Light and the behavior of organisms

Preface

Parts I and II of this volume consist of an essay for which the Cartwright Prize was awarded by the College of Physicians and Surgeons of Columbia University in 1909. The entire volume is the outgrowth of an intensive and extensive study of the processes of orientation in plants and animals, especially those without eyes, i.e., a study of the perplexing and interesting question as to how these organisms regulate their activities so as to bend or move toward or from the source of stimulation. But while the book deals primarily with the question of orientation, it has a broader aspect and may be considered a treatise on the behavior of organisms based on their reactions to light. The generality of the treatment of the subject of actions in organisms, including plants as well as animals, it is hoped will make the work of value to all students of nature, especially to those interested in comparative psychology, zoology, botany and physiology.

Throughout the work it has been my aim first of all to state precisely what organisms do under different conditions of illumination, and then to consider the bearing of the observed reactions on the various theories that have been formulated regarding reactions in general. This aim has made it necessary to present somewhat lengthy and detailed descriptions of methods of stimulation and responses which, it is feared, may be rather tedious to those who are interested only in the general aspect of the problem. To such it will be of particular advantage to consult freely the table of contents and the summaries.

The historical chapters which are found in Part I deal with the origin and development of theories regarding the activities of organisms, especially those associated with light. No attempt has been made in these chapters to

^\^: ., 26585

IV PREFACE

review all the literature on behavior. Only such works are here referred to as appear to have a theoretical bearing, but many others are considered elsewhere.

Part II is dc\oted largely to the description and discussion of experimental observations on orientation made by the author during the past five years. Only a few of these have been previously published. The remaining parts of the book are more general and contain relatively much less original matter.

A large part of the experimental work connected with this volume was done at Johns Hopkins University during my residence as Johnston Research Scholar. To this institution I am greatly indebted, not only for the scholarship, but also for exceptional facilities placed at my command by the late Professor W. K. Brooks, Director of the Zoological Laboratory during my residence, and for friendly courtesies extended on every hand by other members of the University. I am also under obligation to the Marine Biological Laboratory of Woods Hole, Massachusetts, for research facilities during the summer of 1907, and to the United States Bureau of Fisheries for similar privileges during the following two summers, and especially to the Director of the Laboratory of the Bureau of Fisheries at Woods Hole, Massachusetts, Doctor F. B. Sumner, for generously supplying my needs. It is a pleasure to acknowledge my further indebtedness to Professor H. S. Jennings for his enthusiastic interest and support in the work at all times and for critically reading the manuscript; to Professors G. H. Parker and J. B. Watson for valuable suggestions after reading much of the work in manuscript; to Professor R. M. Yerkes for his thorough criticism regarding both composition and contents; and to my wife, Grace Tennent Mast, for invaluable literary aid and criticism. The author however must be held responsible for

all of the subiect-matter*. c. r\ tv/t

•' Samuel Ottmar Mast.

Baltimore, Maryland, February 4, 1910.

CHAPTER I General Introduction

CHAPTER H

Historical Review Concerning the Origin and Development of Ideas and Theories Regarding Movements in Plants and Animals with Special Reference to the Question of Tropisms

PAGE

1. Early Investigations and Ideas concerning Movement in Or-

ganisms S

2. First Attempts at Mechanical Explanation of Life Phenomena

— Galen, Harvey, Descartes, Borelli, Ray 6

3. Period of VitaUsm 8

4. Return to Mechanical Explanations — Johannes Miiller, De

CandoUe 8

5. Evolution and its Effect on the Study of Behavior in Plants

and Animals — Darwin, Bert, Graber, Romanes, Lubbock, Preyer 9

6. Introduction of the Term " Tropism " and Development of its

AppHcation to Different Reactions — De Candolle, Knight, Frank, Hofmeister, Darwin 11

7. Further Analysis of Reactions in Plants to Light — Sachs,

Strasburger, Engelmann, Darwin 13

Chapter III

Historical Review Concerning the Origin and Development of Ideas and Theories Regarding Movements in Plants and Animals with Special Reference to the Question of Tropisms (continued)

I. The Application of the Underlying Principle of Tropisms in the Study of Animal Behavior as opposed to this Study from the Point of View of Comparative Psychology — Loeb,

Verworn, Davenport, Radl and Others 23

V vi TABLE OF CONTEXTS

2. More Thorough Experimental Analysis showing the Relative page

Importance of Internal and External Eactors in Behavior — Jennings, Holmes and Others 44

3. Summary of Historical Review ' 51

4. Various Definitions of Tropisms 53

5. Statement of Important Problems in the Study of Reactions

to Light 57

Part II

EXPERIMENTAL OBSERVATIOXS AND DISCUSSIOXS BEAR- ING ON THE QUESTION AS TO HOW ORGANISMS {ESPECIALLY THOSE WITHOUT EYES) BEND OR TURN AND MOVE TOWARD OR FROM A SOURCE OF STIMULATION

Chapter IV

Processes Involved in the Bending of Different Parts of Higher Plants toward the Source of Light

I. Observations on Plumules of Indian Corn (Zea mays) and

Leaves of Tropaeolum 5v

a. Introduction; h. Apparatus; c. Experiments; d. Results; e. Discussion.

Chapter V

Observations on Unicellular Forms in the Process of Attaining and Retaining a Definite Axial Position with Reference to the Source of Light

1. Myxomycetes and Rhizopods 74

2. Euglena 80

a. Description; h. Historical account; c. Orientation in light from two sources; d. Material; e. Method of locomotion; /. Accuracy of orientation; g. Mechanics of orientation in Euglena x in the crawling state; h. Discussion; i. Orientation of Euglena in the swimming state; 7. Threshold or sensitiveness when different surfaces are exposed to light; k. Function of the eye -spot.

3. Summary no

TABLE OF CONTENTS

PAGE

2. Blowfly Larvae — Musca sp. (?) 175

a. Introduction; b. Locomotion; c. Accuracy of orientation; d. Orientation in light from two sources; e. Orientation and movement — (ij perpendicular to the direction of the rays — (2) toward a source of hght; /. Sensitive region; g. Effect of light intensity on rate of locomotion; //. Method; i. Mechanics of orientation; j. Discussion; k. Summary.

3. Earthworms ig8

4. Planaria 206

5. Echinoderms 211

CHAPTER X

Concerning the Question of Orientation in Mollusks, Arthropods AND Vertebrates, with Special Reference to Circus Movements AND their Bearing on this Question

1. General Account of Orientation 214

2. Circus Movements 215

3. Frogs and Toads 218

A. Bufo americanus.

a. Method; b. Orientation in light from two sources; c. Orientation with one eye destroyed.

4. Caprella 223

a. Orientation; b. Discussion.

5. General Summary and Conclusions of Part II 228

I

Part III General Consideration Of Reactions To Light

CHAPTER XI

Adaptation, Formation of Aggregations in Regions of a Given Light Intensity and Different Methods of Response in Attaining this Region and Remaining in it.

1. Introduction showing that Reactions in general are Adaptive. 236

2. Different Reactions observed in the Process of Collecting in

Regions having a given Condition of Illumination 239

TABLE OF CONTENTS

CHAPTER VI ye ^age

Observations on Unicellular Forms in the Process of Attaini AND Retaining a Definite Axial Position with Reference •' THE Source of Light (continued)

PAGE

1. Stentor cocrulcus 1 13

a. Introduction; b. Orienting reactions; c. Difference in sensitiveness witli different surfaces illuminated; d. Localized stimulation; e. Summary.

2. OeJ Jgjniura Swarm-spores 1 23

a. Description; b. Material; c. Locomotion; d. Orientation in light.

3. Trachelomonas 1 28

4. Chlamydomonas alboviridis (Stein) 131

5. Chlorogonium i34

6. Paramecium i34

CHAPTER VII The Factors Involved in the Process of Orientation in Colonial

Forms

1. Volvox globator and minor 136

2. Pandorina and Eudorina 146

a. Function of the eye-spots.

Chapter VIII

Observations on the Responses Involved in the Regulation of Movement toward the Source of Light in Coelenterates

1. Hydra viridis 149

a. Historical review; b. Effect of light intensity on activity; c. Orientation and locomotion; d. Reactions of negative specimens; e. General conclusions.

2. Eudendrium Planulae I59

3. Eudendrium Hydranths , 163

4. Reactions of Medusae 164

Chapter IX

Regulation in the Direction of Movement with Reference to the Source of Light in Vermes, Fly Larvae, and Echinoderms

I. Arenicola cristata — Larvae 166

a. Description; b. Locomotion; c. Orientation; d. Mechanics of orientation; e. Discussion; /. Orienting stimulation; g. Summary.

TABLE OF CONTENTS ix

a. Random movements and avoiding reactions; b. Orienta- page tion, change in sense of orientation, and avoiding reactions; c. Orientation and extent of movement limited by environment; d. Orientation and movement directly toward the place where the organism comes to rest; e. Random movements and coming to rest in a given place.

CHAPTER XII

Reactions to Light which do not Result in Aggregation or

Orientation

1. Reactions to Shadows — Protective 247

2. Reactions to Shadows — Procuring Food 249

3. Reactions to Sudden Increase of Light Intensity 250

4. Reactions to Light caused by the Effect of Continued Illumi-

nation 252

5. Classification of Reactions to Light — Phototropism, Pho-

topathy 253

6. Reclassification of Reactions to Light 256

(i) On the basis of the character of the stimulus. a. Reactions to change of intensity; b. Reactions to constant illumination; c. Reactions of questionable cause. (2) On the basis of the fundamental causes of the response. a. Reactions caused by the direct effect of light on the reacting tissue; b. Reactions caused by an indirect effect of light; c. Reactions due, not to any effect of light in itself, but to what a given light condition or configuration may represent.

7. Evolution of the Reactions to Light 262

CIL\PTER XIII

Factors Involved in Regulating Reactions to Light — Variability

and Modifi ability in Behavior

I. Change in Sense of Reactions 265

a. Effect of intensity of light; b. Effect of change in temperature — Original observations; c. Effect of chemicals — Original observations; d. Effect of concentration of the medium and mechanical stimuli; c. Effect of internal

changes.

X TABLE OF CONTENTS

CHAPTER XIV

Factors Ina'olved in Regulating Reactions to Light — Variability ANT) MoDiFiABiLiTY IN BEHAVIOR (continued)

PAGE

1. Changes in Sensitiveness, in the Optimum, and in Various Other

Features regarding Reactions 288

2. General Summary of Part III 298

Part IV Reactions In Light Of Different Wave-lengths Or

COLORS

CHAPTER XV Energy, Photochemical Reactions, and Brightness

1. Energy Distribution in the Spectrum 304

2. Brightness Distribution in the Spectrum 305

3. Distribution of Actinic Effect in the Spectrum 308

CHAPTER XVI Effect of Different Rays on the Reactions of Sessile Plants I. Summary 319

CHAPTER XVII

The Relative Effect of Different Rays on the Reactions of

Unicellular Forms

1. Strasburger's Experiments 321

2. Engelmann's Experiments 322

a. Diatoms and Oscillaria with different species of Navicula and Pinnularia as types; b. Ciliates which have chlorophyll with Paramecium bursaria as a type; c. Flagellates with P3uglena \iridis as a type.

3. Verworn's Experiments 326

4. Experiments of Harrington and Leaming on Amoeba 327

5. Original Observations on Amoeba 328

a. Experiments with color filters; b. Experiments with the solar spectrum.

TABLE OF CONTENTS xi

CHAPTER XVIII

Reactions of Multicellular Animals in Light Consisting of Waves

Differing in Length

PAGE

1. Experiments of Wilson on Hydra 333

2. Bert's Experiments on Daphnia 336

3. Lubbock's Experiments on Daphnia 337

4. Experiments of Yerkes on Simocephalus 341

5. Experiments of Graber on Various Animals 343

6. Loeb's Observations 346

CHAPTER XIX

Brief Consideration of the Reactions of Multicellular Animals With Well-developed Eyes in Light Differing in Color — With Special Reference to Color Vision.

3. Higher Crustacea — Experiments of Minkiewicz 355

4. Fishes 35^

5. General Summary and Conclusions of Part IV 360

CHAPTER XX

Index 393

LIGHT AND THE BEHAVIOR

OF ORGANISMS

Chapter I General Introduction

That plants and animals respond to stimulation by light is a matter of common information. It is also well known that many of the motile forms collect in regions of a given intensity of light; that many orient, some moving or turning toward a source of light, others away from it; and that many go toward a source of light under certain conditions and away from it under others. The distribution of the power to respond to stimulation by light in the plant and animal world has likewise been quite fully ascertained,^ and numerous accurate observations concerning the precise methods of response have been recorded. There is however still much contention as to the explanation of these phenomena, and it is this that concerns us chiefly in this work. In what manner and for what reasons do organisms collect in regions of certain light intensity? How do they

The works of these authors referred to by means of the dates following each name, as well as those of all other authors similarly referred to in the text, will be found in the bibliography.

I

j

N. C State Collegt behave in light of different colors? What are the factors involved in orientation, i.e., in attaining a definite axial position with reference to the source of stimulation? How do organisms regulate the direction of movement; how do they remain oriented? What is the cause of reversal in the sense of orientation? What controls variability and modifiability in reactions to light? Are the reactions adaptive? These are the principal problems before us, problems which cannot fail to be of interest to all who are in any way concerned with the activities of organisms. ; \'arious solutions of these problems have been offered by different investigators. Some say that motile plants and animals orient and collect in light of a given intensity because the particular intensity in which they congregate pleases them more than any other, implying that there are psychic phenomena involved in the process and indicating that it is difference of light intensity in the field which controls the direction of movement. Others say the reactions are not fundamentally adaptive and can be explained mechanically; that the movements of organisms are, with few exceptions, regulated by the direction in which the rays of light penetrate the tissue or by the angle which the rays make with the sensitive surface or by the relative intensity on symmetrical opposite sides. Light is supposed by these investigators to act constantly as a directive stimulus. The organisms are automatically controlled by external factors. Still other authors claim that the reactions to light are in general useful to the organism, but that they can be accounted for mechanically and that the essential controlling factor is a change of intensity on the surface of the organism; that the other external factors mentioned are of importance only in so far as they make such a change possible; that light does not act constantly as an orienting stimulus, and that internal physiological processes have much to do with the reactions. Some maintain that only the more refrangible rays of the spectrum, those toward the violet end, are efficient in stimu-

GENERAL INTRODUCTION 3

lating the organisms, others appear to be equally positive that all rays are active in this process, and still others say that the stimulating efficiency of different rays varies in different organisms and in the same organism under different conditions.

Many investigators have apparently not thoroughly analyzed the problems concerning reactions. To them the question regarding orientation, e.g., has been merely: Is it ray-direction or intensity difference that regulates this? And with regard to this question they have failed to see that there may be a vast difference in effect between direction of rays in the field and direction through the organism; between diversity in light intensity in the field and variation on different parts of the surface of the organism. Moreover they have failed to appreciate the importance of difference in sensitiveness of different parts of the reacting organism, and the consequent effect of change in position on stimulation.

An illustration will serve to emphasize the importance of distinguishing these characteristics. Suppose we have an elongated opaque organism the anterior end of which is more sensitive than the posterior, and suppose that this organism is in a field of direct sunlight without any other obstruction. Now it is evident that under such conditions the intensity in the field is uniform, but the intensity on the illuminated side of the organism may be almost infinitely higher than that on its shaded side, since no light can get through the organism, and if the organism changes its axial relation with reference to the ray direction the intensity on the surface may change just as much as it would if the organism moved about in a field in which the intensity was not uniform. Moreover if the organism takes a position in which the sensitive anterior end is shaded by the rest of the body it is of course in a lower effective intensity of light than it would be if this end were illuminated. Here again we see that a change in axial position in a field uniformly illuminated may produce the same effect as movement from a region of one intensity to that of another. And all this is dependent upon the direction of the rays in the field whereas ray direction through the organism could have no such effect. It is evident then that the question " intensity difference or ray direction" may mean any one of several things. This loose way of stating the problems has led to much confusion.

Let us then, first of all, attempt to get a clear understanding of the questions involved in the reactions to light. With this in view we shall consider the origin and development of ideas concerning the movements in general of plants and animals, and those induced through stimulation by light in particular.

Chapter II

HISTORICAL REVIEW CONCERNING THE ORIGIN AND DE- VELOPMENT OF IDEAS AND THEORIES REGARDING MOVEMENTS IN PLANTS AND ANIMALS WITH SPECIAL REFERENCE TO THE QUESTION OF TROPISMSi

I. Early Investigations and Ideas concerning Movement in

Organisms

To primitive man motion was the criterion of life. Everything that moved was aUve, not only plants and animals but also various elements in nature, — water, wind, fire, and the heavenly bodies. Motion was thought to be under the control of higher beings, or the result of the action of mind with which all living things were endowed. The philosophers of early civilized races abandoned the idea that all things which move are alive, but they still considered that all physiological processes are due to vital spirits. Aristotle (384-322 B.C.), thought that plants as well as animals had souls. The pith was supposed to be the seat of the soul in plants and all movements and other phenomena characteristic of living things were regarded as due to its activity. During this period, all but a few thinkers seemed to rest content that nothing more could be learned about the cause or sequence of physiological processes, and these few made only feeble attempts from a

1 The following works are the main sources of information regarding the earlier views on plant and animal activity: History of Botany, by Julius von Sachs (1875), translation revised by I. B. Balfour, Oxford (1890); General Physiology, by Max Verworn (1894), translation second edition by F. S. Lee, New York (1899); Contemporary Psychology, by Guido Villa, translated by H. Manacorda, London, 1903.

philosophical point of view at further analysis of causation. Not until the work of Galen (131-200 ± A.D.), four hundred years later, was there anything approaching experimental analysis.

2. First Attempts at Mechanical Explanation of Life

Phenomena

Galen studied the structure of animals by direct observation, even practicing vivisection on pigs and monkeys, and thus he sought to learn the functions of the various organs. But others did not continue the experimental work begun by him, and nearly thirteen centuries passed without any progress. It was not until early in the sixteenth century that interest in vital phenomena was again aroused, and it was a century later before Harvey made his important discovery on the circulatory system and presented mechanical explanations for many factors involved in the process of circulation, all of them based on experimental evidence.

A few years later, building on Descartes' idea " that the bodies of animals and men act wholly like machines and move in accordance with purely mechanical laws," Borelli undertook to reduce the movements of the organic motor apparatus to purely physical principles. The work of Borelli formed the foundation of the iatromechanical school, the members of which sought to explain all vital phenomena in animals by the application of physical principles. Other investigators of this period recognized the importance of chemical reactions in animal activity, and, under the leadership of Sylvius, founded the iatrochemical school, a school which admitted the importance of physical principles in explaining animal activity, but which strongly emphasized the influence of chemical phenomena in vital processes. The seventeenth century, and part of the eighteenth, formed a period in which mechanical explanations were offered for practically all reactions and other physiological phenomena in animals, and the same may be said with regard to plants, as will be shown in the following pages.

Toward the close of the seventeenth century, the striking movements of the sensitive plant, Mimosa, imported from America, attracted considerable attention. Ray described the movements of this plant in his " Historia Plantarum" (1693), and although an apparent believer in the soul of plants as defined by Aristotle, he tried to explain the movements mechanically. He thought that they were due not to sensations but to physical causes, — " Planta est corpus vivens non sentiens." The leaves remain erect, he said, because of the constant flow of sap into them. When touched, the tubes which carry the sap to them are partially closed, and thus the supply of sap is diminished to such an extent that the leaves are no longer held erect and consequently droop. He was of the opinion, that plants bend toward a window because of difference in rate of growth on opposite sides due to difference in temperature. It was known in a general way that an increase in temperature causes an increase in the rate of growth in plants; and Sharroc had found that the stem on which he was experimenting grew toward that part of a window where air entered through an opening It was from these observations that Ray reached his conclusions.

At about the same time Dodart came to the conclusion that physical contraction of the fibers on the moister side of roots and their expansion on the moister side of stems caused the former to turn down and the latter up.

Du Hamel, after studying the effect of light, temperature and moisture on the direction of growth, concluded that the " Richtung der Dampfe " in the vessels and around the plant is of prime importance, and that if heat, light and moisture have any influence on the direction, it is through their effect on the gases. Ridiculous explanations, all of them, in the light of present knowledge! But, even so, their importance cannot readily be overestimated, for they formed the foundation of later work which led to most fruitful results.

3. Period of Vitalism

It was fully realized before the close of the eighteenth century that the mechanical explanations thus far presented were inadequate to account for many fundamental phenomena at which they were directed. Especially was this true with reference to movements of various kinds, in both plants and animals. It led to the postulation of a controlling principle in living beings, foreign to chemistry and physics, a hypermechanical principle known as vital force. Those who believed in this principle were called vitalists. Some vitalists considered the postulated force inscrutable, and consequently abandoned all hope of gaining an insight into vital processes through experimental means. Others, however, among the foremost of whom were the botanist, De Candolle, and the famous physiologist, Johannes Miiller, held the opinion that this force was subject to further experimental analysis.

The prevalence of the former view was however undoubtedly the chief cause of stagnation in general physiology'' in its broadest sense, during this period, for there was no corresponding unproductive period in the development of physical sciences. As a matter of fact many who had been prominent investigators in both biological and physical sciences, now abandoned the former, and devoted their entire energies to the latter.

4. Return to Mechanical Explanations

Miiller realized the weakness of the iatromechanical school as well as the inadequacy of pure philosophical speculation. On the one hand he recognized the importance of speculation in guiding and unifying experimental work; on the other he saw the necessity of founding philosophical speculation on experimental facts. This broad view resulted in much comparative work especially In physiology and psychology, work which had a direct bearing on the nature of psychic processes as well as on the nature of physlologrcal activity. Miiller worked on the higher animals almost exclusively. His aim was to analyze the phenomena of life as he found them in these organisms. His followers, Wohler, Llebig, Helmholtz, du Bois-Reymond, Lotze, Weber, Fechner and others, perpetuated this aim, but they did not retain his breadth of spirit. Some confined their investigations to the chemical side of physiology, others to the physical side, and still others to pure psychology. The question as to the origin and evolution of vital phenomena, especially psychic phenomena, was not yet prominent, if indeed it had been at all considered. The behavior of lower animals had been studied to some extent, but the Cartesian doctrine that there is no resemblance between the mind of man and that of animals was still very generally accepted.

5. Evolution and its Effect on the Study of Behavior of

Plants and Animals

With the establishment of the theory of evolution, there appeared a new incentive in the study of animal behavior. Darwin had demonstrated in a convincing manner the structural interrelationship between various animals, including man. It seemed clear that the complex anatomical structures found in the higher animals had ^'^^'V origin in the simpler structures found In tl-. !^v%er. Coula u? same be said with reference L^ oehavlor? Did the mental phenomena in mr*- >'-ave their origin in the lower animals? If so, then there must be some evidence of mental activity in the lower animals, the psychic phenomena in these organisms must resemble those in man. and the Cartesian doctrine must be wrong.

The importance of this problem was at once recognized and a number of able investigators undertook its solution. Prominent among these may be mentioned Darwin, Paul Bert, (irabcr, Romanes, Lubbock and Preyer. It should be emphasized that these in\estigators were not primarily interested in explaining behavior either mechanically or otherwise. Their principal aim was to throw light on the origin of mental phenomena in man. Do the lower animals have sensations? Do they have memory? Do they reason? were questions which shaped their investigations. These questions they sought to answer by studying the behavior of animals under various conditions. Their results seemed to indicate that the psychic phenomena in animals differ from those in man in quantity rather than in quality.

With reference to reactions to light they used what is known as the preference method. Experimental conditions were so arranged that the animals could get into light of different intensities or different colors. The kind of light in which they collected was supposed to be the kind they preferred. The w^ork was weak in that only end results of the experiments were considered; it was never ascertained precisely how the animals got into the region in which they finall>' remained. Variation in the color or in the intensity of light in the field was to these investigators the controlling factor in the movement of animals. They failed to consider the possible effects of the direction of the rays, of variation in light intensity on the surface of the animals, and of various internal factors. This led to many erroneous conclusions. Still it must l)e said that whatever one may think as to the point of view of these investigators and the validity of ^heir conclusions in general, one cannot read with unprejudiced rp'*:d the account of their work, especially that of Darwin, Lubbock, and Romanes, without greatly admiring the keenness of their

HISTORICAL REVIEW II

observations and the ingenuity of their experiments. The point of view of these men dominated the field of animal behavior from the middle of the nineteenth century until the appearance of Verworn and Loeb well on toward 1890. As has been stated, they studied the behavior of animals with the express purpose of demonstrating the evolution of psychic phenomena in man. These investigators were therefore not primarily interested in a physico-chemical explanation of animal behavior.

The study of behavior in plants during this period was however pursued with a very different aim. The question as to the origin of mental phenomena influenced this study but little, for it was generally conceded that plants were devoid of all traces of psychic activity. There was consequently nothing left but to attempt to account for their behavior by means of physico-chemical analysis. Even the vitalists realized that in the attempt of such analysis lay the only hope of progress.

6. Introduction of the Term *' Tropism'' and Development of its Application to Different Reactions

In 1806 De Candolle, a vitalist, succeeded in reversing the daily periodic sleep movements of leaves by exposing them to artificial light during the night and to darkness during the day. The same year Knight showed by attaching developing seedlings to a rapidly revolving wheel that the direction of growth of roots and stems is regulated by gravitation. He explained the directive action of gravitation by assuming "that the root, being of a semifluid consistence, is bent downwards by its own weight, while the nutrient sap in the stem moves to the underside and causes stronger growth there, until by means of the curvature so produced the stem assumes the upright position." In 1828 Johnston found that roots in growing downward can overcome considerable resistance and that the direction of growth is therefore not due to their weight as Knight had assumed. About the same time Dutrochet a[)i)Hed endosmose and exosmose to explain the movement of plants mechanically. In 1S33 He Candolle proved that it is light which causes plants to grow toward a window and not difference in temperature on opposite sides as Ray had thought one hundred forty years earlier. De Candolle discovered that light retards growth in i)lants and concluded that they turn toward a source of light because growth is retarded on the more highly illuminated side. The reaction according to De Candolle is due to difference in intensity of light on opposite sides.