have walls which are 
 freely permeable to water-vapor and to gases. Because 
 these cell- walls, like all others in the plant except those 
 forming the outermost bark-layers, are saturated with 
 water, the. passage of oxygen, carbon-dioxide, and nitrogen 
 (if this is taken up at all) through them must be in solu- 
 tion, by osmosis, just as salts enter the cells. The cell-walls 
 bordering upon the unconfined air, however, are so modified 
 chemically by cutinization, suberization, and various im- 
 pregnations (e. g. with SiO 2 ), and by being overlaid with 
 wax, that they are far less permeable to gases and still less 
 to water-vapor. The slow exchange of gases through epi- 
 dermis with closed stomata or with none has been re- 
 peatedly demonstrated, f most recently and certainly most 
 simply, however, by the use of dry filter paper impregnated 
 with cobalt chloride and by the iodine test for starch. t 
 It is well known that cobalt paper, blue, when dry, will 
 
 * Stahl, E. Einige Versuche iiber Transpiration und Assimilation. Botan- 
 ische Zeitung, 1894. 
 
 tSee discussions of this in Pfeffer, Pflanzenphysiologie, I., 21, 29, 30. 
 Engl. transl., vol. I., 21, 29, 30. 
 
 t Stahl, E. L. c. 
 
144 PLANT PHYSIOLOGY 
 
 change to a pink hue when exposed to dampness. This fact 
 has been employed by Stahl in demonstrating that transpira- 
 tion through the walls of epidermal cells is much slower than 
 through open stomata. The iodine test for starch demon- 
 strates whether carbon-dioxide enters the leaf through cell- 
 walls in sufficient quantities for starch manufacture. In 
 most plants the epidermal walls are too impermeable for this. 
 
 Through the relatively impermeable superficial tissues 
 epidermis and cork there must evidently be openings of the 
 intercellular aerating passages. The most important and 
 the most perfect of these openings are the stomata, found 
 on leaves and other young parts. On older and persisting 
 parts the enclosing cork layer may be interrupted by lenti- 
 cels masses of rounded cells, unchanged as to their walls, 
 and enclosing intercellular spaces continuous with those 
 deeper in the body of the plant. Cork may be replaced on 
 submersed organs, or on those growing in the mud of 
 swamps and marshes, by a homologous tissue, like that 
 composing the lenticels, and called a eren chyma.* Lenticels 
 can be closed, and the passage of gases through aerenchyma 
 can be stopped, only by the growth of new tissue, of cork, 
 which seals the openings. The stomata, on the contrary, are 
 intercellular spaces bounded by a pair of delicately balanced, 
 and in most cases freely movable, epidermal cells. Lenticels 
 are found especially in the bark of stems and branches, 
 though also on older bark-covered roots. Aerenchyma is 
 found almost exclusively on roots. These organs secure 
 mainly the entrance of oxygen in sufficient quantities into 
 the older and not necessarily very active tissues, and at the 
 same time the exit of carbon-dioxide. When they occur on 
 aerial organs they of course facilitate transpiration. Sto- 
 mata, on the contrary, form most abundantly in the epi- 
 dermis immediately covering chlorophyll-containing cells. 
 
 Oxygen forms about twenty per cent, of the atmosphere 
 and carbon-dioxide one-twentieth of one per cent. The chloro- 
 phyll-containing cells must be very active during the hours 
 when they are supplied by sunlight with the energy neces- 
 
 * Schenck, H. fiber das Aerenchym : ein dem Kork homologes Gebilde. 
 Jahrb. f. wiss. Botanik, Bd. XX., 1889. 
 
ABSORPTION AND MOVEMENT OF WATER 145 
 
 sary for photosynthesis. Other cells can work at a slower 
 rate and still accomplish as much, for, being independent of 
 light, they can work longer. These constitute some of the 
 reasons for the position and for the number of stomata. 
 
 Stomata are sometimes called " breathing pores." They 
 do admit oxygen, and whenever respiration exceeds photo- 
 synthesis (e. g. in the dark), the unused carbon-dioxide 
 may pass out through them. For these reasons they are 
 breathing pores, but this is neither their sole nor their main 
 function. Through open stomata water-vapor passes out- 
 ward and carbon-dioxide inward. Through the stomata on 
 the surface of the parts of the flower, little carbon-dioxide 
 passes inward, for these are not food-manufacturing organs. 
 On the contrary water-vapor and expired carbon-dioxide 
 pass outward. On such organs the stomata are essentially 
 organs for increasing and controlling transpiration.* 
 
 A stoma consists fundamentally of a pair of epidermal 
 cells not completely connected together and, therefore, either 
 constantly separated from each other by a slit-like space, or 
 separated whenever the cells draw or are drawn apart. The 
 pair of incompletely connected cells known as guard-cells 
 are usually strikingly different in size, form, and contents 
 from other epidermal cells. The cells immediately adjacent 
 to them may also depart from the type of epidermal cells 
 both in appearance and in function. Such cells when present 
 are known as accessory or auxiliary cells, for they either sup- 
 plement the guard-cells in their task of opening and closing 
 the stoma, or they themselves open and close the stoma by 
 drawing the guard-cells apart or pushing them together. 
 Strictly speaking, the stoma is the slit-like opening (Spalt- 
 offnung) between these cells, just as the mouth is the cavity 
 opened and closed by the lips. 
 
 The opening and closing of stomata are accomplished by 
 physical means only, but these means may or may not de- 
 pend upon the irritability of the living protoplasm of the 
 guard-cells and auxiliary cells. Fundamentally, the opening 
 
 * Chester, Grace D. Bau und Function der Spaltoffnungen auf Blumen- 
 blattern und Antheren. Ber. d. Deutsch. Bot. Gesellseh., Bd. XV.. 1897. 
 Older literature here cited. 
 10 
 
146 PLANT PHYSIOLOGY 
 
 and closing of the stomata are due to changes in the tur- 
 gor,* consequently in the form and dimensions, of the 
 guard-cells. These changes in turgor may be due wholly 
 to the absorption and withdrawal of water by means only 
 remotely if at all controlled by the living cells, or they may 
 be due to stimulations of the living cells which cause them 
 to absorb or to expel water or otherwise to change the 
 density of their cell-sap with consequent changes in turgor. 
 The accompanying figures illustrate a stoma without aux- 
 iliary cells (Fig. 3) and with auxiliary cells (Fig. 4). 
 From figures 5 and 6 the differences in thickness of the 
 
 FIG. 3. FIG. 4. 
 
 Figure 3. Lower epidermis from leaf of Vicia faba showing stomata 
 without auxiliary cells. Figure 4. Lower epidermis from leaf of Tradescan- 
 tia zebrina showing stoma with auxiliary cells. 
 
 different walls of the guard-cells are evident. The thin walls 
 joining the guard-cells to their neighbors permit rapid 
 osmotic transfer, the thin walls opposite are pliant. If, for 
 any reason, the guard-cells absorb water, the volume of the 
 cell cavities will increase, the form of the cells will change, 
 and they will occupy the position indicated by the heavy 
 outline in figure 5 ; the stoma will be open. If the reverse 
 is the case, if the amount of water in the cell-sap is dimin- 
 ished by evaporation or expulsion, the volume of the cell 
 cavities will decrease, the thin adjacent walls of the guard- 
 cells will bend out, approach each other and finally come 
 into close contact; the stoma will be closed. From the 
 figures it will be noticed that the guard-cells, provided 
 
 * Mohl. H. von. Welche Ursachen bedingen die Erweiterung und Vereo- 
 gung der Spaltofmungen ? Bot. Zeitung. 1856. 
 
ABSORPTION AND MOVEMENT OF WATER 
 
 147 
 
 with chlorophyll grains, are capable of manufacturing for 
 themselves substances which are osmotically active, con- 
 tributing to the turgor as well as to the nutrition of the 
 cell. The other epidermal cells of most plants, being devoid 
 of chlorophyll, must absorb from their neighbors the osmoti- 
 cally active substances upon which their turgescence depends. 
 But their turgescence will vary according to the proportion 
 sustained between the amounts of water which they absorb 
 and give off. If, as is shown in figure 4, the shape, size, 
 position, and other characters of some or all of the epider- 
 mal cells adjoining the guard-cells are different from the 
 
 Fig. 5. Fig. 6 
 
 Figure 5. Diagram (after Schwendener) to illustrate changes in form of 
 guard-cells in opening and closing stomata. Figure 6. Cross section of 
 stoma of Tradescantki zebrinn, showing relations of auxiliary cells to me- 
 chanics of opening and closing. 
 
 other epidermal cells, their turgidity will vary at times, at 
 rates, and in degrees different from the ordinary epidermal 
 cells. With these changes in the turgor of the ordinary 
 epidermal cells, of the auxiliary, and of the guard-cells, there 
 will necessarily be changes in the size of the openings, in the 
 degree to which the stcmata are open or closed. As re- 
 cently re-stated by Darwin,* in opposition to the extreme 
 view prevailing of late that the guard-cells alone effect the 
 opening and closing of the stomata, "the pressure of the 
 guard-cells and that of the surrounding epidermis should be 
 looked at as correlated, not as opposed and independent 
 
 
 Darwin. Francis. Observations on Stomata. 
 Society of London, Series B , vol. 190, 1898. 
 
 Philos. Trans. Royal 
 
148 PLANT PHYSIOLOGY 
 
 factors." Yet it is easily conceivable owing to the different 
 rates at which water may be absorbed or given off by the 
 different cells of the epidermis, that in some plants, or under 
 certain conditions of culture, etc., there may be such differ- 
 ent conditions in the epidermal cells that while the guard- 
 cells may be expanding and so tending to open the stoma, 
 the other epidermal cells may also be expanding and so 
 tending to push the guard-cells closer together and to close 
 the stoma.* According to Benecke,f the auxiliary cells 
 assist in the opening and closing of the stomata only indi- 
 rectly and by neutralizing the mechanical strains brought 
 to bear on the guard-cells by the changes in form of the 
 epidermal and even of other leaf cells. 
 
 Granting that changes in the turgor of epidermal cells, 
 and especially of those peculiar epidermal structures, the 
 guard-cells, bring about the opening and closing of the 
 stomata, we must enquire how these changes are effected 
 and what are the results as regards transpiration. 
 
 Whenever a cell loses more rapidly than it absorbs water, 
 the turgor of the cell will decline proportionally. This dif- 
 ference will occur whenever a cell is unable to supply itself, 
 directly or through its neighbors, with water enough to 
 make good the loss by evaporation. The decrease in turgor 
 of the guard-cells of the stomata, and their consequent 
 flattening, are the result of such external conditions; the 
 stomata are closed. The difference in the amount of water- 
 vapor given off from leaves with open and with closed stom- 
 ata has been estimated by various means. The most striking 
 way of demonstrating this is perhaps Stahl's cobalt paper 
 method. t Leaves with closed and with open stomata are 
 compared, under as like conditions as possible, as to their 
 rates of changing the color of dry filter paper impregnated 
 with cobalt chloride. Whereas the sheet of cobalt paper 
 placed against the leaf with open stomata will change 
 
 * Stahl, E. I.e.. p. 121. 
 
 f Benecke, W. Die Nebenzelleri der Spaltoffnungen. Bot. Zeitung, p. 
 602-3, 1892. 
 
 t Stahl, E. Einige Versuche tiber Transpiration und Assimilation. Bot. 
 Zeitung, 1894. 
 
ABSORPTION AND MOVEMENT OF WATER 149 
 
 within a few seconds from blue to pink, the color remains 
 unchanged for an hour or more (with Tradescantia zebrina, 
 for four hours) in the paper in contact with the closed 
 stomata. In spite of the very minute size of the opening, 
 even at its fullest extent, the great number of stomata give 
 us some idea of the effectiveness of these gates at the en- 
 trances of the intercellular passages. Pfeffer,* quoting 
 figures as to the size and number of stomata, says the 
 effective area of the opening seldom reaches .0046 sq. mm., 
 but that the number of such openings is from 100 to 300 
 per sq. mm. Estimating 100 to the sq. mm. at the size 
 .0046 sq. mm., we see that nearly one-half the surface can 
 be opened. Assuming that the stomata are on one side of 
 the leaf only, and that on that side they have the propor- 
 tions just given, we see that roughly one-quarter of the 
 leaf-surface can be the free path for the exchange of gases 
 and vapors. 
 
 The disparity between absorption and evaporation which 
 for physical reasons forces the stomata to close, is often 
 supplemented by the irritability of the protoplasm of the 
 guard-cells. Evaporation insufficient to produce any dimin- 
 ution in turgor visible as wilting may still be enough to 
 irritate the g^ard-cells into reducing their turgor by physi- 
 cal or chemical changes accomplished in the cells by the 
 living protoplasm. Thus various influences which cannot 
 bear directly upon turgor, but which can irritate the proto- 
 plasm, bring about the closing and opening of the stomata. 
 The statements of older authors regarding these influences 
 have been lately tested by Stahl,t and still more recently by 
 Francis Darwin,! from whose papers the following conclu- 
 sions are abstracted. The stomata of most plants are widest 
 open in bright light, less widely open or completely closed 
 in darkness. This one would in general expect, for photo- 
 synthesis in chlorophyll-containing cells ( and the guard-cells 
 contain chlorophyll) is most rapid in bright light. There 
 
 * Pfeffer, W. Pflanzenphyeiologie, I., p. 177. Eng. transl., I., p. 195. 
 Compare also Brown (Fixation of Carbon, Nature, 1899), and pp. 45, 50 
 of this book. 
 
 f 7. c. J 1. c. 
 
150 PLANT PHYSIOLOGY 
 
 would then be the greatest demand for carbon-dioxide to be 
 elaborated into food, and the gateways of the intercellular 
 passages should be open to allow the free entrance of car- 
 bon-dioxide. If the stomata close in bright light to guard 
 against excessive transpiration, food production diminishes 
 greatly. The closing of stomata more or less completely 
 in darkness, the decrease in the size of the openings as the 
 light diminishes, may be coupled with these facts : the pro- 
 portion of oxygen in the air is so much larger than that of 
 carbon-dioxide, and the rate of respiration so much lower 
 than that of photosynthesis ( see p. 65 ) , that the stomata 
 may well decrease in size, or even close for a time, with- 
 out interfering with respiration ; transpiration on dry nights 
 might be excessive or at all events would tend to cause in 
 the cells an accumulation of salts unnecessary at the time 
 in amount and kinds; the leaf is cooled by transpiration, 
 and loss of heat by this means might be undesirable; in 
 nyctitropic plants, and in others habitually living where the 
 air is damp, the closing of stomata in darkness is less com- 
 mon than in plants growing under more ordinary condi- 
 tions. Stomata which have remained closed during the 
 night, begin to open at daylight in the morning. Heat tends 
 to open the stomata. This may have injurious or fatal re- 
 sults. If the leaves of an evergreen, for example of holly, 
 are warmed by air and sun while the roots are encased in soil 
 so cold and dry that the root-hairs absorb too little water, 
 the opening of the stomata is very likely to be followed by 
 excessive transpiration, by the drying out and death of the 
 plant. Indeed, most cases of " winter-killing" are to be at- 
 tributed to the inability of the plant to balance transpira- 
 tion by absorption, rather than to actual freezing to death. 
 The majority of plants close their stomata when their 
 leaves are wet. This can be demonstrated by putting in 
 water strips of epidermis from the leaves. The advantage 
 is more obvious than the means by which it is attained. If 
 the stomata are the entrances of the passages through 
 which the necessary exchange of gases and vapor takes 
 place, these entrances must be kept free from whatever 
 would hinder the exchange when the stomata are open. 
 
ABSORPTION AND MOVEMENT OF WATER 151 
 
 Kain and dew collecting over the stomata or passing into 
 the intercellular spaces would prevent the diffusion of gases 
 except as they are dissolved hi the water. This disadvant- 
 age is avoided in the first place by placing the stomata ordi- 
 narily on the lower side of the leaf, the one least likely to 
 be wet by rain. It is further avoided by the fact that when 
 the leaf is wet, transpiration virtually ceases while absorp- 
 tion may continue for a time, thus producing such a degree 
 of turgor in the guard and other epidermal cells that their 
 expansion closes the stomata. 
 
 Certain plants living in extremely moist places, where the 
 danger of excessive transpiration at any season is reduced 
 to the minimum, are unable to close their stomata. Herbs, 
 shrubs, and even trees (notably the willows) possess this 
 character. It is for this reason that these plants cannot 
 bear transplanting to dryer places for cultivation, but are 
 distinctly swamp plants. 
 
 GASES AND THE MOVEMENT OF GASES 
 
 If the plant or any part of it dries that is, loses water 
 faster than it absorbs it air takes the place of the evapo- 
 rated water. If the cell-walls and intercellular spaces are 
 equally permeable to gases and to water-vapor, air will pass 
 in as rapidly as evaporation removes the water, but if 
 water-vapor makes its way through cellulose, cuticula and 
 cork faster than air, drying will tend to produce in the 
 plant a gas pressure lower than that of the surrounding air. 
 If again the component gases of the air diffuse and diosmose 
 at different rates, the evaporating water will be replaced by 
 nitrogen, oxygen, and carbon-dioxide in proportions "different 
 from those hi which they occur hi the air. But if the living 
 cells of the plant use any of these gases, the composition of 
 the air hi the plant will be influenced by this means also. 
 In all living cells of higher plants under normal conditions 
 oxygen is consumed in respiration and carbon-dioxide is 
 liberated, usually hi equal volumes. In all chlorophyll-con- 
 taining cells carbon-dioxide is consumed and oxygen lib- 
 erated, under the influence of light. Carbon-dioxide diffuses 
 and diosmoses more rapidly than oxygen, and the latter 
 
152 PLANT PHYSIOLOGY 
 
 more rapidly than nitrogen, but these gases make their way 
 less rapidly through drying cellulose and lignified mem- 
 branes, though more rapidly through cuticula and cork, 
 than does water-vapor. We have seen that the wood, which 
 consists mainly of the walls of dead cells, is the path of the 
 water-currents from root to leaves, that only rarely if ever 
 are the wood elements filled with water, and that generally 
 they contain air and water in alternating columns ( Jamin's 
 chains ) . Whenever transpiration is more rapid than water- 
 transfer, the air-pressure within the plant will decrease ; there 
 will be the largest volume of air under the least pressure 
 when transpiration has most reduced the amount of water, 
 and when the vessels are fullest of water there will be the 
 least volume of air under a pressure equal or nearly equal to 
 that of the atmosphere. The constant changes in the rate 
 of transpiration, causing differences in the water content of 
 every cell, living and dead, and in the amount of water-vapor 
 in the intercellular spaces, will cause constant changes in the 
 volume and pressure of the gases within the plant. 
 
 More strictly vital activities also affect the gas pressures. 
 As may most conveniently be demonstrated on submersed 
 aquatics, photosynthesis tends, other things being equal, to 
 produce a gas pressure within the plant greater than that 
 outside. Because the stomata of a land plant are usually 
 open while the plant is manufacturing food, the free ex- 
 change of gases between the plant and the outside air keeps 
 the pressures about equal. But in submersed aquatics e. g. 
 Elodea, Ceratophyllum, Myriophyllum, etc. the gas press- 
 ure may be made evident in two ways. In the first place, 
 the buoyancy of the plant increases while it is illuminated, 
 indicating the accumulation of gas in its body ; and second, 
 the classical experiment of inverting and cutting or prick- 
 ing the stem* shows that a stream of bubbles, often 
 countable and therefore offering an index of the photosyn- 
 thetic activity, issues from the cut surface or from the 
 wound. The explanation of this phenomenon of increased 
 pressure is this : the carbon-dioxide used in the elaboration 
 
 * For description see Darwin and Acton's or Detmer-Moor's laboratory 
 manuals. 
 
ABSORPTION AND MOVEMENT OF WATER 153 
 
 of sugar passes by osmosis more rapidly into the body and 
 into the cells of the plant than the liberated oxygen passes 
 out. The latter, therefore, accumulates in the large and well 
 developed intercellular spaces and passages, escaping only 
 slowly from these by osmosis, escaping rapidly only when 
 the plant is wounded. 
 
 The reverse of this result is attained during the night, 
 when the plant is photosynthetically inactive but is steadily 
 respiring, taking in oxygen as fast as it needs it, and giving 
 out the more rapidly diosmosing and diffusing carbon-di- 
 oxide. But since respiration is never so active as photo- 
 synthesis, the negative pressure is never so high as the 
 positive. 
 
 Since plants are subjected to inconstant but frequent 
 movement by the winds, by passing animals, by the rise and 
 fall of the tide, by waves, etc., the gases contained in their 
 bodies are subjected to varying pressures, are forced out 
 and drawn in, are moved from part to part. These me- 
 chanical influences brought to bear upon plants play a very 
 important role in contributing to the movements of enclosed 
 gases and vapors. Besides these, temperature-changes and, 
 as we have already noted in connection with transpiration 
 (p. 139), the temperature of the plant-body as compared 
 with the temperature of the air outside, also affect the 
 movements of gases in the plant. The effects of these in- 
 fluences, however, are mainly upon the gases enclosed hi the 
 intercellular spaces while the other influences which we have 
 considered affect more directly the gases within the cells. 
 But all of these influences contribute to the perfect aeration 
 and ventilation of the plant-body just as the elaborate 
 musculature of the higher animal automatically maintains 
 the movements of the gases needed by the cells of its body. 
 In both animals and plants, osmosis and diffusion underlie 
 all gas movements in the body, but both are controlled by 
 the vital activities, that is, by the amounts of the gases 
 consumed and liberated by the living cells. 
 
 From the foregoing it is evident that, in most plants, and 
 under ordinary conditions, the composition of the enclosed 
 gases can differ only slightly and temporarily from that of 
 
154 PLANT PHYSIOLOGY 
 
 the surrounding air. In certain species, gas of decidedly 
 and permanently different composition from that of the sur- 
 rounding air accumulates in chambers of considerable size. 
 As reported in a preliminary paper by Wille, * the bladders 
 of the Fucaceze contain absolutely no carbon-dioxide and a 
 varying percentage of oxygen, thus 
 
 Bladders wholly immersed in water contain 35-37% oxygen 
 lying for 10 hours in air " 20% " 
 
 darkened for 12 hours " 2.7% (( 
 
 These figures, in connection with what has just been said 
 about the different diffusibilities of oxygen and carbon- 
 dioxide, are significant. The buoyancy of the plant depends 
 upon its photosynthetic activity and upon the consequent 
 accumulation of oxygen which is collected in specially differ- 
 entiated reservoirs. Nitrogen necessarily makes up the 
 greater part of the total volume of gas, but this inert gas 
 varies in proportional amount only because of the produc- 
 tion of oxygen in photosynthesis and of the consumption of 
 oxygen in respiration during the hours of darkness. So in 
 all plants the proportion of oxygen in the intercellular 
 spaces decreases in darkness, and increases, especially in the 
 intercellular spaces of photosynthetically active tissues, in 
 the light. The impermeability of the cutinized membranes 
 of the epidermal cells of the Fucacese permits the develop- 
 ment of high gas-pressure by means of the abundant pro- 
 duction and tardy diffusion of oxygen. Consequently, in 
 spite of their large intercellular spaces, these plants do not 
 collapse even in very deep water, f Their buoyancy is thus 
 maintained largely by gas-pressure, and though the form of 
 the cells is maintained by their own turgescence, the form of 
 
 * Wille, N. Abstract, in Just's Jahresbericht der Botanik, vol. XVII.. 
 p. 226, 1889, of a " vorlaufige Mittheilung," in Norwegian. Also Gasarten 
 in den Blasen der Fucaceen. Chem. Centralbl., 1890. 
 
 f Berthold, G. Uber die Vertheilung der Arten im Golf von Neapel. 
 Mittheil. a. d. Zoolog. Station in Neapel, Bd. III., 1882. In this connec- 
 tion it should be mentioned that all seaweeds living between the tide- 
 marks are subjected twice daily to very different pressures. Where the 
 range of the tide is great, as in the Bay of Fundy on the north Atlantic 
 coast, the algae living near the low-tide mark must adapt themselves to 
 the great differences in pressure. 
 
ABSORPTION AND MOVEMENT OF WATER 155 
 
 the whole plant is due to the high gas-pressure in the inter- 
 cellular spaces as well as to the turgescence and form of its 
 component cells. How much of the size, and to a certain 
 extent of the form also, of plants depends upon maintaining 
 the air-spaces in expanded condition can only be roughly 
 guessed from these figures : * 
 
 | to of the volume of the leaves of most land plants 
 is air-space. 
 
 71% of the volume of Pistia texensis (a floating plant) is 
 air-space. 
 
 3.5% of the volume of Begonia hydrocotylifolia (succulent) 
 is air-space. 
 
 53% of the volume of the leaf of Polypodium setigerum} 
 is air-space. 
 
 TRANSLOCATION OF FOODS 
 
 There remains for us to consider in this chapter the trans- 
 location of foods. Through the xylem elements, especially 
 through the ducts and tracheids, aqueous solutions of cer- 
 tain food-materials are transferred from the absorbing root- 
 hairs to the elaborating chlorophyll-containing parenchyma- 
 cells of the leaves. To these same chlorophyll-containing 
 cells the other food-material, carbon-dioxide, makes its way 
 through the stomata and the intercellular spaces. In these 
 cells water and carbon-dioxide are consumed in elaborating 
 a carbohydrate, a food which accumulates hi the same or in 
 chemically closely related form in the cells which manufac- 
 ture it. From these cells the food must be removed, unless 
 it is also to be stored there, to parts needing it at once or 
 , in which it can be kept in reserve for future use. 
 
 In order to secure the transfer of the non-nitrogenous 
 food manufactured hi the green tissues during the hours of 
 daylight, it is usually necessary to change the chemical con- 
 stitution or composition of the food. If starch or oil is 
 the form in which the carbohydrate elaborated by the 
 
 * Pfeffer W. Pflanzenphysiologie. I., p. 164. Engl. transl., I., p. 182. 
 Various papers there referred to. in wMch other data may be found. 
 
 I Estimated from Stahl's figure in Botan. Zeitung. plate IV., fig. 7, 1894. 
 This is a land plant from one of the rainiest regions in the tropics. 
 
156 PLANT PHYSIOLOGY 
 
 chlorophyll-granule is temporarily deposited in the cell, the 
 conversion of these into portable compounds is advan- 
 tageous. Although oil will pass through cell-wall and 
 living protoplasm, the same amount of nutritious material 
 will pass through the same distance much more rapidly 
 as sugar dissolved in water. Starch, being insoluble, 
 is not only innutritious as such, but is not portable, and 
 hence it must be converted into a soluble substance, also 
 sugar. What is true of the non-nitrogenous foods elabo- 
 rated in those organs receiving the special form of energy 
 needed for their manufacture, is also true of the nitrogenous 
 foods, elaborated probably in all the living cells of the 
 plant, whether illuminated or not. The nitrogenous foods, 
 if temporarily deposited in insoluble form in the cells elabo- 
 rating them, will be much more portable if converted into 
 soluble forms. As we have already seen, the non-nitro- 
 genous foods are transferred mainly as sugars, the nitro- 
 genous mainly as amides. 
 
 We have seen (p. 116) that the transfer of water and 
 dissolved mineral salts from the absorbing organs to those 
 parts in which they are used as food or from which water 
 is evaporated, would be too slow to secure an adequate 
 supply if the movement were wholly osmotic. In order to 
 satisfy the needs of cells consuming food, and to free the 
 manufacturing cells from elaborated food so that they may 
 continue to make it, the transfer of foods must also be by 
 more rapid means than by osmosis merely. In small and 
 simple plants the manufacture, consumption, and storage of 
 food may go on in the same cells simultaneously. In larger 
 plants, division of labor and differentiation of tissues secure 
 greater efficiency and economy. In these plants there dif- 
 ferentiates a system for distributing elaborated foods from 
 the places of manufacture. The earliest and simplest con- 
 ducting system is for this purpose. We find such a system 
 well developed in the large marine algae (Fucus, Laminarix, 
 Nereocystis, etc. ) . When plants come upon the land it be- 
 comes hard to obtain water and easy to lose it. When plants, 
 by becoming erect, limit their water-absorbing part to one 
 end and place their food-manufacturing cells at the oppo- 
 
ABSORPTION AND MOVEMENT OF WATER 157 
 
 site end of the body, they must^ supply themselves with a tis- 
 sue system for the rapid conduction of water and salts from 
 roots to leaves. The wood, which carries mainly food mate- 
 rials, is so much more conspicuous than the bast, which car- 
 ries foods, that the wood seems the more important and the 
 earlier needed of the two. This may be true of land-plants, 
 but it is not true of aquatics. Phylogenetically the food- 
 conducting system is the older; in aquatics it is the main 
 or only tissue for the rapid transfer of aqueous solutions. 
 
 The anatomical distinctions between wood and bast 
 are so evident that it is easy to infer that there is per- 
 fect division of labor between these two sets of tissues. 
 But the living cells of the bast need mineral salts, perhaps 
 to maintain turgor (p. 99), to neutralize injurious by- 
 products (p. 100), to assist in the construction of proto- 
 plasm (p. 101) and will receive, transfer and use them 
 just as the living cells of the wood need sugar and amides 
 as food and will receive, transfer, and consume them. 
 
 From illuminated chlorophyll-containing cells the carbo- 
 hydrates not laid down in solid form like starch or in slowly 
 transferable form like oil, will pass by exosmosis to adjacent 
 cells containing less. Such osmotic transfer will continue 
 while the chloroplastids are manufacturing diffusible food 
 and until osmotic equilibrium has been attained. Osmotic 
 transfer will begin so soon as the carbohydrate deposited as 
 starch or oil is converted into diffusible form, and it will con- 
 tinue so long as starch and oil are converted into sugar. 
 From cell to cell in the leaf parenchyma, and from this into 
 the cortical parenchyma of branch and stem, osmotic transfer 
 will take place ; but it will take place always most rapidly in 
 the direction of least osmotic pressure. This will obviously be 
 toward and into the sieve-tubes of the vascular bundles. The 
 sieve-tubes composed of comparatively large, long cells with 
 thin lateral walls and perforated cross-walls are continu- 
 ous for considerable distances, often forming, by means of 
 anastamoses, a system uninterrupted from tip to base of the 
 plant. In the sieve-tubes, as in the ducts and tracheids, 
 there will tend to be less pressure than in the adjacent cells. 
 In the ducts and tracheids the danger of collapse is avoided 
 
158 PLANT PHYSIOLOGY 
 
 by the thickened and strengthened walls. In the sieve-tubes 
 permanent collapse always occurs sooner or later ( as may 
 be seen at any time in the older parts of the phloem in 
 perennials or late in the season in annuals ) . It may be pro- 
 duced at any time by cutting off the part leaf, branch, or 
 stem whereupon examination will reveal the sieve-tubes 
 and their contents in the abnormal condition figured in even 
 the most recent text-books. Collapse of the sieve-tubes un- 
 doubtedly occurs more or less completely, though only tem- 
 porarily, in the healthy plant whenever the leaf or branch or 
 stem is unduly bent and whenever the removal of food from 
 the sieve-tubes by the adjacent cells for use or for storage is 
 more rapid than the supply of food from the places of man- 
 ufacture. The pressure in the sieve-tubes will vary, just as 
 pressure varies in any cell, according to the prevailing condi- 
 tions; but because the sieve-tubes are continuous and are 
 in contact with cells which consume or store as well as with 
 cells which manufacture food, the osmotic and other pressures 
 in them are likely to be lower than in the food-manufacturing 
 cells. Because they are composed of long cells, the cavities of 
 which are continuous with one another through the pores in 
 the cross-walls, the sieve-tubes are especially adapted to the 
 translocation of the osmotically less portable nitrogenous 
 foods, especially the proteids. It would appear from the 
 investigations of Czapek* that it is especially in them, not 
 even in the companion and cambiform cells of the phloem, 
 that the diffusible carbohydrates also are transferred. It 
 does not necessarily follow, from the discovery of minute 
 starch-grains in the sieve-tubes, that they pass as such 
 through the sieve-like cross- walls. On the contrary, starch 
 occurring in the sieve-tubes must be regarded as carbohy- 
 drate merely temporarily deposited there after being con- 
 verted from portable to stationary form. These starch- 
 grains in the sieve-tubes, like the great majority if not all 
 of the solid particles in other cells, are too large to pass 
 through pores in the cross-walls. \ 
 
 * Czapek, F. Zur Physiologic des Leptoms der Angiospermen. Ber. d. 
 Deutsch. Bot. Gesellsch.. Bd. XV. 1897. 
 f Sachs, J. von. Lectures on the Physiology of Plants. Engl. trans. , p. 325. 
 
ABSORPTION AND MOVEMENT OF WATER 159 
 
 The presence of starch-grains in the sieve-tubes has given 
 occasion to the hypothesis that either the sieve-tubes them- 
 selves, or their companion-cells, are the chief elaborators of 
 carbohydrates into nitrogenous matters.* Such is, how- 
 ever, hardly likely to be the fact (p. 69 ). Although very 
 likely the sieve-tubes do form nitrogenous food from nitrates 
 and carbohydrates, this function seems not yet to have 
 been appropriated by any special tissue. Apart from the 
 classical experiments of ringing or girdling plants, with the 
 result that the downward transfer of food is nearly or quite 
 stopped, it must be admitted that exact experiments and 
 definite knowledge on the special functions of the sieve-tubes 
 remain for the future, t The recent discovery by Raciborski J 
 of a substance resembling the haemoglobin of higher ani- 
 mals in that it readily gives up oxygen to the sieve-cells 
 and lactiferous tubes, in which it regularly occurs, suggests 
 that besides conducting and perhaps contributing to the 
 elaboration of foods, these tissues may also obtain needed 
 oxygen from the substances which they conduct. But as 
 Raciborski says in his second paper, this discovery made in 
 the tropics and away from laboratories must be tested and 
 made the starting-point for research by plant-physiologists 
 with laboratory facilities. 
 
 The milk-tubes or lactiferous ( laticiferous ) tubes or ves- 
 sels, occurring in a very considerable number of plants and 
 forming continuous systems often as extensive as the sieve- 
 tubes, are filled with a mixture of the most diverse com- 
 pounds dissolved or suspended in water. Some at least of 
 these substances are the often very useful by-products 
 formed in nutrition, respiration, or in other vital processes. 
 On the other hand, some being subjected to more or less 
 profound chemical changes, serve as sources of energy in 
 respiration, as materials for the construction of cell- wall, 
 
 * Sachs. J. von. Lectures on the Physiology of Plants. Engl. transl.. 
 p. 325. 
 
 f Trelease (Sixth Annual Report. Missouri Bot. Garden. 1894) finds no 
 sieve-tubes at all in Leitnerin floriflana. 
 
 i Raciborski. M. Ein Inhaltskorper des Leptoms. Ber. d. Deutsch. Bot. 
 Gesellsch.. Bd. XVI.. 1898. Weitere Mittheilungen iiber das Leptomin. ibid. 
 
160 PLANT PHYSIOLOGY 
 
 and as building-material for protoplasm. Because latex 
 the contents of the milk-tubes is composed in part of such 
 nutritious substances as starch, sugars, fats and oils, and 
 proteids, and because the milk-tubes form a system wholly 
 uninterrupted throughout its extent by cross-walls of any 
 kind, the suspicion is irresistible that these tubes offer the 
 easiest course for the transfer of food from part to part. 
 They occur in plants relatively few r in number and conse- 
 quently cannot be indispensable to food transfer. Their 
 functions too need further investigation. 
 
 The opposite process to that which goes on in the chloro- 
 phyll-containing cells of the leaves, whereby the elaborated 
 carbohydrate temporarily deposited as starch is dissolved 
 for transport elsewhere, takes place in the organs where 
 carbohydrates are stored in solid form. In roots, rhizomes, 
 and tubers, in pith and medullary rays, and in seeds, 
 parenchyma cells remove the sugar from the solution in 
 which it comes to them by depositing it as starch grains in 
 the protoplasm or as cellulose lining their walls. How the 
 carbohydrates are acted upon by the protoplasm, or by its 
 special organs the leucoplastids, how sugars are converted 
 into cellulose or into starch and by this means are removed 
 from solution, are still unanswered questions. It is easy to 
 see that if sugar is removed from the cell-sap more will go 
 by osmosis to the cells removing it from solution. This 
 last is a necessary physical consequence of the physiological 
 (that is, in this case, of the combined chemical and physical) 
 action of the living protoplasm. But this storing action of 
 the protoplasm is as little understood as the first secretion 
 of sugar in nectaries (p. 126). 
 
 Still less comprehensible at the present time are the ac- 
 cumulations in limited regions of the plant of substances 
 which ordinarily would move osmotically in one direction as 
 readily as in another. The storage of carbohydrate in the 
 iorm of inulin, dissolved in the cell-sap of dahlia-tubers and 
 in the underground parts of other Composite, is not to be 
 explained by the ordinary laws of physics. The living pro- 
 toplasm which, in one part of the plant, elaborates food 
 and permits its exosmosis, accumulates food and prevents 
 
ABSORPTION AND MOVEMENT OF WATER 161 
 
 its exosmosis in another part. Similarly in the filamentous 
 alga, the accumulation of sugar in its cells bounded on all 
 sides by membranes permeable by water is conceivable only 
 on the hypothesis, strengthened by analogy ( p. 107 ) , that 
 the living protoplasm in some way interferes, either physi- 
 cally or chemically, with the exosmosis of the sugar. 
 
 To summarize the results of the discussions in this chapter 
 we may say that diffusion and osmosis underlie the pro- 
 cesses of absorption and transfer of food-materials and of 
 foods, but that the movements of these gases and solutions, 
 and of the separate substances in the solutions, are con- 
 trolled in direction, rate, and amount by the living proto- 
 plasm. Its needs, and the amount and kind of work it 
 does, change or establish physical conditions and set in 
 operation physical and chemical laws. 
 

 CHAPTER V 
 GROWTH 
 
 The processes so far discussed supply the plant with the 
 materials and with the energy to carry on other processes 
 popularly regarded as functions peculiar to living beings, 
 and at all events performed with a greater degree of inde- 
 pendence and self-control by them than by lifeless objects. 
 In this respect, however, these processes do not differ from 
 those already examined, for we have seen that respiration 
 is oxidation controlled and, to a certain extent, carried on 
 by the living organism ; that nutrition depends upon chemi- 
 cal syntheses accomplished by living protoplasm, which uses 
 simple substances obtained by itself, through its application 
 of the laws of diffusion and osmosis; that the absorption 
 and transfer of food-materials and of foods are physical 
 processes made possible and regulated by the living organ- 
 ism. The processes which we have still to study growth, 
 irritability, and reproduction involve and consist in chemi- 
 cal changes not confined to living organisms but controlled 
 by them. 
 
 Growth cannot be understood unless the sensibility of the 
 growing substance is constantly borne in mind. A crystal 
 of copper sulphate in a concentrated and slowly evaporating 
 solution of this salt could not grow if the molecules of cop- 
 per sulphate were not mutually attractive, if those which 
 were still moving freely in the solution did not respond to 
 the attraction exerted upon them by those already settled. 
 The limits of form and size, of rate and direction of growth, 
 of copper sulphate crystals, are fixed by physical laws, but 
 though these laws are universal, the balance in any one 
 spot may be very different from that elsewhere, and the 
 
GROWTH 163 
 
 growing crystal will conform and must conform to its en- 
 vironment, It is determined by physical law that copper 
 sulphate molecules will not arrange themselves in the crys- 
 talline form without a definite amount of water; if the 
 amount of water be excessive or deficient the copper sulphate 
 molecules, having always the same attraction for one an- 
 other, will still be unable to obey it, to approach one an- 
 other, and to arrange themselves in their due order. The 
 crystals of the same substance will be small or large accord- 
 ing as they are formed fast or slowly. And what is true of 
 the behavior of one substance alone in solution in another 
 is also true of many substances together in solution, 
 whether these find themselves in a cell or wholly outside a 
 living body. The molecules of the different substances will 
 mutually attract or repel one another, they will be indiffer- 
 ent or they will decompose one another, and when a state 
 of balance is attained, the molecules which result from all 
 the changes w r ill arrange themselves in their characteristic 
 ways. Because the living protoplasm itself is composed of 
 molecules forming a definite structure, this structure, like 
 the whole crystal, is subject to physical influences, and its 
 component molecules are obedient to physical laws. So the 
 molecules and groups of molecules forming the living proto- 
 plasmic structure are pulled down by gravitation. The 
 molecules vibrate with lesser amplitude and draw together 
 in cold, vibrating with greater amplitude and so tending to 
 move apart when warmed. And as warmth makes molecu- 
 lar movements in any substance freer, so water makes pos- 
 sible still greater freedom of molecular movement in those 
 substances which dissolve in it. There are substances, how- 
 ever, which neither repel water as do the fats and oils, nor go 
 into solution in it as do many salts, but which still absorb 
 water in great quantity. This absorption results in greater 
 freedom of molecular and even of massive movement. Pro- 
 toplasm is one of these substances; it swells as it absorbs 
 water, and its circulation, as well as its molecular move- 
 ments, becomes more rapid, up to the optimum. If still 
 more water is forced into it, the molecules and groups of 
 molecules composing it will be forced so far apart by the 
 
164 PLANT PHYSIOLOGY 
 
 molecules of water enclosed, that the definite protoplasmic 
 ; structure will be strained or destroyed, and the massive 
 movements will accordingly change or cease. 
 
 Without going further into the subject now (see the next 
 chapter), it may be stated that the growth of the living- 
 organism, like that of the crystal, is in accordance with the 
 sensibility of its component molecules and groups of mole- 
 cules to physical and chemical influences. The difference 
 between the lifeless crystal arid the living organism can be 
 suggested not definitely stated, however in this way : the 
 molecules of the crystal, and the crystal as a whole, are 
 entirely subject to the prevailing balance of physical forces ; 
 the living organism, on the contrary, is able to modifj^, 
 change, or maintain the balance of physical forces. When it 
 ceases to be able to do this it dies, it becomes like the 
 crystal. This power peculiar to living organisms, whatever 
 it may be dependent upon, does not necessarily imply any 
 greater sensitiveness to physical forces than would be repre- 
 sentable by the sum of the sensibilities of all the substances 
 composing and contained within the living organism (p. 
 185). But the organism is sensitive, and its growth cor- 
 responds to the forces or influences to which the organism 
 is subjected quite as much as to the matter with which it 
 is supplied. 
 
 What is growth? Of this no adequate definition can be 
 given, although for each mind the word possesses a' certain 
 significance. Physiologists have always attempted to state 
 in what growth consists, and have always failed. Our con- 
 ception of growth should be clarified at once by distinguish- 
 ing this phenomenon from the others ordinarily accompany- 
 ing it. Growth is a part of the process of development. 
 Growth and differentiation together accomplish develop- 
 ment. Differentiation is that specialization in structure 
 which follows and contributes to specialization in function. 
 Differentiation is limited by the number of parts and of 
 cells, in other words, by the size of the body. In a small 
 body little differentiation is possible, in* a large one much. 
 But it does not follow from this" that the largest bodies are 
 the most highly differentiated in structure and function; 
 
GROWTH 165 
 
 they are not yet, nor will they ever be unless differentia- 
 tion keeps pace with growth. Does growth then consist 
 in increase in size? A cell may elongate, but this will 
 not necessarily be growth. We can easily imagine a cell 
 being subjected to a pull which would elongate it, or to 
 a pressure which would extend as well as compress it; or, 
 indeed, within the cell itself a pressure resisted less in one 
 direction than in others would cause the cell to elongate. 
 In the last case there is stretching because of the pressure 
 developed within the cell by its osmotically active contents. 
 In the two preceding cases, the cell changes its dimensions 
 because it is subjected to a force from outside itself which 
 it cannot resist. In none of these cases has growth taken 
 place. If, however, these changes in dimensions are made 
 permanent by work done by the living cell itself in translo- 
 cating and depositing insoluble material in such positions 
 as to fix the new form or the new dimensions, growth will 
 have taken place. Growth, the fixing of the new form or 
 new dimensions, follows the change accomplished from with- 
 out. But permanent change of form or of dimensions may 
 be accomplished by forces wholly outside the plant,; for 
 example, by stretching the cell beyond the limit of elasticity 
 of its walls. This would not constitute growth. 
 
 Growth, then, does not necessarily consist in an increase 
 in volume, for there are evidently cases of unquestioned 
 growth without this. The boy increasing in stature and the 
 vine increasing in length, decreasing in breadth or thickness 
 meanwhile, are growing though not increasing in volume. 
 There is necessarily increase in volume of parts or organs, 
 but not of the whole organism, in such cases of growth. 
 On the other hand a rise in turgor which increases the 
 volume of a part is not growth ; this is merely expansion. 
 Growth is a process dependent upon the formation of new 
 protoplasm, and though it usually results in increase in 
 volume, in increase in weight or mass, and in increase in 
 substance, it is not essentially any of these. 
 
 Growth is made possible by cell-division, but it does not 
 consist in the formation of new cells, for new cells can be 
 formed by the mere division of old cells. Each cell, each 
 
166 PLANT PHYSIOLOGY 
 
 kind of cell, and hence each organism, has a maximum 
 which its size normally never exceeds. Cells which have at- 
 tained their maximum size can continue to contribute to 
 the growth of an organ only after doing what will make 
 possible the formation of new protoplasm. Cell-division, a 
 process of rejuvenation, accomplishes this. The smaller 
 daughter cells, each with its own nucleus instead of with 
 only a share of the single nucleus of the undivided cell, are 
 more vigorous than the mother-cell at maturity ; they form 
 new protoplasm as well as other products from the food 
 furnished them ; and they then increase in size. In a meris- 
 tematic tissue, such as that at the tip of the stem or root 
 or in the cambium, we have the first and the fundamental 
 stage in the process of growth, namely, the formation of new 
 protoplasm and of new cells. Behind the tip of stem and 
 root, and on either side of the cambium, we have the later, 
 the evident stage, when increase in size or volume, in mass 
 and in substance, takes place. 
 
 Evident growth takes place when the body or any part of 
 it permanently increases in volume or in size. Increase in 
 substance, which results in increase in weight, may take 
 place after all growth has ceased. It may be merely the 
 storing up of food elaborated at one time to be used later, 
 or it may be the absorption of water. This last, however, 
 invariably results in the increase in volume of the parts, 
 or in the increase in turgor, or both at once. Any ab- 
 sorption of water by dry animal and vegetable substances 
 is followed immediately by swelling a phenomenon from 
 which hypotheses regarding the minute structure of organ- 
 ized bodies have been more or less successfully deduced. 
 Beyond the point at which swelling by the intussusception 
 of molecules of water between the molecules or groups of 
 molecules of the formed substances would cease, increase in 
 volume may still go, by reason of the structure and compo- 
 sition of cells. The presence of osmotically active substances 
 in cells which can absorb water ensures increase in pressure 
 within the cells, and this pressure will distend the enclosing- 
 protoplasmic and cellulose or other walls. Up to a certain 
 point, easily conceived but not easily determined, the in- 
 
GROWTH 167 
 
 crease in volume due to the absorption of water is genuine 
 growth, and the second stage in growth, the one which I 
 have designated as evident growth, consists mainly if not 
 wholly in the absorption of water. But as the water-con- 
 tent of any cell or tissue or organ is subject to fluctua- 
 tion, and as the turgor and, consequently, the volume 
 also fluctuate with the water-content, it is difficult to tell 
 when growth ceases and turgor-swelling begins. Both are 
 vital processes in the sense that they depend upon the 
 physical and chemical conditions established by the living 
 protoplasm, but they are distinct processes, fluctuations in 
 size due to changes in turgor taking place even long after 
 true growth has ceased to be possible (see pp. 167, 168). 
 Whether growth depends upon turgor or vice versa is still 
 to be conclusively shown by experiment. 
 
 That evident growth consists mainly in the increase of the 
 water-content of the growing part has long been known to 
 be the case in plants, though only recently properly em- 
 phasized for animals. * A longitudinal section through the 
 tip of a growing stem or root, or a cross-section through 
 the stem of a dicotyledonous plant during its period of 
 growth in diameter, w r ill show at least three distinguishable 
 regions. These are indicated in the accompanying figures ( pp. 
 168, 169 ) of a longitudinally sectioned root of Azolla. In the 
 diagramatic Figure .7 the three regions of cell-formation ( 1 ) , 
 cell-growth (2), and cell-differentiation (3), are indicated. 
 These are shown in detail in the figures 8, 9, 10. Figure 8 
 (corresponding to 1 in 7) represents the tip of the root 
 with its cap (Cap), dermatogen and epidermis (Ep.), 
 cortex (Cor. ), and central cylinder (c. c. ), aU of which 
 come directly or indirectly from the division of the large 
 apical cell. The meristematic and embryonic cells are full of 
 dense protoplasm. Figure 9, taken from further up in the 
 same root, corresponds to region 2 in figure 7, and shows 
 that the increase in size of the cells of the different layers is 
 accompanied by a great increase in the volume of the cell- 
 
 * Davenport, C. B. The role of water in growth. Proc. Boston Soc. 
 Nat. History, vol. 28, 1897. Experimental Morphology. Part II.. 1899. 
 and the literature there cited. 
 
168 
 
 PLANT PHYSIOLOGY 
 
 sap, which accumulates in large vacuoles, without there being 
 any considerable increase in the amount of protoplasm in 
 each cell. Figure 10 corresponds to region 3 of figure 7, and 
 shows how the cells change in taking on their definitive 
 characters, Ves. indicating some of the changes taking place 
 during the formation of a vessel in the central vascular 
 bundle. In this region the amount of protoplasm decreases 
 
 Ep. Cor. Ves. 
 
 FIG. 7. FIG. 9. FIG. 10. 
 
 Figures 710. Longitudinal sections of Azolla root. Fig. 7 diagram- 
 atic showing region of cell-formation (1). cell-growth (2). cell-differenti- 
 ation (3). Fig. 9. = 2 in Fig. 7 more highly magnified. Fig. 10. = 3 in 
 Fig. 7 more highly magnified. 
 
 not only proportionally but absolutely. There can be little 
 more if any permanent increase in volume in this region, 
 although there may be increase as well as decrease in vol- 
 ume because of differences in turgor solely. 
 
 The factors contributing to make growth possible may be 
 grouped under three heads : 1st, there must be an adequate 
 supply of material ; 2d, there must be an adequate amount 
 of room ; 3d, there must be the impulse. Physiologists are 
 not able to reduce to definite physical and chemical terms 
 what is comprehended under this last head, and it has still 
 
GROWTH 
 
 169 
 
 to be ascertained whether the necessary impulse comes from 
 within or from without, whether it is inherited or is new. 
 But without the impulse there will be no growth. 
 
 Growth will not be possible without the needed materials 
 and space. The substances essential for growth are those 
 essential for life, but they may be grouped into two cate- 
 gories nutritious substances, and otherwise useful sub- 
 stances. The nutritious substances furnish the materials of 
 which the protoplasmic structure and the cell- wall are built, 
 and those compounds which in respiration yield the energy 
 needed by the part to complete the first stage of growth. 
 
 If the supply of food is constantly sufficient during the 
 period of growth, both construction and enlargement 
 
 c.c 
 
 Cor 
 
 FIG. 8. 
 
 Figure S. Tip of root of Azolki showing apical cell and region of cell- 
 formation =1 in Fig. 7 more highly magnified. 
 
 will be uniform, other things being equal; but if the sup- 
 ply varies in amount, the rate of growth will vary cor- 
 respondingly. It is found, for example, that the growth of 
 green and independent plants is periodic in a much more 
 marked degree than that of plants which obtain their food 
 ready-made. During the day, while food is being made 
 and accumulated in the organs photosynthetically active, 
 
170 PLANT PHYSIOLOGY 
 
 growth is slower than during the night, when food is sup- 
 plied in abundance to the growing parts. * This periodicity 
 is far less marked in seedlings, with an abundant food- 
 supply in the cotyledons or in the endosperm, and in young 
 plants growing up from bulbs, tubers, and other parts in 
 which food is stored. The growth of parasites and sapro- 
 phytes may also vary periodically if they are subjected to 
 periodically varying conditions. Light affects growth quan- 
 titatively, as well as directing it in the ways to be described 
 in the next chapter ( p. 208 et seq. ) . If plants furnished with 
 a constant food-supply are subjected to otherwise constant 
 conditions, their growth rate will be constant for a time. 
 For reasons not wholly understood, but certainly including 
 other factors than food-supply, the growth-rate of any part 
 or organism will rise to a maximum and afterward fall 
 again. For each cell and for each individual there is what 
 has been rather pompously termed "the grand period of 
 growth." This means simply that from its formation by 
 the division of its mother-cell until the time when it ceases 
 to increase in volume, each cell passes through a period dur- 
 ing which it can grow, and during which its rate of growth 
 gradually rises from nothing and falls again to nothing. 
 After growth ceases, differentiation may still go on as a 
 separate process. The maximum growth-rate is not neces- 
 sarily coincident with the maximum food-supply or with the 
 maximum of any other tangible factor. 
 
 Sachs and other physiologists f have called attention to 
 the fact, without fully explaining it, that the growth-rate 
 
 * Sachs, J. von. Physiology of Plants. Oxford, 1887. Miss Gardner 
 (Trans, and Proceed., Bot. Soc. Pennsylvania. Vol. I, No. 2. 1901) 
 claims that the growth of roots is faster by day than by night. This re- 
 result is probably due to the favorable action of light on processes upon 
 which growth depends rather than upon growth itself. The question de- 
 serves critical investigation. 
 
 f Sachs, J. von. Uber den Einfluss der Lufttemperatur und des Tages- 
 lichts auf die stiindlichen und tfiglichen Anderungen des Langenwachs- 
 thums (Streckung) der Internodien. Arbeiten des bot. Institute Wiirzburg, 
 Bd. II., 1872. Gesammelte Abhandlungen , Bd. II. Lectures on the Physi- 
 ology of Plants, English transl., p. 552. Kraus, Gregor. Physiologisches 
 aus den Tropen, I. Annales du Jardin Botanique de Buitenzorg, vol. XII., 
 1895. 
 
GROWTH 171 
 
 does not steadily rise and fall to and from the maximum, 
 but that there are " discontinuous" (stossweise) variations 
 apparently quite independent of the environment of the or- 
 ganism. It may be suggested that our analysis of growth, 
 according to which it consists in two distinct stages the one 
 fundamental, in which new protoplasm is formed, the other 
 evident, in which the cells expand may suggest a partial 
 explanation. Without a sufficient number of new cells, and 
 without a sufficient amount of new protoplasm, no expan- 
 sion can take place. Unless the two processes keep pace 
 with each other, the mensurable one will necessarily be ir- 
 regular. 
 
 In this connection the fact already referred to ( pp.1 67, 168 ) , 
 that changes in volume may take place quite independently 
 of growth and because of turgor changes only, may be con- 
 sidered in somewhat more definite fashion. Kraus* pointed 
 out long ago, and has confirmed his observations made in 
 Europe by others in the tropics, that there are daily varia- 
 tions in the length and thickness of stems and branches, 
 leaves, buds, and fruits. "The diameter of a tree-trunk, for 
 instance, increases measurably till the early morning hours ; 
 it then decreases till nightfall, when it begins to increase 
 again." This is due to the variation in volume of the cor- 
 tical and other parenchyma cells caused by the difference in 
 the rate of transpiration at different hours of the day. Ab- 
 sorption by the root-hairs continuing at a rate much more 
 uniform than that of transpiration at night slightly 
 higher, by day slightly lower the turgor and the volume 
 of all living and sufficiently thin-walled cells will vary ac- 
 cordingly. This variation, wholly independent of all vital 
 functions, except those which govern the composition of 
 the cell-sap and the permeability of the protoplasm, con- 
 tinues in organs no longer growing, but may also, during 
 growth, contribute to the irregularities in the curve of 
 growth. 
 
 The otherwise useful substances referred to above (p. 169) 
 
 * Kraus, Gregor. /. c. II, find earlier in Die Wasservertheilung in der 
 Pflanze, 1881. nnd Die Gewebespannung des Stammes und ihre Folgen. 
 Botanische Zeitung, 1867. 
 
172 PLANT PHYSIOLOGY 
 
 are mainly water and certain salts not directly entering 
 into the construction of living protoplasm. Some of 
 the water forms an integral part of the protoplasmic 
 structure ( pp. 6-8 ) , but the greater part of it serves as the 
 vehicle of nutritious substances brought to the cell, and as 
 the solvent of all the soluble substances in the cell. As the 
 essential and invariable ingredient of cell-sap, it is the mate- 
 rial which maintains the second or evident stage of growth. 
 The volume of the cell depends upon the water and upon 
 the compounds dissolved in it. The composition of the cell- 
 sap is regulated by the living protoplasm which adds to 
 or takes from it soluble compounds of diverse sorts, 
 assimilable and excreted matters, such as the sugars and 
 organic acids respectively. Besides these, it is claimed that 
 the cell owes its turgescence to certain soil-constituents, 
 especially the salts of potassium. According to Copeland, * 
 the degree of turgescence in ordinary roots, stems, and 
 leaves is only slightly dependent upon food-manufacture, 
 and is mainly due to a substance or to substances which 
 cannot be used to keep the plant from starving. Copeland 
 concludes from his experiments on the effects of light and 
 darkness, heat and cold, that the rate of growth has much 
 more effect upon the turgor of the growing part than vice 
 versa. 
 
 The fundamental stage of growth, consisting in the for- 
 mation of new protoplasm, implies the intussusception or 
 interpolation of new particles between the older parts of the 
 structure, or the application or apposition of new particles 
 upon the older, or both of these processes. In either some 
 force must be exerted. If the cell is distended while the new 
 particles are being formed and placed, the introduction of 
 new particles between the older will be proportionally easier. 
 So the turgescence of the cell, tending to keep all parts 
 stretched, may contribute to such growth. But Copeland 
 had in mind evident growth, increase in volume, rather 
 than the formation of new protoplasm. The turgor of 
 the cell may be sufficient to stretch it, to increase its vol- 
 
 * Copeland, E. B. fiber den Einfluss von Licht und Temperatur auf 
 den Turgor. Inaug. Diss., Halle, 1896. 
 
GROWTH 173 
 
 ume, but this does not necessarily mean growth, as we 
 have already seen; and again, if the cell is stretched, if 
 its volume is increased, by other means, its turgor must 
 either keep pace with this increase in volume, or fall. If it 
 keeps pace, because of the composition of the cell-sap and 
 of the abundance of water to be absorbed, it will be im- 
 possible to determine whether evident growth is dependent 
 upon turgor and is regulated by it, or not. If the turgor 
 fall during the increase in volume, it must be shown that 
 this fall is due to no other cause. On this point decisive 
 experimental evidence is still wanting. 
 
 From experiments by True* on the different rates of 
 elongation in the roots of seedlings grown in water-culture 
 and suddenly transferred to culture media of higher or 
 lower density, it would appear "that growth and turgor- 
 pressure here stand in no directly proportional relation to 
 each other." Furthermore, Pfefferf has shown, in a case 
 where turgor would be at least equal!}' helpful, namely, 
 in the formation of cell-wall, that it is not necessary. 
 The question resolves itself then into this : is turgor- 
 pressure, so useful and so necessary in maintaining the 
 form of cells, tissues, organs, and organisms the force by 
 which increase in volume is attained, or only the means by 
 which increased volume is maintained? Apparently the 
 latter is more likely to be the case ; but if this is true, what 
 is the force by which the living protoplasm expands, and 
 by which it stretches its bounding walls? If turgor is not 
 the force by which visible growth is accomplished, then 
 the increase in the amount of water and in the volume of 
 cell-sap in the growing part is only the evidence, not the 
 intrinsic quality, of visible growth. After all, we are forced 
 to confess that the physiologist's knowledge of the forces by 
 which the living protoplasm works is very incomplete. 
 
 Room is needed. Without it growth cannot take place. 
 
 * True. R. H. On the influence of sudden changes of turgor and of 
 temperature on growth. Annals of Botany, vol. 9, 1895. 
 
 t Pfeffer. W. Druck und Arbeitsleistung durch wachsende Pflanzen. 
 Abhandlungen d. K. Sachs. Gesellsch. f. Wissensch., Bd. XX., Heft 3. p. 
 429. 1893. 
 
174 PLANT PHYSIOLOGY 
 
 Cell-division may take place even where there is not enough 
 room for growth, but it does not constitute an essential 
 part of the process of growth, though it usually precedes 
 growth and makes it possible. An organ consisting of two 
 hundred cells has not grown in any sense when these cells 
 by mere division have become four hundred. Under normal 
 conditions, however, growth will follow, increase in volume 
 and in amount of protoplasm taking place when there is 
 enough room.* 
 
 Growing parts exert, or may exert, great mechanical 
 force. Illustrations of the truth of this assertion may be 
 observed almost daily. \ Until recently, however, the at- 
 tempts to determine the amount of force which a growing 
 part can exert have yielded only inadequate results. It is 
 necessary to employ such apparatus that all the force de- 
 veloped by the growing plant will be exerted directly upon 
 the recording instrument. The best instrument so far de- 
 vised is Pfeffer's.]; 
 
 Pfeffer's description of his apparatus will explain the ac- 
 companying illustration, reduced from the original figure. 
 "The spring is supported on an iron bar (d) 14 mm. thick 
 which is rigidly attached by means of double screw clamps 
 (e) to the upright posts (ss) of the stand. The measuring 
 spring (/) can be changed, for the plate (7) which carries it 
 is fastened by the screws (kk) to the solid brass plate (#). 
 By raising or lowering this plate the spring can be moved 
 up or down, to or from the plaster-of-Paris block (a). 
 For this purpose the plate (#) rests upon three screws (7;) 
 which pass through correspondingly threaded holes in the 
 flattened and expanded part of the bar (d). Upon the 
 metal plate on the upper side of the spring is fastened the 
 
 * On the effects of mechanical restraint on the growth and other behavior 
 of plant-cells and parts, consult Newcombe, F. C. Influence of mechanical 
 resistance on the development and life-period of cells. Botanical Gazette, 
 vol. 19, 1894. Pfeffer, W. Druck und Arbeitsleistung durch wachsende 
 Pflanzen. Abhandlungen d. K. Sachs. Gesellsch. f. Wissensch., Bd. XX., 1893. i 
 Also p. 187 of this book. 
 
 fFor striking observations under this topic see Kerner and Oliver's 
 Natural History of Plants. Vol. I, part 2. pp. 513-17. 
 
 J Pfeffer, W. Druck und Arbeitsleistung. 
 
GROWTH 
 
 175 
 
 glass plate ( c ) by means of a small amount of plaster of 
 Paris. The upper needle is fastened within the spring by 
 means of shellac, while the lower one is adjustable by means 
 of the screw (i). The flower-pot (n) containing the root 
 set as in the figure in plaster of Paris is firmly pressed into 
 the iron ring (777) fastened by two screws to the upright 
 ( s ) . The small plaster block ( b ) is now fitted to the root- 
 
 no. 11. 
 
 Figure 11. Pfeffer's apparatus for measuring the force exerted by grow- 
 ing roots or stems. (Reduced from the original figure.) 
 
 tip and fastened by plaster of Paris to the glass plate (c) 
 so that the root-tip is over the middle of the spring. The 
 screws ( h ) are now turned up so that the two plaster blocks 
 (a and b) are pressed lightly together. The saw-dust in 
 the pot should be kept moist. The plaster blocks may be 
 constantly moistened by wrapping them with filter-paper 
 wet through a strip of paper connecting with a reservoir. 
 The horizontal rod (o) pressed down on the pot (n) and 
 
176 
 
 PLANT 
 
 fastened by screws to the uprights ( ss ) holds it securely in 
 the ring." By appropriate modifications of this apparatus, 
 growth in diameter as well as in length can be investigated, 
 in stems as well as in roots. 
 
 The following table indicates the effective pressures de- 
 veloped by growing roots : 
 
 LONGITUDINAL PRESSURE 
 
 NAME OF PLANT. 
 
 PRESSURE PER SQ. MM. 
 
 PRESSURE IN ATMOSPHERES. 
 
 Faba vulgaris 
 Zea mais 
 Vicia sativa 
 Aesculus hippo- 
 eastanum 
 
 98+ grams 
 138 + 
 111 + 
 
 68+ 
 
 9.5 
 
 12.4 
 10.7 
 
 6.6 
 
 
 
 Mean = 9.8 
 
 TRANSVERSE PRESSURE 
 
 Faba vulgaris 
 Zea mais 
 
 44 + 
 68 + 
 
 4.3 
 
 6.5 
 
 These figures, obtained by averaging those in Tables I and 
 II of Pfeffer's paper, indicate that although the growing 
 parts are composed of such soft materials, they are capable 
 of developing under resistance a force which makes con- 
 tinued growth possible under all ordinary conditions. Cross 
 and longitudinal pressures developed by stems are probably 
 equal to those developed by roots, but the evident difficul- 
 ties in the way of measurements as exact as those for roots 
 cause the figures reported by Pfeffer to be somewhat lower, 
 as for example, 5.8 and 5.5 atmospheres for the longitu- 
 dinal and cross pressures developed by the stems of seed- 
 lings of Faba vulgaris. 
 
 The growth of plants and animals is as a rule so slow 
 that accurate measurements are difficult to obtain. Besides 
 this, their behavior in other ways is very likely to compli- 
 cate any attempt to determine either the amount or the 
 rate of growth. The almost constant movement of plant- 
 parts out of doors under the influences of wind, sunlight, 
 warmth, etc., and the constant spontaneous movements 
 
GROWTH 
 
 177 
 
 called circumnutation, make ifc seem absolutely necessary to 
 experiment upon small plants indoors; but when this is 
 done, other complications ensue, such as are due to watering 
 the soil and its subsequent drying, the jarring of the meas- 
 uring instruments, etc., etc. Furthermore, not all plants 
 grow in straight lines. The tips of the stems of many are 
 sooner or later so curved that the length of these parts can 
 only be estimated. To overcome these various difficulties 
 many instruments have been devised. From the direct ob- 
 servation and measurement of small organisms or parts by 
 means of microscopes, vertical or horizontal, to the com- 
 plicated self-recording auxanometers of the well-equipped 
 physiological laboratory, the utmost variety in methods 
 and means exists. Illustrated descriptions of instruments 
 are so accessible that space need not be taken here for 
 them.* Most of these instruments are but modifications 
 and improvements of those invented by the masters in 
 plant-physiology, especially by Sachs. The principle of all 
 is to magnify the growth so that the evidence or the record 
 of it is visible. The need of this is made evident by the 
 following figures 
 
 Observer. 
 
 Plant. 
 
 Part. 
 
 Growth per 
 minute. 
 
 growth 
 per 
 minute. 
 
 Truef 
 
 Vicia Faba 
 
 roots growing in 
 
 0.012 mm. 
 
 
 
 
 water 
 
 
 
 KrausJ 
 
 Bambusa sp? 
 
 stalk 
 
 0.040 mm. 
 
 
 Askenasy 
 
 Triticum sp? 
 
 stamens 
 
 1.05 mm. 
 
 37.5 
 
 Hofmeister^ 
 
 Spirogyra 
 
 cell 
 
 
 7.5 
 
 Brefeldff 
 
 Coprinus etercorarius 
 
 stalk 
 
 0.225 mm. 
 
 
 * See Ganong. in Botanical Gazette, vol. XXVII., 1-899. 
 
 Arthur, XXII., 1896. 
 
 Stone, XXII., 1896. 
 
 Golden, XIX., 1894. 
 
 Frost, ' Minn. Bot. Studies, " XVII.. 1894. 
 
 f True. R. H.. in Annals of Botany, vol. IX.. p. 371, 1895. Figures in 
 Table I give this average. 
 
 i Kraus G.. in Annales du Jardin Botanique de Buitenzorg, vol. XII.. 1895. 
 Askenasy. in Verhandl. d. naturh- med. Vereins in Heidelberg, 1879. 
 
 * Hofmeister, in Jahreshefte d. Vereins f. Vaterl. Naturkunde in Wurttem- 
 berg, 1874. 
 
 H Brefeld. Untersuchungen iiber Schimmelpilze. Heft 3. 1877. 
 
178 
 
 PLANT PHYSIOLOGY 
 
 These numbers, however, are much above the average, even 
 the Bamboo being a notoriously rapid grower. The extra- 
 ordinarily rapid growth of the stamens of wheat takes place 
 w r hen they are released from mechanical hindrance by the 
 spreading apart of the scales. 
 
 Probably the average rate of growth for plants does not 
 exceed, if it equals, 0.005 mm. per minute. Certainly there 
 are many plants which grow so slowly that no one has had 
 the patience and skill to make accurate measurements. The 
 lichens are among such slow growers, though these must 
 
 GO" 
 
 30,u 
 
 20/i 
 
 70.03 
 
 g o 
 
 Figure 12. Curve of growth in part of a filament of Bacillus ramosus. 
 (From Ward.) 
 
 grow at different rates as is evident in the parts of California 
 where the very large " lace-lichen" (Rainalinti reticuhita) 
 and crustaceous and small foliose lichens live side by side. * 
 In connection with these figures as to the rate and the 
 percentage of growth of larger and in some cases "higher" 
 plants, it may be of some interest to compare the curve of 
 growth obtained by Marshall Wardf while studying bacte- 
 ria. The accompanying diagram gives the curve of growth 
 
 * See Peirce. G. J. On the mode of dissemination and on the reticu- 
 lations of Itamalma reticulata. Bot. Gazette, vol. XXV., 1898. Ditto. 
 The nature of the association of alga and fungus in lichens. Proc. -Cal. 
 Acad. Sci.. Series III., Botany, vol. I., 1899. Ditto. The relation of fungus 
 and alga in lichens. American Naturalist, vol. XXXIV., 1900. 
 
 tWard. H. M. On the biology of Bacillus ramosus. Proc. Roy. So- 
 ciety vol. LVin.. 1895. 
 
GROWTH 179 
 
 in part of a filament of Bacillus ramosus, 27.30 /* * long at 
 the beginning, 70.03 :>. long at the end of the period of 
 observation, two hours. Cell-divisions occurred at the 
 points indicated by the arrows. Between the first and the 
 second cell-divisions there was an increase of 6.79 /* in 
 length, between the second and third of 9.10 //, between the 
 third and fourth of 14.56 //. Between the first and second 
 cell-divisions there was a lapse of thirty-three minutes, be- 
 tween the second and third, twenty-six minutes, between the 
 third and fourth, thirty-one minutes, an average of thirty 
 minutes. The average growth during these three periods 
 was 10.14 n between each two divisions, or a growth of 
 about one-third of a , per minute. This would appear to 
 be slow growth in comparison with that indicated by the 
 table on page 177 ; but the growth of many-celled organ- 
 isms represents the combined increase in length accom- 
 plished by a large number of comparatively large cells 
 working together. Ward's bacillus is a unicellular organism 
 of minute size. A moment's calculation will show that its 
 average increase in length in every thirty minutes, that 
 is, between each two cell-divisions, is 25%. This is certainly 
 a much higher rate of growth than is possessed at any time 
 by higher organisms, t and it is merely the average rate dur- 
 ing a half-hour. Doubtless its maximum growth is decided- 
 ly higher. Probably the growth-rate of bacteria is higher 
 under favorable conditions than that of any other group 
 of organisms. Their high growth-rate, the rapidity with 
 which they attain the size when cell-division is possible, the 
 promptness with which they divide, the immediate growth of 
 the daughter cells at a high rate, all contribute to the 
 effectiveness of these minute organisms. 
 
 It was stated on page 166 that each cell, each kind of 
 cell, and hence each organism, has a maximum which its size 
 normally never exceeds. Let us seek a reason for this. 
 
 * A fi or micron equals yoVfr millimetre. 
 
 + Since going to press, a review of Buechner's paper (Zuwachsgrossen 
 und Wachsthumsgeschwindigkeiten bei Pflanzen. Dissertation. Leipzig. 
 1901) has appeared (Bot. Centralbl.. Bd. 90, p. 500,1902) in which the 
 growth of the pollen-tube of Imptitiens Hawkeri is reported at 220% and 
 of branches of higher plants as \% in a unit of time and length. 
 
 
180 PLANT PHYSIOLOGY 
 
 Each of the component cells of a multicellular tissue, organ, 
 or organism, is limited in all its behavior by the cells which 
 surround it. The single cell of the unicellular organism is 
 not so limited, being constrained only by the conditions 
 prevailing in itself and in its lifeless surroundings. 
 
 Attempts have been made to attribute the maximum size 
 ordinarily attained by organisms to one or two of the three 
 conditions named on page 168 as making growth possible. 
 It is said that an organism or a cell cannot grow beyond 
 a certain size because there may not be room. If this be 
 true, then the word room must be used with a broader 
 meaning than that attached to it in our discussion on 
 pages 173 and 174 : it must mean, as stated on page 6, 
 freedom from interference of every sort. This last is un- 
 doubtedly true, for a plant with such an impulse to grow 
 that it might otherwise cover the whole earth would be 
 prevented by the presence, if not by the attacks, of the other 
 organisms living at the same time. So the individual is 
 kept within a certain size. 
 
 Again, it is said that nutrition fixes the limit of growth. 
 In a growing spherical cell, the increase in surface and in 
 mass are to each other as the square to the cube, "in other 
 words, the smaller the cell, the greater is the surface in 
 proportion to the mass; and the more the cell grows, the 
 less does the surface grow in proportion to the mass."* It 
 is said that since all food-materials are taken in through 
 the surface, the supply of food will be insufficient when the 
 surface becomes too small in proportion to the mass. This 
 implies, however, that the absorbing power does not in- 
 crease proportionally with the mass. The absorbing power, 
 as we have seen, is the osmotic force exercised by the cell- 
 sap upon the solutions and the constituents of the solu- 
 tions outside the cell. This osmotic force depends upon the 
 differences in the composition, absolute and proportional, of 
 -oil-sap and surrounding liquid. The larger the volume of 
 cell-sap, the more slowly can it be made like the liquid out- 
 side, i. e. the more slowly will the osmotic force be dimin- 
 
 * Verworn, M. Allgemeine Physiologie, Ite Aufl., p. 511, 1895. Engl. 
 transl. by Lee, General Physiology, p. 530. 1899. 
 
GROWTH 181 
 
 ished. We have no reason to think that the osmotic force 
 is less, for the turgor is not lower in large than in small 
 cells. So it cannot be merely the ratio of surface to mass 
 which is the determining factor. 
 
 The high turgor and osmotic force maintained in large 
 cells imply, besides the density of the cell-sap, that the 
 enclosing membranes of the cells are not freely permeable. 
 Upon the impermeability of the membranes and upon the 
 composition of the cell-sap depend the turgescence, the 
 plumpness, of cells of large size. The membranes must 
 possess strength as well as impermeability, but it is their 
 impermeability which makes it necessary that they should 
 also be strong. It is this change in the quality of the sur- 
 face rather than in the mere ratio of surface and mass, 
 which is the important factor in limiting size. But this is 
 not all. 
 
 Whatever may be the distinct functions of nucleus and 
 cytoplasm, it is safe to conclude that neither can be greatly 
 increased or diminished in amount or in activity without 
 affecting the other and the cell as a whole. By chilling cul- 
 tures of Spirogyra while in a state of cell-division, Geras- 
 simow * regularly secured the formation of cells without 
 nuclei, normal cells, and cells with double the usual amount 
 of nuclear substance. He found that the non-nucleated cells 
 grew very slightly in length, that normal cells grew nor- 
 mally, that cells with more than the normal amount of nu- 
 clear substance attained a larger size and divided later than 
 normal cells. Since the volume of the nucleus does not keep 
 pace with the volume of the cytoplasm or of the cell as they 
 both increase shortly after the cell is formed by division, 
 the disparity in the amounts of nuclear and cytoplasmic 
 substances increases. It is conceivable that growth ceases 
 when the amount of cytoplasmic in proportion to nuclear 
 substance has attained the optimum or maximum ; in other 
 words, that the limit of growth is fixed in the first and fun- 
 damental stage, the subsequent increase in size going only 
 to the limit set by the amount of protoplasm formed. 
 
 * Gerassimow. J. J. Uber den Einfluss dee Kerns auf das Wachsthum 
 der Zelle. Moskau, 1901. 
 
182 PLANT PHYSIOLOGY 
 
 If the nucleus were larger and the cytoplasm proportion- 
 ally abundant, if the permeability of the enveloping mem- 
 branes and their tensile strength were proportionally in- 
 creased, if the absorbent power (osmotic force) of the cell 
 were also raised proportionally, is there any reason why the 
 cell, the organ, and the organism should not grow larger? 
 The question cannot be answered. Many organisms do not 
 attain larger size when, so far as we can now see, it would 
 be possible for them to do so. Are the bacteria so small be- 
 cause they have so little nuclear substance in proportion to 
 cytoplasmic? On the contrary some authors* have claimed, 
 from the behavior of bacterial cells toward staining agents, 
 that they are mainly nuclear substance w r ith but a thin layer 
 of enveloping cytoplasm. The amount of room and of food 
 which the individual bacteria could occupy and consume 
 would certainly suggest that they might attain larger size. 
 Their enveloping membranes appear to be sufficiently per- 
 meable and strong for larger organisms. Yet the bacteria 
 remain minute, and no reasons now known can account for 
 their size. If our mechanical explanations fail on these organ- 
 isms, are they any more certain to be correct when applied 
 to the growth of larger ones? What limits the size must 
 have to do with the sensitiveness, the irritability, of the 
 living matter, and this leads us to the subject of the next 
 chapter. 
 
 * See in Migula's System der Bakterien, Bd. I., pp. 72-80. the discussion 
 of this point and the references pro and con. 
 
CHAPTER VI 
 IRRITABILITY 
 
 The preceding chapters have taught us that living or- 
 ganisms are composed of chemical compounds and that they 
 work by physical force. The body of a living plant consists 
 of living protoplasm and of lifeless substances The func- 
 tions of a living plant consist in chemical changes some of 
 which liberate energy, or store it, while others result in the 
 accumulation of matter, lifeless and living. These functions 
 are carried on by the living organism, they do not simply 
 take place ; but the organism lives and carries on these func- 
 tions only by using chemical compounds and physical forces. 
 Just as chemical and physical processes are affected by pre- 
 vailing conditions, so the living organism is affected by each 
 factor of its environment. When the factors change, the 
 organism is differently affected, just as, with changing^ 
 conditions, ordinary chemical and physical processes also 
 change correspondingly. As there is an optimum condition, 
 which consists hi temperature, illumination, supply of water 
 and of other substances, etc., for each chemical reaction 
 taking place in the laboratory and in nature, so there is an 
 optimum condition for that complex of chemical reactions 
 constantly taking place hi the actively living organism. - 
 Any departure from the optimum modifies some or all of the 
 chemical changes in the organism so that there is a different 
 and less favorable balance in the complex. Conversely, any 
 approach to the optimum condition so modifies some or all 
 of the chemical changes that their balance is more favor- 
 able. When we conceive a living organism, even the simplest 
 and smallest, as being a definite structure (protoplasm) 
 consisting of simple water molecules and of other molecules 
 highly complex and therefore comparatively destructible, 
 
184 PLANT PHYSIOLOGY 
 
 built together and enclosing many other compounds, simple 
 or complex, we have the ground- work for a rational concep- 
 tion of the sensitiveness of living organisms to their sur- 
 roundings, that is, of their irritability. 
 
 A solid mass of the metals and jewels ordinarily employed 
 in the construction of a chronometer may be subjected to 
 much harsher treatment without danger of destruction than 
 the same weight of the same substances arranged as a 
 chronometer. Though the substances are the same, their 
 arrangement in the two cases is the reason for their sensi- 
 tiveness, or their power of resisting violence. Living sub- 
 stance, protoplasm, is a structure infinitely finer and hence 
 more delicate than a chronometer. Furthermore, proto- 
 plasm is composed of many more chemical compounds than 
 those entering into the structure of a chronometer. The 
 molecules of each of these compounds are composed of so 
 many atoms of so many different elements that they are far 
 less coherent and stable than the simple one, or two, or 
 three atomed molecules forming the substances in a chro- 
 nometer. Besides all this, these large numbers of atoms are 
 combined into molecules, these complex molecules are ar- 
 ranged in groups, these groups enclose and are surrounded 
 by water, and the water holds in solution oxygen and 
 a variety of compounds. The component atoms of these 
 compounds have affinities for other atoms as well as for 
 those with which they are combined. Furthermore, the sub- 
 stances may not all be in the molecular state in the solu- 
 tion; the component atoms of some substances may be 
 more or less completely dissociated. * In this condition they 
 will be still more susceptible to physical and chemical influ- 
 ences than if combined into molecules, and they will affect 
 the protoplasm with correspondingly greater promptness. 
 The intimate contact of the aqueous solution, the cell-sap, 
 with the living protoplasm, and the complete distribution of 
 
 *See Ostwald, W. Outlines of general chemistry, Eng. transl. by 
 Walker, London and New York, 1895. Nernst, W. Theoretical chem- 
 istry, Eng. transl. by Palmer, London and New York, 1895. Jones. 
 H. C. The theory of electrolytic dissociation and some of its appli- 
 cations. New York, 1900. 
 
IRRITABILITY 185 
 
 the solution throughout the cell, insure the thoroughness 
 with which the protoplasm will be affected. The complexity 
 of protoplasm hi composition and structure, and the physi- 
 cal and chemical properties of the cell-sap which is every- 
 where within it, help us to see that protoplasm is neces- 
 sarily as well as actually the most unstable and the most 
 complex, structure known. 
 
 Comprehending these facts, we see reasons for the sensi- 
 tiveness of protoplasm to outside influences. But lifeless 
 protoplasm imagining such a thing for the moment al- 
 though it possesses all this complexity of structure and of 
 composition, and is therefore sensitive to influences from 
 without, is not the seat of the physiological processes, of 
 the destructive and constructive chemical changes, w r hich are 
 constantly going on in living protoplasm. Indeed the dry 
 seed is far less sensitive, as we have already seen (pp. 9, 10 ), 
 than the same seed after water has been absorbed and ger- 
 mination has begun. Wherever there is actively living proto- 
 plasm, i.e. a complex structure among the compounds of 
 which and within which chemical changes are constantly 
 taking place, we have the conditions for irritability : the 
 more complex the structure and its component and enclosed 
 compounds, or the more varied and the more rapid the 
 chemical changes taking place hi the structure, the greater 
 will be its sensitiveness to external influences. The higher 
 the organism, the more complex is its structure, the more 
 varied or the more rapid are the chemical changes taking 
 place in it, and therefore it is the more sensitive. The or- 
 ganism is dormant, unsensitive, unirritable, when its physi- 
 ological processes are slow or simple ; the organism is dead 
 when its physiological chemical changes cease and its struc- 
 ture breaks down. Its component molecules may still be 
 there intact after the organism ceases to live, but their ar- 
 rangement is changed, the sensitiveness of the whole struc- 
 ture and of all its parts is diminished in proportion as the 
 arrangement of the molecules is modified. 
 
 The irritability, then, of living organisms consists in a 
 sensitiveness to external conditions which is due to the 
 complexity in structure and in composition of the organ- 
 
 X^13T*AU5S, 
 f Of THE 
 
 G UNIVERSITY J 
 
 \ OF J 
 
186 PLANT PHYSIOLOGY 
 
 isms themselves. To put this in more definite terms we may 
 say that the irritability (which we may represent as x) of 
 a cell, an organ, or an organism, consists in the sum of 
 these factors, viz. 
 
 (a) the sensitiveness of the component atoms of complex 
 molecules to other forces and affinities as well as to the 
 affinities which hold them together in these compounds. 
 
 (b) the instability of the groups of molecules. 
 
 (c) the instability of the protoplasmic structure which 
 consists of water and these complex molecules. 
 
 (d) the number, variety, and speed of the chemical 
 changes taking place in this structure. 
 
 (e) the number, variety, and speed of the chemical 
 changes taking place between its component molecules and 
 others enclosed among them or outside. 
 
 ( f) the number, kinds, and degree of dissociation, of the 
 atoms and molecules of the substances dissolved in the 
 water in the cell. 
 
 Thus x='a + 5 + c+d+e + /*, a sum greater than is 
 attained in any known combination except the living cell. 
 
 Let us pass on now from these general considerations as 
 a starting-point, to examine certain phases of irritability : 
 first, the relations of irritability to the amount, kind, etc., 
 of growth ; second, the direction of growth, and movement, 
 as depending upon irritability; third, the growth move- 
 ments not evidently connected with irritability. 
 
 IRRITABILITY AND THE AMOUNT AND KIND OF GROWTH 
 
 Every actively growing organism must have, besides an 
 adequate supply of material, an adequate amount of room, 
 and the impulse to grow ( see p. 168 ) , at least the power to 
 direct its growth according to its environment. This power 
 is dependant upon the irritability of the organism and of its 
 separate organs, their sensitiveness to forces and influences 
 wholly external. Since the material of which it is composed 
 came into existence, every organism has been subjected to 
 external influences, some momentary or unequal, some per- 
 sistent and uniform. The effects of these influences are more 
 or less enduring, like the influences themselves, but presuma- 
 
IRRITABILITY 187 
 
 bly are far from permanent at the utmost. While the effect 
 of one influence still persists, other influences are operating 
 on the organism. The status of the organism at any one 
 time represents the results of all the influences to which it 
 has been subjected up to that time; its form, size, position, 
 etc., are its response to all these influences. The condition 
 of the organism and the influences bearing upon it, together 
 determine how it will grow. The study of irritability at 
 any one time, therefore, necessarily includes the results of 
 irritability at other times earlier in the life of the in- 
 dividual. 
 
 We may begin our study with those mechanical influences 
 to which plants are or may be subjected. When a growing 
 part is enclosed within a bandage of plaster of Paris, two 
 results follow, one of which has already been mentioned 
 (p. 174).* The other w r e may consider now. The part not 
 only lacks room to grow, but the plaster ligature relieves 
 the enclosed part, as it does a broken arm or leg, of me- 
 chanical strain of nearly every sort. The tissues primarily 
 contributing to the mechanical strength of the growing 
 organ attain within the ligature neither such size nor such 
 strength as ordinarily, f From this we may conclude that 
 the mechanical strength of a part depends upon the strain 
 to which it is subjected. This conclusion, reached from ex- 
 periments in which the mechanical strain was reduced as 
 much as possible, is enforced by experiments of the opposite 
 sort, in which the mechanical strain was increased.* The 
 strain consisted in traction, effected by means of weights 
 
 *Newcomb. F. C. The influence of mechanical resistance on the de- 
 velopment and life-period of cells. Botan. Gazette, vol. 19, 1894. Reg- 
 ulatory formation of mechanical tissue. Ibid. vol. 20. 1895. 
 
 f Pfeffer. W. Druck und Arbeitsleistung. Abh. d. K. Sachs. Gesellsch. f. 
 Wissensch., Bd. XX., 1893. Also Pflanzenphysiologie. 2te Aufl.. II., pp. 
 144-7. 1901. 
 
 % Hegler, R. Einfluss des mechanischen Zugs auf das Wachsthum der 
 Pflanze. Cohr's Beitrage zur Biologic der Pflanzen. Bd. VI.. 1893. Older 
 literature here cited. See also Pfeffer. W. Besprechung Hegler's Unter- 
 suchungen. Berichte d. K. Sachs. Gesellsch. f. Wissensch.. Sitzung vom 7ten 
 Dec., 1891. A ko Pflanzenphysiologie II.. 36. Derschau. M. von. Einfluss 
 von Kontakt und Zug auf rankende Blattstiele. Inaug.-Diss.. Leipzig. 1893. 
 
188 
 
 PLANT PHYSIOLOGY 
 
 suspended from threads passing over pulleys and fastened to 
 growing parts. The following figures will illustrate the 
 results obtained 
 
 WEIGHT. 
 
 SEEDLINGS. 
 
 PETIOLES. 
 
 
 Helianthus. 
 
 Phaseolus. 
 
 Heleborus. 
 
 Enough to break. 
 
 160 gr. 
 
 180 gr. 
 
 400 gr. 
 
 Used to strengthen 
 
 150 " 
 
 165 " 
 
 
 " test on 2d day. 
 
 250 " 
 
 
 
 " " 3d - 
 
 300 " 
 
 
 
 " " after several days, 
 
 400 " 
 
 
 
 " " on 7th day, 
 
 
 650 " 
 
 
 5th ci 
 
 
 
 3^ K. 
 
 Subjecting otherwise weak stems to pull induces them to 
 form strengthening tissues which would not ordinarily de- 
 velop at all. Stems thus acted upon decrease their growth 
 in length in proportion as they are stimulated to grow in 
 thickness. Even a strain too slight to produce any stretch- 
 ing will have this effect. 
 
 The formation of strengthening tissues is proportional to 
 the need. This is shown each year by fruiting plants. The 
 fertilization of the egg-cells in the ovules of flowering plants 
 leads to the production of seeds, the growth of fruit, and 
 the considerable increase in weight of these and of the adja- 
 cent parts. The development of the fruit and its contents 
 demands a corresponding growth both of conducting and 
 also of mechanically strengthening tissues extending into 
 regions quite distant from the fruit as well as in those near 
 it.* A similar response to mechanical strain is shown by 
 many mosses and liverworts. The stalks bearing the fruits 
 grow greatly in length for a time. Later, when the fruits 
 increase in weight, the stalks cease to elongate and become 
 much thicker and stronger. 
 
 Plants are everywhere in nature exposed to mechanical 
 strains of more or less force and constancy. The winds, 
 flowing water, tides, waves, and the movements of animals 
 
 *Pieters. A. J. Influence of fruit-bearing on the development of me- 
 chanical tissue in some fruit trees. Annals of Botany. X. 1896. 
 
IRRITABILITY 189 
 
 apply mechanical force to living plants, and to these influ- 
 ences they respond by the increased or modified activity of 
 the living protoplasm. 
 
 The most evident effects of winds are those deformities 
 which usually, however, are more the result of injury than 
 of stimulus. Trees in exposed places are unsymmetrical, 
 their limbs short and broken on the side toward the strong 
 or violent prevailing wind, while on the other side the limbs 
 look as if they had been drawn along with the wind. But 
 the root-syste n of such trees shows the stimulating 'effect of 
 the wind without the deformities exhibited in the branches, 
 the roots being longer and stronger on the windward 
 than on the leeward side, the greatest strength develop- 
 ing where there is the greatest mechanical force to be 
 resisted. 
 
 The ordinary swaying of stems and branches, and even of 
 leaves on their stalks, acts as a stimulus to the living 
 cells of a plant. The difference in the amounts of mechan- 
 ical tissue in plants which carry their own weight and in 
 those which lean, twine, and otherwise climb, is partly due 
 to the difference in mechanical stimulus to form strength- 
 ening tissues. By tying an erect plant so firmly to a sup- 
 port that it cannot sway in the wind, or by supporting its 
 weight on a frame, the plant will be deprived of those move- 
 ments, stresses, and strains which normally stimulate it 
 to develop strength. 
 
 Water-currents exercise similar effects to those of wind- 
 currents, whenever they are rapid enough to develop any 
 considerable force. Even slow water-currents have been 
 found to stimulate and direct growth and movement in 
 rather peculiar fashion. Thus the roots of certain plants, if 
 suspended in clean running water, will bend so that the tips 
 point and grow up stream. This phenomenon is known as 
 rheotropism. * The plasmodia of certain Myxomycetes will 
 grow on a vertical strip of filter-paper always in the direc- 
 tion opposite to that of the current of water which is sup- 
 
 * Juel, H. O. Untersuchungen iiber den Rheotropismus der Wurzeln. 
 Jahrb. f. wiss. Bot., Bd. 34. 1900. Newcombe. F. C. Rheotropism cf 
 roots. Bot. Gazette vol. 33 1902. 
 
190 
 
 PLANT PHYSIOLOGY 
 
 plied to them, whether this be upward or downward. This 
 phenomenon is called rheotaxis, * but it is not certain that 
 it is not a response to obscure chemical stimuli rather 
 than to a current of water merely. Rheotaxis and rheo- 
 tropism are therefore probably distinct phenomena. The 
 significance of rheotropism is not understood. 
 
 FKJ. 13. 
 
 Figure 13. PosteMa Palmspformis. Sea-palms at Point Lobos, near 
 Monterey, California. Height about 2 feet. Photograph by Dr. W. A. 
 Shaw. 
 
 Waves and tides have not been studied experimentally in 
 their mechanical relation to plants. It may be inferred, 
 perhaps, that they produce movements which stimu- 
 late the plants exposed. Certainly plants which are to 
 withstand the pounding of the waves must grow propor- 
 tionally resistant. Is this simply a case of the survival 
 of the accidentally toughest and fittest, or do tide and surf 
 plants irritably react to the rude stimuli to which they are 
 
 Stahl, E. Zur Biologic der Myxomyceten. Bot. Zeitung, 1884. 
 
IRRITABILITY 191 
 
 subjected? Of these buffet-ted forms the Sea Palms (Pos- 
 telsia paJm&formis) of the Pacific Coast are the most strik- 
 ing. Living between the tide-marks, always in the most 
 exposed positions, these upright plants hold on and grow in 
 spite of the tremendous pounding to which they are almost 
 continually exposed. In toughness, strength, and elasticity 
 their upper parts are equalled only by the closeness of the 
 attachment and the strength of the hold-fasts. The ,accom- 
 panying figure suggests how rough their habitat may be in 
 a storm. 
 
 From the foregoing we may conclude that a certain 
 amount both of freedom to move and also of actual agita- 
 tion is good for plants. This is exercise, apparently as de- 
 sirable for plants as for animals, and presumably for the 
 same reasons. It facilitates the transfer of nutrient sub- 
 stances and it stimulates the living protoplasm. Which 
 factor is the more important it remains for experiment to 
 determine. 
 
 In trees and shrubs the mechanical tissues are found espe- 
 cially in the wood. Where the seasons are sharply con- 
 trasted, as over the greater part of the temperate zones, the 
 wood presents the familiar appearance known as annual 
 rings. In mechanical strength the different parts of the 
 wood vary considerably, the so-called ''spring- wood," be- 
 cause of the larger size of the cells and the comparative 
 thinness of their walls, being decidedly weaker than the 
 thicker-walled, more compact, and often more abundant 
 ''autumn wood." It is through the wood, especially the 
 ducts and tracheids, that the transfer of food-materials 
 from roots to leaves takes place (see pp. 119-124). The 
 wood is, therefore, both a mechanical and a vascular tissue. 
 The one function or the other predominates at different 
 times during the growing season and affects the growing 
 and developing tissues accordingly. 
 
 Where growth is always possible and is practically con- 
 tinuous, annual rings are not formed. It is only where 
 growth is periodic because of changing seasons, like winter 
 and summer, dry and rainy seasons, that there are decided 
 differences in the character of the wood. Certain other 
 
192 PLANT PHYSIOLOGY 
 
 phenomena almost or quite coincide with the formation of 
 annual rings. "Spring wood" forms (seep. 123) when sap- 
 pressure is greatest, when the buds open and the leaves 
 expand, when there is a sudden extension of the surface 
 from which water will evaporate. At this time water must 
 be abundantly supplied to the parts just emerged from the 
 bud so that the new cells may expand to their proper size ; 
 food must be furnished these growing parts so that new 
 cells and new protoplasm may form and the parts may con- 
 tinue to increase in size. When the buds unfold there is an 
 immediate and great demand upon the conducting tissues, 
 but as the parts increase in size and weight, the mechanical 
 strength of branchlets, branches, and stem must increase 
 also. With an increasing weight each spring and early sum- 
 mer there is an annually increasing mechanical strain upon 
 the tree or shrub. This increased strain is yearly met by 
 increased strength, and this is contributed largely by the 
 "autumn wood." 
 
 The time during which the cambium cells give rise by 
 division to new cells differentiating into wood and bast ele- 
 ments is much briefer than the season during which growth 
 is apparently possible. According to Jost, * the greater part 
 of the increase in thickness of stems and branches takes 
 place in May and June ( in Germany ) . This indicates that 
 the activity of the cambium cells and of their immediate 
 derivatives is controlled by influences outside of themselves. 
 These influences are doubtless many, but we may distinguish 
 some of them at least. 
 
 The young parts growing and developing from opening 
 buds in the spring need much food and water, and they 
 certainly transpire greater or less quantities of water-vapor. 
 There is at this time an especially great and a fairly steady 
 demand upon the conducting tissues for both food and 
 water, not so much for transpiration, perhaps, as for 
 growth in the full sense of the word; for food so that new 
 protoplasm may be formed, for water so that it may prop- 
 erly expand. This demand would make itself felt first in the 
 
 * Jost, L. Beobachtungen iiber den zeitlichen Verlauf des Dickenwachs- 
 thums der Baume. Ber. d. Deutsch. Bot. Gesellsch., Bd. X., 1893. 
 
IRRITABILITY 193 
 
 parts nearest opening buds. It is here that the cambium 
 first resumes its activity, the more and more distant parts 
 coming only successively into activity again.* It is pre- 
 cisely the parts nearest the fully expanded leaves and the 
 maturing terminal buds in which the cambium also first 
 ceases to be active. In plants which form no terminal buds 
 such as roses, briars, etc. the cambium continues to be 
 active so long as the temperature, moisture, and other ex- 
 ternal conditions make growth possible. The cambium cells 
 divide more or less early, and a larger or smaller number of 
 times. To a considerable extent at least this is according 
 to the behavior of the parts developing from the opening 
 buds. The living cells in rapidly growing leaves and elon- 
 gating internodes, demanding much food and water, stim- 
 ulate by this demand the earliest cells cut off by division 
 of the cambium cells to grow to such size and to take on 
 such characters that they will best conduct what is needed 
 above. If the ground is so dry that only insufficient quanti- 
 ties of water can be absorbed, or if in the preceding year 
 only insufficient quantities of food were made and stored, 
 the parts coming from the opening buds will develop less 
 rapidly or less perfectly in size, etc., and will be able to exert 
 and will exert less of a stimulating demand upon the con- 
 ducting tissues for food and water than in a better season. 
 In this way nutrition affects everything, the formation of 
 wood as well as the development of new organs. In the 
 various living cells composing the embryonic organs in the 
 bud, the impulse to grow is given by returning favorable 
 conditions. Warmth, light, moisture, etc., stimulate the 
 cells to grow, to divide, to grow again, to differentiate, 
 etc. These cells, stimulated by purely physical influences 
 from outside themselves, develop needs proportioned to 
 their physiological activities. The needs must be met, if the 
 cells are to continue their activities, by materials drawn 
 from their neighbors. So this demand, extending from cell 
 to cell by the osmotic transfer of nutrient solutions, pres- 
 ently reaches the living cells adjoining? the conducting ele- 
 
 * Jost. L. Beziehungen zwischen der Blattentwickelung und der Gefass- 
 bildung in der Pflanze. Bot. Zeitung, 1893. 
 13 
 
194 PLANT PHYSIOLOGY 
 
 ments, the cambium and its daughter cells. According to 
 the demand, the stimulus, thus exerted, these daughter cells 
 develop into the so-called "spring wood." 
 
 The hypothesis thus outlined would be worthless if it were 
 not for the early growth of the roots which makes possible 
 the supply of the relatively large volume of water demanded 
 by the parts coming from the bud. Goff* has recently 
 shown that growth of the root begins in the spring before 
 there are any signs of growth in the parts above ground. 
 Thus the plant is early provided with the absorbing agent 
 needed. This growth of the root also must be regarded as 
 the irritable response to the stimulus exerted upon it by the 
 moisture and the increasing warmth of the soil. 
 
 The other half of the annual ring remains to be accounted 
 for. What has already been said regarding the effect of in- 
 creasing the mechanical strains to which growing parts are 
 subjected (see pp. 174, 187-8) prepares us for an hypothe- 
 sis, deserving more experimental tests, to account for the 
 change in the character of the season's growth of wood. 
 After the buds open, the leaves expand and grow, the inter- 
 nodes lengthen, and all the parts and their component cells 
 attain their definitive dimensions, weights, etc. As a result, 
 the mechanical strain upon the parts behind increases. It 
 increases not only with the weight and with the change in 
 position of the weight, which produces a greater leverage, 
 but also with occasional sudden and often very great addi- 
 tions to the weight by wind and rain. Meantime there is 
 little or no increased demand for food, and transpiration, 
 being controllable by the stomata, is not likely to increase 
 greatly. There is ordinarily, therefore, no great addition to 
 the conducting system (see pp. 123-4). 
 
 Strengthening tissues fibres, tracheids, thick-walled ele- 
 ments are formed by the differentiation of the young cells 
 derived from the cambium. From Hegler's investigations 
 (p. 187 ) it is evident that strengthening tissues develop ac- 
 cording to the strain to which a part is subjected, that an 
 increasing strain is accompanied by the formation of more 
 
 * Goff, E. S. The resumption of root-growth in spring. Wisconsin 
 Agric. Exp. Sta.. 15th Annual Report 1898. 
 
IRRITABILITY 195 
 
 and stronger mechanical tissues, and that this development 
 is a response to irritation. We have only to apply these 
 conclusions to the changes taking place in growing wood as 
 the season progresses to gam an idea as to one of the most 
 important influences contributing to the formation of "au- 
 tumn wood." 
 
 Ordinarily only one ring is added to the wood each year. 
 Many woody plants, if defoliated by frost, caterpillars, etc., 
 so early in the season that growth has not ceased, will 
 open their latent buds, and develop a second set of leaves. 
 Under these conditions, with the sudden increase in the 
 demand for new conducting tissues, the } r oung derivatives of 
 cambium will develop into elements resembling but not 
 quite equalling those of normal "spring wood." In this 
 way woody plants may form in nature two rings of wood in 
 a single year. It is claimed* that "spring" and "autumn" 
 wood may be found repeatedly alternating with one another 
 in a single season's growth in pine, if only during the grow- 
 ing season there are repeatedly alternating and sharply 
 contrasting rainy and dry periods. Pfeffer's caution f "that 
 an apparently similar result may sometimes be produced in 
 various ways" applies to this observation as well as to 
 experiments. 
 
 The careful study of plants subject to the attack of gall- 
 insects and other pests should throw light on the relation 
 of the growth of wood to the demand made upon it. For 
 example, cross-sections of the younger branches of Monterey 
 Pine (Pinus radiata) which have been attacked by the leaf- 
 galling insect Diplosis pirn-radiate ^% show abnormalities in 
 the vascular tissues. Instead of the clearly marked annual 
 rings of wood, these branches have semi-annual rings which 
 correspond in size, position, and composition with the times 
 at which the plant is attacked by the gall-insect, at which 
 
 * Lutz, K. G. Beitrage zur Physiologic der Holzgewachse. Fiinfstiick's 
 Beitrage z. wise. Bot.. Bd. I.. 1897. 
 
 t Pfeffer, W. Pflanzenphysiologie. 2te Aufl.. Bd. II., p. 274, 1901. 
 
 J Papers by Cannon, W. A. The Gall of the Monterey Pine. American 
 Naturalist, vol. 34. 1900; and Miss Mills in Entomological News, vol. 
 XI., 1900. 
 
196 PLANT PHYSIOLOGY 
 
 the morbid growth begins and ends, and with the differences 
 in the activities of the galled leaves from those which are 
 normal. 
 
 The formation of wood and of its different kinds and 
 elements in a season is affected by all the vital activities 
 of the plant and all the external influences which bear 
 upon it. The character of the wood is not the result of 
 any one set of factors. At the same time that we must 
 constantly recognize that the living plant is sensitive to a 
 great many influences and that it responds to these, we may 
 distinguish in the complex of influences some which are more 
 effective than others. We may therefore accept, at least un- 
 til a better one is advanced, this hypothesis : the two kinds 
 of wood in the year's growth are formed in their different 
 ways in response to the different demands, or stimuli, 
 brought to bear upon the cambium and its young deriva- 
 tives; the "spring wood," composed of large elements, essen- 
 tially for the conduction of liquids; the "autumn wood/' 
 composed of small and thick-walled elements, essentially for 
 mechanical strength; and between these two, the wood 
 which is both "spring" and "autumn" in character, formed 
 when there is still need of more conducting tissues and when 
 the need of strengthening tissues is already beginning. So 
 we have the adaptation of the wood to the different needs 
 of the plant in different parts of the growing season, the 
 adaptation being accomplished through the irritability of 
 the growing and differentiating cells. 
 
 INFLUENCE OF GRAVITATION. 
 
 So far we have studied mainly the effects of evidently 
 mechanical influences upon the living organism. It is, how- 
 ever, sensitive to many other influences, some of them quite 
 as important. Of these only one is constantly and uni- 
 formly operative the force of gravitation. The plant may 
 change, by growth and movement, the relation of its parts 
 to the force, but the amount of force acting upon the plant 
 continues the same. The other influences are variable, 
 periodic, or occasional. The mechanical influences so far 
 
IRRITABILITY 197 
 
 discussed may be said to control growth by limiting it. 
 The influences which we are about to study control growth 
 by directing it. Yet this distinction is suggestive rather 
 than exact, and must not be accepted without reserve. 
 
 The action of gravitation may be considered from its 
 effects on the kind, rate, and direction of growth, and on 
 the position, of plants. Gravity exerts upon all objects a 
 pull toward the centre of the earth. This pull is propor- 
 tioned in intensity to the weight ( mass ) of each object, the 
 heavier the object the stronger the pull. The pull is resisted 
 more or less completely by the medium in which the objects 
 are. An object in a fluid is buoyed up with a force equal to 
 the weight of the fluid which it displaces. Thus the down- 
 ward pull of gravity upon a plant living submersed in fresh 
 water is resisted by a force equal to the weight of the water 
 which the plant displaces. This is 750-800 times greater 
 than the force which an equal volume of air would exert. 
 The average specific gravity of sea- water is 1.2. Hence a 
 plant living in sea- water is supported still more, by a force 
 1.2 times greater than that of an equal volume of fresh 
 water, and therefore 900-950 times greater than air. The 
 parts of a plant growing in a solid medium, the soil, are 
 completely supported. The soil will ordinarily support 
 much more than the weight of the plants growing in it. It 
 is evident, therefore, other things, being equal, that the 
 mechanical strength which the plant or plant-part must 
 develop is proportioned to the fraction of the force of 
 gravitation which is not balanced by the buoyancy or sup- 
 porting power of the medium in which it lives. The force of 
 gravity exerts by this means a direct influence upon the 
 kind of tissue which the plant forms, the kind of growth 
 which it makes. The force of gravity is one of the most 
 important factors in the complex which constitutes the 
 environment. 
 
 The rate of growth in most plants seems to be tolerably 
 independent of gravity, other forces being more effective. 
 Parts which normally stand in one direction may grow at 
 a somewhat different rate when their position is changed. 
 Also, when gravity is opposed by an equal or greater force, 
 
198 PLANT PHYSIOLOGY 
 
 as can be done by a centrifugal machine, the rate of growth 
 may be changed. But when the position of a plant is con- 
 stantly changed or is changed at frequent and regular inter- 
 vals, as by a regular or by an intermittent clinostat, the 
 rate of growth does not seem to be materially affected.* 
 
 Gravity is one of the most important influences determin- 
 ing the direction of growth. It affects direction and kind 
 much, the rate of growth little. Its action in directing 
 growth is called geotropism. Those organs which grow 
 toward the source of gravitation, downward, are said to be 
 positively geotropic. Roots and rhizoids are positively 
 geotropic organs. Stems which grow in the opposite direc- 
 tion, upward, are called negatively geotropic. Other organs, 
 such as branches, which grow horizontally, are said to be 
 diageotropic, while obliquely growing organs like lateral 
 roots are called plageotropic. For reasons of convenience, 
 roots have long been the favorite objects for the study of 
 the effects of gravitation. Such horizontal organs as leaves 
 owe their position quite as much to the influence of light as 
 to gravitation. Though we may say that gravitation is the 
 chief force directing the growth of stems and roots, it must 
 be remembered that it is only the chief of several or many. 
 The position which any part or organ finally assumes repre- 
 sents the combined influence of all of those forces to which 
 it is sensitive and which act upon it. 
 
 It may be stated as a general rule that the stems and 
 roots of higher plants, and the corresponding parts of 
 many lower plants, tend to grow in opposite directions. 
 Each organism begins as a single cell. Upon this one cell 
 all the influences which affect the new plant are converged. 
 The directions of growth and of division of this one cell are 
 the result of these influences. In consequence of the first as 
 well as of subsequent divisions, the different cells of the 
 embryo are differently placed, some opposite to others. 
 From this oppositeness in position there follows an oppo- 
 siteness in behavior, which expresses a difference in the cells 
 and organs themselves. It is easy and natural to suspect 
 
 * For a discussion of this topic see Pfeffer's Pflanzenphysiologie, 2te 
 Aufl., Bd. II., 29, 1901. 
 
IRRITABILITY 199 
 
 that the direction of the divisions of the fertilized egg-cell 
 and of the first cells in the embryo bears a definite relation 
 to the force of gravitation, and this appears in many 
 plants to be really the case, but other influences may co- 
 operate or predominate in producing the same effect, as, 
 for example, in the ferns.* Whatever may be the origin 
 in the embryo of the different responses which the differ- 
 ent parts make, it is evident that, from the germination 
 of the seed onward, the plant is sensitive to gravitation 
 and is directed in its growth by it as well as by other 
 forces. 
 
 The responses to the force of gravity are much better 
 known than are the immediate effects of the force in the 
 sensitive parts. Most of our knowledge and interest in the 
 subject are due to Ciesielski,t Darwin,! Sachs, and Pfeffer 
 and their followers. 
 
 The young root is more highly differentiated, anatomi- 
 cally and physiologically, than its simple external appear- 
 ance suggests. At the extreme tip is the cap, a protective 
 covering of the "growing point." The "growing point" is 
 a mass of permanent meristem which gives rise to all the 
 root structures. Behind this is the region of evident growth, 
 where the young cells are increasing in volume by the ab- 
 sorption of water from their older neighbors (see fig. 9, 
 p. 168). Still further back and adjoining this region is the 
 zone where, through the root-hairs, water is principally 
 absorbed from the soil. If a young seedling with a straight 
 root be laid so that its root will be horizontal, care being 
 taken that the root remain moist, the tip will begin to 
 
 * See Goebel, K. Organographieder Pflanzen, pp. 188-f, 1898. Campbell, 
 D. H. Mosses and Ferns. 1895. McMillan. C. The orientation of the 
 plant-egg and its ecological significance. Botanical Gazette, vol. 25. 1898. 
 
 f Ciesielski. T. Untersuchungen iiber die Abwartskrummiing der Wurzel. 
 Beitrage z. Biologic d. Pflanzen. Bd. I., 1872. 
 
 \ Darwin. C. The Power of Movement in Plants. 1880. 
 
 Sachs, J. von. Different papers from 1873 to 1879, collected in his 
 Gesa melte Abhandlungen iiber Pflanzenphysiologie. Bd. II., 1893, and in 
 his Lectures on the Physiology of Plants. Eng. transl.. 1887. 
 
 Pfeffer, W. Geotropic sensitiveness of the root-tip. Annals of Bcftany, 
 vol. VIII., 1894. 
 
200 PLANT PHYSIOLOGY 
 
 turn downward within an hour from the time when its posi- 
 tion was changed.* The tip is carried downward by the 
 elongation and curvature that take place in the part most 
 rapidly growing, 3-4 millimetres back of the tip. In this 
 case gravity acts as a stimulus. Gravity cannot be the sole 
 force pulling the tip into the soil, for the tip is too light, 
 and the resistance of the soil is too great, for any such 
 result. 
 
 Darwin believed that the tip of the root, like the brain, is 
 a sense organ, receiving the stimulus of gravitation and 
 sending back to the elongating part tha impulse to respond 
 to it. Sachs and others contended that only the growing 
 part received the stimulus and acted upon it. For years the 
 matter stood thus, and Kothert,t very carefully reviewing 
 the whole subject, had just published his opinion that it 
 could not be decided experimentally when Pfeffer J announced 
 the results of the ingenious experiments conducted under his 
 direction by Czapek. In order to decide the matter it was 
 necessary to employ means which would not in any way 
 injure the root or introduce any new factors into the experi- 
 ment ; decapitation, wounding, and the other devices re- 
 sorted to before being obviously open to objections serious 
 enough to invalidate the conclusions drawn from experi- 
 ments involving such procedure. This was accomplished by 
 taking advantage of the plasticity of the growing tip, caus- 
 ing the roots to grow into tubes, 3-4 millimetres long, of 
 thin glass, bent in the middle at a right angle. While the 
 tips were growing into these tubes, the plants were revolved 
 on a clinostat, so that no directive geotropic irritation 
 
 * For experiments on roots, seeds may best be germinated in damp 
 tannin-free sawdust. (Stone. Bot. Gazette. XIX.. 1894.) Many experi- 
 ments are described in the laboratory manuals previously named. 
 
 t Rothert, W. Die Streitfrage iiber die Function der Wurzelspitze. Flora. 
 1894. 
 
 t Pfeffer, W. Cber die geotropische Sensibilitat der Wurzelspitze. Sit- 
 zungsber. d. K. Sachs. Gesellsch. d. Wissensch., Sitzung vom 2ten Juli, 1894. 
 Geotropic sensitiveness of the root-tip. Annals of Botany, vol. VIII., 
 1894. 
 
 Czapek, F. Untersuchungen iiber Geotropismus. Jahrb. f. wiss. Bot., 
 Bd. XXVII.. 1895. 
 
IRRITABILITY 201 
 
 should be set up. "If now a specimen," prepared as shown 
 in the accompanying figure, "is placed so that the terminal 
 part points vertically downwards, whilst the rest of the root 
 is horizontal, no geotropic curvature takes place. This, 
 however, always took place, and with about the same 
 promptness as in straight roots, when the terminal portion 
 was placed horizontally, or in general at an acute angle 
 with the normal position. From these experiments it fol- 
 lows that the root thus treated is perfectly capable of 
 reaction. . . . By this means therefore it is proved with 
 the most perfect certainty, that in an uninjured root only 
 the root-tip is geotropically sensitive." With this is also 
 proved that from the 
 part which is sensitive 
 the part of the root 
 
 ^j\ " 
 
 with the largest amount (^ 
 
 of living protoplasm in 
 
 proportion to its vol- 
 
 Fierure 14. Root-tip in bent glass tube, 
 ume-the stimulus is (From Czapek . } 
 
 transmitted to cells cap- 
 able of responding to the stimulus. These are the cells 
 at that time increasing in volume, growing. The posi- 
 tion of the tip is determined by its own sensitiveness, but 
 its position can be changed, except by artificial means, 
 only by the action of the responding part. The stimulus is 
 transmitted from living cell to living cell. The transmission 
 takes time, but the interval, known as the latent period, 
 between exposure and response to the stimulus, is necessarily 
 employed in preparing to execute the response as well as in 
 transmitting the stimulus. The latent period may mean 
 still more, but at least it means these two things. 
 
 The transmission of the stimulus cannot be understood 
 until it is known in what the stimulus exerted by gravity 
 consists and what it does in the sensitive cells. All the 
 ponderable parts of an organism and of a cell are subject 
 to the physical pull of gravity. From this it follows that 
 while these parts are in place their position is maintained 
 as the result of the activity of living protoplasm. When the 
 position of these parts is changed, either by the living pro- 
 
202 PLANT PHYSIOLOGY 
 
 toplasm or by means outside of it, the relations of the parts 
 and of the protoplasm to gravity are also changed. The 
 force of gravity acts constantly. Therefore, whether the 
 ponderable parts of a cell are changed or are constant in 
 position, they and the protoplasm are constantly influenced 
 by gravity. Only when there is a change in the relation of 
 the parts to gravity is the protoplasm called upon to 
 change its response. The action of gravity in a cell and 
 upon its parts must be the same as upon any ponderable 
 substance. The heavier (or lighter) the substance, the 
 greater (or less) its specific weight as compared with the 
 protoplasm in which it is imbedded, the more promptly 
 will it exert a different pressure upon the protoplasm 
 and change in position when the position of the cell 
 is altered. In every living cell there are solid particles of 
 greater or less size, living or lifeless, presumably differing 
 among themselves and from the protoplasm in weight. In 
 certain plants, e.g. some Desmids, in rhizoids of Chara, * in 
 the starch-sheath of stems and in the root-cap, etc., of higher 
 plants, f solid particles are conspicuous in size or arrange- 
 ment. The force of gravity will, however, act similarly, 
 though less evidently, upon the solid contents of all cells. 
 
 The reaction of the living protoplasm to the pull of 
 gravity is very different from that of lifeless material. The 
 reaction may show Itself in a variety of ways, chemical and 
 physical. CzapekJ has shown that a chemical change takes 
 place in root cells stimulated geotropically. In the periblem 
 cells of sensitive root-tips there is present, in larger quantity 
 
 * Giesenhagen, K. Uber inneie Vorgange bei der geotropischen Kriim- 
 mung der Wurzeln von Chara. Ber. d. Deutsch. Bot. Gesellsch., XIX., 1901. 
 
 f Haberlandt, G. Uber die Perception des geotropischen Reizes. Ber. d. 
 Deutsch. Bot. Gesellsch.. Bd. XVIII., 1900. Uber die Statolithenfunction 
 der Starkekorner. Ibid., Bd. XX., 1902. Nemec, B. Uber die Art der 
 Wahrnehmung des Schwerkraftreizes bei den Pflanzen. Ibid., Bd. XVIII., 
 1900. Uber die Wahrnehmung des Schwerkraftreizes bei den Pflanzen. 
 Jahrb. f. wiss. Bot., Bd. XXXVI.. 1901. Noll, F. Uber Geotropismus. 
 Ibid., Bd. XXXIV.. 1900. Zur Controverse tiber den Geotropismus. Ber. 
 d. Deutsch. Bot. Gesellsch., Bd. XX., 1902. 
 
 f Czapek, F. Ein mikroskopischer Befund an geotropisch gereizten Wur- 
 zeln. Ber. d. Deutsch. Bot. Gesellsch., XV., 1898. Also Jahrb. f. wiss. Bot. 
 Bd. 32, 1898. 
 
IRRITABILITY 203 
 
 than elsewhere, a readily oxidized substance, perhaps also 
 another which is a vehicle of oxygen. In the irritated root- 
 tip, and directly in proportion to the irritation, the amount 
 of aromatic oxidizable substance increases and the substance 
 or substances serving as a vehicle for oxygen decrease. This 
 transfer of oxygen from one compound to another within 
 the cell is accomplished not by gravity but by the living 
 protoplasm stimulated by gravity. 
 
 The transmission of a stimulus which caused only mechan- 
 ical changes in the stimulated cell would be very difficult to 
 conceive. On the contrary, when there is a change in the 
 amount or the composition of diffusible substances, it is 
 much easier to form some notion of the means by which the 
 impulse to grow in a definite way is given by the meriste- 
 matic cells at the root-tip to those in the growing region 
 behind. Any change of this sort in one cell is necessarily 
 followed by corresponding diffusion currents between this 
 cell and its neighbors more and more remote. Wherever ST 
 new substance enters the living protoplasm of a cell it will 
 affect the protoplasm chemically or physically, thus pro- 
 ducing a stimulus* The irritated periblem cells of the 
 root-tip give up by diffusion, the changed substances 
 which they have formed. Finally those cells in the grow- 
 ing zone are reached by the diffusing compounds,/ and 
 they change the direction if not also the rate of growth 
 of the organ. -^ 
 
 Diffusion undoubtedly occurs between the sensitive cells of 
 the root-tip and the cells more or less remote. The trans- 
 mission of an impul&e by diffusion alone is not rapid, and 
 it is difficult to prove. To avoid both of these objections 
 Nemec* has recourse to protoplasmic fibrils to which he 
 attributes the function of conducting stimuli from cell to 
 cell. These fibrils can be seen in suitably prepared fixed 
 material and in certain living cells (e.g. stamen and slime 
 hairs of Tradescantia) , both in tissues which are irritable 
 and which exhibit visible reactions to stimuli > and also in 
 tissues in which no response to stimuli has ever been de- 
 
 * Nemec. B. Die Reizleitung und die reizleitenden Structuren bei den 
 Pflanzen. Jena. 1901. 
 
204 PLANT PHYSIOLOGY 
 
 tected.* Since we conceive all living cells to be affected by 
 external influences, whether they give a response which 
 can be seen at any time or not, there is every reason to 
 believe that all the living cells in a body are connected to- 
 gether. Indeed, Strasburger's investigations go far toward 
 proving that such is really the case.f By Nemec's fibrils and 
 Strasburger's continuity of protoplasm the living parts of a 
 plant are united in one system. Granting this to be the 
 case, it is nevertheless difficult to see how these structures 
 convey a purely mechanical stimulus like that of gravita- 
 tion. Nemec's fibrils suggest the nerve structures in ani- 
 mals. Like these structures, their manner of working is a 
 mystery. We may therefore cling to our diffusion hypothe- 
 sis as comprehensible, if difficult of proof, leaving the future 
 to determine the value and the functions of protoplasmic 
 fibrils and protoplasmic continuity. 
 
 Sachs asserted that when a root-tip is horizontal it is in 
 position to be most strongly stimulated by gravity, and 
 that then the curvature of the growing region will be most 
 rapid and most pronounced. Until Czapek J determined the 
 necessary duration of the stimulus, Sachs's assertion was un- 
 challenged. Czapek ascertained that the root-tips of vari- 
 ous plants require from fifteen to fifty minutes' exposure 
 to the action of gravity in order to bend. The longer the 
 exposure, the more pronounced in rapidity and angle is the 
 bending. Thus roots of Lupinus albus will bend if laid 
 horizontal for twenty minutes, but the maximum effect will 
 follow an exposure of four hours. When the root-tip points 
 upward at an angle of 45, not when it is horizontal, the 
 root bends most, the time of exposure and other conditions 
 being the same. Above this angle, the spontaneous growth- 
 movements, known as nutation or circumnutation, interfere 
 
 * Haberlandt. G. Uber Reizleitung im Pflanzenreich. Biolog. Central- 
 blatt, XXI.. 1901. tber fibrillare Plasmastructuren. Ber. d. Deutsch. Bot. 
 Gesellsch., XIX., 1902. 
 
 f Strasburger, E. fiber Plasmaverbindungen pflanzlicher Zellen. Jahrb. 
 f. wise. Bot.. Bd. 36, 1901. 
 
 J Czapek, F. Untersuchungen iiber Geotropismus. Jahrb. f. wise. Bot.. 
 Bd. 27, 1895. Weitere Beitrage zur Kenntniss der geotropischen Reizbewe- 
 gungen. Jahrb. f. wise. Bot., Bd. 32 1898. 
 
IRRITABILITY 205 
 
 with the action of gravity. Czapek's conclusion is sup- 
 ported by a study of grass-haulms.* 
 
 When the root-tip is stimulated by the action of gravity a 
 bending takes place further back, in the region which is most 
 rapidly elongating. This region, called the motor zone, can 
 bend only if there are differences in the rate of growth or 
 in the tissue tensions in its different parts. It is not neces- 
 sarily the case that all roots behave alike in the means 
 any more than hi the manner of curvature, although it 
 seems probable that in roots of similar structure the me- 
 chanics of curvature will be similar. This may account hi 
 part for the diverse views regarding the mechanics of geo- 
 tropic curvature. Thus, according to Ciesielski, f the cells of 
 the lower (concave) side of the root are forcibly com- 
 pressed, even wrinkled, by the more than average growth of 
 the cells of the upper ( convex ) side. Sachs J denies that the 
 cells of the upper side always grow faster, and attributes 
 the pronounced bending to the diminished growth rate of 
 the cells of the lower side. MacDougal, employing the 
 imbedding methods now in general use but unknown to 
 Sachs when he wrote on this subject, comes to essentially 
 the same conclusions as the great master hi plant physi- 
 ology more than twenty-five years before. Pollock,! study- 
 ing curvatures induced by injury ( traumatropic ) instead of 
 by gravity (geotropic), concludes that the mechanism of 
 root-curvature consists in changed tissue-tensions in the 
 stimulated roots, the normal tension between cortical paren- 
 chyma and axial cylinder increasing on the upper ( convex ) 
 side and decreasing on the lower (concave) side. Though 
 this last view may be correct as regards some if not all 
 roots, it must be conceded that growth makes permanent 
 
 * Pertz. Miss D. F. M. On the gravitation stimulus in relation to posi- 
 tion. Annals of Bot., XIII., p. 62<X 1899. 
 
 t Ciesielski. Th. Untersuchungen iiber die Abwartskrummung der Wur- 
 zel. Conn's Beitr. z. Biol. d. Pflanzen. Bd. I.. 1871. 
 
 J Sachs, J. von. Gesammelte Abhandlungen, Bd. II.. p. 852. 
 
 MacDougal. D. T. The Curvature of Roots. Bot. Gazette, vol. 23^ 
 1897. 
 
 " Pollock, J. B. Mechanism of root-curvature. Bot. Gazette, vol. 29,, 
 1900. 
 
206 PLANT PHYSIOLOGY 
 
 the changes induced by irritation, whether these changes in 
 direction are first effected by changes in the tissue-tensions 
 of the motor zone or by growth itself. 
 
 What has been said about the irritability and means of 
 response of roots which grow vertically ( orthotropic ) , ap- 
 plies equally to lateral roots which grow obliquely ( plagio- 
 tropic). The difference between these organs lies in their 
 different relations to irritation by gravity. They may not 
 differ in sensitiveness, but they will respond in different 
 degrees to the same force. We may conceive this to be due 
 to the different balances of all the influences to which the 
 two kinds of roots are subject. If both lateral and pri- 
 mary roots grew vertically, they would interfere w r ith one 
 another in position and in functions, failing to attain their 
 utmost usefulness and irritating one another by their prod- 
 ucts. This may be the main factor in determining the po- 
 sition of lateral in relation to primary roots. The response 
 of lateral roots to the stimulus of gravity will be changed 
 when the primary root is removed or so injured that its 
 further vertical growth is impossible. The lateral roots will 
 then bend down and one or more will attempt to take the 
 place in position, direction of growth, etc., of the primary 
 root. The relation of any organ to any one influence or 
 combination of influences depends upon the relation of all 
 the other organs to each one of the influences composing 
 the total to which the organism is subject.* 
 
 Among stems as among roots there are erect (ortho- 
 tropic) and horizontal ( plagiotropic ) organs, differing 
 from one another, not in their sensitiveness or in the effi- 
 ciency of their response to gravity, but in their relation to 
 the force. In the ordinary orthotropic stem, unlike the root, 
 there is no separation into sensory and motor zones, though 
 Francis Darwin f has shown, in the seedlings of certain 
 grasses, that the first leaf-sheath is sensitive to gravitation 
 and the bending is accomplished only by the segment of 
 
 * Consult Czapek, loc. cit., and Schober, Das Verhalten der Neben- 
 wurzeln in der verticalen Lage. Bot. Zeitung, 1898. 
 
 f Darwin, F. On geotropism and the localization of the sensitive re- 
 gion. Annal* of Bot., XIII., 1899. 
 
IRRITABILITY 207 
 
 stem immediately below. In ordinary ortnotropic stems geo- 
 | tropic sensitiveness and response are possible only in elon- 
 gating parts, and since the nodes and internodes soon cease 
 to grow in length, geotropic phenomena soon cease. In 
 such stems the cortical parenchyma, uniform in structure 
 and usually containing chlorophyll, is the sensitive tissue, 
 all the other tissues cooperating in the response to the 
 stimulus. In grass haulms, on the other hand, there being 
 a persistent meristem at the base of each internode and 
 in this no visible differences among the cells, we may as- 
 sume that the whole meristem is sensitive as well as re- 
 sponsive. When a grass haulm is laid prostrate by wind 
 or rain, the meristem cells on the lower side of each inter- 
 node begin again to divide, growth is resumed, and by this 
 means the parts nearer the tip are gradually carried again 
 into the vertical position. 
 
 Prostrate and underground stems and branches, and es- 
 pecially leaves, though sensitive to gravity and assuming 
 very definite positions when acted upon by gravity alone, are 
 so much more strongly affected by light that their position, 
 as well as their form, size, and structure, must be ascribed 
 mainly to its influence rather than to gravity (see pp. 214- 
 215)." 
 
 After the growing (motor) zone has executed the response 
 to the stimulus received by the organ, the whole organ 
 returns, like the sense organs and muscles of an animal, to 
 that state of delicate balance and readiness in which the 
 first stimulus found it. If such a return is made impossible 
 by continued stimulation, the response will be continuous 
 until fatigue, insensibility, and impotence are produced, or 
 until the response carries the organ into such a position 
 that other influences will modify or prevent any further re- 
 sponse. This has been shown by Frank,* Darwin,f and 
 Copeland, t who found that, on confining the sensitive part 
 
 * Frank. A. B. Beitrage zur Pflanzenphysiologie : I. t!ber die durch die 
 Schwerkraft verursachten Bewegungen von Pflanzentheilen. Leipzig, 1868. 
 
 t Darwin. F. On geotropism and the localization of the sensitive re- 
 gion. Annals of Bot.. XIII.. 1899. 
 
 t Copeland. E. B. Studies on the geotropism of stems. Bot. Gazette, 
 XXIX., 1900. 
 
208 PLANT PHYSIOLOGY 
 
 to a horizontal position, an almost continuous bending takes 
 place. Similar to this is Elfving's observation* that when a 
 fully grown grass internode is revolved horizontally on a 
 clinostat, its meristem resumes its activity, its cells divide 
 uniformly under the stimulus of gravity applied successively 
 to all its parts, and the internode passes through another 
 period of growth in length. 
 
 The force which a geotropically bending organ exerts is 
 manifestly considerable, e.g. that employed in restoring a 
 prostrate grass haulm to the vertical position. The force 
 thus exerted is developed by the growing and bending part, 
 and we have seen (p. 176) that growing organs may exert 
 force equalling a pressure of ten atmospheres. As in ordi- 
 nary growth, stems and roots bending because of geotropic 
 stimulation will develop only the amount of force needed to 
 accomplish the bending. This has been proved experiment- 
 ally by Meischke.t He ascertained the force ordinarily 
 exerted by geotropically bending parts, and found that, 
 when compelled to do so, they developed from several to 
 many times this force in order to accomplish the bending. 
 Thus 
 
 Grass internodes can develop 4 times the ordinary bending force. 
 Cucurbita seedlings 13 " 
 
 Lupinus 17 " 
 
 Phaseolus 28 ' 
 
 Helianthus " 30 ' 
 
 If any evidence of geotropic stimulus were needed, more 
 striking than the geotropic curve itself, these figures, in- 
 dicating the increase in power under stimulus, would fur- 
 nish it. 
 
 INFLUENCE OF LIGHT 
 
 Light as a form of energy does certain kinds of work and 
 accomplishes certain chemical changes within as well as out- 
 side living cells. Living protoplasm is composed of chemi- 
 
 * Elfving. F. Verhalten der Grasknoten am Klinostat. Ofversigt af 
 finska Vet. Soc., Forhandlinger, Bd. 26, 1884. 
 
 t Meischke. P. Uber die Arbeit sleistung der Pflanzen bei der geotro- 
 pischen Kriimmung. Jahrb. f. wiss. Bot.. Bd. 33, 1899. 
 
IRRITABILITY 209 
 
 cally unstable substances. Some of its constituents and 
 organs bear also definite physical relations to light. Light 
 is neither constant nor uniform in amount, its direction and 
 intensity change, hence its action upon the same organ or 
 organism is different, may even be opposite, at different 
 times. Light affects the living protoplasm through the 
 chemical processes and physical conditions of the cell 
 at all influenced by light. Sunlight and the ordinary ar- 
 tificial lights are composed of rays of different sorts which 
 separately affect the substances, living and lifeless, exposed 
 to them. The effect of ordinary white light is then the sum 
 of the effects of its constituent rays. These rays, visible and 
 invisible, fall into three classes, the thermal or heat, the 
 luminous or light, and the actinic or chemical rays. Their 
 effects are partly suggested by their names. The luminous 
 rays, being the ones most concerned in food-manufacture, 
 affect the protoplasm in ways which we have already studied 
 (see pp. 53-57). The heat rays we have also examined in 
 their relation to transpiration, etc. (pp. 136-151), and we 
 shall study them further in the succeeding section of this 
 chapter (pp. 219-222). We have here to consider mainly 
 the influence of the visible and invisible (" ultra-violet") 
 chemical rays. 
 
 Some idea as to why and how actinic rays affect living 
 protoplasm so strongly in other words, why living proto- 
 plasm is so very sensitive to light may be gained by con- 
 sidering how powerfully those rays influence lifeless sub- 
 stances and many processes taking place under the more 
 readily understood conditions prevailing outside the living 
 organism. For example, * olive oil oxidizes when exposed to 
 light, oxalic acid in solution will break up into carbon- 
 dioxide and formic acid in the light, alcoholic solutions of 
 chlorophyll decompose more rapidly in light than in dark- 
 ness, and many enzyms are destroyed by exposure to light. 
 These are all organic compounds occurring in plant cells. 
 The most significant in the list are the enzyms. Enzyms 
 accomplish the conversion of insoluble substances in the 
 
 * Davenport. C. B. Experimental Morphology. Part I., pp. 162-5, 
 1897. 
 
 14 
 
210 
 
 PLANT PHYSIOLOGY 
 
 cell into soluble, portable, and useable compounds (see 
 pp. 29-30). If the work of the enzyms is interfered with 
 by light, the functions of the cell must be altered or become 
 deranged. The substances composing the living protoplasm 
 may also be as sensitive to light as its products. The sensi- 
 tiveness to light of the com- 
 pounds in and perhaps compos- 
 ing their cells is the reason for 
 the sensitiveness of living organ- 
 isms to light. 
 
 The influence of light, like the 
 influence of gravity, may show 
 itself in the kind, rate, and direc- 
 tion of growth, and in the posi- 
 tion of the organ and organism. 
 Plants kept in darkness will be 
 longer than plants under nor- 
 mal illumination. Seeds sprouted 
 in darkness, and seedlings grow- 
 ing where no light falls upon 
 them, grow under the influences 
 of all other forces than light. 
 When the influence of light is 
 wholly eliminated, root, stems, 
 and branches grow longer but 
 are proportionally more slender 
 than in ordinary sunlight, the 
 leaves are smaller and weaker, 
 flowers do not form (see pp. 
 271-4). That stems are more 
 slender and mechanically weak- 
 er in darkness than in light 
 may be due to the less than nor- 
 mal weight of the small weak leaves to the absence of me- 
 chanical strain (see p. 188). The leaves require a certain 
 amount of light in order properly to develop the food- 
 manufacturing tissues. Yet, comparing the total growth of 
 plants of the same species in light and in darkness, it will 
 be clear that the growth is greater in darkness so long as 
 
 Figure 15. Branch of Cactus, 
 the young parts of which grew 
 in darkness. (From Goebel.) 
 
IRRITABILITY 211 
 
 the plant is adequately nourished. Light retards the 
 growth of those organs which do not depend upon it for 
 energy for food-manufacture. 
 
 The form as well as the size of parts is greatly influenced 
 by light. If a branch of cactus, Opuntia, be potted and 
 kept in darkness, the buds will develop into branches which 
 are small and cylindrical, while the branches formed in the 
 light are larger and flat.* Marchantia ' gemmse, germinated 
 in the light but on a clinostat, develop in two months into 
 small erect tubular plants, wholly unlike the flat expanded 
 thalli which form when light and gravity act normally.! 
 
 The daily periodicity of light and darkness are almost 
 coincident with the daily periodicity in growth-rate. This 
 was first shown by Sachs, J who devised machines (auxan- 
 ometers ) for autographically recording the growth of plants 
 during extended periods. These records show that the rate 
 of growth in length, of plants furnished with all the food 
 they need, will reach its maximum about sunrise, its mini- 
 mum about sunset. That growth is not most rapid when 
 there is least light, or slowest when the light is strongest, 
 shows that the effect of light, like that of gravity, is cumu- 
 lative (p. 204). Maximum growth-rate and minimum 
 temperature almost coincided in Sachs's experiments, but 
 this indicates, not that coolness favors growth, but that 
 Sachs did not make his experiments at constant tempera- 
 tures. Sachs's experiments, repeated at constant tempera- 
 tures, show that the maximum and minimum growth-rates 
 occur at approximately the same times as before, and that 
 light and not heat is the predominant influence in con- 
 trolling the rate of growth. 
 
 It is claimed that light favors the growth of the vegeta- 
 tive organs of submersed aquatics. A certain minimum 
 
 * Vochting, H. Uber die Bedeutung des Lichtes fiir die Gestaltung 
 blattformiger Kakteen. Jahrb. f. wise. Bot., Bd. 26, 1894. Goebel, K. 
 Organographie, I., pp. 211+, 1898. Also Flora. Bd. 80, 1895. 
 
 f Czapek, F. 7. c., 1898. p. 261. 271, etc. 
 
 J Sachs, J. von. Uber den Einfluss der Lufttemperatur und des Tag- 
 eslichtes auf die stiindlichen und taglichen Anderungen des Langenwachs- 
 thums (Streckung) der Internodien. Arb. d. Bot. Inst., Wiirzburg, 1872. 
 Gesamirelte Abhandlungen, Bd. II.. 1893. 
 
212 
 
 PLANT PHYSIOLOGY 
 
 amount of light is required by them to carry on the essen- 
 tial process of food-manufacture. A larger amount acts as 
 a stimulus to the formation of reproductive organs (pp. 
 264-8 ) . Otherwise the relations of these plants to light are 
 similar to those of land plants. Since the water and the 
 materials at the bottom of a natural body of water absorb 
 
 
 
 
 
 1000 
 
 [umber per c.c. 
 2000 
 
 3000 
 
 4C 
 
 00 
 
 
 
 
 o 
 
 
 
 2 
 
 In'te 
 4 
 
 nsity of lig 
 60 
 
 lit 
 80 1 
 
 "&*+ 
 
 
 
 
 
 2 
 
 
 
 
 
 -^ 
 
 
 
 
 
 
 
 4 
 
 
 
 
 ^' 
 
 f 
 
 
 
 
 
 
 
 6 
 
 
 
 y 
 
 / 
 
 
 
 
 
 
 
 1 
 
 8 
 
 
 
 J 
 
 FIGURE 16. DIAGRAM SHOWING THE GROWTH OF 
 DIATOMS AND THE INTENSITY OF LIGHT 
 
 ,s 
 
 1 
 
 
 
 
 
 1 
 
 3 
 
 2 
 
 
 
 III 
 
 AT VARIOUS DEPTHS. 
 
 Water in Lake Cochituate, Massachusetts, Nov. 29, 
 1895. Examined Dec. 9, 1895. Temperature 40-44. 
 Color 0.33 (pt. standard) . Diatom schiefly Asterion- 
 ella and Melosira. Intensity of light at different depths 
 calculated on assumption that layer of aq. 1 ft. in depth 
 absorbs 35 per cent, of light falling upon it. 
 (AfterWhipple). 
 
 
 4 
 
 
 i/i 
 
 
 6 
 
 
 
 
 8 
 
 
 
 
 10 
 
 _ 
 
 4 
 
 
 
 
 
 
 light, plants living in such a situation receive less light than 
 the plants living on the bank near by. Plants living on or 
 over a dark or black bottom receive light only or mainly 
 from almost vertically above. The quality as well as the 
 quantity of light reaching water plants differs from that 
 falling upon plants in the air. Water absorbs the different 
 kinds of rays unequally, the luminous rays least, the actinic 
 rays more. Whipple* cultivated diatoms in bottles at dif- 
 
 * Whipple, G. C. Some experiments upon the growth of diatoms. Tech- 
 nology Quarterly, vol. IX., 1896. Also Microscopy of Drinking Water, 
 1901. 
 
IRRITABILITY 213 
 
 ferent depths in a reservoir. Taking the numbers present 
 as an index of the suitableness of the illumination at differ- 
 ent depths a standard not above criticism the largest 
 number, and inferably the best illumination, are found a 
 few inches below the surface. The accompanying diagram 
 indicates the results of the experiment. On the surface, 
 where there was no absorption of light, and where the 
 organisms could therefore receive most light, the growth ( 7.^. 
 the reproduction ) was less than when a thin layer of water 
 absorbed the actinic rays. 
 
 Certain seeds and spores appear to germinate better, and 
 perhaps only, in light. * The European mistletoe ( Viscum 
 ,'ilbuin), the seeds of some grasses, and the spores of some 
 of the vascular cryptogams are said not to germinate in 
 darkness. In these cases we have the stimulating influence 
 of light upon processes which precede growth ( e.g. respira- 
 tionf ) . After their germination, the influence of light upon 
 growth is the same as in other plants. 
 
 The influence of light upon the rate of growth is evident 
 from the foregoing; light lowers the growth-rate. If un- 
 equal amounts of light fall on different parts of a growing 
 organ, the growth of the organ will be unequal. By this 
 means the direction of growth of an organ or organism is 
 influenced by light. The stems of plants growing indoors 
 near a window turn toward the light, the leaves spread 
 themselves at right angles to the incident rays, the roots 
 (when visible at all) turn away from the light. The be- 
 havior of the parts of such plants is readily accounted for 
 on the basis of what has already been said. The growing- 
 cells on the side of the stem away from the window receive 
 less light and are less checked in growth than those on the 
 opposite side, and hence push the tip of the stem over to- 
 ward the window. The influence of light upon the direction 
 of growth, though we see that it is due to the effect of light 
 on the rate of growth, is known as heliotropism or photo- 
 tropism. 
 
 * Davenport, C. B. Experimental Morphology, Part II., pp. 423 - 5, 1899. 
 f Kolkwitz. R. t^ber den Einfluss des Lichtes auf die Athmung der 
 niederen Pilzen. Jahrb. f. wise. Bot., Bd. 33. 1898. 
 
214 PLANT PHYSIOLOGY 
 
 Most stems are positively heliotropic, that is, bend toward 
 light of ordinary intensity and composition. The English 
 Ivy (Hedera), to a certain extent Ampelopsis, the touch- 
 me-not (Impatiens), the garden nasturtium in its older 
 stages (Tropceolum majus),* and some other plants, are 
 negatively heliotropic toward intense light, though not to 
 light of lower intensity. These are plants which either 
 climb against objects which absorb much light and hence 
 appear dark, or thrive best in comparative shade. The 
 behavior of the roots which attach these climbers to their 
 supports is consistent with the relation of most roots to 
 light. 
 
 Roots are not markedly sensitive to light, light serving, 
 as a rule, only to supplement gravity. So much more 
 does gravity influence the direction of growth of roots that 
 the influence of light is scarcely apparent! until all parts 
 are uniformly subjected to gravitation by means of the 
 elinostat. 
 
 Underground and other horizontally growing stems exhibit 
 a peculiar adjustment to the influences of gravity and light, 
 their position being the resultant of their reactions to these 
 two opposite stimuli. The prostrate shoots of Rubus, J the 
 runners of strawberry, etc., and the root-stocks of Nuphar 
 are examples. Goebel shows that the root-stock of Nuphar 
 luteum creeps horizontally in the mud, possessing at this 
 time a somewhat dorsi-ventral structure. If covered with 
 soil, such a rhizom would bend, grow vertically upward 
 until it came to the light, the structure of the vertical part 
 being radial. At the surface of the soil it would bend to 
 the horizontal and grow through the muddy twilight as 
 before. 
 
 The position and direction of growth of leaves is also a 
 resultant of the two forces, gravity and light. If a plant is 
 
 * Wiesner, J. Die heliotropischen Erscheinungen. Denkschrift der Wiener 
 Akadernie, Bd. 39 u. 43, 1878 u. 1880. 
 
 f Wiesner, J. /. o. 
 
 $ Czapek, Fr. 7. c. 1898, p. 245, 257, etc. 
 
 Goebel, K. Organographie der Pflanzen, I., p. 198, 1898. Stahl, E. 
 Einfluss des Lichtes auf den Geotropiemus einiger Pflanzenorgane. Ber. d. 
 Deutsch. Bot. Gesellsch., Bd. II.. 1884. 
 
IRRITABILITY 215 
 
 revolved horizontally around its long axis on a clinostat, 
 thus exposing it uniformly on all sides to gravity and to 
 light, its leaves will assume no uniform and definite posi- 
 tion. However, when the plant is illuminated from one di- 
 rection only while it is being revolved on the clinostat, the 
 leaves will take a very definite position, such that the blades 
 are at right angles to the incident rays. 
 
 Light and gravity work very closely together in determin- 
 ing the direction, size, and even form into which organs 
 and organisms grow. When one influence is reduced or ex- 
 cluded, e.g. light, there is a change in the organism in its 
 relations not only to the influence excluded but also to 
 those that remain. The influence of the one force or the 
 other may predominate in one set of organs as the roots 
 are affected mainly by gravity, the leaves mainly by light 
 or they may be nearly or quite balanced. Profoundly as 
 they affect the parts sensitive to them, these forces are 
 only factors in the complex of influences, mechanical, nutri- 
 ent, and other, which together determine the character of 
 the organism. No figures as to the relative effectiveness of 
 gravity and light acting upon the whole organism can be 
 given, yet Czapek* presents some figures of the same objects 
 under like conditions, indicating the minimum times of ex- 
 posure for reactions to gravity and light. 
 
 GEOTROP. HELIOTROP. 
 
 Avena sativa, ( etiolated ) 15 minutes 7 minutes 
 
 Sinapis alba, hypocotyl (etiolated) 15 " 10 " 
 
 Beta vulgaris, " 15 " 10 " 
 
 Zea Mais, primary root 20 " 
 
 Helianthus annuus, hypocotyl 20 ". 20 " 
 
 Phaseolus multiflorus, epicotyl 50 " 50 " 
 
 Phy corny ces nitens, sporangiophore 15 " 7 
 
 The direction of locomotion in motile plants is influenced 
 more by light than by any other one influence except that 
 of food or of other stimulating compounds. An organism 
 
 * Czapek, F. Weitere Beitrage zur Kenntniss der geotropischen Reiz- 
 bewegungen. Jahrb. f. wiss. Bot.. Bd. 32, p. 185, 1898. 
 
216 PLANT PHYSIOLOGY 
 
 able to move about will finally reach the position in which 
 it is most favorably affected by all the influences to which 
 it is susceptible. Other positions might be more favorable 
 for separate influences, but, the influences continuing con- 
 stant, the position occupied may be regarded as the best. 
 If the influences remained constant we could conceive that 
 the sensitive motile organism, which had come into the 
 place and position in which the balance of stimuli was most 
 favorable, would never move again. The influences are all 
 changing, except gravity, and hence the organism is exposed 
 to changing degrees and kinds of stimulation. We may 
 imagine that the movements of freely motile organisms 
 indicate three things the rate at which the stimuli change, 
 the degree of sensitiveness, and the rate of response of the 
 organism. The stimuli may change very rapidly,. but a dull 
 or feeble organism will not move correspondingly fast. 
 Conversely, an extremely sensitive and strong organism will 
 be in rapid motion even when conditions are in the main 
 unchanged. But since conditions are not absolutely identi- 
 cal at two different points, locomotion itself introduces the 
 organism to new stimuli. 
 
 Diatoms, desmids, Oscillatoria, swarm-spores, ordinary 
 bacteria, sulphur bacteria ( both the red and the colorless ) , 
 plasmodia, and many other motile organisms, move toward 
 light until they reach the point of optimum illumination. 
 Locomotion directed by light is called phototaxis or helio- 
 taxis. Movement toward the light is said to be positive 
 phototaxis, away from the light negative phototaxis. Most 
 organisms are both positively and negatively phototactic, 
 depending upon the degree of illumination to which they are 
 exposed. Thus the various species of red sulphur bacteria 
 now growing in my laboratory collect on the brightest 
 parts of the jars in which they are living when the jars are 
 on a table six feet from the nearest window, and on the less 
 or least brightly lighted parts when the jars are on the 
 window-sills. 
 
 Where locomotion, or at least a change in position of the 
 whole organ or organism, is impossible, the organs of the 
 cell are, in many cases, able to move in accordance with the 
 
IRRITABILITY 
 
 217 
 
 intensity and direction of the light. This is shown most 
 clearly by the movements of chromatophores. * The ac- 
 companying figures a, b, c, were drawn on a warm sunny 
 morning from leaves of a moss ( Funaria ) growing under a 
 bell-glass in the laboratory. Figure a is from a leaf taken 
 from a plant when the dish was on a table six feet from the 
 nearest window : the chlorophyll grams are against the 
 upper and lower walls of the cells, presenting their maximum 
 area to intercept the light. This is the typical daylight posi- 
 tion. Figure b is from a leaf taken from a plant after the 
 
 Figure 17. Cells from leaves of Funaria Sp. 
 a cell from leaf in ordinary diffuse daylight, 
 b position of chlorophyll-grains in intense light, 
 c position of chlorophyll-grains in darkness. 
 
 dish had been standing 5 to 10 minutes in brilliant sun- 
 shine on the window-sill inside the laboratory : the chloro- 
 phyll grains are against the lateral walls of the cells, pre- 
 senting their minimum area to intercept the light. Figure 
 c is from a leaf which had been kept in the dark for 5 to 10 
 minutes ; the chlorophyll grains are against the lateral and 
 bottom walls of the cells. These figures are drawn at the 
 same magnification, but the cells and chromatophores differ 
 considerably in size. Changes in position of the chlorophyll 
 grains take place very rapidly, and the drawings must be 
 
 * Frank, A. B. Uber die Veranderung der Lage der Chlorophyllkorner 
 und des Protoplasmas in der Zelle und deren innere und aussere Ursachen. 
 Jahrb. f. wiss. Bot., VIII., 1872. Stahl, E. Uber den Einfluss von Rich- 
 tung und Starke der Beleuchtung auf einige Bewegungserscheinungen im 
 Pflanzenreiche. Bot. Zeitung, 1880. Oltmanns, F. Uber photometrische 
 Bewegungen der Pflanzen. Flora, Bd. 75, 1892. 
 
218 
 
 PLANT PHYSIOLOGY 
 
 made rapidly and at once, else changes of considerable ex- 
 tent occur. Within two minutes after the darkened leaf is 
 put on the microscope, the chlorophyll grains have gone 
 halfway to the day position (a). 
 
 The behavior of the plate-like chromatophores of the alga 
 Mesocarpus is similar.* When intense light falls upon a 
 
 Figure 18. Cells of Mesocarpus Sp. 
 a cell with chromatophore in diffuse daylight position, 
 b chromatophore bending in too intense light, 
 c chromatophore bent to reduce absorption of light. 
 
 Mesocarpus cell, the chromatophore presents only its edge 
 to the incident rays, but when the light is less intense the 
 chromatophore bends, as in c, so that the greater part 
 of it presents its profile to the light. Still less intense yet 
 strong light induces the position indicated in b, and light 
 of only moderate intensity or less is indicated by the chro- 
 matophore being expanded at right angles to the incident 
 rays as in a. The chromatophores change their positions 
 only with a change in the direction or the intensity of the 
 illumination, but under favorable conditions the changes 
 can be accomplished rapidly. 
 
 Whether heliotropism and heliotaxis are or are not funda- 
 mentally similar phenomena there is no experimental evi- 
 dence for determining. The effect of light upon the cell is 
 not precisely known, though it is probable that it chemi- 
 
 Oltmanns F. 1. c.. p. 209. 
 
IRRITABILITY 219 
 
 cally affects substances contained in the cell, and that the 
 changes in these stimulate the protoplasm (see p. 210). 
 The protoplasm of one or more cell-members of a tissue may 
 react by changing the form, size, turgor, or some other 
 quality of the cells, thus causing a change in the form or 
 direction of an o^gan ( heliotropism ) . This change may be 
 made permanent by growth without being directly accom- 
 plished by growth. The same change in form, size, turgor, 
 or other quality of isolated cells may result in locomotion 
 ( heliotaxis ) . In both instances the effect of light might be 
 intrinsically the same, the reaction of the protoplasm to 
 chemical stimulation also the same, the result being different 
 only because one cell could move freely while the other was 
 obliged to move more than itself.* 
 
 INFLUENCE OF HEAT 
 
 The degree of heat prevailing in the medium air, water, 
 soil in which a plant is growing, and the radiant heat fall- 
 ing upon the plant and its immediate surroundings, exert 
 definite influences upon its activities. Without its minimum 
 degree of warmth, the plant will survive only in a resting 
 condition, if at all. There is also a maximum temperature 
 which can be withstood only in the resting condition. Ap- 
 parently there is no temperature so low as to be fatal to 
 resting protoplasm seeds, spores, etc. and the maximum 
 for dry resting protoplasm is surprisingly high. There are, 
 however, minimum, optimum, and maximum temperatures 
 for active protoplasm, which are definite and definitely re- 
 lated to each other. Organisms living where the mean 
 temperature is low have low minimum and maximum tem- 
 peratures. The native plants of warm situations have cor- 
 respondingly high extreme temperatures, and plants accus- 
 tomed to living where there is only slight variation in 
 temperature can be cultivated only under similar con- 
 
 * Loeb, J. Zur Theorie der physiologischen Licht und Schwerkraft- 
 wirkungen. Archiv f. d. gesam. Physiologic Bd. 66. 1897. Einleitung in 
 die vergleichende Gehirnphysiologie und vergleichende Psychologic. Leipzig, 
 1899. 
 
220 PLANT PHYSIOLOGY 
 
 ditions. These statements are illustrated by the following 
 
 table : * 
 
 
 
 
 
 min. 
 
 opt. 
 
 max. 
 
 Hydrurus foetidus 
 
 
 
 10 
 
 below 16 
 
 Ulothrix zonata 
 
 C 
 
 below 15 
 
 24 
 
 Zea mais 
 
 9 
 
 34 
 
 46 
 
 Cucurbita pepo 
 
 14 
 
 34 
 
 46 
 
 Bacillus thermophilus 
 
 42 
 
 63-70 
 
 72 
 
 " tuberculosis 
 
 30 
 
 38 
 
 41 
 
 It is greatly to be regretted that those organisms which 
 apparently transgress the general laws expressing the rela- 
 tions of active protoplasm to heat have been so little 
 studied by physiologists. The vegetation of geysers and 
 hot-springs, and of the very cold waters from glaciers and 
 persistent snow-deposits, has been studied too exclusively by 
 systematists. The temperatures of the waters are known 
 only in a general way, the temperature of the water actu- 
 ally bathing the organisms and the exact condition of the 
 organisms have not been accurately determined.! Each 
 actively fermenting manure-heap, each mass of organic 
 matter so rapidly decomposing that high temperatures 
 develop in it (e.g. where hay "heats" in cock or mow), is 
 the seat of bacteria, the optimum temperatures of which are 
 above the maxima for most forms. The difficulty of suc- 
 cessfully imitating the natural conditions hampers experi- 
 mental study of these organisms. 
 
 Continental bodies of land are subject to considerably 
 wider extremes of temperature than prevail in oceanic 
 islands and in certain limited and peculiarly situated areas 
 near the coast. These show limited ranges of temperature, 
 but it is possible to cultivate there plants which naturally 
 occur much farther north or south. This is partly the rea- 
 son why semi-tropical and decidedly northern plants can be 
 successfully cultivated in the region about the Bay of San 
 
 * See Pfeffer's Pflanzenphysiologie, Bd. II., p. 87, 1901. 
 
 f Davis, B. M. Vegetation of the hot-springs of the Yellowstone Park. 
 Science. VI., 1897. Tilden, J. E. Observations on some West American 
 thermal algre. Bot. Gaz., vol. 25. 1898. 
 
IRRITABILITY 221 
 
 Francisco, in California, for there the respective minimum 
 and maximum temperatures of these forms are never at- 
 tained. The optimum temperatures for most plants in this 
 region are maintained for only very short times, owing to 
 the great daily range in temperature, but are frequently 
 repeated, owing to the slight seasonal ranges. The frequent 
 occurrence of optimum temperatures may more than com- 
 pensate for their brief duration a matter, however, which 
 requires proof. 
 
 Movements and locomotion, as well as the rate of growth, 
 are more or less controlled by heat. Thermotropism and 
 thermotaxis are the irritable responses to the stimulus 
 of heat. A certain degree of warmth will induce positive 
 thermotropism or thermotaxis respectively, growth and 
 locomotion towards the source of heat while a higher or 
 lower degree of heat may induce the opposite, negative 
 thermotropism or thermotaxis. Roots, for instance, grow 
 toward but not into contact with steam or hot-water 
 pipes passing through the soil between buildings. Roots of 
 Indian Corn* will grow toward the source of heat till they 
 reach a point where the temperature is 37.5 C. They will 
 bend and grow away from any warmer part of the soil. 
 Ordinarily in nature, however, thermotropic sensitiveness is 
 of little advantage to the plant. There is no differentiation 
 into sensory and motor zones, the whole growing region 
 being equally sensitive. A certain degree of warmth is re- 
 quired in order that the organism may be sensitive to other 
 influences. Thus heat must arouse the parasitic dodder 
 ( Cuscuta) to a certain degree of sensitiveness else it will not 
 be able to respond to the contact stimulus ( see pp. 244-5 ) 
 upon which its attachment to the host absolutely depends, f 
 
 The sensitiveness and the response of motile organisms to 
 heat is manifestly important. The sensitiveness to heat 
 
 The organs especially active in absorbing nutrient aqueous 
 
 * Wortmann, J. Uber den Einfluss der strahlenden Warme auf wachsende 
 Pflanzentheile. Bot. Zeitung, 1883. Tiber den Thermotropismus der Wurzel. 
 Bot. Zeitung, 1885. 
 
 fPeirce, G. J. A contribution to the physiology of the Genus Cuscuta. 
 Annals of Bot.. Vol. VIII., 1894. 
 
222 PLANT PHYSIOLOGY 
 
 which causes a man who is cold to approach the fire but 
 not to draw too close, and when w r armed to move to a 
 slightly cooler spot, is not peculiar to him or to higher 
 animals. This example of thermotaxis expresses the sensi- 
 tiveness of all living protoplasm and the habit of all motile 
 organisms to seek positions which are most comfortable 
 because most favorable to the accomplishment of their vital 
 functions. Thus slime-moulds* and zoosporest are known 
 to move toward sources of heat. 
 
 Whether, corresponding with the influence of light, the 
 direction of movement of the internal organs of the cell is 
 affected by the direction of the source of heat seems not 
 to have been studied. The rate of movement of the 
 organs of the cell in relation to the degree of warmth has 
 been carefully examined. For example, the chlorophyll 
 grains in Elodea, Valisnerhi, and Chara circulate with in- 
 creasing rapidity up to a temperature of 34 to 38 C., but at 
 44 all movement ceases, though it will be resumed when 
 the temperature is lowered.]: The temperature at which the 
 protoplasm circulates most rapidly in these plants is near 
 the optimum for most of the activities of the cell. 
 
 INFLUENCE OF WATER 
 
 A sufficient quantity of water is a necessary condition 
 for active life (p. 6 ) . Water is also an essential component 
 of the living structure, protoplasm (p. 7). Given the 
 minimum amount of water for the erection of the living 
 structure after a period of inaction, the organism will re- 
 sume its functions. The germination of seeds and spores 
 shows this. Increase in the amount of water from the 
 minimum to the optimum is followed by increase in all the 
 vital activities. This is consistent with chemical and physi- 
 cal experience in the laboratory, a minimum quantity of 
 water being indispensable to many reactions, a larger ( opti- 
 
 * Stahl, E. Zur Biologic der Myxomyceten. Bot. Zeitung. 1884. 
 
 f Strasburger, E. Wirkung des Lichtes und der Warme auf Schwarm- 
 sporen. Jenaischer Zeitschrift, 1878. 
 
 % Velten. W. Einwirkung der Temperatur auf die Protoplasmabewegung. 
 Flora, 1876. 
 
IRRITABILITY 223 
 
 mum ) amount causing them to take place more rapidly or 
 more energetically. There are, therefore, minimal and opti- 
 mal quantities for the ptrvsical states and the chemical 
 processes in the living cell and the living organism, but 
 until water is sufficient in amount to become the vehicle of 
 mechanical forces, there cannot be said to be any maximum 
 quantity. The minimum and optimum vary with the or- 
 ganism. 
 
 An organism will absorb water up to a given amount, 
 but it can absorb no more (see p. 110) because the sub- 
 stances which it contains do not offer the physical condi- 
 tions making further absorption possible. The absorption 
 of water gives rise to changes in the physical conditions pre- 
 vailing within the cells ( pp. 210-11 ) , the changes manifesting 
 themselves most conspicuously in an increase in volume or 
 pressure, or both. These physical changes directly affect the 
 living protoplasm. The greater pressure or larger volume 
 of individual cells enables them to exert greater mechanical 
 force upon their surroundings and to occupy more space ; it 
 enables the organ which they compose to expand or to come 
 into a more favorable position, etc. Besides these mechani- 
 cal effects of water, which influence the living protoplasm, 
 the greater or less quantity of water in the cell exercises a 
 direct physiological effect upon the activities of the cell. 
 When the plant or the cell needs water, its activities will be 
 reduced, within certain limits, in proportion. Conversely, 
 any increase, under these conditions, hi the amount of water 
 available will stimulate the plant or the cell. 
 
 Water exerts two distinct influences upon the plant. This 
 can best be shown by examining the directive influence of 
 water upon growth. First, let the downward-growing pri- 
 mary root of a seedling growing in air not excessively damp 
 be exactly measured as to diameter and length by the eye- 
 piece micrometer of a horizontal microscope; then plunge 
 this root into water of the same temperature as the air and 
 note the increase in dimensions which promptly follows. 
 This is a purely mechanical effect, swelling. Again, let seed- 
 lings be grown in any apparatus which will expose the 
 roots to unequal amounts of moisture on opposite sides ; for 
 
224 PLANT PHYSIOLOGY 
 
 instance, Sachs's familiar cylinder of wire-netting filled with 
 damp moss or moist earth.* If this cylinder be hung 
 slanting at an angle of about 45, the roots will not grow 
 vertically downward away from the cylinder, but will follow 
 along its under side in spite of the geotropic stimulus (p. 
 204). In this experiment the roots bend in the direction 
 from which they obtain water. In the first experiment the 
 absorption of water is followed by expansion, swelling. 
 In the second experiment that side expands which is not 
 able directly to absorb water, while that side which does 
 absorb water does not expand as much. In the first 
 experiment the physical influence is uniform on all sides 
 of the root, which swells uniformly. In the second ex- 
 periment, the physical influence was greater on one side 
 than on the other, the tendency to swell on one side 
 was offset by the increased activity of the protoplasm, 
 the more rapid growth, of the opposite side, in response to 
 stimulus. Such a reaction as this is called hydrotropism. 
 
 The hyphse of Mucor grown on moist bread over water 
 are deflected when they reach the surface of the water, grow- 
 ing instead horizontally over it and against the bread. 
 This bending away from water is not negative hydrotro- 
 pism, but rather the reaction of the hyphae to other stim- 
 uli such as food, oxygen, etc., which are more abundant in 
 the bread than in the water. 
 
 Sachs's experiment (see above), with roots deflected by 
 moisture from the vertical position, reveals a very important 
 principle. It is manifestly more important for the root as 
 an absorbing organ to grow where it will obtain needed 
 aqueous solutions than for it to be in any particular posi- 
 tion with relation to the vertical; but in order that the 
 older parts of the root may give the needed mechanical 
 support to the plant, the root must grow downward into 
 the soil. There must therefore be a delicate balance in the 
 sensitiveness and the reactions to geotropic and hydro- 
 tropic stimuli. 
 
 * Sachs, J. von. Uber die Ablenkung der Wurzeln von ihrer normalen 
 Wachsthumsrichtimg durch feuchte Korper (Hydrotropismus). Arb. d. 
 bot. Inst. Wiirzburg, Bd. I., 1871-4. Gesam. Abhandl., Bd. II., 1893. 
 
IRRITABILITY 225 
 
 solutions roots, rhizoids, * pollen-tubes, f etc. are the ones 
 most sensitive to differences in the proportions of water in 
 their environment. Other organs are relatively or quite 
 indifferent. Most stems are not hydrotropically sensitive, 
 but the peculiar stem of the parasite, dodder (Cuscuta), 
 which must twine closely about living plants and extract 
 nourishment from them, is somewhat sensitive.! 
 
 The size and number of root-hairs is inversely propor- 
 tional to the ease with which the plant obtains the water it 
 needs. The difficulty of absorbing water in sufficient quan- 
 tity to meet the demand made by transpiration and evapo- 
 ration ( see p. 141 ) from the aerial parts acts as the stim- 
 ulus to the young epidermal cells of the root to grow larger and 
 into such form that they will be able more readily to absorb 
 the water held on the surfaces of the soil particles. An 
 amount of water in the soil less than the optimum is fol- 
 lowed by elongation of the epidermal cells, and they grow 
 out as hairs. Seeds germinated in moist air instead of in 
 damp soil develop a larger number of still longer hairs. On 
 the other hand, the roots of seedlings growing in water are 
 nearly or quite bare of hairs. The stimulus to the produc- 
 tion of root-hairs may develop within the plant itself. It 
 may consist in the osmotic withdrawal of water from the 
 epidermal cells to make good the losses above. In any case, 
 water exerts the stimulus to growth, and the living proto- 
 plasm reacts to the stimulus. 
 
 The root-hairs grow toward and around soil particles. 
 These hold films of water on the surface. The growth of the 
 root-hairs toward the soil particles is evidently positively 
 hydrotropic. The growth of the hairs around the particles 
 may be the combined result of the stimulus of contact with 
 the solid (see p. 241) and of water. 
 
 The opposite result, the growth of parts away from water, 
 
 * Molisch, H. Untersuchungen iiber den Hydrotropismus. Sitzb. d. K. K. 
 Akad. der Wiss v Wien, 1884. 
 
 f Miyoshi, M. Uber Reizbewegung der Pollenschlauche. Flora, Bd. 78, 
 1894. 
 
 J Peirce, G. J. Contribution to the physiology of the Genus Cuscuta. 
 Annals of Botany, vol. VIII. . 1894. 
 
226 PLANT PHYSIOLOGY 
 
 is shown by the sporangiophores of some of the fungi. It 
 may well be that the fruiting organs of other plants depen- 
 dent upon dry air for the best dispersal of their seeds or 
 spores are also negatively hydrotropic. * 
 
 Hydrotaxis is exhibited by many of the slime-moulds, for 
 example .Ethalium (Fulligo). Stahlt shows that the dis- 
 tribution of these plants on and in the rotting wood, etc. 
 where they live, corresponds to the distribution of water 
 there; they are positively hydrotactic. 
 
 The locomotion of such plants as are motile at any time 
 is dependent upon water, for this is the only medium 
 through which they can propel themselves. Many other 
 movements than those of locomotion are also dependent 
 upon water; the withdrawal of water, or variation in the 
 amount of water, may cause purely physical changes in cell- 
 walls or dead tissues which result in the exercise of me- 
 chanical force. These hygroscopic, movements, such as the 
 opening of the peristome in mosses, the action of the elaters 
 of liverworts and Equisetum, the opening of fern-sporangia, 
 .the curling of the awns of Geraniaceae, the splitting open 
 of pods, etc., etc., are free from the action of living proto- 
 plasm, and are mentioned in connection with irritable phe- 
 nomena only to make the difference clear. 
 
 INFLUENCE OF OTHER SUBSTANCES THAN WATER 
 
 In the whole range of chemical substances there are ex- 
 tremely few which never in any way affect living protoplasm. 
 Many substances, however, affect protoplasm only mechani- 
 cally as obstacles in the way of its growth or movement, 
 or, like the nitrogen of the air, which blows against and 
 subjects it to strain, etc. (For the effects of these see p. 
 189). Unless a substance is soluble in water it can af- 
 fect protoplasm in no other than mechanical ways. Of the 
 soluble substances some affect protoplasm favorably, others 
 unfavorably, others not at all. When an effect is produced, 
 
 * Preliminary experiments on Fimhriaria California, lead me to believe 
 that the sporophyte of this and of other liverworts (e. g. Anthoceros) 
 may be negatively hydrotropic. 
 
 f Stahl, E. Zur Biologic der Myxomyceten. Bot. Zeitung, 1884. 
 
IRRITABILITY 227 
 
 it may be purely physical or it may be chemical, involving 
 some change in composition. The same substance in differ- 
 ent concentrations, may produce favorable or unfavorable 
 effects or apparently none, and the effects may be first 
 physical and then chemical, or vice vei'sa. The effect pro- 
 duced is dependent upon, 1st, the acting substance, 2d, the 
 organism, 3d, the conditions prevailing at the time. The 
 organism will vary at different times in its activities, con- 
 tents, and composition, and will therefore vary in its reac- 
 tion or response to external influences. 
 
 In studying the influence of gravity upon plants, w r e were 
 dealing with a constant force and a varying organism. In 
 studying the influence of light, we were dealing with a vary- 
 ing force as well as var} r ing organism, but the force pro- 
 duces its effects always by the same means. In studying the 
 influence of water, we were dealing with a single, simple 
 compound varying in quantity, but in its effects only 
 because of this fact and of the varying organism. In study- 
 ing the influence of other substances upon protoplasm 
 we have to deal with their relations to water and to each 
 other as well as to the organism. This materially compli- 
 cates the subject. The effects of common salt illustrate 
 these several points. A very dilute solution of common salt 
 (Nad) acts unfavorably upon plants the roots of which 
 are bathed in it. The sodium and chlorine atoms become 
 dissociated and act as independent poisons. Whether the 
 poisonous action of these atoms is due to their own chemi- 
 cal properties or to their electrical charges is not certain 
 and at all events need not now be discussed.* A solution of 
 such concentration that the atoms are not dissociated and 
 that molecules remain intact will have no effect. Still fur- 
 ther concentration will cause plasmolysis of the living cells. 
 
 * Kahlenberg. L.., and True, R. H. On the toxic action of dissolved 
 salts and their electrolytic dissociation. Bot. Gazette, XXII., 1896. True, 
 R. H. The physiological action of certain plasmolyzing agents. Bot. Ga- 
 zette, XXVI., 1898. Heald. F. D. On the toxic effect of dilute solutions 
 of acids and salts upon plants. Bot. Gazette, XXII., 1896. Loeb. J. On 
 ion-proteid compounds and their role in the mechanics of life-phenomena. 
 I. The poisonous character of a pure NaCl solution. Amer. Journ. 
 Physiol., vol. III.. 1899-1900. 
 
228 PLANT PHYSIOLOGY 
 
 This in itself is injurious, though the cells may recover their 
 turgor, becoming adjusted to the greater density. Plants 
 continually exposed in nature to salt solutions of considera- 
 ble density present a very different appearance from those 
 growing under more usual conditions.* They are usually 
 fleshier and coarser, often more compact in habit, than ordi- 
 nary land plants ( compare Solidago sempervirens with the 
 other Solidagos of New England, the marsh grasses with 
 those of meadows, etc.). 
 
 The theory of electrolytic dissociation of the atoms of 
 substances dissolved in proportionally large volumes of ' 
 water is extremely useful to the physicist and chemist. It 
 has been profitably employed by some physiologists in dis- 
 cussing the poisonous effects of certain substances ordinarily 
 harmless. The theory is useful, but it is very far from ex- 
 perimental proof. Nevertheless it may be employed in plant 
 physiology to suggest possible explanations of otherwise 
 incomprehensible phenomena. We may conceive the cell-sap 
 an aqueous solution of a great number of substances in very 
 various and inconstant amounts as containing not only 
 substances still in the molecular state, but also substances 
 in a state of atomic dissociation, in which the component 
 atoms are scattered at considerable distances from each 
 other throughout the solution. Whether these dissociated 
 atoms or groups of atoms smaller than molecules are 
 charged with electricity some positive, others negative or 
 whether the atoms possess only their chemical properties 
 and affinities is by no means known. Certain it is, how- 
 ever, if there are dissociated atoms in the cell-sap, that 
 these atoms are more susceptible to physical and chemical 
 influences within and without the cell, and more prompt 
 and active in response, than are atoms associated in mole- 
 cules. Dissociated atoms exhibit the properties of the 
 chemical elements, whereas associated atoms present only 
 the properties of compounds. The elements are more ready 
 for combination than compounds are for recombination. A 
 dilute solution, one containing dissociated atoms, is there- 
 
 * Schimper, A. F. W. Pflanzengeographie auf physiologiscker Grund- 
 lage. Jena, 1898. 
 
IRRITABILITY 229 
 
 fore more sensitive to external influences and is more likely 
 to change than is a concentrated solution. Some of the 
 substances in the cell-sap may be present in such minute 
 quantities as to be dissociated. In this case these dissoci- 
 ated atoms, and in consequence the cell-sap as a whole, 
 would be very sensitive to external influences, physical and 
 chemical. Anything influencing the cell-sap in any way 
 immediately affects the protoplasm through the cell-sap 
 which permeates every part of it. This influence upon the 
 protoplasm may be chemical or it may be electrical. Only 
 the future can decide which it is, but meantime this concep- 
 tion assists us in appreciating how the protoplasm may be 
 affected and how the stimulus may be promptly transmitted 
 from cell to cell.* 
 
 Any substance or any force which disturbs the balance 
 between the constructive and destructive processes main- 
 tained by the organism or the cell will affect the organism, 
 producing an irritation equal to the disturbance. If the new 
 balance necessitated by the introduction of the new sub- 
 stance or the new force is more or less favorable to the 
 accomplishment of the vital processes than the old, the liv- 
 ing cell or organism will respond accordingly. Organisms 
 living in certain situations are exposed to very frequent and 
 very rapid changes in their environment. Thus the plants 
 living between the high and low tide marks on the sea- 
 coast are subjected to a twice daily change in their rela- 
 tions to water. If rain falls upon them while they are 
 bared by the tide, they are subjected to still another change. 
 Marine plants living at the mouths of fresh-water streams 
 are also exposed to frequently repeated and very rapid 
 changes in the composition and density of the medium which 
 surrounds them. Such changes in osmotic influences imply 
 a very great range of adaptability on the part of the 
 plants regularly exposed to them. Plants living in situa- 
 tions where osmotic influences change only gradually or 
 slightly, as in pools which dry up during the summer, or in 
 bodies of water which are added to only occasionally or 
 
 * Matthews, A. P. The nature of the nerve impulse. Century Magazine, 
 vol. 63, 1902. Abstract also in Science, vol. 15, p. 344. 1902. 
 
230 PLANT PHYSIOLOGY 
 
 seasonally, are far more susceptible to changes in the den- 
 sity of the water. It has been suggested that the recourse 
 of fresh water algae to their various modes of reproduction 
 (see pp. 268-9) may be a reaction to osmotic differences 
 in the water in which they live,* but this is by no means 
 assured. 
 
 Certain substances, nutritious and other, stimulate the 
 growth and other activities of plants as of animals. Small 
 amounts of poisonous matters, such as ether, t metallic 
 salts, cocaine, and morphine]: act as stimulants. The prin- 
 ciple governing the action of these substances is the same 
 as that controlling the use of tea, coffee, and tobacco by 
 human beings, for all of these drugs are dangerously de- 
 pressing poisons when taken in concentrated form. Alcohol 
 may, in both animals and plants, serve as a source of 
 energy by oxidation and thereby stimulate the organism. 
 
 One word must be said about the startling claim that 
 definite chemical stimuli may replace the male elements in 
 sexual reproduction. Loeb reports that he has reared sea- 
 urchins through the earlier larval stages from unfertilized 
 eggs by subjecting the fggs to suitable chemical stimuli. 
 From this he argues that the main function of the sperm 
 is to supply the eggs with certain compounds indispensable 
 to further development, and that when, no matter by what 
 
 * Livingston, B. E. Nature of the stimulus which causes change of 
 form in polymorphic algae, Bot. Gazette, vol. 30, 1900. Further notes 
 on the physiology of polymorphism in green algae. Bot. Gazette, vol. 32, 
 1901. 
 
 t Townsend, C. O. The correlation of growth under the influence of 
 injuries. Annals of Bot., XI.. 1897. 
 
 t Richards, H. M. Die Beeinflussung des Wachsthums einiger Pilze durch 
 chemische Reize. Jahrb. f. wiss. Bot., Bd. 30, 1897. See also Raciborski, 
 M. Uber den Einfluss ausserer Bedingungen auf die Wachsthumsweise des 
 Basidiobolus ranarum. Flora, Bd. 83, 1896. 
 
 Atwater. W. O. Metabolism of matter and energy in the human body. 
 Bull. U. S. Dept. Agriculture, Washington, 1901. 
 
 Loeb, J. On the nature of the process of fertilization and the artificial 
 production of normal larvae (plutei) from the unfertilized eggs of the 
 sea-urchin. On the artificial production of normal larvae from the un- 
 fertilized eggs of the sea-urchin (Arbacia) , Amer. Journ. Physiol., vol. III., 
 1899-1900. 
 
IRRITABILITY 231 
 
 means, these compounds are supplied, development will go 
 on., This may be true, but that it is true is far from proved. 
 There are well-known cases of parthenogenesis among both 
 animals and plants. From these one might infer either that 
 no eggs require fertilization in order to develop, or that 
 these eggs are chemically stimulated to develop by the sub- 
 stances in air or water. Both of these inferences would be 
 unjustified. Sea-urchin eggs may not develop partheno- 
 genetically when let alone, though they may do so when 
 chemically stimulated. But it is a far cry from Loeb's 
 brilliant experiments to the generalization that fertilization 
 is principally chemical stimulation. In many cases, the 
 structure of the egg must be completed by the material of 
 the sperm before normal development can begin. This, in a 
 sense, is chemical stimulation, but it is also a morphological 
 process. Fertilization, in most cases at least, consists in the 
 completion of the structure of the egg as weU as in the 
 stimulation of its living protoplasm to grow, etc. 
 
 The direction of growth, as well as the rate and kind, is 
 influenced by other substances than water. This response of 
 the living plant to external stimuli is called chemotropism. 
 It is well known that roots grow toward, around, and into 
 especially fertile clods of earth, leaving the less nutritious. 
 The distribution of oxygen, carbon-dioxide, and possibly of 
 other gases in the soil (e. g. illuminating gas leaking from 
 underground pipes) affects the growth of the roots. * They 
 will not grow far down into water ; they will turn up when- 
 ever their supply of air faUs below the optimum. They will 
 grow toward the sources of many gases, but so soon as 
 they reach a point where the amount of gas becomes at all 
 considerable, they will bend and grow away. 
 
 The chemotropic sensitiveness of hyphaef and of pollen- 
 tubes J is marked and important. Miyoshi finds that the 
 
 * Molisch. H. Fber die Ablenkung der Wurzeln von ihrer norraalen 
 Wachsthumsrichtung durch Case (Aerotropismus) . Sitzb. d. Akad. d. Wiss.. 
 Wien. Bd. 91. I.. 1884. 
 
 4 Miyoshi. M. Uber Chemotropismus der Pilze. Bot. Zeitung 1894. 
 
 i Miyoshi, M. Uber Reizbewegungen der Pollenschlanche. Flora. Bd. 
 78. 1894. Lidforss, B. fl)er den Chemotropismus der Pollenschlauche. 
 Ber. d. Deutsch. Bot. Ges. Bd. 17. 1899. 
 
232 PLANT PHYSIOLOGY 
 
 spores of various fungi, germinated on finely perforated films 
 of collodion or of stomata-bearing epidermis floating on 
 water or solutions of various substances, or on the stomata- 
 bearing surface of Tradescantia leaves injected under the air- 
 pump with solutions of the substances to be tested, send 
 out hyphae which grow toward and through the perforations 
 into the solution, or away from the perforations, according 
 as the substances used are attractive or repellent. The 
 attractive substances are not all nutritious, the repellent 
 not all poisonous. Osmotic and dissociation phenomena 
 may be concerned. Glycerine and gum arabic, nutritious 
 though they are, do not attract. Miyoshi and Lid- 
 forss report that the direction of growth of pollen-tubes is 
 influenced by water, oxygen, sugars, and easily diffusible 
 proteids, especially those composing and associated with 
 enzyms. All of these are present in various amounts in the 
 different parts of stigma, style, and ovary. Lidforss says 
 that the direct growth of the pollen-tube toward the micro- 
 pyle is due solely to hunger certainly a graphic expression. 
 On scattering various kinds of pollen and the spores of 
 Mucor with ovules of Scilla on moist agar-agar, Miyoshi 
 found Mucor hyphse and pollen-tubes growing toward and 
 even into the ovules. 
 
 The meeting of the conjugation tubes of Spirogyra cells 
 could hardly occur with such regularity if it were merely a 
 matter of chance. On the other hand, experimental proof 
 that their growth toward one another is chemotropically 
 directed is so difficult that no attempts have yet succeeded. 
 It has been noticed* that bacteria are attracted to those 
 portions of Spirogyra cells which grow out into tubes. 
 From this it is inferred that some special substance diffuses 
 from the tubes. This substance may not induce the cell in 
 an adjacent filament to put out a tube, yet it may so di- 
 rect the growth of a tube already forming that the two will 
 meet. Too much reliance should not be placed on inference 
 from such evidence, however, for though the behavior of 
 
 * Overton, C. E. tiber den Conjugationgvorgang bei Spirogyra. Ber. d. 
 Deutsch. Bot. Ges., VI., 1888. Haberlandt, G. Zur Kenntniss der Conju- 
 gation bei Spirogyra. Sitzb. d. K. Akad. d. Wigs., Wien, Bd. 99, 1. 1890. 
 
IRRITABILITY 233 
 
 bacteria where only one kind is concerned may be definitely 
 known, the behavior of a swarm of several or many kinds 
 is due partly to the influence of the bacteria themselves 
 upon each other. 
 
 One of the most striking responses to chemical irritation 
 is that exhibited by the tentacles on the leaves of the vari- 
 ous species of the carnivorous Sundew, Drosera. The 
 thorough investigation of this by Darwin* has placed our 
 knowledge on a very definite basis. He found that the ten- 
 tacles would bend over and the leaves curl with a prompt- 
 ness and to a degree proportional to the irritating quality 
 of the substance used. That it was not mere contact that 
 caused the irritation was experimentally demonstrated by 
 placing bits of wood, quartz, etc., and drops of milk, beef 
 infusion, etc., upon the tentacles of expanded leaves. The 
 wood, quartz, etc., did not induce the tentacles to close over 
 them ; the drops of nutrient liquid did. Darwin then tested 
 the influence of a great variety of substances and found 
 that some did, others did not, stimulate the tentacles to 
 close over them. 
 
 The effects of fragrant substances in directing the locomo- 
 tion of animals is a fact of such universal experience that 
 we may well use it as the starting-point of our discussion of 
 chemotaxis. Flies are attracted by the odors of cooking 
 and repelled, as every camper knows, by a smoky fire. 
 Walking, crawling, and swimming animals are attracted or 
 repelled by soluble substances, either in the solid form or in 
 solution. Ants will follow a trail of grains of sugar to its 
 source, but they will be turned from their path by un- 
 pleasant substances placed in their way.f Insects, and 
 doubtless birds also, concerned in the cross-pollination of 
 flowers, are attracted from a distance by the odorous sub- 
 stances formed in them. Only when the animal comes near 
 enough for the flower to be within its range of vision 
 
 * Darwin, C. Insectivorous Plants. On cytological changes in gland 
 cells following stimulation see Huie, L. H., in Quart. Journ. Microsc. Sci.. 
 vols. 39 and 42 ? 1896 and 1899; and Rosenberg. O.. Physiolog.-cyto- 
 log. Untersuch. iiber Drosera rotundifolia. Upsala, 1899. 
 
 f Lubbock, Sir John. Ants. Bees, and Wasps. New York, 1884. 
 
234 PLANT PHYSIOLOGY 
 
 is it attracted by the distinguishing color and form. The 
 direction of locomotion by taste and scent is but the special 
 form of chemotaxis exhibited by higher animals. In the 
 lower animals and in plants, in which the separate senses 
 are not seated in specially differentiated organs, the same 
 sensitiveness of the living protoplasm is exhibited, in a de- 
 gree not always less striking. 
 
 The habit of motile aerobic bacteria of gathering around 
 air-bubbles and at the edges of the coverglass in micro- 
 scopic preparations is of common observation. Correspond- 
 ingly, in preparations made without air-bubbles and cut off 
 from a supply of air by sealing the margins with vaseline, 
 the bacteria become uniformly distributed and presently 
 come to rest. This indicates the dependence of such bac- 
 teria upon oxygen as a source of energy. Their uniform 
 distribution when the supply of oxygen is uniform, and 
 their collecting around the greater quantities when the 
 supply is scattered irregularly, indicate the importance of 
 oxygen in directing as well as maintaining their movements. 
 The chemotactic movement of certain bacteria in relation to 
 oxygen was carefully studied by Engelmann* and made use 
 of by him in determining which of the component rays of 
 sunlight are most efficient in the manufacture of food by the 
 chlorophyll grain (see p. 56). By placing a filamentous 
 alga under the coverglass with the bacteria, and duly illu- 
 minating the preparation, the alga will become photosyn- 
 thetically active and liberate oxygen, the bacteria gather- 
 ing about it. If a short spectrum, instead of white light, be 
 thrown on the alga, the cells exposed to the yellow light 
 will give out most oxygen, as shown by the largest num- 
 ber of bacteria collecting there. 
 
 The chemotactic movements of bacteria with relation to 
 conjugating Spirogyra filaments have already been referred 
 to (p. 232). Bacteria are known to collect on the surface 
 
 '* Engelmann, T. W. Eine neue Methode zur Untersuchung der Sauer- 
 stoffausscheidung pflanzlicher und thierischer Organismen. Archiv. f. d. 
 ges. Physiol., Bd. 25, 1881. L'emission d'oxygene sous Tinfluence de la 
 lumiere par les cellules a chromophylle, demonstres au moyen de la 
 methode bacterienne. Archives Neerland., T. 28. 1894. 
 

 IRRITABILITY 235 
 
 of decaying matters, attracted thither by the substances 
 diffusing from the dead organic material. The sensitiveness 
 of the bacteria of disease to the dissolved substances in the 
 body of their host enables them to move toward and into 
 the places most favorable to them. The chemotactic sensi- 
 tiveness of bacteria, flagellates, Volvocinste, and motile 
 reproductive bodies was studied very thoroughly by Pfeffer. * 
 His method consisted in the employment of capillary tubes 
 closed at one end and partly filled under the air-pump 
 with solutions of the substances to be tested. These tubes, 
 after rinsing, he introduced into drop-cultures or under 
 the coverglass on the slide. From the mouths of such tubes 
 the molecules of the dissolved substances diffuse in all direc- 
 tions into the surrounding liquid and presently impinge 
 upon the motile organisms present. Such impinging mole- 
 cules set up an irritation within the cell. When this irrita- 
 tion is sufficient, the organism reacts by changing its posi- 
 tion, moving so that its long axis becomes parallel to the 
 line of advancing molecules, and presently going either to- 
 ward or away from the source of the irritating substance, 
 according as f this substance attracts or repels. Pfeffer 
 found that certain substances attract, others repel, and still 
 others induce no change in the direction of locomotion. 
 Pfeffer's list has been considerably lengthened by one of his 
 pupils, Buller, who worked on the antherozoids of ferns.t 
 As a result of Pfeffer's work it is now generally accepted 
 that the antherozoids of ferns, liverworts, and algae make 
 their way to the egg-cells under the definite attraction of 
 soluble substances diffusing from the egg-cells themselves, or 
 from a part or the whole of the oogonium or archegonium. 
 The specifically attractive substances are unknown, though 
 it is inferred in the case of ferns that malic acid is the at- 
 
 * Pfeffer, W. Locomotorische Richtungsbewegungen durch chemische 
 Reize. Unters. a. d. bot. Inst. z. Tubingen. Bd. I.. 1884. Uber chemo- 
 taktische Bewegungen von Baktericn, Flagellaten und Yolvocineen. ibid. 
 Bd. II. 1888. See also Jennings and Crosby on manner in which bacteria 
 react to stimuli, especially to chemical stimuli. Amer. Journ. Physiol.. 
 vol. VI.. 1901. 
 
 f Buller. A. H. R. Contributions to our knowledge of the physiology of 
 the spermatozoa of ferns. Annals of Botany vol. 14 1900. 
 
236 PLANT PHYSIOLOGY 
 
 tractive agent, and cane-sugar in the liverworts. This 
 inference is based on experience only to this extent : malic 
 acid is present in fern prothalli bearing sexual organs, and 
 capillary tubes containing a 0.05% solution of malic acid 
 collected one hundred fern antherozoids in an hour from a 
 drop of water in which there were many. The evidence for 
 cane-sugar is similar. Presumably the free egg-cells of 
 Fucus, etc., are fertilized by chemotactically attracted 
 antherozoids, but proof of this is wholly lacking. Much 
 physiological work of this kind remains to be done on the 
 reproduction of the fresh and salt water algae. 
 
 Pfeffer's work on chemotaxis was quantitative as well as 
 qualitative. He showed that Weber's law, that " the smallest 
 change in the magnitude of a stimulus which will call forth 
 a response always bears the same proportion to the whole 
 stimulus," applies to chemotactic as to other stimuli. In 
 the case of "Bacterium termo"now known to be a mixture 
 of several species of short rod-shaped bacteria Pfeffer 
 found, in the first place, that there must be more of the 
 attractive substance in the capillary than outside, and then 
 that if the hanging-drop culture contain 0.01% meat extract, 
 the bacteria will not swim toward and into a capillary 
 containing less than 0.03% and that they are attracted more 
 and more strongly by solutions containing 0.05%, 0.08%, 
 and 0.1%. The same organisms in a culture containing 
 0.1% meat extract will be attracted only by capillaries con- 
 taining 0.3%, 0.5%, 0.8%, and 1.0% solutions of meat extract. 
 Similarly, the bacteria are drawn from a 1% solution only 
 by a 3% solution or stronger. We see then that- the at- 
 tractive substance must be increased by stages of three in 
 order that there may be a change in the direction of loco- 
 motion. This does not imply that there is no stimulation 
 with less, for there may well be, only that there will not be 
 a response in the form of a change in direction of locomo- 
 tion. In order to induce an organism in an environment of 
 a certain degree of favorability to leave this for another, 
 there must be a decidedly larger proportion of the stimulat- 
 ing substance. 
 
 Chemotropism and chemotaxis may be due to funda- 
 
IRRITABILITY 237 
 
 mentally the same causes, though there is no proof of this. 
 (See discussion of heliotropism and heliotaxis on page 219. ) 
 If the chemical theory advanced hi explanation of the influ- 
 ence of light upon the direction of growth and of locomotion 
 be true (p. 210), the influence of light upon the living 
 protoplasm is only through those compounds in the cell 
 which are acted upon by light. So far, then, as the proto- 
 plasm only is concerned, it may be that heliotropic and 
 heliotactic phenomena are intrinsically chemotropic and 
 chemotactic. The sources of the influence upon the chemical 
 compounds within the cell are different, the influence which 
 directly affects the living protoplasm may be only that of 
 chemical compounds, not of light. 
 
 INFLUENCE OF ELECTRICITY 
 
 The electrical conditions in soil and air are constantly 
 changing, but except at rare intervals no great amount of 
 electrical force becomes evident or is concentrated in any 
 one place. Under natural conditions living organisms are 
 ordinarily exposed to feeble currents only, and to such they 
 are accustomed. Under the artificial conditions prevailing 
 in the planted streets and in the parks of cities, in the soil 
 of which powerful electric currents are induced or through 
 which they are transmitted in consequence of the very 
 general tfse of electricity, the roots of plants are peri- 
 odically or constantly subjected to more violent electri- 
 cal influence. Not all electricity, however, comes from 
 without, for in the plant itself, under normal conditions, 
 electrical as well as other currents are constantly developed 
 and maintained. These are very likely due to the chemical 
 changes going on in the plant and in its different parts, and 
 just as these processes affect the balance of other forces of 
 heat, osmosis, and chemical energy so they may readily 
 alter the balance of electrical tensions in different parts of 
 the organism and thereby set up electrical currents. If, 
 by chemical and physical changes in the organism, elec- 
 trical currents develop in it, it is natural to suppose that 
 electrical influences brought to bear upon the organism 
 
238 PLANT PHYSIOLOGY 
 
 from without will effect chemical and physical changes 
 within it. These changes, whether in the living protoplasm 
 itself or in its lifeless contents, cannot be without influence 
 upon the protoplasm itself. 
 
 According to Loeb* " probably all electrical effects on 
 living things are only indirect. What we call electrical 
 effects are really due only to the chemical and molecular 
 action of the ions or the compounds formed from these" 
 set free in the living cells, or in the living organism, by 
 electrical force. Such an hypothesis as this is certainly com- 
 prehensive enough. Protoplasmic tensions and the strength 
 of the protoplasmic structure would surely vary with the 
 chemical and physical rearrangements taking place in the 
 dissolved salts of the cell-sap throughout the living cell. 
 
 In spite of considerable experimental study of the possible 
 influence of electricity upon the rate and amount of growth 
 no general statement regarding it is now possible. Some 
 authors f claim that plants screened from atmospheric 
 electricity are smaller than plants grown under otherwise 
 similar conditions. Others J assert that increasing the at- 
 mospheric electricity by discharges from wires strung above 
 the plants, increases both the growth and the yield. The 
 eminent German agricultural physicist, Wollny, having 
 critically re-examined the subject, denies both claims. We 
 may, therefore, safety leave this question to the sanguine 
 students of applied botany, only hoping that the farmer 
 will not be led to cumber the ground and mar the land- 
 scape with unsightly screens of telegraph wire. 
 
 * Loeb, J. Zur Theorie des Galvanotropismus. Archiv. f. d. ges. Physio- 
 logic, Bd. 67, 1*97. 
 
 t Grandeau, L. De Finfluence de 1'electricite atmospherique sur la nutri- 
 tion des vegetaux. Ann. de Chem. et de Physique., T. XVI., 1879. Aloi, 
 .A. DelP influenza dell' elettricita atmospherica sulla vegetazione delle 
 piante. Bull. Soc. Bot. Italiana, 1895. 
 
 t Lemstrom, S. Om elektricitens inflytande p& vaxterna. Helsingfors, 
 1890. Abstract by Bailey in Trans. Mass. Hortic. Soc., 1894. Also Elec- 
 tricultur. Erhohung der Ernteertrage aller Culturpflanzen durch elek- 
 trische Behandlung. f bers. von Pringsheim. Berlin, 1902. 
 
 Wollny, E. Elektrisch > Culturversuche. Forsch. a. d. Gebiete d. 
 Agrik.-Physik., Bd. XVI. ? 1893. 
 
IRRITABILITY 239 
 
 The net result of the many attempts to hasten the germi- 
 nation or to increase the germinating-power of seeds by 
 subjecting them to electrical action seems to be that there 
 are such great differences between plants of different species 
 that no general rule can be formulated.* Experiments on 
 the possible effects of the X or Rontgen rays upon the 
 I growth of plants, from the pathogenic bacteria up, have 
 led mainly to negative results. 
 
 It is claimed that the direction of growth is influenced 
 by electrical currents, that plants are galvanotropic or 
 electrotropic, positively or negatively, at least as to their 
 roots.t If Loeb's hypothesis (p. 238) be true that electri- 
 cal currents set up chemical changes in the cell, it would 
 be surprising if these produced no effect as shown by 
 the direction of growth. Still, the subject is far from ex- 
 hausted. 
 
 Electrotaxis in plants is almost uninvestigated. Some 
 work on the movements of bacteria J with relation to electri- 
 cal currents has been done, but with meagre results. Zoo- 
 spores would suggest themselves as favorable objects of 
 study. 
 
 INFLUENCE OF CONTACT 
 
 By the term contact is implied much the same idea as is 
 expressed in the physiology of higher animals and in com- 
 mon parlance by the word touch. When the human finger 
 gently touches a motionless clean glass rod, the temperature 
 and other physical properties of which are similar to those 
 
 * For description of method and the detail of beneficial effects of elec- 
 trical stimulation see Kinney. Electro-germination. Bull. Hatch Experi- 
 mental Station Amherst Mass.. Jan. 1897. and Stone. Influence of 
 Electricity on plants. Bot. Gazette, vol. 27. 1899. 
 
 \ Elfving. F. Uber eine Wirkung des galvanischen Stroms auf wachsende 
 Keimlinge. Botan. Zeitung. Bd. 40. 1882. Brunchorts. J. Uber Galvano- 
 tropismus. Ber. d. Deutsch. Bot. Ges.. Bd. II.. 1884. and Bot. Centralbl., 
 Bd. XXTTT. 1885. 
 
 J Yerworn. M. Die polare Erregung der Protisten durch den galvani- 
 schen Strom. Arch. f. d. ges. Physiol.. Bd. 46. 1890. p. 291. Lortet, L. 
 I/influence des courants induits sur 1* orientation des Bacteries vivantes. 
 Co.iptes Rendus de 1'Acad. des Sciences Nat., Paris. T. CXXII., 1896. 
 
240 PLANT PHYSIOLOGY 
 
 of adjacent objects, the mind is conscious of the contact. 
 Solid object and solid object exert on one another certain 
 influences. The finger imparts a certain amount of 
 warmth, of moisture perhaps, possibly of other forces and 
 substances, to the rod, and the rod also acts upon the 
 finger. The contact of solid objects with plants and plant- 
 parts enables them directly to influence each other. Only 
 when the plant or plant-part is more responsive than the 
 lifeless object, only where there is a visible reaction to the 
 influence thus exerted, can we tell either the character or 
 the extent of the influence. By studying the reactions to 
 the influence exerted by contact we can divine something re- 
 garding the nature of the stimulus. 
 
 When zoospores and other motile and floating spores of 
 sessile plants come to rest, attaching themselves to the sub- 
 stratum, the attachment is effected through means not yet 
 wholly clear. The course of events is briefly this. When a 
 spore, e. g. of Vaucherm, (Edogonium, Fucus, etc., comes to 
 rest against the surface of a solid object a dead leaf or 
 branch or stone its foi*m changes and the naked mass of 
 protoplasm invests itself with a cell-wall. The part of the 
 cell against the solid object flattens, spreads out somewhat, 
 conforms accurately to the surface of the object, and adheres 
 closely to it. In this way the holdfast of the new plant is 
 begun. The cause of this local and peculiar growth cannot 
 be due to the stimulation of gravitation, for such spores 
 attach themselves as frequently to oblique and vertical as 
 to horizontal surfaces. Nor can it be warmth or moisture, 
 or even chemical stimulation. It must be either light or 
 contact or both. Light or darkness must play a part in 
 controlling the local growth of holdfasts in many plants. 
 It appears, however, to influence locomotion and the direc- 
 tion of division of the spores* rather than to stimulate 
 growth in the parts touching the solid object. We may 
 conclude, then, that contact with the solid stimulates the 
 protoplasm, but it remains for experiment actually to show 
 that the increased rate and the changed direction of growth 
 
 * Winkler, H. Einfluss ausBerer Factoren auf die Theilung der Eier von 
 Cystosira barbata. Ber. d. D. Bot. Ges., Bd. 18, 1900. 
 
IRRITABILITY 
 
 241 
 
 e 19.-Tendrils of Ampelopsis Veitchii. 
 * before > ft ' a!ter atta * hment - 
 
 in such cases among plants are due to contact, though it 
 seems to be the case in animals.* 
 
 Among higher plants, familiar examples of growth stimu- 
 lated by contact are afforded by those species of Ampelopsis 
 which form the curiously branched organs of attachment 
 shown in the accompanying figures (A. Veitchii). The 
 young branches, "ten- 
 drils," (a) are soft, 
 weak, tapering, long 
 and slender, slightly 
 enlarged at the tips 
 and green only there. 
 After the tips come into 
 contact with a suita- 
 ble vertical surface, they 
 broaden and flatten out 
 pressing closely against 
 the surface, forming the little discs which presently be- 
 come very strongly attached (b). If the "tendrils" do 
 not touch a surface suitable for attachment, the tips 
 do not enlarge, the whole organ remains weak, and 
 finally dies and falls away. Stability of the support and 
 prolonged contact with it seem to be indispensable to the 
 formation of the disc-shaped bodies. These are the larger 
 the rougher the surface of the support, other things being 
 equal. The subsequent thickening and strengthening of the 
 "tendril" are due to mechanical pull (see pp. 187-8) rather 
 than to the contact of the tips with a rough object. 
 
 The direction of growth is controlled in many instances 
 by contact with solid objects. Thigmotropism or stereo- 
 tropism is the name proposed for this phenomenon. The 
 most striking examples are furnished by tendrils, f which 
 have been studied by many observers. 
 
 The sensitiveness of tendrils to contact varies greatly 
 with the species, the prevailing conditions, and the age 
 
 * Loeb J. rntersuchungen zur physiologischen Morphologic der Thiere. 
 1. Uber Heteromorphose. Wiirzburg, 1891. 
 
 f See Darwin's " Movements and Habits of Climbing Plants" and Pfef- 
 fer's P"anzenphysiologie Bd. II.. 2ter Theil 2te Auflage. 
 16 
 
242 PLANT PHYSIOLOGY 
 
 of the tendrils. They are most sensitive when they are 
 half or three quarters grown and until after they have at- 
 tained their full length. A considerable degree of warmth 
 and humidity increases their sensitiveness, warmth being 
 the more important factor. Most tendrils are more sen- 
 sitive on one side than on the others. The part most 
 sensitive to contact is that between the tip and middle, 
 the tip itself and the base not being sensitive to contact. 
 The base is, however, sensitive to gravity and in this re- 
 spect differs from the rest of the tendril. The zones of 
 greatest sensitiveness and of greatest growth, as in roots 
 (see p. 201), are not coincident. 
 
 A tendril of P&ssiflora in the right stage of growth and 
 under favorable conditions will soon bend toward the 
 side touched, if gently stroked with a pencil. The extent 
 and duration of the bending will be proportioned to the 
 degree of irritation. The rougher the surface of the object 
 which the tendril touches, the more pronounced the bend- 
 ing. Because the surface of tendrils is so smooth, they so 
 slightly irritate each other when they chance to touch that 
 they rarely bend about each other. A succession of light 
 touches, each too light to induce a pronounced or per- 
 manent bending, will stimulate a tendril to the same degree 
 as continuous contact with the same object, but unless the 
 contacts are made at very short intervals, so that the ten- 
 dril finally catches and entwines the stimulating object, 
 thereby producing an enduring contact and a prolonged 
 stimulus, the tendril will cease to bend more and will finally 
 straighten out. Presumably loss of sensitiveness precedes 
 loss of ability to bend, but since the reaction is the only 
 evidence we now have of the sensitiveness, it is impossible 
 to say positively. This experiment shows, among other 
 things, that there is an accumulation of stimuli, that the 
 response is not immediate but induced, and that the ten- 
 dril finally ceases to respond to one degree of inconstant 
 stimulation, becoming either accustomed and unsensitive 
 or else fatigued and unresponsive. This condition has its 
 parallel in the familiar state of the fatigued muscle or 
 organism. A fresh horse lightly touched by the whip re- 
 
IRRITABILITY 243 
 
 spends to the slight stimulus by increasing his speed, but 
 the same horse at the end of a hard day's journey can- 
 not be made by the same stimulus, or even by a greater, 
 to give the same response. This indicates two things : in 
 the first place, that the touch of the whip or the repeated 
 contact with a solid body do not of themselves increase 
 the speed of the horse or accomplish the bending of the 
 tendril, but that they are merely stimuli, that these slight 
 influences set in operation other forces. 
 
 Our conception of the stimulus as merely the feeble force 
 which sets other forces in operation is justified by further 
 consideration of the behavior of horse and tendril. Though 
 the fresh horse promptly responds to the light touch of the 
 whip, there is actually the lapse of some time between 
 stimulus and reaction, but this time is brief because the 
 nerve and muscle systems of the horse are exquisitely 
 adapted to receiving and responding to such stimuli. They 
 are especially differentiated to accomplish a few purposes. 
 The tendril responds more slowly to contact. There is 
 what is termecT the "latent period " between stimulation 
 and reaction, ranging from five seconds or less to an hour 
 or more according to the species, but during this period 
 the forces set in operation by the stimulus are working. 
 
 The means by which a tendril curves around its support 
 is not definitely known, and it may well be that the me- 
 chanics of curvature are not the same in all cases. The 
 older plant physiologists, like Sachs* and de Tries, f as- 
 serted that the bending is due to changes in the rate of 
 growth on the two sides, the side in contact with the sup- 
 port growing less rapidly, the opposite side more rapidly, 
 than before the contact was made. MacDougalJ dissents 
 from this generally accepted opinion, believing that there 
 are not such changes in growth-rate, but that the side in 
 contact with the solid object contracts decidedly and that 
 
 * Sachs. J. von. Physiology of Plants. Eng. Edition. 1887. 
 
 f De Vries. H. Langenwachsthum der Ober- und Untereeite krummender 
 Ranken. Arb. d. Bot. Inst. Wurzburg. Bd. I.. 1873. 
 
 J MacDougal. D. T. Mechanism of curvature in tendrils. Annals of Bot- 
 any, X.. 1896. 
 
244 PLANT PHYSIOLOGY 
 
 the opposite side may grow less rapidly than when the 
 tendril is free. Presumably either the contraction of the cells 
 on the side touched, or the expansion ( fixed by growth ) of 
 the cells of the opposite side, would develop the mechanical 
 force needed to accomplish the bending, for the resistance 
 to be overcome is slight. Indeed, until they have attached 
 themselves to a support and have developed strengthening 
 tissues as a result of strain (see pp. 187-8), tendrils are 
 mechanically weak as well as irritable. 
 
 The irritability of tendrils varies greatly, the tendrils 
 of grape-vine requiring prolonged contact with a compara- 
 tively rough object, those of passion-vine ( Passiflora ) 
 responding to extremely slight and transient stimuli in 25 
 to 30 seconds. The size of the solid object must be pro- 
 portioned to the size, structure, and sensitiveness of the 
 tendril if it is to be successfully clasped. The tendrils of 
 grape, for example, cannot twine about objects of small 
 size, while tendrils of Echinocystis will coil about a spider's 
 thread.* A piece of thread weighing 0.00025 milligram, 
 placed as a rider on a tendril of passion-vine, causes no 
 stimulus if motionless, but induces bending if swinging 
 on the tendril. f All solids except moist gelatine irritate; no 
 harmless liquid, free from solid particles, not even mercury, 
 stimulates the most sensitive tendril to bend. From this 
 we may infer that unless there be a certain amount of 
 adhesion between the tendril and the object by which it is 
 touched, there will be no irritation. This adhesion changes 
 the pressure in the cells of the part touched, increasing 
 or decreasing the pressure according to circumstances. 
 
 The petioles of several well-known plants are more or 
 less sensitive to contact, e. g. Lophospermum scandens. 
 Solanum j&sminoides, Trop&olum majus, Clematis. The 
 stems of dodder (Cuscuta) are also periodically sensitive 
 to contact. % This parasite twines about its host, forming 
 
 * MacDougal, Joe. cit., p. 376. 
 
 f Pfeffer, W. Zur Kenntniss der Contaktreize. Untersuch. a. d. Bot. 
 Inst. Tubingen, Bd. I, 1885. 
 
 I Peirce, G. J. Contribution to the physiology of the genus Cuscuta. 
 Annals of Botany, VIII., 1894. 
 
IRRITABILITY 245 
 
 alternately the steep spirals typical of twining plants and 
 short close spirals like tendrils. After it has sent haus- 
 toria into the tissues of its host, dodder elongates very 
 rapidly for a few hours. During this time the stem is not 
 sensitive to contact and it behaves like an ordinary twining 
 plant, circumnutating and obeying the force of gravity. 
 Presently, however, the rate of growth decreases and then 
 it begins to be sensitive to contact. When irritated by con- 
 tact with an object of suitable size, dodder will make two 
 or three or even more close tendril-like turns . about the 
 support. It will not form these close coils about moist 
 gelatine, it cannot twine about objects too large or too 
 small. The longer the contact and the rougher the surface 
 of the support, the more prompt and pronounced will be the 
 bending. Only prolonged contact will induce permanent 
 bending. This is the first effect induced by contact. 
 
 A second effect consists in the formation of haustoria, 
 lateral root-like organs which the parasite sends into the 
 tissues of its host and through which it draws needed food. 
 Without contact these organs never form, even in rudimen- 
 tary conditions. Without contact with an object able to 
 furnish food as well as mechanical support to the parasite, 
 the haustoria will not fully develop. Contact stimulus in- 
 duces the stem to bend closely about the support and to 
 form haustoria, but chemical stimulus is needed besides 
 to secure the development of the haustoria. In fact, the 
 seedling dodder will not even twine about innutritions 
 supports. Furthermore, the dodder stem is sensitive to 
 gravity and will not twine closely or otherwise about a 
 suitable, even nutritious, support unless the position of 
 this be vertical or only slightly inclined. 
 
 The dodders, and a few tropical plants like them ( e. g. 
 Cassytha), are exceptional twining plants. The great 
 majority of twining plants are not sensitive to contact, 
 and though they twine about vertical or nearly vertical 
 supports of suitable size and form, they do so by the com- 
 bined action of their spontaneous nutation movements 
 and of gravitation. As Darwin so clearly demonstrated,* 
 * Darwin, C. The power of movement in plants. 
 
246 PLANT PHYSIOLOGY 
 
 all growing parts, at least of vascular plants, are in con- 
 stant motion, owing to their growth not being equal on all 
 sides at any one time. The rate of growth changes in the 
 different parts, and because this change in rate is fairly 
 regular and takes place in adjacent parts successively 
 around the plant or part, the motion is regular, circular 
 or elliptical in a horizontal plane, spiral in space. Darwin 
 called it circumnutation. Apparently the regularity of 
 the motion is due to external stimuli rather than to causes 
 inherent in the organism. Experiment* seems to show 
 that the cooperation of the forces to which plants are 
 constantly and successively subjected e. g. gravitation, 
 light, heat, etc. reduces the otherwise wholly irregular 
 movements to order and system, and that without these 
 stimuli these movements produced by unequal growth are 
 entirely irregular. The circumnutation of twining plants is 
 through a somewhat wider orbit than that of other plants, 
 probably because of their greater length in proportion to 
 their thickness and mechanical strength. The tips of the 
 stems and branches do not stand out straight for any 
 great distance ; they tend to droop somewhat. The negative 
 geotropism and the ample nutation of the slender and 
 elongated stems of twining plants cause them to grow 
 spirally upward around suitable supports, t 
 
 Sensitiveness to contact and later their parasitism were 
 probably acquired by dodder and its allies after these plants 
 had developed the twining habit. The nearest relatives of 
 the dodder are still independent twiners, closely resembling 
 the behavior of dodder in its unsensitive periods. Further- 
 more, under stress of insufficient food, the dodder is able to 
 manufacture some food for itself. It will develop chloro- 
 phyll in the usually rudimentary and often otherwise col- 
 ored chromatophores which it contains in large numbers. 
 
 Thigmotropic sensitiveness of other organs and of lower 
 
 * Fritzsche, Curt. Uber die Beeinflussung der Circumnutation durch 
 verschiedene Factoren. Inaug.-Diss. Leipzig, 3 899. 
 
 t For a careful study of the mechanics of twining see Kolkwitz, R. 
 Beitrage zur Mechanik des Windens. Ber. d. D. Bot. Ges., Bd. XIII., 1895. 
 The literature is here cited. 
 
IRRITABILITY 247 
 
 plants has been demonstrated by various authors, * though 
 Sachs's claim that roots are sensitive to contact seems to 
 have been successfully disproved by Newcombe.f New- 
 combe shows that roots do not bend about harmless sub- 
 stances ( e. g. glass rods and splinters of tannin-free wood ) 
 although they do coil around pins, brass wire, and rods 
 made of wood containing tannin or other injurious mat- 
 ters. The bendings which Sachs described were responses 
 to injury (traumatropic) rather than to contact. J Studies 
 of the thigmotaxis of plants are very few, if any at all 
 exist, yet the behavior of such motile organisms as the 
 diatoms, Oscillatoria, Beggiatoa, etc., and the relations of 
 zoospores to the solid substances to which they attach 
 themselves, offer objects as interesting as they are difficult 
 for investigation. 
 
 Something must now be said about the so-called "sensi- 
 tive plants." These respond to a touch, a blow, or even a 
 sudden breath of air, to a drop or stream of water upon 
 the leaves, as well as to changes in illumination, tempera- 
 ture, etc. These plants have compound leaves, the petioles 
 of the leaves and the stalks of the leaflets being supplied 
 with cushion-like enlargements called pulvini, which serve as 
 an articulation between petiole and blade or between petiole 
 and branch. A pulvinus is composed mainly of parenchyma 
 tissue, the cell-walls of which are elastic and readily per- 
 meable to water. The fibre-vascular bundles which, in the 
 petiole and in the blade of the leaf, are separated from 
 one another by plates of parenchymatous tissue, are placed 
 close together in the pulvinus forming an axial strand. 
 This axial strand is the part of the pulvinus possessing 
 greatest tensile strength, but the several layers of paren- 
 
 * Sachs, J. von. Uber das Wachsthum der Haupt-und Nebenwurzeln. Arb. 
 d. Bot. Inst. Wiirzburg. Bd. I, p. 437, 1873, and Ges. Abhandl. Bd. II. p. 826, 
 1893. Errera. L. Die grosse Wachsthumsperiode bei den Fruchttragern 
 von Phycomyces. Bot. Zeitung. Bd. 42. 1884. Wortmann, J. Zur Kennt- 
 niss der Reizbewegungen. Bot. Zeitung, Bd. 45. 1887. 
 
 fNewcombe, F. C. Sachs' angebliche thigmotropische Kurven an 
 Wurzeln waren traumatisch. Beihefte z. Bot. Centralbl, xii., 1902. 
 
 Spaulding, V. M. The traumatropic curvature oi roots. Ann. of Bot., 
 viii., 1894. 
 
248 
 
 PLANT PHYSIOLOGY 
 
 chyma cells by which it is surrounded are what expand or 
 close, raise or lower, the leaflets and the whole leaf by 
 changes in the amount of water which they contain, in 
 other words, by changes in their turgescence. These par- 
 enchyma cells act as the cushion on which the blade 
 of the leaf rests. As has been repeatedly described in the 
 text-books, and in numberless works on "the wonders of 
 Nature/'* the position of the leaves and leaflets varies, 
 or may be made quickly to change, according to the con- 
 ditions surrounding the plant. The accompanying figures 
 illustrate the periodic changes occurring in the position 
 
 Figure 20. " Sensitive Plant ' ' ( Mim osa pudica ) by day ( a ) , by night ( b ) , 
 and in light and air of excessive brightness and dryness (c) . From 
 MacDougal. 
 
 of the leaves and leaflets of Mimosa pudica under normal 
 conditions. Figure 20 a, is the ordinary " day position," when 
 the light is fairly strong, the air warm, and water suffi- 
 ciently abundant to allow rapid transpiration without harm. 
 Figure 20 b is the ordinary "night or sleep position/' when 
 the light is weak, the air cool and moist so that dew will 
 form. Figure 20 c is the position taken when the illumina- 
 tion is so intense and transpiration so rapid that there 
 is danger of excessive loss of water, t 
 
 * For example, see " Living Plants and their Properties," by Arthur and 
 MacDougal, Chapter IV., New York, 1898, from which the following de- 
 scription is mainly drawn. 
 
 t For a discussion of these movements with relation to transpiration 
 eee pp. 141-2. 
 
IRRITABILITY 
 
 249 
 
 The plants of Mimosa pudica shown in the accompanying 
 figures (21) were stimulated not by touch but by a small 
 flame, though a touch or blow of suffic- 
 ient force would produce the same ef- 
 fect. The figures therefore illustrate 
 what there is to say of the relations of 
 this plant to contact. Comparing 
 these with figure 20 a on p. 248 we find 
 that under favorable conditions of 
 light, temperature, moisture, etc., 
 lightly touching one or more of the leaf- 
 lets of a plant with the finger, a pencil 
 or some similar harmless instru- 
 ment, will induce the leaflets to move 
 quickly upward toward one another 
 and slightly forward, so that they 
 come to lie closely face to face along 
 the leaf-stalk. An exceedingly light 
 touch may induce only one lea flee to 
 move, a touch less light will induce 
 both leaflets of a pair to move, one 
 still stronger will stimulate adjacent 
 pairs and finally all the leaflets of 
 the compound leaf to close. At last 
 the main petiole of the whole leaf 
 sinks, bending at its pulvinus. as is 
 shown in figure 21 b. If the touch 
 be sufficiently strong, a blow rather 
 than a touch, the other leaves and 
 leaflets will behave in all respects 
 similarly ( figure c ) until finally the 
 appearance of the plant will be such 
 as indicated by figure d. 
 
 Although evidently there is still 
 much work to be done before the 
 subject will be quite clear, it appears 
 from already published investiga- 
 tions that the means by which 
 
 Figure 21. Sensitive Plant in 
 
 the leaves and leaflets are moved various stages of stimulation, 
 are found in the parenchyma cells From MacDougal. 
 
250 PLANT PHYSIOLOGY 
 
 of the pulvini.* These cells, taking up water from the 
 adjacent vascular elements in the axial strand of bun- 
 dles, expand, become turgid, and exercise sufficient force 
 to raise the petiole or the blade respectively of the leaf 
 or leaflet above. The opposite effect follows, leaflet, leaf, 
 and petiole droop when, for any reason, the parenchyma 
 cells of the pulvini, giving up the water which they con- 
 tain to the vascular elements, become smaller, flabby, and 
 contracted, if not collapsed. 
 
 A great variety of stimuli set in motion the mechanism 
 by which the leaflets and leaves are closed. What is the use 
 of these movements, and what is the means of transmit- 
 ting the impulse to move, are by no means clear. Stahl,f 
 from observations made in the tropics, confirms Darwin's 
 claim that the closing of the leaves is an effective protec- 
 tion, in connection with the thorns which the plant bears, 
 against hungry herbivorous mammals. It is seemingly 
 probable that contact and various other stimuli cause the 
 contraction of some of the cells in the leaf and that, in their 
 contraction, these cells expel water into the vascular ele- 
 ments or intercellular spaces. The contraction of any cell or 
 group of cells will necessarily affect the tensions of adjacent 
 cells. Thus, though we cannot conceive of the transmission 
 of the impulse itself from cell to cell, yet we can readily 
 conceive of the extension to an increasing number of cells of 
 the conditions first produced by the irritant acting upon a 
 small number of cells. Mimosa is not able to respond to 
 stimulus (whether it is then sensitive or not is another 
 question ) when under the influence of an anaesthetic or in a 
 state of chill, heat-rigor, etc. For this reason the inference 
 is easy that the physical changes induced by stimuli are 
 
 * Stahl, E. Ober den Pflanzenschlaf und verwandte Erscheinungen. 
 Bot. Zeitung, 1897. Cunningham, D. D. The causes of the fluctuations 
 in the motor organs of leaves. Annals of the Botanic Garden, Calcutta, 
 vol. VI., 1895. MacDougal, D= T. The mechani m of movement and 
 transmission of impulses in Mimosa and other "Sensitive" Plants: a 
 review with some additional experiments. Bot. Gazette, XXII., 1896. 
 Haberlandt. G. Das reizleitende Gewebe der Sinnpflanze, Leipzig, 1890. 
 Sinnesorgane im Pflanzenreich. Leipzig, 1901. 
 
 f Stahl, E. 7. c. 
 
IRRITABILITY 251 
 
 transmitted only through living cells. This is not neces- 
 sarily the case, as MacDougaPs experiments tend to prove. * 
 Haberlandtf claims the transmission of the stimulus 
 through tubular series of cells hi the phloem of the vascu- 
 lar bundles. Since Fischer! has shown the continuity of the 
 sieve-tubes throughout the plant, it is not unlikely that 
 Haberlandt's tubes are also continuous and may therefore 
 furnish at least one course along which the changed turgor 
 of any group of cells could affect other cells. 
 
 The stamens of barberry and of some of the Composite, 
 and the stigmas of Mimulus, Torenia, etc., contract on 
 being touched. Presumably these, and other similar move- 
 ments, are due to decreased turgor in the parenchyma cells 
 forming a considerable part of the organ, as well as in 
 
 (Mimosa. 
 CONCLUSION 
 So far we have separately considered the operations of the 
 more evident influences which direct plants in growth and 
 in other activities. This process of analysis leads us to 
 more definite views regarding the effects of these different 
 forces acting as stimuli, but in nature the plant is subjected 
 to all of these forces more or less constantly and more or 
 less simultaneously. Thus light and gravitation may be 
 acting simultaneously and, because of the action of both, 
 the response of an organ to either force will not be the 
 same as if that one were acting alone (see p. 214). Mois- 
 ture, warmth, contact, and chemical substances may also 
 be acting upon the plant at the same time. The behavior 
 of a plant, then, expresses its adjustment to all the influ- 
 ences operating upon it. Its size, form, color, vigor, etc., 
 represent its response to all the stimuli it has received. 
 
 Furthermore, as conclusively proved by recent experi- 
 ment^ whatever stimulates or otherwise affects one part or 
 
 * MacDougal, D. T. 7. c. t Haberlandt. G. 7. c. 
 
 J Fischer, A. ^eue Beitrage zur Kenntniss der Siebrohren. Ber. d. math.- 
 phys. Classe d. K. Sachs. Ges. d. Wiss., 1886. 
 
 Hering, F. f ber Wachsthumscorrelationen in Folge mechanischer Hem- 
 mung des Wachsens. Jahrb. f. wiss. Bot.. Bd. 29, 1896. Kny, L. Corre- 
 lation in growth of roots and shoots. II. Ann. of Bot.. XV., 1901. Other 
 papers cited by these authors, and by Pfeffer Pflanzenphysiologie Bd. II., 
 Theil 1 1901. 
 
252 PLANT PHYSIOLOGY 
 
 organ affects all the other parts of the plant. Injury or 
 loss of a part is always followed in healthy plants by the 
 replacement of the part, either by new tissues, or by an- 
 other part assuming the work of that injured or lost. Thus, 
 if the tip of a pine or other perennial with excurrent stem j 
 is injured, the lateral branches change their direction of 
 growth, and finally the strongest and most rapidly growing 
 of these assumes the direction and functions of the main 
 stem. "Cutting back" stem or root is followed by copious 
 branching. When, as in floating specimens of Marsilia, etc., 
 enough water is absorbed by other parts, the roots soon 
 cease to grow and may even finally disappear. 
 
 We must conclude, then, that the plant is sensitive as a 
 whole because of the sensitiveness of its parts, that the 
 condition of one part affects all the other parts, that cor- 
 relation is a necessity if the organism is to act as an in- 
 dividual or even as an association of cooperating members. 
 
 Besides the forces which, through analysis, we have been 
 able to recognize distinctly, there are complex influences 
 consisting of forces and influences which have so far eluded 
 analysis. The analytical method has not yet exhausted the 
 subject ; more detailed physical and chemical knowledge will 
 come presently. Meantime it is more or less the fashion, 
 under the name of ecology, to view things in the large way, 
 and by feeling rather than by trt application of exact 
 physiological methods, to reach conclusions regarding the 
 effects of environment and of association. The trees of a 
 given species, presenting one appearance when they grow as 
 members of a thick forest, are very different when growing 
 separately in the open. These differences in appearance are 
 due in great part doubtless to differences in the amounts of 
 light and food, of mechanical strain, and of room, but these 
 are not all, nor do we know the relative importance of the 
 separate influences. We do not know why small plants of 
 characteristic species are the regular associates of certain 
 kinds of trees.* Such phenomena show that plants are 
 
 * For example, see Hock, F. Begleitpflanzen der Kiefer in Norddeutsch- 
 land. Ber. d. Deutech. Bot. Ges., Bd. XI., 1893; Coulter, Plant Relations. 
 New York, 1899, etc. 
 
IRRITABILITY 253 
 
 sensitive to all the forces and influences which we combine 
 without analysis under the name environment, and that the 
 cooperation of these influences induces, as i-eactions in the 
 living plants, the qualities which we see and call character- 
 istic of the species, order, or class reactions which are 
 realty characteristic only of the living protoplasm. Proto- 
 plasm is not all equally sensitive to any one influence or to 
 the whole complex of influences which constitute its living 
 and lifeless environment. These different degrees of sensi- 
 tiveness, coupled with different powers of response, are what 
 bring about, in the same environment, the forms, sizes? 
 colors, etc., which characterize individuals and species. 
 
 The various forces operating upon the living organism set 
 up and maintain in it physical and chemical conditions 
 which can be changed only by the introduction of a new 
 force or by a change in the relative proportions of the old 
 forces. The living differs from the dead organism and from 
 all lifeless compounds and structures in the supreme deli- 
 cacy of its structure, due to the arrangement, complexity, 
 and instability of the compounds composing and contained 
 within it (see pp. 183-6). The sensitiveness of living or- 
 ganisms is different in degree and not in kind from that 
 of lifeless things; the sensitiveness of both is a matter of 
 physics and chemistry. 
 
CHAPTER VII 
 
 REPRODUCTION 
 
 THE subject of reproduction has been more fully and more 
 exactly studied by morphologists than by physiologists. 
 It has been meditated upon more than it has been investi- 
 gated through experiment. Yet there are certain results of 
 comparatively recent work, and there are certain hypothe- 
 ses, which must be considered in any discussion of the 
 physiology of plants. 
 
 In the vegetable as in the animal kingdom the span of 
 life of the individual organism is limited. In many cases it 
 is limited by perfectly obvious influences; in most it is 
 limited by means little understood even if apprehended at 
 all. In many plants and animals there are no evident rea- 
 sons inherent in the organisms themselves why they should 
 not continue to live indefinitely. Influences wholly external 
 and only slightly controllable by the organism determine 
 and terminate its career. The effects of these influences are 
 to be distinguished from the irritable responses which we 
 have studied in the preceding chapter. Irritable response 
 depends upon a degree of sensitiveness possessed only by the 
 living organism, although this sensitiveness is dependent 
 upon the sum of the physical and chemical conditions pre- 
 vailing in the organism (p. 186). But the influences which 
 terminate its career exceed the powers of resistance, reac- 
 tion, or response, of the organism. They act upon it as 
 upon any lifeless thing of similar composition and structure, 
 and they produce on the lifeless similar effects to those pro- 
 duced on the living body. For example, the heavy wind 
 which uproots a tree would bring it to the ground were it 
 alive or dead. Uprooted it would dry faster than if its 
 roots were still in the soil. It is after all the drying, follow- 
 ing the uprooting, and not the uprooting itself, which is the 
 
REPRODUCTION 255 
 
 fatal thing. Again the frost which kills a living herb 
 would produce in another similar but lifeless herb the same 
 injurious mechanical changes, which neither living nor lifeless 
 herb could obliterate or reverse and recover from. Ex- 
 cessive shade or light injure and may be fatal to living 
 plants, the former entailing insufficient food-manufacture, 
 the latter causing undue activity in this or in other ways. 
 Lifeless structures as sensitive to light will also be affected 
 by the same means; the photographic plate is injured or 
 ruined by insufficient or by excessive light. 
 
 From these examples the inference is obvious that, if the 
 conditions which make life possible ( p. 6 ) were maintained, 
 many organisms which now die at the end of a season 
 or a cycle, would continue to live. The inference could 
 not, however, be extended to all plants, because what de- 
 termines the span of life of the organism is often within it, 
 not outside. This every one knows. Wheat harvest comes 
 long before frost or heat or drought can terminate the lives 
 of the wheat plants. In California the wheat plants are 
 dead before harvest begins. They have ceased to live when 
 they have matured their fruits. They have transferred to 
 the embryos in the fruits the life which they themselves 
 possessed. The one living wheat stalk has formed several 
 or many kernels, each containing a living plantlet. Among 
 these new individuals the life of the parent has been com- 
 pletely distributed. The parent stalk ceases to live, its life 
 is ended, the parent lives only in its offspring. No external 
 influences have contributed to the death of the stalk except 
 as they have contributed to its successful life and to its 
 production of successors. So it is with other plants which 
 fruit once and then die, whether fruiting take place in a 
 month, a season, or a "century." By preventing fruiting, 
 man can artificially prolong the life of many such plants. 
 The flowering plants of our gardens are induced to continue 
 blooming and to live longer by being kept from setting 
 seed. By constantly picking the flowers of sweet-pea, pansy, 
 etc., larger, handsomer and more numerous flowers may be 
 had for a longer time than if the life of the individual plant 
 were allowed to pass over into the new individuals repre- 
 
256 PLANT PHYSIOLOGY 
 
 sented by the embryos in the seeds. The life cannot pass 
 over, however, without a physical basis; the substance of 
 the parent is given to and forms the offspring. The giving 
 of substance and of life are simultaneous if not identical. 
 When this gift exceeds a certain amount, the parent ceases 
 to be, it abandons its own body, it lives only in its off- 
 spring. Internal causes are what terminate the lives of 
 plants like these, but as we shall presently see (pp. 263- 
 76), these are profoundly affected by external influences. 
 
 There are other internal causes which may terminate the 
 life of an individual. If its parent endow it so badly with 
 substance, form, or food that it cannot maintain the bal- 
 ance of constructive and destructive processes in which liv- 
 ing consists, it will die. Its span of life is determined by its 
 own substance, structure, and energy, all of which, or the 
 means of gaining which, were given it by its parents. Such 
 an individual, unable to live long enough to produce off- 
 spring, ceases to live because its internal balance breaks. 
 Its life ceases because destructive processes exceed the con- 
 structive. Its lifeless substance thereupon becomes the 
 source of matter and of energy for living individuals of 
 other kinds. 
 
 Since living may be said to consist in maintaining the 
 balance between construction and destruction, life may be 
 said to represent the balance of energy-storing and energy- 
 liberating processes. There is as much substance and as 
 much energy when there is no balance. When the balance is 
 attained, there is life; when the balance is not attained, 
 there is no life. When an individual dies without offspring, 
 there is less life in the world but no less substance or energy. 
 When an individual, having given sufficient substance in 
 suitable form to its offspring to start them in their careers, 
 continues to live for a time, there is less life when it dies 
 but no less matter or energy. There is simply a different 
 adjustment of forces, a different arrangement of matter. 
 But this different adjustment is merely local, more or less 
 individual. The population of a given locality may greatly 
 increase, but with the increase in number of human beings 
 there is a decrease in the number of living organisms of 
 
REPRODUCTION 257 
 
 Certain other sorts. The multiplication of the individuals of 
 one species may often result in an increase in the number 
 of living organisms in the locality, but there is no evidence 
 that, as a whole, the living population of the earth is in- 
 creased. Keproduction is not a process by which more is 
 made out of a given amount, for, if this were so, the be- 
 ginning would be the making of something from nothing. 
 Force and matter remaining constant, reproduction can 
 only maintain, it cannot, beyond a certain point, increase 
 the total number of living organisms. 
 
 We are thus led to see that reproduction is a process of 
 which the chief end is maintenance and increase of the spe- 
 cies rather than the increase in life. The command "Be 
 fruitful and multiply" is coupled with the law which irre- 
 sistibly forbids increase beyond a certain point. As char- 
 acteristic of the living substance as breathing or feeding is 
 the tendency to maintain and to perpetuate itself. Only by 
 securing more supplies of matter for replacing worn-out 
 parts, and of energy for carrying on its functions, does the 
 living organism maintain itself. When it secures more mat- 
 ter than is needed to repair, and more energy than is 
 needed to operate, already existing parts, it grows. WTien 
 for any reason growth beyond a certain characteristic size 
 is impossible or difficult, while the supply of matter and 
 energy continues the same, there must be some other mode 
 of increase of the individual, whether cell or organism. 
 The one individual, cell or organism, after forming new sub- 
 stance (protoplasm) and furnishing it with energy (heat, 
 etc.), may finally separate this as a new individual. The 
 mother cell divides it off as a new cell, the parent organism 
 separates it as a reproductive body, a sperm or egg, a 
 spore or seed, an embryo or a larva. Reproduction has 
 been defined as "Growth beyond the limits of the indi- 
 vidual." Like growth, it depends upon the successful per- 
 formance of the ordinary functions, and it simply insures 
 their continuance. 
 
 Again, reproduction is said to be "the effort to bridge the 
 gap of death." This definition is suggestive but misleading. 
 By reproduction living is continued ; the life, the substance, 
 
258 PLANT PHYSIOLOGY 
 
 and the activities, of the offspring were those of the parents. 
 There is no gap or break, there is perfect continuity. If a 
 gap were to occur, nothing could bridge it, there would be 
 nothing with which to bridge it, the species or the race 
 would be extinct. A new creation would be necessary, and 
 experience does not encourage belief in new creations. Re- 
 production, then, prevents the formation of a gap. When 
 death comes to an organism which has already formed a 
 new individual consisting of and continuing the substance, 
 structure, and activities of the old, death effects no break, 
 there is no gap. When death comes to an organism which 
 has not yet grown beyond itself, a gap is formed, but 
 formed too soon ever to be bridged or closed. 
 
 The chief end of reproduction is, then, the maintenance of 
 life, the continuity of the species. Another end is only less 
 important, that of increasing the number of individuals. 
 The parent stalk of wheat which has given all its living 
 substance to the two dozen or so new individuals enclosed 
 within the kernels it has produced, is contributing to in- 
 crease the number of wheat individuals living next year. As 
 we know, there will be no real increase, however, unless 
 these kernels fall into spots where no plant or only a weaker 
 one lived before. Under natural conditions, which have so 
 long remained the same that the possibilities of the situa- 
 tion are as fully exploited by the organisms living there 
 now as can be the case at the present stage in evolution, 
 there will be no vacant spots in which an increased number 
 of individuals can live. Under these conditions reproduction 
 cannot effect an increase, it can only continue the life, of the 
 species. However, when any new factor is introduced, when 
 seismic disturbance, or dimatic change, or the entrance of 
 some new arid powerful organism, modifies the conditions, 
 each new individual in a brood or crop has a different 
 chance from before, a better chance or a worse according to 
 the relative characters of the new individual and of the 
 changed environment. Those organisms which have young 
 ready and able to take advantage of the changed environ- 
 ment will thereby have a better chance to increase their 
 kind as well as to maintain it. As the maintenance of the 
 
REPRODUCTION 259 
 
 species is always more important than its increase, and as 
 the increase of the species is only sometimes possible, such a 
 system of reproduction as will best serve the chief end has 
 been developed by every successful organism. Many or- 
 ganisms possess one means of reproduction which combines 
 these two ends, others have different means leading to the 
 two ends separately.* 
 
 Two modes of reproduction, almost universal in their 
 occurrence among plants, can be distinguished, the sexual 
 and the non-sexual. In the former, two cells from two 
 different sources unite and thereby form a new individual. 
 In the latter, the new individual is developed from one cell 
 or from a group of cells. The difference between these two 
 would seem very clear were it not for the fact that it some- 
 times happens spontaneously in nature, and may be made 
 to happen in a considerable number of cases in the labora- 
 tory, that one of the sexual elements (cells) develops into 
 a new individual without first fusing with the other sexual 
 element (cell). The development of one sexual cell into a 
 new individual without first fusing with the other sexual cell 
 is called parthenogenesis. Parthenogenesis is, in effect, the 
 same as non-sexual reproduction. Morphologically, sexual 
 reproduction differs from non-sexual reproduction in the 
 fusion of two cells into one. Physiologically, sexual repro- 
 duction differs from non-sexual reproduction in the causes 
 which lead to it and in the results produced by it. In 
 sexual reproduction, one cell is " fertilized" by the fusion 
 with it of the other sexual element. In non-sexual reproduc- 
 tion the cells are "fertile" without this fusion. "Fertiliza- 
 tion" has been regarded as giving the stimulus needed by 
 the sexual cell for development as a new individual. Non- 
 sexual reproductive cells do not require this stimulus to 
 develop as new individuals. In natural parthenogenesis, the 
 one sexual cell, the egg-cell, also develops as a new indi- 
 vidual without the stimulus of fertilization. In partheno- 
 genesis artificially produced in the laboratory, chemical or 
 other stimuli applied to the eggs cause them to develop as 
 
 * What these means are. may be learned from Campbell's University 
 Text Book of Botany. New York, 1902. 
 
260 PLANT PHYSIOLOGY 
 
 new individuals.* So far, individuals thus produced have 
 not developed to maturity, though this may be expected to 
 be attained. There is, however, an essential difference be- 
 tween individuals produced on the one hand by non-sexual 
 means, by natural parthenogenesis, and from sexual cells 
 artificially stimulated, and on the other hand by sexual 
 means the fusion of two cells. In the latter case, the one 
 cell absorbs not only simple inorganic compounds, but also 
 all the complex organic compounds contained in the other 
 sexual element ; the structure as well as the substance of the 
 two cells unites ; two sets of organs are combined into one ; 
 nucleus fuses with nucleus, cytoplasm with cytoplasm; 
 the individual characters of the two cells are combined in 
 the new individual. We see, then, that fertilization is more 
 than stimulation ; it is union. f In this union of structure, 
 organs, and characters, although these are composed of 
 chemical compounds, the definite arrangement of the parts 
 in the sexual elements is as essential as the composition of 
 the parts. The result in the new individual will vary with 
 the arrangement of the parts in each sexual element. In 
 other words, there is not only a chemical basis to fertiliza- 
 tion; there is also a mechanical. Fertilization cannot be 
 complete and perfect without both chemical and mechanical 
 effects. What the significance, and what the relative im- 
 portance, of the chemical and mechanical factors in fertili- 
 zation may be is now the subject of hypothesis and experi- 
 ment. The matter is far from settlement. 
 
 That there are advantages in a mode of reproduction 
 which unites chemical and mechanical effects, and in develop- 
 ment which follows only after these are produced, would 
 seem to be indicated by the development of sexual repro- 
 duction by so many different types of organisms. If non- 
 
 * Loeb. J. Artificial production of normal larva? from the unfertilized 
 eggs of the Sea Urchin (Arbacia). Amer. Journ. Physiology, vol. III., 
 1900. Experiments on artificial parthenogenesis in Annelids (Chaetop- 
 terus). Ibid. vol. IV., 1901. Nature of the process of fertilization. Ibid. 
 vol. IV., 1901. Matthews, A. P. Artificial parthenogenesis produced by 
 mechanical agitation. Amer. Journ. Physiol., vol. VI., 1901. 
 
 t Strasburger, E. f ber Befruchtung. Bot. Zeitung, 1901. Boveri, Th. 
 Das Problem der Befruchtung. Jena, 1902. 
 
REPRODUCTION 261 
 
 sexual reproduction were the more successful mode of repro- 
 duction, would its relative importance be so greatly reduced 
 among higher organisms, both plants and animals? Yet 
 what the advantages of sexual over non-sexual reproduction 
 are, is by no means certain. 
 
 Between animals and plants there are striking resem- 
 blances in the processes which constitute and succeed sexual 
 reproduction. Fertilization and the formation of new indi- 
 viduals are the same in the two kingdoms, and the care of 
 the young between the times of their conception in the body 
 of the mother and of their separation from it also cor- 
 respond. Only among the highest animals, the Mammalia, 
 is the care of the offspring carried beyond what is found 
 among plants. The plant-mother cannot nourish her off- 
 spring after it has been separated from her body, though she 
 has supplied it with a store of food. The mammalian mother 
 can and does. This difference of degree in development 
 does not, however, make our comparison less suggestive. 
 
 One point more needs emphasis in this connection. Since 
 the continuity of the species is the chief end of reproduction, 
 precautions to ensure its attainment must be taken. The 
 lower plants like the lower animals, simple in structure and 
 small in size, as a rule rely on the survival of some of their 
 numerous and but slightly developed offspring rather than 
 upon the better equipment of a smaller number of more 
 completely developed young. Among higher organisms, the 
 opposite is the general rule. The relatively small number of 
 the seeds of the Leguminosse, each seed containing a large, 
 well fed, and highly developed embryo, may be regarded as 
 typical of the higher plants, while the small, not fully de- 
 veloped embryos of the Orchidaceae, Ericaceae, etc., in the 
 seeds of which only small quantities of food are stored, 
 represent a distinctly lower type. 
 
 There are two kinds of non-sexual reproduction, the vege- 
 tative and the spore. In the former a mass of tissue, capable 
 of carrying on many functions and of developing into a new 
 plant, is separated from the single parent. In the latter, 
 a single cell or, at most, two cells together are cut off. 
 These, though formed sometimes by the cooperation of 
 
262 PLANT PHYSIOLOGY 
 
 many cells, are never formed by the fusion of cells. Each 
 of the cells cut off is able, by successive divisions, to form 
 a new plant like its parent. The one parent or parent-cell 
 or mass of cells gives its own matter and energy, its own 
 protoplasm, to the new individual. Because the substance 
 of only one parent goes into the offspring there is a radical 
 difference between this non-sexual mode of reproduction and 
 the sexual mode. With the transfer from parent to off- 
 spring of its own substance and means of obtaining energy 
 there are transferred also the same degree of sensitiveness, 
 the same powers of reaction to stimuli, and the results of 
 the reaction to stimuli already accomplished by the living 
 protoplasm of the parent. The offspring are not only like 
 the parent, they are really branches or continuations of 
 the parent. There is introduced into the new individual 
 nothing new, but as soon as it is cut off from the parent, 
 it is subjected to influences some of which may be new 
 and different and which will stimulate the individual to 
 corresponding reactions. The gemmules of certain Mar- 
 chantiaceae, the lateral branches which gradually become 
 the separate plants of Azolla, the runners of the straw- 
 berry, etc., are means of non-sexual (and vegetative) repro- 
 duction. The matter and energy, the substance and struc- 
 ture, of the one parent are carried directly over into these 
 offspring. The condition of the parent at the time of re- 
 production, representing the resultant of all its reactions 
 to all the stimuli to which it has been subjected, is also 
 the condition of offspring made in this way. This fact is 
 still clearer in such a plant as Spirogyra, which, breaking 
 up into the cells of which the filament is composed, * forms 
 new individuals which share its qualities by sharing its sub- 
 stance and structure. In many other low plants the proc- 
 ess of vegetative reproduction is as simple and clear as 
 possible. Similar but multicellular bodies vegetatively re- 
 produce higher plants, continuing and at times multiply- 
 ing the species with nearly or quite the same characters 
 in the new as in the old individuals. The runners of straw- 
 
 * Benecke, W. Mechanismus und Biologie des Zerfalles der Conjugaten- 
 faden in die einzelnen Zellen. Jahrb. f. w. Bot., 32, 1898. 
 
REPRODUCTION 263 
 
 berry, the suckers of lilac and Sequoia,* the stolons, off- 
 sets, runners, etc., of other plants, illustrate this matter. 
 
 In the spore mode of non-sexual reproduction single cells 
 or pairs of cells, each capable of forming a new plant, are cut 
 off by the parent. These reproductive bodies, like the vege- 
 tatively reproductive bodies above mentioned, contain and 
 continue the substance and the condition of the one parent. 
 So soon as they are separated from the parent, they may 
 be so influenced by the factors of their environment as to 
 depart from the character of their parent, to vary. 
 
 The new individuals produced by non-sexual means are 
 usually more numerous than those produced sexually. Non- 
 sexual reproduction is especially the means by which the 
 species is multiplied. Many species, however, have only one 
 means of reproduction, hence the two ends, of maintaining 
 and of multiplying the species, are met by the same means. 
 Many organisms, however, endowed with both means of 
 reproduction, employ one at one time, another at another 
 time. What determines the organism in its behavior? And 
 what are the advantages of each mode? The first ques- 
 tion is answerable at the present time through the already 
 available means of experimenting; the second is not defi- 
 nitely answerable now, though the interpretation of what 
 is observed in nature, and speculation partially supported 
 by experiment, suggest probable answers. From (Edogomum 
 and Coleoch&te among the green algae to the highest of the 
 flowering plants, the phenomenon of alternating generations 
 is one of the most conspicuous in the life of plants. How 
 this has originated and the influences which now control it 
 constitute one of the most intricate problems in morphol- 
 ogy and physiology, the solution of which remains, how- 
 ever, hidden in the future. 
 
 The preceding chapter has shown us that the size, form, 
 color, rate of growth, direction of growth and of move- 
 ment, and many other characters, represent the reaction of 
 the organism to the external influences, its response to the 
 stimuli, which are collectively termed its environment. We 
 
 * Peirce, G. J. Studies on the coast redwood. Proc. Cal. Acad. Sciences. 
 3d series, Botany, vol. II.. 1901. 
 
264 PLANT PHYSIOLOGY 
 
 are, therefore, inclined to believe that its reproduction and 
 its reproductive processes may also be similarly affected. 
 Yet this belief is so modern as to be almost heretical. The 
 morphology of reproduction has been studied by many; 
 very few have engaged in experimental studies of the phys- 
 iology of reproduction among plants. Of these few Sachs, 
 Vochting, and Klebs merit first mention. These men have 
 shown that upon external influences quite as much as upon 
 so-called inherited impulses depend the various kinds and 
 stages of reproduction. 
 
 Klebs* studied certain algae and fungi in order to de- 
 termine the conditions under which they reproduce them- 
 selves. By experiment in culture he found that when cer- 
 tain conditions prevail, Vaucheria will form zoospores, 
 under other influences it will develop sexual organs and 
 reproduce itself through them. These conditions are repre- 
 sented in tabular form thus : 
 
 VAUCHERIA SESSILIS f 
 
 ZOOSPORES SEXUAL ORGANS 
 
 Darkness after sufficient illumina- Light absolutely necessary for the 
 tion for adequate food-manufacture formation of sex-organs. Light suf- 
 invariably induces zoospore forma- ficient for healthy growth may be 
 tion. This will continue in darkness insufficient for formation of sex- 
 till there is no longer enough food, organs. Strong light needed, 
 though there may still be enough Light, apart from its effect on 
 for slow growth. nutrition, is a direct stimulus to 
 
 form sex-organs. 
 
 The addition of water to a culture Sex-organs form either in damp 
 in damp air induces active zoospore air or in water ; better in the for- 
 formation. mer. 
 
 Temperature of 3 C. inhibits zoo- Same for sex-organs, 
 spore formation in all but accli- 
 mated forms. 
 
 * Klebs, G. Die Bedingungen der Fortpflanzung bei einigen Algen und 
 Pilzen. Jena. 1896. The literature is here cited. A second volume, treat- 
 ing of the general questions of the physiology of reproduction in low or- 
 ganisms, is promised, but not yet published (1902). See also Jahrb. f. 
 wiss. Bot., Bd. 32, 33, 35. Falck, R. Die Bedingungen und die Bedeutung der 
 Zygotenbildung bei Sporodinia grandis. Beitrage z. Biol. d. Pflanzen, Bd. 
 VIII, 1901. 
 
 f Klebs divides this species into three. The names of these, and the 
 reasons for division, not being essential in this connection, I retain the 
 old name. 
 
REPRODUCTION 
 
 265 
 
 ZOOSPORES 
 
 3-8 are constant stimulus to 
 zoospore formation. 
 
 26 ' is maximum temperature for 
 zoospore formation but not for 
 growth. No acclimation to this. 
 
 10~-20 r are indifferent tempera- 
 tures, exercising neither stimulating 
 nor depressing effects in passing 
 up or down. 
 
 Raising temperature from indiffer- 
 ent to high, or lowering from in- 
 different to low, exercises no stimu- 
 lus; but raising from 3 to 15 
 stimulates. 
 
 Culture in inorganic nutrient solu- 
 tion favors nutrition and growth, 
 but not zoospore formation. Trans- 
 fer from such to less nutritious acts 
 as stimulus to zoospore formation. 
 
 Sugars and other nutritious or- 
 ganic compounds do not stimulate 
 to formation of zoospores. 
 
 More oxygen required than for 
 growth. 
 
 Flowing water, by favoring nutri- 
 tion and growth, retards or pre- 
 vents zoospore formation. Con- 
 versely, transfer to still water acts 
 as a stimulus. Accommodation to 
 still water followed by rapid growth 
 and cessation of zoospore forma- 
 tion. 
 
 From this table certain influences stand out as especially 
 efficient stimuli to the formation of reproductive bodies. 
 Thus light, which furnishes the energy for the manufacture 
 of the materials needed to form the sexual organs and ele- 
 ments, must fall upon the plant in intensity, not only 
 sufficient for this, but also great enough to act as a distinct 
 stimulus to use the manufactured substance in this special 
 way. Light furnishes the energy for one kind of work ; more 
 energy of this same kind sets and keeps other processes in 
 operation. Food, when available in abundance, thus reliev- 
 ing the plant of the necessity of diligently manufacturing 
 
 SEXUAL ORGANS 
 
 Sex-organs form, but develop more 
 slowly than at higher temperatures. 
 Same for sex-organs. 
 
 Same for sex-organs. 
 
 Same for sex-organs. 
 
 Same for sex-organs. 
 
 Favor formation and develop- 
 ment of sex-organs. 
 
 More oxygen required than for 
 zoospore formation. 
 
 No fertile plants in running water. 
 Become fertile in still water. 
 
266 PLANT PHYSIOLOGY 
 
 food for itself, stimulates to sexual activity, as is shown by 
 the formation and activity of the sex-organs. This seems 
 to be universally true in nature. Given the need of food 
 and the means of manufacturing or obtaining it, the plant 
 will be so especially engaged in the processes of nutrition 
 that reproduction is not undertaken. But on the other 
 hand, given a sufficient supply of food stored in its own 
 body and diminished or suspended means of manufacturing 
 more food, the plant will produce for a time in still water 
 both zoospores and sexual elements, while in the dark the 
 formation of zoospores (not of sex-organs) will continue 
 for some length of time. 
 
 If plants are healthy, certain conditions will invariably 
 induce them, whether they are old or young, to remain 
 sterile. Certain other conditions will induce them to form 
 bodies for their non-sexual reproduction, still other condi- 
 tions will induce them to reproduce sexually. Reproduction 
 then must be regarded, like movement, as a reaction or 
 response to stimuli. But just as stimuli the same in kind 
 and degree induce one response or the opposite or none at 
 all, according to the kind of organism, so in reproduction, 
 a stimulus or a combination of stimuli which induces one 
 kind of plant to reproduce itself sexually may induce other 
 kinds to reproduce themselves non-sexually or not at all. 
 This may b^ illustrated by Klebs's studies of Hydrodictyon. 
 This plant, the water-net, lives in still or only slowly run- 
 ning water, under normal conditions floats, and is physio- 
 logically as well as mechanically a very sensitive organism. 
 It possesses two very distinct methods of reproduction, by 
 non-sexual zoospores, and by sexual motile spores or 
 gametes. Gametes and zoospores are formed in the ordi- 
 nary vegetative cells, not in specially differentiated organs. 
 Until these cells have attained a certain, though very 
 minute, size they cannot form reproductive bodies. Whether 
 there is a maximum size or not cannot be positively stated. 
 Since gametes and zoospores form in vegetative cells indis- 
 tinguishable from each other, and since all or some of the 
 cells of a net will form zoospores or gametes or neither, it is 
 obvious that each cell possesses in equal measure the ability 
 
* 
 
 REPRODUCTION 
 
 267 
 
 to reproduce itself by zoospores or by gametes. Observa- 
 tion shows fchat the same cell never reproduces itself by both 
 means. External influences must, therefore, swing the 
 balance hi one direction or the other. 
 
 The following is a table showing the results of Klebs's 
 experiments on Hydrodictyon : 
 
 HYDRODICTYON UTRICULATUM 
 
 ZOOSPORES GAMETES 
 
 Light absolutely necessary to zoo- 
 spore formation, strong light most 
 favorable. 
 
 Darkness checks zoospore forma- 
 tion. 
 
 Transfer from darkness to light 
 stimulates. 
 
 Flowing water retards or pre- 
 vents zoospore formation, still 
 water stimulates, considerable 
 volume needed. 
 
 Inorganic nutrient salts very 
 strongly induce zoospores to form. 
 Zoospores especially abundant 
 when transfer in the light from nu- 
 trient solution to water. 
 
 Cane sugar not stimulating. 
 Maltose very stimulating in light. 
 
 32 maximum temperature for 
 zoospores. 
 
 8 too low temperature for zoo- 
 spores. 
 
 Only floating nets form zoospores. 
 
 Light not absolutely necessary. 
 
 Darkness favors gamete over zoo- 
 spore formation. 
 
 Transfer from light to darkness 
 stimulates. 
 
 Small volume of still water stimu- 
 lates gametes to form. 
 
 Inorganic nutrient 
 gamete formation. 
 
 salts retard 
 
 Cane sugar in O.Sjg-lO^ stimulates. 
 
 Maltose and cane sugar stimulate 
 in darkness. 
 
 Cells with slight tendency to zoo- 
 spore formation produce zoospores 
 in lighted, gametes in darkened 
 maltose solution. 
 
 33 r -34 maximum temperature for 
 gametes. 
 
 Same for gametes. 
 
 Slightly higher temperature will 
 retard zoospore and stimulate ga- 
 mete formation. 
 
 Nets grown on filter paper wet 
 with water, or better, with cane 
 sugar solution, form gametes. 
 
268 PLANT PHYSIOLOGY 
 
 This table shows that those conditions which contribute 
 to increase and to maintain those activities which supply 
 the plant with food, also favor reproduction by non-sexual 
 rneans. Light, enabling the plant to manufacture carbohy- 
 drates, and the salts needed in elaborating carbohydrates 
 into amides and proteids, stimulate zoospore formation. On 
 the other hand, the copious supply of already elaborated 
 carbohydrates, especially when no unusual amount of in- 
 organic salts stimulates to the further elaboration of carbo- 
 hydrates into amides and proteids, is most favorable to the 
 formation of gametes. Light is a specific stimulus to zoo- 
 spore formation, apart from its influence on nutrition. 
 Given a vegetating cell, healthy and of such size that it can 
 divide into zoospores or gametes, it will continue to vege- 
 tate until it is subjected to some influence which will cause 
 it to grow more slowly or to cease growing, and which will 
 stimulate it to reproduce itself in the one or the other of the 
 two ways in which it can reproduce. Hydrodictyon shows 
 plainly that the time and the manner of reproduction are 
 not fixed, but that both are determined by the influences to 
 which the plant is subjected and is sensitive. Additional 
 evidence of this is furnished by Klebs's observation that 
 Hydrodictyon nets with an already marked tendency to 
 form zoospores can be so influenced by external conditions 
 that they will cease to form zoospores and will thereupon 
 produce gametes instead. The same net, darkened at one 
 end and illuminated at the other, will form gametes in the 
 one end, zoospores in the other, respectively. 
 
 Livingston* has studied one of the polymorphic algae, a 
 StigeoclonJum, particularly with a view to determining the 
 influence of varying concentrations of the medium upon the 
 form and reproduction of the plants. He finds that de- 
 creased osmotic pressure acts as a stimulus to zoospore- 
 formation as well as favoring vegetative activities, and that 
 high osmotic pressure checks zoSspore-formation and the 
 
 * Livingston. B. E. Nature of the stimulus which causes change of 
 form in polymorphic algre. Bot. Gazette vol. 30. 1900. Further notes 
 on the physiology of polymorphism in green algae. Bot. Gazette, vol. 32, 
 1901. 
 
REPRODUCTION 269 
 
 vegetative activities. This corresponds with Klebs's obser- 
 vations, but puts one part of them in somewhat more defi- 
 nite terms. 
 
 We see, then, that so far as the fresh-water algae and 
 certain fungi are concerned, plants react in reproduction, as 
 in other ways, to external influences. In regions where 
 there are clearly marked seasons, during one of which active 
 vegetation is impossible by reason of extreme cold or dark- 
 ness or dryness, these violent conditions determine the 
 behavior of the organisms living there. In milder seasons 
 and in more constantly temperate regions, more moderate 
 influences determine their behavior. Of the forces which act 
 upon algap, stimulating processes which can be accom- 
 plished without them, light and heat are evidently the most 
 important. A certain minimum amount of heat is a neces- 
 sary condition of life; without it action is impossible; but 
 given this minimum amount, more heat will stimulate to 
 reproduction. A certain minimum amount of light is the 
 necessary source of the energy for food-manufacture, but 
 given this amount, more light will stimulate to some form 
 of reproduction. Neither this larger amount of external 
 heat nor the more intense light is necessary as a means of 
 carrying on reproduction; they only set in operation that 
 succession of processes which terminates in the formation of 
 new individuals. Among the fungi, Klebs* claims that 
 changes in the nutrition furnish the stimulus to a well- 
 nourished mycelium to develop reproductive organs. 
 
 Vochting's work on the conditions of reproduction in 
 flowering plants t yielded results with which those of Klebs 
 harmonize. Flowers are the visible agents of sexual repro- 
 duction in higher plants. When they are suppressed, or even 
 are imperfect, sexual reproduction does not take place. 
 Sexual reproduction in higher plants consists, as in lower 
 forms, in the union of the microscopically small sexual 
 elements. The forces which institute, stimulate, and favor 
 
 * Klebs G. Zur Physiologic der Fortpflanzung einiger Pilze. Jahrb. f. 
 wiss. Bot.. pp. 146-7, Bd. 35. 1900. 
 
 \ Vochting. H. f ber den Einfluss des Lichtes auf die Gestaltung und 
 Anlage der Bliithen. Jahrb. f. wiss. Bot.. Bd. 25, 1893. 
 

 270 PLANT PHYSIOLOGY 
 
 sexual reproduction can, therefore, be ascertained by study- 
 ing those forces which influence the conspicuous parts, the 
 flowers. A force which must attain a certain strength before 
 it will stimulate the plant to blossom, and without which 
 the plant blossoms but imperfectly if at all, is one which 
 controls sexual reproduction. Without this force the plant 
 may reproduce itself by non-sexual means, by vegetative 
 processes such as the formation of runners, suckers, etc. 
 Under the influence of this force it will multiply only by 
 sexual means. In such a case, the one force would control 
 reproduction, both sexual and non-sexual. The effect of the 
 force as a controlling agent is due to its own direct influ- 
 ence upon the plant and also to the irritable response of the 
 plant to its influence, the response setting in motion other 
 forces in its own body. The force from without is but the 
 initial energy which releases other quantities of energy, thus 
 setting in operation other processes than those which it 
 can directly affect. 
 
 Of the many plants studied by Vochting, Mimulus Ti- 
 lingi serves best to illustrate the facts which he has brought 
 out. Figures 22 and 23 in the accompanying illustration 
 indicate the normal habit of potted plants of this species, 
 respectively a blooming and vigorous plant from a last 
 year's cutting, and a much younger one not yet ready 
 to bloom. The vegetative functions, as well as the vegeta- 
 tive mode of reproduction, of the plant, are carried out by 
 the usually more or less creeping leafy branches originating 
 on the stem just above the surface of the soil. When the 
 vegetating period draws toward the close, the new in- 
 ternodes of these branches are successively shorter, the tips 
 of the branches forming finally rosette-like bunches of leaves 
 on the surface of the soil. Such a bunch cut off and trans- 
 planted will presently give rise to the flowering erect 
 branch shown in figure 22. The rosettes will form erect 
 branches, however, only when warm enough. The upper 
 part of an erect branch bears flowers. Just below these are 
 a few pairs of leaves in the axils of which short vegetative 
 branches begin to form. The flowering part of the branch 
 may bear lateral flowering branchlets if the plant is vig- 
 
REPRODUCTION 
 
 271 
 
 orous. After fruiting, the whole branch usually dies to the 
 ground, and then the rosette from which it arose will give 
 rise to the lateral creeping branches by which new rosettes 
 will be formed. Thus non-sexual vegeta- 
 tive reproduction follows the sexual mode, 
 the same plant being capable of both but 
 not of both simultaneously. Before it has 
 flowered, however, a rosette is not likely 
 soon to branch and to form new rosettes. 
 If vigorous young plants which, under 
 ordinary conditions, would presently bloom 
 are so placed that they have only enough 
 light for active vegetation but otherwise 
 are very favorably situated, they will not 
 form the erect flower-bearing branches, but 
 will continue to grow and will presently 
 form creeping branches, spreading and 
 maintaining themselves in this way. Plants 
 may be kept from blooming by this means 
 
 Fig. 23 Fig. 22 
 
 Figures 22. 23. Mimulus Tiling}. Fignre 22. a blooming plant from 
 a cutting of the preceding year. Figure 23, a younger plant not yet 
 ready to bloom. (After Vochting). 
 
 for an indefinite length of time. Vochting reports having 
 kept plants sterile for three years. The effect of such 
 enforced sterility on the health of the plants appears 
 to be neither favorable nor unfavorable. Under other- 
 wise like favorable conditions plants which have not flow- 
 ered present as vigorous an appearance as those which 
 
272 PLANT PHYSIOLOGY 
 
 have flowered and seeded. In every way they behave alike. 
 It would seem then that these plants require, as the stim- 
 ulus to form flowering branches, an intensity of light which 
 is more than sufficient to maintain the vegetative activi- 
 ties at a perfectly healthy pitch. 
 
 Vochting further shows that various degrees of illumina- 
 tion above what is sufficient for the vegetative processes, 
 but insufficient for those connected with sexual reproduc- 
 tion, will produce effects exactly corresponding to the de- 
 gree of illumination. Light enough to induce the formation 
 of an erect branch and its growth to a height of several 
 centimetres may be insufficient to induce the formation of 
 any flower-buds upon it. Still stronger illumination will in- 
 sure the formation of flower-buds and their attaining a very 
 considerable size, but will not induce them to open. The 
 bearing of this fact on the formation of cleistogamous 
 flowers Yochting considers very important. Such a degree 
 of illumination, instead of inducing the perfect development 
 and opening of flower-buds, will stimulate the vegetative 
 buds in the axils of the pairs of leaves, just below the 
 flowering part of the erect branch, to develop. Still stronger 
 light, inducing the opening of the flower-buds, if still in- 
 sufficient, is indicated by the small size of the flowers or 
 by the rudimentary condition of some of the floral organs. 
 The corolla, and in two-lipped flowers the upper lip, give 
 evidence by their imperfect development of slightly in- 
 sufficient light, while to suppress or to reduce the cal^yx 
 still less light must fall upon the plant. The essential 
 organs, stamens and pistils, are least dependent upon light. 
 However, the flower as a whole, all of its parts, and even 
 the branch which bears it, are dependent for their forma- 
 tion upon illumination of sufficient intensity and duration. 
 
 The light evidently acts as a stimulus, inducing certain 
 effects, for if the formation of a flowering branch has be- 
 gun, its growth will continue for a time even in darkness. 
 If the formation of flowers on the branch has begun, they 
 too will continue to grow in darkness. But to insure the 
 perfect development of the flowers of most plants, suffi- 
 ciently intense illumination, repeated with sufficient fre- 
 
REPRODUCTION 273 
 
 quency, is absolutely necessary. In other words, the stim- 
 ulus is but transient. Repeated stimulation is needed to 
 continue in operation the processes by which the formation, 
 growth, and perfect development of flowers, and their ac- 
 cessory parts, are accomplished. In some few plants, flowers 
 which have been duly induced to form will develop per- 
 fectly in the dark, but in most plants abnormal characters 
 will appear, varying in importance from an insufficiency of 
 color to deformity or suppression of parts, or the failure of 
 the flowers to open. 
 
 Sachs's hypothesis, * that certain specific substances, pre- 
 sumably made in the leaves under the influence of light, are 
 necessary for the formation of flowers, breaks down under 
 Vochting's experiments, for we cannot reasonably admit the 
 presence in the plant of corolla-forming material as distinct 
 from calyx-forming, etc. 
 
 When a plant is so well nourished, has attained such a 
 size, and is under such favorable conditions generally, that 
 it is able to form the organs and substances of sexual re- 
 production, it must be that it will do so whenever the proc- 
 esses concerned in sexual reproduction are set and main- 
 tained in operation by a stimulus from outside. This 
 stimulus is the light. Without it, the higher plant cannot 
 reproduce, or even prepare to reproduce, itself sexually. 
 The plant is sensitive to that force which, in cases where the 
 plant is dependent upon insects for cross-pollination, will in 
 so far as possible ensure the visits of the necessary insects. 
 It would be interesting to determine whether other than 
 entomophilous flowers are so sensitive to light and so 
 dependent upon it. 
 
 The influence of light is to be distinguished from the in- 
 fluences of food, water, and of the other substances, of heat 
 arid of the other forces, essential to active life. With in- 
 sufficient warmth, food, or water, the plant will be imper- 
 fectly able, or quite unable, to reproduce itself by sexual 
 means ; but this inability is due to the effect of unfavorable 
 conditions upon all of its vital functions. Warmth, food, 
 
 * Sachs. J. von. Lectures on the physiology of plants. English Ed., 
 Oxford 1887. and his special papers therein referred to (pp. 530. 534. etc.) 
 
 18 
 
276 PLANT PHYSIOLOGY 
 
 to change their behavior accordingly. So, also, changes 
 of other sorts, making the conditions suitable not only for 
 the growth and healthy life of the individual but also for 
 the production and active life of new individuals, will induce 
 plants to form these by sexual or by non-sexual means, 
 according to the species of plant. In the spring, when the 
 conditions for active vegetation are becoming more and 
 more favorable, the Equisetums complete the development 
 of the spores non-sexually formed in the foregoing season, 
 and shed these spores, which germinate at once, if at all, 
 and form new plants, the prothalli. These, if conditions 
 favor, soon develop sexual organs and produce new plants 
 by sexual means. In these alternating generations, the non- 
 sexual and the sexual, which follow one another in suc- 
 cessive years, the conditions of the later growing period 
 of one year induce the mature plants to form spores by 
 non-sexual means. The different conditions of the earliest 
 growing period of the following year induce the same plants 
 to complete the development of the spores already formed. 
 The still different conditions of the days and weeks suc- 
 ceeding induce these spores to germinate, the prothalli to 
 form archegonia and antheridia, the embryos to develop 
 into the larger non-sexual plants. These last vegetate 
 for an indefinite length of time. The successive changes in 
 the seasons, in the soil, and in the water-supply, fix the 
 succession of generations which recur according to the 
 peculiar adjustment of the species to all the factors of its 
 environment. Species, genera, families, and orders differ in 
 their adjustments and in their reactions to the stimuli given 
 by the many different factors composing the environment. 
 Comparing sexual and non-sexual reproduction, we see 
 that the advantage of the parents determines what mode 
 of reproduction, if any, shall be carried out. For a long time 
 it has been supposed that sexual reproduction is distinctly 
 advantageous to the species, if not occasionally altogether 
 necessary to those forms capable of reproducing themselves 
 by this means. Such a view is not quite correct. Many 
 species of plants, among them forms which are eminently 
 successful as judged by their numbers and activity, are 
 
REPRODUCTION 277 
 
 wholly incapable of reproducing themselves by sexual means. 
 The bacteria, the higher fungi, and many independent 
 plants illustrate this. Some of the higher algae, archegoni- 
 ates, and flowering plants may go on indefinitely, without 
 reproducing themselves by other than purely vegetative 
 means, and with no evidence of injury to the individual or 
 to the species. It is only when special conditions produce 
 special effects on plants, that they need to reproduce them- 
 selves by sexual or non-sexual spores. Under special con- 
 ditions, reproduction becomes a necessity by becoming 
 advantageous to the individual. The species profits ac- 
 cordingly. 
 
 Certain advantages to the species are conceivable in the 
 spore method of reproduction over any vegetative method, 
 and certain advantages are conceivable in the sexually 
 produced offspring over those non-sexually produced, but 
 these are conceptions rather than proved facts in all but a 
 very few cases. These few cases may or may not be typical 
 of the majority. It is conceivable, for example, that the 
 formation of new individuals by runners, as in the straw- 
 berry, and by suckers, as in the coast red- wood (Sequoia 
 .semper virens ), is advantageous to the species. The new 
 individuals hold the territory won by their parents and 
 may even extend it somewhat. The young are nourished 
 by the parent until they have attained size, strength, and 
 development which give them an advantage in competing 
 with other plants for space and for the means of existence. 
 These means of reproduction are, however, defective in 
 that they do not secure the dispersal of the new individuals 
 and consequently do not tend to insure the survival of 
 the species or to extend its territory rapidly enough. The 
 survival of the well-equipped offspring formed by vegetative 
 means is certain so long as the conditions in the space 
 occupied by the parent remain favorable, but such repro- 
 duction is dangerous, because wide dispersal will secure for 
 the species some positions at least in which it can be main- 
 tained whatever may befall individuals in other places. 
 A puddle may become densely populated through the vege- 
 tative reproduction of the alga? and animals started there- 
 
276 PLANT PHYSIOLOGY 
 
 to change their behavior accordingly. So, also, changes 
 of other sorts, making the conditions suitable not only for 
 the growth and healthy life of the individual but also for 
 the production and active life of new individuals, will induce 
 plants to form these by sexual or by non-sexual means, 
 according to the species of plant. In the spring, when the 
 conditions for active vegetation are becoming more and 
 more favorable, the Equisetums complete the development 
 of the spores non-sexually formed in the foregoing season, 
 and shed these spores, which germinate at once, if at all, 
 and form new plants, the prothalli. These, if conditions 
 favor, soon develop sexual organs and produce new plants 
 by sexual means. In these alternating generations, the non- 
 sexual and the sexual, which follow one another in suc- 
 cessive years, the conditions of the later growing period 
 of one year induce the mature plants to form spores by 
 non-sexual means. The different conditions of the earliest 
 growing period of the following year induce the same plants 
 to complete the development of the spores already formed. 
 The still different conditions of the days and weeks suc- 
 ceeding induce these spores to germinate, the prothalli to 
 form archegonia and antheridia, the embryos to develop 
 into the larger non-sexual plants. These last vegetate 
 for an indefinite length of time. The successive changes in 
 the seasons, in the soil, and in the water-supply, fix the 
 succession of generations which recur according to the 
 peculiar adjustment of the species to all the factors of its 
 environment. Species, genera, families, and orders differ in 
 their adjustments and in their reactions to the stimuli given 
 by the many different factors composing the environment. 
 Comparing sexual and non-sexual reproduction, we see 
 that the advantage of the parents determines what mode 
 of reproduction, if any, shall be carried out. For a long time 
 it has been supposed that sexual reproduction is distinctly 
 advantageous to the species, if not occasionally altogether 
 necessary to those forms capable of reproducing themselves 
 by this means. Such a view is not quite correct. Many 
 species of plants, among them forms which are eminently 
 successful as judged by their numbers and activity, are 
 
REPRODUCTION 277 
 
 wholly incapable of reproducing themselves by sexual means. 
 The bacteria, the higher fungi, and many independent 
 plants illustrate this. Some of the higher algae, archegoni- 
 ates, and flowering plants may go on indefinitely, without 
 reproducing themselves by other than purely vegetative 
 means, and with no evidence of injury to the individual or 
 to the species. It is only when special conditions produce 
 special effects on plants, that they need to reproduce them- 
 selves by sexual or non-sexual spores. Under special con- 
 ditions, reproduction becomes a necessity by becoming 
 advantageous to the individual. The species profits ac- 
 cordingly. 
 
 Certain advantages to the species are conceivable in the 
 spore method of reproduction over any vegetative method, 
 and certain advantages are conceivable in the sexually 
 produced offspring over those non-sexually produced, but 
 these are conceptions rather than proved facts in all but a 
 very few cases. These few cases may or may not be typical 
 of the majority. It is conceivable, for example, that the 
 formation of new individuals by runners, as in the straw- 
 berry, and by suckers, as in the coast red-wood (Sequoia 
 .semper virens), is advantageous to the species. The new 
 individuals hold the territory won by their parents and 
 may even extend it somewhat. The young are nourished 
 by the parent until they have attained size, strength, and 
 development which give them an advantage in competing 
 with other plants for space and for the means of existence. 
 These means of reproduction are, however, defective in 
 that they do not secure the dispersal of the new individuals 
 and consequently do not tend to insure the survival of 
 the species or to extend its territory rapidly enough. The 
 survival of the well-equipped offspring formed by vegetative 
 means is certain so long as the conditions in the space 
 occupied by the parent remain favorable, but such repro- 
 duction is dangerous, because wide dispersal will secure for 
 the species some positions at least in which it can be main- 
 tained whatever may befall individuals in other places. 
 A puddle may become densely populated through the vege- 
 tative reproduction of the algae and animals started there- 
 
278 PLANT PHYSIOLOGY 
 
 in, but unless these give rise to spore or other resting 
 stages all the species will be locally destroyed when the 
 puddle dries. Vegetative reproduction is in many cases 
 the most effective means of multiplying the number of in- 
 dividuals, but it is seldom effective in securing wide dis- 
 persal or in insuring the permanence of the species. These 
 two needs are generally met by the formation of non- 
 sexual or sexual spores and their products (seeds, fruits). 
 The non-sexually formed spores of the ferns, and arche- 
 goniates generally, and of the lower plants, fungi and alga?, 
 are especially adapted by their small size and in various 
 special ways to secure wide dispersal. They may or may 
 not be very resistant. According to the plant which forms 
 them, and according 'to the season at which they are formed, 
 they will be resting-spores or such as must germinate at 
 once if at all. 
 
 New individuals vegetatively produced, and those from 
 germinating non-sexual spores, will contain material from 
 their single parent only. By sexual means, there are united 
 in the offspring substances from both parents. Among the 
 lowest plants ( and animals ) the visible differences between 
 the two parents are only slight. In higher plants, however, 
 where the new individuals are formed by the fusion of 
 sexual elements themselves obviously unlike, formed in 
 organs and by parents also obviously unlike, it is evident 
 that a new balance of forces and matters may be, but not 
 necessarily will be, possessed by the offspring. Maternal 
 and paternal characters may offset or intensify one another ; 
 maternal influences during the development of the spore or 
 embryo may or may not neutralize the paternal influence 
 carried into the germ-cell by the sperm. The offspring 
 sexually produced, representing a new adjustment, may be 
 better adapted to prevailing conditions than was either 
 parent. These are all possibilities, seldom probabilities, 
 almost never certainties. The chances are even that the 
 offspring will be like the average of the species. Some may 
 be worse, a few may be better, but unless the better, when 
 it comes their turn to reproduce sexually, mate with others 
 equally good, their offspring will almost certainly be like the 
 
REPRODUCTION 279 
 
 average of the species. The statistical study of variation,* 
 now so much the fashion among biologists, shows how wide 
 the range of variation is. The inevitable result of combin- 
 ing two different things is the production of a mean or 
 balance. In this balancing or equalizing ( Ausgleich ) , Stras- 
 burgerf sees the essential character of sexual reproduction 
 and its fundamental advantage over all other modes of 
 reproduction. 
 
 In certain species sexual reproduction seems to be abso- 
 lutely necessary to escape degeneration and extinction. 
 This has been shown to be the case among certain Infusoria 
 and Diatoms, but these are special cases in which the indi- 
 viduals by prolonged division (vegetative reproduction), 
 incompletely offset by growth, have become unduly small 
 and weak. By the fusion of two such individuals the nor- 
 mal amount of substance and the normal means of secur- 
 ing energy are furnished to the new individuals. Except 
 in these special cases, and in the highest animals, sexual 
 reproduction does not seem to be necessary to the mainte- 
 nance of the species in a healthy condition, in which it 
 stands a good chance to succeed in the struggle for ex- 
 istence. 
 
 HEREDITY 
 
 The physiologist is confronted with the problem of hered- 
 ity. He sees plants resembling one another in succeeding 
 generations and he hears discussions of the means by which 
 the characters and the tendencies of the parents are trans- 
 mitted to the offspring. What is "heredity," and what are 
 the means by which the offspring are made to possess the 
 characters and tendencies of their parents? "Heredity is the 
 biological law by which living beings tend to repeat their 
 characteristics in their descendants."! The means are of 
 two sorts first, the continuity of substance from parent to 
 
 * Shull, G. H. A quantitative study of variation in bracts, rays, and 
 disk florets of Aster Shortii etc. Arner. Naturalist, vol. 36, 1902. 
 
 t Strasburger. E. f ber Befnichtung. Bot. Zeitung. 1901. 
 
 J See Ritter. W. E. The power and methods of heredity. University 
 Chronicle vol. III.. Berkeley Cal. 1900. 
 
280 PLANT PHYSIOLOGY 
 
 offspring, and second, the continuity of influences to which 
 organisms are exposed. These means are continually em- 
 phasized and for the most part ignored, respectively. The 
 former are for morphology to describe, the latter are within 
 the province of physiology to discuss. Granting the con- 
 tinuity of substance, which we have already considered 
 (pp. 255-63), what are the influences which are continu- 
 ous? 
 
 The environment of an organism is made up of many 
 factors, some great, some small, as judged by their visible 
 effects, some continuous, some constant, some variable, 
 some occasional, some periodic. Among continuous factors 
 the following may be mentioned. First, the atmosphere, the 
 composition and pressure of which are unchanging. Second, 
 water, which has the same composition, the same solvent 
 power, and the same carrying power always. Third, grav- 
 ity, a force of uniform strength. Fourth, the earth as a 
 whole, the character of which changes only with inconceiv- 
 able slowness. If these factors are not absolutely continu- 
 ous, taking infinite time into account, they are continuous 
 and unchanging so far as millions of generations of organ- 
 isms are concerned. They are exactly the same for the 
 offspring as for its parents. 
 
 On the other hand, light and heat are forces which act 
 periodically with great regularity, but which are constant 
 only within rather wide limits. Of the influences which are 
 occasional, other and motile living organisms are perhaps 
 the most striking. 
 
 The continuous and the periodic influences are the ones 
 which have received least attention from those interested in 
 the problem of heredity. These influences must conserve if 
 the variable and occasional influences introduce differences. 
 But as a basis for all conceptions of heredity and variation, 
 we must concede the irritability of living protoplasm. 
 
 The egg, which is a part of the living substance of the 
 mother, is penetrated by the spermatozoid, a part of the 
 living substance of the father, and these two fuse into one 
 mass of living substance. Each particle of living substance, 
 while still within the body of the parent, was influenced by 
 
REPRODUCTION 281 
 
 all the forces, constant and otherwise, which were then 
 operating. Assuming the irritability of living protoplasm, 
 any force which influences it will set up a reaction of some 
 sort in it, though this reaction may not be visible at the 
 time. Between the time of leaving the parent and meeting 
 and fusing with its mate, each sexual element was influenced 
 by and reacted to all the forces then operating. By the 
 fusion of these two living masses into one, the resultant 
 effects of all preceding influences upon the two separate 
 masses are united. From the moment of fusion, the single 
 mass is influenced by all the forces and matters, continuous, 
 constant, and variable, which constitute its environment. 
 If the fertilized egg remain within the body of the parent, it 
 is subjected, during the period of gestation, to mechanical 
 and other influences, some of which absolutely limit it in 
 form and size. For instance, the young zygospore of Spiro- 
 gyra, the young oospore of (Edogonium, the foetal colt, in 
 the body of the mother, are limited as to form and size by 
 the enclosing walls of cell or uterus. They could not grow 
 beyond a certain size, though they may never reach this 
 size, because growth would be mechanically hindered and 
 stopped by the surrounding cell or uterine walls. In form 
 also they are similarly limited, not as the mould fixes 
 the size and shape of the cast, but because, by excess- 
 ive growth in one direction or part, they would encounter 
 the mechanical resistance of the enclosing walls of cell, or 
 uterus. 
 
 The young and forming individual, whether one-celled 
 spore or many-celled embryo, is affected also by food, 
 warmth, position with relation to gravity, etc. Of these 
 influences, we know least about the most constant. The 
 most constant influences have so far baffled the experi- 
 menter to eliminate. Since they are the most difficult to 
 eliminate, it is reasonable to conclude that they are also the 
 most powerful. If they oppose the experimenter, they must 
 also affect the young and forming individual. 
 
 If the fertilized egg is not enclosed within the body of the 
 mother, it too is subjected to all the influences composing 
 its environment, to influences which have changed with in- 
 
282 PLANT PHYSIOLOGY 
 
 conceivable slowness if at all, as well as to changing influ- 
 ences. 
 
 The dominant power of the unchanging influences is shown 
 by man's inability to make fundamental changes in living 
 organisms by experiment, and by the absence of any proof 
 of the * so-called "inheritance of acquired characters." If 
 man could eliminate the force of gravity in experimenting 
 on an organism, he might obtain other results than now, 
 when all he can do is to expose the organism on all sides to 
 the force, or to oppose gravity by centrifugal or other force. 
 If he could change the composition of water, or find a sub- 
 stitute for it, he might make some fundamental change in 
 the organism under experiment. He cannot remove the air 
 from around a plant or animal without at once changing 
 the conditions of his experiment in other ways also, and the 
 result of such experiment is of little value in the subject 
 under discussion, because it is produced by a combination 
 of changes. 
 
 We may safely conclude, then, that the unchanging factors 
 in its environment, to which it irritably responds, are 
 among the most powerful, if they are not the only, influ- 
 ences which make the offspring like its parents. The chang- 
 ing factors cause it to vary. Heredity is possible only 
 because of the irritability and continuity of living proto- 
 plasm, and the continuity of certain influences acting upon 
 this. 
 
 Heredity is a mystery, but so is the form of a crystal of 
 common salt. Spirogyra plants are no more and no less 
 alike than are crystals of common salt. Does common salt 
 " inherit 77 its crystalline form? Crystals of common salt 
 represent the reaction of NaCl molecules to their environ- 
 ment, of which some factors are constant and others chang- 
 ing. Spirogyra cells represent the reaction of the molecules 
 and combination of molecules of which its living protoplasm 
 consists, to their environment, of which some factors are 
 constant and others changing. Given the continuity of the 
 irritable substance (protoplasm) from parent to offspring, 
 heredity and variation are the inevitable results of the 
 constant and of the changing factors respectively, which, 
 
REPRODUCTION 
 
 283 
 
 taken together, compose the environment. These influences 
 at least we can more or less definitely study in their rela- 
 tions to heredity. Though there doubtless are other factors 
 contributing to heredity, they are not yet included within 
 the domain of physiology and need not be discussed here. 
 
INDEX. 
 
 Absorption, Chapter IV., 103-161 
 
 Absorption bands, see Chlorophyll 
 
 Aerenchyma, 144 
 
 Aerobic respiration, see Respiration 
 
 Age of cells, 123 
 
 Air passages, 143; variation in size, 
 142, 143 
 
 Air space, proportion, 155 
 
 Alcohol, see Fermentation, Intramo- 
 lecular Respiration, and Respir- 
 ation 
 
 Alpine plants, brilliancy of flowers, 
 * 274 
 
 Aluminum, 95 
 
 Amides, occurrence, 70; as waste 
 products, 70,71 ; as stages in pro- 
 teid synthesis, 71 
 
 Anaerobic respiration, see Intramolec- 
 ular respiration 
 
 Animals, differences from plants, 1 ; 
 resemblances, 261 
 
 Annual rings, 123; causes of forma- 
 tion, 191-196 ; more than one ring 
 per annum, 195 
 
 Antherozoids, chemotaxis, 235 
 
 AKCEUTHOBITJM, 90 
 
 Arctic plants, brilliancy of flowers, 
 274 
 
 Ascent of water, 119; Sachs's hypo- 
 thesis, 119, 121; Godlewski's, 
 119, 121, 122; Strasburger's, 120, 
 122, 123; sap-pressure hypothe- 
 sis, 120, 127, 131, 133; gas press- 
 ure, 120; Jamin's chains, 120 
 
 Ash constituents, 92+; minimum 
 amounts, 101, 102 
 
 Associated organisms, 252. See also 
 root tubercle plants, 72-j- ; humus 
 plants, 78+ ; carnivorous plants, 
 81+; parasites, 85+; lichens, 91, 
 92 
 
 Autumn wood, 123; causes of forma- 
 tion, 194 
 
 Auxanometers, 177 
 
 B 
 
 Bacteria, respiration of, 20, 21 ; in 
 fermentations, 30-37; Engel- 
 mann's bacteria method, 56; nitri- 
 fying, 68; in root tubercles, 72- 
 76 ; N-fixing, 76 ; associated with 
 carnivorous plants, 83; sulphur 
 bacteria, 20, 98; iron bacteria, 
 20 ; nitrite and nitrate bacteria, 20 
 
 Bacteroids, 74 
 
 Bleeding, 127, 132 ; influence of tem- 
 perature, 134 ; pressure in bleed- 
 ing, 134, 135; amounts, 136 
 
 Breathing pores, see Stomata 
 
 BRUGMANSIA, 90 
 
 Calcium, 99,100; Loew's hypothesis, 
 100 
 
 Calories, liberated in respiration, fer- 
 mentation, intramolecular respir- 
 ation, see these topics 
 
 Carbon, source of, 43; percentage in 
 air, 44; quantity in air, 44 
 
 Carbon dioxide, rate of absorption, 
 45; means of maintaining sup- 
 ply, 45, 46 ; change in percentage 
 in air, 46 ; effect of increased per- 
 285 
 
286 
 
 INDEX. 
 
 centage, 46; principles of diffu- 
 sion, 47; aeration of the plant 
 body, 47, 48, diffusion through 
 cell wall, 48, 49; stomata, 49; 
 Stahl's test, 49; conditions for 
 absorption, 50; for elaboration, 
 50 ; rate of diffusion, 50 
 
 Carnivorous plants, 81; bacteria as- 
 sociated with, 83 ; enzymes, 83 
 
 Carotin, 52 
 
 Cell-division, relation to growth, 165; 
 relation to gravity, 198, 199 
 
 Cell-sap, active agent in absorption, 
 106; density and composition, 
 how maintained, 111, 112 
 
 Chasmogamy, 274, 275 
 
 Chemical fertilization, 230, 231, 260 
 
 Chemical stimuli, 226, 237 ; conditions 
 of influence, 227 
 
 Chemotaxis, 233-237 
 
 Chemotropism, 231-233, 236, 237, 245 
 
 Chlorophyll, location, 51; physical 
 characteristics, 51; mixture of 
 pigments, 51; associated pig- 
 ments, 51; carotin, 52; solvents, 
 52; fluorescence, 53; spectrum, 
 53 ; absorption bands, 53 ; amount 
 of light absorbed, 54; efficiency 
 of chlorophyll screen, 55; pro- 
 portion of light absorbed, 55; 
 amount, 55; dependence upon 
 iron, 57, 58, 101; upon light, 57; 
 in guard ceils of stomata, 147 
 
 Chlorophyll grains or granules, see 
 Chromatophores 
 
 Chloroplastids, see Chromatophores 
 
 Chlorosis, 101 
 
 Chloro vaporization, 137 
 
 Chromatophores, body and pigment, 
 57; photosynthesis in isolated 
 Chromatophores, 57; position af- 
 fected by light, 217, 218. See also 
 Chlorophyll 
 
 Circumnutation, 246 
 
 Cleistogamy, 272, 274, 275 
 
 Climate, continental and island, 220, 
 221 
 
 Colloids, 129 
 
 Colored screens, 56 
 
 Common salt, 93, 227 
 
 Comparison of photosynthetic and 
 respiratory activities, 65 
 
 Conditions essential to life, 6 
 
 Conducting tissues, 156, 157 
 
 Contact, irritation by, 239-251; ef- 
 fect 011 amount of growth, 240, 
 241 
 
 Correlation, 251, 252 
 
 Corrosive action of roots, 125 
 
 Crystalloids, 129 
 
 CUSCUTA, 88-90.. 244-246 
 
 Cytoplasm, ratio between, and nu- 
 cleus, 181 
 
 Cytoplasmic membranes, 106 
 
 D 
 
 Dew, 128 
 
 Diastase, 64 
 
 Diffusion, principles of, 47; rate of 
 CO 2 diffusion through minute 
 openings, 50 ; physics of, 108 
 
 Dissociated atoms, effect of, 228, 229 
 
 Dodder, 88-90, 244-246 
 
 DROSERA, 82, 83 
 
 E 
 
 Ecology, 252, 253 
 
 Ectoplast, 106 
 
 Electricity, 237-239; influence on 
 amount of growth, 238; on ger- 
 mination, 239; on direction of 
 growth, 239; on locomotion, 239 
 
 Energy, need of, 12 
 
 Engelmann's bacteria method, 56 
 
 Enzymes, formed in connection with 
 respiration, 19, 29, 30, 33; zy- 
 mase, 33; diastase, 64; effect of 
 light on, 209, 210; in carnivorous 
 plants, 83 , 
 
 EQUISETUH, reproduction, 276 
 
 Excretion, by roots, 125; of water, 
 126-128 
 
 Exercise, advantage of, 191 
 
INDEX. 
 
 287 
 
 " Fatness " of leaves, 64 
 
 Fermentation, definition, 30; distinc- 
 tion from decay and disease, 30 ; 
 alcoholic, 31; by-products in al- 
 coholic, 31 ; zymase in alcoholic, 
 33 ; amount of alcohol formed by 
 fungi, 31, 33, 34; lactic, 34; bu- 
 tyric, 34; oxidizing, 34, 35; ace- 
 tic, 35; disease, 35; diphtheria, 
 35; mixed, 36 
 
 Fermentations, 30-37 
 
 Fertilization, essential feature of, 259; 
 advantage, 260, 261; processes 
 succeeding, 188; chemical, 230, 
 231, 260 
 
 Fischer's hypothesis, 62 
 
 Flower-forming substances, 273 
 
 Food, influence on reproduction, 265, 
 266, 273, 274 
 
 Food distribution, Chapter IV., 103- 
 161 ; translation, 63, 155+ 
 
 Food manufacture, see Photosynthe- 
 sis, Nitrogenous foods 
 
 Force exerted in growth, 174-176; in 
 geotropic bending, 208; by tur- 
 gor, 110, 111, 130, 131 
 
 Formic aldehyd hypothesis, 61; Spi- 
 rogyra cultures as a test, 61 
 
 Gall-insects, effect on growth of 
 
 wood, 195 
 
 Galvanotaxis, see Electricity 
 Galvanotropism, see Electricity 
 Gases, movement, 103, 104 ; exchange 
 through stomata, 142 ; rate of dif- 
 fusion through stomata, 50 ; rate 
 through epidermal cells, 143; 
 movement, 151+ ; pressures with- 
 in plant, 151-154; composition of 
 enclosed gases, 152-154; propor- 
 tion of air space in plants, 155 
 Geotropism, definition, 198; of roots, 
 198-206; sensitive region, 200, 
 201; region of response, 200; la- 
 
 tent period, 201 ; transmission of 
 stimulus, 201, 203, 204; nature 
 of stimulus, 201, 202; effect of 
 stimulus in cell, 202 ; position of 
 greatest sensitiveness, 204; rela- 
 tion of sensitive parts to light, 
 207, 214, 215; force exerted in 
 bending, 208; comparison with 
 heliotropism, 215 
 
 Germination, 9, 10; influence of elec- 
 tricity, 239; influence of light, 
 213; influence of contact, 240, 
 247 
 
 Gravitation, 196-208; opposed by air, 
 water, soil, 197; effect on rate of 
 growth, 197 ; effect on first divi- 
 sions of egg, 198, 199 ; immediate 
 effects in sensitive parts, 197 ; ef- 
 fect on parts of different specific 
 gravity in cell, 202; chemical 
 changes in cell in response, 202. 
 See also Geotropism 
 
 Growth, Chapter V., pp. 162-182; 
 effect on respiration, 38 ; depend- 
 ence upon irritability, 162-164, 
 183, 240, etc. ; definition, 164, 
 165; relation of cell division, 
 165; stages, 166; relation of 
 water, 167; factors making pos- 
 sible, 168; time of most rapid, 
 169-171 ; periodicity, 170, 171 ; ir- 
 regularity, 171 ; relation of tur- 
 gor, 171-173 ; relation of mechan- 
 ical restraint, 174, 187; force 
 exerted, 174-176; measuring in- 
 struments, 177; rates of, 177- 
 179; maximum size, 180-182; pro- 
 portion between cytoplasm and 
 nucleus, 181 
 
 H 
 
 Halophytes, 94, 95; resemblance to 
 Haustoria. 245; xetrophytes, 95 
 
 Heat, one of the conditions of life, 6 ; 
 minimum, optimum, and maxi- 
 mum temperatures, 219, 220; ef- 
 fect on rate of movements, 221, 
 
2 88 
 
 INDEX. 
 
 222 ; on direction of movements, 
 221 ; as an element of climate, 
 220, 221 ; liberated in respiration, 
 fermentation, intramolecular res- 
 piration, see tliese topics 
 
 Heights of trees, 119 
 
 Heliotaxis, 216. See Light 
 
 Heliotropism, 213-215. See Light; 
 comparison with geotropism, 
 215 
 
 Heredity, 279-283; definition, 279; 
 means, 279, 280; environmental 
 factors, 280; prenatal influences, 
 281; subsequent influences, 281- 
 283 
 
 Hot springs, plants of, 220 
 
 Humus, 78; humus plants, 78; asso- 
 ciation with other plants, 78-80 
 
 Hydathodes, 128 
 
 HYDKODICTYON, reproduction, 266- 
 268 
 
 Hydrotaxis, 226 
 
 Hydrotropism, 224-226; balance be- 
 tween, and geotropism, 224. See 
 Water 
 
 Hygroscopic movements, 226 
 
 Insectivorous plants, see Carnivorous 
 Intercellular spaces, 142, 143 
 Intramolecular respiration, 16. See 
 
 Respiration 
 
 Intramolecular respiration, 16 ; defini- 
 tion, 25, 26 ; inherent in all organ- 
 isms, 25; duration limited in 
 higher plants, 25, 27; substances 
 concerned in, 28; products, 8. 
 See Fermentation and Respiration 
 Inuliu, 160, 161 
 Iodine, in marine plants, 113 
 Iron, 101 ; as stimulant, 101 ; relation 
 to chlorophyll formation, 101; in- 
 crustations, 126 
 
 Irritability, Chapter VI., 183-253; 
 possible physical reasons for, 
 184-186; effect on growth, 186. 
 
 See Geotropism, Heliotropism, 
 Water currents, etc. 
 
 Jamin's chains, 120 
 
 Latent period, 201, 243 
 
 Laticiferous tubes, 159, 160 
 
 Leguminous plants, growth in steril- 
 ized and unsterilized soil, 75; 
 seeds of, 261. See Root-tuber- 
 cles 
 
 Lenticels, 144 
 
 Leptom, 158, 159 
 
 Leptomin, 159 
 
 Leucoplastids, 160 
 
 Lichens, 91, 92 
 
 Life, definition, 256; essential condi- 
 tions, 6 ; how limited, 254-256 
 
 Light, one factor essential to life, 6; 
 relation to food manufacture, 50, 
 58-63, 69, 70; relation to chloro- 
 phyll, 52-58; component rays, 56; 
 values of different rays, 57 ; rela- 
 tion to chlorophyll formation, 57 ; 
 effect on organic substances, 208- 
 210; effect on rate of growth, 
 210-213; on form, 211; on perio- 
 dicity of growth, 211 ; on sub- 
 mersed aquatics, 211-213; on 
 germination, 213; on direction of 
 growth, 213-215; relation to geo- 
 tropism, 214; comparison of he- 
 liotropism and geotropism, 215; 
 effect on locomotion, 216; effect 
 on position of cell organs, 217, 
 218; comparison of heliotropism 
 and heliotaxis, 218, 219; influence 
 on reproduction, 264-268, 271- 
 276; on brilliancy of flowers, 274, 
 275 
 
 Lime incrustations, 126 
 
 Living, definition, 4 
 
 Locomotion, influences affecting, see 
 Chemotaxis, Phototaxis, etc. 
 
INDEX. 
 
 289 
 
 Magnesium, 100, 101 
 Maple sap, 132, 133, 136 
 Mechanical effect of water, 189-191 ; 
 
 of other substances, 226 
 Mechanical force, see Force 
 Mechanical pull, effect on growth, 
 
 187, 188 
 Mechanical restraint, effect on 
 
 growth, 174, 187 
 Milk-tubes, 159, 160 
 MIMOSA, 247-251 
 MIMULUS, reproduction, 270-273 
 Mistletoe, 86-88 
 Motor zone, in roots, 201 ; in tendrils, 
 
 242 
 Movement, of gases, 103, 104, 142; of 
 
 water, Chapter IV., 103-161 
 Movements, see Growth, Irritability, 
 
 Mimosa, etc. 
 Mycorhiza, 79, 80 
 
 N 
 
 Nectaries, 126 
 
 NEPENTHES, 84, 85 
 
 Nitrates, 69 
 
 Nitrifying bacteria, 20, 68 
 
 Nitrogen, distribution in plant, 66; 
 occurrence in nature, 67 ; sources, 
 67 ; nitrifying bacteria, 68 ; N-fix- 
 ing bacteria, 75, 76; cycle of N 
 in nature, 77 
 
 Nitrogen bacteria, 20, 68, 75, 76 
 
 Nitrogenous foods, occurrence, 70; 
 origin, 70; manufacture, 70, 71; 
 storage, 71, 72; use, 71, 72. See 
 Amides, Proteids 
 
 Non -sexual reproduction, advantages 
 277-279 
 
 Nucleus, relation to growth, 181 
 
 Nutrient solutions, means of transfer, 
 116-125 
 
 Nutrition, Chapter III., 40-102; rela- 
 tion to respiration, 40, 41 ; stages, 
 41; furnishes material, 42; food 
 materials, 42 ; essential elements, 
 
 TO 
 
 42; characteristic element, 43. 
 See Photosynthesis, Nitrates, Ni- 
 trogenous foods, etc. 
 
 Osmosis, 106, 108-110 
 
 Osmotic pressure, 110; effect of 
 changes in, 229, 230 
 
 Oxidation, conditions, 13, 14; in res- 
 piration, 18-20; of NH, com- 
 pounds, 20; of H 2 S and S, 20; of 
 Fe compounds, 20, 21. See Res- 
 piration 
 
 Oxygen, optimum percentage, 14; 
 effect of excess, 15; in respira- 
 tion, 18-20 
 
 Parasites, 85-92; Arceuthobium, 90; 
 
 bacteria in root-tubercles, 76; 
 
 Brugmansia, 90; Cuscuta, 88-90; 
 
 lichens, 91, 92; PJwradendron, 
 
 86-88; Rqfflesia, 90; Basoumow- 
 
 stoa, 90; Viscum, 86-88 
 Parasitism, 85; advantageous, 86 
 Parthenogenesis, 259, 260 
 Petioles, irritable by contact, 244 
 Phosphorus, 96, 97 
 Photosynthesis, 58-66 
 Phototaxis, see Light 
 Phototropism, see Light 
 Physiology, aim, 2 
 Plageo tropic organs, 198 
 Plants, differences from animals, 1; 
 
 resemblances, 261 
 Poisons, stimulating and other effects, 
 
 230 ; action of dissociated atoms, 
 
 228, 229 
 
 Potassium, 98, 99 
 Protoplasm, a structure, 7 
 Pulvinus, 247 
 
 R 
 
 RAFFLESIA, 90 
 
 RAZOUMOWSKIA, 90 
 
 Removal of manufactured foods, 63 
 
290 
 
 INDEX. 
 
 Reproduction, Chapter VII., pp. 
 254-283 ; chief end, 257, 258 ; defi- 
 nition, 257; modes, 259; sexual, 
 259, 271, 272, 275-279; non-sex- 
 ual, 259, 277-279; precautions to 
 secure, 261; vegetative, 261-263; 
 influence of environment, 263- 
 276 ; stimuli determining mode in 
 Vaucheria, 264-266, 275; in Hy- 
 drodictyon, 266-268; in Stigeodo- 
 nium, 268, 269; influence of os- 
 motic pressure, 268, 269; stimuli 
 affecting reproduction in flower- 
 ing plants, 269-275; in Mimulus 
 Tilingi, 270-273; in Viola odo- 
 rata, 274, 275 ; in Equisetum, 276 ; 
 in Sequoia, 277 
 
 Resin, 126 
 
 Respiration, Chapter II., pp. 12-39; 
 definition, 13; rates at different 
 times, 15; regulated by proto- 
 plasm, 16; reduction of, 16; sus- 
 pension, 16; object, 16; yield in 
 energy, 17, 22; substances con- 
 cerned, 17, 18; products, 18; 
 characteristic product, 21; heat 
 of combustion of sugar, 22, 23-; 
 heat of alcoholic fermentation^ 
 23, 24; of butyric, 24; of acetic, 
 24; of combustion of alcohol, 24; 
 relation of enzymes to respiration, 
 19, 29, 30, 33 ; optimum temper- 
 ature, etc., 37; effect of injuries, 
 37, 38 ; rate in relation to growth, 
 etc., 38; ratio of O 2 to CO 2 , 38; 
 variation in ratio, 38, 39 ; amounts 
 of CO a given off, 39; effective- 
 ness of bacteria, 39; summary, 
 39. See Fermentation, Intramo- 
 lecular respiration 
 
 Response to stimuli, see Geotropism, 
 Irritability, etc. 
 
 Rheotaxis, 190 
 
 Rheotropism, 189 
 
 Roots, 113; corrosive action, 125; 
 early growth in spring, 194; geo- 
 tropism of, 198-206 ; heliotropism, 
 
 214; thermotropism, 221; hydro- 
 tropism, 224 ; chemotropism, 231 ; 
 galvanotropism, 239 ; thigmotrop- 
 ism, 247 
 
 Root-hairs, 114, 115 
 Root-pressure, see Sap-pressure 
 Root-tubercles, occurrence, 72; struc- 
 ture, 74; contents, 74; mode of 
 infection, 75; bacteria parasitic 
 in, 76 ; fix free N, 75 
 
 Salt, 93, 227, 228 
 
 Salt plants, see Halophytes, 94, 95 
 
 Sap, composition of maple, 132 
 
 Sap-flow, 132, 133, 136 
 
 Sap-pressure, distinction from turgor 
 pressure, 131; figures, 135, 136; 
 relation to ascent of water, 120, 
 127, 131, 133; "root-pressure," 
 135 
 
 SAKHACENIA, 84 
 
 Sea-water, 94, 95, 197 
 
 Secretion, 125-130 
 
 Seeds, respiration in air-dry, 9; dur- 
 ation of vitality, 10 
 
 Selective power of roots, etc., 112, 
 113 
 
 "Sensitive plant," see MIMOSA 
 
 SEQUOIA, reproduction, 277 
 
 Sexual reproduction, see Reproduc- 
 tion ; necessary?, 276-279; in in- 
 fusoria, 279; in diatoms, 279 
 
 Sieve-tubes, 157-159 
 
 Silica, 93 
 
 Soil, comparison of, in Eastern and 
 Western States, 11 
 
 Soil water, 104, 105 
 
 Span of life, how limited, 254-256 
 
 "Spring wood," 123, 191-194 
 
 Staining living protoplasm, 107 
 
 Stamens, sensitive to contact, 251 
 
 Starch, proportional and structural 
 formulae, 59; possible mode of 
 formation, 61, 62; translocation, 
 63, 64, 99; in sieve-tubes, 158; 
 storage, 160 
 
INDEX. 
 
 291 
 
 Stems, geotropism of, 306, 207 
 
 St reotropism, see Thigmotropism 
 
 STIGEOCLOXIUM, reproduction, 268, 
 269 
 
 Stigma, secretion on, 129; sensitive 
 to contact, 251 
 
 Stimulants, 230 
 
 Stomata, 49, 142; structure, 145; 
 mechanism of opening and clos- 
 ing, 145-147 ; function of auxiliary 
 cells, 145, 148; size, 149; propor- 
 tion to leaf surface, 149; condi- 
 tions of opening and closing, 148- 
 150; with fixed guard cells, 151 
 
 Storage of foods, 160 
 
 Sulphur, 97, 98 
 
 Sulphur-bacteria, 20, 98 
 
 Sunlight, see Light 
 
 Temperature, fatal, 8, 9 
 
 Tendrils, 241-244 
 
 Thermotaxis, 221 
 
 Thermotropism, 221 
 
 Thigmotropism, 247 
 
 Tides, effect on growth, 190, 191 
 
 Tonoplast, 106 
 
 Toxic substances, see Poisons 
 
 Traction, iufluence s on growth, 187, 188 
 
 Transfer of nutrient solutions, 116-125 
 
 Translocation of foods, 63, 155, 156 
 
 Transpiration, 136-141 ; conditions, 
 136, 137; difference from evapora- 
 tion, 137 ; means of reducing, 137, 
 138; means of increasing, 138; in 
 moist tropics, 138-140; move- 
 ments increasing, 142 
 
 Traumatropism, 247 
 
 Trees, heights, 119 
 
 Tropical plants, transpiration in, 138- 
 140 
 
 Turgor, definition, 111; relation of 
 potassium salts, 99; relation to 
 
 growth, 171-173 
 Twining plants, 245, 246 
 
 U 
 
 UTRICULAKIA, 85 
 
 Vascular bundles, 117 
 
 VAUCHERIA, reproduction, 264-266, 
 
 275 
 
 VIOLA, reproduction, 274, 275 
 VISCUM, 86-88 
 Vitality, suspended, 10 
 
 W 
 
 Water, vehicle of food materials, 6; 
 essential component of active 
 protoplasm, 6-8 ; in air-dry seeds, 
 8; in soil, 104, 105; conducting 
 tissues, 117, 118; ascent of, 119- 
 123, 127, 131, 133; storing tissues, 
 124; secretion, 126-128; pores, 
 128; glands, 128; flow, 132, 133, 
 136; relation to growth, 167; ef- 
 fect on growth, 222-226 
 
 Water-currents, effect on growth, 189, 
 190 
 
 Waves, effect on growth, 190, 191 
 
 Weber's law, 236 
 
 Weeping, 132, 134-136 
 
 Winds, 189, 226 
 
 Winter-killing, 150 
 
 Xerophytes, 95 
 
 Zinc, 95 
 
 "Zinc soil flora," 95 
 
 Zoospores, influence of contact, 240, 
 
 247 
 Zymase, 33 
 

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