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 The Cambridge Manuals of Science and 
 Literature 
 
 PLANT-ANIMALS
 
 CAMBRIDGE UNIVERSITY PRESS 
 
 SLonDon: FETTER LANE, E.G. 
 
 C. F. CLAY, MANAGER 
 
 N>. PRINCES STREET 
 
 Hantson- H. K. LEWIS, 136, GOWER STREET, W.O. 
 WILLUM WESLEY & SON, 28, ESSEX STREET, STRAND 
 Berlin: A. ASHER AND CO. 
 leipjifl: F. A. BROCKHAUS 
 efo $ork: G. P. PUTNAM'S SONS 
 Bornbau ant) Calcutta: MACMILLAN AND CO.. LTD. 
 
 All rightt reserved
 
 
 B 
 
 A. The Green Plant-animal (Convoluta roscoffenns). 
 
 B. The Yellow-brown Plant-animal ( Convoluta parado.v(i). 
 
 40 times natural si/e.
 
 PLANT-ANIMALS 
 
 A STUDY IN SYMBIOSIS 
 
 FREDERICK KEEBLE, Sc.D 
 
 PROFESSOR OF BOTANY IN 
 
 UNIVERSITY COLLEGE, READING ; 
 
 AUTHOR OF "PRACTICAL PLANT 
 
 PHYSIOLOGY" 
 
 Cambridge : 
 
 at the University Press 
 
 IQ12
 
 First Edition 1910 
 Reprinted 1912 
 
 With the exception of the coat of arms at 
 the foot, the design on the title page is a 
 reproduction of one used by the earliest known 
 Cambridge printer John Siberch 1521
 
 39 
 T3 
 
 PREFACE 
 
 X 
 
 DURING some ten years' work in a small marine 
 laboratory in Brittany it has fallen to me 
 not infrequently to attempt to explain to curious 
 ^visitors what were my objects in going to and fro 
 <o upon the shore, in wading among the sea-weeds and 
 ^ in bringing into the laboratory minute, worm-like 
 animals which represented often my sole "catch." 
 
 I discovered that many of the visitors to the 
 ^laboratory became interested in the work that 
 rwas going on, and that, though they disclaimed a 
 Jj knowledge of biology, they followed with under- 
 y, standing and interest the story of the behaviour 
 and life histories of " the worms " : indeed, they 
 succeeded generally in putting to me pertinent and 
 unanswerable questions with respect to these " plant- 
 animals." 
 
 The pleasant recollection of hours spent in this 
 way is responsible primarily for my undertaking to
 
 vl PREFACE 
 
 contribute this volume to The Cambridge Manuals 
 of Science and Literature. 
 
 If it succeeds in interesting the layman, success 
 will be due to the severe educational regime to 
 which my visitors submitted me in their cross- 
 questionings as to the bearings and objectives of 
 my biological work. 
 
 If it fails, they must bear the blame : for had they 
 not exhibited a fondness for "Convoluta" I should 
 scarcely have ventured to publish its doings to the 
 world at large. Of these friends I would mention 
 particularly Mr Alfred Dutens, whose interest in 
 "Convoluta roscoffensis" has been a source of constant 
 encouragement to me. 
 
 The biological facts recorded in this volume are 
 the outcome of researches carried on for some years 
 by Professor Gamble and myself, and, subsequently, 
 without Professor Gamble's co-operation. 
 
 Throughout the whole time during which the work 
 has been in progress, it has benefited more than may 
 be stated explicitly by the unremitting assistance 
 rendered by my wife. To her, are due the long and 
 patient records of the periodic changes of behaviour 
 of the plant-animals Convoluta roscoffensis and 
 C. paradoxa : records which entailed visits to the 
 Convoluta colonies at all phases of the tide and at 
 all hours of the day and night. Though an adequate
 
 PREFACE vii 
 
 expression of my thanks to my wife were out of place 
 here, I beg leave to give myself the pleasure of 
 acknowledging how great has been her share in 
 this work. 
 
 The original memoirs, giving detailed accounts of 
 the life histories of the plant-animals, have appeared 
 in the Quarterly Journal of Microscopic Science. A 
 list of these memoirs and of other researches to which 
 reference is made in the text is included in the short 
 bibliography appended to this volume. The dates, 
 enclosed within brackets in sundry places in the text, 
 refer each, to the year of publication of the research 
 which is cited and indicate that the title of the re- 
 search in question may be found in the bibliography 
 under that date. 
 
 The black and white illustrations have been pre- 
 pared specially for this volume by Mrs Seward from 
 the original drawings made by Miss Dorothea 
 Richardson in the laboratory at Tregastel. I am 
 deeply indebted to Mrs Seward and Miss Richardson 
 for their kind assistance, and to the skill and patience 
 which they have bestowed on the drawings I offer 
 a sincere and admiring tribute. 
 
 Should the reader find that the main arguments 
 exposed in the course of the volume are intelligible, 
 he may, perhaps, be inclined to forgive the use, 
 which I hope is as occasional as inadvertent, of un-
 
 ^ PREFACE 
 
 familiar biological terms. I have endeavoured to 
 "this pAll, but have doubts as to the com- 
 pleteness of my success. I shall be obliged therefore 
 ? eaders wffl point out passages which require 
 elucidation, so that, in the event of another edition 
 being published, the defects may be remedied. 
 
 FREDERICK KEEBLE. 
 
 COTES-DU-NORD, 
 
 FRANCE. 
 September, 1910.
 
 CONTENTS 
 
 PART I 
 
 THE BEHAVIOUR OP THE PLANT-ANIMALS 
 CHAP. PAGE 
 
 I. Introductory : the worms, Convoluta roscofi'ensis 
 
 and Convoluta paradoxa : their habits and habitats 3 
 
 II. The origin and significance of the habits of Con- 
 voluta roscoffensis and Convoluta paradoxa . 37 
 
 PART II 
 
 THE NATURE OP THE PLANT-ANIMALS 
 
 III. The green cells of Convoluta roscoftensis and the part 
 
 they play in the ecouomy of the plant-animal . 75 
 
 IV, The origin and nature of the green cells of Convoluta 
 
 roscoftensis 100 
 
 V. The significance of the relation between coloured 
 
 cell- and animal-constituents of the plant-animals 130 
 
 BIBLIOGRAPHY 159 
 
 INDEX 161 
 
 WITH TEXT-FIGURES, 1 22. 
 
 COLOURED FRONTISPIECE. The Green Plant- Animal, Con- 
 voluta roscoffensis and the Brown Plant- Animal, 
 Convoluta paradoxa.
 
 PART I 
 
 THE BEHAVIOUR OF THE PLANT-ANIMALS
 
 CHAPTER I 
 
 INTRODUCTORY: THE WORMS: CONVOLUTA R08- 
 COFFBNSIS AND CONVOLUTA PARADOXA- THEIR 
 HABITS AND HABITATS. 
 
 BIOLOGISTS who devote themselves to the investi- 
 gation of the life histories and life processes of the 
 lower animals are apt to encounter the criticism: 
 why expend pain and labour on insignificant creatures 
 when so much remains to discover with respect to 
 the higher animals, including man himself? 
 
 This perfectly legitimate criticism admits of a con- 
 clusive reply and, since it is possible that a question 
 of the kind may arise in the mind of anyone taking 
 up this book, it shall be answered forthwith. The 
 reply may take one of three forms. In the first place, 
 it may be urged that the most important modern 
 biological discoveries have resulted from researches 
 into the life histories of the lower organisms. Modern 
 surgery relies for much of its technique on the results 
 of investigations into the physiology of the bacteria. 
 Yet more recently, the experimental elucidation of 
 the life-histories of the protozoa the lowest group of 
 
 12
 
 4 PLANT-ANIMALS [en. 
 
 animals has laid the foundation of a great and 
 increasing body of knowledge with respect to the 
 cause of malaria, sleeping sickness, and other tropical 
 
 In the second place, it may be urged that, the 
 more complex the organism, the more difficult it is to 
 use the results of observations upon it for the purpose 
 of generalising on important biological problems such 
 as those of the origin of instinct and habit, or of the 
 meaning of heredity and the course of evolution. 
 The higher the organism, the more it has covered up 
 the tracks along which the species to which it belongs 
 has travelled. For this reason alone, the study of the 
 loAver organisms is not only to be justified but also 
 urged on zoologists as one bound to lead to results 
 of the greatest value. 
 
 In the third place, it has yet to be proved that 
 the higher animals differ in any fundamental respect 
 from more lowly forms of life. Hence, if, as a 
 physiologist must hold, such differences as exist 
 between higher and lower forms are differences of 
 degree and not of kind, it follows that an increased 
 knowledge of the nature of the lower organisms 
 connotes also an increase in knowledge with respect 
 to the higher organisms. 
 
 On these grounds, the patient and exhaustive 
 study of the lower organisms is to be justified. 
 Nay more, if the reasons for this study are valid they
 
 I] INTRODUCTORY 6 
 
 should serve to induce some of the younger genera- 
 tion of physiologists to devote their attention to a 
 field of research both rich in promise and too little 
 cultivated by the men of science of this country. 
 
 Though the results recorded in this volume are 
 but modest, throwing here and .there only a faint 
 light on the problems which they raise, nevertheless 
 they suffice to demonstrate that more skilful observers 
 would, by taking up similar subjects of investigation, 
 make notable contributions to the science of com- 
 parative physiology. 
 
 Having vindicated the importance of research on 
 the lower organisms, let us proceed to our task. 
 
 The plant-animals whose life histories and habits 
 form the subject of this volume are two simple, 
 marine worms, Convoluta roscqffensis and Convoluta 
 paradoxa (Frontispiece). Both are small, though large 
 enough to be seen easily by the unaided eye, and both 
 are conspicuous by reason of their colours. C. ros- 
 coffensis is dark, spinach green, and C. paradoxa 
 yellow-brown. 
 
 Even among worms they occupy a lowly place. 
 Unlike the higher members of this group, C. ros- 
 coffensis and C. paradoxa are unsegmented. Instead 
 of consisting, like garden worms, of a series of ring- 
 like pieces, the bodies of our plant-animals are in one 
 piece and, consequently, bear no ring-like markings
 
 PLANT-ANIMALS [OH. 
 
 
 Fig. 1. The distribution of the colonies of Convoluta roscoffensis on 
 the sea-shore. I. at spring-tidal periods (low water): II. at 
 neap-tidal periods (low water). Though a colony remains fixed 
 in position, its size waxes with the spring tides and wanes with 
 the neap tides. C, C = the colonies. 8. = sea
 
 i] INTRODUCTORY 7 
 
 on their surfaces (Frontispiece). Imagine a minute, 
 elongated fragment of a most delicate leaf, some in. 
 long by T ^ in. broad, and you have a picture of 
 C. roscoffensis. Imagine, further, myriads of such 
 green, filmy fragments lying motionless on moist, 
 glistening patches of a sunny beach between tide-marks 
 and you see the species in its native habitat (Fig. 1). 
 To find C. paradoxa at home it is necessary to follow 
 the receding tide, to gather handfuls of the brown 
 seaweeds (Fig. 2) which are exposed towards the low- 
 water limit of the larger tides and to allow the tips 
 of the weeds to dip into water in a white dish. Singly 
 from their hiding-places chubby, brown C. paradoxa 
 come gliding down with rounded "head" and pointed 
 "tail" to swim uneasily in the water of the dish. 
 C. roscoffensis is pre-eminently gregarious, C. paradoxa 
 by comparison is solitary. Sand from a Convoluta 
 patch scooped up in a cup contains many thousands 
 of C. roscoffensis ; a patient fishing throughout the 
 time of low tide may result in a catch of fifty, or at 
 most a hundred, specimens of C. paradoxa. 
 
 The surface of the bodies of the plant-animals is 
 somewhat slimy ; particularly in C. roscoffensis, and 
 is covered by fine cilia (Fig. 3) which, during the life 
 of the animals, are in constant motion. The cilia, 
 which are protoplasmic projections from the super- 
 ficial cells, serve, by their unceasing movements, to 
 row the animal through the water.
 
 PLANT-ANIMALS 
 
 [CH. 
 
 C. paradoxa possesses, in addition to cilia, occa- 
 sional, stouter, bristle-like structures which stick out 
 from its body, chiefly in the "tail" region (Fig. 16, 
 p. 84). These structures serve, when put in action 
 by the animal, to pin it down and thus enable it to 
 stop and stick in any position. 
 
 Fig. 2. Convoluta paradoxa (C) attached to sea-weeds 
 of the paradoxa zone. (Magnified eight times.) 
 
 In both animals, the sides of the body are flexed 
 beneath the under surface, and together form a groove 
 which, in C. paradoxa, serves to fit the animal saddle- 
 wise to the fine sea- weeds over which it glides (Fig. 2). 
 This animal, in its general progress, appears almost 
 to flow over the substratum on which it is moving.
 
 I] THE STRUCTURE OF CONVOLUTA 9 
 
 Occasionally, however, on meeting with an obstacle it 
 rears its head-end, caterpillar-wise, relaxes the grip 
 of its flexed sides, readjusts them to the surface and 
 glides on with stealthy motion. 
 
 Though we have called C. roscoffensis and C. 
 paradoxa simple worms, it is not to be inferred that 
 the structure of their bodies is really simple. Both 
 species possess a well-defined nervous system and 
 efficient sense-organs. At the front or "head" end 
 of the body, on the upper surface, a little way behind 
 the anterior end, lie two eyes right and left of the 
 median line (Frontispiece and Fig. 3). Though of the 
 simplest construction, each consisting of a minute 
 spot of orange pigment lying over nervous tissue, 
 the eyes are efficient for distinguishing light of 
 different intensities. Numerous orange-pigmented 
 glands, scattered over the surface of the body, function 
 probably as accessory eyes. Between the two eyes, 
 in the median line on the dorsal (upper) side of the 
 body of either species, lies the otocyst (Frontispiece 
 and Fig. 3, OT), It consists of a hollow sphere of 
 nervous tissue enclosing a space within which lies a 
 small lump of chalk. 
 
 Like a pea in a thimble, the heavy, chalky mass, 
 or otolith, lies freely in the otocyst, and, if the position 
 of the animal change with respect to the line of 
 action of gravity, the vertical the otolith falls or 
 rolls on a new part of the otocyst- walL Pressing on
 
 10 PLANT-ANIMALS [CH. 
 
 this area it acts as a stimulus to the nervous tissues 
 beneath. As the result of stimulation of this tissue, 
 nervous impulses may be despatched to the muscles 
 of the body, and, causing them to contract, give rise 
 to movements of the body which are definite in 
 direction. 
 
 Thus the otocyst serves as an indicator of the 
 line of gravity ; in other words it acts as the organ 
 
 Fig. 3. Young Convoluta paradoxa. C = cilia covering the surface 
 of the body. OT = otocyst. OC=eyes. V = empty digestive 
 vacuoles. 
 
 for gravi-perception. By its means, the animal is able 
 to orientate itself with respect to the vertical, and 
 so to find fts way downward or upward. 
 
 That the otocyst does indeed serve this end 
 has been established by experiments with other 
 animals, and may be inferred in the case of C. ros-
 
 I] THE STRUCTURE OF CONVOLUTA 11 
 
 coffensis from the following facts. Occasionally, 
 among just-hatched larvae specimens occur which 
 fail to respond like their fellows to gravitational 
 stimulus. Such specimens are found, on microscopic 
 examination, to lack properly developed otocysts. 
 For example, if numbers of C. roscoffensis larvae are 
 taken up with water into a glass tube and the tube 
 is shaken slightly, the animals come down, some 
 tumbling, some curvetting. These animals in general 
 respond to vibration by a geotactic movement that 
 is, one having reference to the line of action of 
 gravity but the one or two, devoid of otocysts, fail 
 to descend, remain glued to the side of the tube and 
 are dislodged with the greatest difficulty. 
 
 As indicated already, the bodies of Convoluta 
 possess a well-developed system of muscles by the 
 ordered contractions of which the movements of the 
 animals are effected. 
 
 The digestive system is of a primitive order. A 
 well-developed mouth, capable of a wide gape, occurs 
 on the under side of the body rather nearer the 
 " head " than the " tail " end. The mouth communi- 
 cates by a short gullet, not with a distinct digestive 
 tube, but with a loose, central tissue. Hence food 
 which is ingested passes through the mouth to the 
 gullet whence it is distributed to improvised spaces or 
 vacuoles in the tissues (Fig. 3). In these vacuoles the 
 food is digested. The undigested residue is discharged
 
 12 PLANT-ANIMALS [CH. 
 
 at any point of the body, generally, however, toward 
 the hinder end. 
 
 Neither species of Convolutapossesses a circulatory 
 system. In the absence of heart and blood-vessels, the 
 distribution of the nutritive substances derived from 
 the food is effected in a primitive manner, the materials 
 being passed from cell to cell. 
 
 Fig. 4. Convoluta paradoxa. a. Seen from ventral surface, showing 
 the folds of the sides of the body. b. An animal with nearly 
 ripe eggs (E). 
 
 There is, moreover, no excretory apparatus, and 
 the waste products are not discharged from the body 
 but remain and accumulate in the tissues. 
 
 Both C. roscoffensis and C. paradoxa are herma- 
 phrodite, each animal possessing male and female 
 reproductive organs, the essentials of which are, re- 
 spectively, spermatozoa and egg-cells. The eggs are 
 numerous and attain to so considerable a size that
 
 I] THE STRUCTURE OF CONVOLUTA 13 
 
 they may be seen lying in rows in the bodies 
 of "ripe females," that is, animals in the female 
 stage (Fig. 4, b, E). The eggs are fertilized in the body, 
 though the spermatozoa which effect fertilization 
 are derived from another individual of the same 
 species. After fertilization, the eggs are discharged 
 in groups or clutches of from about eight to fifteen 
 or more. As it is extruded from the body the 
 egg-clutch becomes surrounded by a transparent, 
 mucilaginous, sticky capsule secreted by the glands 
 on the surface of the skin. A clutch of eggs of 
 
 Fig. 5. Egg-capsule of Convoluta paradoxa. Each egg is contained 
 in an egg-membrane and the group of eggs is enclosed by a 
 common capsule. (Magnified twenty times.) 
 
 C. roscoffensis is recognisable to the trained eye as 
 a minute, more or less transparent sphere of about 
 the size of a small pin's head. The egg-clutch of 
 C. paradoxa is of a similar size ; but, owing to the 
 presence of pigmented granules, it is of a rufous 
 colour (Fig. 5). 
 
 C. roscoffensis lays its eggs on the beach just 
 beneath the surface of the sand : C. paradoxa deposits 
 them on the fine sea-weed lower down the shore.
 
 14 PLANT-ANIMALS [CH. 
 
 The habitat of C. roscoffensis is restricted and 
 localised (Fig. 1). This gregarious species occurs 
 within a well-defined zone of the foreshore of sandy 
 beaches of Normandy, Brittany, and the Channel 
 Islands. Elsewhere it is unknown. 
 
 An observer, walking at low tide seaward across 
 a golden beach in Brittany, passes scattered granite 
 rocks scantily clad with yellow-brown patches of 
 seaweed adventuring landward and before he reaches 
 the main belt of brown seaweeds, some yards land- 
 ward of the thin line of green Cladophora which lies 
 bleaching in the sun, he may see dark, spinach-green 
 glistening patches the colonies of C. roscoffensis. 
 He must tread softly lest the patches melt away 
 at his approach. The colonies may extend for 
 many yards as dark green, irregular strips running 
 more or less parallel with the shore-line, or they may 
 consist of apparently disconnected patches varying in 
 size from an inch or so to a yard or more across. From 
 the intervals between the colonies, the animals are 
 not absent. Though they are not to be seen, they 
 may be smelt. Sand from a part of this roscoflensis 
 zone where no animals are visible, when squeezed 
 between the fingers, emits from the crushed, occasional 
 Convolutas contained in it a pungent and evil smell. 
 The odour, which is like that of decaying fish, is due 
 to the volatile trimethyl amine which is produced 
 by the animal.
 
 I] THE HABITAT OF CONVOLUTA 15 
 
 On a peaceful beach, in quiet times, when storms 
 and tourists are absent, the colonial patches of C. 
 roscoffensis keep their respective outlines with sur- 
 prising constancy. Day after day the several patches 
 may be recognised, waxing in size with the spring tides, 
 waning with the neap or slack tides (Fig. 1) : larger, 
 also, on any day soon after the tide has receded 
 from their borders; smaller, just before the rising 
 tide invades them. At certain times, the multitude of 
 individuals which make up a patch may be seen lying 
 lethargic and motionless, bathed in the sunlight and 
 the film-like stream of drainage sea- water which oozes 
 from the sand and flows over them seaward. On days 
 of bright sunshine, in particular, the animals lie very 
 still ; on duller days, a constant gliding too and fro of 
 these minute films of living matter is to be observed 
 within the confines of a colony. It is on such occasions 
 that the observer must tread softly, for C. roscoffensis 
 is so sensitive to vibration that his heavy, approach- 
 ing tread may send it to earth with lightning speed. 
 How quickly the animals may make their descent 
 from the surface may be judged from the illustration 
 (Fig. 6) which depicts two photographs of a colony, 
 the second taken at an interval of five minutes after 
 the first. Three gentle taps on the sand, after the first 
 photograph was taken, served as the signal for retreat. 
 At that signal, the army, many millions strong, vanished 
 with amazing swiftness and took cover underground. 
 Lest the words "many millions" should seem to
 
 16 
 
 PLANT-ANIMALS 
 
 [CH. 
 
 savour of exaggeration, it may be said that one colony 
 of moderate size extending over some two square 
 yards was found by estimation to contain 5,600 
 million individuals. Of such flaky thinness are these 
 animals that as many as 28,000 may be packed in a 
 
 Fig. 6. Besponse of C. roscoffensis to vibration. Reproductions of 
 photographs of a colony. I. before, II. five minutes after the 
 sand had been tapped lightly with the foot. The dark patches 
 in I. represent vast numbers of the animals which in II. have 
 disappeared almost entirely below the surface of the sand. 
 
 space measuring one cubic centimetre. A search on 
 dark nights at low tide in the roscoffensis zone fails to 
 reveal any of the animals upon the surface. In such 
 circumstances they remain just beneath the sand. 
 On moonlight nights, some, but not many, may be
 
 i] THE HABITAT OF CONVOLUTA 17 
 
 seen, by the light of a lantern, lying in the river-films 
 of their diurnal stations. 
 
 Except for a rich micro-flora and -fauna of diatoms, 
 bacteria and infusoria, except for a rare, solitary 
 enemy another worm, a species of Plagiostoma, which 
 shovels live Convolutas by the hundred into its capa- 
 cious body except for an occasional, small shore-crab, 
 picking its way with rolling but deliberate gait over 
 the patches, C. roscoffensis enjoys undisputed posses- 
 sion of its tract of foreshore. Though the wastage 
 from each colony must be prodigious, every incoming 
 tide taking toll, yet the species, fecund and resource- 
 ful, rises superior to the circumstances of its environ- 
 ment and maintains itself in the strange situation 
 which fate has chosen for it. 
 
 The roscoffensis zone (Figs. 1 and 7) is as localised 
 as the range of distribution of the species is restricted. 
 The upper limit of the zone is marked by the level 
 reached by high water at the slackest of the neap tides : 
 for, further landward, C. roscoffensis could not obtain 
 at all tidal periods the diurnal plunge-bath without 
 which it does not thrive. Risk of desiccation bars 
 its more landward advance. The lower limit of the 
 zone is but a few yards seaward, for C. roscoffensis 
 loves the light and ensues it. 
 
 At every making tide, this zone is submerged and 
 C. roscoffensis becomes a submarine plant-animal 
 sheltering beneath the surface of the sand out of
 
 18 PLANT-ANIMALS [CH. 
 
 reach of the shock of the waves. At every falling 
 tide, as the receding waters lay bare the zone, C. ros- 
 coflensis rises to the surface of the sand and becomes 
 a land plant-animal, or rather, a sedentary denizen 
 of the filmy rivers which have their sources in 
 the sand flooded by water when the tide is full. 
 Where the springs of drainage- water reach the sur- 
 face and become rivulets cutting seaward courses, is 
 the upper limit of the C. roscoffensis zone. Thus the 
 colonies are so situated on the beach that they are 
 bathed continuously in running water and receive the 
 maximum of light-exposure during low water at all 
 tidal periods. Records kept during a lunar month 
 show that the time of exposure during low tides is 
 very fairly constant. The time during which C. 
 roscoffensis lies on the surface is, on the average, five 
 and a half hours, and ranges from four and a half to 
 six hours. Twice during twenty-four hours the ros- 
 coifensis zone is submerged and the animals live a life 
 of darkness underground : twice the zone is uncovered 
 and the animals are free to rise to the surface of the 
 sand (Fig. 7). By fixing its station and adjusting its 
 habits, C. roscoflensis succeeds to a remarkable degree 
 in simplifying its environmental conditions. In that 
 station, periods of inundation succeed periods of ex- 
 posure at fairly regular intervals, and, by synchronising 
 its rhythmic movements up to the surface and down 
 below the surface with the movements of the tides,
 
 Ij 
 
 THE HABITAT OF CONVOLUTA 
 
 19 
 
 C. roscoffensis adjusts its working days to the rhyth- 
 mic changes of its environment. How remarkable is 
 the rhythmic movement up and down we shall pre- 
 sently discover. 
 
 Fig. 7. Habitats of C. roscoffensis and C. paradoxa shown in relation 
 with the rise and fall of the tides during a lunar period. 
 S = spring tides. N = neap tides. Rose, zone = habitat of 
 C. roscoffensis. Parad. zone = habitat of C. paradoxa. The 
 position of colonies of C. roscoffensis is just below the high-water 
 level of the slackest tides. The habitat of C. paradoxa is un- 
 covered at low water except during the slacker neap tides. 
 
 Leaving the C. roscoflensis zone and passing the 
 rank, brown sea- weeds left high and dry by the tides, 
 the observer paddles into the shallow water, or, if the 
 tide is a big one, walks almost dry shod and sees the 
 long, yellow bands of another sea-weed (Ascophyllum) 
 swaying beneath the water of the pools or lying 
 
 22
 
 20 PLANT-ANIMALS [CH. 
 
 prone on the soft, grey ooze of the sea-floor. The 
 extremities of the Ascophyllum are clothed with tufts 
 of fine, epiphytic brown and red sea-weeds. Further 
 out, as the tide continues to fall, the browner weeds 
 are becoming uncovered ; first, the dichotomous straps 
 of Himanthalia which spring from button or saucer- 
 like stalks attached to the rocks, and then the finger- 
 like Pycnophycus (Fig. 2) which extends beyond the 
 seaward limits of even the biggest spring tides. It is 
 among the fine weeds attached to Pycnophycus that 
 C. paradoxa is to be found. On dangling these weeds 
 in water, the animals come out, but as single spies 
 not in battalions like C. roscoffensis which lies in 
 swarms thirty yards further up the beach. The 
 abode of C. paradoxa is less circumscribed than that 
 of C. roscoffensis and shifts with the tides. At the 
 onset of the spring tides, minute specimens may be 
 taken from among the epiphytic weeds attached to 
 the most landward of the brown sea- weeds (Fucus). 
 During subsequent spring tides, the animals must be 
 sought lower down the beach in the zone occupied 
 by Himanthalia and Ascophyllum ; whilst, yet later 
 in the same series of spring tides, C. paradoxa is to 
 be found only in the Pycnophycus zone. Just where 
 that dark brown weed ceases to be exposed at low 
 water of the largest spring tides is the further limit 
 of the paradoxa zone (Fig. 7). Like the Greek sailors 
 described in Eoilien C. paradoxa hugs the shore. Ex-
 
 i] THE HABITAT OF CONVOLUTA 21 
 
 posed now to the violence of the sea and now to the 
 hot sun striking on the drying, emerged rocks and 
 weeds, C. paradoxa has chosen its abiding place. But, 
 unlike C. roscoffensis, C. paradoxa fails to finds in its 
 station a regular recurrence of change, and hence it 
 is constrained to shift its station during the lunar 
 periods. At times of slack tide, the seaward part of 
 the C. paradoxa zone is submerged continuously and 
 the light which reaches the animals clinging to weed 
 some feet below the surface is too feeble for their 
 requirements. Hence, during such tides, C. paradoxa 
 edges up landwards to the shallower water and reaches 
 so far as the Fucus zone. During the spring tides, this 
 latter zone is left high and dry for hours and hapless 
 C. paradoxa stranded there would suffer from the 
 intense insolation and also run the risk of desiccation. 
 So, as the tides increase, it works its way down the 
 beach, reaching, at the median spring tides, to the 
 more seaward weeds, and at the largest springs, when 
 these weeds may no longer harbour it in submerged 
 peace, it treks again yet further toward the sea and 
 takes up its station among the tangle of fine weeds 
 which hang in tassels from the finger-like, dark brown 
 Pycnophycus. During the slack periods, at low water, 
 when the landward part of the Pycnophycus zone is 
 uncovered, C. paradoxa creeps into the deepest re- 
 cesses of the matted, emerged weeds. Soon, the 
 making tide covers the Pycnophycus with an in-
 
 22 PLANT-ANIMALS [CH. 
 
 creasing load of water and C. paradoxa, clinging 
 painfully to the floating, swaying weed, finds itself 
 exposed to a light intensity none too high for its 
 requirements. 
 
 Unlike C. paradoxa which, as we see, migrates 
 periodically, its Sittings coinciding with the phases 
 of the moon, C. roscoflensis, having selected its station 
 on the beach, maintains it in spite of time and tide. 
 Small wonder therefore that the latter organism 
 has learned to respond so swiftly to vibrations 
 that it sinks below the sand at the approach of 
 heavy feet. How sure and swift are the uprisings 
 and downlyings of C. roscoffensis may be learned by 
 standing at the water's edge near by the situations 
 known to be occupied by C. roscoffensis colonies. 
 Scarcely has the tide run oft 7 them when a faint green 
 discolouration of the sand marks the contours of each 
 colony, and before the water has receded more than 
 a few yards the dark greenness of the patch indicates 
 that all the animals have risen to the surface. Or if, 
 when the sea is smooth, we watch the incoming tide 
 making its way with gentlest approach toward the 
 patches, we see the animals inert and lying massed 
 together, bound into scum-like lumps by the muci- 
 laginous excretion of their bodies. They lie motion- 
 less, oblivious of the lapping waves a yard or so away. 
 Then, as the latest wave washes over the patch, 
 lethargy gives place to action and, in an instant,
 
 i] THE HABITAT OF CONVOLUTA 23 
 
 C. roscoffensis is gone. On stormy days, when the 
 making tide announces its landward progress angrily 
 thundering like ramping clouds of warrior horse 
 the reverberations of the sand send signals to the 
 colonies which make their dispositions underground 
 long before the breaking waves can reach or damage 
 them. All these ordered goings and comings may 
 the observer see on any day on any beach in Brittany. 
 But, to discover more precisely the physiological 
 methods of these purposeful movements, the labora- 
 tory must take the place of the beach, and simple 
 scientific methods must supplement bare observation. 
 In this way, it is possible to refer movements, so pur- 
 poseful as to suggest volition, to simple, non-conscious, 
 nervous responses to one or more of several stimuli, 
 the chief of which are gravity and light. 
 
 Before, however, we investigate the living animals 
 in the laboratory we may note yet another example 
 of rhythmic behaviour in our plant-animals. 
 
 However carefully the observer seeks at low water 
 among the exposed weeds of the paradoxa zone, he will 
 find no animals bearing ripe eggs. As the tides be- 
 come large enough to permit of approach on foot to 
 that zone, the animals which he obtains are, for the most 
 part, minute, immature specimens. On succeeding days, 
 the catch consists of larger animals, till, during the 
 latest spring tides, it is composed chiefly of adults, many 
 of which may contain unripe eggs. Then comes a period
 
 24 PLANT-ANIMALS [CH. 
 
 of slack tides when the paradoxa zone is constantly 
 submerged beneath ten feet or more of water. At 
 the succeeding spring tides, the same sequence of 
 immature, young and adult animals is obtained by 
 the collector. The absence of mature females and 
 of deposited egg-capsules is not to be explained by 
 a migration of gravid females to some other place 
 more convenient for the purpose of egg-laying ; for, 
 now and again, a solitary capsule may be found during 
 the latest spring tides glued to the weed of the 
 paradoxa zone. By hatching experiments carried 
 out in the laboratory, it may be demonstrated that 
 the time of maturing of the animals coincides with 
 a definite tidal period. It takes either a month 
 or a fortnight for the animals to become mature. 
 They reach maturity at neap-tidal periods. At the 
 beginning of these periods, or soon after, when the 
 zone is submerged continuously for some seven or 
 eight days, C. paradoxa lays its eggs. No matter 
 how the conditions are altered in artificial hatching 
 experiments, C. paradoxa is faithful to its habit. As 
 indicated by the diagram (Fig. 8), which records the 
 results of such experiments, the females lay their 
 eggs only during the neap tides. 
 
 Nor is it without significance that the large 
 yellow-brown eggs of C. paradoxa, rich in food- 
 yolk, hatch with extraordinary rapidity. Within 
 twenty-four to forty-eight hours of the time of lay-
 
 I] 
 
 PERIODICITY IN CONVOLUTA 
 
 25 
 
 ing, the larvae, after circling actively within the 
 capsule, burst the walls thereof and escape. Thus 
 they have, during the remainder of the neap-tidal 
 period, some days of comparative tranquillity and 
 uniformity of conditions. Not for some days yet will 
 they be exposed to the full to the chances and changes 
 which must beset their adult lives. They are born 
 as submarine animals, and in their earliest days are 
 spared to some extent the buffetings which shall 
 be theirs when, with the advent of the spring tides, 
 they are, now, clinging to fragile weed dashed against 
 
 Fig. 8. Periodicity of egg-laying and hatching of C. paradoxa. The 
 shaded band shows the position of the Paradoxa zone with 
 respect to low water-marks of spring and neap tides. The 
 undulating line, joining up the low water-marks of successive 
 day- tides, is obtained by marking off, along the verticals indi- 
 cating successive days, from a zero line above, the amount of 
 vertical descent (in decimeters) of each day's tide. On those 
 days when the undulating line falls below the shaded band, the 
 Paradoxa zone is uncovered during low water; on those days 
 when the low water-line lies above the shaded band, the zone is 
 continuously submerged. The dots represent egg-capsules, the 
 crosses signify larvas hatched ; the positions of dots and crosses 
 give the dates on which the capsules were laid and larvas 
 emerged.
 
 26 PLANT- ANIMALS [CH. 
 
 the rocks and, now, still clinging to the weed, left 
 stranded, prone upon the ooze beneath the glare of 
 an August sun. 
 
 With C. roscoffensis, also, egg-laying is a periodic 
 phenomenon, though, in this species, the times at 
 which it occurs coincide not with the beginnings of 
 neap tides but with the onset of the springs. A 
 colony of C. roscoffensis is indeed a well-drilled army. 
 Not only do all its members take cover as one unit at 
 a given signal, not only do the individuals keep their 
 ranks when the order comes to climb to the surface 
 once again, but they are born together, grow up to- 
 gether, mature at the same time and lay their eggs 
 simultaneously. As a consequence, it is easy to obtain 
 large numbers of egg-capsules, though only at definite 
 tidal periods. To secure them, all that is necessary 
 is to visit a fertile colony at low water during one of 
 the earlier spring tides, tap with the foot and thus 
 drive C. roscoffensis below the surface, scoop up a 
 little sand, shake it with sea-water in a glass tube, 
 and isolate the slow-sinking, transparent capsules. 
 It is still easier, however, to rear them in the labora- 
 tory. By collecting a cupful of sand and C. roscoffeu- 
 sis just before the onset of a series of spring tides, 
 bringing the cup into the laboratory, adding a little 
 sea-water, leaving it till the plant-animals have col- 
 lected on the surface, scooping them off with a 
 watch-glass and putting them with sea-water in a
 
 i] PERIODICITY IN CONVOLUTA 27 
 
 large glass vessel, hundreds of egg-capsules may be 
 obtained within a few days. The laying continues 
 for a week or more and then, when the time of the 
 slack tides arrives, it ceases, even though some of 
 the animals are yet carrying mature eggs. After a 
 barren fortnight, egg-laying begins again. Both the 
 animals which failed to bear and those which pro- 
 duced eggs contribute to the fortnightly crop. The 
 mode of egg-laying of C. roscoffensis is in some 
 respects peculiar. Occasionally, the eggs are dis- 
 charged separately one or two at a time ; but more 
 often they are contained, as has been stated, in a 
 common, gelatinous capsule. It happens frequently 
 that oviposition results in a rupture of the tissues 
 of the parent. The body becomes torn and may even 
 break across the middle. The anterior end crawls 
 away and, behaving like an intact animal, heals its 
 wounds, regenerates its lost parts and recovers 
 completely. The tail end remains near the egg- 
 capsule, and exhibits ceaseless, revolving, "circus" 
 movements, swimming in devious spirals ; then it 
 comes to rest and finally disintegrates. Unlike 
 C. paradoxa, C. roscoffensis is slow in hatching. 
 After about four days, the larvae begin to revolve 
 actively within the capsule-membrane, then at the 
 fifth to the seventh day after the eggs were laid, the 
 egg-membranes split equatorially and the society of 
 larvae is set free to creep and swim within the common
 
 28 PLANT-ANIMALS [CH. 
 
 capsule. Suddenly they leave it, passing with ease 
 through the mucilaginous wall, though they not 
 infrequently return now and again to the capsule 
 after enjoying a short spell of activity a fact the 
 significance of which we shall have occasion to 
 comment upon later. 
 
 In seeking an explanation of the significance of 
 the fortnightly periodicity in egg-laying, it is easy to 
 conclude that the periods chosen by C. roscoffensis 
 are the most convenient for this purpose. For in 
 summer, to which period of the year these observa- 
 tions apply, low water of spring tides occurs about 
 midday and midnight Now, as we have learned, when 
 the roscoifensis zone is uncovered during the darkness 
 of night the animals do not remain on the surface. 
 Hence, during the spring tides, C. roscoffensis has an 
 uninterrupted period of some eighteen hours in which 
 to lay its eggs. At other tidal periods, its leisure 
 would be less, for, as the tide runs off the patch, the 
 animal must come up to the light and it must re- 
 main up till the returning tide gives the signal that its 
 vigil is at an end. In short, whereas, during the 
 neap-tides, at which periods low water occurs in 
 early morning and late afternoon, C. roscoffensis has 
 two up- and two down-periods, during the springs it 
 has only one period of compulsory " upness " in each 
 twenty-four hours. 
 
 The weak point, however, of all such teleological
 
 ij PERIODICITY IN CONVOLUTA 29 
 
 explanations is that they tend to exercise the in- 
 genuity of men of science rather than to advance 
 our knowledge of physiology. To which it may be 
 answered that adaptation is as much a property of 
 protoplasm as weight is a property of matter, and 
 that the biologist is performing a service in showing 
 how deep-bitten into the organism are the adaptations 
 whereby it adjusts itself to its environment. 
 
 The critic replies : That is very true, but to rest 
 content with a teleological explanation, to say that 
 this animal does such and such a thing because 
 it is convenient or useful for it to do that thing 
 is to renounce profound investigation. Before this 
 can be regarded as the proper philosophical attitude 
 toward life, the resources of chemistry and physics 
 must be exhausted, and the behaviour under con- 
 sideration must at least be proved not to be due to 
 a chemical or physical change induced by some factor 
 or factors of the environment. 
 
 In other words, the least the physiologist can do 
 is to attempt to discover how the adaptive trick is 
 performed by the animal which exhibits it. 
 
 An admirable example of an apparently adaptive 
 character, which is capable of a simple physical 
 explanation, is given by Loeb (1909) in his brilliant 
 essay on the influence of environment on animals. 
 
 The two species of Salamander, Salamandra atra 
 and S. maculosa occupy distinct stations. The former
 
 30 PLANT-ANIMALS [CH. 
 
 species occurs in dry alpine regions of relatively low 
 temperature ; the latter, in lower regions with plenty 
 of water and of higher temperature. 
 
 In the dry, alpine regions S. atra deposits eggs 
 which hatch out as land-animals ; in the wet lowlands, 
 the eggs laid by S. maculosa contain embryos in a 
 less advanced stage of development. The young, 
 when born, are gill-bearing and complete their de- 
 velopment whilst leading an aquatic life. Thus each 
 species is adapted to the physical conditions of its 
 environment. 
 
 But it has been shown that if S. atra is exposed 
 to lowland conditions, that is, to a moist atmosphere 
 and a relatively high temperature, it lays its eggs 
 earlier, the young hatch out in the gill-bearing stage 
 and development is completed during their life 
 as independent, aquatic animals. Conversely, if 
 S. maculosa is exposed to alpine conditions, oviposition 
 does not take place till the embryos have passed 
 beyond the aquatic, gill-bearing phase. Therefore, 
 in these circumstances, they are born as land- 
 animals. 
 
 Hence the adjustment of each species to its 
 environment is due to the direct effect of certain of 
 the physical conditions of that environment on the 
 course of development of the embryos. The fact of 
 adaptation is not denied, but the mechanism whereby 
 it is effected is discovered, and the way made clear
 
 i] PERIODICITY IN CONVOLUTA 31 
 
 for a fuller physiological analysis of the mode of 
 reaction of protoplasm to physical stimuli. 
 
 The problem with respect to periodicity of egg- 
 laying by Convoluta requires us to ascertain whether 
 it is possible to refer the periodicity to any definite, 
 recurrent physical condition of the natural environ- 
 ment of the animal. 
 
 The facts about to be related appear to indicate 
 that this is possible. 
 
 It may be premised that if adult C. roscoffensis 
 are kept in darkness for some time previous to the 
 full development of their eggs, no egg-capsules are 
 laid. The lack of egg-production on the part of 
 dark-kept animals is due to the fact that animals 
 kept under such conditions become starved and, as 
 a consequence, incapable of supplying the eggs with 
 food-materials. But if a similar experiment is made 
 with animals containing eggs in an advanced stage of 
 development and already supplied with plenty of 
 food-materials, it is found that the number of egg- 
 capsules produced by animals kept in darkness is 
 actually greater than that produced by animals which 
 are exposed throughout the day to the light. Hence 
 we may infer that exposure to long spells of twelve 
 or more hours of light is unfavourable to the maturing 
 or deposition of eggs. Further experiments on similar 
 lines show that egg-laying reaches its maximum when 
 the animals are subjected daily to one short spell of
 
 32 PLANT-ANIMALS [CH. 
 
 six hours' light-exposure followed by a long spell of 
 eighteen hours' dark-exposure. But and the fact is 
 remarkable these conditions of light and darkness 
 are precisely those to which C. roscoifensis is exposed 
 during the spring tidal periods at which its eggs are 
 laid habitually. At such periods, low water of suc- 
 cessive tides occurs about the middle of the day and 
 of the night, and hence, in twenty-four hours, the 
 C. roscoffensis zone is uncovered once during day- 
 time and once during night-time. So it comes about 
 that, during the spring tides, C. roscoffensis is exposed 
 for about six hours to the light and for the rest of 
 the twenty-four hours it is in darkness. Therefore, 
 as the laboratory experiments show, of all the daily 
 changing light conditions to which it is subjected 
 throughout a lunar period, those which obtain at 
 spring tide are most favourable to the deposition of 
 egg-capsules. 
 
 In ascribing to light a leading rdle in determining 
 the periodicity of egg-laying we have the support of 
 not a few well-established biological facts. Thus the 
 profound influence which light exerts on plants, both 
 on their development in general and on their flower- 
 production in particular has long been recognised. 
 Perhaps the best-known example of this influence 
 is afforded by the common ivy. It is a fact of 
 general observation that ivy growing on a wall 
 rarely if ever flowers, though when climbing over
 
 i] PERIODICITY IN CONVOLUTA 33 
 
 an arch exposed on all sides to the light it blooms 
 freely. These effects of illumination on flower- 
 formation have been investigated by Vb'chting, whose 
 researches are summarised by Goebel (1900). In 
 order that plants may form flowers in a normal 
 way, the illumination must not sink below a certain 
 amount which is very unequal in different species. 
 If illumination is allowed to fall below the required 
 amount, the size of the whole flower or of its individual 
 parts is diminished and, with decreasing illumination, 
 a stage is reached at which the formation of flowers 
 ceases. 
 
 Similar phenomena are doubtless common among 
 animals though they have not been investigated 
 systematically. Thus, though the phenomenon is 
 not one of reproduction in the strict sense, we may 
 cite Loeb's account (loc. cit.) of the effect of light in 
 inducing regeneration of the polyps of the Hydroid 
 Eudendrium racemosum. If a stem of this Hydroid, 
 covered Avith polyps, is put into an aquarium, the 
 polyps fall off very soon. If the aquarium is in 
 darkness, no regeneration of the polyps takes place 
 even after several weeks ; but, when they are exposed 
 to the light, new polyps form in the course of several 
 days. We may suppose that light favours the forma- 
 tion of definite substances which are the pre-requisites 
 for polyp formation. 
 
 Similarly, we are bound to conclude from our 
 K. 3
 
 34 PLANT-ANIMALS [CH. 
 
 experiments on C. roscoffensis that a spell of illumi- 
 nation of brief duration favours one or other of 
 the series of processes which results in egg-laying, 
 that a longer or shorter spell of illumination is un- 
 favourable to this process, and that, when animals are 
 subjected to these unfavourable conditions, many of 
 them, though they are carrying eggs in an advanced 
 stage of development, remain sterile. 
 
 With respect to the periodicity of egg-laying by 
 C. paradoxa it is not so easy to refer the periodic 
 character of this event to the influence of light. It 
 is noteworthy that other littoral, marine organisms, 
 (certain brown sea- weeds) living in almost identical 
 habitats, exhibit an identical periodicity with respect 
 to their reproductive processes. 
 
 This, according to Williams (1898), is the case 
 with the brown sea-weed Dictyota dichotoma, and 
 subsequent observers have shown it to be true of 
 other marine algse. Not only does Dictyota liberate 
 successive crops of fertile eggs at fortnightly periods 
 but it sheds them at the same point in the tidal 
 period as that chosen by C. paradoxa for the dis- 
 charge of its egg-capsules. In either case, the eggs 
 are liberated some three to five tides after the 
 greatest springs. During the subsequent tides of 
 smaller amplitude, the zone which forms the habitat 
 of both sea- weed and plant-animal is continously sub- 
 merged. Hence we can scarcely escape the conclusion
 
 I] PERIODICITY IN CONVOLUTA 35 
 
 that the period selected for egg-laying has reference 
 to the greater security which is offered during the 
 first days of larval life. Born into the world at this 
 period, the animals and the plantlets have some days 
 of submerged grace before they become subjected 
 twice daily to such extreme environmental changes as 
 occur at other phases of the tidal sequence. 
 
 Thus, driven back provisionally on a teleological 
 explanation, we may interpret the significance and 
 origin of periodicity of egg-laying in the following 
 way. One condition for survival of the species 
 C. paradoxa and of Dictyota is that the just-liberated 
 young shall be for some days after birth continually 
 submerged : that, for one reason or another ultimately 
 connected with nutrition, the maturing of these 
 marine organisms and the development of their 
 sexual cells requires a period of fourteen days, and 
 that the organisms fit their fortnightly periods into 
 the tidal periods in such a way that they reach their 
 climaxes at the most convenient moments. As the 
 waving flag of the guard gives the signal for the 
 train to start, so change of light intensity appears 
 to give the signal for the maturing of the sexual 
 organs and thus secures their liberation at the proper 
 moment. 
 
 Whether such bi-lunar periods of fertility ex- 
 hibited by littoral, marine organisms have any bearing 
 
 32
 
 36 PLANT-ANIMALS [CH. i 
 
 on similar periodic phenomena exhibited by the 
 higher land-animals it is impossible to say ; though 
 it is tempting to think with "The Lady from the 
 Sea," "that we all are descended from sea-animals, 
 and that if we had only accustomed ourselves to live 
 our lives in the sea we should by this time have been 
 far more perfect than we are."
 
 CHAPTER II 
 
 THE ORIGIN AND SIGNIFICANCE OP THE HABITS OF 
 CONVOLUTA ROSCOFFENSIS AND CONVOLUTA 
 PARADOXA. 
 
 THE fact which stands out most prominently from 
 open-air observations of C. roscoffensis and C. para- 
 doxa is that the behaviour of these animals is 
 complex and purposeful. By some means or other 
 they create for themselves an ordered life, in spite of 
 the welter of change in their environment. Through 
 the ever-varying conditions of the world in which 
 they live, they thread their consistent way as surely 
 as we, with conscious self-control and agility, pick our 
 ways safely through the crowded traffic of the street. 
 
 We have now to endeavour to ascertain the 
 nervous components of the complex behaviour of 
 our plant-animals; to learn, by the method of ex- 
 perimental analysis, whether it is possible to refer the 
 ordered complexity of this behaviour to some few, 
 simple, nervous acts. 
 
 It is a matter of common knowledge that many 
 organisms, both plants and animals, orientate them-
 
 38 PLANT-ANIMALS [CH. 
 
 selves with reference to the directions in which light 
 and gravity act upon them. A geranium in a cottage 
 window so disposes its leaves that they receive the 
 maximum of such light as may reach them. Each 
 leaf places itself at right angles to the direction of 
 the incident light. The stem of the plant behaves 
 differently, bending till its tip is parallel with 
 the rays, it grows toward the source of light. Light 
 is the agent, or stimulus, which induces these orien- 
 tations. The mode of orientation is determined by 
 the plant itself and has, in each case, a purposeful 
 significance. The leaves in the window are none too 
 well illuminated ; the work which they have to per- 
 form depends on ample light and thus, by their 
 orientations, they secure for themselves the most 
 favoured light-treatment. 
 
 The root of a plant grows vertically downward 
 through the soil. When, from one cause or another, 
 the tip is displaced from the vertical line, the rate of 
 elongation of the growing region of the root becomes 
 faster on the upper than on the lower side. A growth- 
 curvature results, and the tip is carried by the bending 
 root once more into the vertical line. Here gravity is 
 the stimulus, and the result of the stimulus is a motor 
 response a purposeful growth-curvature. Cut away 
 the root-tip, and the root, although it be displaced from 
 the vertical, grows indifferently in any direction till a 
 new root-tip is regenerated. From such and similar
 
 n] HABITS OF CONVOLUTA 39 
 
 observations Charles and Francis Darwin (1880) 
 concluded that the power of gravi-perception is 
 localised in the root-tip. Nor has subsequent criticism 
 succeeded in invalidating this conclusion. 
 
 Now, since the growth-curvature, which results 
 from the perception by the root-tip of the stimulus 
 of gravity, occurs in a region of the root the elon- 
 gating region which is separated from the tip by 
 a region which is not increasing in length, it follows 
 that perception by the root-tip results in an excitation 
 of the living substance of its tissues, and that this 
 excitation gives rise to some change in the tissues 
 which intervene between the perceptive and motor 
 regions. This change, of unknown nature, we may 
 call a nervous impulse, and we may say that, as the 
 result of excitation consequent to perception, a 
 nervous impulse is transmitted from the root-tip to 
 the motor region. Of the nature of this impulse we 
 know nothing ; nor, for the matter of that, is any- 
 thing definite known of the nature of any nervous 
 impulse ; for example, that which travels along a 
 nerve to a muscle in one of the higher animals. The 
 "nervous impulse" may well be of chemical nature, 
 and transmission of such an impulse through living 
 tissues does not connote definite specialised nerves. 
 It is as much the property of protoplasm to transmit 
 nervous impulses as it is of fire to burn, or of a lit 
 fuse to explode a charge of gunpowder. Protoplasm
 
 40 PLANT-ANIMALS [CH. 
 
 is an apparatus for that purpose : as well, of course, 
 as for other purposes. 
 
 Arrived at the motor region, the nervous impulse 
 sets up an excitation in the protoplasm of that region. 
 As the result of this excitation, there arises a definite 
 modification in the hitherto uniform rate of elonga- 
 tion of the cells of the motor region. The cells of 
 one side grow faster than those of the other, and a 
 growth-curvature results by which the tip is carried 
 back to the vertical position. Such a mode of 
 nervous action is called a reflex. In every case, 
 in the simplest, unicellular organism and in the 
 highest animals, reflex action involves perception, 
 excitation, transmission to the motor region, excita- 
 tion of, and motor (or other) response by, that region. 
 All protoplasm, as we know it, contains the apparatus 
 required for this series of events, and evolution, as 
 we know it, has but resulted in the perfecting and com- 
 plicating of these reflex arcs. We may take the reflex 
 as the base or primal manifestation of all nervous 
 activity. In the reflexes of root and stem and leaf, 
 the stimuli light, gravity, etc. which induced them 
 give rise to the assumption by the root or stem 
 or leaf of a definite position with respect to the 
 direction whence the stimulus proceeds. Such 
 stimuli are therefore called directive stimuli. When 
 a fixed plant or animal responds to a directive 
 stimulus by a definite, purposeful curvature we de-
 
 n] HABITS OF CONVOLUTA 41 
 
 scribe the response as tropistic. If the plant or 
 animal is not fixed but free, it responds by moving 
 in a definite direction and the response is described 
 as tactic. Inasmuch as the end of either reaction 
 is the achievement of a definite orientation and inas- 
 much also as the fixed plant, not only curves till it 
 assumes a definite position, but also, having done so, 
 moves by growth in the direction to which its curva- 
 ture has brought it, we may use the term tropistic to 
 describe the reactions of both fixed and free organisms 
 to directive stimuli. 
 
 We have now to consider the tropisms of our 
 plant-animals. 
 
 Brought into the laboratory and placed in sea- 
 water in a glass vessel near the window, C. ros- 
 coffensis behaves precisely like the leaf of the 
 geranium in the cottage window. Each animal turns 
 to the light, moves toward it and finally exposes 
 che surface of its body athwart the line of light* 
 Within a minute or two the reaction is completed. 
 Swiftly and, as it would seem, inevitably the animals 
 assemble on the side of the vessel toward the 
 light, and form a green scum on the surface of 
 the water. If the vessel is turned round, the 
 animals release their holds and, either falling like 
 a precipitate to the bottom or edging round the 
 side of the vessel, arrive once again at the surface 01 
 the water on the side of the vessel directed toward the
 
 42 PLANT-ANIMALS [CH. 
 
 light. This mode of response we speak of as a 
 positively (+) phototropic response. 
 
 C. paradoxa, which lives in a shadier situation, 
 responds to the light of the laboratory by an opposite 
 movement a negatively (-) phototropic response 
 (Fig. 9, a). 
 
 I 1 
 
 
 
 Fig. 9. Phototropism of C. paradoxa : the influence of light-intensity 
 on phototropic response, a. Mode of response when the light- 
 intensity is high. b. Mode of response when the light-intensity 
 is low. "The glass troughs containing the animals are represented 
 (in plan) by oolongs. The troughs standing on a black ground 
 are represented by the shaded, those on a white ground by the 
 clear oblongs. The animals are indicated by dots, and the arrows 
 show the direction of the light. 
 
 It is easy to prove, however, that neither the 
 positive phototropism of C. roscoffensis, nor the oppo- 
 site mode of reaction of C. paradoxa, is, in reality, an 
 inevitable reaction. Expose C. roscoffensis suddenly 
 to a bright light and it recoils (see Fig. 12) as we 
 ourselves in similar circumstances may recoil. Place 
 it in a dim light and it exhibits no phototropistic re-
 
 ii] HABITS OF CONVOLUTA 43 
 
 sponse : it has become non-phototropic. On the other 
 hand, in the gloom of an ill-lit cellar, in which the 
 light-intensity approximates to that of its habitat 
 among the masses of brown sea-weed, C. paradoxa 
 becomes somewhat + phototropic (Fig. 9, 6). 
 
 It is urged not infrequently that reflexes are the 
 nervous units, unalterable in form, of which behaviour 
 and higher phases of nervous activity are composed. 
 The briefest study of the lower animals demonstrates 
 that, though reflexes may be regarded as units by 
 the physiological summation of which behaviour 
 and habit are composed, they may not be regarded 
 as unalterable and inevitable. No more than con- 
 scious acts are reflexes the masters of the organism 
 which exhibits them. They are but servants, and 
 tropistic reflexes serve the master-organism, to 
 draw it this way or that according as it is well 
 that, this or that route be taken. Under un- 
 changing conditions, both with respect to environ- 
 ment and with respect to the state of the organism, 
 the reflex is inevitable. But under such conditions 
 a conscious act is likewise inevitable. Since un- 
 changing internal and external conditions are all 
 but unknown in nature, there will always be scope 
 for modification in the reflex as in the conscious act. 
 If the physiologist is called in to act as umpire in 
 the dialectical game between the advocates of free- 
 will and determinism, he pronounces the game a
 
 44 PLANT-ANIMALS [CH. 
 
 draw. Under abnormal and well-nigh impossible 
 conditions, the organism, high or low, is an automa- 
 ton the creature of inevitable nervous responses 
 reflex or conscious. Under normal conditions of life, 
 it responds now this way and now that to external 
 or internal stimuli and so appears to act as a free 
 agent. 
 
 The apparent inevitability of reflexes is but an 
 indication of habit. When the environmental cir- 
 cumstances to which an organism is exposed are 
 comparatively simple or when the organism itself is 
 not highly differentiated, one or two external agents 
 may serve it as guides. The organism takes the habit, 
 for example, of relying implicitly on the stimuli of light 
 and gravity. By responding to these stimuli, it finds 
 its proper place with such certainty that other modes 
 of response to other stimuli are ignored habitually. 
 Hence, by playing on its habitual tropisms, it is 
 easy in the laboratory to lure an organism to its 
 doom. 
 
 This we may illustrate by exposing C. roscoflensis 
 to simultaneous stimulation by light and heat. It 
 must be premised that the animal, though, for a 
 marine organism very tolerant of high temperatures, 
 is negatively thermotropic at about 35 C. At this 
 temperature it moves in the direction of the colder 
 water. In order to investigate the behaviour of the 
 animal with respect to simultaneously applied light-
 
 ii] HABITS OF CONVOLUTA 45 
 
 and-heat stimuli, large numbers of C. roscoffensis are 
 placed in sea- water contained in a long glass trough, 
 the axis of which is parallel with the direction of the 
 light. Promptly, the animals mass themselves at the 
 end of the vessel directed to the light. The water 
 at that end is heated gradually ; but in spite of the 
 rising temperature, and in spite of its powers of 
 negative thermotropism, Convoluta remains faith- 
 fully at its light station, and dies in thousands at its 
 post. Habitual obedience to the command of light 
 renders it oblivious of the warning of increasing 
 temperature, which warning suffices to bring about 
 the withdrawal of less pre -occupied animals from 
 dangerous regions. We see in the behaviour of the 
 plant-animals thus subjected to simultaneous stimu- 
 lation not an illustration of the inevitableness of a 
 reflex, but an example of the limitations attaching 
 to all nervous actions, both reflex and conscious. 
 
 The behaviour of C. roscoflensis with reference 
 to black and white backgrounds supplies a striking 
 illustration of the fact that circumstances alter re- 
 flexes. These "background" responses we will now 
 consider. The choice of a definite background is a 
 phenomenon exhibited by many sea-shore and aquatic 
 animals. When offered the alternative of a white or 
 black background, some animals take up a position 
 on the one, some on the other. Thus among the 
 marine Crustacea, certain prawns and also species of
 
 46 
 
 PLANT-ANIMALS 
 
 [CH. 
 
 B 
 
 Fig. 10. Diagram illustrating the influence of background (white 
 or black) on the movements of Convoluta roscoffeusis. The 
 animals are placed in shallow porcelain troughs, the bottoms of 
 which are half white and half black (shaded). Each dash 
 represents a Convoluta. A. In uniform light. Aj. At beginning 
 of experiment, Convoluta fairly uniformly distributed. A 2 . After 
 forty minutes, Convoluta all on white ground. B. In lateral 
 light (arrows show direction of light). Fifty Convolutas placed 
 in the white half of lx , and fifty in the black half of 2 t . 1 2 and 2 a 
 show the results after two minutes : in 1 2 ratio on black and 
 white = f$ ; in 2 2 ratio = ^ . See text.
 
 n] 
 
 HABITS OF CONVOLUTA 
 
 47 
 
 Mysis station themselves on the black part of a 
 dish the bottom of which is half black, half white. 
 The chameleon shrimp, Hippolyte varians, on the 
 contrary, selects the white ground. A similar be- 
 haviour is exhibited by various fishes, trout among 
 others. 
 
 1 1 
 
 Fig. 11. Phototropism of C. paradoxa : the influence of background 
 on phototropic response. The flat, porcelain troughs containing 
 the animals are represented (in plan) by the oblongs. The bottom 
 of each trough is half white and half black. In the diagram, the 
 white ground is indicated by the unshaded, the black ground by 
 the shaded part of the oblong. The animals are represented by 
 dots, and arrows show the direction of the light. a. In bright 
 light. 6. In weak light, c. "Choice" of black ground in 
 preference to white ground. 
 
 So striking is the behaviour both of C. roscof- 
 fensis and C. paradoxa with respect to background,
 
 48 PLANT-ANIMALS [CH. 
 
 that when specimens of the two animals are put 
 together into a dish, the bottom of which is half 
 black and half white, they segregate rapidly and 
 completely ; C. roscoffensis takes up positions on the 
 white ground, C. paradoxa on the black ground 
 (cf. Figs. 10 and 11). The distribution is in accord 
 with reasonable expectation based on knowledge of 
 the natural habitats of the two species. C. ros- 
 coffensis, attuned to a high light intensity, with its 
 place in the sun, is evidently a bright background 
 animal ; C. paradoxa, lurking in the shadows of the 
 weeds, though it also needs light for its growth and 
 development, is unused to well-lit situations and 
 seeks in preference the darker background. 
 
 But though the selection of ground seems bio- 
 logically reasonable the question remains, how is it 
 done? The hypothesis may be hazarded that it is 
 a phenomenon of association. C. roscoffensis is, as 
 we know, positively phototropic. In effecting a 
 phototropic response, it is bound in ordinary cir- 
 cumstances to pass from a darker to a lighter 
 background. The performance of the phototropic 
 movement is associated with the darker ground, the 
 achievement or consummation that is, a state of 
 immobility is associated with the brighter back- 
 ground. If therefore we adopt the hypothesis, 
 proposed by Semon, (1904) that environmental con- 
 ditions, which are contemporaneous with a particular
 
 ii] HABITS OF CONVOLUTA 49 
 
 stimulus, are recorded in the "nmeme" or unconscious 
 memory of an organism as integral parts of the 
 nervous operation initiated by the stimulus and con- 
 summated by the reaction which it calls fortli ; then 
 it may well follow, as it follows in organisms endowed 
 with conscious memory, that these environmental 
 conditions acting alone and in the absence of the 
 stimulus, may suffice to set in action the nervous 
 apparatus in the same manner as the stimulus itself 
 originally set that apparatus in action. Hence the 
 attendant environmental conditions may produce the 
 reaction originally called forth by the stimulus. 
 
 An example borrowed from Semon's work (loc. rit.) 
 may make the idea clearer. A boy throws a stone 
 at a puppy. The dog is hit and hurt, whimpers and 
 runs away. The next time the puppy grown older 
 and wiser sees a boy stoop, as though to pick 
 up a stone, it whimpers and runs away. Linked in 
 the memory are the hurt, the stone, and the stooping 
 boy. The hurt supplied the stimulus for whimpering 
 and flight ; but memory, the constable of the body, 
 charges the stooping boy with being an accessory to 
 the act. Henceforth it will advise the avoidance of 
 stooping boys. Experience consists in the discovery 
 of short cuts to safety. 
 
 So, assuming with Samuel Butler and Bering, 
 an unconscious memory, or mueme, Semon suggests 
 that the lower organisms may react to the attendant
 
 50 PLANT-ANIMALS [CH. 
 
 or accessary stimulus in the absence of the prin- 
 cipal. 
 
 Applying this hypothesis to background reaction, 
 we assume that dark background, from constant 
 association with unilateral light, has come to suffice 
 to stir up the mneme and so to set going the nervous 
 apparatus which induces a precise muscular move- 
 ment. Thus, C. roscoffensis, placed on a dark back- 
 ground, begins to crawl about and continues to do 
 so. That this interpretation of the origin of back- 
 ground reaction, contains something of the truth 
 seems probable from the fact that the movements 
 performed by the animals on a dark background are, 
 compared with the business-like, phototropic move- 
 ments, aimless as to direction. They are non-directive, 
 chance movements ; but since they continue so long 
 as the dark background is there to call them forth, 
 they conduct the animals sooner or later to the white 
 ground of the particoloured vessel. Arrived there, 
 the stimulus ceases from troubling and C. roscoffensis 
 is at rest. 
 
 We conclude, therefore, that background, from 
 being a mere attendant circumstance, an environ- 
 mental accessory to unilateral light, has come itself 
 to serve as a stimulus to movements which, by the 
 devious paths of chance, direct the animals to the 
 lighter ground. 
 
 In nature, under all ordinary conditions, back-
 
 n] HABITS OF CONVOLUTA 51 
 
 ground co-operates with unilateral light to bring 
 C. roscoffensis to its proper light station its place 
 in the sun. Nevertheless background stimulus may, 
 under artificial conditions, act antagonistically to 
 that of lateral light and even dominate it. 
 
 Thus if two half-black, half-white porcelain 
 troughs containing a little sea-water are so placed 
 that, in one, the white half, in the other, the black 
 half is directed towards the source of light, then, if 
 some fifty C. roscoffensis are placed in each of the 
 troughs, whereas in a very short time all the animals 
 are congregated on the white ground of the first 
 trough, only about forty per cent, manage to arrive 
 at and maintain themselves upon the black half of 
 the second trough (Fig. 10, B\ 
 
 When positive phototropism involves the passage 
 from black to white the movement is executed with 
 certainty and despatch ; but when it demands a 
 passage from white to black a movement against 
 the grain of habit there are hesitation, uncertainty, 
 and many failures. Under yet other conditions, 
 when, for example, the intensity of the unilateral 
 light is lowered the stimulus of background may 
 prove more potent altogether than that of unilateral 
 light. In such circumstances, C. roscoffensis remains 
 on the black half of the dish, although the rays 
 of light signal to it to approach their source. It 
 looks as though the facts or illusions which, in 
 
 42
 
 52 PLANT-ANIMALS [CH. 
 
 higher animals, are named choice and volition are 
 illustrated in our plant-animals by the simplest of 
 working models. If this view is accepted, it would 
 seem to follow that the intricacy and mystery of 
 complex habits and instincts are begotten of the 
 ever-increasing complexity of conditioning or ac- 
 cessory stimuli which have first attached them- 
 selves to and then replaced the original series of 
 stimuli 
 
 Some further facts with respect to the photo- 
 tropic responses of C. roscoffensis are worthy of a 
 passing word. 
 
 In the first place, just-hatched animals, though 
 they respond to the directive stimulus of gravity, do 
 not respond to that of light. After a few hours of 
 free existence, they acquire the faculty of responding 
 tropistically to unilateral light, which henceforth be- 
 comes a masterful factor in determining their habits. 
 
 In the second place, the rays of light to which 
 C. roscoflensis responds tropistically are not those 
 which induce phototropic curvatures in plants. As 
 is well known, a plant exposed, unilaterally, to rays 
 of the less refrangible part of the spectrum the red 
 for example shows no phototropism ; whereas a plant 
 subjected on one side to blue-violet light reacts as 
 readily as to white light. Convoluta roscoffensis, on 
 the other hand, responds not to blue or violet light, 
 but to green light. The diagram (Fig. 12) represents
 
 II] 
 
 HABITS OF CONVOLUTA 
 
 53 
 
 WA,?e Light 
 
 Creen Light 
 
 Red Light 
 
 Blue Lijht 
 
 I c 
 
 Fig. 12. Tropistio reaction of Convoluta roscoffensis to Monochro- 
 matic Light. Each circle represents a ground-plan of a shallow 
 porcelain vessel containing a central heap of sand, a little sea- 
 water, and many Convolutas (represented by dots). The break 
 in the circle (Series 1) indicates the position of the window in 
 a blackened bell-}ar placed over each porcelain vessel. The 
 arrows represent the direction of the light. Series 1 shows 
 the disposition of Convoluta at the time of removal of the 
 covers. Series 2 shows the instant negative phototropic reaction 
 set up by removing the covers (raising the light intensity). 
 Series 3 (only one example shown) shows the recovery (a few 
 seconds after Series 2) of the positive phototropism. The green 
 light was produced by passing daylight through three of Baker's 
 green gelatine films ; the red by using three of Baker's red films ; 
 the blue by using four of Kirchmann's blue films and one 
 green film. (The blue and green lights were not absolutely 
 monochromatic.)
 
 54 PLANT-ANIMALS [OH. 
 
 the results of a series of experiments with unilateral, 
 monochromatic light. As the illustration shows, 
 the animals represented by dots mass themselves 
 toward the source of light when that light is 
 white or green. In blue light, they remain distri- 
 buted with fair uniformity around the periphery of 
 the containing vessel. In red light they show some 
 sign of a negative reaction. 
 
 The most probable explanation of this re- 
 sponse to green light is that the orange eye-spots 
 and pigment glands are the organs of light-per- 
 ception ; and that the pigment contained in the eye- 
 spots and glands absorbs principally the green light. 
 From observations on other animals it would appear 
 probable that green light not infrequently serves 
 marine organisms for purposes of perception : nor, 
 when we reflect upon the green-blue colour of sea- 
 water, will this appear surprising. 
 
 In the third place, C. roscoifensis, with its well- 
 defined tropisms, is an admirable subject in which to 
 study what without a great violation of language we 
 may call the problem of the parallelogram of physio- 
 logical forces: in other words, the problem of the 
 mode of response of an organism to two directive 
 stimuli, simultaneously applied and acting along dif- 
 ferent lines. As might be expected, when two stimuli 
 act on C. roscoiFensis one not infrequently dominates 
 the other, so that the resulting reaction is that which
 
 n] HABITS OF CONVOLUTA 55 
 
 would occur were the dominating stimulus alone 
 applied. This happens, as we have seen, when C. 
 roscoffensis is subjected to both light- and heat- 
 stimulation. 
 
 So also, in the case of light and gravitational stimuli 
 acting simultaneously, the mode of response of C. 
 roscoffensis shows that the latter stimulus may be 
 ignored. 
 
 Thus, if animals are placed in water in a tall 
 glass cylinder on a steady table, they rise to the 
 surface of the water and congregate on the side 
 toward the light. If the light-conditions are modi- 
 fied by the interposition of a black card or plate 
 of ground glass between the source of light and the 
 top of the water, C. roscoffensis relaxes its hold and 
 swims downward to take up a new position just 
 below the edge of the screen. Geotropism is sub- 
 ordinated to phototropism. 
 
 Subjected to simultaneous stimulation by light 
 and gravity, C. roscoffensis behaves exactly like a 
 green plant placed under similar conditions. Though 
 the stem of a green plant is negatively geotropic, 
 yet, if it is illuminated from below, the plant, 
 ignoring gravitational stimulation, directs the tip of 
 its stem downwards toward the source of light. 
 
 The behaviour of both plant and animal would 
 seem to indicate that, in the reflex groundwork 
 of nervous activity, something akin to the pheno-
 
 56 PLANT-ANIMALS [OH. 
 
 menon of attention in psychic life may manifest 
 itself. 
 
 Even though he may not be concerned with 
 problems of reflex action, the biologist who would 
 investigate the life histories or structure of such 
 animals as C. roscoffensis must pay some heed 
 to their tropistic behaviour, for on this knowledge 
 his successful manipulation of the living animals 
 depends. 
 
 Thus, to obtain plentiful supplies of eggs we may 
 make effective use of the phototropic reaction. Large 
 numbers of mature C. roscoffensis are placed near a 
 window in a flat, glass dish containing sea- water. The 
 animals move up to the light. At nightfall, the dish 
 is turned round. The operation, if performed care- 
 fully, does not disturb the animals. They remain 
 throughout the night in that part of the vessel which 
 was nearest the light during the previous day. Cer- 
 tain of the animals deposit egg-capsules. In the 
 morning, the animals, responding to the directive 
 stimulus of light, cross over to the side turned 
 toward the window. The egg-capsules thus left 
 behind are readily visible to the eye and may be 
 picked out by means of a pipette. In this way, several 
 hundred egg-capsules may be obtained in the course 
 of a few days. 
 
 Again, young C. roscoffensis are so minute that 
 they may be found only by a practised eye. Never-
 
 n] HABITS OF CONVOLUTA 57 
 
 theless, by exploiting their background reaction, they 
 may be picked out easily. 
 
 By laying a small sheet of white paper on a black 
 cloth and standing the dish containing the animals 
 partly on the black and partly on the white ground, 
 the animals are caused to accumulate above the 
 latter. There, however, they are almost invisible; 
 but by turning the vessel round so that the part 
 above the white paper is brought over the black 
 cloth, the animals may be seen distinctly and picked 
 out by means of a fine pipette before they scuttle 
 off again to the white ground. To transfer a young 
 C. roscoffensis from one vessel to another is difficult 
 enough till its geotropism is pressed into service, when 
 the operation becomes quite easy. The animal is lifted 
 in a pipette ; but on endeavouring to expel it from the 
 tube by pressing the indiarubber nipple of the pipette, 
 only the water is discharged, and C. roscoffensis is left 
 sticking to the side of the glass tube. To avoid this, the 
 pipette is held vertically and no effort made to eject the 
 water. The slight, involuntary shaking of the hand 
 suffices to render the animal negatively geotropic. 
 Down it swims till it reaches the drop of water at the 
 point of the pipette, whence the gentlest pressure 
 suffices to transfer both drop and animal to another 
 
 So far, this study of the behaviour of our plant- 
 animals has been confined to the investigation of their
 
 5.8 PLANT-ANIMALS [CH. 
 
 tropistic responses to light and gravity. A stimulus 
 may however induce responses in an organism of a 
 very different kind. It may give rise, not to a change 
 of place, but to a change of state. Such effects of 
 stimulation are called tonic effects, and the organism 
 which responds to them is said to be in a state of 
 tone or tonus. Certain peculiar effects of this kind 
 are well known among human beings and may serve 
 us as illustrations. People who work all day by 
 artificial light, specially by unmitigated electric or 
 incandescent gas light become irritable and depressed. 
 Professional photographers, who spend long hours in 
 "dark rooms" developing photographs in red light, 
 suffer mentally in a similar way. The change of 
 state induced by such abnormal conditions we may 
 describe as a change of tone. We will assume that 
 light is indispensable to the human race, that men's 
 bodies are attuned to light and that this harmony 
 is maintained by an unceasing sequence of light- 
 stimuli which contribute to the well-being of the 
 nervous system. Then, if we adopt this view, it will 
 be easy to imagine that a cessation of the rain of 
 stimuli may prejudice the well-being of the nervous 
 system and be the origin of disorders of more or 
 less severity. 
 
 How far the normal nervous state of human 
 beings is determined by the tonic effect of light it 
 is not possible to say ; but there is no doubt that
 
 H] HABITS OF CONVOLUTA 59 
 
 this phototonic effect is of considerable and even 
 fundamental importance to the well-being of many 
 animals and plants. 
 
 If a green plant is placed in darkness the 
 mechanism of its growth is thrown out of gear. 
 Though it grows, and, if supplied with proper food, 
 might, for all we know, go on living indefinitely, its 
 nervous state is changed, its tone has been affected. 
 It becomes "drawn," as gardeners say, its stem grows 
 long and supple, and its leaves remain small and un- 
 developed. As, without the controlling baton of the 
 conductor, the unity of the orchestra is lost, and, it 
 may be, harmony is replaced by discord, so, without 
 the constant influence of light, the harmony of growth 
 which obtains normally throughout the plant is dis- 
 turbed. Since, therefore, light-stimuli contribute to 
 the maintenance of the normal nervous state of the 
 plant, we say that light exerts a tonic influence. In- 
 asmuch, however, as even in the absence of light 
 the plant remains alive and capable of some sort 
 of growth and development, we must conclude that 
 the state of tone in which it lives is not the result 
 of light but that it is modified by light. This 
 effect of light in modifying to the advantage of the 
 organism its state of tone is called phototonus. The 
 language is clumsy but the ideas which it conveys 
 are clear enough, though lacking in precision. 
 
 C. roscoffensis is, as we know, attuned to a high
 
 60 PLANT-ANIMALS [OH. 
 
 light intensity. It exposes itself on the beach to the 
 bright light of the midday sun with nothing between 
 it and desiccation but a constant, filmy stream of 
 salt, drainage-water. In such situations, it retains its 
 powers of activity unimpaired for hours. If, however, 
 it is placed in a vessel with some sand and water, taken 
 to the laboratory and kept in darkness, it passes after 
 some days into a lethargic condition. In this state 
 of dark-rigor C. roscoffensis remains on the surface of 
 the sand and may even fail to respond by downward 
 migration when subjected to the stimulus of vibration. 
 So also, after prolonged exposure to high light 
 intensity, a similar lethargic condition a light-rigor 
 comes over the plant-animals. Even in their natural 
 positions on the beach, after long hours of exposure 
 to the sun's glare, colonies of C. roscoffensis may be 
 observed in which all the members appear to be 
 overcome by light-rigor. They lie roped together 
 by the slimy excretion of their skin, inert, floating 
 in water-puddles. At times, chunks of a colony in this 
 state may be detached by running water, and small 
 green masses, each of many thousand individuals, are 
 borne seaward by the drainage stream. In this 
 lethargic state of light-rigor, which both young and 
 old animals exhibit, C. roscoffensis is difficult to 
 manipulate ; for example, attempts to transfer them 
 from one vessel to another by means of a pipette 
 result generally in damage to the animals. To this
 
 n] HABITS OF CONVOLUTA 61 
 
 lethargy or light-rigor here attributed to exposure 
 for long periods to high light intensities is due the 
 ftuct that, whereas, on some days, the C. roscoffensis 
 patches on the shore disappear before the water of 
 the making tide reaches them, yet, on other days, 
 the multitude of animals composing a patch lies 
 motionless and indifferent to the approach of the in- 
 coming tide. Not till the first wave sweeps over them, 
 do the animals throw off their sloth and disappear. 
 
 We have now to attempt to apply the knowledge 
 we have obtained of the tropistic responses of C. 
 roscoffensis to light and gravity, and of the tonic 
 effects of light, to the elucidation of the most strikingly 
 picturesque feature of the behaviour of this animal, 
 that of its tidal uprising and downlying. Almost as 
 soon as the water of the falling tide has run off the 
 roscoffensis zone, the green colonies appear, and, 
 before the making tide invades it, they vanish. The 
 purposes of ascent and descent are obvious. By its 
 ascent, the animal reaches the light without which 
 for reasons we shall discover subsequently it cannot 
 live ; by its descent, C. roscoffensis maintains its 
 situation on the shore and escapes the waves. 
 
 As our study of its tropisms makes clear, these 
 movements of ascent and descent may be induced in 
 the laboratory by subjecting the animals to appro- 
 priate stimulation. Vibrations produced by tapping
 
 62 PLANT-ANIMALS [CH. 
 
 the sand or the containing vessels send them down, 
 only to reappear when the tapping has ceased. But, 
 as we have seen, a colony may disappear before the 
 tide has mounted high enough to disturb it The 
 eyes of C. roscoffensis mere pigment spots are too 
 rudimentary to allow us to suppose that it sees the 
 water coming in and so takes warning and descends 
 betimes. Indeed a simple experiment suffices to 
 demonstrate, not only that this is not the case, but 
 also that whole colonies of C. roscofiensis may descend 
 beneath the sand in the total absence of an apparent 
 external stimulus. 
 
 Thus, if a batch of animals from a roscoffeneis patch 
 is scooped up with sand and water by means of a cup 
 and taken into the laboratory, the shaking to which, of 
 necessity, the specimens are subjected in the process 
 causes their swift descent. By the time the cup is 
 brought indoors, not a trace of green may be visible 
 in it ; but, in the calm of the laboratory, the animals 
 reascend once more and lie as a thick, dark green 
 scum upon the surface of the sand. They remain in 
 this state for hours, then suddenly disappear. 
 
 Wondering at this swift retreat, and as we wonder, 
 staring through the laboratory window at the shore 
 a stone's throw away, we note first vaguely and then 
 with quickening curiosity that the rising tide is just 
 about to flood the roscoifensis zone. Curious, this 
 descent of the green scum of animals in the cup !
 
 n] HABITS OF CONVOLUTA 63 
 
 Some hours elapse, and, as the tide is running off 
 the roscoffensis zone, curiosity, or its after-effect, 
 provokes us to inspect the cup on the laboratory 
 table. Even as we look into it, a faint green colour 
 steals over the surface of the sand, and, in a minute 
 or two, it is almost black with a dark crowd of 
 C. roscoftensis. Now curiosity joins with astonish- 
 ment to beget a new idea. More cups are found 
 that the observation may be repeated and coincidence 
 put out of court. Each time we repeat the observation 
 on fresh batches of animals we obtain the same result. 
 As on the shore in the roscoftensis zone, so in the 
 laboratory the upward and downward movements of 
 Convoluta march with the movements of the tide. As 
 the tide recedes from their home upon the shore, the 
 sojourners in the laboratory rise up : as the tide rises 
 over it, they sink down. In the absence of all apparent 
 external stimulus, C. roscoffensis, obedient to its 
 custom, yet keeps time with the tide. The rhythm of 
 the tides is reflected by the movements of the animal. 
 For eight successive tides (Fig. 13) the animals in the 
 laboratory maintain their rhythm, synchronous with 
 the ebb and flow of the waters over the roscoffensis 
 zone : then, though the rhythmic movement up and 
 down may yet continue, its temporal periodicity 
 loses precision, and, finally, the rhythm is worn down. 
 This stage reached, the animals exchange a working 
 day of double, six-hour shifts, two up, two down, for
 
 PLANT-ANIMALS 
 
 [CH. 
 
 one of a single, twelve-hour spell of " upness " with a 
 like twelve-hour spell of " downness." In other words 
 they phototrope themselves up to the light as day 
 breaks and sink down with the sun. 
 
 Whence comes the power whereby C. roscoffensis 
 acts as a tide-indicator? What orders its rhythmic 
 coming and going? 
 
 Shore, 
 
 Ugh* 
 LfcWt 
 
 D*rfc ^Q 
 
 Fig. 13. The rhythmic tidal movements of C. rosooffentua. The 
 curves represent the rise and fall of the tide. The horizontal 
 lines included within the tidal curve indicate the "up" or 
 "down" positions assumed by the animals. "In Laby. light" 
 shows that, with animals kept in the laboratory and exposed to 
 light during the day, the rhythm is lost after seven or eight 
 periodic tidal movements up or down. "Light agitated" shows 
 that animals exposed to constant vibration lose their periodicity 
 more quickly. " Dark " that in constant darkness periodicity does 
 not manifest itself. 
 
 A French biologist, Dr Bohn (1903), who has 
 also observed this periodicity of upward and down- 
 ward movement, rejects the view which is put forward
 
 n] HABITS OF CONVOLUTA 65 
 
 below, and regards the phenomenon as a manifestation 
 on the part of C. roscoffensis of " memory of the shock 
 of the waves." Certainly, if all other explanations fail 
 if we can discover no agent which serves to jog this 
 memory we must accept this suggestion ; though in 
 doing so, it might be well to ask ourselves whether it 
 is to be regarded as an explanation or as a succinct 
 statement of our ignorance. 
 
 Experimental investigation of the phenomenon 
 would appear to indicate that no such large demand 
 on memory or mneme as that which is implicit in 
 the above hypothesis need be made. 
 
 In the first place, as we have noted already, 
 C. roscoffensis does not remain on the surface of 
 the sand at night. Hence we must suppose, on the 
 memory hypothesis, either that it forgets to arise from 
 the dark sand when it is dark on the surface, or that 
 it remembers, rises, and finding nothing better to do, 
 goes to bed again. 
 
 The behaviour of C. roscoffensis in constant dark- 
 ness is yet more difficult of interpretation on this 
 hypothesis. For, when kept in continuous dark- 
 ness, C. roscoffensis ceases to exhibit periodicity of 
 alternate up and down movement. There may be 
 one movement downward and one upward ; but, after 
 that, the animals remain upon the surface of the 
 sand day after day until they die (Fig. 13). 
 
 Again, if the downward movement is due to a 
 
 K. 5
 
 66 PLANT-ANIMALS [CH. 
 
 memory of past vibrations caused by the making 
 tides invading periodically the C. roscoffensis zone, 
 how much more certain should be the effects of 
 present vibrations. Yet, if the vessel containing the 
 animals is so exposed that a steady drip of water falls 
 upon the surface of the sand contained in the vessel, 
 C. roscoffensis clings to its periodic habit. As soon as it 
 perceives the vibrations it descends and remains below 
 the sand. When, however, the time for its uprising 
 arrives, it rises to the surface, and, in spite of injuries, 
 remains upon the surface. It seems difficult of belief 
 that the memory of a particular kind of blow can be 
 a more powerful spur to action than the actual 
 receipt of an unceasing series of blows of a like kind. 
 The original suggestion which, though it is not ac- 
 cepted by the author of the memory hypothesis, 
 seems to fit the facts, seeks to explain the periodicity 
 of upward and downward movement exhibited by C. 
 roscoffensis by connecting it with tonic light effect. 
 
 In support of this it may be mentioned that 
 C. roscoffensis fails to exhibit its tidal rhythm except 
 when it is subjected to a fairly high light intensity 
 during its period of " upness." Thus, even in a room 
 at some little distance from the window, the movement 
 does not keep tidal time. 
 
 Again, other observations indicate that the spell 
 of illumination counts for something in determining 
 the precision of the movements. Thus, if three
 
 n] HABITS OF CONVOLUTA 67 
 
 batches of C. roscoffensis, collected directly after 
 the colonies emerge, are put in darkness for periods 
 of one, two, and three hours respectively, and are 
 then exposed to the light, that which had only one 
 hour's run in darkness descends first, and that which 
 had two hours' darkness descends next. 
 
 Taking the results of these experiments into con- 
 sideration and bearing in mind the condition of 
 lethargy which C. roscoifensis may manifest, in its 
 natural station, after long light-exposures, we are led 
 to frame some such hypothesis as the following, in 
 order to account for the periodic tidal movements 
 exhibited by this animal. 
 
 Phototropism and background reaction lead C. 
 roscoffensis to the most illuminated parts of that 
 region of the beach which provides it with a con- 
 tinuous, filmy stream of water. 
 
 Independently of its tropistic effect, light exerts a 
 tonic effect on the physiological state of the animals. 
 Under the combined influences of tropistic and tonic 
 light-stimuli, C. roscoffensis is held at attention 
 in the "up" position: in other words, whilst subject 
 to this constant rain of phototonic stimuli, it remains 
 negatively geotropic. True, if the sand be tapped, 
 the vibrations set up suffice to change the sign of the 
 animal's response to gravity and send it geotroping. 
 Nevertheless it is easy to show that the response of 
 C. roscoffensis to the vibration-stimulus is less marked 
 
 52
 
 
 68 PLANT-ANIMALS [OH. 
 
 at the beginning of an " up " phase than it is toward 
 the end of that phase. Thus, if specimens are col- 
 lected as soon as the tide is off the colonies and are 
 brought in a vessel into the laboratory, they swarm 
 up to the surface almost as soon as the vessel ceases 
 to be shaken, whereas animals collected after a long 
 light-exposure and placed in a similar position, may 
 remain down till the next tidal " up " phase is due. 
 
 Thus it is reasonable to conclude that, after some 
 five or six hours of light-stimulation, internal changes 
 are induced which act as stimuli and cause the animal 
 to change the sign of its response to gravity. It 
 becomes positively geotropic and descends beneath 
 the sand. In the darkness of the sand, recovery of the 
 original, normal state takes place gradually, and the 
 animals now respond to the stimulus of gravity by 
 a movement in the opposite sense. They ascend to 
 the surface. In its simplest form, the hypothesis 
 involves the assumption that prolonged light-exposure 
 and prolonged dark-exposure modify the tone or state 
 of nervous irritability of the animals, and that these 
 changed conditions manifest themselves by a changed 
 mode of response to gravitational stimulus. After 
 a prolonged light-exposure, the animals are positively 
 geotropic ; after a corresponding sojourn in the dark, 
 they become negatively geotropic. The reversal of 
 the direction of a tropistic movement is by no means 
 unusual among plants and animals. Thus, in order
 
 n] HABITS OF CONVOLUTA 69 
 
 to cause horizontally growing lateral roots to take up 
 vertical positions, it suffices merely to remove the 
 main root. As a result of the operation, the physio- 
 logical state of the whole* root-system is so changed 
 that members formerly transversely geotropic become 
 positively geotropic, and tertiary roots which previous 
 to the operation were ageotropic (non-geotropic) and 
 hence grew indifferently in any direction, become 
 transversely geotropic. 
 
 Similar changes in sign of tropistic response 
 may be induced by definite changes in the environ- 
 ment. For example, as Loeb has pointed out, fresh- 
 water Copepods, (small Crustacea) taken from the 
 same pond at the same time, may exhibit, some a 
 positive, some a negative, phototropic response and 
 others may be non-phototropic. If, however, a little 
 carbon-dioxide is added to the water they all become 
 positively phototropic. It is not improbable that this 
 uniform migration of the animals in the direction of 
 the light which follows on the addition of carbon- 
 dioxide is an example of response to associated 
 stimuli. Copepods feed no doubt on algse, which 
 can only live and grow in the light. In the course of 
 their nutrition, algso decompose carbon-dioxide and 
 liberate oxygen, so that the amount of carbon-dioxide 
 contained in the water in their immediate neigh- 
 bourhood is less than that contained in the darker 
 regions of the pond. Much carbon-dioxide will be
 
 70 PLANT-ANIMALS [CH. 
 
 correlated with limited food supply. Now it has 
 been shown definitely in the case of other animals, 
 e.g. the caterpillars of Porthesia, that they are only 
 positively phototropic so -long as they are not fed. 
 If this holds good for Copepods, their response to 
 increased carbon-dioxide becomes at once intelligible 
 on the mneme or associated stimulus hypothesis. 
 Thus hunger affects the tone or physiological state in 
 such a way that the Copepods respond to light by 
 directive movements whereby food supplies become 
 available. The movement brings the animals from a 
 part of the water which contains a maximal amount 
 of carbon-dioxide to a part where, thanks to the 
 presence and activity of the green algae, the food 
 sought by the Copepods the water is not fully 
 saturated with carbon-dioxide. When the animal 
 encounters carbon-dioxide conditions which are 
 normally associated with hunger conditions, it takes 
 the hint and photo tropes just as though it were 
 hungry. For a hungry man, a cook-shop window has 
 an irresistible attraction, whereas to the well-fed 
 person it may offer no seduction, or even be 
 repulsive : nevertheless, " si par impossible " the 
 odour which emanates from it is very agreeable, the 
 well-fed may deign to sniff. 
 
 What internal changes, chemical or other, resulting 
 from the prolonged light-exposure of C. roscoffensis 
 on the beach, give the signal for its dismissal from
 
 ii] HABITS OF CONVOLUTA 71 
 
 the attitude of attention which it takes up during 
 the " up " period we do not know. Nor may we say 
 with confidence that the explanation of the periodic 
 rhythm which we have offered is complete or final. 
 The subject deserves more detailed study than it has 
 yet received, both for its own sake and for the light 
 which it may throw on the origin of habit and, it 
 may be, also, of instinct.
 
 PART II 
 
 THE NATURE OF THE PLANT-ANIMALS
 
 CHAPTER TIT 
 
 THE GREEN CELLS OF CONVOLUTA ROSCOFFENSIS 
 AND THE PART THEY PLAY IN THE ECONOMY 
 OF THE PLANT-ANIMAL. 
 
 IT is not only on account of their behaviour, as 
 exhibited by the tropistic movements and periodic 
 phenomena which we have recorded, that the plant- 
 animals C. roscoffensis and C. paradoxa attract the 
 attention of the biologist. The most superficial 
 microscopic examination is sufficient to convince 
 him that their tissues are not like those of other 
 animals. The green cells of C. roscoflensis and 
 the yellow-brown cells of C. paradoxa arrest his 
 attention. In regular and close rows, just beneath 
 the surface of the body, lie the green cells of 
 C. roscoffensis, each so minute as to be invisible to 
 the unaided eye and yet so numerous as to be the 
 source of the dark, spinach-green colour of the 
 animals (Frontispiece and Fig. 14). Though less 
 numerous and less regularly arranged, the yellow- 
 brown cells which lie beneath the skin of C. paradoxa 
 are, like the green cells of the former species, striking
 
 76 
 
 PLANT-ANIMALS 
 
 [CH. 
 
 and puzzling objects (Frontispiece and Fig. 15). 
 Puzzling because, whilst they seem to be just as 
 much integral parts of the bodies of the animals as 
 any other tissue-elements, they have nevertheless 
 a foreign and plant-like appearance. So plant-like 
 
 Dtat* 
 
 Fig. 14. A young Convoluta roscoffensis. Magnified about 60 times. 
 GC= green cells. Diat. and R = remains of diatoms ingested 
 and digested. OT=otocyst. 
 
 indeed is the aspect of C. roscoffensis as seen under 
 the microscope, that a botanist might well be excused 
 for mistaking it lor a fragment of a leaf, endowed 
 with an uncanny kind of movement.
 
 in] GREEN CELLS OF CONTOLUTA 77 
 
 In yet another and no less remarkable way, 
 C. roscoffensis exhibits a plant-like character. The 
 bodies of normal, mature animals never contain the 
 slightest trace of food-substances. Though it is 
 kept for days in pure sea-water till any ordinary 
 marine animal would be ravenous, in point of 
 fact most marine animals are always ravenous an 
 adult C. roscoffensis makes no attempt to ingest any 
 
 YB. 
 
 Fig. 15. The superficial tissues of Convoluta paradoxa. Highly 
 magnified. Y.B.= yellow-brown cells. 
 
 food-substances which may be added to the water. 
 Though tempted with diatoms, green algsB, starch 
 grains, oil drops, milk, or lamp-black, it remains with 
 its capacious mouth so pursed up as to be invisible 
 and refuses to ingest any solid food whatever. Till 
 last year it seemed that there was no exception to 
 this fasting habit of adult C. roscoftensis ; but during
 
 78 PLANT- ANIMALS [CH. 
 
 observations on animals which had been kept for a 
 month in darkness in pure sea- water, certain individuals 
 were discovered which had so far condescended from 
 this ascetic mode of life as to have become cannibals. 
 Instead of being straight and slim, they carried a 
 large pouch-like distension about the middle of their 
 bodies. Microscopic examination showed that the 
 pouch was occupied by another adult Convoluta as 
 large as that which had engulfed it. Hence it follows 
 that normal adult C. roscoffensis in its natural state 
 does not refrain from food because it cannot swallow, 
 but because it does not want to eat. 
 
 Now green plants do not take up solid food : they 
 manufacture it. From inorganic substances, water 
 and carbon-dioxide, which are absorbed from without, 
 the green cells of plants manufacture sugar. This pro- 
 .cess, which is a preliminary to nutrition, is termed by 
 botanists, photosynthesis, since the energy required 
 for the manufacture of the carbohydrate (sugar) is 
 derived from the radiant energy of the sun. The 
 green pigment, chlorophyll, which is associated 
 in the green cells of the plant with specialised, 
 granular bodies called chloroplasts, absorbs light, 
 and in some way, as yet imperfectly understood, this 
 radiant energy is utilized by the protoplasm of the 
 chloroplasts in the manufacture of sugar. The plant 
 possesses also the power of synthesising yet more 
 complex substances. Beside carbohydrates such
 
 in] GREEN CELLS OF CONVOLUTA 79 
 
 as sugar, which consists of carbon, hydrogen and 
 oxygen, the plant prepares synthetically its own 
 nitrogenous food-substances, the proteins. Though 
 next to nothing is known of the details of protein- 
 synthesis as carried on by the plant, this much is 
 known, that the nitrogen contained in the proteins 
 is derived by the green plant from inorganic sources, 
 chiefly from nitrates which are absorbed in solution 
 from the soil or water in which the plant is growing. 
 Having thus manufactured its food-substances from 
 raw, inorganic materials, the plant is free to feed 
 upon them, that is, to use them either to build up 
 and repair its living substance (protoplasm) or to 
 convert them directly or indirectly into substances 
 (secretions) which enter into the composition of its 
 tissues. Thus, for example, from the photosynthesised 
 carbohydrate, are derived the cellulose substances 
 which form the enclosing shell or cell-wall within 
 which is contained each individual mass of protoplasm 
 which we call a cell or protoplast. But beside serving 
 such constructive purposes, much of the manufactured 
 food-substance, particularly the carbohydrate material, 
 is used for respiratory purposes, that is, for supplying 
 the energy wherewith the plant does the work of 
 living. By inducing compounds like sugar to unite 
 with oxygen, their decomposition and oxidation are 
 effected, with the result that energy is liberated and 
 simpler substances, e.g. carbon-dioxide and water, are
 
 80 PLANT-ANIMALS [OH. 
 
 produced. The liberated energy serves for the per- 
 formance of the work which the living plant must do, 
 and also, converted into heat, contributes to maintain 
 the temperature of the plant's tissues at a proper 
 level. The surplus of carbohydrate and of protein 
 not used for constructive or respiratory purposes the 
 plant puts by for future use. The starch, oil and 
 nitrogenous substances contained in seeds, tubers, 
 and other storage-organs of plants represent this 
 reserve food-material. 
 
 The power possessed by the green plant of manu- 
 facturing food-materials in excess of its immediate 
 needs is the lever which makes the whole world of 
 animal life to move. For the animal has no such 
 synthetic powers, and yet it requires the same food- 
 substances as the plant. Hence it is constrained to 
 take them from the plant. The aphorism "all flesh 
 is grass" is no mere figure of speech, but a terse 
 statement of truth. 
 
 Though the foregoing facts are, of course, the 
 commonplaces of plant-physiology, yet they require 
 mention here, for it follows from them that, if C. ros- 
 cofiensis does not take in solid food, it must either 
 absorb it in solution or manufacture food for itself. 
 Since the plant-animals not only live very well but also 
 increase and multiply in pure sea- water, and since pure 
 sea- water contains but the merest traces of any organic 
 substances which might serve them as food, we are
 
 in] GREEN CELLS OF CONVOLUTA 81 
 
 driven to accept the latter alternative, and to conceive 
 of C. roscoffensis as an animal which lives like a plant, 
 in other words, as a plant-animal. This conclusion 
 forces us to direct our attention to the plant-like 
 green cells which form such a prominent tissue in 
 the body of C. roscoffensis. 
 
 From the general considerations which we have 
 just advanced, it would appear to follow that the green 
 cells possess the power, common to those of green 
 plants, of manufacturing carbohydrate food-materials 
 from the simple, inorganic, soluble substances, water 
 and carbon-dioxide, and possibly also of manufacturing 
 complex nitrogen-containing food-substances, such as 
 proteins, from simpler bodies. If we succeed in 
 proving that the green cells of C. roscoffensis possess 
 these powers, other problems will present themselves. 
 Thus, we shall want to know, what are the green 
 cells? Is C. roscoffensis born with them or does it 
 acquire them ? If it acquires them, how do they get 
 into the body and what are they like before they 
 become constituents of the body of the animal ? 
 
 As a preliminary to the investigation of these and 
 other problems on the origin, significance and fate of 
 the green cells, we will turn back to consider further 
 the behaviour of C. roscoffensis and C. paradoxa with 
 respect to food. The various observers who have 
 occupied themselves with investigations into the 
 mode of life of C. roscoffensis have all reached the
 
 82 PLANT-ANIMALS [CH. 
 
 conclusion that this animal does not take up solid food. 
 A similar apparent total abstinence has been recorded 
 in cases of other animals which contain green or 
 yellow cells not unlike those which occur in our 
 plant-animals. Thus no food has been seen in the 
 bodies of certain adult Radiolaria, Ciliata, Hydro- 
 corallines and-Madreporaria, and in all these animals 
 from which remains of food are absent, coloured cells 
 are present. Hence the natural inference has been 
 drawn that such animals subsist on the food-materials 
 manufactured synthetically by their green or yellow 
 cells. 
 
 If, however, the evidence which we have now to 
 bring forward with respect to C. roscoffensis is 
 applicable to the other green- or yellow- celled 
 animals, then, though the conclusion may contain 
 a large measure of truth, the premise on which that 
 conclusion is based is erroneous. 
 
 When referring to the abstemious habit of 
 C. roscoifensis we were careful to state that it is the 
 mature animal which does not take up solid food. 
 As a matter of fact, from the time of hatching to 
 the period of maturity, C. roscoffensis feeds, and 
 feeds voraciously. Indeed, its catholicity of taste 
 is remarkable. Diatoms, unicellular algae, spores of 
 various kinds and, in the absence of more nutritious 
 substances, grains of sand are swallowed with avidity 
 (cf. Fig. 14). Arrived at maturity, it ceases to ingest
 
 in] GREEN CELLS OF CONVOLUTA 83 
 
 solid food-substances. As old age comes on, it begins 
 to feed upon its green cells. Groups of such cells in 
 all stages of digestion and varying in colour from 
 yellowish-green to brown may be seen lying in large 
 vacuoles in the central digestive tissue of the bodies 
 of old specimens of C. roscoffensis. Thus, though, 
 as we shall see presently, the green cells of C. ros- 
 coifensis play an all-important part in the economy 
 of that organism, they are not the sole purveyors of 
 nourishment to it. Throughout a considerable part 
 of its life, C. roscoffensis is able to help itself to the 
 solid food supplied by the micro-flora and fauna of 
 its environment. 
 
 Unlike C. roscoffensis, its ally, C. paradoxa, knows 
 no abstemious fits. Throughout its life it is a 
 glutton. A glance at the larval animal (Fig. 16) 
 gives the impression of a marine museum, so accom- 
 modating is the body of C. paradoxa. There, may 
 be seen the remains of several scores of diatoms of all 
 shapes and sizes. When examined immediately after 
 capture, a young or old C. paradoxa may be found to 
 contain, not only diatoms, but two or three Copepods, 
 each half as large as the animal itself, and, if it be late 
 in the summer, rows of tetraspores of red algse show 
 through the transparent body like so many cardinal 
 buttons. In C. paradoxa therefore, as in C. roscoffen- 
 sis, though the coloured, plant-like cells may well 
 play an important and even indispensable part in the 
 
 62
 
 PLANT-ANIMALS 
 
 [CH. 
 
 life of the animal, they are not called upon to cater 
 for all the food that it requires. 
 
 As a first step toward the investigation of their 
 origin and rdle, we will make a microscopic examina- 
 tion of the coloured cells of our plant-animals. 
 
 -YB 
 
 Fig. 16. A larval Convoluta paradoxa magnified 60 times showing 
 cilia and bristle-like projections from skin. YB = the only yellow- 
 brown cell contained in the body. M= mouth and gullet. 
 OC = eye spots. OT = otocyst. 
 
 The tissue of the body is crowded with large numbers of 
 diatoms which have been ingested. 
 
 When a living C. roscoffensis is examined under 
 the low power of the microscope, its green cells are 
 seen to be, some spherical, some pear-shaped and
 
 in] GREEN CELLS OF CONVOLUTA 85 
 
 some of irregular form. As the animal moves along, 
 its muscles contract and the shapes of the green cells 
 change somewhat (cf. Fig. 14). Seen under a higher 
 power, the green cells present the appearances indi- 
 cated in Fig. 17. Each cell or protoplast is made 
 up of a large green, and a small colourless part. 
 The former consists of the chloroplast, the latter of 
 
 F.G. 
 
 Fig. 17. Cross-section of the superficial tissues of Convoluta roscoffen- 
 sis. Highly magnified. G.C. = green cells in rows. N= nucleus 
 of green cell. Pyr=pyrenoid. Mes.C. = nucleus of attendant 
 cell. F.G. = fat granules. Oil = cilia at the surface of the animal. 
 Ep epidermis. 
 
 colourless protoplasm. Embedded in the mass of 
 colourless protoplasm, but not visible without special 
 methods of preparation, is a denser, oval body, the 
 nucleus which is an integral part of plant and animal 
 cells. Lying in the chloroplast is a dense body sur- 
 rounded, halo-like, by a clearer margin. This body, 
 which is called a pyrenoid, consists of proteins and is
 
 86 PLANT-ANIMALS [CH. 
 
 characteristic of the cells of many of the lower algae 
 (Fig. 21, p. 123). 
 
 If a green cell is treated with a solution of iodine, 
 the nucleus and the pyrenoid are stained brown, and 
 round the latter a thin, blue, granular layer may be 
 distinguished. This layer is known as the starch 
 sheath. As the action of the iodine continues, 
 minute, lens-shaped starch grains distinguished by 
 their blue colour may be seen lying in the chloro- 
 plast. Unlike algal cells in general, the green cells 
 in the body of C. roscoffensis have no cellulose wall, 
 but are bounded each by an elastic layer of protoplasm. 
 
 The yellow-brown cells of C. paradoxa are built 
 on somewhat different lines. Each cell contains a 
 number of irregularly oval, or polygonal, yellow- 
 brown, discoidal chloroplasts which occupy about 
 half of the cell (Fig. 18). The other half of the yellow- 
 brown cell consists of clear, transparent, vacuolated 
 protoplasm. By suitable treatment, involving the 
 dissolution of the pigment, a nucleus may be made 
 out, slung in the centre of the cell by threads of 
 protoplasm which stretch from the periphery. When 
 the cells are treated with alcohol, the yellow-brown 
 pigment is dissolved away and green chlorophyll, 
 previously screened by the yellow-brown pigment, is 
 seen in the chloroplasts. The reaction is useful in 
 that it enables us to distinguish the chloroplasts 
 of the yellow-brown cells from the orange-coloured
 
 in] GREEN CELLS OF CONVOLUTA 87 
 
 glands which occur in the surface-tissues of the body 
 of the animal. 
 
 The green pigment of C. roscoffensis is chlorophyll, 
 identical in its spectroscopic properties with that 
 contained in the green tissues of plants. Moreover, 
 
 I. n. 
 
 Fig. 18. Yellow-brown cells of Convoluta paradoxa, highly magnified. 
 I. As seen in an animal some hours after capture. II. As seen 
 immediately after capture. The spherical masses lying in the 
 cells and also outside them represent the fat-globules referred to 
 on pp. 89 and 91. 
 
 The shaded oval bodies at the periphery of the cells represent 
 chloroplasts. 
 
 as Geddes (1879) has demonstrated, the green cells 
 of C. roscoffensis are capable of photosynthesis. When 
 the animals are exposed to light, they decompose 
 carbon-dioxide, give off oxygen, and manufacture 
 carbohydrates, the excess of which is stored in the 
 chloroplast in the form of starch. 
 
 That the starch which occurs in the green cells
 
 88 PLANT-ANIMALS [CH. 
 
 of C. roscoffensis owes its origin to photosynthesis, we 
 demonstrate by the method which is used for a similar 
 purpose in the case of plants. The living animals 
 are kept in darkness and examined daily for starch. 
 After a time about seven or eight days in young 
 C. roscoffensis, about fourteen days in older animals 
 when, as indicated by the samples tested, starch has 
 disappeared having been converted into sugar and 
 used as food or in respiration the animals are 
 brought into the light and tested at intervals for 
 starch. As is the case with green plants treated 
 similarly, photosynthesis is resumed as soon as light 
 falls on the green cells, and within less than ten 
 minutes starch, which represents the reserve form of 
 the photosynthesised carbohydrate, makes its appear- 
 ance in the green cells. Moreover, the light which 
 is most efficient for photosynthesis in plants, that of 
 the red end of the spectrum, is also most efficient for 
 photosynthesis in the green cells of the plant-animal. 
 
 It is not so easy to obtain rigid proof that the 
 yellow-brown cells of C. paradoxa are capable of 
 photosynthesis. Nevertheless, the indirect evidence 
 supports strongly the view that they do actually 
 function in this manner. 
 
 In the first place, like similarly coloured algae, 
 which are known to manufacture their food photo- 
 synthetically, they possess a screening pigment and 
 also chlorophyll.
 
 in] GREEN CELLS OF CONVOLUTA 89 
 
 In the second place, when examined immediately 
 after capture, the transparent reticulum of the cells 
 (Fig. 18, n) is found to contain colourless, refractive 
 globules or droplets, which, when treated with suit- 
 able reagents (osmic acid, etc.), may be recognised to 
 consist of, or at all events to contain, fat. Now, it 
 is well known that certain plants, some algse among 
 others, store their reserve, photosynthesised carbon- 
 compounds, not as starch, but as oil. That these 
 globules are of the nature of reserve substances 
 derived from the products of photosynthetic activity 
 is rendered probable by the following facts. First, 
 when a catch of animals is divided into two lots, and 
 one is kept in darkness and the other in the light, the 
 reserve fat-globules disappear more quickly from the 
 yellow-brown cells of the dark-kept animals than 
 from those of the animals kept in the light. Second, 
 if two similar batches of animals are kept in darkness, 
 one in pure (filtered) sea- water, the other in sea-water 
 containing sea-weed from the C. paradoxa zone, fat 
 disappears from both, but more quickly from the 
 yellow-brown cells of the starved animals. If the 
 fat were derived from the food (sea-weed with its 
 micro-flora and fauna) there would seem to be no 
 reason why it should disappear at all from the yellow- 
 brown cells of the fed animals. On the other hand, 
 assuming that the fat-globules serve as food-material, 
 not only for the yellow-brown cells but also for those
 
 90 PLANT-ANIMALS [CH. 
 
 of the animals, we should expect that the latter, when 
 deprived of other supplies, would make larger de- 
 mands on the reserve fat of the yellow-brown cells 
 than when the animals had access to other food 
 supplies. Third, if C. paradoxa are kept in filtered 
 sea-water, and hence deprived of all food except that 
 which it can obtain from the yellow-brown cells, 
 then, so long as they are exposed to the light, the 
 yellow-brown cells continue to contain fat-globules. 
 Since animals deprived of food get just as hungry 
 in the light as in darkness, it would appear to follow 
 that the reason why the fat does not disappear from 
 the yellow-brown cells of the light-kept animals is 
 that, as fast as it is removed to serve for the nutrition 
 of the animal, it is reformed by the yellow-brown 
 cells. It is therefore to be concluded that the fat- 
 globules are reserve products of the photosynthetic 
 activity of the yellow-brown cells. 
 
 Thus we reach a definite stage in the course 
 of our enquiry into the significance of the green 
 and yellow-brown cells of C. roscoffensis and C. 
 paradoxa. These cells are capable, in the same way 
 as the chlorophyll-containing cells of plants, of manu- 
 facturing organic, carbon-containing substances from 
 inorganic materials and of storing the surplus in the 
 form of starch or fat. 
 
 Our next step must be to determine whether the 
 products of the photosynthetic activity of the coloured
 
 in] GREEN CELLS OF CONVOLUTA 91 
 
 cells are available for the nutrition of the tissues of 
 the animals which contain them. 
 
 The evidence which suffices to demonstrate that 
 the coloured cells do actually make contributions to 
 the nutrition of the animals is not far to seek. If 
 C. paradoxa is examined microscopically immediately 
 after capture, it is seen that the tissues of animals, 
 whose yellow-brown cells are rich in droplets of 
 reserve fat contain also large numbers of globules 
 of a similar nature (Fig. 18). Moreover, the appear- 
 ance of the fat-globules contained in the yellow- 
 brown cells suggests most forcibly that the fat lying 
 in the tissues of the animal owes its origin to the 
 secretion of fat by the yellow-brown cells. The 
 appearance of the yellow-brown cells recalls, in this 
 respect, that of cells of a mammary gland in its active 
 stage. Just as the fat contained in the milk which 
 is secreted by the cells of a mammary gland is 
 liberated in droplets by the rupture of the clear 
 vacuolated parts of the secreting cells, so droplets 
 may be seen in course of extrusion from the yellow- 
 brown cells into the tissues of the animals (Fig. 18). 
 The large, clear, anterior end of the yellow-brown 
 cell only to be seen in fresh-caught animals which 
 have been exposed in their natural habitat to a fairly 
 high light intensity contains often one, large fat- 
 globule. In some yellow-brown cells, one or more 
 droplets lie in the deeper part of the clear anterior
 
 92 PLANT-ANIMALS [CH. 
 
 end, whilst, in others, a single, large drop lies close 
 against the anterior margin of the cell, separated 
 from the tissues of the animal only by the thinnest 
 of membranes. Finally, other large globules may 
 be seen lying just outside the colourless borders of 
 yellow-brown cells and presenting the appearance 
 of having been extruded from them. We conclude 
 that the fat-globules, formed in the yellow-brown 
 cells of C. paradoxa, pass by a process of secretion 
 from these cells to those of the animal and serve the 
 animal for nutritive purposes. 
 
 It is very probable that a similar secretion occurs 
 in C. roscoffensis. For, in the first place, starch, 
 which, as we have learned, appears in the green 
 cells as the result of photosynthesis, does not occur 
 in the other tissues of C. roscoffensis. In the second 
 place, this animal does not possess the power of 
 digesting starch. When supplied with starch grains, 
 it ingests them readily, transfers them to the 
 vacuoles which lie in its digestive tract, but is 
 unable to dissolve them. They remain for a time 
 in the vacuoles, and are then discharged by a 
 temporary rupture of the surface of the body. 
 In the third place, in carefully prepared and 
 stained sections through the body of C. roscofl'ensis, 
 there may be seen rows of fatty granules passing 
 from the green cells to the neighbouring animal 
 cells (Fig. 17, F.G.). Nor does a conversion of
 
 in] GREEN CELLS OF CONVOLUTA 93 
 
 starch into fat present any difficulty to vegetable 
 cells. For example, in many trees, the tissues of the 
 trunks contain, in autumn, large stores of starch ; as 
 winter advances the starch is replaced by oil or fat, 
 and, again, when spring arrives, the oil gives place 
 once more to starch. It is of course open to us to 
 suppose that, just as the sugar formed photosyn- 
 thetically by the green cells of a leaf is translocated 
 as fast as may be through the tissues of the leaf-stalk 
 and stem to meet the demands of the colourless cells 
 of the plant which depend for their food supplies on 
 the activity of the green cells, and just as the starch 
 which appears in the green cells of the leaf represents 
 only the surplus which is stored temporarily in a 
 convenient form to be changed to sugar and dis- 
 tributed later on ; so, in C. roscoffensis, the photo- 
 synthesised sugar streams away as such to the 
 colourless cells of the animal, only the surplus 
 being stored as starch. 
 
 Whether it travels as sugar or, as the former 
 observations seem to indicate, as fat, there is no 
 doubt that the organic, carbon-containing food- 
 material, produced photosynthetically by the green 
 cells of C. roscoflensis, serves for the nutrition of the 
 animal's tissues. 
 
 Indeed, as we show presently, unless the green 
 cells are present in the body of the animal, and 
 unless they increase and multiply therein, the animal 
 does not grow at all.
 
 94 PLANT-ANIMALS [CH. 
 
 The phase in the relation between coloured, 
 chlorophyll-containing cells and animal tissues which 
 we have just described, presents the closest parallel 
 with the relation which obtains between the green 
 and non-green cells of any chlorophyllous plant. 
 In both plant and plant-animals, the chlorophyll- 
 containing cells manufacture carbohydrates in excess 
 of their own requirements, and, in both, the excess is 
 translocated to the colourless tissues and used by 
 them as food-material. 
 
 But, in certain circumstances, C. paradoxa and, 
 to a somewhat less degree, C. roscoffensis may exploit 
 their coloured cells in a more summary manner. 
 
 Thus, when animals are kept in darkness in sea- 
 water filtered through a Pasteur-Chamberland filter 
 they become reduced greatly in size. The reduction 
 in size is, as we know, greater, and takes place more 
 rapidly, in dark-kept than in light-kept animals. 
 In one experiment in which the animals were 
 measured, those which had been kept in darkness 
 were, on the average, two and a half times as small 
 as those which had been kept in the light; the 
 average superficial dimensions of the dark-kept 
 C. paradoxa being '08 square inch and those of 
 the light-kept animals '2 square inch. 
 
 The powers of resistance to starvation of both 
 C. roscoffensis and C. paradoxa are extraordinary. 
 Thus, it is possible to maintain C. paradoxa alive for
 
 in] GREEN CELLS OF CONVOLUTA 95 
 
 upwards of a month in filtered water ; that is, under 
 conditions, in which it is deprived of all external 
 supplies of food. When subjected for long periods 
 to these conditions, the animals become reduced in 
 size and as is the case to a yet more marked degree 
 with those kept in darkness also show an extra- 
 ordinary reduction both in the number and magnitude 
 of their yellow-brown cells. 
 
 In prolonged darkness, the yellow-brown cells, once 
 the reserves of food-material have been extracted 
 from them to meet the needs of the animal, are 
 digested wholesale by C. paradoxa. If the water in 
 which it is contained be altogether devoid of food 
 supplies, the attack by the animal on its coloured 
 cells occurs all the sooner. Even in the light, if 
 external food supplies are withheld from C. para- 
 doxa, a time comes when, although the yellow-brown 
 cells are supplying the animal with photosynthesised 
 food-materials as fast as they can under the difficult 
 circumstances, it turns upon them ; killing and di- 
 gesting the goose which laid its golden eggs. 
 
 Microscopic examination of animals kept in pro- 
 longed darkness supplies evidence that the degenera- 
 tion of the yellow-brown cells is not a mere decay 
 within the body, but is the result of a true process 
 of digestion exerted on them by the animal. The 
 first sign of digestive action is a reduction in size 
 of the yellow-brown cells. They assume a more
 
 96 PLANT-ANIMALS [OR 
 
 spherical shape and their chloroplasts become smaller 
 and rounder. Each reduced algal cell is now seen 
 to lie in a distinct, digestive vacuole containing a 
 pink fluid. Next, the pigment of the chloroplasts is 
 dissolved and, diffusing out of the cell, may impart a 
 brown colour to the vacuolar fluid. At this stage, 
 the chloroplasts are greenish ; later, they become 
 colourless. Finally, heaps of few or many, colourless, 
 curiously persistent granules are all that remain of 
 the algal cells. 
 
 It is interesting to observe, in this connection, that 
 if animals are brought, after a prolonged sojourn in 
 darkness, into the light and supplied with fresh 
 sea-water, yellow-brown cells make their appearance 
 again in their bodies. As they grow and increase 
 in .numbers, the animals also begin again to grow. 
 
 So also in the case of C. roscoffensis, if the green 
 cells fail to make their appearance in the body, the 
 animals remain of microscopic size. If, on the other 
 hand, the green cells appear, increase and multiply 
 to form the characteristic green tissue, the animals 
 begin to grow rapidly. 
 
 Thus in various ways it has been demonstrated 
 that C. roscoffensis and C. paradoxa depend for 
 their food on their coloured cells. Without them, 
 they fail to grow. When, by exposure to dark- 
 ness, the coloured cells are put out of photosyn- 
 thetic action, the animals become reduced in size,
 
 in] GREEN CELLS OF CONVOLUTA 97 
 
 and, after giving their coloured cells a respite 
 of some weeks, they turn on these algal cells and 
 digest them. In C. paradoxa, this raiding of the 
 coloured cells occurs only under special, artificial 
 conditions ; as, for instance, during prolonged dark- 
 ness. But, in the case of C. roscoffensis, it is a regular 
 procedure with animals which have reached a certain 
 age. Nor is the reason for this difference of be- 
 haviour between the two plant-animals far to seek. 
 Whereas C. paradoxa retains its habit of ingesting 
 solid food and looks to its yellow-brown cells for sup- 
 plementary supplies only, C. roscoffensis, at a certain 
 stage, shuts its mouth and cultivates its garden of 
 green cells. Now, inasmuch as hunger cell-hunger 
 may be due to one or more of many different 
 lacks, lack of carbohydrate, lack of nitrogenous food- 
 substances, or of mineral compounds, it is bound to 
 happen sooner or later that the animal part of C. 
 roscoffensis, in its phase of total abstinence from 
 food, will feel the pinch of one kind of hunger or 
 another. Goaded by this all-powerful stimulus it 
 turns upon its green cells, and, biting the hand that 
 fed it, seeks, by devouring them, to satisfy its cravings 
 for some special food-substances. 
 
 To the question what particular kind of hunger 
 is it that drives the animal to devour its plant-like 
 cells, we shall address ourselves, after we have in- 
 vestigated, in the next chapter, the origin of these 
 
 K. 7
 
 98 PLANT-ANIMALS [OH. 
 
 cells. For the present, we content ourselves with 
 summing up what we have learned of the relations 
 between the animals and their plant-like cells. 
 
 The coloured cells manufacture photosynthetically 
 food-materials, storing the surplus as starch or fat. 
 The animal receives from the coloured cells supplies 
 of food-material. So plentiful are these supplies in 
 C. roscoffensis that the animal comes, in course of 
 time, to rely altogether upon them for its nutrition. 
 Ceasing to take up food, it grows, bears eggs, and 
 produces young at the expense of the materials 
 supplied by its green cells. This life of curious 
 asceticism leads, however, to trouble. Though the 
 green cells continue to supply organic carbon com- 
 pounds, something or other is lacking from the 
 prepared food which the animal thus receives. To 
 make up for this lack, it digests in detail its green 
 cells, coming often in old age to present a strange 
 appearance head-end green, tail-end white. Hav- 
 ing exhausted its stores of green cells, without 
 apparently satisfying all its needs, it pines away 
 and dies. 
 
 In its earlier youth, C. roscoffensis feeds, after the 
 manner of animals in general, on other plants or 
 animals. This is the first phase. In the course thereof, 
 green cells appear in the body, increase, multiply, 
 photosynthesise and distribute food materials to the 
 animal's tissues. For a while, C. roscofiensis receives
 
 in] GREEN CELLS OF CONVOLUTA 99 
 
 food from two sources from ingested plants and 
 animals and from its green cells. 
 
 This second phase is succeeded by a third, in 
 which C. roscoffensis, having ceased to ingest solid 
 food, is nourished, in the same manner as the colour- 
 less non-chlorophyllous tissues of a green plant are 
 nourished, by the products of the photosynthetic 
 activity of its green cells. 
 
 Last stage of all which ends this strange eventful 
 history: the animal digests its green cells, and, 
 having done so, dies. 
 
 In the first phase, the mode of nutrition is 
 animal- wise : in the second, part animal-, part plant- 
 wise : in the third, altogether plant- wise or holo- 
 phytic : and in the fourth, autotrophic, that is by 
 living on itself. 
 
 Convoluta paradoxa is like unto C. roscofiensis, 
 except that its experiments in nutrition stop, under 
 normal circumstances, at phase two. Under artificial 
 conditions, however, it behaves like its ally, lives for 
 a while like a plant at the expense of the products 
 of photosynthesis of its yellow-brown cells, and, 
 finally, driven to digest these cells, prolongs its life 
 autotrophically. 
 
 72
 
 CHAPTER IV 
 
 THE ORIGIN AND NATURE OF THE GREEN CELLS 
 OF CONVOLUTA ROSCOFFENSIS. 
 
 GREEN, yellow or brown cells, resembling in a 
 general way those contained in the bodies of C. ros- 
 coffensis and C. paradoxa, are found in many different 
 kinds of animals belonging to the lower groups of the 
 animal kingdom. 
 
 Such cells are known to exist in representatives 
 of every division of the free-living Protozoa the 
 lowest group of animals. They occur in certain 
 sponges, in many sea-anemones and in various 
 species of coral-forming animals. In higher groups, 
 they are rare though they are known to occur in 
 isolated cases, for example, in Zoobothrium, a 
 member of the Polyzoa, in Elysia (a Mollusc), and 
 in Echinocardium (an Echinoderm). 
 
 The best known example of an animal containing 
 green alga-like cells is the common, freshwater hydra, 
 Hydra viridis. 
 
 In certain of the animals which are characterised 
 by the possession of coloured cells, these peculiar
 
 CH. rv] GREEN CELLS OF CONVOLUTA 101 
 
 elements are invariably present. In other animals, 
 the coloured cells may occur in some individuals, 
 but not in others. The former, general association 
 we may call obligate, and the latter, occasional 
 association, facultative. 
 
 Hydra viridis, Convoluta roscoffensis and C. 
 paradoxa are examples of organisms in which the 
 association is obligate. 
 
 Facultative association may take one of two 
 forms. Either some specimens living in a given 
 region may possess coloured cells, whilst other speci- 
 mens of the same region lack them, or a given 
 species may consist, in one part of its range, of 
 individuals all of which contain coloured cells, and, 
 in another part of its range, of individuals none of 
 which possess them. For example, Noctiluca is colour- 
 less in the North Atlantic, but green in the Indian 
 Ocean. British Alcyonium have no chlorophyll- 
 containing cells, whereas the nearly allied Alcyonium 
 ceylonicum possesses them. It seems probable and 
 this is a point of which we shall make use presently 
 that association between animal and plant-like cells 
 is commoner in the warmer than in the colder seas. 
 
 The problem of the origin and nature of the green, 
 yellow and brown cells which occur in animals has 
 engaged, from time to time, the attention of zoologists. 
 Long ago the name Zoochlorella was given to the green 
 cell and Zooxanthella to the brown or yellow-brown
 
 102 PLANT-ANIMALS [OH. 
 
 cell. Since, however, these names are applied, re- 
 spectively, to any green and any brown plant-like 
 cell which occur in any animal their value is but 
 limited. 
 
 That Zoochlorellse and Zooxanthellse are plant- 
 like cells is undisputed. They contain chlorophyll, 
 decompose carbon-dioxide with evolution of oxygen, 
 may, in the case of Zoochlorellae, contain starch : a 
 substance for the manufacture of which plants and 
 not animals possess the secret. Further, in some 
 cases, at all events, the coloured cells possess a wall 
 of cellulose, another substance the formation of 
 which is confined exclusively or almost exclusively 
 to members of the vegetable kingdom. 
 
 Beside one or more chloroplasts, a nucleus and 
 a pyrenoid, the coloured cells have been shown in 
 some cases to contain a small, bright red body known 
 as an eye-spot (Fig. 21, p. 123). In free-living, uni- 
 cellular algse, the eye-spot serves the purpose of light- 
 perception and thus is part of the nervous machinery 
 for the performance of photo tropic movements. Hence 
 its occurrence in green cells imprisoned in the bodies 
 of animals may be regarded as a strong indication 
 that the green cell which possesses it had once a 
 free-living existence. 
 
 Nevertheless, though such facts as these lend 
 powerful support to the hypothesis that Zoochlorellse 
 and Zooxanthellse are algal cells which have aban-
 
 iv] GREEN CELLS OF CONVOLUTA 103 
 
 doned their free and independent modes of life and 
 Lave taken up their abodes in the tissues of animals, 
 yet they do not constitute a final proof of the truth 
 of this hypothesis. Indeed, the problems presented 
 by the chlorophyllous cells of animals are too 
 numerous and important to be dismissed by means 
 of a loosely-drawn inference of this sort. To the 
 possession of chlorophyll the plant owes its powers 
 of photosynthetic manufacture ; and to the absence of 
 this pigment from the cells of animals is due the 
 dependence of the animal world on the world of 
 plants for food supplies. Yet, low down in the 
 animal kingdom, organisms exist which, though un- 
 doubtedly possessed of distinct animal characteristics, 
 contain chlorophyll and use it for the manufacture 
 of carbohydrate food. Thus, species of Eugleua 
 (e.g. E. viridis), which stand near the parting of the 
 ways which lead, the one to the animal kingdom, 
 the other to the vegetable kingdom, contain chloro- 
 phyll and use it for photosynthetic purposes. Now 
 Euglena viridis is undoubtedly an animal. The single 
 cell or protoplast of which it consists is provided 
 with a gullet, into which solid particles may pass 
 and thus be ingested by the animal. The membrane 
 which encloses the organism is not composed of 
 cellulose the cell-wall substance of typically vege- 
 table organisms ; and in yet other ways Euglena 
 gives evidence of its " animal " nature.
 
 104 PLANt-ANIMALS [CH. 
 
 Although zoologists and botanists are agreed that 
 the genus Euglena belongs to the animal kingdom, 
 yet it possesses the power of constructing a green 
 pigment chlorophyll which is identical in physical 
 properties with that which occurs in the chloroplasts 
 of plants. Here there is no question, apparently, of 
 any swallowing by Euglena of plant cells. The 
 animal cell makes the pigment in the same way as 
 a plant cell makes it, and, having made it, uses it 
 for photosynthetic purposes. 
 
 In certain circumstances, chlorophyll disappears 
 from the body and Euglena viridis passes into a colour- 
 less phase. When in this state the animal, if it is to 
 feed at all, must do so by ingesting ready-made food. 
 That is, from being a holophytic organism one with 
 a typically plant-like mode of nutrition it becomes 
 heterotrophic, that is, it feeds on ready-made, organic 
 materials, obtained from its environment. After a 
 time, it may reconstruct its chlorophyll and become 
 free once more to manufacture by photosynthesis its 
 organic food-substances from the raw, inorganic 
 materials of its environment. 
 
 If one species of animal can do this, why should 
 not other, even more highly developed species, 
 possess like powers ? Why should there not appear, 
 here and there, animals which resume the habit 
 possessed by their ancestors, construct chlorophyll and 
 become independent, photosynthesising organisms ?
 
 iv] GREEN CELLS OF CONVOLUTA 105 
 
 Or, to pursue another line of argument. The 
 Zoochlorellee of some animals are typically plant-like 
 cells. They possess a chloroplast, a nucleus and a 
 cellulose cell-wall. But in other animals, in C. ros- 
 coffensis, for example, the green bodies are of simpler 
 build. Each consists of a naked protoplast which is 
 made up of a green chloroplast and a colourless mass 
 of eccentrically lying protoplasm in which a nucleus 
 may be included (Fig. 17, p. 85). The green tissue, 
 composed of vast numbers of these elements, appears 
 to be as much a part of the animal as any other of its 
 tissues. So much is this the case that aU attempts to 
 cultivate the green cells of C. roscoffeusis outside the 
 body end in failure. They are no more capable of inde- 
 pendent existence than are the chloroplasts of the 
 chlorophyllous tissues of a green plant. 
 
 What is there to prevent us from assuming, as 
 Haberlandt has assumed, that the green cells of C. 
 roscoffensis are not complete cells but merely chloro- 
 plasts, and that, like the chloroplasts of the green plant, 
 they are transmitted as colourless particles (leuco- 
 plasts) from the organism to its eggs, and, multiplying 
 as the egg divides to form the embryo, reappear as 
 green chloroplasts in the tissues of the new genera- 
 tion ? On this hypothesis the colourless part of the 
 green cell of C. roscoflensis (Fig. 17) is an animal cell 
 which attends upon the green chloroplast. In other 
 words, just as a green cell of a flowering plant
 
 106 PLANT-ANIMALS [OH. 
 
 consists of colourless, nucleated protoplasm contain- 
 ing chloroplasts, so, on Haberlandt's hypothesis, the 
 green cell of C. roscoffensis consists of a colourless, 
 animal part containing a green chloroplast. 
 
 Pursuing this hypothesis to its natural conclusion, 
 it is easy to imagine, with Haberlandt, that, in some 
 remote past, algal cells came to exist in symbiosis 
 with colourless C. roscoffensis ; that the animal offered 
 such a congenial lodging as to induce the algse to give 
 up going out altogether. They abandoned their cell- 
 wall as an enclosing apparatus no longer of service 
 to them. In return for security and all the comforts 
 of a home the green cells prepared the food both for 
 themselves and for their host. Submitting itself to 
 the guidance of the animal, the green cell aban- 
 doned its nucleus and became reduced to a naked 
 chloroplast. 
 
 So it might have come about that the only powers 
 retained by this relic of a once complete and free 
 algal cell are those possessed by the chloroplast of 
 a green plant, namely, the powers of photosynthesis 
 and of division to form new chloroplasts. Moreover, 
 just as the chloroplasts contained in the egg-cells of 
 plants lose their green pigment, and become colourless 
 leucoplasts, which, dividing as the cells of the plant- 
 embryo divide, give rise to the chloroplasts of the 
 next generation, so, on this hypothesis, it would follow 
 that the green chloroplasts of C. roscoffensis might
 
 iv] GREEN CELLS OF CONVOLUTA 107 
 
 give rise to colourless leucoplasts which pass into 
 the egg and provide the rudiments from which the 
 chloroplasts of the larval animal are developed. 
 
 Yet again, if this were indeed the course of events 
 in C. roscoffensis, if, from their free, complete con- 
 dition, the original green cells which gained access 
 to the body of our plant-animal have become reduced 
 to mere chloroplasts, might not this animal provide an 
 illustration of the mode of origin of the higher green 
 plants themselves? In a remote past, a symbiosis 
 was struck up between a colourless organism and 
 a green alga such a communal mode of life, for 
 example, as that presented by lichens at the present 
 day. Convenient models these to show us the 
 relation between colourless organism and undoubted 
 algal cells. So happy is the hypothetical partnership 
 between alga and colourless organism that fresh 
 developments ensue. A new and composite form of 
 life comes into existence. The colourless tissues 
 burrow in the earth and supply, along well-defined 
 conduits, the water and minerals required by the 
 green cells. They form tall trunks and spreading 
 branches to lift the chloroplasts the representatives 
 of the algal cells nearer to the sun. The green plant 
 is in being. 
 
 Of this alluring picture, evoked by syren-voiced 
 hypothesis, we are bound to ask the simple, sober 
 question, is it true? To this question we can give
 
 108 PLANT-ANIMALS [CH. 
 
 no answer until we have discovered experimentally 
 the origin of the plant-like cells occurring in each 
 species of the many animals which possess them. 
 
 This we proceed to do in the case of C. roscoffensis. 
 Two methods are open to us for the purpose. We must 
 either trace back the green cells to the earliest stage 
 at which they make their appearance in the animal 
 and ascertain whether they may be then identified 
 with any known, free-living alga. If we succeed in 
 this, we shall have obtained, not absolute proof, but 
 strong ground for believing that the green cells are 
 of intrusive origin. Or and this is the only certain 
 way we must cultivate the alga, and having ob- 
 tained animals which are free from it, and having 
 demonstrated that such animals remain indefinitely 
 colourless, we must infect the animal with the algae 
 of our pure algal-culture and synthesise the green 
 plant-animal. 
 
 As we have indicated already, all attempts to 
 isolate living green cells from the body of C. ros- 
 cofiensis have failed ; and so it would seem that the 
 former, less satisfactory method alone remains. The 
 application of the method is simple enough. It 
 consists in the microscopic examination of larval 
 C. roscoffensis in all stages, from the time of hatch- 
 ing up to the time when green cells, resembling those 
 of the adult, may be recognised within the body.
 
 iv] GREEN CELLS OF CONVOLUTA 109 
 
 When just-hatched C. roscoffensis are examined with 
 the high power of the microscope, no green cells are 
 to be seen in their bodies, nor are there present any 
 colourless cells resembling in shape or structure the 
 green cells, nor do either eggs or larvae appear to 
 contain leucoplasts. 
 
 If just-hatched animals are transferred to sea- 
 water filtered by means of a Pasteur-Chamberland 
 filter, though, in the course of time, some may become 
 green, many remain colourless. Therefore it is highly 
 probable that the green cells do not owe their origin 
 to colourless antecedents (leucoplasts) present in the 
 eggs. For, were such forerunners of the green chloro- 
 plasts present, they would develop into chloroplasts 
 in all, or at all events in the great majority, of the 
 larvae. On the other hand, if young animals are 
 hatched and allowed to remain in ordinary unfiltered 
 sea-water, green cells make their appearance with 
 certainty in the animals in the course of one or two 
 days. In the youngest larvae, there are to be seen 
 no more than two or four green cells, each of them 
 lying in a clear vacuole and occupying a fairly definite 
 situation in the body. Two such cells lie right and 
 left, a little behind the otocyst and two right and 
 left about the middle of the body. By their repeated 
 division is produced ultimately the whole contingent 
 of green cells of the adult body. 
 
 At stages earlier than this, no green cells are to
 
 110 PLANT-ANIMALS [CH. 
 
 be found, but a larger or smaller, colourless body may 
 be seen lying in a central vacuole in such a situation 
 as to suggest that it has been taken up through the 
 mouth. The larger body consists of two closely 
 apposed cells, the smaller of a single cell. In 
 either case, the colourless body is surrounded by a 
 mucilaginous wall which swells considerably as its 
 contents divide, in the case of the single cell into 
 four, in that of the large cell, into eight daughter 
 cells. The colourless cells, each from 15 to 16 /* in 
 length (= about j^ inch), are discharged by the 
 bursting of the vacuole and take up positions similar 
 to those in which the four green cells are found. 
 Though colourless and of granular content, a large, 
 oily looking pyrenoid may be made out in each cell (cf. 
 Fig. 22, B, Pyr., p. 125), and by appropriate methods 
 of staining, the presence of a nucleus may be demon- 
 strated. The colourless cells increase in size and, in 
 each, a red eye-spot makes its appearance as a little, 
 lateral patch near the margin. Soon a distinction is 
 to be seen between a colourless plug of protoplasm 
 and a cup-shaped, granular mass which occupies the 
 major part of the cell (Fig. 22 O). A faint yellow 
 colour steals over the cup-shaped, granular mass, the 
 yellow colour deepens and, becoming green, enables 
 us to identify the cup-shaped, granular mass as a 
 chloroplast. The cell, now a green cell, has no cell- 
 wall, and differs only from a green cell of an adult
 
 iv] GREEN CELLS OF CONVOLUTA 111 
 
 C. roscoffensis in its more regular, oval shape and in 
 the possession of an eye-spot. Each green cell divides. 
 The daughter cells formed by the division lack the 
 oval shapes of the mother cell : they lack also eye- 
 spots. The colourless plug or neck of protoplasm no 
 longer occupies the position of a cork in a flask, but 
 lies eccentrically to the chloroplast and in it the 
 cell-nucleus is contained. In short, they are identical 
 with the green cells of the adult animal. Thus we 
 reach two conclusions of importance. First, that the 
 green cells of C. roscoflensis are preceded by colour- 
 less cells. Second, that the mass of colourless proto- 
 plasm attached to the green cell is not, as Haberlandt 
 suggests, an animal cell standing in close relation 
 with a chloroplast, but is an integral part of the 
 green cell. As the young green cell continues to 
 divide, a significant change may be observed in the 
 shape and state of the nucleus. Distinct and spheri- 
 cal in the colourless and original green cells, it becomes, 
 in the cells produced by successive divisions, more 
 granular and indistinct, till, when the number of 
 green cells has increased considerably, some only 
 among them may be seen to contain nuclear material 
 fine granules in a clear area : the rest contain no 
 trace of nuclear material. In other words, the great 
 majority of the green cells of the adult animal are 
 not complete cells, but cells which show all stages 
 of diminishing nuclear substance (Fig. 17). Inasmuch
 
 112 PLANT-ANIMALS [CH. 
 
 as it is a well-established fact that the nuclear part 
 of the protoplasm plays an important rdle in the 
 life and work of the cell, these observations throw 
 light on the subordination of the green-celled tissue of 
 C. roscoffensis to that of the animal. Since, also, the 
 nucleus is known to play a part in cell- wall formation, 
 we are no longer surprised that a cell-wall fails to 
 form in the green cells. Further, in this progressive 
 nuclear degeneration, we have the explanation of the 
 inability of the green cells to survive separation from 
 the tissues of the animal. Those green cells whose 
 nuclei are least degenerate are capable of division, 
 but even they have suffered. They are no longer 
 able to form a cell-wall nor to exist as independent 
 organisms. As division succeeds division, the nuclear 
 material becomes further reduced till, in the adult 
 animal, it is often difficult to find any sign of nucleus 
 in the large majority of the green cells. 
 
 It is highly probable that the advent of this 
 enucleate stage in the green cells is the signal to 
 the animal to devour them. Though still capable 
 of photosynthesising, the green cells, unable to offer 
 resistance to those of the animal, are surrounded by 
 the latter, devoured and digested. A significant 
 phenomenon is revealed by the drawings (Fig. 17) of 
 sections through the green cells of C. roscoffensis. 
 In places, ingrowing rows of cells may be seen budded 
 off from the outermost green cell and, of these rows,
 
 iv] GREEN CELLS OF CONVOLUTA 113 
 
 only the outermost contain a distinct nucleus, others 
 possess deep-staining granules, and others no nuclear 
 material whatever. A parallel suggests itself between 
 the green cells of C. roscoffensis and the red blood- 
 corpuscles of the higher vertebrates. As the red 
 discs are enucleate, partial cells budded off from the 
 nucleated red cells, so may the green cells be regarded 
 as enucleate, partial cells budded off from the outer- 
 most, nucleated green cells ; and, as the red blood cor- 
 puscles are of limited life and specialised (respiratory) 
 function, so are the green cells of C. roscoffensis of 
 limited life and of specialised (photosynthetic) 
 function. 
 
 The green cell, devoid of nucleus, would not, 
 however, appear to be shut off from all nuclear 
 influences. For the enucleate green cell may be 
 connected by fine processes with another green cell 
 still possessed of nuclear substance (Fig. 17, p. 85). 
 Moreover, such green cells as are without nuclear 
 material are accompanied by a large attendant nucleus 
 of animal origin. This close association of " attendant 
 nucleus" and green cell is shown in Fig. 17, Mes.C. 
 It may be that the attendant nuclei are those of 
 "wandering cells" of the animal which lie in wait 
 for enucleate green cells and, at a subsequent stage, 
 digest them. 
 
 The astonishing closeness of the relationship 
 between animal and green cells offers some support 
 
 K. 8
 
 114 PLANT-ANIMALS [CH. 
 
 for the hypothesis, suggested independently by 
 Schimper and Lankester, which we have already 
 outlined as to the composite nature of higher green 
 plants. The algal cells of C. roscoffensis are on 
 the road which leads to complete loss of inde- 
 pendence. In the higher green plant this loss is 
 complete. The green cells of C. roscoffensis lose 
 cell-wall and nucleus, but retain some colourless 
 protoplasm ; the green elements of the flowering 
 plant if they are regarded as the descendants of 
 originally free algae have lost everything except the 
 photosynthesising organs the chloroplasts. 
 
 But a wide gap remains between the state of 
 affairs in C. roscoffensis and that in the higher green 
 plant. Sooner or later C. roscoffensis destroys and 
 digests its green cells, and none of them, nor any 
 colourless representatives of the green cells, pass into 
 the egg-cells ; whereas the higher green plants pro- 
 vide for the future crop of chloroplasts in their 
 descendants by transmitting colourless rudiments of 
 the chloroplasts to their egg-cells. 
 
 An adult C. roscoffensis is a complex of two 
 organisms one, the colourless animal, the other, the 
 chloroplast-remainders of the original, green, nucle- 
 ated, algal cells. In its case, unlike that j ust imagined 
 for the green plant, the synthesis is not a permanent 
 one. It endures but for the lifetime of the animal 
 and has to be recommenced in every larval Convoluta*
 
 iv] GREEN CELLS OF CONVOLUTA 115 
 
 The discovery that the green cells of C. roscoffensis 
 arise, in the larval animal, from a colourless cell 
 which lies in a vacuole near the mouth of the animal, 
 makes it all but certain that they are of extraneous 
 origin, and that, in the course of ingesting solid food, 
 the larvae take up also these antecedents of the green 
 cells. 
 
 Failing the isolation and cultivation of the green 
 cells, and failing the discovery of the colourless 
 or green cells in the sea- water, all that seemed pos- 
 sible to do more was to demonstrate that animals 
 hatched in pure, filtered sea-water, remain colour- 
 less. The method adopted for this purpose was as 
 follows. Large numbers of animals were scooped up 
 in a watch-glass as free from sand as possible. They 
 were brought into the laboratory, allowed to geotrope 
 into a white cup and washed repeatedly with filtered 
 sea- water. Their habit of sticking to the surface of 
 the cup after it had been tapped gently, permitted 
 of the water being poured off without any consider- 
 able loss of animals. After washing them many 
 times, the animals were transferred to filtered sea- 
 water in large glass dishes. In due season, the egg- 
 capsules were formed and since, though very minute, 
 they are visible readily to the practised eye, they 
 could be picked out and transferred yet again to 
 filtered sea-water. In this they were allowed to 
 hatch. Though the results of such experiments, 
 
 82
 
 116 PLANT-ANIMALS [CH. 
 
 which were repeated many times, confirmed the 
 infection-hypothesis they were not sufficiently uni- 
 form to establish it absolutely. Time after time, the 
 minute larvae showed, when examined microscopic- 
 ally, a general absence both of green cells and colour- 
 less precursors of green cells ; but, time after time, 
 also, an occasional animal reared under these ap- 
 parently sterile conditions was found to contain 
 green cells. Though the occurrence of green cells 
 among animals hatched in filtered sea-water was 
 infrequent and sporadic, yet there such animals were 
 and, to make matters worse, the longer the larvae 
 were kept under observation, the larger was the 
 number of specimens which contained green cells. 
 
 Evidently, either the infection-hypothesis was 
 wrong or there was some defect in the experiment. 
 A careful examination of the conditions of the ex- 
 periment revealed ultimately a source of error and 
 opened up the possibility of a new origin for the 
 green cells. The mucilaginous capsules, enclosing 
 groups of eggs, were discovered to be infested with 
 all sorts of minute organisms. On the capsules, and 
 in them, were many different kinds of microscopic 
 animals and plants diatoms, infusoria and forms of 
 life unlike any to be seen elsewhere. Since the egg- 
 capsule is formed by a secretion of the skin, and since 
 the skin of the animal is covered with slime, it was at 
 once clear that repeated washing in filtered sea-water
 
 iv] GREEN CELLS OF CONVOLUTA 117 
 
 had not succeeded in making the animals biologically 
 clean. It was clear, also, that, if the infecting organism 
 came from the egg-capsule, it might be derived not 
 from the sea- water but from the body of the parent. 
 At the time of hatching, there might be liberated 
 from the body of the animal, colourless or green 
 cells which, though they could not live in sea-water 
 and were not to be cultivated artificially, might well 
 be capable of living inside, or in the walls of the egg- 
 capsules. 
 
 Fortunately, the elimination of this source of 
 error though laborious is not impossible. When 
 ready to hatch, a very little help suffices to enable 
 the young to escape, not only from the thin mem- 
 brane which encloses each, but also from the common, 
 mucilaginous egg-capsule. Thus, by drawing a clutch 
 into a small pipette and then ejecting it and the 
 water from the pipette, the capsule bursts and the 
 young escape. In this way, it is possible to separate 
 larvse from their capsule-remnants. By employing 
 this method, large numbers of larvse, white, minute 
 and active, were isolated in filtered sea-water in 
 which the only source of infection lay in such in- 
 visible shreds of the capsule as might have happened 
 to get themselves transferred with the larvse to the 
 filtered sea-water. 
 
 The method proved successful. In one case, out of 
 forty -four larvae isolated from capsule-remnants and
 
 118 PL ANT- ANIMALS [OH. 
 
 kept under observation for nearly three weeks, only 
 five animals contained green cells. In another case, 
 not a single animal of a total of forty-seven was 
 found to contain any trace of green cells or colourless 
 precursors. 
 
 Further, on transferring such uninfected animals 
 to ordinary, unfiltered sea- water, they became uni- 
 formly green in the course of one or two days. 
 
 We conclude therefore that the green cells of 
 C. roscoffensis are algse ; that the species to which 
 they belong exists as a free-living, independent, 
 marine plant; that this alga has a colourless stage, 
 as well as a green stage, in its life-history ; that the 
 alga lives on the egg-capsule as well as in sea- water ; 
 that it is ingested with the food, and, resisting 
 digestion, is planted in the body where it increases 
 and multiplies and forms the green tissue of adult 
 C. roscoffensis. 
 
 The questions remain : What is this alga and what 
 does it look like in its free stage? 
 
 All sorts of attempts, some ludicrous in their 
 extravagance, were made to isolate the infecting 
 organism ; whilst all the time it was lying under the 
 eye. None of the attempts succeeded, and, during the 
 winter, when experimental work could not be carried 
 on, there was nothing to do but to contemplate rue- 
 fully the note-books recording the failures. But some 
 wise person once observed, " You learn to play cricket
 
 iv] GREEN CELLS OF CONVOLUTA 119 
 
 during the winter and to skate during the summer " ; 
 a paradox which contains a biological truth, for 
 the effects of exercise sum themselves up and write 
 the addition in our experience, not during the exer- 
 cises, but in the intervals between them. At all 
 events, it was in the winter that a scrutiny of results 
 of the previous summer's work showed that when 
 green animals appeared among larvae left with their 
 capsules in filtered sea-water, the manner of their 
 appearance was like that in which an epidemic 
 declares itself. At first, it marks down a single 
 victim, then some of the neighbours are affected till, 
 by and by, the disease is general. So it was with 
 the green-cell infection of C. roscoffensis. For 
 days after animals hatched in sea-water had become 
 quite green, those hatched from capsules kept in 
 filtered sea-water remained colourless. Then one 
 green, among many non-green, appeared. The 
 numbers of green animals increased, till, finally, 
 all became green. At once the conclusion presents 
 itself. In our experiments, the colourless stage of 
 the larvae, which lasts so long as fourteen days, is an 
 incubation period, not for the animal but for the 
 infecting organism. Here or there, in spite of many 
 washings, a single algal cell which had settled on 
 the surface of the body has remained entangled in 
 the slime covering the animal. Transferred during 
 egg-laying to the capsule, it grows and divides. It
 
 120 PLANT-ANIMALS [CH. 
 
 increases till it forms a multitude. Then the capsule 
 bursts and swarms of infecting organisms are liberated 
 and, ingested eagerly by C. roscoffensis, give rise to 
 the chlorophyllous cells of its body. The idea sug- 
 gests the simple method : isolate the empty capsules, 
 as well as the just-hatched young, and in a week or 
 two some of the capsules will be found teeming with 
 the infecting organism. 
 
 On returning to Brittany in the following summer 
 the first thing done was to test the hypothesis. 
 Animals were washed and put to lay in filtered 
 sea- water ; the egg-capsules were washed likewise and, 
 when the larvse were hatching out, the latter were 
 put in one vessel and the remains of their capsules 
 in another. The animals remained colourless, though, 
 when samples of them were put into ordinary sea- 
 water, green cells made their appearance in their 
 bodies with uniform regularity. After seventeen 
 days, several small, green, globular bodies, each as 
 large as a big pin's head, made their appearance in the 
 water of the vessel containing the capsule-remnants 
 (Fig. 19). Their hue was the dark spinach-green of 
 C. roscoffensis. On microscopic examination, under 
 the slight pressure of a cover-glass, the dark green 
 mass dissolved and formed a cloud of active, green 
 flagellated cells, emerging from an egg-capsule (Fig. 20). 
 Though these free cells differed in various details from 
 the green cells which occur in the body of C. roscoffen-
 
 iv] GREEN CELLS OF CONVOLUTA 121 
 
 sis, it was evident at once that they represented a 
 stage in the life history of the infecting organism. 
 
 Fig. 19. Egg-capsule of Convoluta roscoffensis occupied by a dark 
 mass consisting of vast numbers of the "infecting organism." 
 (Magnified forty times.) 
 
 The final proof was applied. To the vessel contain- 
 ing colourless, uninfected Convolutas, some of the 
 free, green cells were added. Within two days all 
 
 Fig. 20. The capsule shown in Fig. 19 enlarged and compressed 
 during microscopic examination: the "infecting organism" 
 escaping.
 
 122 PLANT-ANIMALS [CH. 
 
 the animals were green. The synthesis of the plant- 
 animal had been effected. As the result of introducing 
 an undoubted green alga to a colourless, larval C. 
 roscoffensis, a green plant-animal was formed. 
 
 Since a similar last stage has not been reached in 
 the case of C. paradoxa though to reach that stage 
 is but a matter of time and experiment we will 
 devote ourselves not to a description of the incomplete 
 evidence of its infection by a yellow-brown algal 
 cell (see Keeble, 1908), but to a continuation of 
 the study of the life-history of the infecting organism 
 of C. roscoffensis. The securing of material for this 
 purpose is comparatively easy once the art of culti- 
 vating the organism on the egg-capsules has been 
 learned. It is facilitated also by the fact that the 
 motile, green cells are, like C. roscoffensis, positively 
 phototropic and assume, in a vessel of sea-water 
 exposed to unilateral light, a position identical with 
 that taken up by C. roscoffensis. Thus, though each 
 algal cell is far too small to be visible to the unaided 
 eye, the green, motile cells aggregated together at 
 the surface of the water on the side toward the light 
 become collectively visible as a green scum at or just 
 above the water-line. The fact that they react to 
 light by a vertically upward movement as well as 
 by a movement toward the source of light suggests 
 that the pyrenoid (Fig. 21), a dense mass of protein 
 surrounded by protoplasm, may perform for the green
 
 iv] GREEN CELLS OF CONVOLUTA 123 
 
 cell a function similar to that which the otocyst 
 performs for the animal. Just as the granule of chalk 
 which is contained in the otocyst serves, by its gravi- 
 tational movements, to stimulate C. roscoffensis to 
 orientate itself, so may the pyrenoid, falling now this 
 way and now that, serve to stimulate the protoplasm 
 
 A 
 
 Fig. 21. The infecting organism a green alga belonging to the 
 Chlamydomonadinese seen under the high power of the micro- 
 scope. A=macrocyte. B=microcyte. Chi. = chloroplast (repre- 
 sented by a reticulum) occupying the greater part of the cell. 
 Nuc. = nucleus. St. = eye-spot, indicated in A, but not in B, in 
 which, however, it occupies a similar position. Py p. = pyrenoid. 
 The four long threadlike projections represent the flagella. 
 
 of the green cell to perform like movements of 
 orientation. 
 
 If the green cells which have taken up their 
 position along the water-line are examined micro- 
 scopically, they are found to include many which are 
 in active movement Each such cell resembles, in
 
 124 PLANT-ANIMALS [CH. 
 
 essentials, the first green cells which appear in the 
 body of C. roscoffensis. The bulk of the cell (Fig. 21) 
 is occupied by a flask-shaped chloroplast (CM.} in the 
 middle of which lies the pyrenoid surrounded by its 
 starch sheath. In the "neck" of the flask-shaped 
 cell lies a colourless plug or core of protoplasm in 
 which the nucleus is suspended. On one side of the 
 chloroplast, a red eye-spot is placed (St.). So far, the 
 description of the active cell corresponds exactly 
 with that of one of the first green cells to be seen 
 in the body of the infected animal. But, in addition 
 to these structures, two others are met with in the 
 free, active green cell which are absent from the 
 green cells contained in the body of C. roscoflensis. 
 These new structures are flagella and cell- wall. 
 
 The flagella (Fig. 21) consist of four equal, delicate 
 protoplasmic threads each about twice as long as the 
 green cell. They project from the anterior end of 
 the colourless plug of protoplasm, and by their active, 
 contractile movements serve to row the animal 
 through the water. The cell- wall which invests the 
 alga is extremely delicate and gives, when treated 
 with appropriate reagents, the reaction not of cellulose 
 but of chitin. 
 
 The flagellated cells are remarkable in that they oc- 
 cur in two sizes (Fig. 21). The large green cells macro- 
 cytes are about twice as big as the small microcytes. 
 Such a diflerence in size occurs not infrequently among 
 unicellular green algse, and in cases where it occurs it
 
 iv] GREEN CELLS OF CONVOLUTA 125 
 
 has been shown that the macrocytes and microcytes 
 fuse in pairs. As a result of this fusion, a single cell 
 (or zygote) is produced, which, after passing through 
 a period of rest, gives rise by division to four or more 
 daughter cells. Since a similar fusion of two cells 
 (or gametes) is the essential characteristic of sexual re- 
 production in all plants and animals, this fusion may be 
 regarded as a process of sexual reproduction. Under 
 ordinary circumstances, however, the macrocytes and 
 
 c 
 
 D 
 
 Fig. 22. A = Resting cell (green or colourless) of the infecting 
 organism. B = Colourless cell dividing to form daughter cells. 
 C= Besting cell containing four daughter cells. D = Eesting 
 cell with eight daughter cells (six shown). Nuc. = nucleus. 
 Pyr. = Pyrenoid. 
 
 microcytes of the infecting organism do not fuse with 
 one another. 
 
 Other cells taken from the green streak along 
 the water-line possess no flagella (Fig. 22). They 
 have settled down, withdrawn these structures 
 and have surrounded themselves each with a thick
 
 126 PLANT-ANIMALS [CH. 
 
 wall of mucilage. So rapidly may the wall form 
 about the encysting green cell, that individuals are 
 sometimes observed in which the flagella may be 
 seen in undulating movement within the enclosing wall. 
 
 The resting cells (Fig. 22) vary remarkably both 
 in form and behaviour. Thus, a single, flagellated cell 
 may come to rest, surround itself with a thin wall 
 and divide longitudinally into two or four daughter 
 cells. Each daughter cell, at first naked, organises a 
 delicate cell-wall, develops flagella and escapes from 
 the deliquescent mother wall as an active, flagellated 
 cell. Or an active cell comes to rest, surrounds itself 
 with a thick wall, takes on a spherical shape and 
 becomes uniformly green (Fig. 22, A ). From such cells 
 the pyrenoid and eye-spot disappear. Within each 
 such resting cell, four daughter cells arise, develop 
 flagella and escape (Fig. 22, C). 
 
 A third form of resting cell occurs (Fig. 22, A). 
 It is identical with that just described, except that 
 it is colourless. Like its green counterpart it may 
 divide to form four colourless daughter cells, which 
 may be extruded from the mother cell-wall or divide 
 yet further (Fig. 22, B, C, and D). 
 
 Again, paired resting cells occur. Two active 
 green cells settle down together, become pressed 
 against one another, and surround themselves with 
 a common envelope. Such paired resting cells are 
 either green or colourless.
 
 iv] GREEN CELLS OF CONVOLUTA 127 
 
 The existence of paired and single colourless 
 resting ceils formed by the infecting organism in 
 its free state completes the evidence as to the 
 identity of this alga with the green cells of C. ros- 
 coffensis. For, as we have seen, though the alga 
 may be ingested in its green, flagellated stage, 
 the more usual mode of infection is by means of 
 a colourless cell surrounded by a thick wall. This 
 cell, lying in the central vacuole, undergoes division 
 into daughter cells, which escape subsequently from 
 the mother wall and are sown about the body of 
 C. roscoffensis. The cell originally taken into the 
 body may be paired or single. In the former 
 case it gives rise to eight, in the latter case, 
 to four daughter cells. We conclude, therefore, 
 that the alga which is the infecting organism of 
 C. roscoffensis, lives a double life. At times, it has 
 the form of a green cell, at others, of a colourless 
 cell. As a green cell it is holophytic, that is it manu- 
 factures photosynthetically its food materials. As a 
 colourless cell it is a saprophyte, feeding like an 
 animal on ready-made organic material. In its 
 active stage, it is green and seeks the light : in its 
 passive stage, it may be colourless and may live in 
 darkness. Beneath the sand, therefore, where C. 
 roscoffensis is born, abound vast numbers of the 
 colourless infecting organism. On its emergence 
 from the egg-membrane, the larva encounters them
 
 128 PLANT-ANIMALS [OH. 
 
 in plenty, lying and dividing on its egg-capsule and 
 on any other organic debris. Should it escape in- 
 fection a rare contingency C. roscoffensis may, as 
 we have learned, return to the egg-capsule and thus 
 incur it. Or, becoming positively phototropic, the 
 larva moves up to the light. There, at the upper 
 edges of the water-films, it finds assembled the green, 
 flagellated organism. Beset in darkness and in light 
 by the infecting organism, swallowing eagerly all the 
 minute particles that come its way, C. roscoffensis 
 cannot escape its destiny. A colourless or green 
 cell is taken into the body and the plant-animal is 
 formed. 
 
 So pleomorphic is the infecting organism that it 
 occurs in yet other forms beside those described 
 already. It may give rise by repeated divisions to 
 groups of rounded cells lying together in a colony. 
 Such a colonial form is known as a palmella stage 
 and occurs in the life histories of various green 
 algse. It is remarkable that a colourless palmella 
 form also exists. At any moment, a green member 
 of the palmella may slip its mucilaginous coat and 
 appear as a flagellated, active cell. 
 
 As to the name and position in the plant kingdom 
 of the infecting organism we need say but little. Its 
 characteristics are those of a group of primitive, 
 green algse know as the Chlamydomonadinese. Like 
 the infecting organism, the members of this group
 
 iv] GREEN CELLS OF CONVOLUTA 129 
 
 are unicellular, bear flagella of equal length and store 
 their reserve food-material in the form of starch; 
 they possess each an eye-spot, a pyrenoid and a cup- 
 shaped chloroplast, enclosing a core of colourless, 
 nucleated protoplasm. The only important respect in 
 which the infecting organism differs from a typical 
 chlamydomonadine cell is that, whereas the cell-wall 
 of the latter consists of cellulose, that of the former 
 has not given in our hands the reactions indicative 
 of this substance. 
 
 Carteria, a genus of the Chlamydomonadinese, is 
 characterised by the possession of four flagella, and 
 so also is one species of Chlamydomonas (C. multifilis). 
 Therefore the infecting organism should perhaps be 
 referred to one or other of these genera. Or it may 
 be that it belongs to a yet lower group. These are 
 matters, however, for the systematist to decide. 
 
 What is certain is that the green cells of the body 
 of C. roscoflensis once saw independent days, and 
 that, for those cells, naked and deprived of nuclear 
 material, these independent days are gone never to 
 recur.
 
 CHAPTER V 
 
 THE SIGNIFICANCE OF THE RELATION BETWEEN 
 COLOURED CELL- AND ANIMAL-CONSTITUENTS 
 OF THE PLANT-ANIMALS. 
 
 WE have discovered enough already, with respect 
 to the relations which obtain between algal and animal 
 cells in our composite plant-animals, C. roscoffensis 
 and C. paradoxa, to convince ourselves that the re- 
 lation is not one of casual intimacy, lightly entered 
 upon and lightly abandoned ; but one which is of 
 fundamental importance to the animals. Before it 
 was realised how vital was this association, we 
 entertained hopes of raising a colourless race of 
 C. roscoffensis, and, having done so, of comparing 
 the colourless with the green adult, with the purpose 
 of ascertaining what changes had arisen in the 
 organism as the result of the symbiosis. Though 
 this hope has not been fulfilled, though it has proved 
 a task, if not impossible, yet beyond our powers, 
 nevertheless the attempts to accomplish it have 
 brought to light facts which show how ingrained in the 
 lives of the worms is this habit of association with 
 the algal cells of their respective infecting organisms.
 
 CH. v] NATURE OF PLANT-ANIMALS 131 
 
 The attempts have also provided us with informa- 
 tion with respect to the full significance of the 
 association. In order to make clear the nature of 
 this information, AVC will first consider very briefly 
 the normal course of events in the development of 
 C. roscoffensis in ordinary sea-water containing the 
 infecting organism and then its behaviour in filtered 
 sea-water which contains no infecting organisms. 
 
 Within a day or two of hatching, C. roscoffensis, 
 maintained in ordinary sea-water, is found to have 
 become infected. The colourless cell, the first sign of 
 infection, divides and the colourless daughter cells are 
 sown about the body. They become green, divide and 
 re-divide till ultimately the many thousands of algal 
 cells making up the green tissues of the organism, lie 
 in dense masses in the body. For a time, the animal 
 continues to feed : but, after a while, it abstains from 
 ingesting solid food and lives a simple, easy life, fed by 
 the products of the photosynthetic activity of its green 
 cells. Later on, unsatisfied with the amount or kind 
 of tribute which it thus receives, the animal begins to 
 digest its algal cells and may continue the habit until 
 the green tissue has, in larger measure, disappeared. 
 In the meantime, however, C. roscoffensis has matured 
 and produced eggs, the substance of which has been 
 supplied by the green cells ; so, even though it now 
 dies as the consequence of its ill-considered greedi- 
 ness, the continuance of the species is assured. 
 
 9-2
 
 132 PLANT-ANIMALS [CHL 
 
 C. paradoxa exhibits a like behaviour. Though it 
 never becomes a total abstainer from ingested, solid 
 food, there are times when it makes inroads upon its 
 yellow-brown cells, and indeed, if supplies of solid 
 food are withheld from it, C. paradoxa exhibits no less 
 scruple than C. roscoffensis in raiding its algal cells. 
 
 But if C. roscoffensis is maintained in filtered sea- 
 water, and hence prevented from becoming infected 
 with its green algal associate, the course of events is 
 very different. Even though diatoms and other micro- 
 organisms are added to the filtered sea-water, C. ros- 
 coffensis, after a few days of active feeding, ceases from 
 ingesting solid food-substances. For a while, the fat 
 and other reserve food-substances contained in its body 
 suffice for the needs of the animal, but, when these 
 reserves are exhausted, starvation begins. In spite 
 of the addition of food-material to the filtered sea- 
 water food material which its infected green fellows 
 are enjoying uninfected C. roscoffensis abstains 
 obstinately from ingesting it It is waiting for the 
 development of its green tissues which ought by this 
 time to have been laid down in the body. Thus 
 it waits, and starves, dwindles till it becomes in- 
 visible to the eye, and, ultimately, after weeks of 
 waiting, dies. If, before this happens, the algal in- 
 fecting organism is added to the water, the animal 
 may should exhaustion be not too pronounced 
 ingest it Having thus achieved infection, it is a
 
 v] NATURE OF PLAKT-ANIMALS 133 
 
 changed being. Activity takes the place of lethargy 
 and growth, of degeneration. In a few days C. ros- 
 coffensis becomes, instead of a microscopic, trans- 
 parent object, a visible, green organism. 
 
 The immediate problem is, how to explain this 
 arbitrary behaviour of the uninfected organism. That 
 it is at once a pathetic tribute to the dependence 
 of C. roscoffensis on the infecting organism and a 
 justification of the title of this book, is evident 
 Without the green cells, life to it is not worth living, 
 and it dies though surrounded by a plentiful micro- 
 flora of which in happier, infected circumstances it 
 avails itself without stint 
 
 Let us suppose that the tenor of normal develop- 
 ment of an organism is not smooth and even, but 
 abruptly intermittent ; that in the complex business 
 of growing up a business which involves many 
 simultaneous processes and many processes which are 
 necessarily consecutive the consummation of one 
 phase serves as the signal for the commencement of 
 the next Then, if, for one cause or another, one 
 process does not complete itself, there will be no 
 signal for the beginning of the consequent process. 
 So, in respect of this series of processes, the organism 
 never grows up. It exhibits the phenomenon of 
 arrested development The signal for full steam 
 ahead with the next growth-process may be produced 
 internally ; or it may be of external origin. The ivy.
 
 134 PLANT-ANIMALS [CH. 
 
 which grows on the wall, awaits in vain the signal 
 of high light intensity which is required to call forth 
 the development of its flowers. Many plants exhibit 
 transient or permanent youth-forms ; that is, they 
 pass through or remain in juvenile stages of de- 
 velopment. A like phenomenon, with respect to the 
 organism as a whole, or with respect to single 
 organs is exhibited by animals and by man himself. 
 In the great number of cases, it is to be supposed 
 that the signal which is given or not given is of 
 internal origin. It is very probable that such signals 
 for development are of a chemical nature. The 
 important work of Starling (1906) has supplied 
 physiologists with a new method, capable of precise 
 application, in their work of analysing nervous 
 responses in animals and in plants. Thus, he has 
 demonstrated that a secretion may be the result, not 
 of a nervous stimulus, but of the arrival at the 
 secreting organ of a definite chemical substance. 
 To give but one illustration of Starling's discoveries : 
 some time after food has been swallowed, the pancreas 
 begins to discharge pancreatic juice into the small 
 intestine. Hence, the food, partially digested by the 
 stomach, is met, soon after its arrival in the small 
 intestines, by the pancreatic juice and acted on 
 in such a way that the materials it contains are 
 rendered soluble and diffusible and so capable 
 of passing into the blood-stream. This purposeful,
 
 v] NATURE OF PLANT-ANIMALS 135 
 
 automatic process of the production of pancreatic 
 juice is independent of the nervous system. It occurs 
 in the absence of all nervous connections between 
 the intestines and pancreas. Now Starling has 
 shown that the stimulus which induces secretion in 
 the pancreas is due to a definite, chemical sub- 
 stance (secretin). This substance is produced in the 
 small intestine as the result of the passage of food 
 into that organ. It passes from the intestine into 
 the blood-stream, is carried to the pancreas and gives 
 the signal to that organ to commence its secretive 
 activity. Such specialised, chemical stimulators 
 Starling calls "hormones," and it is not to be doubted 
 that they play an important part in inducing 
 many of the normal processes which, as we know, 
 arise as the consequences of antecedent processes. 
 In plants in particular, it would seem that we must 
 look to hormones, or chemical stimulators to pro- 
 vide us with an understanding of many phenomena 
 which are at present ignored, or ascribed vaguely to 
 nervous action. For example, the living tissue in the 
 stem of plants, known as cambium, which is re- 
 sponsible, by continued growth and division, for the 
 increase in thickness of the stem, occurs in young 
 plants as definite, localised patches or sheets lying 
 between the vascular-bundles. After the plant has 
 reached a certain stage, the non-dividing cells of the 
 cortex which are coterminous with the cambium
 
 136 PLANT-ANIMALS [OH. 
 
 cells, become, as it were, infected, and commence to 
 divide. Then cells neighbouring these begin to divide 
 and play the part of cambium, till finally a complete 
 ring or hollow cylinder of actively dividing cells may 
 be formed in the stem ; and from this ring, which lasts 
 as long as the plant lives, are produced new wood 
 and new bast. Though no definite, chemical stimu- 
 lator has been discovered in this case, we may feel 
 sure that that is due to the fact that it has not been 
 sought. 
 
 Applying the conception of chemical stimulators 
 or hormones to the case of arrested development 
 exhibited by C. roscofiensis, we may suppose that in 
 this animal, the signal for the commencement of the 
 later phases of development owes its origin to the 
 presence, within the body of the animal, of the green 
 algal cells, that, in the absence of these cells, the 
 signal is not given, and that, consequently, develop- 
 ment does not proceed. 
 
 Hence it would follow that no amount of feeding, 
 either with diatoms or any other elements of the natural 
 micro-flora and fauna existing in the environment of 
 C. roscoffensis, can compensate for the lack of the 
 hormone entrusted by custom with the task of sig- 
 nalling to the animal to proceed with the business of 
 ordered development. On this view, the 'failure 
 which has been complete to rear C. roscoifensis on 
 artificial food, starch, sugar, peptone, protein, milk
 
 v] NATURE OF PLANT-ANIMALS 137 
 
 and prepared " human foods" of various kinds, is 
 intelligible ; nor may we expect success to attend our 
 attempts to raise a colourless race of C. roscoffensis 
 till we have discovered the signalling substance pro- 
 duced by the green cells. 
 
 We turn now to another phenomenon exhibited by 
 larval C. roscoffensis. Considered attentively, the 
 rapid development of the green tissue in the infected 
 animal is no less remarkable than the arrest of develop- 
 ment in the uninfected animal. How comes it that 
 an alien organism, intruding itself among the tissues 
 of a young animal, is able to multiply so rapidly and 
 extensively therein? It might be supposed that it 
 is but a case of simple parasitism ; that the green 
 cell lives and multiplies directly at the expense of 
 the animal's cells. This, however, can scarcely be the 
 case, for, in their early stages at least, the green 
 cells keep themselves to themselves. They lie in 
 vacuolar spaces out of direct contact with the animal 
 cells. Hence any food-materials* which they obtain 
 from the body of the animal must be in a state of 
 solution. Again, there is no evidence whatever that 
 the green cells obtain access to any soluble food- 
 substances which the animal has prepared for its 
 own use. The time during which the increase of the 
 green cells is greatest soon after infection has taken 
 place is also the time when the animal itself is 
 growing most rapidly. It is true that during this
 
 138 PLANT-ANIMALS [OH. 
 
 period, the animal is ingesting solid food and there- 
 fore it is not impossible that the food-materials 
 obtained from this source may be shared alike by 
 the cells of the animal and the green, algal cells. 
 But, before we accept this view, we must enquire 
 into the conditions which obtain in the body of C. 
 roscoffensis at the time of infection, with the object 
 of ascertaining what sort of a "seed bed" for the 
 growth of the algal cells is provided by the body of 
 the animal. 
 
 The first fact which is brought to light by an 
 enquiry of this nature is that the association between 
 green cells and animal does not begin with the en- 
 trance of the green cell or its colourless antecedent 
 into the body. Before the relationship reaches this 
 condition of intimacy, animal and free green alga have 
 struck up an acquaintance based on the identity of their 
 mode of phototropistic response. Under the stimulus 
 of unilateral light, they both move in the direction 
 of the light and both proceed to the upmost edge of 
 the sea- water pools or streams in which they occur. 
 But this is not all. The free-living alga settles 
 down from time to time in the mucilage which forms 
 a slimy coating over the body of C. roscoffeusis and, 
 withdrawing its flagella, passes into the condition of 
 a resting cell (Fig. 22). Inasmuch as the skin of the 
 animal provides the capsules which enclose the 
 clutches of eggs, it follows that, not infrequently, one
 
 v] NATURE OF PLANT-ANIMALS 139 
 
 or more of the resting cells of the infecting organism 
 come to be included in the capsule-wall. Further, 
 apart from such chance inclusions, thanks to which 
 we were enabled to produce our pure cultures of the 
 alga, the egg-capsules appear to exert a definite 
 chemical attraction on the motile green cells. Thus, 
 if to a bulk of filtered sea-water containing egg- 
 capsules which have been laid under the cleanest 
 possible conditions, a number of the flagellated cells 
 are transferred, then, after a few hours, one or more 
 of the green cells will be found to have settled down 
 on each capsule. Yet more striking results are 
 obtained if a capsule is suspended in a hanging drop 
 that is, a drop of sea-water which depends from 
 the under side of a microscope cover-glass and if 
 a number of the flagellated cells are added to the 
 drop. On observing such a preparation under the 
 microscope, the motile green cells are seen to approach 
 the capsule, to swarm about it, to press in close ranks 
 into the soft, gelatinous wall and so embed themselves 
 in the envelope. We conclude, therefore, that the 
 egg-capsule exercises an attractive (chemotactic) 
 influence on the flagellated algal cells ; or, in other 
 words, that a definite substance diffusing out from 
 the capsule-walls induces a tropistic (tactic) move- 
 ment in the motile, algal cells of such a nature that 
 they approach the source whence the chemical sub- 
 stance emanates. The behaviour of the green cells,
 
 140 PLANT-ANIMALS [OH. 
 
 which thus settle on and in the capsule, proves that 
 they find in it a favourable medium for growth. 
 Within a few hours, each green cell, having with- 
 drawn its flagella, increases considerably in size and, 
 whilst retaining its green colour, takes on a granular 
 appearance. The eye-spot and pyrenoid become 
 fainter and the cell undergoes division. In the 
 daughter cells thus produced, a series of successive 
 divisions occur till a loose colony of green cells is 
 formed such a colony, in short, as that which enabled 
 us to determine the nature of the infecting organism 
 (p. 120). In egg-capsules, some of the eggs of which 
 have died, the green cells find yet richer supplies of 
 food-material and increase the more rapidly. These 
 observations give us a hint as to the nature of the 
 food-materials contained in the capsules, which serve 
 for the rapid increase in the green cells. For though, 
 as we have learned, green plants have at their 
 command unlimited supplies of the raw materials, 
 carbon-dioxide and water, for the manufacture of 
 carbohydrates, they are by no means in so happy 
 a situation with respect to the raw materials for 
 the synthesis of organic, nitrogen-containing com- 
 pounds. A green plant growing with its roots in 
 the soil, and relying on inorganic salts nitrate of 
 potash, etc. for its supplies of nitrogen, is often 
 hard put to it to obtain enough of these nitrogen 
 compounds wherefrom to manufacture its proteins,
 
 v] NATURE OF PLANT-ANIMALS 141 
 
 and thus to augment its living substance, integral 
 parts of which consist of organic, nitrogen-containing 
 compounds. That such plants suffer not infrequently 
 from nitrogen-hunger is one of the most important 
 agricultural discoveries of the last century. As a 
 consequence of the recognition of this fact, many 
 thousands of tons of nitrate of soda from the nitrate 
 beds of S. America and equally vast quantities of 
 sulphate of ammonia a bye-product of the distil- 
 lation of coal are added annually by the farmer to 
 his land. Nor is the origin of this nitrogen-deficit 
 far to seek. The nitrogen contained in the nitrates 
 of the soil comes in the plant to form a constituent 
 of the organic nitrogen compounds, such as the pro- 
 teins. The plant dies and decays, or is eaten and 
 the eater decays. Ultimately, as the result of these 
 processes of decay, water and carbon-dioxide are 
 liberated and may at once be brought again, by the 
 agency of the green plant, into the vital circulation. 
 Synthesised to form carbohydrates, these substances 
 are once more available for the nutrition of plants 
 and animals. But with respect to nitrogen it is 
 otherwise. The organic nitrogen compounds of the 
 dead animal or plant are broken down by the bacterial 
 and fungous agents of decay into a series of simpler 
 forms which, acted on by yet other of the ordered 
 army of saprophytic micro-organisms, yield finally 
 ammonia and nitrogen. The nitrogen leaks away 
 into the atmosphere and contributes to the 79 per
 
 142 PLANT-ANIMALS [OH. 
 
 cent, of nitrogen gas which is contained in the 
 air. The ammonia may leak away also as every 
 dung-hill testifies or it may be fixed in the soil 
 by the agency of certain nitrifying micro-organisms. 
 These bacteria convert the ammonia into nitrates 
 and the nitrates so formed become available to the 
 roots of the green plant. On the other hand, the 
 nitrates of the soil may be seized upon by yet other, 
 denitrifying micro-organisms and, becoming converted 
 into ammonia compounds, may be lost to the vital 
 circulation. The constant leakage of nitrogen from 
 combined forms to the free and inert form of nitro- 
 gen gas results in a shortage of the nitrogen available 
 for the formation of the nitrogenous food of plants. 
 We may thus speak of the problem which besets all 
 living organisms that of obtaining adequate supplies 
 of organic nitrogen compounds as the nitrogen 
 problem, and we may well believe that the sum-total 
 of life supported on our planet is determined ulti- 
 mately by the amount of available nitrogen present 
 in the earth and sea. Occasionally, organisms are 
 met with which have solved the nitrogen problem 
 in a fundamentally satisfactory manner. Among 
 such organisms are nitrogen-fixing bacteria, legu- 
 minous plants and man. Each of these organisms 
 has evolved methods of bringing back into vital 
 circulation the nitrogen which has escaped as nitrogen 
 gas into the air. 
 
 The nitrogen-fixing bacteria which occur in the
 
 v] NATURE OF PLANT-ANIMALS 143 
 
 soil and also in the sea, possess the power of 
 causing free nitrogen to enter into combination with 
 other elements and so to serve as material for the 
 construction of the vitally necessary proteins. The 
 leguminous plants, clovers, peas, lupins, etc., do it 
 or rather get it done for them by entering into 
 association with a certain species of nitrogen- 
 fixing micro-organism. This organism, Pseudomonas 
 radicicola, enters the root and increases in its 
 tissues. Under the stimulus of this micro-organism, 
 the root swells locally to form nodules or tubercles. 
 Later, when the nodule-organism has accumulated 
 considerable quantities of organic, nitrogen com- 
 pounds, the tissues of the root destroy it, raid its 
 stores and, .living on the nitrogen -plunder, are able, 
 unlike other plants, to grow in soils which are 
 deficient or even lacking in inorganic, nitrogen com- 
 pounds. Thus, the gorse occupies vast tracts of 
 sterile, sandy wastes in Brittany and elsewhere, and 
 the traveller in spring may journey for miles between 
 tree-like groves of gorse aflame with golden blossom, 
 every particle of which owes its presence in the air 
 to the nitrogen-fixing bacteria at work in the roots 
 underground. These bacteria it is which have provided 
 the essential, organic nitrogen compounds without 
 which the tissues of the flowers could not have been 
 formed. Large tracts of waste land in Germany, 
 America and other parts of the world have been
 
 144 PL ANT- ANIMALS [CH. 
 
 rendered amenable to cultivation by planting with 
 lupins. The roots of these plants, beset with nodules, 
 decay in the ground, release nitrogen-compounds, 
 hitherto deficient in the soil, and thus, by their decay, 
 admit of the growth of plants which rely entirely on 
 "fixed" or combined nitrogen. It is computed by com- 
 petent authorities that in Germany alone no less than 
 500 million pounds of nitrogen are secured annually 
 from the air through the activity of the root-tubercle 
 bacteria associated with leguminous crops. 
 
 It is a grim commentary on the mode and rate of 
 progress of agricultural science that these discoveries 
 of the men of science yesterday were among the 
 accepted commonplaces of the ancients. Thus Pliny 
 observes that "the bean ranks first among the legumes 
 and it fertilizes the ground in which it has been sown 
 as well as any manure." 
 
 Man solves the nitrogen problem by including 
 legumes in his crop-rotations, by transporting nitrates 
 from Chili to his European fields and more re- 
 cently by effecting a combination of the nitrogen 
 of the air with oxygen or other elements, utilising 
 for this purpose electrical energy. Where water- 
 power is available for the generation of electricity, 
 factories, destined to play an increasingly important 
 part in the solution of the nitrogen problem, are at 
 present at work turning out large quantities of cal- 
 cium nitrate or other nitrogen-containing compounds.
 
 v] NATURE OF PLANT-ANIMALS 145 
 
 These compounds, put into the soil, are each a source 
 whence the green plant may obtain the raw materials 
 for the synthesis of organic nitrogen and thus increase 
 the supplies of material essential for the development 
 of brain and muscle in animals and man. 
 
 The fact of nitrogen-hunger is, then, no small matter 
 of mere academic importance. It touches the future 
 of man himself and presents a problem which every 
 living organism must solve. The supply of available 
 nitrogen is a limiting factor of life. Let us see what 
 bearing the fact of nitrogen-hunger has on the 
 economy of C. roscoffensis and C. paradoxa. 
 
 That nitrogen-hunger presses as hardly on marine 
 organisms as on those which live on the land is 
 undoubted. Recent investigations have shown that 
 the amount of combined nitrogen present in sea- 
 water, in a form available to plants for synthetic 
 purposes, is extremely low. Thus, according to 
 Johnstone (1907), the amount of nitrogen compounds 
 in Baltic and North Sea water may be taken as about 
 "2 millegrams (= '003 grains) in a litre, or about two 
 parts in a million. No wonder that marine animals 
 are always hungry ! No wonder either that the 
 free, flagellated infecting organism of C. roscoflensis 
 settles down on the egg-capsules to avail itself of any 
 crumbs of nitrogen compounds that it may find there. 
 Nor is it remarkable that, finding a certain amount 
 
 K. 10
 
 146 PLANT-ANIMALS [OH. 
 
 of combined nitrogen, it begins to divide and soon 
 forms a colony of numerous green cells. 
 
 Now, as we have indicated previously, C. ros- 
 coffensis and C. paradoxa are remarkable among the 
 Turbellarian worms in possessing no excretory sys- 
 tem. Unlike their allies, they possess no apparatus 
 for the systematic discharge of the waste products of 
 their metabolism. Hence such products, compounds 
 of nitrogen of a kind useless to the animal, are stored 
 in the tissues of the body. But such substances, 
 though useless for the nutrition of the animal, serve 
 well for plants. Even a terrestrial green plant is 
 very catholic with respect to the compounds of nitro- 
 gen which it takes up and utilises for the synthesis 
 of proteins. Thus, experiment has shown that the 
 root-system of a green flowering plant is capable 
 of absorbing, not only nitrates and, in many cases, 
 ammonium salts, but also such organic, nitrogen- 
 containing substances as urea, uric acid, asparagine 
 and many others. Now the infecting organism of C. 
 roscoflfensis occurs, as we know, in a colourless as well 
 as in a green stage, and, in the colourless form, it can 
 obtain its food materials only after the manner of an 
 animal, that is, in combined organic form. So that 
 its powers of taking up and utilising organic nitrogen 
 compounds are likely to be even more marked than 
 those of a self-supporting green plant. This con-
 
 v] NATURE OF PLANT- ANIMALS 147 
 
 jecture is confirmed by experiment. Comparative 
 cultures of the free stage of the infecting organism 
 have demonstrated that the alga flourishes better when 
 supplied with nitrogen in the form of uric acid than 
 when it is supplied with a nitrate (potassium nitrate). 
 Thus our argument brings us to the following 
 position : We have evidence that the infecting 
 organism increases rapidly as soon as it gains access 
 to the body of the plant-animal. We know that it 
 is able to utilise organic nitrogen compounds such 
 as uric acid for the construction of its proteins. We 
 know, further, that no apparatus for the removal of 
 waste nitrogen compounds, uric acid, urea, etc., occurs 
 in the bodies of C. roscoffensis or C. paradoxa. The 
 conclusion forces itself upon us that the green and 
 yellow-brown cells in the bodies of their respective 
 hosts obtain access to and utilise the stores of waste 
 nitrogen-compounds accumulated therein. Or, to 
 put the same idea m another way, green cells and 
 yellow-brown cells constitute the excretory organs 
 of C. roscoffensis and of C. paradoxa respectively. 
 The plants flourish in the bodies of these animals 
 because there they discover large accumulations of 
 waste nitrogen compounds: the animals, looking to 
 the algse to come and take charge of the work of 
 getting rid of these waste substances, have ceased 
 to construct any excretory apparatus whatever. 
 Hence it is not surprising that, when the algae fail 
 
 10-2
 
 148 PLANT-ANIMALS [CH. 
 
 to appear in their bodies, the animals suffer. It may 
 be that the death of uninfected animals is not merely 
 the consequence of starvation, but is at all events 
 hastened by poisoning due to the accumulation in 
 the tissues of the products of nitrogenous metabolism. 
 According to this view, uninfected C. roscoffensis dies 
 as the consequence of an aggravated attack of " uric 
 acid trouble." 
 
 Evidence is not lacking in support of this some- 
 what fantastic suggestion. Thus, if larval C. ros- 
 coffensis are protected from infection and kept without 
 food, as their large store of reserve food-material 
 derived from that contained in the eggs, disappears, 
 numerous vacuoles charged with long, acicular, 
 crystalline bodies make their appearance in the 
 tissues. The vacuoles and crystals increase in 
 numbers till they present a most striking appear- 
 ance. These crystals represent, in all probability, 
 the waste products of nitrogen-metabolism. 
 
 Now, in infected animals, the crystalline bodies 
 do not occur, and if animals in which they are present 
 are caused to become infected by the green algal 
 cells, the crystals disappear as fast as the green cells 
 develop. Whence we may infer that the materials 
 of which the crystalline bodies consist are used for 
 the nutrition of the green cells. 
 
 The evidence which C. roscoffensis provides in 
 favour of our hypothesis is, of course, but slender.
 
 v] NATURE OF PLANT-ANIMALS 149 
 
 Let us appeal therefore to C. paradoxa. A far more 
 greedy feeder than the green species, its accumula- 
 tions of nitrogenous waste substances are much larger 
 than are those of its ally. Inspection of the Frontispiece 
 or of Fig. 4 shows well-marked, granular bands across 
 the body of the animal. These bands consist prob- 
 ably, as von Graff has suggested, of urates. They 
 are slight in the young animal, increase as it matures, 
 but may disappear as the period of egg-laying arrives, 
 at which time the yellow-brown cells have developed 
 to their full extent. 
 
 In order to establish our hypothesis we must 
 demonstrate that the yellow-brown cells of C. para- 
 doxa actually make use of such substances pre- 
 sumably uric acid or urates as are stored in the 
 body. 
 
 For this purpose, two modes of experimentation 
 were adopted. In the first method, batches of animals 
 of similar sizes and origin were maintained in the 
 light in filtered sea-water to which uric acid was 
 added and their condition was compared with that 
 of animals kept in filtered sea- water containing no 
 uric acid. Preliminary observations showed that the 
 uric acid added to the sea- water was taken up readily 
 by the animals and stored in vacuoles in the tissue 
 of the digestive tract. Examination and measurement 
 of animals from the two batches those in filtered 
 sea-water only and in filtered sea-water plus uric
 
 15 o PL ANT- ANIMALS [CH. 
 
 ac id proved that the latter were, after twenty-one 
 
 days, considerably larger than the former. 
 
 The experiment was continued. During the follow- 
 ing weeks the animals in filtered water, dwindled, lost 
 all their yellow-brown cells, became of microscopic 
 size and died. On the other hand, after upwards of 
 thirteen weeks, specimens of the animals in filtered 
 water plus uric acid were alive, of a recognisably 
 brown colour and possessed of many normal, yellow 
 brown cells. 
 
 We thus have proof that when C. paradoxa is 
 kept in the light, so that its yellow-brown cells may 
 photosynthesise, and when uric acid is supplied, this 
 substance serves as a source of nitrogen to the yellow- 
 brown cells. Moreover, in these circumstances, the 
 materials manufactured by the yellow-brown cells 
 serve not only for the nutrition of the alga but also 
 for that of the animal. This, however, means that 
 the yellow-brown cells contribute not only fatty but 
 also nitrogenous, protein-forming material to the 
 animal. That this is the case the results of the 
 second mode of experimentation render highly 
 probable. 
 
 Here, in lieu of determining the effect of uric 
 acid on the life of algal cell and animal, its influence 
 on egg-laying was investigated. The experiment 
 consisted in maintaining equal numbers of similar 
 animals in filtered sea- water, under conditions which
 
 v] NATURE OF PLANT-ANIMALS 151 
 
 were identical except for the fact that one lot received 
 uric acid. The animals supplied with no extra nitrogen 
 laid nine clutches of eggs, whereas the animals sup- 
 plied with extra nitrogen laid twenty-seven clutches. 
 The results of the two sets of experiments just 
 described serve to account for the rich development 
 of algal cells within the bodies of the plant-animals. 
 In their free state, these algse, like all marine plants, 
 run grave and frequent risk of nitrogen-starvation, 
 or at all events of having their increase limited by 
 the shortage of available nitrogen in the sea. Wherever 
 there is any leakage of nitrogen compounds and 
 traces of combined nitrogen must be given off from 
 such animals of C. roscoffensis and C. paradoxa 
 marine, motile plants will congregate. Congregating 
 about our plant-animals, such minute organisms are 
 ingested indifferently. Out of this mixed infection C. 
 roscoffensis and C. paradoxa make each a pure culture, 
 the one of green cells the other of yellow-brown cells. 
 Established in the body, the algal cells find them- 
 selves transferred from a region of scarcity to a land 
 of plenty. Outside, in the open sea, the amount of 
 nitrogen available is but small and the claimants for 
 a share of it innumerable : within the body, the 
 amount of suitable, combined nitrogen is large and 
 at the exclusive disposal of the algal visitors. In 
 such Capuan circumstances, the algal cells grow and 
 divide luxuriantly. Their photosynthetic activities
 
 152 PLANT-ANIMALS [OH. 
 
 increase, for only in the presence of plentiful sup- 
 plies of nitrogen does the chlorophyll-apparatus work 
 well. Large quantities of carbohydrate material 
 are produced in the algal cells enough for the needs 
 of these cells and also for those of the animal. All 
 goes well, so well indeed that C. roscoffensis, less 
 conservative than its ally, contents itself entirely 
 with the supplies of food-material, of fat and also 
 of organic nitrogen compounds, provided by its green 
 cells and abandons the practice of fending for itself. 
 The ample tribute which it receives suffices for its 
 needs and also for the provision of its eggs. But the 
 weakness of the system here discloses itself. This 
 handing of nitrogen-containing substances to and fro 
 from animal to plant and from plant again to animal 
 cannot go on indefinitely or without loss. Sooner or 
 later, the animal finds itself lacking in essential, 
 nitrogen-containing food-materials. Supply fails to 
 equal the demand. Then the animal is under the dire 
 necessity of digesting its algal cells. To satisfy an 
 imperious, present need, the plant-animal destroys 
 the source of its supplies. 
 
 Thus the animal repudiates the association and, 
 having digested its green cells, C. roscoffensis dies 
 of the very complaint nitrogen-hunger which the 
 green cells sought to avoid by their intrusion into 
 the body of the animal. To dismiss the association 
 between animal- and plant-constituent of the plant-
 
 v] NATURE OF PLANT-ANIMALS 153 
 
 animals by labelling it symbiosis is to miss the vary- 
 ing significance of the association. Looking at the 
 relationship from the standpoint of the animal, it is 
 one of obligate parasitism. Apart from their algal 
 cells, C. roscoifensis and C. paradoxa are unable to 
 live. The existence of either species depends upon 
 the infection of the individuals of each successive 
 generation. Where the infecting organism is absent, 
 there C. roscoffensis does not exist. Hence its re- 
 stricted range. From the standpoint of the species, 
 "infecting organism," the relation of certain of its 
 individuals with C. roscoffensis or C. paradoxa is an 
 episode without significance. Unlike the animal, 
 which bears the inherited impress of the relation 
 in lack of excretory system and in the habit of 
 patient waiting abiding the question of infection 
 the alga is free. Of a swarm of flagellated green 
 cells, some small percentage meet the picturesque 
 fate of forming a tissue in the body of an animal. 
 The others pursue a less romantic adventure, either 
 as green, self-supporting organisms or as colourless 
 cells which batten on the offal of the sea. 
 
 From the standpoint of the ingested algal cell, 
 association with the animal means a successful solu- 
 tion of the nitrogen problem. It sacrifices its 
 independence for a life of plenty. This universal 
 nitrogen-hunger is a misery which makes strange 
 bed-fellows. 
 
 105
 
 154 PLANT-ANIMALS [OH. 
 
 It is noteworthy that the interpretation, in terms 
 of the hypothesis of nitrogen-hunger, of the relation 
 between animal and algal cell throws light on the 
 facts, already referred to, concerning the distribution 
 of algal cells in various marine animals. Analyses 
 have demonstrated (Johnstone, 1907) that the amount 
 of combined nitrogen present in sea-water is less 
 during the warm months (e.g. August) than during 
 the cold months of the year, and that it is less 
 in the warmer seas (Mediterranean) than in the 
 colder seas (Baltic and North Sea). Now, as we 
 have mentioned, certain animals possess green or 
 brown algal cells in one part of their range of dis- 
 tribution but lack them in other parts. Thus 
 Noctiluca, colourless in the North Atlantic, is green 
 in the Indian Ocean. Whence it would appear to 
 follow that where the stress of nitrogen-hunger is 
 more acute, there the association between algal cells 
 and animals manifests itself. 
 
 One word more and one more speculation and our 
 work is done. The colourless phase in the life-history 
 of the infecting organism of C. roscofiensis, the colour- 
 less state of the just-ingested algal cells both in 
 C. roscoffensis and C. paradoxa, and the rapid as- 
 sumption of their proper pigments by the infecting 
 cells after they are established in their respective 
 animal quarters suggest that the colourless phase is 
 itself the outcome of nitrogen hunger. Such colour-
 
 v] NATURE OF PLANT-ANIMALS 155 
 
 less phases are known to occur in the life histories 
 of other micro-organisms, in diatoms, in various species 
 of Chlamydomonas and in Flagellates (Euglena), and 
 it is stated generally that they may be induced by 
 increasing the amount of soluble carbohydrate in 
 the culture medium. But in the cases of the algal- 
 infecting organisms of our plant-animals, the rapid 
 development of the chlorophyllous pigment appears 
 to be associated with the increase in the amount of 
 available nitrogen. So that, if this is the case, the 
 colourless phase would appear to be brought about, 
 not by excess of food-material, but by lack of nitrogen. 
 It may well prove to be that the colourless sapro- 
 phytic phases exhibited by such organisms as those 
 just mentioned diatoms, etc. are each a symptom 
 of nitrogen-hunger. For, failing proper supplies of 
 nitrogen compounds, no amount of carbohydrate photo- 
 synthesis will keep the organism from starvation. 
 Indeed, the more the carbohydrate photosynthesis, in- 
 volving as it must the wearing out and reconstruction 
 of the nitrogen-containing chlorophyll machinery, the 
 acuter will be the nitrogen-hunger ; whereas, on the 
 contrary, a shutting down of the photosynthetic process 
 will effect economies in the use of organic nitrogen 
 compounds and thus postpone the evil day of nitrogen- 
 starvation. Though the facts are not yet available 
 for a confident statement, the hypothesis may be 
 proposed that saprophytism generally depends for
 
 156 PLANT-ANIMALS [CH. 
 
 its' inception on nitrogen-hunger. It is tempting 
 to push this hypothesis to its limits, and to imagine 
 that the great saprophytic groups of the fungi 
 and the bacteria owe their origin to the changed 
 mode of nutrition imposed upon them by lack of 
 nitrogen. That the fungi are examples of descent 
 by reduction is undisputed. All the evidence points 
 to their derivation from chlorophyll-containing algal 
 ancestors. Having lost their chlorophyll, and, with 
 it, their powers of photosynthesis, they are now con- 
 demned to obtain both carbon and nitrogen in the 
 form of organic compounds and hence are compelled, 
 with the bacteria, to play the part of Nature's 
 scavengers. In their quest for food, they settle 
 either on the dead remains of plants or animals, 
 or, invading the living organism, they exchange a 
 saprophytic for a parasitic mode of life. 
 
 The hypothesis suggested here is that the first 
 and fatal step from independence to dependence was 
 the outcome of the nitrogen scarcity which exists 
 in Nature. Confronted with inadequate supplies of 
 nitrogen, the photosynthetic activity of their chloro- 
 phyll apparatus was brought to a standstill. The 
 organisms, unable to obtain a sufficiency of inorganic 
 nitrogen compounds, were constrained to resume their 
 powers, never wholly lost, of absorbing nitrogen com- 
 pounds in organic form. But such organic nitrogen- 
 containing compounds contain also carbon. Hence
 
 v] NATURE OF PLANT-ANIMALS 157 
 
 supplies of this element were obtained together with 
 nitrogen. In these circumstances, the expensive chlo- 
 rophyll apparatus ceased to be worth its upkeep and, 
 wearing out, proved to be too costly in nitrogen to be 
 replaced Thus the organism, now devoid of chloro- 
 phyll, was reduced to a condition in which it obtains 
 directly from its environment as much carbon in 
 combined form as is of use to it and as much combined 
 nitrogen as it can get. It has become a saprophyte. 
 
 Should this hypothesis of the origin of sapro- 
 phytism be established, C. roscoffensis and C. para- 
 doxa will rank high in interest among organisms as 
 suggesting the route along which far-reaching evo- 
 lution has travelled. In any case, it may be claimed 
 for our plant-animals that they have anticipated the 
 advice of Candide and live to cultivate their gardens. 
 
 Both C. roscoffensis and C. paradoxa possess self- 
 sown, well-tended, highly productive gardens, and if 
 they could but learn how to bequeath packets of 
 vegetable seed to their descendants, they might lose 
 their animal characteristics altogether and become, 
 C. roscoffensis a green plant, and*C paradoxa a yellow- 
 brown plant. As it is, the garden has to be replanted 
 in the individuals of the successive generations and 
 so they remain plant-animals.
 
 BIBLIOGRAPHY 
 
 For more complete lists of the literature dealing with the 
 subject of symbiosis between animals and plants see the Biblio- 
 graphies attached to the memoirs published by Messrs Gamble 
 and Keeble in the Quarterly Journal of Microscopic Science 
 (1903, 1907, 1908). 
 
 1879. Geddes, P. Observations on the Physiology and Histology 
 of Convoluta Schultzii. Proc. Roy. Soc. xxvm. pp. 449 457 
 
 1880. Darwin, C. and F. The Movements of Plants, p. 523. 
 1898. Williams, J. Lloyd. Reproduction in Dictyota dichotoma. 
 
 Ann. of Bot. xn. pp. 559560, 1898; and The Periodicity of 
 the sexual cells in Dictyota dichotoma. Ann. of Bot. xix 
 pp. 531560. 1905. 
 
 1900. Goebel. Organography of Plants. Eng. Trans. Univ. 
 Press, Oxford, p. 244. 
 
 1903. Bohn, G. Sur les mouvements oscillatoires des Convoluta 
 roscoffensis. C. R. Ac. Sc. Oct. 1903. 
 
 1903. Gamble, F. W. and Keeble, F. The Binomics of Convoluta 
 roscoffensis. Q. J. M. S. LVIL 1903. 
 
 1904. Semon, R. Die Mneme. W. Engelmann. Leipzig, 1904. 
 
 1906. E. H. Starling. Recent Advances in the Physiology 
 of Digestion. London, 1906. 
 
 1907. Johustone. The Law of the Minimum in the Sea. Sci. 
 Progress, n. No. 6. Oct. 1907 ; and Life in the Sea. Univ. 
 Press, Cambridge. Biological Series.
 
 160 BIBLIOGRAPHY 
 
 1907. Keeble, F. and Gamble, F. W. The Origin and Nature of 
 the Green Cells of Convoluta roscoffensis. Q. J. M. S. LI. 
 Part 2. 1907- 
 
 1908. Keeble, F. The Yellow-brown cells of Convoluta paradoxa. 
 Q. J. M. S. LII. Part 4. 1908. 
 
 1909. Loeb, J. Experimental study of the influence of Environ- 
 ment on Animals. Essay in Darwin and Modern Science. 
 Univ. Press, Cambridge.
 
 INDEX 
 
 Alcyonium (British), 101 
 Alcyonium ceylonicum, 101 
 
 Bohn, G., 64 
 Butler, Samuel, 49 
 
 Carteria sp., 129 
 Chemical stimulators (hormo- 
 nes), 135 
 
 Chemotactism, 139 
 Chlamydomonadineae, 128 
 Chlatnydomonas, 129 
 Chlorophyll, 87 
 Chlqroplast, 85, 86, 105 
 Convoluta paradoxa : 
 
 Background, influence of, 45, 
 48, 50 
 
 Behaviour in constant dark- 
 ness, 65 
 
 Bristles, 8 
 
 Cilia, 7 
 
 Digestion of green cells by, 
 83, 97 
 
 Digestive system, 11 
 
 Eggs, 13; periodicity of pro- 
 duction of, 24 
 
 Egg-laying, conditioned by il- 
 lumination, 31 
 
 Eyes, 9, 54 
 
 Fat, 89 
 
 Feeding habits, 81, 83, 97, 98 
 
 Convoluta paradoxa (eont.): 
 General aspect, 5 
 Glands (pigmented), 9, 54 
 Gravi-perception, 10 
 Gullet, 11 
 Habitat, 7, 19 
 Mouth, 11 
 Otocyst, 9 
 Paradoxa zone, 17 
 Periodicity of egg-laying, 24, 
 
 34 
 
 Phototropism, 42 
 Secretion of fat by yellow- 
 brown cells of, 92 
 Starvation, resistance to, 94 
 Tidal migration, 21 
 Tropistic response to light, 
 
 42 
 Uric acid, effects on egg-laying, 
 
 150 
 
 Vacuoles, 11 
 Yellow-brown cells of, 75, 84, 
 
 91, 95, 147 
 
 Convoluta roscoffensis : 
 Background, influence of, 45, 
 
 48, 50 
 
 Chlorophyll in, 87 
 Cilia, 7 
 Dark-rigor, 60 
 Digestion of yellow-brown cells 
 
 by, 95
 
 162 
 
 INDEX 
 
 Convoluta roscoffensis (cont.) : 
 Digestive system, 11 
 Eggs and egg-capsules, 13, 56 ; 
 
 periodicity of laying of, 26 
 Excretory organs (absence of), 
 
 147 ' 
 Eyes, 9 
 
 Feeding habits of, 77, 81, 97 
 General aspect, 5 
 Gravi-perception, 10, 89 
 Green cells of, 75, 84, 105; 
 
 algal nature of, 118; life 
 
 history of, 132; origin, 108 
 Gullet, 11 
 Habitat, 7, 14, 18 
 Light-rigor, 60 
 Mouth, 11 
 Nucleus, 110 
 
 Nuclear degeneration, 112 
 Otocyst, 9 
 Periodicity of ascent, 62; of 
 
 egg-laying, 26 
 Photosynthesis by the green 
 
 cells of, 81, 87 
 Phototonic effect of stimulation, 
 
 59 
 
 Phototonus, 59 
 Phototropism, 41, 52 
 Eeaction to monochromatic 
 
 light, 54 
 
 Eegeneration of, 27 
 Response to vibration, 15 
 Khythmic ascent and descent, 
 
 19, 62 
 
 Koscoffensis zone, 17 
 Simultaneous stimuli. 44, 45 
 Size, 16 
 
 Starch in green cells, 87 
 Starvation, resistance to, 94 
 Tidal rhythm, 63, 67 
 Tonic influence of light, 67 
 Tropistic response to light, 41 
 
 Convoluta roscoffensis (cont.) : 
 
 Vacuoles, 11 
 
 Vibration, response to, 62, 
 
 67 
 Copepods, tropism of, 69 
 
 Dictyota dichotoma, 34 
 Directive stimuli, 40 
 
 Echinocardium sp., 100 
 Elysia sp., 100 
 Eudendrium racemosum, 33 
 Euglena viridis, 103 
 Eye-spot, 102, 110 
 
 Flagella, 124 
 Fungi, 156 
 
 Geddes, P., 87 
 
 Goebel, K., 33 
 
 Gravi-perception (by roots), 38 
 
 Green cells of animals, 82, 
 100 
 
 Gxeen cells of 0. roscoffensis, 
 algal nature of, 128 ; colourless 
 phase of, 126; cultivation of, 
 115 ; structure and life history, 
 110, 123 
 
 Green light, and marine organ- 
 isms, 54 
 
 Haberlandt, G., 105 
 Bering, Prof. E., 49 
 Hippolyte varians, 47 
 Hormones, 135 
 Hydra viridis, 100 
 
 Ivy, 32 
 
 Lankester, Sir Bay, 114 
 Leucoplast, 105, 109 
 Lichens, 107
 
 IM)EX 
 
 163 
 
 Light, influence of, on plants, 32 ; 
 
 on regeneration of polyps, 33 
 Loeb, J., 29, 69 
 
 Macrocytes, 124 
 
 Microcytes, 124 
 
 Mneme (memory hypothesis), 49 
 
 Monochromatic light, 54 
 
 Mysis sp., 47 
 
 Nervous impulses, 39 
 Nitrogen-compounds in sea-water, 
 
 145, 154 
 
 Nitrogen-fixing bacteria, 143 
 Nitrogen-hunger, 145 
 Nitrogen-problem, 142 
 Noctiluca, coloured cells of, 101 
 
 Otocyst, 9 
 
 Palmella, 128 
 
 Photosynthesis by green plants, 
 
 78; by C. roscoffensis, 87 
 Phototropism of Copepods, 69 
 Prawns, 45 
 Protoplast (cell), 79 
 Pseudomonas radicicola, 143 
 Pyrenoid, 85 
 
 Keflex action, 43 
 
 Reflex arcs, 40 
 
 Reproduction, periodicity of, in 
 
 brown sea-weeds, 34 
 Bhizobium leguminosarum (= 
 
 Pseudomonas radicicola), 143 
 Roscoffensis zone, 17 
 
 Salamandra atra, 29 
 S. maculosa, 29 
 Saprophytism, origin of, 157 
 Schimper, A. F. W., 114 
 Secretion, 135 
 Semon, R., 48 
 
 Simultaneous stimulation, 54 
 Starch, 87 
 
 Starling, E. H., 134 
 Starvation, 94 
 Symbiosis, 106, 143, 153 
 
 Tactic response to stimulation, 
 
 41 
 Tonic effect of light-stimulation, 
 
 58,67 
 Tropistic response to stimulation, 
 
 41,69 
 
 Unconscious memory, 49 
 Uric acid, absorption of, by C. 
 paradoxa, 149 
 
 Vacuoles (digestive), 11 
 Von Graff, 149 
 Vochting, H M 33 
 
 Williams, J. L., 34 
 
 Yellow-brown cells of animals, 
 82, 100 ; of C. paradoxa, 75, 84, 
 91, 95, 147 
 
 Zoobothrium sp., 100 
 Zoochlorella, 101, 105 
 Zooxanthella, 101 
 
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