WANS CANA ME ES ie me) ‘ a Se * : ‘ \ ee id an LIBRARY —) ae! ——————— \\ UNIVERSITY EXTENSION MANUALS EDITED BY PROFESSOR KNIGHT LIBRARY NEW YORK BOTANICAL GAKDEN. CHAPTERS IN MODERN BOTANY GENERAL PLAN OF THE SERIES. This Series ts primarily designed to aid the University Extension Movement throughout Great Britain and America, and to supply the need so widely felt by students, of Text-books for study and reference, in connection with the authorised Courses of Lectures. Volumes dealing with separate sections of Literature, Science, Philosophy, History, and Art have been assigned to representative literary men, to University Professors, or to extension Lecturers connected with Oxford, Cambridge, London, and the Universities of Scotland and Ireland. The Manuals differ from those already in existence in that they are not intended for Elementary use, but for students who have made some advance tn the subjects dealt with. The statement of details is meant to illustrate the working of general laws, and the development of principles ; while the historical evolution of the subject dealt with ts kept in view, along with its philosophical significance. The remarkable success which has attended University Extension in Britain has been partly due to the combination of scientific treat- ment with popularity, and to the union of simplicity with thorough- mess. This movement, however, can only reach those resident in the larger centres of population, while all over the country there are thoughtful persons who desire the same kind of teaching. It ts for them also that this Series ts designed. Its aim is to supply the general reader with the same kind of teaching as ts given in the Lectures, and to reflect the spirit which has characterised the move- ment, viz. the combination of principles with facts, and of methods with results. : The Manuals are also intended to be contributions to the Literature of the Subjects with which they respectively deal, quite apart from University Extension ; and some of them will be found to meet a general rather than a special want. and VIII.) Vi. é S| nS fy % Z) — rae N ca FOR OPICAL > X A TI Chapters Modern Botany BY-PATRIGK GEDDES PROFESSOR OF BOTANY, UNIVERSITY COLLEGE, DUNDEE LIBRARY NEW YORK ROTANICAL UAK UL. LONDON JOHN MURRAY, ALBEMARLE STREET 1893 All rights reserved LIBRAR . NEW y ORK BOTANi( Ai PREFACE ee Tuis little book makes no attempt to condense a survey of its science; even within the fields through which it passes it seeks only to be suggestive, not exhaustive ; its chapters have actually grown out of the syllabus and notes of University Extension Lectures, with their neces- sary limitations. In matter and form its appeal is to the general reader ; yet, in method and spirit, to the student also,—in some measure even to the teacher. In botany, as in other studies, educational methods alter with the times. In the Linnean period the “best botanist was he who knew the most plants,” however little of each; while a later and still dominant school has founded upon Cuvier a type-system which makes him know much,—but of few. Hence the student has come no longer to load his vasculum and memory in a single vacation, with all things from the cedar to the hyssop; but, seeing that cedar and hyssop have been selected as types by the highest authority, scrutinises these, and these only, for his term. Analysis is great, and the anatomist is its prophet; yet such Elementary Biology is but Necrology, its so-called “‘life-histories ” being but histories of form. It is the misfortune of biology that Darwin was not a teacher. It is no easy matter for us professors, trained SFP 20 1907 vi Preface from our youth in Linnean or Cuvierian schools, to change our thought methods, and see how, instead of merely appending the dogma of evolution to our old curriculum of morphological training, to organise one truly Darwinian in spirit from the beginning. But, as teacher and student usually end as they begin, let them begin as they would end; neither with conning an inventory of plant-mummies, nor with the tissue-unwrapping of samples of these ; but with childlike watching, scene after scene, of the actual drama of nature, in which life interacts with ‘life, and fate with all. Only then, indeed, can Linnaeus, can Cuvier, have his due, and do his work for us. It is surely in the measure of our intelligent interest in the play that we notice more and more of the dramatis persone, or have keener scrutiny of each actor in his hour. Hence‘the plan of this little book, which seeks to lead from small scenes to great. Were it worthy, it should be dedicated to the memory of Darwin. PATRICK GEDDES. ‘« For him the woods were a home and gave him the key Of knowledge, thirst for their treasures in herbs and flowers. The secrets held by the creatures nearer than we To earth he sought, and the link of their life with ours : And where alike we are, unlike where, and the veined Division, veined parallel, of a blood that flows In them, in us, from the source by man unattained, Save mark he well what the mystical woods disclose.’ MEREDITH, Melampus. CONTENTS CHAPTER Ret PITCHER PLANTS Darlingtonia—Sarracenia—Origin of Darlingtonia Pitchers—In- sect-Catching—Other Relations to Insects—Minute Structure of these Pitchers—Australian Pitcher Plant (Cephalotus)\—The Pitcher Plant proper gcse, — es Glands and Nectaries ° : . Pages 1-20 CHAPTER Il PITCHER PLANTS—continued Use of Pitchers, “‘ Bionomics” —Bionomics—Bionomics of Nepenthes —Morphology of the Pitcher — Bladderwort — Bionomics of Bladderwort—Allied Forms . : . SSSe CHAPTER: It OTHER INSECTIVOROUS PLANTS—DIFFICULTIES AND CRITICISMS _ Fly- Traps (Dionea and Aldrovanda)—Sundews and Birdlime Traps —Butterworts—Sundews proper (Drosera)—Details, Functional and Structural — Digestion— Movements —Absorption— Utility x Contents —Other Insectivorous Plants—Legends—Dofficulties—Further Difficulties and Criticisms—Direction of further Investigation ; Possible Compromise. : : . Pages 36-59 CHAPTER AV MOVEMENT AND NERVOUS ACTION IN PLANTS Climbing Plants—Darwin’s Observations, with Summary—Inter- pretation of Movements—Movements of Seedlings—Methods of Observation— Theory of Circumnutation E . 60-75 CHAPTER. V MOVEMENTS OF PLANTS—continued Movements in relation to Gravitation—Light-seeking and Light- © avoiding Movements—Rationale of Light-seeking and Light- avoiding Movements—The Sleep of Plants—Mr. Francis Darwin's recent Discussion of Plant Movements—Summary and Conclusion ; ; : ; . 76-94 CHAPTER VI THE WEB OF LIFE Struggle among Plants—-Perched Plants or Epiphytes—Parasttic Plants — Mistleto — Dodder — Root - Parasites — Toothwort— Broom - rapes — Saprophytes — Parasitic Fungi — Bacteria — Symbiosis. : : : : - 95-119 CHAPTER VII RELATIONS BETWEEN PLANTS AND ANIMALS Plants and Snatls—Plants and Ants—Domatia—Myrmecodia— Galls—Plants and Aphides—Cats and Clover - 120-142 Contents xi CHAPTER Vill SPRING AND ITS STUDIES—-GEOGRAPHICAL DISTRIBUTION AND WORLD-LANDSCAPES—SEEDLING AND BUD Spring Studies—Mode of Study in Botany—Phenology and Dis- tribution—A spects of Nature, Vegetation and Landscapes of the World—Germination— Buds and Bud-Scales—Arrangement of. Leavesin the Bud. : : . Pages 143-160 CHAPTER IX LEAVES General Facts tn regard to the Life of Leaves—Experiments, rough and exact—Summary of Leaf Functions—The Structure of . the Leaf—FPalisade Cells and Chlorophyll Grains—Shapes of Leaves—Leaves adapted to special Functions—Substitutes for Leaves—Vitality of the Leaf—Fall of the Leaf . 161-189 CHAP Thre SUGGESTIONS FOR FURTHER STUDY Root and Stem—Flower, Fruit, and Seed—The Web of Life once more—Systematic ane hee ae of Organs and Tissues —Evolution : : ; 190-201 LIST OF ILEUSTRATIGNS A Tropical Forest . : . : : . Frontispiece FIG. PAGE 1. Darlingtonia california . : : : - 3 2. Leaves of Sarracenia purpurea . ) : : ; 9 3. Pitcher of Mepenthes distillatoria . ; : : : 16 4. Pitcher of Mepenthes bicalcarata showing downward- directed prickles. : P ; : ; é 27 5. Elk’s-horn Fern (Platycertum grande) . ‘ p - 99 6. Patch of Lichen grown synthetically by Bonnier (from sowing of fungus spores on algze) under bacteriological precautions against entrance of foreign spores . . ag 7, Assai Palm (Zuterpe oleracia) : : ; . . > ie 8. Apparatus of Bonnier and Mangin, for analysis of gases given off by plants . : : : ‘ : . 166 CHAPTER I PITCHER PLANTS Darlingtonia—Sarracenia— Origin of Darlingtonia Pitchers—In- sect-Catching— Other Relations to Insects— Minute Structure of these Pitchers—Australian Pitcher Plant (Cephalotus)— The Pitcher Plant proper (Nepenthes) — Secreting Glands as Nectaries. IN attempting to arrange a suitable introduction to the study of botany, a teacher may incline to one or other of two distinct methods. The first, and in many ways the more satisfactory, is to take the commonest plants around one, begin with the simplest knowledge of these, extend it by what is easily to be observed or obtained, deepen this by closer study, and next extend to less familiar forms the growing experience and practical power of the student. Most courses of biological instruction now actually run upon this principle: they place before the student some common type, some frog or crayfish, some common fern and flower, from which he may work his way towards a wider survey of the science. The other method, which also has its favourable side, is to start with something rare or strange—at any rate unfamiliar,—and so not only evade the prejudice that botany deals mainly in hard names B 2 Chapters in Modern Botany CHAP. or the like, but obtain the immense advantage of ministering to some measure of reawakened curiosity, some freshened feeling of the varied marvellousness of nature. The plan here adopted is practically a compromise of the two, Beginning indeed with some of the strangest forms and processes of the vegetable world, it is not pro- posed to exhibit these merely as a vegetable menagerie of rarities and wonders, but to use them as a convenient means of reaching, as speedily as may be, not only (a) some general comprehension of the processes and know- ledge of the forms of vegetable life, but also, and from the very first, (2) some intelligent grasp of the experimental methods and reasoning employed in their investigation. For these purposes a very convenient beginning may be made with pitcher plants. Moreover, they will be found to lead us more rapidly than would many more familiar types to the point of view of Darwin, and the reading of his actual works ; this being of course most central and characteristic in modern botany. Darlingtonia.—Beginning then far afield, in the land of big trees and vegetable wonders, we find not the least of these in a marsh plant discovered just fifty years ago by the botanist of an exploring expedition in the region of the Sierra Nevada. A fresh expedition nine years later gathered flowering specimens, but it was not until 1855 that the systematist Torrey formally introduced the plant to the world as Darlingtonia californica (the surname given in compliment to a friend). At first a rarity of botanic gardens, it is now not uncommon in greenhouses, and is very easy of cultivation. The flowers are large and strange, resembling those of Sarracenia, described below (p. 5). The leaves, however, are yet stranger; they rise in stemless clumps above their mossy bed to a height of 12 or 18 inches, slender tubes extending upwards like I Pitcher Plants 3 organ pipes, but each recurving into a large and well-arched hood or helmet, with a framework of strongly - marked veins. This helmet is brightly splashed with red, and glistens with innumerable translucent spaces in which green Fic. 1.—Darlingtonia californica. tissue is absent, and only the epidermis of either side remains. Its small downward-directed opening is con- cealed, not only by its position, but by a gaily-tinted and banner-like streamer which hangs in front. To this open- ing, however, there ascends a gently-curved pathway which 4 Chapters in Modern Botany CHAP. runs upwards from the very ground, as if expressly built for ants and other wingless creepers, while at its top it is no less useful as a landing stage for winged explorers. The finger-tip can easily be inserted, and finds the edge to be incurved all the way round, while the descent into the pitcher is of course close by. Slitting open one of the old pitchers a gruesome sight presents itself—two, five, nay, it may be more likely twenty or fifty mouldering corpses, chiefly, in our greenhouses at least, those of bluebottles and wasps, but with now and then also a moth or bee. Sarracenia.—To understand all these peculiarities of form and life we may best pass to the allied genus Sarracenia, of which there are a good many different species of similar habit‘and habitat, but wider distribution, the genus ranging from Florida to Canada. The form is less perplexing, the hollow leaves are now simply trumpet- shaped, and instead of being rolled through a semicircle and curiously narrowed, open widely towards the sky, while the forked pennon of Darlingtonia is obviously represented by an almost circular or somewhat pointed leafy expansion, sometimes sloping over the mouth, like a half open lid or cover, large enough to throw off rain, but often also standing erect and conspicuous, the whole effect being often no less attractive to botanist or bluebottle than that of the Darlingtonia itself (Fig. 1, p. 3). Here clearly we have the simple form; and passing from the empirical facts of geographical distribution towards that interpretation the healthy childish or scientific mind cannot but demand, we can hardly fail to suspect that Darling- tonia is but the most outlying of the Sarracenias, one which has wandered to the farther side of the Rocky Mountains, and in that region become so much specialised beyond the ordinary type as to warrant re-naming as a new genus. Comparing the flowers, we find essential kin- I Pitcher Plants = ship yet sufficient generic difference ; curiously enough, it is the flower of Sarracenia which is the more specialised. Each is solitary upon its lofty stalk, the long dull-red petals quite surpassed in conspicuousness by the curiously- dilated style, recalling the peltate leaf of the common Indian Cress (7vope@olum), which climbs over so many cottage walls, or in dwarf forms brightens the garden ~ border. In Darlingtonia the style merely shows the faintest beginnings of such an arrangement. From this one of the perplexities of evolution becomes evident ; we often cannot say that one plant is more evolved than another as a whole ; but only it may be on this or that respect, e.g. the form of the leaf in Darlingtonia, of style in Sarracenia. Origin of Darlingtonia Pitchers.—-But how should such a change come about? the thoughtful student will ask, so beginning to raise all the enigmas of evolution. Was it by change of climate and soil? or by spontaneous internal variations, trifling differences of the kind visible in every patch of seedlings, of which some, useful in some way to the plant, helped their lucky possessors, which therefore survived while their fellows died, and transmitted these to a new series of similar divergent seedlings, which again had to struggle for life in the same way? ‘Thus we see how in course of generations we might obtain important and obvious differences from the simpler parent form; and this of course would have been Darwin’s view ; it is a special case of his famous theory of “the Origin of Species, by means of Natural Selection, or the Preservation of Fortunate Varia- tions in the Struggle for Life,” while the speculation as to the possible but unknown influence of climate and soil represents the position of the earlier evolutionists, Lamarck, Erasmus Darwin, etc. Here, then, at the very outset of our studies, the riddle of origins comes up and refuses to admit itself fully solved. For though each of these 6 Chapters tn Modern Botany CHAP. answers alone successively satisfied many minds, Darwin himself at least inclined towards some varying compromise of them. And as the student learns the really scientific atti- tude, that of not learning answers but of asking questions, new puzzles arise. Thus, what influence are we to place upon the geographical isolation through many generations of the incipient Darlingtonia from its kindred Sarracenias ? How far does it modify our natural selectionist position with its characteristic insistance upon adaptation to external uses when we note that the odd hood of the Darlingtonia is developed merely by increasing the relative rate of growth of the outer and upper surface of the Sarracenia pitcher, especially as we approach its mid-rib, so producing not only the inflation but the curvature, and even the stretch- ing apart of the leaf-tissue so as to leave the pretty window-like patches. But if this be clear we have already got a step below the conventional Darwinian level of external adaptation, below the idea of progress merely through the cumulative patenting of mechanical improve- ments in our fly-traps. Helpful though that explanation is, so far as it goes, we have in fact reached a new and deeper plane of thought on which it may become necessary to work out an entirely new set of evolutionary interpretations. The outer world of external and mechanical adaptations once left behind, we are at once brought face to face with the internal and vital processes, and have to grapple with the problems of organic growth, both general and special ; in other words, it is in terms of the laws of growth that we have to reinterpret the phenomena of development. We are familiar with these differences of growth of different regions of the leaf, asin young leaves of ferns, but the experimental study in detail lies still before us. Deliberately to arrange new conditions for our Sarracenia so that it shall at least begin to roll its leaf into the form of Darlingtonia is, how- I Pitcher Plants 7 ever, at present “impracticable,” ze. remains a problem for the experimental physiologist. Yet even the natural selectionist most satisfied with accounting for the change on the principle of the mechanically improving fly-trap is notwithstanding the very man empirically to help, if not anticipate us, by finding us a seedling varying in the required direction among a patch of young Sarracenia ; for on this and its offspring the experimentalist might best begin. All such experimental researches are as yet only in their infancy, but it is becoming admitted on all hands that as the past of the science lies mainly in systematic collections, in anatomical and microscopic analyses, so its future has to be sought in the physiological laboratory, the greenhouse, and the garden. Insect-Catching.—But how, the active minded observer will ask, does our curious helmet-like Darlingtonia pitcher keep its victims,—for though it is natural for an insect to creep into what doubtless appears to it a very inviting new kind of flower, why should it not creep out again? A _first difficulty is afforded by the incurved margins ; but this would not be enough to detain it ; and here comes in the use of the relatively widely distended helmet-space with its innumerable transparent glassy panes let into its whole upper surface. The insect has eyes on the top and sides of its head, and sees abundant light above to spread its wings and beat itself upon the resisting roof and walls of the pitcher as persistently and as vainly as he does within our own window-casement. No doubt when he becomes tired and falls down over the entrance he may at times escape, but is more likely only to rest over it till he can again begin his struggles on the wing, while the adjacent opening, that of the fatal oubliette, is not only of much larger diameter, but of gently-sloping sides instead of repellent recurved margins. The walls of the leaf-tube substantially 8 Chapters in Modern Botany CHAP. correspond to those of Sarracenia, which we may therefore describe more minutely. The gaily-coloured lid of the Sarracenia pitcher is bedewed in spring and early summer with drops of nectar, which lie on its inward surface, at least for the most part ; not on both, as in the pennon of the Darlingtonia. A closer examination of its surface shows that these drops are at once helped to form, and if sufficiently large to trickle downwards by a coating of fine but short and stiff hairs which arise from the epidermic surface. Here, in fact, is in every way an admirably-constructed ‘attractive surface,” and it is obvious as well as natural that the insects which sip the honey should travel down into the interior of the pitcher to seek for more. Beyond the lid surface with its hairs and nectar-glands they come upon the smooth and glassy ‘“‘conducting surface,” a well- paved path leading indeed towards destruction. In S. purpurea there are indeed a few fresh nectaries to be reached by this descent, a new secreting surface below the conducting one—in S. flava and other species not even this,—but in all cases we soon reach the ‘‘ detentive surface” of the whole lower part of the pitcher. This is covered with long, stout, bristly hairs, averaging say } inch long, all sloping downwards into the cavity of the pitcher, and so presenting no obstacle towards descent, but much resistance towards return, as the finger can easily verify, or as the dead inmates of the tubular prison still more conclusively show. That so comparatively powerful an insect as a wasp or bluebottle can be thus detained may be at first sight perplexing ; but we see that there is no scope to use the wings for escape, while legs and wings alike become entangled and held back by the stiffly-pointed hairs, which the struggling insect can at most only thrust along, and thus not break. Another captive soon comes on top; = Pitcher Plants 9 ventilation becomes checked, and the foul air rising from dead predecessors must still further check respiration ; little wonder then that life must fail. Even in our greenhouses Fic. 2.—Leaves of Sarracenia purpurea. A, attractive surface of lid; B, conducting; C, glandular, and D, detentive surface, magnified. (A and D are taken from S. flava.) the leaf thus becomes filled, not only 1 or 2, but often 5 or 6 inches deep with dead insects; while observers on the spot, notably Dr. Mellichamp, to whom our know- ie) Chapters tn Modern Botany CHAP. ledge is mainly due, have shown that there is normally a considerable amount of fluid secreted by the pitcher, although this does not seem to appear in European culti- vation, and that this fluid has distinctly anzsthetic and fatal properties to insects immersed in it. Other Relations to Insects.—It is an odd fact that while with us the bluebottle falls an easy and natural prey to this unwonted trap, being doubtless attracted like the wasp by that odour of decomposing carrion to which the bee and butterfly in turn owe their safety, a shrewder American cousin (Sarvcophaga sarraceni@) lays a few eggs over the pitcher edge, where the maggots hatch and fatten “on the abundant food. In April three or four of these larvz are to be found, but in June or July only one sur- vives, the victor who has devoured his brethren. But nemesis is often at hand in the form of a grub-seeking bird, who slits up the pitcher with his beak, and makes short work of all its eatable contents. For this bird in turn the naturalist has next to lie in wait, and so add a new link to the chain. The larve of a moth (Xanthoftera semicrocea) also inhabit the pitcher, but devour its tissue, not its animal inmates ; in fact, they spin a web across its diameter, as if to exclude further entrance of these, and then devour the upper part of the tissue, especially, it would seem, the nectar-glands, finally passing through their chrysalis stage within the cavity of the pitcher, and not, as in the case of the Sarcophaga larva, making their exit into the ground. It is said that spiders also spin their webs over the mouths of the pitchers and wait to reap the profit of their attractiveness—again a point of almost human shrewdness. An American entomologist, Professor Riley, has de- scribed the ways in which these associated living insects (commensals, we may perhaps call them, by a not extreme I Pitcher Plants II stretch of technical language) are adapted to life in such dangerous conditions. The moth has long spurs upon its tibize (second leg joints), which cross many of the hairs as it walks, and so prevent its legs from sinking among them ; while its larva, destitute of this snow-shoe arrangement, spin their silken strands over the tips of the detentive ~ hairs, and so keep out of danger. The larve of the blow- fly, on the other hand, have peculiarly long claws and large cushions on the last tarsal joints, and so grip down through the hairs and hook themselves firmly into the very tissue of the trumpet-leaf itself. The question naturally arose—are not these treacherous plants victimising the very insects which fertilise them? But this seems little or not at all to be the case; for S. variolaris, at least, our good observer Dr. Mellichamp has shown that fertilisation is effected by the ‘‘melancholy chafer” (Euphoria melancholica), nor has he ever beheld the moth Xanthoptera so act. So far, at any rate, it seems we have quite distinct and separate inter-adaptations of flower and leaf, and to distinct and separate insects. Minute Structure of the Pitcher.—Before leaving this subject one may have a useful first lesson in ‘vegetable histology,” since the tissues here are not only peculiarly in- teresting and intelligible, but very easily handled. Opening the pitcher with one’s penknife it is easy to make out with the naked eye, and clear with the pocket lens, the essential character of these surfaces, attractive, conductive, and detentive respectively ; but to see the exquisite beauty and perfection of their details we must multiply lens above lens, so developing our simple microscope, noting, of course, that we are passing to no separate ‘science of microscopy,” but that we are merely adding in front of our own eye lens first one artificial lens, and then more as we need them. How these lenses need to be held together, 12 Chapters in Modern Botany CHAP. and how one combination of these is brought near the eye (‘‘eye-piece”), so as to multiply still further the image already magnified by another held nearer the object (and hence naturally termed object-glass), is of course the ele- mentary common sense of that exquisite marvel of detailed perfection, the compound microscope. The further develop- ments, as that of shutting off side light above the object- glass by the microscope tube, and below it by the stage- diaphragm, of placing the object upon a transparent stage, and this upon a perforated one, or of getting the instrument when we wish to examine a transparent object out of the inconvenient horizontal position at first necessary into the more convenient vertical or sloping one by the simple device of reflecting the window light through the tube to ‘ the eye by means of a mirror fixed below the stage, are again no less obvious. This elementary instrument once constructed, a new set of considerations would naturally arise, among which the necessity of focussing and the diffi- culty of getting rid of the prismatic colours which would as yet enhalo our magnified image may be cited as specially important. These have to be met by mechanical and optical devices respectively, which are familiar enough ; and sO we might work on, a whole volume being needed to do justice to the history of the instrument, as, indeed, are special journals to its unending developments. The bare outline given above is but to emphasise the idea, com- monplace in phrase but too little habitual in practice, that the scientific study of anything, be it a natural or social product, ought always to proceed from the known towards the unknown, and rationally from its beginnings onwards wherever possible. Given, then, the compound microscope, we may first attempt to examine the epidermis more in detail, beginning with the attractive surface of the lid by shaving off thin I Pitcher Plants 13 slices from its surface, and mounting them in a drop of water between slide and cover-glass. These, however, we probably find to be comparatively opaque and confused, because too thick, and including much of the deeper leaf-tissue or parenchyma as well as epidermis. We may, it is true, improve our preparation by the use of . methods familiar to every microscopist, e.g. by washing the preparation in spirit to dissolve out the green colouring matter or chlorophyll, by treatment with caustic potash solution to destroy even the protoplasm, by dyeing or staining the cell-walls conveniently with a solution of one of the anilines to bring out their outlines more clearly, and by mounting in glycerine instead of water, so as to give greater transparency to the whole. Instead of all this trouble, which after all will not make a good preparation out of a badly- made section, we may learn much from even a thick slice in the fresh state by observing merely its edges, which are sure to be somewhere thin enough. A better method how- ever, which, with a little practice, will be found to give excellent preparations, not only of all the tougher-leaved insectivorous plants, but of any tolerably strong epidermis, is to lay the morsel of leaf face downwards upon a slide in a large drop of water, and then holding it firmly at one edge, to scrape away with a sharp scalpel or penknife the other epidermis and the green leaf-tissue, the veins too, as far as they will come, washing the debris from the prepara- tion from time to time, and scraping more carefully and lightly as the lower epidermis is exposed, and of course threatens or begins to tear. When tolerably clean the preparation may be turned over and examined, a funda- mental principle in all microscopic study being first to make out all one can with the low magnifying power before proceeding to a higher one, while the various operations of histological ‘‘cuzs7ne” above indicated may be applied if 14 Chapters in Modern Botany CHAP. desired, and the preparation mounted permanently for the collection, either simply in glycerine by putting a ring of asphalt or gold size around the cover-glass edge, or by mounting in glycerine jelly. Sufficient technical skill and experience to make very fair botanical preparations will be found to come very rapidly with practice, especially if the beginner can obtain a practical start from any more experienced student or amateur; the help of any work on the microscope is often of much value, although there is nowadays a bewildering wealth of technical devices, each no doubt useful in its own way, like the numberless refinements of the microscope itself, yet like these quite unnecessary until skill has been reached and special problems undertaken. Australian Pitcher Plant (Cephalotus)— Another pitcher plant, farther-fetched than Darlingtonia, and less fre- quent in cultivation in our greenhouse collections—indeed one of the rarest and most peculiar plants in the world —is the curious little Australian pitcher plant Cephalotus follicularis, which occurs only in a small area not far from the capital of Western Australia. It is by far the smallest and least impressive of all the pitcher plants, yet is of some beauty, and also of morphological interest in pos- sessing at once ordinary leaves and well-formed pitchers between which no gradations normally exist. In the large collection of pitchered and other insectivorous plants in the Edinburgh Botanic Garden the late Professor Dickson (than whom these interesting forms have never had a more keen and thoughtful student) was able to collect and figure an interesting series of the monstrous leaves which occa- sionally arise, and thus to show that the pitcher is but a specialised modification of the ordinary leaf, a first em- bryonic dimple near the point deepening backwards and downwards into a pouch, the lid thus arising on the side I Pitcher Plants ts of the pitcher orifice originally nearer the base of the leaf. The histological details of the pitcher are of interest, and of exceptional beauty of colour. The Pitcher Plant proper (Nepenthes).—From this solitary and tiny Australian rarity we may now pass to the abundant and magnificent pitcher plants proper, the genus _ Nepenthes, of which not less than forty species are described in Dr. Macfarlane’s recent excellent revision of the group. They are widely scattered over the Oriental tropics, with their headquarters in the hotter regions of the Malay Archipelago, but thence range northward into Cochin China, southwards into North Australia, and westwards into Cey- lon, Bengal, and even Madagascar. ' In all the species the pitcher is borne at the end of a long tendril-like prolonga- tion of the leaf, and is not only of very beautiful form but great size, varying from an inch to a foot or more in depth. Two varieties of pitcher occur in many species, the first, associated with the lower leaves and developed during the younger state of the plants, are not uncommonly found actually resting on the ground. This form is short and broad, provided with broad, external, wing-like prolonga- tions, up which ants and other ground insects readily make their way to the lip of the pitcher. The adult and more abundant form of pitcher is longer and narrower, with the external wing-like appendages less strongly developed, or it may be even absent. The anterior (lower) surface of the lid stands well open, serving after maturity no longer as a protective cover, save that it may serve to throw off rain, but apparently as an attractive surface or insect lure, being, like that of the forms already examined, more or less baited with nectar. The rim of the pitcher rewards the closest scrutiny, its surface being beautifully fluted and turned inwards and downwards, so as not only to strengthen the pitcher and keep its mouth always stiffly open, but to 16 Chapters in Modern Botany CHAP. lead the insect gently to the dangerous verge, at which the fluid contents of the pitcher come fully into view, and the glassy conducting surface can be easily reached. Dickson discovered also the apparently constant presence of a row Fic. 3.—Pitcher of Nepenthes distillatoria. A, honey gland from attractive surface of lid; B, digestive gland from interior of pitcher, in pocket-like depression of epidermis (opening downwards) ; C, transverse section of the same. of very large flask-shaped glands along the very edge of this incurved rim, and presumably of further attractiveness. The edges of the flutings are often produced downwards into stout hook-like processes, which are sometimes strong enough to retain a small bird. I Pitcher Plants 17 Scraping our microscopic preparations as before, we may rapidly note the nectar-glands of the attractive lid, the flask-shaped marginal glands just referred to, the smooth internal conductive surface, and below this the secreting surface. The former often shows small down- ward - directed crescentic ledges, while when we come to the secreting surface we find these suddenly becoming better developed and crowded, each ridge bearing below it a well-developed gland, which projects slightly, like a watch just beginning to slip out of an inverted watch- pocket. The fluid of the pitcher stands at a tolerably regular level, and so far as the insect visitors are concerned re- places the detentive surface of Sarracenia. That it is a normal and genuine secretion, and not mere collected rain, is evident from its development before the young pitcher has opened; while its analysis by Voelcker shows the presence of oxalic and citric acids, of chloride of potassium, and of carbonate of soda, magnesia, and lime. Lawson Tait again denies the presence of acid in the fluid of a young pitcher. Of much greater importance, however, is the interpretation of its nature and uses first promulgated by Sir Joseph Hooker in a memorable address to the British Association in 1874, in which he gave full details of his experiments on the digestive properties of the fluid, which he tested not only upon insects, morsels of beef, egg, etc., but even upon substances so resisting as cartilage. Lawson Tait in 1875, and subsequently Rees and Will of Erlangen in 1876; Gorup- Besanez, the well-known physiological chemist of Strasburg, in 1877; Vines, and others, con- firmed these results, and extended them by the separation of a digestive ferment, apparently identical with the pepsin of the animal stomach. Rees and Will actually found that fibrin was dissolved even more rapidly by the secretion of Cc 18 Chapters in Modern Botany CHAP. the excited pitchers than in a test experiment with pepsin from the pig’s stomach! This, it must be confessed, seems proving too much, and we shall do well to remember that most samples of prepared pepsin are far from possessing the same digestive potency, still less that of the fresh stomach, not to insist on other sources of fallacy. Still the existence of some appreciable quantity of pepsin seems obvious. Hooker and Tait have shown that fluid removed from a living pitcher into a glass vessel does not digest unless some acid, preferably lactic, be added. During the presence of food, however, they regard the pitcher as con- tinuously stimulated to secrete acid, and to keep up the supply of pepsin. Tait also separated a very deliquescent substance from the secretion of this and other insectivorous plants, which he termed azerviz. To this he ascribed digestive and antiseptic properties, and also drew attention to its remarkable power of wetting surfaces, just as glycerine or paraffin does. Placing living flies in tubes containing distilled water, Nepenthes fluid, and solution of prepared azerin, he observed that “when those in the tube contain- ing the water touch the surface they remain there as long as the water is undisturbed without ever getting completely wetted, and that they live for a very long time,—as long, perhaps, as in a perfectly dry tube. Those in the other tubes, on the contrary, will become completely wetted in a very few minutes after they touch the surface of the fluid, soon become immersed, and seldom live more than a quarter of an hour or twenty minutes.” “This must be due to the peculiar wetting property of azerin, enabling the water to enter their trachee and drowning them. This method of death can be seen in the case of flies placed upon the leaves of Drosera rotundt- folia, for they become wetted in a way which was most I Pitcher Plants 19 astonishing to me until I knew the peculiar properties of azerin. In the process of digestion this penetration of the fluid must also be usefuk” Secreting Glands as Nectaries.—So far we have been looking at the glands of the Nepenthes pitcher as peculiar organs special to their position; but soon after the re- examination of the pitcher by Professor Dickson, which resulted in the discovery of those curious marginal glands (which we may view as the highest specialisation of the gland structures of the lid and pitcher—perhaps, indeed, of the secreting gland yet known in the vegetable kingdom), Dr. Macfarlane made an interesting step towards the determination of the less specialised organ to which the whole of these péculiar structures may be referred, and of which they may be considered developments and modifica- © tions. Closely examining the whole plant, he noticed that “not only is honey secreted by the inside of the lid and the mouth of the pitcher, as we already knew, but the outer surface of the pitcher, as well as that of the lid, also possesses honey glands. Further, the whole so-called ‘leaf,’ or expanded lamina, including the thong-like pro- longation of the midrib, to the end of which the pitcher is attached, may be regarded as a complete insect-lure, seeing it also is found to be studded with honey-secreting glands, thus presenting to unwary insects a long but pleasant passage to the cavity of the pitcher below. The stem, too, was found to possess glands for honey secretion —in some species to a greater extent than in others.” The curator of the Edinburgh Botanic Garden drew Dr. Macfarlane’s attention to the viscid nature of the fluid secreted by Nepenthes when flowering, and it was found that this also was a honey secretion, and glands were dis- covered to be present on the upper epidermis of the sepals. Dr. Macfarlane then made a minute examination 20 Chapters in Modern Botany CHAP. I of the other three genera of pitchered insectivorous plants at present in cultivation—viz. Sarracenia, Darlingtonia, and Cephalotus—with the result that substantially the same condition of things was found to subsist in them all. ‘The pitcher-plants may thus be regarded as ingenious mechan- isms for first attracting insects, in order to receive their aid in fertilisation; and next, for the capture of these insects, and their subsequent appropriation for purposes of nutrition.” These are in fact the ‘‘extra-floral nectaries ” well known in many plants, and which the reader may most conveniently learn to know by looking for them on a shoot of cherry laurel. CHAPTER II PITCHER PLANTS—continued Use of Pitchers, ** Bionomics’’—Bionomics—Bionomics of Nepenthes —Morphology of the Pitcher —Bladderwort—Bionomics of Bladderwort—Allied Forms. Use of Pitchers, “Bionomics.’— The view of the economy of the Nepenthes pitcher held more or less strongly by some older naturalists, that this was a benevolent provision of nature to comfort the weary traveller or refresh the thirsty bird, had of course given way; not so much before the distributional fact that these plants inhabit wet places in tropical forest thickets (where even if travellers were wont to pass, they with the birds would not need to seek so far for water), as from the general decay of this cheaply optimistic teleology. Yet so habitual was this way of look- ing at things that we have even had in more modern times the pitchers of Sarracenia and Darlingtonia described as caves of refuge supplied by a benevolent Providence to conceal insects from their pursuers. Some better explanation was needed, and the new one, in terms of that grim and all-pervading struggle for existence, which naturalists were learning from Darwin and the times to substitute along the whole line for the old-fashioned ‘‘ harmony of nature,” could not but at once arrest attention and quickly win its way to acceptance and 22 Chapters in Modern Botany CHAP. approval, the more so because of its new and dramatic form, the plant, usually the passive prey of the animal, turning the tables and making the animal its victim. A great step was thus made towards realising that view of nature, that physiology not of the machinery of the indi- vidual merely, but of species in their relation to all the life around them, which it is probably the very greatest of all Darwin’s services to have put before us in so many of its scenes. This is what many old writers meant by “ Natural History,” and what too many modern German authors un- fortunately confuse with the well-established general name including all the fields of organic science as ‘ Biologie.” Semper therefore prefers to speak of this his favourite study (see his Animal Life in International Science Series) as the ‘physiology of organisms,” of course in distinction to the physiology of organs. Mr. Wallace terms this the ‘‘higher’ physiology,” while Professor Ray Lankester has suggested the convenient term of bionomics. The last term has many advantages, not the least being that its very sound and form helps us to realise its meaning as expressing the economics of each of the innumerable species with which we share the planet. It is, we trust, likely to come into general use, and to supersede the vague or confused terms above mentioned. Bionomics.—It is important clearly to distinguish in the ’ work and influence of Darwin the various elements ; since putting aside altogether his evolutionary theories, his work in thus reopening the study of natural history in its widest aspects, of constituting Bzonomics as Cuvier did Compar- ative Anatomy or Paleontology, or Linnzus Taxonomy, must always remain of the first magnitude. It is thus worth a little time fully to realise this. The child at first delightedly watches the bees and butterflies upon the flowers; grown a little older he hunts and kills; and II Pitcher Plants 23 by and by, when the civilised ‘‘mania of owning things” has arisen, he collects. At school and college he learns to name and to analyse; normally too, alas! to forget all discontent though grammar be substituted for literature, and form in all things for life, and though every outdoor aspect of nature be forgotten during a whole youth © wasted in imprisonment between the whitewashed school- room and the ball exercise-yard of his school. Thus prepared, circumstances may make him “a _ naturalist” again, but now with a difference from his childish starting- point. His first impulse will be to seek his accounts of nature in books and to comment on his predecessor, as naturalists did all through the middle ages, and as most of us do too much still; if he go beyond this it is in the first place to make a collection; especially as he has here the lucid logic, the consummate academic discipline handed on from Linnzeus to guide him, and so he becomes a systematist—it may be a Bentham or an Agardh; but if so, concentrating himself on his herbarium, group by group, leaving the insects to their keeper over the way in the Zoological Museum. Or a later medical education (itself of course deeply influenced from the schools) may dominate the preliminary one, and thus we get an anatomist—it may be an Owen or a Huxley—and so far indeed our contemporary university and school presentation of natural science has now actually come—witness the accepted text-books (excellent of course from their point of view) of ‘Elementary Biology,” “Practical Botany,” and their various sources and imita- tions. But all this time we have been taking no sufficient note of Darwin. He, happiest as well as greatest of - naturalists, has gone straight through school and college, but obviously with but an irreducible minimum of their result upon him. Still fresh from the gardens and wood- lands of childish and boyish home, he passes to his 24 Chapters in Modern Botany CHAP. “‘ Naturalist’s Voyage.” Year after year, he watches for himself the drama of organic nature, sees it more fully and more deeply than ever great naturalist before had the good hap to do; and so returns to use indeed the museum and the scalpel as well as another, but always as a mere means to an end—that of watching the organic drama, scene by scene, and if it might be deciphering the inner mechanism of the plot. For him, as for not a few pene- trating predecessors, the plot is Hvolution (whatever that may mean), and his special interpretation of its mechanism is his world-famous “theory of natural selection,” at which we have already glanced, and to which we shall come more fully by and by. Now it is plain that this reading of the drama of the universe neither began with Darwin nor can end with him; it is indeed at the very outset frankly to be admitted as one of the purposes of this little book to help the reader towards getting beyond the Darwinian theory of the progress of nature; yet all the more must it be insisted on, not only that we appreciate clearly what that theory is,— and this, of course, in no mere literary fashion,—but as an actual seeing of nature, scene by scene, as it appeared to Darwin’s eyes: this too not merely for the general theo- retic interest, much less the special controversial one just hinted at, but for its intrinsic interest as well—indeed first and foremost. At the drama of evolution mankind are but awakening spectators; here is one who, even if we put aside his general interpretation of its nature and mechanism for the moment altogether, we should still have to appeal to as not only the most patient but the most penetrating of observers. We cannot have, it is true, too full a list of the kinds or “‘ species” of the innumerable and strangely varied dramatis persone, we cannot look too closely into their corpses as they fall, else we shall fail to understand much ; if we dry or pickle these sad remains they will be of II Pitcher Plants 25 use for reference. Even more useful is it to excavate ancestral tombs, and bring out their fossils; indeed none has set us a better example than Darwin himself in all these very ways. But not the smallest living scene—not even this bee upon its flower—is to be understood from our museums and _ her- baria: for this exhaustive division of labour, with its ento- mological and botanical specialists, in winning extension of exact and detailed special knowledge, had lost sight of de- veloping that vague general knowledge with which childhood begins. Watching the bees among the flowers is an old and happy occupation, not only for children but their elders, and many a writer, from the prosaic economist up to the master of all poets, has long ago said his say; their points of view, however, were alike always too impressionist or too anthropomorphic in standpoint to contribute anything to- wards exact natural science, and so mummy-labelling and shelving went on indoors undisturbed. Once only a hundred years ago a childlike old German botanist went out into the garden and watched summer after summer, till he saw what the bees were doing, all unconsciously, to the flowers, and learned how they and the flowers were fitted one to the other in every detail of form like hand and glove; and when he was sure of his facts he could keep the secret no longer; he noted down everything that he had seen, and this too with excellent drawings, calling his book, in naive childlike delight and pride, Zhe Secret of Nature Dis- covered / But the botanists indoors would not look at his book, save at most to say—-What nonsense! what childish fancies! what waste of time! So it was soon forgotten, and lay for a century unnoticed, until a naturalist, not con- ventionalised in the museums, nor over-educated for his intellect, but persistently childlike in questioning and watching, and watching and questioning again, should 26 Chapters in Modern Botany CHAP. II come once more. And so Darwin wrote Ox the Fertilisa- tion of Orchids, and Hermann Miiller, Hildebrand, Kerner, Delpino, followed suit ; while MacLeod and a whole younger generation are following these nowadays. Bionomics of Nepenthes.—It is time to return from the history of bionomics in general to our special scene; that of Nepenthes, luring and entrapping its flying and creeping insect prey. We may now group around this some of the minor incidents which naturalists have gradually described. Thus a recent traveller in Borneo descants upon the superior intelligence of certain ants, who refuse to be inveigled into the pitchers, and succeed in drinking its fluid contents, rich with the sapid juices of their less wary congeners, by piercing and sucking the tendril-like stalk upon which the pitcher hangs. He or another even credits them with knowing that water rises to its own level, and so with taking care not to pierce the stalk higher than the level of the fluid in the adjacent pitcher ! It is a good story, and constructed on excellent, one may almost say standard, lines; the sceptical reader may wish, however, to know whether the ants, however, were not simply licking up the sugary exudation of those glands which, as above mentioned, were left to Dr. Macfarlane to notice outside the pitcher, and which, especially in a species so carefully treated by the ants, might fairly be expected to be more abundant as one descended towards it. Be this as it may, we owe to the same observer another interesting picture, that of the odd little lemur (Zarszus spectrum) prowling over the Nepenthes pitchers, fishing out with its long-clawed fingers their insect contents, and confiscating them to its own use. One species, however (JV. dzcalcarata), he tells us, gets the better of the Tarsius, repelling, and if need be punishing, the robber by help of a pair of long strong prickles which grow from the lower side of the base Fic. 4.—Pitcher of Mepenthes bicalcarata showing downward-directed prickles. (After Burbidge.) 28 Chapters in Modern Botany CHAP. of the lid downwards into the pitcher opening, their points ending just where the intruder would naturally insinuate its neck. And so on, as with Sarracenia, we see that nature’s scenes are like Shakespearian ones: around the main in- cident (or what we take to be that) there may be grouped all manner of by-play, here quaint or picturesque, or again laden with deadly issues. Morphology of the Pitcher.—Baillon, and indeed first of all Linnzus, pointed out that by exaggerating the con- cavity of a (peltate) leaf like that of the Water-lily (Vym- ~hea) we might obtain a pitcher like that of Sarracenia. Baillon has described intermediate forms — incipient pitchers—exhibited by a variety of Peperomia arifolia, a plant allied to the Peppers, and without going so far afield we may see exhibited nearly every year at meetings of botanical or horticultural societies specimens of monstrous pitcher leaves in cabbage, lime, and other plants, where elongated stalk or enrolled leaf forms a well for the rain- drops. A recent writer describes such a pitcher upon a leaf of vetch (Vicia sepium), of which he ascribes the origin to the puncture of an insect. Heckel’s theory of the pitcher of Sarracenia is that it re- presents a hollowed leaf-stalk, the lid corresponding to the blade of the leaf. However this may be, the interpretation already given of the Darlingtonia pitcher as a further develop- ment of that of Sarracenia of course remains unaltered. Many authors have been wont to regard the broad leafy portion of the Nepenthes as but a marginal expansion of the lower portion of the leaf-stalk, the tendril being its upper portion, and the pitcher thus corresponding to the whole leaf, pouched as we have seen is actually the case in Cephalotus. They trace the margins of the leaf in its ex- ternal wings, and point out the little threadlike tip of the leaf just behind the lid. II Pitcher Plants 29 For Hooker the tendril is a simple prolongation of the leaf such as we see in various leaves, ¢.g. the lily-climber Gloriosa, while he describes the development of the pitcher as a simple dimpling and deepening of the upper surface near the extreme tip of the tendril, which survives as the mere rudiment already mentioned. He explains this strange development as finding its possible initial rudiment in those water-secreting glands common at the tip of so many leaves, an apparatus which the reader may see at work in the drop which often hangs at the leaf tip of his white ‘ Lily of the Nile” (Azchardia africana) ; still better, in many greenhouse arums; or, best of all, on the dew- gemmed leaves of Lady’s Mantle (Alchemilla vulgaris) in a summer morning’s walk. Applying our histological experi- ence, too, we may prepare excellent microscopic specimens of these water glands from the leaf tips of Saxifrages or of Indian Cress (Zvof@olum) by carefully scraping away the lower epidermis and parenchyma upon a glass slide in a drop of water, and then turning over the remaining epi- dermis to show its upper surface. Partly from his study of the pitcher leaves of seedling Nepenthes, which appear immediately after the cotyledons, and in which the pitcher seemed to him to develop from _the first as a much more important portion of the leaf, Dickson was led to give up this doctrine of Hooker’s. The curiously ‘‘interrupted” leaves of some Crotons (C. zufer- ruptus), in which the flat portion, the intervening midrib, and even the pitcher in a rudimentary form, are all present, the latter as simply the upper third of the leaf, seemed to him to afford the key to the difficulty ; the detailed develop- ment of the pitcher as a leaf pouching seeming to him essentially similar to that of Cephalotus, above mentioned. Dr. Macfarlane’s conclusions as to the nature of the pitchers are very different. He believes that in Nepenthes, Heliam- 30 Chapters in Modern Botany CHAP. phora, Sarracenia, and Darlingtonia alike, the pitcher is developed from what is originally a compound leaf, con- sisting of from two to five pairs of leaflets. But there is a marked tendency to dorsal fusion of these leaflets from apex to base. Such fused leaflets are seen in the broad basal part of the Nepenthes leaf, and in the flaps and lids of the various pitchers. The pitcher itself is a deep dorsal involution of the midrib just above the termination of the fused upper pair of leaflets, except, indeed, in Cephalotus, where, as Dickson clearly showed, it is an involution of the leaf blade. Professor Bower by no means agrees with Macfarlane. He interprets the lid of Nepenthes as composed of a single pair of leaflets fused together; on the other hand, the lid of Sarracenia as merely the flattened terminal portion of the modified leaf. Goebel lays stress upon the scantiness of the evidence upon which both these ingenious rival theories of the com- plex origin of the pitcher have been erected, and believes that the structure of all the pitchers is very much the same, that all may be derived from a peltate leaf in which a deep involution of the upper surface has occurred. As to the side wings, in which some see the vestiges of leaflets, he regards them as entirely secondary growths. We have cited all these opinions—and_we might have given others—just because in their puzzling divergence they illustrate the difficulty, yet fascination, of morpho- logical studies. It is not important to the student to “ get up” this doctrine or that; indeed the teacher may with advantage postpone or even refrain altogether from express- ing his own judgment; what really is important is that the student should know how such a question is asked and answered—partly by a study of the actual form alike in its obvious and in its microscopic structure; partly by com- I Pitcher Plants 31 paring the form in question with that of nearly related plants ; partly by observation of the young plants and their gradual development; partly by attention to those mon- strosities which often reveal the secret of strange structures. It is only when the student has learned to place himself at the standpoint of several distinct theories, and to state and weigh these impartially, that he becomes able to give his adherence to one or other view. Nor can he fully do. this without reading for himself the original papers, and perhaps reinvestigating the subject for himself after all. Bladderwort.—Our tour round the world in search of pitcher-plants may find appropriate completion in the dis- covery of one not less interesting almost at our own doors, In marshy lochs and mountain tarns the Common Bladder- wort makes itself conspicuous for a month or two in summer, when from the floating stem the flower-stalk rises bearing quaint bright golden blossoms, somewhat orchid- like in appearance, though really akin to the primroses, which are commonly considered to represent the simpler regular ancestral form, much as do lilies to orchids, or potato- blossom to foxgloves and snapdragon. At other times the plant is not so readily seen, for it floats in the water, and its leaves are small. Like some other aquatic plants the water bladderworts have no roots, and the straggling stem bears numerous, much-divided slender leaves. Among these are hundreds of little bladders. From the main submerged stem of U¢ricularia vulgaris, and yet more markedly in tropical species, peculiar thin shoots, which Geebel calls ‘‘ aerial shoots,” rise to the surface, and bear leaves slightly different from those on the other parts of the stem. It seems likely that this part. of the plant is of special use in effecting interchange of gases with the air. Each bladder—shown by its mode of development to be 32 Chapters in Modern Botany CHAP. a modified leaflet-—is a simple but effective trap. It is a hollow chamber, about 4, of an inch in length, entered by a thin transparent door or valve which opens inwards only and allows of no egress, for it shuts instantly, as if with a spring, against an anterior thickened collar or pro- jection around the mouth. | These traps are very fatal to small Crustaceans, popu- larly known as water-fleas, which swarm in every fresh- water basin. Pursued by their enemies, or attracted perhaps by a slight mucilage which is exuded from glands on the door of the trap, or prompted it may be by wayward curiosity, the water-fleas clamber on six or seven long stiff bristle-like processes which project from the mouth of the bladder. So far they are safe enough, but if they explore farther, and push before them the inward-yielding door of the bladder, they are within a prison from which there is no escape. Fora day or two they may live, but the trap becomes crowded with prisoners, and they die. No diges- tion occurs, but the bodies of the animals are decomposed by Bacteria, and the products of decomposition are ab- sorbed. That these products are useful to the bladderwort is confirmed by Biisgen’s observation that those plants from which all water-fleas were artificially excluded did not flourish so well as those in normal conditions. Darwin believed that the four-fold hairs which occur abundantly over the internal surface of the bladder were absorbent structures, but this is by no means certain. Indeed these peculiar hairs are connected by intermediate forms with the slime-secreting hairs found outside the bladders, for instance on the door. It is worth noting that such hairs are to be found in many aquatic plants quite innocent of insect catching; as, for instance, on the leaves of Cad/z- triche verna. Chodat, and after him others, have shown that they arise from cells which under ordinary circum- I Pitcher Plants 33 stances would have formed the stomata of an aerial leaf. (See Chap. IX.) The seasonal life of the aquatic bladderwort is interest- ing. Throughout the summer it floats on the surface of the water, and the straggling stem grows at one end as it dies away at the other. Perhaps too many decomposing Crustaceans, however good for the growth of the plant, may not be altogether good for the leaves which most directly receive this liberal manuring. As the summer ends, and as the water-fleas cease to swarm in the pond, the life of the plant becomes concentrated in a thick-set terminal tuft. This, as in some other aquatic plants, sinks to the bottom of the pond and passes the winter there. In spring, lightened perhaps of what stores of reserve material it contained, the stem rises again, and forming a fresh set of bladders begins to grow vigorously at the surface. The older botanists, eg. A. P. de Candolle, believed that the bladders were floats for the plants; at first they were filled with mucus, and the plant rested at the bottom ; afterwards they were filled with gas, and the plant rose to the surface. \ This was a pretty notion, but not true. The plant will float without any bladders, or when all are full of water ; the bladders have really nothing to do with the floating. Bionomics of Bladderwort.—It may be that the water- fleas enter the bladders in search of Infusorians and other small creatures on which they feed; it seems likely, too, that the bladderwort does really profit by the capture of the water-fleas. There is another strand in the web. Amid the Utricularia one commonly finds certain water- spiders, who make for themselves a diving-bell with air which they carry from the surface, their bubble glistening like silver as they descend. Have not these clever crea- tures come to recognise that the bladders of Utricularia D 34 Chapters in Modern Botany CHAP. are so many larders, and do not they rifle them? Thus, as in former cases, there would be a play within a play. Allied Forms.—Not all the species of Utricularia, how- ever, are aquatic ; some, especially in the Tropics, are terres- trial plants. In these, though the booty is of course different, the bladders sometimes borne by the creeping underground stems may capture small terrestrial animals, larval earth- worms, centipedes, and the like, much in the same way as the aquatic species do. The same is true of an allied terrestrial genus (Polypompholix), The terrestrial bladder worts usually live in damp places, but some are perched on the mossy stems of trees. For each kind of habitat there are special adaptations: the aquatic forms have sometimes air-reservoirs which act as buoys for the upright flower-stalk ; the epiphytes often have water-reservoirs which enable them to survive the dry season; the ter- restrial species, though rootless like all the others, have little processes or rhizoids which descend into the ground, especially at the base of the flower-stalk, and serve to steady the latter as well as to absorb water. Allied to the Utricularia there is another rarer insecti- vorous plant, Genlisea, which is represented by several species from Brazil, Cuba, and Angola. It lives in marshy places, and, like the bladderwort, is rootless. The stem rises upright from the ground, and is thickly beset with leaves, most of which are spathulate, while others are modi- fied into strange twisted descending staircases, These are long-necked and lined with downward-directed hairs, which at once aid an animal in its entrance and prevent its retreat. Pinguicula, Utricularia, and Genlisea all belong to the same order (Lentibulariaceze), and may be grouped, as Goebel points out, in a series. ‘‘ We regard,” he says, ‘forms like Pinguicula with a rosette of slimy leaves and a central flower-stalk as near the starting-point of the II Pitcher Plants 35 series. From this Genlisea has diverged in one direction, Utricularia in another. That they have lost their roots is not to be wondered at, for they are in great part aquatic plants. Genlisea diverges but little from the hypothetical ancestral form; it has indeed very remarkable bladders, but it retains the rosette of leaves seen in Pinguicula. This is still represented in the terrestrial bladderworts, but here there are most marvellous modifications of leaves which obliterate the distinctions between leaf and shoot. In the aquatic bladderworts the terminal floral axis of the seedling is suppressed, but there is still a rosette of leaves, though often only in rudimentary form. The strength of the development lies in the floating shoots, which are homologous with leaves.” Before we leave the pitcher-plants and bladderworts, we shall simply notice another strange plant—the Scaly Tooth- wort (Lathrea sguamaria), which is sometimes found in our woods. It is a parasite on the roots of trees and shrubs, and being without chlorophyll looks wan and strange. We shall return to it in a subsequent chapter, but it is of interest here to notice that its underground toothlike leaves are not only solidly thickened stores for the food-reserve appropriated from their hosts, but contain small hollow traps in which many kinds of small terrestrial animals are ensnared. The underground buds of Barista alpina show a somewhat similar structure, and also imprison minute animals. It is interesting also to note that these Bartsias with some of their nearest allies, like the pretty euphrasy and the curious yellow rattles and louseworts,! and one or two others, are all parasites upon other plants as well, their roots sucking those of grasses; while through them, as we Shall see later, we pass to true parasites. 1 Euphrasia officinalis, Rhinanthus Crista-galli, Pedicularis sylvatica, and P. palustris. CHAPS EE tH OTHER INSECTIVOROUS PLANTS—DIFFICULTIES AND CRITICISMS Fly- Traps (Dionea and Aldrovanda)—Sundews and Birdlime Traps— Butterworts —Sundews proper (Drosera) — Detatls, Functional and Structural—Digestion—Movements—Absorp- tion—Utility—Other Insectivorous Plants—Legends— Diffi- culties — Further Difficulties and Criticisms— Direction of further Investigation ; Possible Compromise. Fly-Traps (Dionza).— Besides those insectivorous plants which we have already studied under the general title of - “pitchers,” there are others more active in insect-catching, which we may call “ fly-traps.” Of these the most famous, cynically nicknamed ‘‘ Venus’s Fly-Trap” (Dionea musct- pula), grows in damp places in the east of North America, occurring in very local distribution in North and South Carolina, especially near the town of Wilmington. It was the first of the insectivorous plants to attract atten- , tion, for in 1768 Ellis, a London merchant, but a shrewd naturalist withal, who discerned the animal nature of coral, sent a description of the plant to Linnzeus, who in his enthusiasm called it “ szraculum nature.” But he sup- posed that the insects were captured accidentally, and subsequently allowed to escape. The Venus Fly-Trap, like its allies the Sundews, grows on the wet moorland. A circle of more or less prostrate CHAP. III Other Insectivorous Plants ay leaves surrounds the base of a flower-stalk which bears numerous flowers at a height of four to six inches from the ground. Each leaf is a fly-trap. The broadly flattened or winged (sfathulate) stalk is constricted to the midrib at its junction with the bilobed blade, the halves of which are movable on one another along the middle, closing together with a snap, as a very tightly-bound book will sometimes do. Around each margin are twelve to twenty long teeth, and, when the leaf closes, those of one side interlock with those of the other, thus forming a very perfect miniature rat- trap. The centre of each half-leaf bears numerous rosy glands, and’on each side there are three hairs, which an old naturalist described as spikes to impale the captured insect, but which are really quite weak, and bend flat on a basal joint when the leaf closes. The student will find it easy and useful to make .a paper model, cutting it out the proper shape, folding it along the middle line, carefully fashioning the teeth at each margin, so as to interlock neatly, and gumming on the three hairs on each half-blade, or more simply snipping them from the texture of the paper. The glands may easily be supplied with a red pencil, and a toy, not to be despised by young or old, is the result. Let us see how the trap works. If we go—preferably on a warm and bright day—to our plant, which is common in green- houses in this country, and with a finger touch the leaf the closure follows. More careful experiment with anything nearer the size of the insect’s leg, say a pencil-point, shows that we may wander all over both surfaces of the leaf or the marginal tentacles with safety until we touch one or more of the projecting hairs ; and repeated experiment shows them to be alone sensitive—the two halves of the blade close. In the plant’s native haunts some insect does what our finger did, and if in the rapid closure of the leaf the prey be 8 Chapters in Modern Botany CHAP. ies) caught, a secretion from the rosy glands immediately follows, and the leaf remains closed for a week or two according to the size of the insect. During that time the leaf acts as a temporary stomach, and the insect is digested and absorbed, as far at least as can be expected, and then the leaf reopens, but remains for a time in a torpid state. Sometimes, however, if the insect caught happen to be a very large one, the leaf never opens again, its meal proving too much for it; and even ina state of nature the most vigorous leaves are rarely able to digest more than twice, or at most thrice, during their life. The attractive rosy patches on the leaf, the rapid closure of the blade on its midrib, the interlocking of the teeth around the margin, the specialised sensitiveness of the six jointed hairs, the copious secretion of the digestive glands, combine to make Dionza a very efficient fly- trap. The secretion poured out from the stimulated glands contains formic acid and a digestive ferment. It further resembles gastric juice in having marked antiseptic quali- ties ; thus when Lindsay fed leaves with such quantities of meat as to kill them with indigestion, the meat inside remained fresh while portions hanging outside putrefied. So abundant is the secretion that when Darwin made a small opening at the base of one lobe of a leaf which had closed over a large crushed fly, the secretion continued to run down the footstalk during the whole time—nine days— during which the plant was kept under observation. That absorption follows digestion is shown by the disappearance of the digestible substances, and Fraustadt was able, by feeding leaves with albumen stained with aniline red, to colour the contents and nuclei of the gland-cells. The three pairs of hairs are exquisitely sensitive to the contact of solid bodies, but are indifferent to wind and III Other Insectivorous Plants 39 rain. The triangular area between the bases of the hairs is slightly sensitive, and if the general surface of the leaf be wounded closure may occur. Inorganic or non-nitro- genous bodies placed on the leaves without touching the hairs do not excite any movement, but nitrogenous sub- stances, if in the least degree damp, cause after several hours the lobes to close slowly. Leaves which have made a mistake and have closed over useless bodies, reopen after a few hours, or at most a day’s rest, and are again ready for action. According to Macfarlane, mechanical stimulus of the fly-trap requires two touches, unless the stimulus be very powerful, and the touches must be separated by an interval greater than one-third of a second. If less than one-third of a second be allowed as interval, no contraction follows, and a third touch is then necessary. As to the movement of the fly-trap, Darwin detected a measurable contraction or alteration of form, and showed that the movement follows a stimulus passing through the cellular tissue from the sensitive hairs. There are really two kinds of movement—one rapid, which follows the irri- tation of the sensitive hairs ; the other slow, excited chemi- cally, as when the leaves gradually tighten their hold on a fly and bring the glands on both sides into contact with it. Burdon Sanderson shows that the electrical conditions associated with the rest and activity of the leaf are closely like those observed in our muscles. Not only is there a normal electric current in the leaf, but at the moment of closure there is what animal physiologists know as a “negative variation,” due to the conversion of electro- motive force into mechanical work. These facts lead us to believe with Burdon Sanderson that ‘“‘ the property by virtue of which the excitable structures of the leaf respond to stimulation is of the same nature as that possessed by the similarly endowed structures of animals.” 40 Chapters in Modern Botany CHAP. Aldrovanda.—Allied to Dionza, and with a some- what similar leaf, is a water fly-trap (A/drovanda vesicu- Zosa), which lives in clear well-sunned ponds in south and central Europe, and also occurs in Australia and India. Like the common bladderwort, Aldrovanda has a thin root- less floating stem, which bears whorls of modified leaves. It dies away at one end as it grows at the other, and is reduced in autumn to a concentrated tuft, which sinks for the winter to the muddy bottom of the pond, thence to rise again in summer after it has exhausted its stores of starch and has become buoyant with gas. The little leaves have a spathulate stalk and a folding two-lobed blade with teeth round the edges. Although in figures of the plant the leaves are commonly represented as open, like those of Dionza, it is said that their lobes really open only about as much as the valves of a living mussel, and are thus the better fitted for capturing small animals. The surface bears numerous sensitive jointed hairs and colourless stalked glands, and the leaf closes on water-fleas, larve of insects, and even diatoms, very much after the fashion of Dionza. Darwin believed that the glands secrete a digestive fluid, and that small four-lobed hairs situated on the outer thinner parts of the leaf absorb decaying animal matter. Gcebel is inclined to think that these four-lobed hairs secrete some slimy substance, perhaps attractive to small animals, Sundews and Birdlime Traps.—lIt is said that Portu- guese peasants use as a Substitute for fly-paper the viscid leaves of Drosophyllum lusitanicum, a common plant in dry, sandy, or rocky places in Portugal and Morocco, and hence, it is worth noting, with a much better developed root system than its marsh-loving allies. It grows eight or ten inches high, and has long narrow strap-like leaves (which are interesting as being rolled up in the bud like those of ferns, but backwards instead of forwards), beset III Other [nsectevorous Plants AI with stalked glands which secrete a viscid dewdrop- like juice. On these leaves insects, attracted it may be by the glistening drops and by the reddish colour, but more probably by the honey-like fragrance, alight, and knocking off drops from the stalked glands become besmeared and choked. They sink on the surface of the leaf and then small unstalked, uncoloured glands exude a dissolvent secretion. On a plant a year old, which had lived in a glass house with open doors, Gcebel on one occasion counted no less than 233 distinctly visible flies, distributed over nineteen leaves. Belonging to the same order as Drosophyllum and the more familiar Drosera, there are two other sticky plants in which the “‘insectivorous habit” is not more than incipient. These are Roridula dentata from the Cape, and Byblis gigantea, the latter with simple glands, scarce differing appreciably from those on many other kinds of plants, though with a more copious and glutinous juice. They are interesting in showing the beginnings of the peculiarity which becomes so marked in the sundews, and in the same connection we should notice that leaves and stems of some geraniums, sedums, and primulas have glan- dular surfaces on which insects become entangled. Butterworts.— The Common Butterwort (Pimguzcula vil- garis) is very common on marshy grounds, especially among the hills. It has a wide geographical distribution, repre- sents a genus with about forty species, and belongs to the same order as Utricularia (Lentbulariacee). It has long been known, though not in connection with insect-catching, for Linnzeus noted that the Lapps used it for curdling milk. Every one who has tramped over high wet moor- lands or followed the banks of a mountain stream up into the hills know the appearance of the plant,—the rosette of plump glistening leaves prostrate on the ground, the beautiful 42 Chapters in Modern Botany CHAP. violet flower raised on an upright stalk. The plump leaves, to which the plant owes its quaint name Pinguicula (= little fat one), have a characteristic fungus-like smell perhaps attractive, and are covered with stalked and unstalked glands, which exude a copious viscid secretion. This obviously catches insects, and further, Darwin tells us, also digests the small flies and midges which carelessly allow themselves to be limed. To the touch of other things, raindrops or sand-grains for instance, the butterwort is indifferent, but a little insect provokes abundant secre- tion. But this is not all; the leaf moves, slightly capable of motion as it seems. When an insect is entangled, the edges of the leaf curl slowly inwards for an hour or two, not fast enough indeed to catch an insect, but that is not neces- sary, yet sufficiently to enclose the booty, or shift it inwards, or at least expose it to the action of a greater number of glands. The result is that the insect’s body is soon dis- solved away, only indigestible chitinous shreds being left. It seems that the butterwort forms the two ferments of most gastric juice—a rennet-like ferment, to which the plant’s power of curdling milk is due; and a digestive or peptic ferment, which dissolves the usable parts of the bodies of insects. It is possible that the antiseptic pro- perties of these ferments justify the old custom of applying the leaves of the butterwort to the sores of cattle, but we should not like to commit ourselves to any such apology, since the coolness and dampness of such a plaster and its special power of keeping flies from the sores are evidently so far sufficient. Professor J. R. Green describes a rennet-forming fer- ment comparable to that of the calf’s stomach—not only in Pinguicula, but in the flowers of the yellow bedstraw (Galium verum), in the stem of the Clematis, in the petals of the Artichoke, and in other plants. A peptonising fer- III Other Insectivorous Plants 43 ment, like that of the gastric juice, occurs not only in insectivorous plants, but in situations so different as the seeds of the Vetch or the milky juice of the papaw-tree (Carica papaya). In warm countries, where of course fresh meat cannot be hung for any length of time, it is common to make a joint rapidly tender by applying to it the leaves of this tree, a practice which probably finds its explanation in this presence of ferment. There is also in some plants an emulsifying and saponifying ferment, which acts on fats and oils as the juice of the animal pancreas does; while the diastase, which, as in germinating malt, turns starch into sugar, is closely comparable to the ferment of the salivary juice in ‘animals, Although from our present physiological standpoint we have delayed mention of the butterwort until coming to the sundews, its structural relations are with the bladderworts and Genlisea. This is shown not only by the characters of the flower, but in minute details, for, according to Gcebel, the two sets of glands in Pinguicula and the slime-secreting hairs of Utricularia and Genlisea are all fundamentally the same. Sundews proper (Drosera).—Beside the butterwort on the marshy moor, we may perhaps find one of the Sundews (Drosera). The genus is a large one, and the species are widely distributed over the northern parts of both hemi- spheres. In almost all countries and languages they bear the same pretty and, so far as description goes, appropriate name—Aossolis, Sonnentau, Sundew. Our commoner British species—Drosera rotundifolia— grows loosely rooted in marshy and peaty ground, often embedded among the bog-moss which forms a fitting back- ground for the rich red colour of the leaves. These, to the number of half a dozen or so, lie prostrate, and from their midst arises a small upright stalk with inconspicuous 44 Chapters tn Modern Botany CHAP. whitish flowers. Each leaf consists of a long narrow stalk, expanded into a more or less circular blade, the edges and surface of which bear scores of club-like “hairs ” or “tentacles,’ apparently tipped with dew. These hairs are complex structures ; the head of each is glandular and well supplied with water-pipes (spirally thickened wood- fibres or ‘‘tracheides”); it is the viscid secretion of the gland which makes the apparent dewdrop. These hairs or tentacles, then, are sensitive, mobile, digestive, and absorptive—most marvellous little structures, indifferent to the drops of rain which often fall upon them, but responsive to the stimulus of a midge. An insect unwary or deluded alights on the leaf, and is forthwith entangled ; as it struggles the secretion becomes more abundant. The tentacles too bend down upon the entangled midge ; first one, and in a few minutes another, and another, till all the two hundred may close upon the prey like so many slow merciless fingers. The leaf may become more con- cave, and after complete closure looks like a closed fist. As the result of the secretion the booty is digested and the products of digestion absorbed. So far the usual general description of the sundew; but now let us take up Darwin’s /msectivorous Plants, and read up the details, for there is no more characteristic example of his patient elaborate way of working than his account of the sundew. Further Details of Functional or Structural Interest. —There are on an average about two hundred glandular tentacles. The stalk of each has the essential structure of a leaf: a small ‘“fibro-vascular bundle,” consisting mainly of spiral tracheides, runs up the centre, and is surrounded by a layer of elongated cells lined by a thin layer of colourless circulating protoplasm, and filled with a purplish fluid. The glandular head of the tentacle con- III Other Insectivorous Plants 45 tains a central mass of spirally thickened cells in immediate contact with the upper ends of the conducting tracheides. Around these, but separated from them by an intermediate stratum of elongated cells, there is a layer of cells filled with purple fluid, and outside these another somewhat similar layer. These two external layers form the really glandular part. Goebel insists that all the glands of Droseracez, whether borne on tentacles, as in the common sundews, or quite unstalked, as in Dionza, have essentially the same structure; the tentacled and the sessile forms are connected by intermediate gradations. The leaves of the sundew seem to have some fascina- tion for insects, but whether this is due to their colour, their glittering secretion, their odour, or to all three, remains uncertain. The drops are so viscid that an insect may be caught if it but touch one or two of the outer tentacles; as they begin to bend and also secrete more copiously the insect is carried inwards and more effectively smeared. Even a large insect—such as a dragon-fly— may be caught by several leaves, though in these cases it is likely that the insect was to begin with in a weak state. The bending of the tentacle takes place near its base, and may be excited in various ways. For although the plant seems to have grown accustomed to gusts of wind or drops of rain, and is to its own advantage indifferent to these, repeated touches will cause the tentacles to bend. On the other hand, contact with any solid particle, even though insoluble and of far greater minuteness than could be appreciated by our sense of touch, will induce movement, and one would think that in natural conditions movements so induced must sometimes occur, and to no purpose. A morsel of human hair, weighing only +34;5 of a grain, and this largely supported too by the viscid secretion, suffices to induce movement. Finally, the absorption of 46 Chapters in Modern Botany CHAP. even a minute trace of certain fluids, especially nitro- genous, acts as a stimulus. During the bending of the tentacle the secretion of the gland becomes more copious, and its chemical reac- tion changes from neutral to acid. Meantime within the stalk of the stimulated tentacle a strange change occurs, marked externally by a somewhat mottled appearance. When examined under the microscope the formerly homo- geneous fluid contents of the cells of the stalk are seen to have separated in purple bead-like masses, of constantly varying number, shape, and size, and suspended in a colourless fluid. ‘This change makes the layer of colour- less circulating protoplasm which lines the cells more distinctly visible. Darwin attached considerable importance to this process, which he termed ‘aggregation of the protoplasm.” It begins in the glands, and gradually travels down the tentacle from cell to cell. After the action of the tentacle is Over, a reverse process of redissolution of the protoplasm proceeds from the base upwards. Darwin believed that it was a vital process, only exhibited when the cells were alive and normal, not necessarily connected with either bending or increased secretion, and quite different from ‘‘ plasmolysis” or the shrinking of the protoplasm from the cell- wall, which is observed when parts of plants are examined in any dense fluid which induces osmosis. Darwin observed a similar aggregation in the sensitive hairs of Dionza, and in the roots of various plants, and believed that it was of wide occurrence and of profound importance in the physiology of the vegetable cell. In connection with the sensitiveness of the Drosera, one of the most interesting of Darwin’s observations was in regard to the salts of ammonia. All the salts of ammonia cause the tentacles to bend—the carbonate III Other Insectivorous Plants A7 strongly, the nitrate even more so, and the phosphate most of all. But the remarkable fact is the sensitiveness of the tentacles to infinitesimal quantities. Thus the immersion of a leaf in a solution of the last-mentioned salt, so weak that each gland could absorb only about spgq/5g07 Of 8 grain, is sufficient to produce complete inflection of the tentacles. Though the particles of solid matter which stimulate the olfactory nerves of animals, and so produce the sensation of smell in animals, must be very much smaller than this, as Mr. Darwin remarked, the fact remains truly wonderful that the absorption of so minute a quantity by a gland should induce some change in it, which leads to the transmission of a motor impulse down the entire length of the tentacle, causing the whole mass to bend, often through an angle of more than 180°, and this too in the absence of any specialised nervous system. It is not much to our present purpose to discuss the numerous experiments which Darwin made on the action of salts and acids, drugs and poisons on the leaves of the sundew ; but the effects of organic fluids are important. Darwin treated sixty-one leaves of Drosera with non- nitrogenous solutions—gum-arabic, sugar, starch, dilute alcohol, olive-oil, tea. The tentacles were not in a single case inflected. He then applied to sixty-four other. leaves various nitrogenous fluids (milk, wine, albumen, infusion of meat, mucus, saliva, isinglass), and sixty-three had the tentacles and often the blades well inflected. Finally, taking twenty-three of the leaves which had served for the first experiment, and treating them with bits of meat or drops of nitrogenous fluids, all save a few,—apparently injured by exosmose caused by the density of the former solution of gum, sugar, etc.,—were distinctly inflected. Digestion.—That the sundew possesses power of digest ing the insects which it catches is evidenced in great detail. 48 Chapters in Modern Botany CHAP. The secretion of the glands, like the gastric juice of animals, contains a digestive ferment and several acids, as Frank- land, Rees and Will, Lawson Tait, and others have shown. But this is corroborated by what happens to little pieces of organic material placed upon the leaves. Darwin fed numerous plants with roast meat and minute cubes of boiled white of egg, and by way of check placed similar cubes in wet moss. Those in the moss putrefied, while those on the sundew were dissolved. Pollen-grains had their protoplasmic contents dissolved, and seeds were usually killed. It is interesting to notice further that the secretion obtained from tentacles stimulated by fragments of glass was not able to digest, showing that the ferment is not secreted until the glands have absorbed a trace of animal matter. Moreover, while the leaves were able to digest beef, egg, cheese, and the like, they could not digest horn, chitin, cellulose, and other such substances— thus completing the analogy with the gastric digestion of animals. Movements.—As to the movements of the tentacles, experiments showed that pricking the leaf or the leaf-stalk did not induce any response, that the stalks of the glands were not stimulated by food, that in fact the glands alone were sensitive. When a tentacle receives an impulse either from its own gland or from the central tentacles, it bends towards the middle of the leaf, the short tentacles on which do not bend at all; in all other cases all the tentacles, even those of the centre, bend towards the point whence the stimulus comes. Thus all the tentacles of a leaf may be made to converge into two symmetrical groups by placing a fragment of phosphate of ammonia in the middle of each half of the blade. . Vivisection showed that the motor impulse travels through the cellular tissue, and not through the fibro- II Other Insectivorous Plants 49 vascular bundles. An impulse thus travels more rapidly along than across the leaf, since from the shape and lie of the cells, fewer cell-walls have to be crossed in a given distance. When the central glands are stimulated they send some influence outwards, which reaches the external tentacles and glands. There we can see one of the results of its arrival in the process of aggregation which descends the tentacles. To explain the actual mechanism of bending is a most difficult problem. Various suppositions have been made. Thus if we suppose the cells at the base of the tentacle to be in a state of high water-tension and to possess great elasticity, a rapid outflow of water might cause shrinkage and bending. Or it may be that the living matter of cells is contractile, like that of muscles. What then causes the outflow, z.e. how does the stimulus set it up? What meaning, again, are we to give to the ‘protoplasmic con- tinuity,” which has been traced between the living matter of cell and cell, and how far shall we grant a share in the matter to the contractility of this living network? And so we are face to face once more with perplexities to muse over; which, as they take form in definite questions, will lead the reader towards the larger treatises and to new reflection beyond their scope in turn. Absorption.—It is difficult to make precise statements in regard to the plant’s power of absorbing the digested sub- stances. Clark fed Drosera with flies soaked in chloride of lithium, and after several days found that all parts of the plants when burned showed the characteristic spectrum of lithium. But this did not conclusively prove more than that the lithium salt had been absorbed by the plant. Lawson Tait, by cultivating plants with roots cut off and leaves buried in pure sand watered with an ammoniacal solution, showed that the sundew can not only absorb nutriment E 50 Chapters in Modern Botany CHAP. from its leaves, but can actually thrive by their aid alone, if supplied with a little nitrogenous material. Bennett described, not only in Drosera, but also in Dionza and Nepenthes, what he termed “absorptive glands” lying beneath the epidermis, and sometimes furnished with papillz, which rise above the surface, Utility Relatively complete as Darwin’s study of Drosera and other insectivorous plants was, it did not adequately meet the scepticism of those who doubted the utility of the habit. Though Knight in 1818 had thought that plants of Dionza which he fed with morsels of beef throve better than others not so treated, many observers have since failed to see any improvement on insectivorous plants when regularly fed, or any disadvantage when prevented from obtaining animal food altogether. And others have gone so far as to assert that animal food was hurtful, having injured or killed their plants by feeding. But it is obviously hazardous to draw conclusions as to the utility of the insectivorous habit from plants under cultivation. For it may be that plants living in the green- house have a richer supply of nitrogenous food from their roots than they can in natural conditions secure. Sundew and butterwort very often grow among bog-moss on the moors, hardly rooted, and therefore less adapted than ordinary plants to absorb nitrogenous salts from their soil, even supposing that these were present in abundance; which, it is worth noting, recent analyses of boggy soils show they are not. Here indeed may be the grounds of a fresh argument, since this relative scantiness of nitrogenous supplies in the natural surroundings of insectivorous plants may render them in part dependent on their peculiar animal diet. Again, although it has often been noticed that a leaf of sundew or fly-trap may suffer, or even die, from the effects UI Other Insectivorous Plants 51 of too large a meal, this can hardly be regarded as a serious objection against the alleged utility of the insecti- vorous habit until we learn that the casualty is common in nature. A similar objection might indeed be urged against eating dinner. The gap left in Darwin’s work was soon filled by his son. Francis Darwin took six plates full of thriving plants of sundew, and divided off each by a transverse bar. Then, choosing the least flourishing side of each, he placed, on 12th June 1877, roast meat in morsels of about 1, of a grain on the leaves, and renewed the dose at intervals. Soon the plants on the fed sides were clearly greener than those on the starved sides, and their leaves contained more chlorophyll and starch. In less than two months the number of flower-stalks was half as numerous again on the fed as on the unfed sides, while the number and diameter of the leaves and the colour of the flower-stalks all showed a great superiority. ‘‘ The flower-stalks were all cut at the end of August, when their numbers were as 165 to 100, their total weight as 230 to 100, and the average weight per stem as 140 to 100 for the fed and unfed sides respectively. The total numbers of seed capsules were as 194 to 100, or nearly double, and the average number of seeds in each capsule as 12 to Io respectively. The superiority of the fed plants over the unfed was even more clearly shown by comparing their seeds, the average weights per seed being as 157 to 100, their total cal- culated number as 240 to 100, and their total weight as 380 to 100.” ‘“‘The fed plants, though at the commencement of the experiment in a slight minority, were at the end of the season 20 per cent more numerous than the unfed. In the following spring the young plants which arose on the fed side exceeded those on the unfed side by 18 per cent 52 Chapters in Modern Botany CHAP. in number and by 150 per cent in total weight, so that, in spite of the relatively enormous quantity of flower-stalk produced by the fed plants during the previous summer, they had still been able to lay up a far greater store of reserve material.” Similar results were independently reached by Rees, Kellermann, and Von Raumer, who used aphides as food for the plants. It is noteworthy that the beneficial effect of insect diet, although distinct in the vegetative system, is much more remarkable in relation to reproduction—a fact which ex- plains the unfavourable opinion of other observers. Other Insectivorous Plants.—Besides the apparently indubitable insect-eaters which we have described, there are some in regard to which fuller information is still desirable. Thus not only Dischidia, an Asiatic genus of Asclepiads, whose pitchers contain internal roots, Martynia, one of the Pedalinez, but Caltha dionefolia, a species of the same genus as our marsh-marigold, and several Aroids have been called insectivorous. Some South American liver- worts (2g. Anomoclada mucosa and Phystotium cochleart- forme) and a fern (Elaphoglossum glutinosum) have been described by Spruce as capturing numerous insects. The basally-united leaves of the common teasel (Dzfsacus) frequently enclose moats of water in which insects are drowned, and Francis Darwin described protoplasmic fila- ments apparently emitted by the cells of certain glands within these cups, and which he supposed to absorb the products of decomposition. A similar process has been described by Ludwig as occurring in Sz/phiwm, a genus allied to the teasel. Zopf has recently described an interesting fungus (Arthrobotrya oligospora), which catches small thread- worms in great numbers in its nooses, riddles their bodies 111 Other Insectivorous Plants 53 with a growth of fine threads (hyphz), and absorbs the tissues. Legends.—The existence of insectivorous plants was not recognised in days when fancy, ever nimbler than knowledge, was allowed to trespass unrebuked in the domain of science ; yet for that very reason our subject, which affords so many convenient types, shows also how nature-legends arise as of old, by the growing exaggeration and distortion of some real image, as it is reflected from mind to mind. Thus in a well-named medium, the Review of Reviews, quoting from “Lucifer” (but it appears not verifiably even there), we recently find this traveller’s tale and modern myth: “Mr. Dunstan, naturalist, who has recently re- turned from Central America, relates the finding of a singular growth in one of the swamps which surround the great lakes of Nicaragua. He was engaged in hunting for specimens when he heard his dog cry out, as if in agony, from a distance. Running to the spot he found him enveloped in a perfect network of what seemed to be fine rope-like tissue of roots and fibres. The plant or vine seemed composed entirely of bare interlacing stems, re- sembling more than anything else the branches of the weeping willow denuded of its foliage, but of a dark, nearly black hue, and covered with a thick viscid gum which exuded from the pores.” Hardly were “the fleshy mus- cular fibres” severed; ‘‘the dog’s body was blood-stained, while the skin seemed to have been actually sucked or puckered in spots ;” the “twigs curled like living sinuous fingers about Mr. Dunstan’s hand”; “its grasp can only be torn away with loss of skin and even of flesh;” “as near as Mr. Dunstan could ascertain its power of suction is contained in a number of infinitesimal mouths or little suckers, which, ordinarily closed, open for the reception of food.” “If the substance be animal, the blood is drawn 54 Chapters in Modern Botany CHAP. off, and the carcase or refuse then dropped.” ‘A lump of raw meat being thrown to it, in the short space of five minutes the blood will be thoroughly drunk off, and the mass thrown aside.” ‘Its voracity is almost beyond be- lief.” Just as in this story we find a magnified and dis- torted image of the actual sundew, so in the same way the sober and scientific natural history treatise of Aristotle gradually, through alternately unthinking and fanciful copy- ing, became all but unrecognisably transformed into that marvellous compendium of fabulous natural history, the Phystologus, whence herald or gargoyle-carver drew his fantastic images. In earlier beginnings we may perhaps detect the same process at work upon more of Darwin’s volumes than the one we have been discussing. Difficulties —When Darwin published his book on /7- sectivorous Plants, there were many who disbelieved on the ground that ‘‘only animals have the power of digestion.” But Morren and others more definitely soon showed the mistakenness of this opinion, for indeed all plants have digestive ferments, and many have two or three. The peculiarity of the insectivorous plant lies then in the out- pouring of the digestive juice, not in the possession of it. For even the absorption of organic material is not unique; it is exhibited by the fungi which live among rottenness and by some parasites like the dodder. How could this exudation of digestive juice begin? May we begin from glands such as those of some Saxi- frages, Primulas, Geraniums, and suppose with Darwin that the exudation of digestive fluid began with an exos- mose induced by the juices of decaying insects caught among the hairs, and that the habit once set up would be perfected by natural selection? Or may we suppose that the glands and their exuded secretion have always had and still have some meaning, apart altogether from in- 11 Other Insectivorous Plants 55 sects; that they express some more direct physiological peculiarity of the plant, and that the insect-catching is after all a minor function? Let us return to the pitcher plants, Further Difficulties and Criticisms.— How far then are we to regard the pitchers with Hooker, Darwin, and so many others, as primarily insectivorous in function, and to account for them as marvels of the perfecting action of natural selection in progressive adaptation to that strange use? Scepticism and even controversy were rife enough fifteen years ago, but gradually these diminished and disappeared, and most botanists (with whom we may apparently reckon the very latest author, Professor Gcebel, although it is to be regretted that the physiological part of his treatise has still to appear) undoubtedly accept this alike as an estab- lished doctrine of vegetable physiology and an admirable special case for the Darwinian theory, explaining every detail of elaborate structure and attractive colour in terms of that view. To this persuasion also the writer was wont fully to belong ; witness the undoubting orthodoxy of his article on ‘“Insectivorous Plants” in the Lucyclopedia Britannica (vol. xiii. 1879). Yet, as better acquaintance with the large Edinburgh collection went on year by year, his faith, it must be confessed, gradually—at first almost in- sensibly—diminished. Experiments on digestion did not come off so well as Dr. Hooker, still less as Rees and Will, would have it ; but this one at first put away as temptations to unbelief—indeed, was ashamed to speak of, for were they not more likely due to some defect in experiment or experi- mentalist, or if not, to some unlucky dyspepsia of these particular pitchers? The old criticisms of the doctrine, too, were often so unphysiological and in evolution so reactionary, it seemed incredible that they could be of any value! Timid comparison of notes with Mr. Lindsay, the curator 56 Chapters tn Modern Botany CHAP. of the Botanic Garden, than whom these plants have never had a more experienced cultivator, led to a mutual confes- sion of diminished certitude. Mr. Lindsay had often been annoyed by the deterioration of specimens of Nepenthes lent to flower shows, until it occurred to him that this might be due to the loss of the fluid in transit. He gave orders that the next plants so treated should have their pitchers refilled to the proper level on arriving at the show, and again on their return; and when this was done he was gratified to find that the plants no longer suffered. Hence then the fluid is of importance to the plant itself; the -pitcher seems a reservoir of the water of transpiration. It is hardly necessary here to recall the distillation of dew from plants by night; the gemmed leaf-tips of the lady’s mantle should be familiar to every one who has ever taken a morning stroll along a dewy lane, while every grower of hothouse arums and the like is familiar with the dropping which goes on from the leaf-tips. In 1885 Kny and Zimmermann called attention to the very considerable development in the veins of the Nepenthes leaf of cells of that spirally thickened type which is associated with the carriage of water, and speculated as to its importance for the internal water supply of the plant ; while Maury in 1887 insisted that the secreting glands of the Cephalotus pitcher were not the special and essential adaptations towards insect capture and digestion, as which they are commonly de- scribed, but mere ‘“water-stomata, which play the part of regulators of transpiration, which give off water when there is an excess, and take it up again when there is a deficiency.” Again, “the presence of fluid up to a certain level is the sole cause of the uniformity and polish of the epidermis ; one should not see in it a surface specialised as detentive for insects.” He denies the digestive agency of the fluid, finds indeed drowned insects, but also infusorians, green 11 Other Insectivorous Plants 57 algae, and zoospores all alive, and insists that if the liquid were really very digestive, these could not survive its action. He sums up that the physiological use of pitchers is a general one (associated with transpiration), and not an exceptionally specialised digestive one, as so commonly believed, and lays considerable stress on the comparatively frequent recurrence of pitchers, monstrous as well as nor- mal, among unassociated plant-forms as additional pre- sumptive evidence of their relation to some constant rather than exceptional function of plant-life. Again, M. Treub, the distinguished director of the famous tropical garden of Buitenzorg (Java), has sug- gested the internal roots of the Dischidia pitcher to be con- nected with the reabsorption of water rather than of insect- broth. Ina book written before Hooker and Darwin had made Nepenthes and its physiological analogues so famous, Grisebach’s Distribution of Plants (vol. ii.), we find much speculation on Nepenthes, apparently overlooked by subse- quent writers, from which a couple of sentences may be profitably extracted: “‘So considerable a loss of water should accelerate the circulation of sap much more strongly than would mere transpiration from the surfaces of the leaves. . . . All that the geographical distribution of Nepenthes (Madagascar to New Caledonia) suggests as to their organisation amounts to this, that they inhabit insular climates, of which the atmosphere, abundantly laden with water-vapour, impedes evaporation.” A still more serious criticism is furnished by the latest experiments on the digestion of pitchers and sundews— those of Professor Dubois of Lyons, who has not only the advantage of being apparently the only trained animal physiologist who has yet worked at the question, but also of having at his command the experience and the resources of that vast development of bacteriological science which 58 Chapters in Modern Botany CHAP. has grown up, one may practically say entirely, since Hooker and Darwin, Tait, and others carried out the experiments upon which current ideas became established. Without denying then the existence of traces of digestive ferment, such as may be prepared from all or almost all living protoplasm, be it of a seed, a fungus, or a morsel of muscle, he affirms that when the fluid of a pitcher is sterilised so as to exclude the action of bacteria, no diges- tion takes place—in short, for him such digestion and dis- solution of the bodies of insects or artificially supplied food material is simply the work of bacteria, and so comes into line not with digestion, but with putrefaction and decay. He even extends this to fly-traps and sundews. Need of Further Investigation ; Possible Compromise. —It may seem at first sight as if we were but returning to the position of the ancients, yet from either side of the dis- pute it is easy to correct that impression. Even if our Dar- winism be vain, a new explanation has come in sight—that associated with transpiration—and has to be applied in turn. For if pitchers be reservoirs, how do they operate? How shall we explain the glutinous and deliquescent “ azerin ”— say, as impeding evaporation, or even itself at times helping to draw water from the atmosphere, as the aerial roots do for an orchid? Or does it act, say on sundews, by aiding the transpiratory current, so necessary to the ordinary processes of vegetative life and growth by compelling an exosmose of water to dilute it? Here, then, a new and as yet practi- cally untried field of experiment opens out before us. In fact, despite Darwin’s volume, the whole subject of tran- spiration in sundews and other insectivorous plants, notably of course WVepenthes, has still to be experimentally investi- gated by the vegetable physiologist, before the function of pitchers or tentacles can be really understood. Yet our Darwinian interpretations will not be so easily dismissed, III Other Insectivorous Plants 59 for the movements, the increased and changed secretion of Drosera, still more of Dionza, are obviously not by mere transpiration to be explained away. And even if the stress hitherto laid upon digestion be more or less given up, and bacteria be admitted as essential factors in the process, must not, even on the new hypothesis, that of regulation of transpiration, the absorption of the soluble products of decay, be all the greater and the more regular, and the importance of insect-catching as a source of nitrogen to the plant be reaffirmed in an altered but even developed form ? Here, indeed, to the writer’s mind, we are nearing the probable solution of the case in a compromise, which may indeed give up insectivorism as the main function, yet re- instate it as a secondary one. In all the preceding descrip- tions of different scenes of the organic drama we have been noting, around what was at first described as the main action, more or less of secondary incident. And now if this ap- parently main action turn out to be itself but secondary and incidental to a deeper lying and more general action, we may be indeed for a moment confused and perplexed, but our whole interpretation settles itself anew into a richer and more complex form, to which all the preceding interpreta- tions have contributed. Nor is even this new view in turn necessarily final, yet the student-spectator has not lost his pains who can feel and say that while nature’s art is long, his time short, experiment fleeting, judgment difficult, yet In Nature’s infinite book of secrecy I can a little read. CHAPTER. IV MOVEMENT AND NERVOUS ACTION IN PLANTS Climbing Plants—Darwin’s Observations, with Summary—Inter- pretation of Movements—Movements of Seedlings—Methods of Observation— Theory of Circumnutation, WE have seen that the sundews and fly-traps have a power of movement as energetic as that of many animals, and a sensitiveness to external stimuli which in its acute- ness is not surpassed by that of our own nerves. In our study we closely followed the work of Darwin, for his researches, though by no means infallible, are funda- mental, and moreover profoundly suggestive of that living conception of nature which was so characteristic of his work. In order to gain more complete possession of his point of view—which is indeed that of Modern Botany— let us still, as it were, continue to walk in his garden and look at plants through his eyes. With this purpose we shall first take a rapid survey of Darwin’s observations on Climb- ing Plants as they are set forth in one of his volumes.! Climbing Plants.——Among many different orders of plants, and in all parts of the world, there are climbers which reach the air and the light on the shoulders of their stronger fellows, They insinuate themselves among the 1 The Movements and Habits of Climbing Plants (London, 1875). cuap.iv Movement and Nervous Action in Plants 61 straggling branches of their neighbours, or twine them- selves around the upright stems, or moor themselves by sensitive elastic tendrils to the twigs of their bearers ; thus reaching out of the crowded life of the ground herbage, or out of the darkness and closeness of the jungle, to room and fresh air and sunlight above. Darwin arranged these climbers in four grades. Rising a little above the crowd of those which merely scramble over surrounding bushes, there are the hook-climbers, such as Jack-run-the-hedge (Galium Afarine), and root- climbers, such as the Ivy. More efficient are the twiners, like the Hop (//umulus) and the Honeysuckle (Lozicera), but the climbing habit is most frequently and most perfectly exhibited by plants with sensitive prehensile organs, either leaves or tendrils. Let us consider these different kinds of climbers more precisely, recognising, however, that the classification is merely one of general convenience, for there are gradations between scramblers and hook-climbers, between creeping plants and root-climbers, between those which climb by their leaves and the tendril-bearers. The hook-climbers are least effective, being little more than scramblers well equipped with hooks which are caught up in the surrounding vegetation. Thus many brambles and roses are merely scramblers, while the New Zealand Rubus sguarrosus and a rose known as Rosa setigera may be fairly called climbers. The habit is very well illustrated by those oriental palms which are often called Rotangs (e.g. Calamus extensus), which with barbed branches in- sinuate themselves and grow up through their more vigorous neighbours. A more familiar example is the Jack-run-the-hedge, whose stem and leaves are beset with backward-directed hooks most efficient in binding the plant to the growth of the hedgerow. 62 Chapters in Modern Botany CHAP. Creeping plants, such as Ground-ivy (Vepeta Glechoma) and Strawberry, spread along the ground or up the woodside bank, sending out long shoots which are at intervals rooted in the soil. These suggest the next set of climbing plants—the root-climbers, such as the common Ivy. These ascend slowly, fixing themselves by rootlets which grow away from the light and become glued to the stems of trees or to the surfaces of rocks. We all know the little brown roots by means of which the ivy clings so closely that if you pull a piece off by force the roots often break at their origin from the stem and not from their attachment. There are many other root-climbers, such as 7ecoma radicans common in the Southern States, some species of Bignonia, and many Figs (Ficus repens, etc.) The beautiful night-flowering Cactus (Cereus nyctt- flora), often called ‘‘ queen of the night,” also affords a not uncommon transition to these true climbers ; scrambling as it does over rocks, and freely giving off at almost any portion of its surface adventitious roots which soon fix the plant, but no doubt also have a genuinely absorbent func- tion as well. The twiners, such as the Hop and the Honeysuckle, differ from those already mentioned, for as they grow their stems have a marked power of movement, bending and bowing to all sides, and thus encircling their support. Most twine in a definite direction ; thus the hop twines in a right-handed spiral (z.e. with the sun, or with the hands of a watch lying face upwards), while the majority resemble the French Bean (Phaseolus multiflorus) in winding to the left. The Bitter-sweet (Solanum Dulcamara) seems to twine indifferently in either direction, and the stem of the Chili-nettle (Zoasa) may change its direction in the course of its climbing. The next two sets of climbing plants are closely united. tv Movement and Nervous Action in Plants 63 The leaf-climbers have clasping petioles as in Clematis and Tropzolum, or hook themselves up by the tips of their leaves as in Gloriosa; most of them also revolve like the twiners, and in this way bring their leaves into contact with adjacent branches. When they are young the leaf- stalk or the leaf-tip, or even the whole surface of the leaf (in the climbing fumitory, Corydalis claviculata), is sen- sitive to contact, bends towards the side on which pressure is exerted, and thus clasps the plant to its support. The tendril-bearers, such as the pea and the vine and the passion-flower, are the most evolved climbers, for they have prehensile organs specially adapted for this function. These prehensile organs or tendrils may be modified leaflets as in the pea, or modified leaves as in Lathyrus Aphaca, or flower-stalks as in the vine, or even branches in some rare cases. The shoots of the tendril-bearers revolve as those of the leaf-climbers do, and the tendrils themselves move round and round. ‘Thus the tendrils are brought into contact with surrounding objects, to the touch of which they are often finely sensitive. They curve to what they touch and link themselves around it, after which they usually grow stronger and thicker, and by coiling into a spiral raise the plant nearer to its support. ‘Darwin’s Observations on Climbing and Twining Plants.—Having now classified the climbing plants as Darwin did, we shall inquire more carefully into what he has told us of their life. In regard to twining plants let us quote one of Darwin’s observations: ‘‘ When the shoot of a hop rises from the ground, the two or three first-formed joints or internodes are straight and remain stationary ; but the next formed, whilst very young, may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun. The 64 Chapters in Modern Botany CHAP. movement very soon acquires its full ordinary velocity. From seven observations made during August on shoots proceeding from a plant which had been cut down, and on another plant during April, the average rate during hot weather and during the day is 2 hours 8 minutes for each revolution ; and none of the revolutions varied much from this rate. The revolving movement continues as long as the plant continues to grow; but each separate internode, as it becomes old, ceases to move.” . The characteristic movement is a turning to all sides in succession. In so doing the stem usually becomes twisted upon itself, and perhaps thus gains in strength, as a rope becomes firmer as it is more twisted. The revolutions continue, though not with equal rapidity, during the night as well as during the day; the orbit described by the tip of the shoot becomes wider and wider as the shoots grow longer, and the chance of meeting with some upright support becomes greater and greater. The shoot is arrested at length by contact with a support, but the free part goes on revolving. ‘‘As this continues, higher and higher points are brought into contact with the support and are arrested ; and so onwards to the extremity ; and thus the shoot winds round its support.” There is a general uniformity in the behaviour of twining plants, but the direction and rate of revolution vary in different kinds. Thus, as we have mentioned, the majority revolve in a direction opposite to that of the hands of a watch, but many follow the sun; one shoot may make its revolution in little more than an hour, while another may take a whole day. But at present we are chiefly concerned with the general fact that these twiners do move round and round. Leaf-climbers are in their behaviour in some respects intermediate between twiners and tendril-bearers. Like tv Movement and Nervous Action in Plants 65 twining plants they show a revolution of young shoots, but with a marked tendency to change the direction of circuit; they approach the tendril-bearers in having petioles or leaf- tips sensitive to contact and able to clasp their support. This sensitiveness is often exquisitely fine, indeed it seems more delicate than the tactile sense of most animals. Thus Darwin observed a petiole responding to the exces- sively slight but continued pressure of a loop of soft thread weighing only #4, of a grain. The response is a bend- ing towards the side which is touched, and sometimes begins a few minutes after contact. After clasping has been effected the leaf-stalks become stronger and more woody, often acquiring a stem-like internal structure. It is interesting to observe that while most species of Clematis and Tropzolum are effective leaf-climbers, there are some species of more sluggish constitution, in which both the mobility and the sensitiveness of the petiole are enfeebled, or even lost altogether. Both mobility and sensitiveness reach their climax in the tendril-bearers. In the common pea the tendrils revolve in ellipses, taking about an hour and a half to complete their orbit. ‘Whilst young and about an inch in length, with the leaflets on the petiole only partially expanded, they are highly sensitive; a single light touch with a twig on the inferior or concave surface near the tip caused them to bend quickly, as did occasionally a loop of thread weighing 1 of a grain.” The long and thick tendrils of the vine—modified flower-stalks in structure— are much less active, but move from side to side, or in narrow elliptical revolutions. In both the pea and the vine, and in most tendril-bearers, the tendrils contract spirally a day or two after they have clasped some object. In this way they become apparently shorter and obviously more elastic, not only drawing the shoot nearer its support, F 66 Chapters in Modern Botany CHAP. but forming cables which do not easily snap. ‘I have,” Darwin says, “‘more than once gone on purpose during a gale to watch a Bryony growing in an exposed hedge, with its tendrils attached to the surrounding bushes ; and as the thick and thin branches were tossed to and fro by the wind, the tendrils, had they not been excessively elastic, would instantly have been torn off and the plant thrown prostrate. But as it was, the Bryony safely rode out the gale, like a ship with two anchors down, and with a long range of cable ahead to serve as a spring as she surges to the storm.” As to the more precise nature of the movement, it is enough in the meantime to notice that the whole tendril —excepting the base and the tip—is continuously curved, bending in succession to each point of the compass. On a thick tendril a line of paint may be drawn; this line, if drawn on the surface which chanced to be convex at the time, would first become lateral, then concave, then lateral, and finally convex as at first. But we should also give some illustration of the great sensitiveness of tendrils. To those of the passion-flower (Passifiora gracilis) Darwin gave the highest place. “A bit of platinum wire 4, of a grain in weight, gently placed on the concave point, caused a tendril to become . hooked, as did a loop of soft, thin, cotton thread 4, of a grain. With the tendrils of several other plants, loops weighing 4, of a grain sufficed. The point of a tendril of Passiflora gracilis began to move distinctly in 25 seconds after a touch, and in many cases after 30 seconds.” Summary.—To sum up after Darwin: the first action of a tendril is to place itself in a proper position ; if a twining plant or a tendril gets by any accident into an inclined posi- tion, it soon bends upwards, though secluded from the light, wv Movement and Nervous Action in Plants 67 the guiding stimulus being the attraction of gravity ; climb- ing plants bend towards the light by a movement closely analogous to the incurvation which causes them to revolve ; the spontaneous revolving movement is independent of any outward stimulus, but is contingent on the youth of the part and on vigorous health, which again depends on a proper temperature and other favourable conditions of life ; tendrils, and the petioles or tips of the leaves of leaf- climbers, and apparently certain roots, all have the power of movement when touched, and bend quickly towards the touched side; tendrils contract spirally soon after clasping a support, but not after a mere temporary curvature (due to pressure which is not permanent), and they ultimately contract spirally if they have not come into contact with any object. Interpretation of Movements.—But the student natur- ally asks what interpretation Darwin put upon these move- ments of climbing plants. It will be easier to answer this after we have considered what he thought of the many other movements which plants exhibit. Meantime, how- ever, a partial answer may be given. In the first place, it is plain that the climbing habit is a useful one, such as would tend to persist in nature. ‘* The advantage gained by climbing is to reach the light and free air with as little expenditure of organic matter as pos- sible.” In the second place, the habit of climbing is not an occasional freak; it is of widespread occurrence among plants. Of the fifty-nine alliances into which Lindley divided flowering plants, thirty-five, according to Darwin, include twiners, leaf-climbers, or tendril-bearers. More- over, the most different organs—stems, branches, flower- stalks, petioles, midribs of the leaf and leaflets, and apparently aerial roots—all possess this power. Not un- 68 Chapters in Modern Botany CHAP. naturally, therefore, did Darwin regard the powers of climbers as inherent in all the higher plants. In crowded surroundings, where it is of advantage to rise, some plants have retained and developed the powers of moving and feeling which are latent in all. In the third place, while the powers of climbers are often markedly influenced by temperature, by light, by gravity, and other factors in their environment, it is certain _ that these do not explain the movement, except, of course, in so far as the health of the plant depends ultimately upon the sufficiency of its surroundings. In short, the move- ments are manifestations of the internal life of the plant. But by what means within the plant are they produced? Of this in his volume on Climbing Plants Darwin says little, nor can we even now say anything very complete. This is not greatly to be wondered at, for as we have little minute knowledge as to the processes of contraction in animals where we know that these are located in definite structures—the muscles—it is not surprising that we know still less as to the movements of plants which have no specialised contractile elements that we can recognise. Sachs, Dr. de Vries, and others have suggested that the rotating movement—say of a twining shoot—is due to un- equal growth now on one side and now on another, and that this again depends on altered water-tension or tur- gescence in the growing cells. This suggestion was accepted by Darwin, although he did not believe that it could apply to those cases where rapid movement follows a slight touch. This is obviously a difficulty. Furthermore, the suggestion that unequal growth on different sides of the stem explains the revolving movements has to face the fact that the rotating movement may continue without there being any observable growth. Without denying that the altered water-tension and un- tv Movement and Nervous Action tn Plants 69 equal growth explain some of the facts, it seems to others more convenient to take refuge in the hypothesis that there are in longitudinal rows along the moving shoot certain cells which retain the power of contracting and expanding —of passing rapidly from one state of water-tension to another—and that these determine the movement of the whole shoot. For our present purpose what is especially important is that we appreciate the plant world as living ; instead, there- fore, of here prolonging our discussion of the theories held in regard to the movements of climbers, let us return to the general standpoint of Darwin’s volume, of which the last paragraph may be quoted :— “It has often been vaguely asserted that plants are dis- tinguished from animals by not having the power of move- ment. It should rather be said that plants acquire and dis- play this power only when it is of some advantage to them ; this being of comparatively rare occurrence, as they are affixed to the ground, and food is brought’to them by the air and rain. We see how high in the scale of organisation a plant may rise when we look at one of the more perfect tendril-bearers. It first places the tendrils ready for action, as a polypus places its tentacula. Ifthe tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends towards or from it, or disregards it, whichever may be most advantageous. During several days the tendrils or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and has done it in an admirable manner.” 70 Chapters tn Modern Botany CHAP. Movements of Seedlings.—We have seen how Darwin began to believe that all plants in some degree possessed the powers of movement which are conspicuously developed in the climbers. To test this opinion he and his son Francis began to watch and experiment with many kinds of plants, and the results of their study are told in the sequel to the work on Climbing Plants, a volume entitled Zhe Power of Movement tn Plants (London, 1880). As before, let us follow Darwin, and first of all in watching the life of a seedling. The seed lies on the damp ground, covered perhaps with leaves which have fallen from the trees. As water finds its way into the seed, as the life within begins to gather strength, the young root or radicle makes its appear- ance. It begins at once to move round and round. But its movements are influenced by gravity, and it bends downwards, following a more or less spiral course towards the ground. Darwin believed that “sensitiveness to gravitation resides in the tip, which transmits some influ- ence to the adjoining parts, causing them to bend.” When the tip of the root reaches the soil it bores into it, aided by the continued movement of the radicle, and this boring is easier if some soil has fallen upon the seed and fixed it, or if fine root-hairs from the top of the radicle have moored the seed to the surface. Then as the radicle grows longer its tip is forced into the soil. There it can no longer bend round and round, but it may try todo so. And sometimes the tip will reach a crevice, or it may be an earthworm’s burrow, in which it can move more freely. ‘‘ When a tip encounters a stone or other obstacle in the ground, or even earth more compact on one side than the other, the root will bend away as much as it can from the obstacle or the more resisting earth, and will thus follow with unerring skill a line of least resistance.” tv Movement and Nervous Action in Plants 71 The tip of the radicle is a very sensitive structure: “it was excited by an attached bead of shellac weighing less than 53, of a grain;” it bends towards moisture and away from light ; it always responds to the attraction of earth even when grown in entirely abnormal conditions. “If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side ; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides.” “It is hardly an exaggeration,” Darwin concluded, ‘to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals.” But the movements of seedlings are not confined to the radicle. From the seed there also emerges the young stem or plumule, almost always bent in the form of an arch, the tender tip remaining within the protecting seed-coats until the first obstacles to growth have been overcome. If the seed be buried the arch breaks through the ground, and is helped in doing this by slight movements. The young stem grows blindly but unerringly upwards towards light and air, as the root downwards towards moisture and dark- ness ; we cannot yet give any full or sufficient mechanical explanation why, though it is here only too easy to cloak our ignorance under learned nomenclature. ! As the arch grows upwards, freeing itself from the soil, the seed-leaves or cotyledons, if not already in use as store- 1 It does not seem necessary to encumber this narrative or the student’s mind at this stage with the numerous technical terms applied to movements in relation to gravitation, light, moisture, etc., since these are only too apt to correspond to no definite mental images of any kind, but be used as mere illusory explanations in terms of ‘‘in- herent properties.” See next chapter. 72 Chapters in Modern Botany CHAP. houses of food, will be raised above the ground and ex- panded in the air. Sooner or later the arch straightens into an upright shoot. Both in the young shoot and in the expanded cotyledons there is power of movement ; they are exquisitely sensitive to light and to gravitation. The power of movement does not cease when the shoot becomes a stem with leaves and branches. ‘“ If we look, for instance, at a great Acacia tree, we may feel assured that every one of the innumerable growing shoots is con- stantly describing small ellipses; as is each petiole, sub- petiole, and leaflet. The latter, as well as ordinary leaves, generally move up and down in nearly the same vertical plane, so that they describe very narrow ellipses. The flower-peduncles are likewise continually circumnutating. If we could look beneath the ground, and our eyes had the power of a microscope, we should see the tip of each root- let endeavouring to sweep small ellipses or circles, as far as the pressure of the surrounding earth permitted. All this astonishing amount of movement has been going on year after year since the time when, as a seedling, the tree first emerged from the ground.” Methods of Observation.—But how can these move- ments be seen and measured? In some of the more marked cases, as of twiners and climbers, there is no difficulty what- ever. We need only observe the position of the plant at intervals of hours and days. Or if we note the position of a growing shoot in the garden as it bends over to one side, and fix a piece of string, by means of a couple of stakes, so that it lies in a line with the shoot when looked at from above, we can, if we return in half an hour or so, plainly see that the shoot has moved through a large angle. But there are of course more delicate modes of observa- tion. Darwin gives many figures representing, e.g. the tv Movement and Nervous Action in Plants 73 path of the young stem of a seedling cabbage during ten hours. How did Darwin get this record ? ‘‘ Plants growing in pots were protected wholly from the light, or had light admitted from above, or on one side, as the case might require, and were covered above by a large horizontal sheet of glass, and with another vertical sheet on one side. A glass filament, not thicker than a horse- hair, and from a quarter to three-quarters of an inch in length, was affixed to the part to be observed by means of shellac dissolved in alcohol. The solution was allowed to evaporate, until it became so thick that it set hard in two or three seconds, and “it never injured the tissues, even the tips of tender radicles, to which it was applied.” (?) To the end of the glass filament an excessively minute bead of black sealing-wax was cemented, below or behind which a bit of card with a black dot was fixed to a stick driven into the ground. The weight of the filament was so slight that even small leaves were not perceptibly pressed down. The bead and the dot on the card were viewed through the horizontal or vertical glass-plate (according to the position of the object), and when one exactly covered the other, a dot was made on the glass-plate with a sharply-pointed stick dipped in thick Indian ink. Other dots were made at short intervals of time, and these were afterwards joined by straight lines.” Of course the result was not a picture, only a record of the plant’s path, showing nothing more than the general character of the movement. Another method, which, however, is only in a few cases practicable, is to allow the moving parts—radicles, for instance—to trace their own paths, to write, as it were, their own diary, on smoked plates of glass. Figures of these root-tracks are given by Darwin in abundance, and the student, in this case as the preceding, may profitably endeavour to make them for himself. 74 Chapters in Modern Botany CHAP. Darwin’s Theory of Modified Circumnutation.— Darwin’s. observations led him to conclude that every growing part of every plant is continually moving, though often on a small scale. And as the most prevalent form of movement is like that of a climbing plant, which bends successively to all points of the compass, so that the tip revolves, Darwin applied to it the term circumnutation. This term he explains as follows: “If we observe a cir- cumnutating stem, which happens at the time to be bent, we will say towards the north, it will be found gradually to bend more and more easterly until it faces the east; and so onwards to the south, then to the west, and back again to the north. If the movement had been quite regular the apex would have described a circle, or rather, as the stem is always growing upwards, a circular spiral. But it gener- ally describes irregular elliptical or oval figures; for the apex, after pointing in any one direction, commonly moves back to the opposite side, not, however, returning along the same line.” In this “universally present movement” Darwin found ‘the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements.” His particular thesis was that the great sweeps made by the twiners and by tendrils, the move- ments of leaves when they go to sleep at night, the move- ments of various organs to the light or away from it, and even the movement of stems towards the zenith and of roots towards the centre of the earth, are all modified forms of circumnutation, which is omnipresent while growth lasts. In regard to the two last sets of movement he believed, of course, in the influence of light and gravitation, but he regarded these as simply operating upon spon- taneous processes of circumnutation, hastening, diminish- ing, or otherwise modifying them. While maintaining this tv Movement and Nervous Action in Plants 75 theory of circumnutation Darwin acknowledged that it did not explain all movements ; he did not propose to apply it to the collapse of the Sensitive plant’s leaves when they are touched, or to the movement of the stimulated Sundew and Venus Fly-Trap, or to the movement of the Barberry’s stamens when they are jostled by an insect’s legs. In regard to the means by which the circumnutating movements are brought about, Darwin expressed himself in his Movements of Plants more definitely than he had done in the previous volume. Following several German botanists, he emphasised the importance of the turgescence or state of water-tension of the cells of the plant, and he also recognised the extensibility of the cell-walls. “On the whole,” he says, ‘‘we may at present conclude that increased growth, first on one side and then on another, is ~a secondary effect, and that the increased turgescence of the cells, together with the extensibility of their walls, is the primary cause of the movement of circumnutation.” Having stated the general thesis of Darwin’s book, namely, that most of the movements of plants are modified forms of circumnutation, and his general conception of the internal processes involved, we may next briefly discuss the movements of plants in their relation to gravitation, light, and other external influences. In so doing we shall not in- sist upon Darwin’s generalisation, for perhaps no part of his work has met with more adverse criticism, as notably on the part of Sachs, than this. It is indeed the opinion of most botanists that Darwin’s theory of circumnutation was over- strained. CHAPTER -Y. MOVEMENTS OF PLANTS—continued Movements in relation to Gravitation—Light-seeking and Light- avoiding Movements—Rationale of Light-seeking and Light- avoiding Movements — The Sleep of Plants — Mr. Francis Darwin's recent Discussion of Plant Movements—Summary and Conclusion. Movements of Plants in relation to Gravitation. —We are so familiar with the fact that stems grow upwards and roots downwards that we perhaps do not think of it as in the least remarkable that one part of a plant should persistently grow against and the other part in the direction of the acting force of gravity. Of course the student may be tempted to ask what roots would do up in the air or what stems would do down in the ground, or how they could grow otherwise than up and down, but these questions are not exactly to the point—which is this, that even when young plants are taken from their natural conditions, when they are turned upside down or grown on a rapidly rotating wheel, still the opposite tendencies of root and stem assert themselves. Let us see how the behaviour of roots and stems to the force of gravity is experimentally demonstrated. If seeds of peas and beans which have germinated in CIBAP. V Movements of Plants 74 loose soil, and have little straight radicles, be carefully removed, and suspended in the air under a damp bell-jar with the radicles pointing upwards or horizontally, in the course of a few hours the radicles will all have turned downwards. In the same way the growing shoots of plants may be placed or artificially forced to grow in a horizontal position, but if the shoot be strong enough and be not, for instance, naturally a creeper, its character asserts itself as soon as it is free from restraint, and the stem grows upwards. As long ago as 1806 Knight tried the effect of growing young plants on a rotating wheel. When an apparatus of this sort is devised, with seedlings suitably fixed on a rotating disc, the young roots always grow outwards and the young stems inwards. When the plane of rotation is vertical the direct influence of gravitation is counteracted, its direction being continually altered ; but in relation to the so-called ‘‘ centrifugal force” the roots and the stems grow in consistently antagonistic directions, the stems against, the roots in the direction of the acting force. There is a more modern apparatus called a klinostat, a clockwork arrangement by which germinating seeds are slowly rotated in a vertical plane, and as the relation of the parts to the earth’s axis is constantly changing, the direct action of gravity is counteracted ; the result being, as we would expect, that the young roots and stems grow in totally indefinite fashion. No one will suppose that in normal conditions the roots simply sink downwards passively, for they will force their way into quicksilver, and besides the stem is influenced in an exactly opposite way. It is certain that the parts of plants are influenced by gravity, but not passively; they are living organs. It seems likely that the roots and the stems, differing as 78 Chapters in Modern Botany CHAP. they do in structure, have the water-tension of their cells, and secondarily, their growth differently affected by the force of gravity; but other explanations are in the field. Some botanists go so far as to suppose a distinct stem- protoplasm and a distinct root-protoplasm reacting in opposite ways to given stimuli. Hence we see that after all the research and discussion which has taken place the subject is still far from being cleared up. Here, as in the allied or, at any rate, intermingled problem of explaining the mechanism of light-seeking and light-avoiding, the student must guard against the too common habit of find- ing an explanation in what is but a technical nomenclature, and not cease to ask himself the child’s puzzling questions of why does this stem grow up and that root grow down merely because he is told to call them in longer words “negatively” or ‘‘ positively geotropic.” For though we may all laugh with Moliére at old-world medicine, it is no easy matter to leave its wirtus dormitiva out of our minds. Darwin laid emphasis on the very tip of the radicle. “In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits an influence to the adjoining upper part, causing it to bend towards the centre of the earth.” When the tips were amputated the power was lost until a new tip was formed. Subsequent experiments do not, however, confirm this opinion, if indeed they have not disproved it. Darwin also made a large number of experiments show- ing how a radicle on whose tip some minute object was fastened, or some slight injury inflicted, bent towards the free or uninjured side, But when the stimulus is applied not to the side of the tip, but to the growing region a little above the tip, the bending takes place towards the stimulus. Vv Movements of Plants 79 In regard to such experiments, however, critics have justly pointed out that we must be exceedingly careful, with plants as well as with animals, in drawing conclusions as to normal life from facts observed when injuries are inflicted, however apparently slight these injuries may be ; and this experiment must therefore be given up. Besides being affected by gravity and by light, roots are very sensitive to moisture. They always grow towards the greater moisture, sometimes even against gravity. Thus if seeds be allowed to germinate in a sieve filled with damp sawdust, the roots first grow downwards as usual, but after they have descended through the sieve into the dry air, the direction of their growth changes, and they grow up again into the moisture. Light-seeking and Light-avoiding Movements.— Every one must have noticed that plants which have grown in the recess of a narrow window are often very markedly curved towards the light. The same light-seeking can be readily shown by experiment, for if seedlings are grown in a box which is illumined by a single aperture they all turn their young stems towards the path of the light. Some sedentary animals, such as the Serpula worms, which make for themselves twisted tubes of lime, have the same habit of bending to the light. Some of Darwin’s observations show the extreme sensitive- ness of certain seedlings to light. ‘The cotyledons of Phalaris became curved towards a distant lamp, which emitted so little light that a pencil held vertically close to the plants did not cast any shadow which the eye could perceive on a white card. These cotyledons, therefore, were affected by a difference in the amount of light on their two sides, which the eye could not distinguish.” Dar- win also noticed that if seedlings kept in a dark place were laterally illuminated by a small wax taper for only two or 80 Chapters in Modern Botany CHAP. three minutes at intervals of about three-quarters of an hour, they all became bowed to the point where the taper had been held. This convinced him that the excitement from the light was due not so much to its actual amount as to the difference in amount between that previously received. ‘‘Light seems to act on the tissues of plants almost in the same manner as it does on the nervous system of animals.” Usually the movement is towards the light, especially in the case of stems, but this is not invariably the case. Thus the shoots of the ivy bend away from, not towards the light, and so do the tendrils of the vine, of the Virginian creeper (Ampelopsis hederacea), and some other plants. We can easily understand that it is advantageous for tendrils to seek shaded recesses, for in so doing they will usually come nearer the support to which they cling, although of course this advantage must not blind us to the necessity of a physiological explanation. As most roots seek the ground their reactions to light are not readily tested, except in artificial conditions. But by means of the klinostat—the rotating apparatus which we have already mentioned—it is possible to grow seedlings illumined in one direction only, and with the influence of gravitation eliminated. Then it is seen that some roots bend towards and others away from the light. Of roots which in natural conditions always bend away from the light, the climbing roots of the ivy are the most familiar examples. Or the following simple experiment may be made, following the directions of Professor Detmer’s laboratory manual of practical vegetable physiology—a work which will be of great use to the student (and which can also be obtained in translation): A glass filled with water is closed with a lid of fine muslin; on the top of this are placed germinating seeds of white mustard (Szvafzs alba); the Vv Movements of Plants 81 whole is covered with a bell-jar and placed in the dark. The young stem grows straight up, the young root grows straight down into the water. Then light is allowed to fall upon the plant, in one direction only, through a narrow slit. In a few hours the young stem has curved towards, the young root away from the light. Darwin’s experiments led him to conclude that the sensitiveness to light was localised in the tips, e.g. of the cotyledons of Phalaris and Avena, of the young stems of Brassica and Beta, and of the radicles of Sinapis. There seemed to be a transmission of influence from the tip to the other parts, causing them to bend. “It is an interesting experiment to place caps over the tips of the cotyledons of Phalaris, and to allow a very little light to enter through minute orifices on one side of the caps, for the lower part of the cotyledons will then bend to this side, and not to the side which has been brightly illuminated during the whole time.” The commonest position of leaves and cotyledons during the day is one more or less transverse to the direction of the light, and this also Darwin believed to be due to a modified circumnutation ; but few would now agree with him in this interpretation. In some plants in which the leaflets are provided with little swollen cushions or pulvini at their base, they move upwards or downwards or twist laterally ’ when the sun shines very brightly upon them, as in the well-known “compass-plant” (Silphium) ; by directing their edges towards the light they avoid the injurious effects of too intense illumination. It is easy to understand that it is advantageous for most plants to bend towards the light, for thus their leaves are in a better position to use the power of the sunlight on which the life of the plant so much depends. On the other hand, it is also advantageous that the aerial rootlets of the ivy or the tendrils of the vine should turn away from the light. G 82 Chapters in Modern Botany CHAP. But it is difficult for us to form any conception of the reason why different plants or different parts of the same plants should be affected by the light in opposite ways. Thus the flower-stalks of the ivy-leaved toad-flax (Lznaria Cymbalaria), which so often drapes our old walls with beauty, at first bend towards the light, but turn in the opposite direction after the flowers are fertilised, and the time approaches for planting the seed in a crevice. As to the causes of the light-seeking or light-avoiding movements, in their usual forms they are possible only as long as the moving parts continue to grow. Now as light usually exerts a retarding influence on growth, the side of the plant which is shaded grows more quickly than the side which is lighted, hence the plant bends towards the light. But this will not explain light-avoiding, and so we are led to recognise, as before, that the effect of the light on the living matter of different parts of the plant or of different plants is not always the same. Rationale of Light-seeking and Light-avoiding Move- ments.—But when we inquire more precisely into the in- fluence of the light, a hundred difficulties beset us. How far was Darwin right in supposing that light simply modifies an existing spontaneous movement of circumnutation? How far are others warranted in assuming that there are in different parts of the plant specially contractile cells which ° are affected in various ways by stimuli? in other words, how far may the movements be independent of growth? Or if the movements be entirely dependent upon growth, how far is this the result of a change in the water-tension or turgescence of the cells exerting pressure on their in- ternal surfaces, as Sachs and De Vries would say; or is Klebs wholly wrong in regarding the turgescence rather as a symptom than as a cause, and in referring growth to un- known properties of the protoplasm ? v Movements of Plants 83 And even if we accept the general conclusion that the altered growth is due to a modification of turgescence in the growing cells, to what is the change of turgescence due? —to a change in the elasticity of the cell-wall, as Sachs, Pfeffer, Wiesner, and others suggest; or to a change in the osmotic properties of the cell-sap, the stimulus pro- moting, according to De Vries, the formation of substances which are osmotically active ; or to a change in the perme- ability of the protoplasm, resulting directly from the influence of light, as Vines believes? Finally, according to Wortmann, growth depends on three factors—the osmotic force within the cell, the extensibility of the cell-walls, and the relative abundance of surrounding water. Or if weturn to a recent Botanical Journal, we find Noll maintaining that curvature is due to a greater extensibility of the cell-walls on the convex side and to a diminished extensibility on the concave side— opposite effects produced by the mysterious activity of the parietal protoplasm ; and we find Wortmann denying this, saying that the curvature is due to a movement of the protoplasm which in the region of growth alters the thick- ness and tension of the cell-wells, distributing its materials so that the two sides are unequally thickened. Thus the student may see that as in regard to structural problems, such as the real nature of the pitchers of pitcher plants, so in regard to physiological problems, such as this of the movement of plants in relation to light and other stimuli, there is much difference of opinion. The reason for this is perfectly obvious—the problem is so com- plex, so many factors have to be considered. ‘The history of the investigation is, as in all other cases, one of pro- gressive analysis. That plants move towards the light in virtue of ‘‘heliotropism” is obviously a truism; that the ‘‘heliotropism” may be due to unequal growth expresses the first step in the analysis ; the unequal growth is asso- 84 Chapters in Modern Botany CHAP. ciated with altered water-tension, etc., in the cells expresses another step, andso on. The problem is to discover all the observable conditions of the movement before finally con- fessing that what remains unexplained is due to some still unknown property or power of the living matter or protoplasm. This above all the student should recognise that the plant is in all its actions no mere mechanical system, completely explicable according to the facts of mechanics, hydrostatics, and the like, but a living organism. And just as no one will pretend to give a ‘mechanical explanation” of how a Serpula worm makes its calcareous tube towards the light, nor pretend to be content with calling the animal “ heli- tropic,” so we must avoid both extremes in regard to plants. The Sleep of Plants.—Every-.one has noticed how the three leaflets of the clover and the wood-sorrel change their position as the light of day grows and wanes; they are expanded during the day, and fold downwards in the even- ing. Many may have observed similar movements in Lupine and Melilot, Mimosaand Acacia. Indeed these movements are very common, and were noticed by early naturalists such as Pliny. Since Linnzus wrote his famous essay called Somnus Plantarum they have been spoken of as the sleep of plants. The movements are sometimes very marked, changing the whole appearance of the plants; thus “a bush of Acacia farnesiana appears at night as if covered with little dangling bits of string instead of leaves.” Although sleep-movements are very general, their amount and their nature are greatly diversified. Thus Darwin enumerates 37 genera in which the leaves or leaflets rise, and 32 genera in which they sink at night. In some species the leaves sleep, but not the cotyledons ; Vv Movements of Plants 85 in a larger number the cotyledons sleep, but not the leaves ; in many both sleep, but in widely different positions. Yet if sleep occur the general result is always the same; the blade is placed in such a position at night that its upper surface is exposed as little as possible to full radiation. The movements are without doubt associated with the daily alternation of light and darkness; but it is the difference in the illumination rather than the darkness which excites the change, for Darwin showed that in several species, if the leaves have not been brightly illuminated during the day, they do not sleep at night. Moreover, in the North, where the sun does not set, Mimosa still goes regularly to sleep; and by artificial light and darkness the daily movements. may be reversed. ““The presence of light or its absence cannot be supposed to be the direct cause of the movements, for these are wonderfully diversified even with the leaflets of the same leaf, although all have of course been similarly exposed. The movements depend on innate causes, and are of an adaptive nature. The alternations of light and darkness merely give notice to the leaves that the period has arrived for them to move in a certain manner. We may infer from the fact of several plants (Tropzeolum, Lupine, etc.) not sleeping unless they have been well illuminated during the day, that it is not the actual decrease of the light in the evening, but the contrast between the amount at this hour and during the early part of the day, which excites the leaves to modify their ordinary mode of circumnuta- tion. As the leaves of most plants assume their proper diurnal position in the morning, although light be excluded, and as the leaves of some plants continue to move in the normal manner in darkness during at least a whole day, we may conclude that the periodicity of their movements is to a certain extent inherited. The strength of such 86 Chapters in Modern Lotany CHAP. inheritance differs much in different species, and seems never to be very rigid; for plants have been introduced from all parts of the world into our gardens and green- houses ; and if their movements had been at all strictly fixed in relation to the alternations of day and night, they would have slept in this country at very different hours, which is not the case; moreover, it has been observed that sleeping plants in their native homes change their times of sleep with the changing seasons.” . The movements Darwin explained in two ways: ‘firstly, by alternately increased growth on the opposite sides of the leaves, preceded by increased turgescence of the cells ; and secondly, by means of a pulvinus’ or aggregate of small cells, generally destitute of chlorophyll, which be- comes alternately more turgescent on nearly opposite sides ; and this turgescence is not followed by growth except during the early age of the plant.” The movement may be almost the same whether a pulvinus be present or not, but in the former case the course of the leaves is more regularly elliptical, and the movements are continued for a much longer period in the life of the plant. The position occupied by the leaves at night indicates that the benefit they derive is ‘‘the protection of their upper surfaces from radiation into the open sky; and in many cases the mutual protection of all the parts from cold by their being brought into close approximation.” In evidence of this Darwin showed that leaves compelled to remain extended horizontally at night, suffered much more from radiation than those which were allowed to assume their normal vertical position. ‘Any one who had never observed continuously a sleeping plant, would naturally suppose that the leaves moved only in the evening when going to sleep, and in the morning when awaking; but he would be quite mis- v Movements of Plants 87 taken, for we have found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours; they move, however, more quickly when going to sleep and when awaking than at other times.” Exceptional Developments of Plant Movement, the Telegraph Plant, Sensitive Plant, etc.—The most re- markable case of continuous movement is that of the Indian Telegraph plant (Desmodium or Hedysarum gyrans). Its leaves have three leaflets—a large median one, and two lateral ones which are rudimentary. The main part moves a little during the day and has a remarkable sleep move- ment, but the lateral leaflets which do not sleep move con- stantly, describing with a series of little jerks minute circles. Each revolution sometimes occupies little over a minute. The term sleep is often applied, as Linnzeus applied it, not only to leaves, but also to the petals of many flowers which close at night. This seems to depend rather upon a difference of temperature determining differences in turgescence than on a difference of illumination, and it has the effect of sheltering the internal parts of the flower from cold winds, rain, and over-radiation. Here certainly Darwin’s suggestion of “modified circumnutation” does not apply. We have not spoken of one of the most familiar of plant movements—that of the sensitive plant (/imosa pudica). As is well known, a touch is enough to make the leaves of this plant suddenly assume their sleep position. This is usually referred to a change in a cushion of cells at the base of the leaf. Even Darwin did not think of explaining it as connected with circumnutation, although he justly re- gards it as an extreme case of exaggeration of the sleep movements. Its outward or bionomic utility we cannot fathom, still less its inner mechanism. It is tempting to correlate it with that remarkable “intercellular continuity 88 Chapters tn Modern Botany CHAP. of the protoplasm” which has been discovered in the tissue of this cushion—at least so far as propagation of impulse is concerned, for contractility of the network cannot be assumed. The most recent theory on the matter is that of Professor Haberlandt, who by means of very careful anatomical researches has been led to conclude that the stimulus travels down a continuous set of tubular conduct- ing cells included in the bast portion of the leaf-strands. He believes that the transmission depends upon changes of hydrostatic pressure induced in the tubular cells and on resulting movements of the cell-sap; in other words, that it is not due to any particular nervous excitability of the living matter. But anatomical researches alone cannot justify such a conclusion ; nor is it easy to see how to prove or disprove it by direct experiment. The movements of the tentacles of the Sun-dew, of the leaves of Dionza, of the irritable stamens of the Rockrose (Helianthemum), Barberry, etc., the closure of the stigma lips of a Mimulus upon a pollen-grain, the bending of a tendril when touched, are also to be distinguished from that series of movements to the discussion of which this chapter has been devoted ; and no doubt reserve a wide and varied field for a future generation of subtle experimentalists, For instead of the animal world alone possessing movement, and the plant standing passive, the phenomena of plant movement, while of course less obvious in amount, seem to be not only the more varied in kind, but perhaps also in cause. Summary.—The different kinds of movement may be arranged as follows :— A. Movements of Growing Parts— (1) ‘Spontaneous ” movements—that is to say those whose conditions are not known, e.g. the revolving v Movements of Plants 89 movements of twiners and climbers, young shoots and roots, etc. (2) Movements in relation to external influences : (z) Downward movement of roots and up- ward movement of stems. (6) Light-seeking and light-avoiding move- ments, especially of stems and leaves. (c) The movement of roots towards the greatest moisture. B. Movements of Adult Parts—- (1) “Spontaneous” movements, e.g. of Desmodium or Hedysarum gyrans. (2) Movements in relation to external influences : (2) In relation to light—sleep movements of leaves. (4) In relation to chemical and physical stimuli other than light, e.g. the move- ments of the Sensitive plant and of Fly- Traps. Mr. Francis Darwin’s Discussion of Plant Move- ments.—At this point it is appropriate that we should turn to what Mr. Francis Darwin, who collaborated with his father in writing Zhe Power of Movement in Plants, has recently said in his Presidential address to the bio- logical section of the British Association, 1891, which con- tains an important discussion of growth -curvatures, a problem towards the solution of which his detailed researches have largely contributed. A brief summary of this address, although including points already touched, may hence be of service to the student. He begins by distinguishing the two main questions :— 1. ‘* How does the plant recognise the vertical line ; how does it know where the centre of the earth is ?”—a ques- tion of irritability. go Chapters in Modern Botany CHAP. 2. ‘In what way are curvatures which bring the plant into the vertical line executed?”—a question of the mechanism of movement. The history of the answers to these questions may conveniently begin with Hofmeister’s researches (1859) on the effects of bending or striking a turgescent shoot. He showed that when a shoot is violently bent the elasticity of the passive tissues (cortical and vascular con- stituents) on the convex side is injured by over-stretching. “The system must assume a new position of equilibrium ; the turgescent pith (the active or erectile tissue) stretches the cortex; but as the passive tissues are now no longer equally resisting on the two sides, the shoot must assume a curvature towards that side on which the passive tissues are most resisting.” Applying the same conception to a cell, Francis Darwin says: “As pith is to cortex, so is cell-pressure to cell-membrane.”’ When a shoot is laid horizontally there is, according to Hofmeister, a tendency for the resisting passive tissue along the lower side to become water-logged, and therefore more extensible. Therefore the shoot bends upwards, So Knight, in 1806, supposed that roots penetrated down- wards, because of the sinking downwards of the juices. But both these explanations are crude; they are too mechanical. As far back as 1824, Dutrochet, who was, however, by no means consistent, had recognised the fundamental biological fact that growth-curvatures were provoked by external influences acting as stimuli, but ‘‘the botanical mind took more than fifty years to assimilate Dutrochet’s view.” In 1868 Frank attacked the problem with true physio- logical insight, showing that earth-seeking is an active curvature, and that it depends, like other growth-curvatures, Vv Movements of Plants gI on unequal distribution of longitudinal growth, Moreover, his experiments on the horizontal runners of the straw- berry, and those of Elfving on rhizomes “paved the way for the theory that there are a variety of different organisa- tions (or, as we now say, irritabilities) in growing plants ; and that, whether a plant grows vertically upwards, or down- wards, or horizontally, depends on the individual and highly sensitive constitution of the plant in question.” Frank’s views were, as we have seen, accepted by the authors of The Power of Movement in Plants, although Frank’s particular interpretation of the irritabilities as due to “polarities ” was not. Francis Darwin then points out how the development of our present views on irritability was delayed by the in- sufficient theory of light-seeking, as implied, for instance, in De Candolle’s explanation, that curvature towards the light was simply due to the more rapid growth of the shaded side. Fuller acquaintance with the facts, e.g. of plants which curve away from the light, showed that this again too mechanical theory was insufficient, and led on to the idea, repeatedly expressed in Zhe Power of Movement, that light and gravitation act merely as landmarks by which the plant can direct itself. Pfeffer, Sachs, Vines, and other botanical physiologists are at one in regarding growth-curvatures as phenomena of irritability, as responses to gravitation, light, and other stimuli. So far the general question of irritability. Mr, Darwin then proceeds to discuss that of mechanism. ‘‘The first step in advance of Hofmeister’s views was the establishment of the fact that the curvatures under consideration are due to unequal growth—that is to say, to greater longitudinal growth on the convex than on the concave side.” Frank made important contributions to the subject, and Sachs thoroughly demonstrated that the con- 92 Chapters in Modern Botany CHAP. vex side grows faster, while the concave side grows slower, than if the organ had remained vertical and uncurved. ‘‘Then it began to be established, through Sachs’s work (1871), that turgescence is a necessary condition of growth.” “De Vries (1879) maintained that growth-curvatures in multicellular organs are due to increased cell-pressure on the convex side: the rise in hydrostatic pressure being put down to increase of osmotic substances in the cell-sap of the tissues in question.” But, as Francis Darwin points out, there are many serious objections to this explanation. Many suggestions followed. Sachs directed attention to the changes in the extensibility of cell-walls. ‘* Wiesner held that the curvature of multicellular organs is due both to an increase of osmotic force on the convex side, and to increased ductility of the membranes of the same part.” ‘“‘ Strasburger suggested that growth-curvatures are due to increased ductility of the convex membranes.” More detailed explanations in similar lines are those of Noll and Wortmann, which differ in this: ‘‘ The former lays the greater stress on the increased extensibility of the convex side, the latter on the diminution of that of the concave side, Again, Wortmann explains the difference n extensibility as due to differences in thickness of the cell-walls. Noll gives no mechanical explanation, but assumes that the outer layer of protoplasm has the power of producing changes in the quality of the cell-wall in some unknown way.” Francis Darwin concludes ‘there is a focussing of speculation from many sides in favour of ‘active’ surface- growth, or, what is perhaps a better way of putting it, in favour of the belief that the extension of cell-membranes depends on physiological rather than physical properties, that it is in some way under the immediate control of the protoplasm.” Beyond that we may choose between rival Vv Movements of Plants 93 suggestions of Wiesner, Strasburger, Wortmann, Noll, Vines, and others. In the last section of his Presidential address Mr. Francis Darwin states his position in regard to circum- nutation. He cites the most important criticism of Zhe Power of Movement, that of Wiesner, who denied that circumnutation was a widespread phenomenon ; that some stems, leaves, etc., grow in a perfectly straight line; that such curvatures as those of geotropism and heliotropism cannot be interpreted as modifications of circumnutation. Yet, for reasons given, Mr. Francis Darwin confesses that he “‘ cannot give up the belief in circumnutation as a widely-spread phenomenon, even though it may not be so general as was supposed.” He adopts Véchting’s concep- tion of ‘rectipetality”—‘‘a regulating power leading to growth in a straight line,” and says, ‘“‘ The essence of the matter is this: we know from experiments that a power exists of correcting excessive unilateral growth artificially produced ; is it not probable that normal growth is simi- larly kept in an approximately straight line by a series of aberrations and corrections? If this is so, circumnutation and rectipetality would be different aspects of the same thing.” “A bicycle cannot be ridden at all unless it can ‘wobble,’ as every rider knows who has allowed his wheel to run in a frozen rut. In the same way it is possible that some degree of circumnutation is correlated with growth, owing to the need of regular pauses in growth. Rectipetality would thus be a power by which irregularities, inherent in growth, are reduced to order and made sub- servient to rectilinear growth. Circumnutation would be the outward and visible sign of the process.” Phases of Botany, Pre-Hellenic to Neo-Hellenic.— Hence, whatever detailed value be retained by the special 94 Chapters in Modern Botany CHAP. V theories of Darwin, the world will always owe him thanks ; for his books have a deeper use and significance. To the dawning intelligence of the race, the forest is vaguely astir with a life which man does not clearly separate from his own—a mystery of growth which has left its mark deep in the history of all religions. A later and more self-conscious mind moulds this omnipresent life into anthropomorphic shapes; so a Dryad hides in every tree, while Pan roams through the glade. These anthro- pomorphic shapes are next formalised away from the living realities they symbolise ; they become mere shadowy gods, then fairies and fables. The tree (or what remains of it) is now something economically useful; it has also a popular and a systematic name; but to Utilitarian or Linnzan alike, the form and substance seems the main thing, not the life. ‘‘Great Pan is dead”; the botanist is as prosaic and unseeing as the woodcutter, in fact essentially is one, at best with finer tools, and like him does his best work away from the wild wood altogether, But as the ages of fetishism, of Hellenic anthropomorphism passed away, so now the formal and utilitarian and analytic spirit is passing also in its turn. Science is entering a new and brighter Hellas; the Dryad, living and breathing, moving and sensitive is again within her tree; nay better, the plant is herself the living Dryad, her naked beauty radiant in the sun. And what of this old naturalist who led us back into the forest sounding with the Protean mystery-play of evolving Life? Now that his rugged face has vanished, it grows more strange yet more familiar in memory; we have seen it of old—we know it now for the returning avatar of Pan. CHAPTER VI. THE WEB OF LIFE Struggle among Plants—Perched Plants or Epiphytes—Farasitic Plants — Mistleto— Dodder — Root - Parasites — Toothwort — Broom - rapes— Saprophytes — Parasitic Fungi— Bacteria— Symbiosts. Struggle among Plants.— In our study of climbing plants we saw that plants as well as animals had difficulties to contend with, and that there was, especially in crowded places, a more or less intense struggle for existence. Of this the tropical forest of our frontispiece is a supreme illustration. To rise out of the perpetual twilight of their depths is a condition of success, and thus we have first tall straight-stemmed trees, then twiners and climbers which strangle and overshadow these, while strange high-perched epiphytes and parasites grow and scatter their seed over the whole tangled roof of verdure. Describing “the struggle for life in the forest,’”’ Mr. James Rodway writes of a scene in British Guiana: ‘“‘We can almost fancy the magnificent forest tree protesting strongly, as octopus-like, the Clusia begins to compress and strangle it. . . . The Clusia grows stronger and stronger, until by and by, as the strangler opens its magnificent waxy flowers to the sun, and glories in its conquest, the poor unfortunate victim droops and 96 Chapters in Modern Botany CHAP" dies. Then the trunk becomes diseased, wood ants begin their work, and finally nothing is left but the hollow cylinder of the strangler.” In the crowded vegetation by the river-side, in the meadow, along the hedgerows, the same struggle for standing room, for air, for light, must occur, and there are many peculiarities of plants which find partial explanation as adaptations of structure which help plants in crowded places to keep their foothold. . When we remember that plants have not such diverse needs as animals have: that they all require very nearly the same kind of food; that most of them get this in precisely the same way—by their roots from the soil, by their leaves from the air—we feel that there must be what may be called struggle or competition between them when they grow crowded together. We get a vivid impression of struggle for space in the crowded rosettes of a patch of house-leeks (Sempervivums), which have been allowed to grow for some time as they list ; we see the younger plants budding from their parents, rising upon their shoulders, and often not only smothering them, but soon coming to crowd upon each other and compete anew. Again, only a fraction of the seedlings which appear above the surface in a plot of ground reach maturity. There is neither room nor food for them all, and the less fit are eliminated. And this is recognised practically by every farmer or gardener, for does he not thin his turnips or onions, knowing that thus alone can he ensure the success of individuals ? No doubt there is some danger of exaggerating this struggle for existence among plants, and yet more of attributing to it results which may have some entirely different origin. Experiment is much needed to sub- stantiate what is often assumed.! But on general grounds 1 A careful statement of facts was given by Mr, Walter Gardiner in vI | The Web of Life 97 it seems much more true of plants than of animals that between those of the same kind the struggle for existence in a crowded area is very keen. For plants cannot migrate nor combine in mutual aid ; in a crowd the slightly weaker must be smothered. In this matter Darwin is again ready for us with exact observation and experiment. ‘From observations which I have made it appears that the seedlings suffer most from germinating in ground already thickly stocked with other plants. . . . The more vigorous plants gradually kill the less vigorous, though fully-grown plants ; thus out of twenty species growing on a little plot of mown turf (3 feet by 4) nine species perished, from the other species being allowed to grow up freely. . . . Seedlings also are destroyed in vast numbers by various enemies ; for instance, on a piece of ground 3 feet long and 2 wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295 were destroyed, chiefly by slugs and insects.” Perched Plants or Epiphytes.—In temperate climates the only perched plants are those mosses and lichens which sometimes clothe the stems and branches of trees, but in tropical countries many Orchids, Aroids, and other flower- ing plants have this habit. Entirely isolated from the ground, and yet not parasitic on their bearers, how do they live? In part, of course, like other green plants, on the air and the power of the sunlight; and it is interesting to notice that they flourish best on trees, such as Cassia and Czesalpinia, whose crown of branches is in the dry season bereft of leaves. For water and mineral matters the a lecture entitled ‘‘ How Plants maintain themselves in the Struggle for Existence,” delivered at Newcastle, September 1889. See abstract in Nature, September 1889, H 98 Chapters in Modern Botany CHAP. perched plants depend on the rain and damp atmosphere, for absorbing which the skin of the roots is often specially adapted, forming a kind of sponge. And although the roots do not come into contact with the soil, it seems that in some cases they may absorb salts from the decaying bark of their bearers, or from such debris as may gather about the branches. From the great Cypress swamps of Florida, or the small but better known surviving fragment of one of these which surrounds the palace-citadel of Chapultepec, the favourite morning ride of every visitor to the city of Mexico, the traveller brings back a unique impression of mournful picturesqueness, The sombre coniferous foliage is draped with long silver-gray streamers, the “ Spaniards’ beards” (7zllandsia usneoides), one of the most conspicuous and widely distributed examples of the perching habit. This has not even roots, but fastens itself to its bearer by means of its long thread-like stems, the scales of which seem to absorb ‘the necessary supply of water. Aroids of the genus Philodendron, often cultivated in our greenhouses, and Orchids belonging to the genera Dendrobium, Oncidium, Phagus, etc., are also good examples of epiphytes provided with aerial roots, which absorb water-vapour from the moist atmosphere of the tropical forest. Here indeed is the solution of .a mystery which has often puzzled the botanist: how transpiration from the leaves could go on without any apparent source of water-supply. A micro- scopic section of the root, however, shows a tissue of characteristically thickened cell-walls, which take up mois- ture from the vapour-laden atmosphere during the coolness of night. The best general account of Epiphytes is that given by Goebel in his valuable Pflanzenbiologische Schilderungen (Part I., Marburg, 1889). With this Schimper’s E¢7phy- 4 The Web of Life 99 tische Vegetation Amerikas (Jena, 1888) should be com- pared. Goebel discusses, for instance, the various ways Fic. 5.—Elk’s-horn fern (Platycertum grande). (After Burbidge.) in which the epiphyte effects attachment to its bearer, usually by a special disc with root-suckers, some- what similar to that in mistletos. The spongy rind of Aroids and Orchids is regarded as primarily an organ of 100 Chapters in Modern Botany CHAP. assimilation, but as a secondary function it absorbs mois- ture. Sometimes, as in Zz//andsia usneotides, the leaves absorb water through their surface, and then the roots tend to disappear. Another fact of much interest, repre- sented in most collections, but which Gcebel describes in detail, is the manner in which some epiphytes, especially ferns, ¢.g. “the bird’s-nest fern” (Asplenium nidus-avis) and the “stag’s-horn fern” (Platycerium), gather nests of humus about their roots, thus ey making soil for them- selves upon the branches. Parasitic Plants.—The perched plants which grow on the shoulders of their fellows naturally suggest parasites, which to a greater or less extent live at the expense of their hosts. As among animals, there are endoparasites, like some bacteria and fungi, which live within their hosts, and ectoparasites, which, though vitally fixed to their hosts, live outside of them. Let us begin with the ectoparasites the outside hangers-on. Some of these, eg. the mistleto, are only in part dependent on their hosts, the parts which do not pierce the host retaining the ordinary powers of green plants ; others, e.g. the adult dodder, are wholly dependent upon their hosts for the sustenance of their life. Mistleto.—Of all parasitic plants the mistleto is probably most familiar, as certainly also one of the most remarkable. The species (Viscum album) which grows in Britain and throughout Europe is a parasite on a great variety of trees, such as apple, pear, sycamore, lime, but its favourite host is the black poplar (Populus nigra), and one of its rarest, in Britain at least, the oak. The strange habit of the plant, the beautiful harmony of colour between stem, leaves, and berries, its verdant and fruit-laden con- spicuousness at the turn of the year towards lengthening days and summer, of which the joyous celebration by the VI The Web of Life IOI northern peoples underlies the festival of Christmas, have -made the mistleto a favourite with men; and, whether it be the sacred plant of the Druids or no, it has become the centre of many beautiful myths and customs. A word for the life-history of the plant. In autumn and winter the white seeds are eaten by birds, especially by thrushes, and. passing undigested from the food-canal are voided on the branches, to the sides of which they eventu- ally adhere. A tree in the botanic garden at Bonn is thus specially noticeable ; its trunk crowded with seedling mistletos just below a mass of branches whose sheltering attractiveness is well marked by the remains of yearly nests. From the seed a little root grows out, bends towards the branch, sticks to it, and expands into a clinging disc. From this there grows a modified rootlet which pierces the bark and reaches the wood. There is no further growth that year. But next spring the growth of new wood encloses the rootlet, which at the same time increases in length. In the second year the rootlet gives off lateral .branches which grow longitudinally between the wood and the outer rind, and give off other rootlets. In proportion to the number of these the mistleto plant flourishes in stem and leaves. From the spreading roots fresh stems may arise, so that from one seed a score of mistleto bunches may arise. The rootlets penetrate into the wood and absorb what that contains—namely, as we shall afterwards see, water and salts,—while the mistleto stem spreads forth its leaves, and behaves in regard to the light and air as any independent green plant. It is in fact a natural graft. Resembling our mistleto in most respects are other species of the same genus Viscum, the American mistleto often called by a different generic name, the more shrubby Loranthus europeus common on oaks and other trees in many parts of the south of Europe. There are about 102 Chapters in Modern Botany CHAP. three hundred species of mistleto-like plants included in the order Loranthacez, while the genus Henslowia—plants of similar habit found in Southern Asia and in the Indian Archipelago—belongs to the order Santalacez. It is interesting to notice that some of these mistleto-like plants are sometimes parasitic on one another. Thus our mistleto may grow on Loranthus europaeus, or one species of Viscum or of Loranthus may grow on another ; indeed the common mistleto has been noticed—as is natural and perhaps common enough—thus sprouting upon itself. Dodder.—The dodders (Cuscwta) are parasites on such plants as clover, flax, nettles, and hops. It is on the two last that the commonest European species (C. europea) is usually found. In many ways it differs from the mistleto ; thus the seed almost always germinates on the ground, and the adult plant is practically destitute of chlorophyll and leaves. Let us follow the life-history of the common dodder. Like most of the other species, it is an annual, dying away in autumn. Before that, however, the seeds have burst explosively from the seed-boxes, and have been swept hither and thither by the wind. All through the winter they lie dormant on the ground, sheltered in many cases by decay- ing leaves, which supply a suitable bed for germination. This does not take place till comparatively late in the following year, not before the nettles and hops have acquired some strength of stem,—a delay which is obviously of advantage to the dodder. Out of the seed there comes a little club-shaped root which seeks the soil, but the young stem remains surrounded by the seed-husks and the store of nutriment which these enclose. It grows—thin as a thread, and somewhat spirally —at the expense of the seed-store. This is soon exhausted and growth practically stops, but the thin stem still circum- VI The Web of Life 103 nutates in sweeping circles, as if seeking about for some plant on which to cling. If this be not found, the stem at length lies prostrate on the ground, and after a strange dormant existence for a month or so longer, dies. But if some plant be near at hand, the dodder stem slings itself around it after the manner of a twining plant. As soon as its stem has embraced that of its bearer—let us say a nettle or hop—it gives off attaching papille which penetrate the rind and bud off numerous little rootlets. These come into close connection with the bast portion of the hop or nettle stem, and thence absorb nutritive materials. Now, in all plants the vessels of the bast form the channels by which the complex organic substances manufactured in the leaves pass towards the root. Such are the spoils which the leafless dodder, unable to manufacture organic stuffs for itself, absorbs from its host. After suitable attachment has been effected the stem continues its twining growth with increased vigour, the basal part dies away and all connection with the soil is lost, and eventually the wan parasite bursts into flower. Root-Parasites.—In some well-shaded part of the wood we may find a large patch of the cow-wheat (J/elampyrum), a delicate plant with a pale yellow flower, akin to toadflax and snapdragon. If we dig it up very carefully along with the plants growing near, we may be able to see that its roots are at intervals tightly bound to those of its neighbours. The connection is a vital one, and effected by peculiarly modified discs which grow round the roots of other plants and send suckers into them. This is a clear case of root- parasitism, but it is difficult to tell how much it amounts to, for the cow-wheat has also independent roots which absorb water, salts, and probably decaying vegetable matter from the soil, and it also isa green plant. If this habit were peculiar to the cow-wheat we should not be inclined to attach 104 Chapters tn Modern Botany CHAP. much importance to it, but it is far otherwise. What is true of Melampyrum is true of several hundred species of plants, especially of the orders Santalaceze and Rhinan- thacez. Thus the louseworts (Pedicularis) have very long surface-roots which attach themselves to the fibrous roots of the grasses and other plants of pastures and meadows. The length of the roots is in part explained, as Kerner points out, by the fact that most species of Pedicularis persist from year to year. For when a root has fixed itself to that of an adjacent annual, and that has died, it becomes necessary for the parasitic root to shift its anchorage and to find another farther on. This case of Pedicularis is further interesting because the root-hairs which are borne by the roots of most plants are here absent, except at those points where parasitic attachment is effected ; not but that the skin of the root may without root-hairs absorb water and salts and the products of decaying vegetable-mould. Of the Alpine Bartsia (2. a/fina), Kerner states that some of the roots are parasitic, while others are adapted for absorbing humus; and among the same group of root- parasites we have also to include the odd-flowered and fruited yellow-rattles (AAznanihus crista-gallz), and even the pretty Eyebright or Euphrasy (Zuphrasia officinalis) of our moorland pastures and roadsides. The root-parasites are often blamed for spoiling pastures, and even the milk of the cattle that graze there. They certdinly often occur in great abundance, and hence must to some extent impoverish the plants whose roots they suck; as yet, however, there is no definite evidence of appreciable damage done. One wonders what constitutional peculiarity distinguishes the members of these two orders (Rhinanthaceze and Santalacez), so many members of which have this strange habit of root-parasitism. Some hints of an answer have VI The Web of Life 105 been discovered by Professor Bonnier, who finds that in Euphrasia, Bartsia, and Rhinanthus (though not in Melam- pyrum and some others), respiration predominates over assimilation (see chap. ix.), the oxygen which is liberated in assimilation being completely masked by the absorption of oxygen in respiration—the very reverse of that gaseous interchange characteristic of the daily life of ordinary green plants. The Toothwort.—One of the strangest British plants is the toothwort (Lathrea sguamaria), which has been already mentioned at chap. il. p. 35. It lives an almost completely underground life upon the roots of poplars, hazels, and other trees, hidden by a growth of ivy, or by a heap of mouldering leaves. No young botanist will ever forget his first finding of the strange pale plant, nor can an old one ever lose his feeling of wonder as he digs down from the thick drooping spike, with its faded lilac-tinged flowers, to the still stranger underground stem, close-set with thick sharp-edged white teeth, more like a witch’s necklace of human incisors than a leafy shoot. The plant, though never abundant, has a wide distribution in Europe and Asia. When we bare the thin roots which spring from the underground ‘stem, we see that they are clasped by small adhesive discs to the roots of the tree at whose base the plant grows. From these adhesive discs, which in another European species (Lathrea clandestina) are about the size of split peas, suckers penetrate the tree-roots, from which without doubt the toothwort steals not only its salts and water, but the abundant starch reserves which help to swell its crowded leaves. Nor does the odd story of life end here ; for each tooth-leaf has a strange recess, no mere hollow of decay, but a normal cavity, narrow of entrance, gland- lined, difficult of interpretation save as an insect tfap, as which it is commonly regarded, carrying back our 106 Chapters in Modern Botany CHAP. thoughts to the terrestrial species of Utricularia (p. 31). Few botanists imagine that this strange and local plant may be cultivated ; it grows well under cherry laurels (“‘ common bay,” Cerasus laurocerasus) in the shrubberies of the Edinburgh Botanic Garden, and hence at least suggests the possibility of its introduction elsewhere. Broom-rapes.—lIn our search for the toothwort we may perhaps find one of the broom-rapes (Ovobanche), which grow parasitically on the roots of thyme, scabious, and other common plants. As in Lathrza, there is no chlorophyll ; but the union between parasite and host is much more inti- mate, so much so indeed that it is difficult to separate them, or to tell where the tissues of the parasite end and those of the host begin. The seedling of the broom-rape and related parasites is very remarkable ; it bears no trace of cotyledons, but is a delicate thread-like structure, one end of which is hidden by the remains of the seed, while the other grows in search of roots to which to fix itself. If these be not soon found, the seedling shrivels and dies, for it seems quite unable to absorb food from the soil. But if a suitable host be found, the seedling becomes ultimately united with it, thickening into a knotted tuber-like structure, from which the flower-bearing stem with its brown useless leaves will afterwards rise above the ground. Thus from the non-parasitic toad-flax and its allies, which we shall come to know better as the natural order Scrophulariace@, commonly gay flowered and free growing, often indeed mischievous as weeds, we have a regular gradation through cow-wheat and Euphrasy, lousewort and Bartsia, to full parasites like toothwort and broom-rape. For still further modified allies of the broom-rapes (Orobanche) we must travel abroad, especially to tropical countries, where there are many strange plants (Balanophorez) of similar habit but strangely crowded and reduced flowers. VI The Web of Life 107 Best known among these, especially to tradition, is the odd Cynomorium, a plant of fungus-like appearance, long known as fungus melitensts. It is found in the islands of Malta and Gozo, and is remarkable for its scarlet colour and blood-like juice, which, according to the “ doctrine of signatures,” were interpreted as providential indications of its value as a cure for all diseases accompanied by bleed- ing. Even stranger are the Rafflesias, found in the Indian islands and in South America. They have neither a developed stem nor leaves, but are reduced practically to a flower closely fixed to the roots of some other plant (usually a species of Cissus). The largest species, dis- covered in 1818 in Sumatra by Dr. Arnold, and sent by Sir Thomas Stamford Raffles to Robert Brown, who hence gratefully named it Rafflesia Arnoldi, measures about a yard in diameter, and is, one need hardly say, the largest of all known flowers. It seems, however, a repulsive giant, fleshy and fungus-like, coarse and gaudy, worst of all, fetid as carrion, and hence swarming with appropriate insects, who even lay their eggs in preparation for its foul decay, and so, too, doubtless render the service of fetching and carrying pollen from flower to flower. Saprophytes.—A large number of plants, especially in the woods, some with and some without chlorophyll, depend in great part at least on the abundant organic material afforded by the decaying leaves and other parts of plants—in other words, on the humus or vegetable mould of the soil. This is true, for instance, of the Bird’s-nest Orchis (Veottza nidus-avis), of the rootless Corallorhiza, of the little twae-blade (Lzstera cordata), and of many other orchids. But it is very difficult to draw any hard and fast line between parasites, which live on living organisms, and the so-called saprophytes, which live on decaying organic matter. Thus the species of cow-wheat (Melampyrum) 108 Chapters in Modern Botany CHAP. and yellow rattle (Rhinanthus), which are undoubtedly root-parasites, appear also to depend upon the humus of the soil; the orchid MJonotropa Hypopitys is said to be a parasite in the pine woods, but a saprophyte on the rotting leaves under other trees ; and some of the epiphytes gather so many decaying leaves around their roots that it is likely that some of them should also be called saprophytes. The staghorn- fern (Platycerium), so commonly grown as a suspended ornament of our greenhouses, may here again be an instructive case (see p. 99). | Parasitic Fungi.—Of infinitely greater importance as mischief-makers in the world than the parasitic flowering plants are the innumerable parasitic Fungi. Those which cause mildew, potato-disease, some diseases of cereals, vine- plants, and timber-trees, and so on, are often disastrous to the prosperity of a fertile region, nay, as modern experience as well as history too often tells, to a province, a whole nation. Nor do they affect plants alone, but also animals, and even man himself; witness respectively salmon-disease and ringworm. Fungi being, like dodder, toothwort, and broom-rape, plants without any chlorophyll, they are unable to use the carbonic acid gas of the air, as green plants do, and are therefore dependent upon ready-made supplies of organic matter. Hence we find them living either as parasites on living plants and animals, or as saprophytes on decaying organisms, or in some cases indifferently on either. As we do not propose here to do more than indicate the part played by fungi in the economy of nature, either in destroying living organisms or in utilising the products of decay, we refer the student to that convenient and in- teresting introduction to the subject supplied by Professor Marshall Ward in his little volume on Z77zmber and some of zts Diseases (Nature Series, Lond. 1889), and for syste- VI The Web of Life 109 matic information to the text-book of Cryptogamic Botany, by Bennett and Murray (Lond. 1889). As an easy intro- duction to the subject, the writer’s article “Fungi” in Chambers’s Encyclopedia may in the first place be read. Bacteria.—Of all parasitic plants, the smallest—the microscopic Bacteria—are the most important. It is true ‘that not all of them are parasitic, for many live in rotten- ness, but they all agree in being minute colourless units unable to live, as green plants do, on water, salts, carbonic acid, and sunlight; able to thrive only when they find organic substances of some sort ready-made for them. We have already seen that their presence may help to explain the disappearance of flies within the traps of some of the insectivorous plants, and we can detect the work of bacteria in a hundred ways all around us, If we leave a piece of meat or fish (whether cooked or raw matters not) in an open vessel filled with clear water, this becomes turbid, and a scum gathers on the surface; if we examine this scum under a high power of the microscope we see incalculable numbers of bacteria. On the other hand, if the piece of meat be placed in a glass flask which is filled with water, then boiled for a short time, and carefully closed, while still boiling, with a plug of cotton-wool, the water will remain quite clear, and the flesh will not decom- pose. The only difference between the two cases is that in the latter the bacteria in the water have been killed by boiling, and are kept out of the flask by the cotton-wool, therefore we may say that the decomposition observed in the first case was due to bacteria. So it is always. As Huxley in aphoristic style puts it, ‘‘ Putrefaction is the result not of death, but of life.” It is more than two centuries since the keen-eyed Leeuwenhoek—that early devotee of the microscope to whom, despite his rude and feeble instruments, the most IIO Chapters in Modern Botany CHAP. abundant laurels of discovery must be awarded-—discovered this living dust, a knowledge of which has not only changed some of our biological conceptions and created the dominant school of medical theorists, but reformed and rationalised surgery and hygiene, and exercised a potent influence upon the greatest industries. To what is the importance of bacteria due? They are very minute, quite invisible individually to our unaided eyes—minute spheres, rods, or spirals they are, the smallest unit masses of living matter. The secret of their strength is in their power of multipli- cation. By repeated division and redivision one soon becomes a thousand ; indeed, given sufficient food, in a few hours a single unit may have become the progenitor of millions. But their importance depends also on their universal distribution. Universal, indeed, for they live in the air and in the soil, in food and raiment, in man, in beast, in plant; not even the water which we drink is free from them. From the mouths of men to the walls of their houses and the flies on the windows, from the hair of the head to the toes of the feet, from the highway dust to the recesses of the forest, and from-wayside pool to sea, bac- teria abound. But while they spread here, there, and everywhere, they become most obvious wherever dead organic matter is abundant, as in the refuse of our towns, or in dead plants and animals. Rancid butter and rotten cheese, ‘‘ high” game-flavoured meats and over-stale bread, blue milk and soured wine, as well as cesspools and dust- heaps, sewers and slums, are their common habitats. Most important, however, is the fact expressed in the ‘“‘germ-theory,” that bacteria are constantly and intimately associated with some of the most fatal of human diseases, such as consumption, diphtheria, smallpox or typhoid, malaria or leprosy. Bacteria, in fact, kill most of us, VI The Web of Life IIL multiplying within us so as to choke the system, at once feeding greedily on the tissues or fluids of the body, and poisoning us in every cell with the waste-products of their loathsome life. It would take us too far from present botanical considerations to discuss the ways in which we are saved from the bacteria which so easily beset us. There are happily many drugs and reagents of inward or outward use, from quinine to carbolic acid, and all the other ever-multi- plying antiseptics—drugs which they cannot stomach. Like the Acacia tree with its bodyguard of ants driving off other dangerous ants, so, some tell us, we may have a protective standing army of bacteria which save us from others ; at all events, the widely accepted theory of Metschnikoff makes us view the animal body as garrisoned by the uncoloured blood-corpuscles or leucocytes which seize the intruding microbes just as amcebz would do, and digest them beyond the reach of mischief. But when the germs become too numerous or the warders enfeebled, the bacterial parasite begins its growth, and the disease has set in. Such, for instance, Dr. Gulland tells us, is the use of the tonsils which lie on either side of the pharynx ; in health they furnish a continual succession of these corpuscle sentries, while in the inflammations to which they are so subject, the bacteria have been too many, the defence too weak. This subject is at present under active controversy, not a few bacteriologists maintaining that the germicidal virtues of the blood lie wholly or mainly in the serum, not in the corpuscles. But the most potent preventive, universal and costless, is a natural one—the sunlight. Miquel, of the excellent bacteriological observatory of the Parc Montsouris at Paris, and several French bacteriologists have shown that the sun’s light, even without the heat, for a few bright hours is able to kill the germs which float in the air, and leave them 112 Chapters in Modern Botany CHAP. harmless. What it requires hard boiling to accomplish, the sunlight does with rapid ease. For although the spores or young bacteria which are carried in the air are peculiarly hardy, able to nurse their dormant virulence for months or even years, ready when they find a suitable resting-place to multiply with appalling swiftness, they cannot withstand the power of the sunlight. What is the light of life to other plants, is to the bacteria death. Per- haps it makes them live for a short time so rapidly that their scanty speck of living substance is worn out and consumed beyond possibility of repair. Quite literally then ‘‘the pestilence walketh in darkness.” Hence then we may interpret much of the history of disease, and see that we have here no mere mysterious visitations, to be crouched before with submissive dread ; but a definite and intelligible part of the order of nature not only to be analysed and grasped by science, but which we may partly modify by art and partly by adapting our other conditions. And thus it is that we find so many dead germs in the air; thus we regard alt dark places as evil, whether they be the lidless coffins of the slums or the dungeon-like sunk flats of west-end houses; thus we begin to appreciate the hygienic importance of sunlight—the most universal, the most economical, the most potent antagonist of our subtlest and deadliest foes. But the part which bacteria play in the economy of nature is often beneficent. For although we may not derive much satisfaction from the fact that bacteria by thinning out the weaker organisms help to keep up the standard of vitality, a little observation leaves the more cheering con- viction that many of them are among the great cleansers of the world. Out of dead plants some form vegetable mould, while others reduce the carcases of animals to simpler and purer substances. Consider the large circle on which bacteria occupy an arc; green plants feed on air, water, and VI The Web of Life I13 the salts of the soil; many animals feed upon plants, even the most purely carnivorous are of course indirectly depend- ent upon them; dead animals are consumed by bacteria, which in so doing set free carbonic acid gas and ammonia, which steal off into the air, and nitrates and the like which soak down into the soil, all to be utilised by the plant- world anew. Here then the bacteria furnish the indispens- able means by which the circulation of the materials of organic life is perpetually renewed. In this connection we may appropriately discuss an interesting story in regard to the relations between bacteria and some other plants. Pull up a bean-plant, wash gently and examine the roots. Here and there may be noted small rounded swellings or tubercles, sometimes as large as peas. ‘The same may be seen on the roots of vetches and other leguminous plants, and also on some cereals. That they are not normal parts of the plants is clear, for there are roots without a trace of them ; and if the seeds be grown in soil that has been calcined at a high temperature or in water that has been boiled, the tubercles are not formed. This becomes yet more evident when the tubercles are examined microscopically, for then they are seen to exhibit, not the tissues of a rootlet, but crowds of minute rod-like or somewhat spherical bodies. What then is the meaning of the tubercles? The re- searches of numerous botanists, with a controversy which has been in progress since 1858, when these tubercles were first noted, have at length convinced most of us that the little rod-like bodies contained in the tubercles are bacteria, and that the tubercles are due to the infection of the plants by these micro-organisms. As to their physiological effect on the plant, the investigators are not yet thoroughly agreed, but the conclusions of Hellriegel and Willfarth, corroborated by some others, are of great interest. By growing legum- I 114 Chapters in Modern Botany CHAP. inous plants and cereals in calcined earth and with a supply of water that has been boiled, they obtained plants without tubercles on their roots; by watering these with water which has been in contact with the soil in which tubercled roots have grown, they were able to infect the plants, or they could also inoculate them directly. After infection or inoculation the plants acquire a new vigour, and they increase in nitrogenous substances to a greater extent than can be accounted for by the nitrogenous salts in the soil. To explain this it seems necessary to believe that with the help of their partner bacteria the legumes and cereals are able to utilise the free nitrogen of the air. If so, they are able to do what, as we shall afterwards see (chap. ix.), is regarded by most botanists as quite impos- sible for other plants. Symbiosis.— Whatever be the final verdict in regard to the alleged partnership between bacteria and _ their leguminous or cereal hosts, there are other cases of mutually helpful partnership in regard to which there is no doubt. To this De Bary in 1879 applied the name Symbiosis (literally, a living together). It is interesting to notice the parallels among animals ; zoophytes clustered on the backs of crabs are comparable to epiphytes ; among animals as among plants there are many external and internal parasites; the well-known partnerships between hermit-crabs and sea-anemones are examples of the beginnings of a truly co-operative symbiosis, The most important case of symbiosis among plants, indeed by far the most fully-developed case in nature, is that of the lichens. These used of course to be regarded, as they no doubt popularly are still, as definite individual plants united by common characters into a well-marked group or “natural order,” which botanists were wont to reckon along with algz and fungi, but no less distinct - Ori ye | vI The Web of Life IIs than these. Their immense variety has given no small amount of labour to the systematists, and a well-developed specialism of “lichenology” with a literature and with collections voluminous enough to surprise any one who first enters upon it, has arisen in consequence. In 1866 the penetrating cryptogamist De Bary threw out the suggestion that the many resemblances shown by the lichens on one hand to fungi and on the other to alge, might really be due to an admixture of these two constituents. The idea was taken up and worked out carefully by his pupil Schwendener, to whose practical insight and industrial training the science owes other important ideas. From the fungus-like tissue of a lichen he isolated little greenish cells like Protococcus and other unicellular algze, and in some cases these green cells were able to live and multiply after they had been isolated. This remarkable fact led Schwendener to develop the hint given by De Bary, and to establish his so-called “dual hypothesis” of the nature of lichens. ‘‘As the result of my researches,” he says, ‘‘all these growths (lichens) are not simple plants, not individuals in the ordinary sense of the word; they are rather colonies consisting of hundreds and thousands of individuals, among which, however, one predominates, while the rest in perpet- ual captivity prepare the nutriment for themselves and their ‘master. This master is a fungus, a parasite which is accustomed to live upon others’ work; its slaves are green alge, which it has sought out, or indeed caught hold of and compelled into its service. It surrounds them, as a spider its prey, with a fibrous net of narrow meshes, which is gradually converted into an impenetrable covering ; but while the spider sucks its prey and leaves it dead, the fungus incites the algz found in its net to more rapid activity, indeed to more vigorous increase.” 116 Chapters in Modern Botany CHAP. The lichenologists were indignant at this proposed revolutionary demolition of their science, and defended the traditional position stoutly from 1869 to 1873; but in that year the leading algologist, M. Bornet, fully confirmed and extended the results of Schwendener, showing, for instance, that a single “lichen” might really contain three or four distinct and familiar species of algze, overrun and woven into a false tissue by a single mould ; while the lichenologist’s contention, that the green alga-like cells developed from the fungus-like filaments, was shown to be based on incorrect observation, the former being really only sucked by close- fitting or ingrowing protrusions of the latter. Another important proof was given by Stahl, who succeeded in making lichens artificially, ze. by taking a known alga, and sowing a known fungus upon it; a lichen, a £vowz lichen, was the result. How obstinately the controversy raged is oddly commemorated in the still current edition of the Encyclopedia Britannica; thus the article BOTANY retains the older view, while FUNGI states the modern, LICHENS . (of course by an eminent and conservative lichenologist) returns to the traditional standpoint, while this is finally corrected at PARASITISM. That, however, no shadow of doubt any longer exists on the matter is well shown by the recent researches of Bonnier, who has not only repeated the synthetic experiments of Stahl with all bacteriological precautions, but varied them by substituting the protonema or filamentous alga-like stage of the life-history of a moss for the ordinary alga constituent. Since then it is not only possible to separate the algze constituent, and to see it live independently while the fungal one as naturally starves, but also to combine the two elements into a unified life, we believe that a lichen is not a single, but a double organism,—an intimate union of an alga and a fungus, living in mutual helpfulness or symbiosis. After VI The Web of Life 117 we have considered the physiology of the leaf, we shall be better able to understand how really helpful the two kinds of elements which compose a lichen are to one another (see Fic. 6.—Patch of lichen grown synthetically by Bonnier (from sowing of fungus spores on algz) under bacteriological precautions against entrance of foreign spores. (After Bonnier.) chap. v.) Yet we already see that we have here a tiny repetition of the organic universe, of the Balance of Nature between plant and animal. The lichenologist is no real loser ; if his specimen has lost its individuality, it has gained 118 Chapters in Modern Botany CHAP. a higher and more complex one; and it is a microcosm of nature to boot. It is also interesting to notice the existence of what Zukal calls ‘‘half-lichens,” in which the mutual partnership has not been thoroughly established. For certain fungi usually occurring as lichens may, in certain conditions, live bereft of their partner-algze, as saprophytes ; while others, which are usually parasites or saprophytes, are sometimes found combined with algze, and forming lichens... We have thus what we may fairly consider lichens in the making. An interesting parallel to the case of lichens, in which the part of the fungus may be taken by animals of very various kinds, is that of the “yellow cells,” first known as practically constant features of the organisation of the Radiolarians, and long supposed to form an essential part of these, but also occurring in the tissues of many much higher organisms, especially Ccelenterates. Notably the large sea-anemone Azzhea cereus, which prospers and multi- plies greatly in consequence. These yellow cells: survive after their host dies, or even after being isolated; they divide like unicellular alge; they contain starch, and a pigment like that of Diatoms; they have a wall of cellu- lose, and they evolve oxygen during sunshine. There- fore they are regarded with justice as partner alge. As they live they remove carbonic acid and nitrogenous waste from their partners, and evolve oxygen which acceler- ates the vital processes of the animal; they form starch, which when dissolved passes out by exosmosis into the animal tissues; when they die they are digested. The partnership is therefore one of benefit to both parties. There has been considerable dispute as to details, but the general facts of this symbiosis between plants and animals are admitted by all. Strasburger has discovered an interesting association VI The Web of Life 119 between a small aquatic plant called Azolla and one of the freshwater algze called Anabzena. On the under surface of each floating leaf of Azolla there is a small opening leading into a cavity in the substance of. the leaf. In this cavity a colony of the alga is always found. The alga may indeed live independently in the water, but the Azolla is never without its partner. We are not, however, certain as to the precise meaning of the association ; and it may be doubted whether we have really here as yet any appreciable measure of true co-operation at all. Yet through such mere mechanical associations or juxtapositions true parasitism and sym- biosis must largely have arisen. One of the strangest kinds of reputed symbiosis is the occurrence of fungi around the roots of certain plants, Frank and others have shown that the root-tips of beeches, birches, hazels, and the like, are invested by a net of fungus- threads. They suggest that the net acts as a sort of sponge © intermediate between the roots and the soil, and this idea of the “ mycorhiza,” as the fungus network is called, receives some corroboration from the fact that in heaths and some orchids the fungus-threads actually penetrate into the sub- stance of the root. The question is, however, under hot discussion, the veteran arboriculturist, Hartig, stoutly main- taining that we have here nothing beyond a mere parasitism of the mould filaments upon the tree-roots. The controversy is one which the student may profitably follow, summarising (and where possible checking by actual observation) the arguments on either side. CHAPTER. VII RELATIONS BETWEEN PLANTS AND ANIMALS Plants and Snails—Plants and Ants—Domatia—Myrmecodia— Galls—FPlants and Aphides—Cats and Clover Relations between Plants and Animals.—Our study of insectivorous plants and of plant-movements have already shown us that the conventional distinctions between plants and animals are by no meansnatural. Of this fundamental unity Linnzeus had some conception when he united plants and animals under the common title Ovgavzsafa, in contrast to the mineral world of matter merely aggregated, not organised, as Cozserta, and to have demonstrated the essential unity of organic life is one of the most important characteristics of modern botany. ‘The reader will do well to consult Claude Bernard’s work, of which the title is a summary, Sur les Phénoménes de la Vie communs aux Animaux et aux Végétaux. Nor can it too clearly be realised that the distinctness of three kingdoms of nature, animal, vegetable, and mineral, however stereotyped in school-books, is, like much else we have learned there, a survival of medizval non-science. It is in fact a doctrine of the alchemists,. of which the whole science of biology, the whole doctrine of evolution is the confutation. How and where to divide the Organisata into Avzmalia and | -_—. he dh. tee cuar. vi Relations Between Plants and Antmals 121 Vegetabilia, with or without a common debatable land of Protista (or it may be a still more fundamental, albeit in its own way strangely specialised group of Myxomycetes), is an important controversy enough, yet after all a mere internal problem for biologists to settle among themselves. Recognising then plants and animals as two main groups of living creatures, which in their life have to solve essentially the same problems, we may profitably continue to study the many points of contact between them, inter- relations such as those which we have already described between insectivorous plants and their prey. Illustrations of these interactions are growing increasingly numerous. The seedlings, whose struggle for existence Darwin was fond of watching, were sometimes thinned by one another, and sometimes by the weather, but often by slugs, insects, and other animals. This is of course the most obvious of relations ; hosts of vegetarian animals feed upon plants. Nor is this relation so one-sided as it looks, for in so doing the animals are often of real service. The thrush which eats the mistleto-berries spreads the undigested seeds from tree to tree ; the bees which rob the flowers of their nectar are at the same time the bearers of fertilising pollen from plant to plant; even the cattle which wholly eat up some kinds of herbage give others more room in which to grow. That animals select one kind of plant and leave others untouched is an undoubted fact, and it is easy to believe that this selective process may have many important results. Let us take the case of snails and plants which was studied in great detail a few years ago by Professor Stahl,! the experimental lichenologist already quoted. Plants and Snails.—There are two kinds of snails— omnivorous eaters and “specialists,” as Stahl calls them. The latter are epicures, feeding daintily on toadstools and 1 Pflanzen und Schnecken, 8vo, Jena, 1888. 122 Chapters in Modern Botany CHAP. the like; the majority, unfortunately for our gardens, will eat almost any verdure they can find. ‘They have large appetites, able to devour an eighth part of their weight of cabbage in three hours; and as every gardener mourns, they are also very abundant ; 150 have been seen around a plot of but a square yard, and an industrious naturalist in one day collected 1200 of a single species from a piece of ground three-quarters of a mile square. But while snails, always excepting the “specialists,” are not fastidious in their diet, they draw the line somewhere. Certain plants they have found do not agree with them, and these they eschew. They try them, suffer for it, remember their experience, and leave the disagreeable plants alone for the future. Here then we have a problem such as a German professor loves, and which no one can tackle with more painstaking industry than he,—not even Darwin with his sundews,—to tempt innumerable snails with all manner of meats, to find out their favourite menu and their zvdex expurgatorius of viands, and to make a theory out of it. Thus Professor Stahl enumerates at least fifteen different ways in which plants have become abhorrent to snails, and are thereby protected. Some plants are too sour, others are poisonous ; some are full of ferments, others are rich in purgative oils. There may be bristling hairs which prick the sole of the snail’s “foot” as it creeps on the plant ; or limy and flinty armature which makes eating too slow and laborious even for a slug ; or slimy secretions which prevent the animals from getting a good grip; or, best of all, the tissues may contain thousands of little crystal needles which stick in the lips and make them smart. If you chew a little piece—let it be only a little piece— of the cuckoo-pint (Arum maculatum), which grows in the shady corner of the wood, your tongue and lips will be vi Relations Between Plants and Animals 123 painfully excited for some time afterwards. This property is much exaggerated in some of its allies; thus the ‘< Dumb-Cane” of the West Indies is associated with ugly tales of slave-torture, and even nowadays with occasional cruel practical joking among the usually gentle and kindly race of gardeners. Even in Arum some acrid poison is present, but the irritating effect is mainly due to the myriads of minute needle-like crystals (the raphides of old books on the microscope) which the plant’s tissues contain, and which pierce the soft skin of the experi- menter’s mouth. After this experience one remembers the cuckoo-pint ; can you believe that the snails, which both Darwin and Romanes credit with good memories, will forget? For years a taste or a smell will remain in the memory, and just as the suggestion of a mouthful of flinty horsetails gives one a “ goose-skin ” shiver, so the snail on a renewed impression of a disagreeable plant writhes its horns in disgust and turns away. Stahl’s research is doubtless valuable ; it is interesting to know of the fifteen different kinds of protection which save plants from being eaten by snails, though when he says that he finds no wild flowering plant—not even a tree —without some kind of protection against snails, the suspicion cannot be repressed that he is proving a great deal too much. And when he goes on to interpret these protective qualities as being in direct relation to the appe- tite of snails, to credit snails with being important factors in the evolution of these qualities, we emphatically protest against his conclusion. The notion is of course a familiar one, but the truth that there is in it may be falsely exaggerated. Something unusual happened within a plant and it became sour; the snails tasted it and left it alone, but ate up its relatives _ which remained sweet. These eaten up, the sour plant 124 Chapters in Modern Botany CHAP. was left to produce other sour plants, on which the snails, this time a trifle less fastidious, have to begin anew. They naturally select the sweeter, and hence “natural selection” preserves and propagates the sourer ; and so on indefinitely, and vegetation thus tends to grow sourer to all eternity. For this the snails are responsible! Meanwhile, too, natural selection, her ministers this time the browsing mammals, is at work producing thorny plants, and so on. It is the Darwinian theory in a nutshell; and its accept- ance or rejection is of fundamental importance to our whole conception of evolution. Not only to Professor Stahl, therefore, but to Mr. Wallace, to Professors Weis- mann and Ray Lankester, perhaps even to the mildly divergent Mr. Romanes, as assuredly to Sir John Lub- bock or Mr. Grant Allen, such interpretations seem not only valid but necessary, and not only necessary but sufficient. You have two dozen apples in your fruit-basket just begin- ning to spoil. Each day you take the two best, and at the end of a week there are ten rotten apples left. To a slight extent, it may be said, you are responsible for the growing rottenness, for you might have periodically selected the two worst, but the rottenness was there ; it not only arose, but increased without you. Your selection, it is true, has been such as to accelerate this disastrous evolution; a different selection would have retarded it; what selection can do then is at most to accelerate or slow the progress of the selected along its definite grooves of natural change. So the acids and ferments, the oils and encrustations, the hurtful hairs and crystal needles, are constitutional peculi- arities which arise in plants, and increase in them apart altogether from any snails. Nature is indeed a marvellous web, but we must not make it more tangled than it is. The sourness, the poison, the ferments, the crystals, are, vi Relations Between Plants and Animals 125 so to speak, “in the blood.” Their occurrence is wide- spread, and in some cases their primary meaning in the internal economy of the plant is well known. The idea that tannin has been developed as a protection against snails and other animal enemies is no doubt at first sight attractive, while we are ignorant of its real nature, and also while we ignore the fact that a plant rich in tannin, like the oak, may be peculiarly a prey to animal enemies, or meet it by inventing a fresh hypothesis of the adaptation of some animal enemies to withstand the defence. But when it, like the protective crystals, becomes viewed as essentially a waste product, and as therefore necessarily turned out in definite chemical proportion by the life-processes of the plant, the natural selectionist argu- ment recedes as far into the background as it would have to do in explaining the origin of the crystalline form or taste of any chemical product, the colour of any precipitate, the lustre or specific gravity of a mineral. We laugh at those who said, “So are fleas black that they may be caught more readily upon a white ground,” but are we becoming wiser now, if it be true, as Professor Stahl thinks, and we fear only with too much justice, that the majority of modern naturalists would corroborate the opinion that the protective characters of plants stand in direct causal connection with the appetite of animals? To give snails credit for evolving plants with crystals, sourness, and poison, to make cattle and the like responsible for the thorns on plants, is like giving snakes the credit of evolving boots which protect our heels, In all these cases alike the possibility of some defensive utility is undenied, nor even of some improvement through selective agency; what is contended for is, however, a change in our evolutionary perspective, laying increased importance upon the definite- ness and cumulativeness of the internal variation and con- 126 Chapters in Modern Botany CHAP. sequently a diminished stress upon the external selection which plays upon this. Hence we have all sympathy with a recent critic, Dr. Jumelle, who, in reviewing a recent endeavour to explain not only the presence but the position of alkaloids, etc., in plants by showing how defensive they are, says: ‘ The final causes to which so many authors constantly appeal, have indeed the advantage of supplying an easy explana- tion of embarrassing facts, but they are hurtful to the progress of science, since the mind duped by an illusory satisfaction is dissuaded from further investigation.” Plants and Ants.—Both gardeners and botanists have long been aware that ants are among the most frequent guests of flowering plants, and such names as J/yrmecodia, Euphorbia formicarum, refer to this. Are these visitors hurtful? In most cases only to a slight extent, for although they rob the flowers of honey, they compensate for this by eating other small insects which would do the plants much harm. They are useful to the plant but not to its flowers, for as the worker-ants are not winged, they are not suited for carrying the fertilising pollen from plant to plant as bees and flies often do. They devour pollen and nectar without fulfilling any useful function, and it is said that when there are crowds of them about a flower the bees are apt to have their noses somewhat rudely pulled when they thrust them into the recesses of the flower, and this will obviously tend to frighten away the bees. But without attaching much importance to this allegation that the ants assault the bees, we see that it is advantageous that the ants should be excluded from the flowers. In many plants this is effectively done. There may be, as in some of the teasel tribe, chevaux-de-frise of stiff, downward - pointing hairs, like those inside of a pitcher plant, against which the ants cannot climb. There may be sticky parts of the stem Lite A vi Relations Between Plants and Animals 127 which the insects cannot cross, as we see in some of the catchflies, e.g. Lychnis viscaria and others. There may be very slippery stems on which the climbers can find no foothold, “and the flowers are often pendulous, as in snowdrop and cyclamen, creeping creatures being thus kept out of them, just as the pendulous nests of the weaver bird are a protection against snakes and other enemies.” Sir John Lubbock quotes from Kerner, whose charming /lowers and their Unbidden Guests is most full of information on this head, the case of Polygonum amphibium, a common pond-weed, which, as its name suggests, lives amphibiously, sometimes in water, sometimes on land. ‘So long as it grows in water it is protected by the water, and its stem is smooth; but, on the other hand, those specimens which live on land throw out certain hairs, which terminate in sticky glands, and thus prevent small insects from creeping up to the flowers. In this case, therefore, the plant is not sticky, except just when this condition is useful.” In many cases, too, the flowers, like those of the snapdragon, are virtually closed boxes which can be opened by the bees, but not by the small ants, unless indeed these bite a hole through the base of the petals. But it may be asked, if the ants are excluded from the flowers in any of these ways, why do they visit the plants - at all? Partly, no doubt, by way of experiment, partly for the sake of the booty of smaller insects which they find about the leaves, but also because the secretion of nectar is not always confined to the flowers, but may take place on other parts of the plant—in what are called extra-floral nectaries, of which something will afterwards be said. But the web has another mesh. As long ago as 1688 John Ray noted the constant occurrence of ants in the hollow stems of the South American Cecrofia palmata, and many other naturalists had called attention to the same 128 Chapters in Modern Botany CHAP. and similar cases. But precise knowledge of the meaning of this partnership was not attained till 1874, when Belt, the naturalist of Nicaragua, and Delpino, an Italian botanist, cleared up the whole matter. Let us quote Belt’s account! of his discovery: ‘The thorns of the bull’s- horn Acacia are hollow, and are tenanted by ants that make a small hole for their entrance and exit near one end of the thorn, and also burrow through the partition that separates the two horns, so that one entrance serves for both. Here they rear their young, and in the wet season every one of the thorns is tenanted, and hundreds of ants are to be seen running about, especially over the younger leaves. If one of them be touched, or a branch shaken, the little ants (Pseudomyrma bicolor) swarm out from the hollow thorns, and attack the aggressor with jaws and sting. They sting severely, raising a little white lump that does not disappear in less than twenty-four hours. They form a most efficient standing army for the plant, which prevents not only the mammalia from browsing on the leaves, but delivers it from the attacks of a much more dangerous enemy—the leaf-cutting ant. For these services the ants are not only securely housed by the plant, but are provided with a bountiful supply of food, and to secure their attendance at the right time and place, the food is so arranged and distributed as to effect that object with wonderful perfection.” There is a sweet gland at the base of each pair of leaflets on the bipinnate leaf, and a little yellow pear-like body at the end of each small division of the compound leaf which is carried off when ripe. The young thorns are soft and filled with sweet pulp, so that the ant finds its house full of food. As it hollows this out, the thorn increases in size and bulges out towards the base. 1 The Naturalist in Nicaragua. Lond. 1874. vi Relations Between Plants and Animals 129 ‘“‘These ants seem at first sight to lead the happiest of existences. Protected by their stings, they fear no foe. Habitations full of food are provided for them to commence housekeeping with, and cups of nectar and luscious fruits await them every day. But there is a reverse to the picture. In the dry season on the plains the acacias cease to grow. No young leaves are produced, and the old glands do not secrete honey. Then want and hunger overtake the ants that have revelled in luxury all the wet season; many of the thorns are depopulated, and only a few ants live through the season of scarcity. As soon, however, as the first rains set in, the trees throw out numerous vigorous shoots, and the ants multiply again with astonishing rapidity.” A more recent traveller, Professor A. F. W. Schimper, who has reinvestigated the whole matter, has gathered some new facts of much interest, and the student may be referred to Schimper’s book! not only on its own account, but for the bibliography of the subject which it contains. He tells us first of the destructive ravages of the leaf- cutting ants, the sight of whose march soon becomes familiar to the traveller in tropical America. In a minute or two a sixpence-like circle is cut from a leaf and the ant marches off with its burden. Only the dry and the very young leaves are spared, and armies of thousands of ants soon make sad havoc of a tree’s foliage. It is not quite certain what the ants do with the leaves which they carry home: Bates believed that they were used as a lining for the subterranean galleries ; Belt supposed that the ants fed upon fungi which grew upon the decaying leaves ; M‘Cook observed, in the case of Atta fervens and A. septentrionalis, that a papery material was manufactured from the leaves and used in the 1 Die Wechselbezichungen swischen Pflanzen und Ameisen in tropischen Amerika, 8vo. Jena, 1888. Cp. E. Huth, IZyrmecophile und Myrmecophobe Pflanzen. Berlin, 1887. K 130 Chapters in Modern Botany CHAP. internal furnishing of the ant’s nest ; but the subject requires further investigation. The destructive leaf-cutters have certain preferences ; imported plants, such as oranges, roses, coffee-plants, and mango, are greedily attacked ; Solanaceee and grasses are left untouched, but in South Brazil Schimper found that the guava, a Caladium, Cassia neglecta, and Alchornea tricurana were peculiarly liable to be ruined. It is certain that the presence of ethereal oils and ferments is not always a deterrent to the ants, whatever it may be to snails, else orange, guava, mango, rose, etc., would not be such favourites. There can be little doubt that the leaf-cutters would soon exterminate these and other imported plants if no precautions were used, and it is likely that they have in this way exterminated many indigenous species. The leaves of the common orange and the bitter orange are eagerly used, but those of the mandarine and the lemon are avoided. Thus in the natural course of things the two first would be eliminated, the two last preserved. This may be taken as a good example of the action of natural selection, but we cannot of course ascribe to the influence of the ants the qualities which save the lemon and the mandarine. In the province of Canton in China it is the custom to place nests of harmless, tree-inhabiting ants upon the orange- trees in order to defend these from the attacks of the leaf- cutters, This is but an intentional imitation of what has taken place in the course of nature. For, as Belt has told us, the ants which inhabit various species of Cecropia, Cassza, and other plants serve as a bodyguard against destructive intruders. Let us follow Schimper’s account of this ‘* symbiosis.” Schimper’s problem, shortly stated, is whether the plants, which are so constantly furnished with a bodyguard of ants, exhibit structures which can be definitely regarded as adapta- vu Relations Between Plants and Animals 131 tions due to the symbiosis. ‘“ Of late,” he says, “‘ naturalists have become more and more accustomed to interpret all structural peculiarities which are seen to be useful for a certain purpose, as if they had originated for that purpose.” This Schimper rightly regards as unscientific. All such interpretations must rest on observation and _ experi- ment, Many ants live in the nooks which plants often afford— in the axils of leaves, among the tangled roots of epiphytes, inside old galls, and so on; others build nests which they fix to the branches; others bore labyrinthine passages in the dead bark of trees. ‘‘The fanatics of biology” (ze. bionomics) ‘are inclined to find adaptations in all such cases of constant or usual symbiosis. The chambers of 77z/landsia bulbosa, the feltwork of aerial roots in the case of many epiphytes, the cavities of the stem and branches in Triplaris, are all for the reception of the protective ants. But we know that these structures have quite another meaning : the cavities in the base of Tillandsia are dried-up cisterns, the feltwork of aerial roots collects moisture and humus, the hollow stems of Triplaris serve to combine maximum elasticity with minimum material.” _ Where it can be shown that certain plants of which the leaf-cutters are fond have a bodyguard of ants, there is no reason to doubt that these are of protective advantage. This has been repeatedly proved by observation, especially in the case of some species of Cecropia (Imbauba or trumpet tree). These trees have smooth upright stems, raised on short aerial roots, and bearing simple branches, the whole appearance suggesting a gigantic candelabra. The leaves are few but very large. Now when one shakes a branch of the Cecropia, one rouses a wild army of ants, which with poisonous jaws resent the intrusion. Where do they all come from? Closer inspection shows little round apertures, 132 Chapters in Modern Botany CHAP. especially on the upper internodes, which lead into the hollow stem. In walking with the well-known naturalist Fritz Miller, Schimper saw a small Imbauba-tree which had been stripped of its leaves by the leaf-cutters. Fritz Miiller ventured to affirm that in this case the bodyguard must have been absent, and in slicing up the stem he found not one. This was not an isolated case; the same has been repeatedly observed, so that we may safely conclude that a Cecropia tenanted by its bodyguard is relatively safe from the leaf- cutters. The hollow stem with its horizontal partitions is plainly a comfortable home for the protective ants, but does it exhibit any structural peculiarities which must be referred to the insects? This cannot be said of the cavities them- selves, but what of the doors? Each internode has at one time a door, which is obliterated by subsequent growth. The door is always in the same position ; it is made by the ants at a spot where the wall of the stem thins away in an oval depression, and this is originally due to the pressure of a bud. There is no doubt that the beginning of the door arises quite apart from the ants, but on the other hand Schimper has detected a number of minute structural peculiarities connected with the door which he cannot explain except as adaptations to the visitors. On Cecropias which grow on the hill of Corcovado near Rio de Janeiro, and have very smooth wax-covered skins, ants are unrepre- sented, and the minute structure around the slight depression at which a door, were there one, would be formed, is quite different. The ants find shelter within the Cecropia stem, but the tree also affords them abundant food. Near the base of the leaf-stalk there is on the under surface a small area covered with brown velvet-like hair, and on the surface of vi Relations Between Plants and Animals 133 this are numerous pear-shaped or oval little bodies which look like insects’ eggs. These (‘‘ Miiller’s bodies”) are eaten by the ants, and they must be very nutritious, for their contents are rich in albuminoids and fatty oil. In their young stages they resemble the glands of other plants which secrete mucus or resin, and Francis Darwin has suggested that they are homologous with glands, but they are very peculiar, and their peculiarities Schimper would connect with the ants, for, strange to say, they are absent on Cecropias, such as those of Corcovado, on which there are no ants. From Cecropia Schimper passes to other myrmecophilous plants, such as Acacia spherocephala, whose hollow thorns afford shelter to a bodyguard, and which also bears little food-bodies comparable to those on Cecropia. But we have given sufficient illustration of this matter. There is, however, another much-discussed subject on which Schimper has something interesting to tell us. We know that many of our flowers have honey-bags or nectaries, and that these are attractive lures to the bees and other insects which, in their search for sweets, carry the fertilising pollen from flower to flower. But often, especially in the Tropics, nectaries occur outside the flowers, as we have seen in Nepenthes. Both Belt and Delpino regard these extra-floral nectaries as adaptations for the attraction of protective ants. Kerner - supposes that they afford such generous supplies of nectaries that the ants leave the floral nectaries undisturbed ; but this is not the way with ants! Bonnier regards them as stores of reserve-material ; but the sugar is almost always stolen by ants or washed away by the rain. Johow has even suggested that they are receptacles for the waste-products formed in the movements of the leaves! but this too is for several reasons most unlikely, since it is enough to notice that 134 Chapters in Modern Botany CHAP: they are especially large and numerous on the bracts which exhibit no movements. Schimper concludes that the theory of Belt and Delpino is the only one which need be seriously considered. But before it can be believed that the extra-floral nectaries have arisen as adaptations to the protective ants, it must be shown, as Schimper rightly observes :— (1) That the visits of the ants thus attracted afford so much protection, that without them the plant would be at a great disadvantage ; and (2) That the nectaries have not some other use in the economy of the plant, to which they are primarily referable. By removing the extra-floral nectaries from various plants, Schimper has convinced himself that they are unnecessary and not demonstrably important for the wellbeing of the plant. They are most abundant where there are most ants. They certainly attract the ants, and these visitors sometimes, but by no means always ward off leaf-cutters. The plants which possess them may be worsted, but none the less they had an advantage as far as it went. In short, Schimper accepts the suggestion of Belt and Delpino. This con- clusion has been further corroborated by W. Burck,! who finds that almost all plants with extra-floral nectaries or food-bodies are truly myrmecophilous, and proves by direct observation that the bodyguard of ants attracted by the nectaries are of important service in driving off marauding insects which spoil the foliage or perforate the corollas of the flowers without effecting fertilisation. Domatia.—The homes which ants find inside the Acacia’s thorns, or within the hollow stems of Cecropia, are not unique; Dr. Lundstrém has described under the title of domatia (meaning little homes) a large number of shelters on plants which are tenanted by harmless little 1 Ann. Jardin Bot, Buitenzorg, x. (1891). vi Relations Between Plants and Animals 135 insects and mites. Sometimes they are like little wig- wams formed from converging hairs, sometimes minute caves or pits. They are not formed by the tenants, at least not now, for they are natural to the plants, but they are none the less well adapted to the use they serve. A’ simple instance may be seen on the leaves of the lime-tree, on the under surface of which, at points where two veins of the leaf cross, there are little nooks tenanted by mites. These do not injure the plant, but rather help it, for they clear away minute fungi, and it is also possible that their nitrogenous waste-products are absorbed by the leaves. Myrmecodia.—A pretty tale is that of Myrmecodia tuberosa, a rubiaceous plant from the Malayan Archipelago. The worthy Rumphius, still memorable as the pioneer naturalist of these regions, ‘‘describes it in his Herbarium Amboynense (1750) under the formidable but appropriate name of Widus germinans formicarum rubrarum, and terms it ‘‘Arodigtum nature.” He seems to have been uncertain whether the whole was a vegetable, or whether the tuber was an ant’s nest from which the plant sprung ; he says it is to be regarded as a zoophyte among vege- tables! It presents the form of a large irregular tuber growing on the branches of old trees; from this spring a few thick fleshy stems, having a small number of smooth, leathery, oblong leaves crowded together at their summits. The small white sessile flowers are situated at the base of the petioles, and almost concealed by the large persistent stipules. The tuber is tenanted by small and very fierce red ants, which rush out upon the intruder if their dwelling is attacked. The way in which these ants take possession of the MWyrmecodia, and the intimate relation which exists between the plant and the insect, are thus feferred to in Professor Caruel’s recent paper upon the genus.! The 1 Nuovo Giornale Botanico Italiano, iv. pp. 170-176 (1872), 136 Chapters in Modern Botany CHAP. account is quoted from a manuscript note by Dr. Beccari, who collected the plant in Borneo :— ‘‘ | have carefully followed the development of this tuber, having been able to observe the young plants in all stages of growth from the period of germination. The seed is surrounded by a viscid pulp, resembling that of our mistle- toe, which readily attaches itself to the branches of the trees upon which it falls, Its dissemination is probably caused by means of the birds which eat the fruit, the undigested seed passing through them and adhering to the branches. The seed soon germinates and unfolds its cotyledons, especially if it has fallen in an opening of a branch where lichens have collected, or if it be placed in mould ; the stem develops itself to the length of from three to six millimetres, widening towards the base, acquiring a somewhat conical shape, with the two cotyledons at its apex. In this condition it remains until a particular species of ant burrows a small lateral cavity at the base of the stem; if this does not happen, the stem does not develop itself, and the plant dies. The wound caused by the bite of the ant determines a great development of cellular tissue, in the same way as the sting of the cynips causes the galls on the oak. The tuber now enlarges and the stem develops ; the ants soon find sufficient space for forming a colony, and excavate galleries in the interior of the tuber in all directions, thus making for themselves a living habita- tion—a circumstance which is necessary to the existence of the plant. The plant could not live or even arrive at maturity unless the ants contributed to the formation of the organ which must be the source from which it derives its support, while in all probability the ants could not exist or propagate thémselves unless they had discovered this mode quoted by Britten, ‘‘Ant-supporting Plants,” Popular Science Review. vi Relations Between Plants and Animals 137 of constructing so ingenious a habitation. The fleshy sub- stance of this formicarium is formed of cellular tissue; the channels and galleries with which it is perforated have their entrance near the lower part of the tuber.” Unluckily for Caruel and Beccari, the development of Myr- mecodia has been reinvestigated lately by Treub,' in Java, who sadly diminishes the wonder by showing that the plant can thrive without its guests; and that the galleries are formed and grow as congenital peculiarities without the aid of ants. By rearing plants from the seed, in the absence of all ants, he has been able to study the growth of the tuberosities. They seem to him to be at first reservoirs in which water is stored for the needs of the plant, the firm outer surface (devoid of stomata or lenticels) preventing eva- poration. The ants simply use these when dry as dwellings, without in any marked way benefiting the plant. This isa fresh lesson of caution, and of the risks of Darwinising over-much. A good specimen may be observed at Kew. Galls. — These interesting domatia above referred to must of course be distinguished from the galls which many insects form on plants, Every one knows the large gall-nuts (rich in tannin and gallic acid, both useful in ink- making, etc.) formed on oak-leaves by wasp-like insects of the genus Cynips, and the strange ‘ Bedeguar” tufts so common as mossy excrescences on wild roses. These abnormal growths are produced by a remarkable vegetative increase of the tissues of the plants. This is due to the irritation produced by the gall-flies, or rather gall-wasps, which lay their eggs in the soft substance of the plant. Here again, as with lichens or ants, or what not, we are but at the threshold of a new subject with its literature and its specialists. For along every main road of the organic 1 «* Nouvelles Recherches sur le Myrmecodia de Java,” Annales du Jardin Botanigue de Buitenzorg, vol, vii. (1888), p. 191. 138 Chapters n Modern Botany CHAP. sciences there branch off innumerable pleasant byways, each leading into a tiny world of its own, a minor infinity ; at first sight no doubt a hazy labyrinth, yet on deepening study an ordered microcosm of evolutionary law. Plants and Aphides.—From ants we naturally pass to the plant-lice or Aphides, for these little insects which form sweet juice receive much attention from the ants, who sometimes use them as cows.! Many of them, such as those which infest roses, fruit-trees, and hops, are exceed- ingly injurious to the plants, for they suck the sap, choke the pores of the leaves with their honey-dew, and do other damage. The honey-dew, of which the ants are so fond, has been for long the subject of much discussion. Pliny, and many later naturalists who should have known better, said that it fell from heaven; many have described it as an exudation from plants, while other — perhaps most— naturalists speak of two kinds—an animal honey-dew formed by the Aphides, and a vegetable honey-dew exuded in some way or other from the plants. Biisgen’s recent observations ? lead him to affirm con- clusively that all honey-dew, excepting sugary exudations caused by parasitic fungi, is an excretion of the Aphides, The only flow of sap from the cells of the plants is into the mouths of the insects; the sweet juices are slightly changed in the food-canal, but the quantity of glucose in this is out of proportion to the animal’s wants, and hence what is unused as food passes out. As a single aphis may form as many as forty-eight drops of honey-dew in twenty- four hours, it is not surprising that a very rain of nectar should sometimes fall from trees (especially limes) which 1 See conveniently J. Arthur Thomson, The Study of Animal Life. Lond. 1892. 2 «(Das Honig-Tau,” Jenaische Zeitschritt fiir Naturwissenschaft, vol, xxv. (1891), pp. 339-428. 2 Pls. vi Relations Between Plants and Animals 139 are infested by thousands of these insects. Besides sapping the strength of the plant, and choking the leaves with honey-dew, the Aphides by their secretion make it easier for injurious parasitic fungi to establish themselves upon the leaves. On the other hand, they attract ants, whose presence in moderate numbers at any rate may be useful to the plants.! But as Herr Biisgen calculates that in one case the quantity of carbo-hydrate material absorbed by the Aphides from a plant was about one sixth of that required to furnish the whole foliage, we must agree with him that this is too high a price to pay for problematical benefits. The Ants and Aphides must serve as types of the injurious insects, between which and plants there are numerous interesting relations. We have to consider how far various structures of plants, such as hairy stems, viscid stems, pendent flowers, and the like, serve to save plants from their enemies, as may be true in the case of unwel- come ants; we have “also to notice what changes the injurious insects, such as corn-insects, Phylloxera, Weevils, etc., may effect on the plants which they infest ; and we must also observe how the hostile insects, which affect forest trees and vegetation generally, may occasion changes which have far-reaching influences on the fauna, flora, scenery, and even climate of a country-side. Readily available information will be found in Miss E. A. Ormerod’s valuable work on Jzjurious Insects (second edition, Lond. 1891), while the more advanced student would do well to 1 With all respect to this observer, one may still maintain that during prolonged fine weather, especially in June, while leaf tissues are fresh and young, and Aphides not yet abundant, such an excess of assimilation sometimes takes place as to create an overflow of nectar from leaves; the very ferns sometimes showing this with- out Aphides upon them, or at any rate in adequate numbers. 140 Chapters in Modern Botany CHAP. consult the researches of Riley, Packard, and others in the Bulletins of the United States Entomological Commission, from which many detailed illustrations of the web of life may be gleaned. Cats and Clover.—We have only given a few illustra- tions of the infinite number of interactions between plants and animals ; other examples and of more importance, e.g. in connection with the fertilisation of the flowers and the scattering of seeds, will be referred to again, As with insectivorous or moving plants, the writer’s aim is to familiarise the reader with the essential point of view at which Darwin has placed us,—his appreciation of the dra- matic complexity of nature; as also of the task of the least. Nature is no longer a mere confused multitude of specimens to be collected and analysed, but each organism is linked with others as consecutively as in the ‘‘ House that Jack Built”; nay, with indefinite cross relations as well: what seemed a unit is a link; what seemed a chain is but a thread within the labyrinthine web of nature. That in thus opening out for us this new and fascinating study of bionomics, he has exaggerated the importance of the selective factor in evolution, and that the drama is not merely of incident and adventure but of character also, is a secondary consideration, though an important one; since but for Darwin we might hardly yet have realised that there is an organic drama at all. He takes a ball of mud from off the leg of a bird, and finds that out of it no less than eighty seeds germinate! He shows how cattle absolutely determine the existence of the Scotch fir on the Surrey heath, or how insects deter- mine the absence of wild cattle and horses in Paraguay. “If certain insectivorous birds were to decrease in Para- guay, the parasitic insects would probably increase ; and this would lessen the number of flies which destroy the vi Relations Between Plants and Animals 141 young horses and cattle—then the latter would become feral, and this would certainly greatly alter the vegetation ; this again would largely affect the insects; and this the insectivorous birds, and so onwards in ever-increasing circles of complexity.” So, too, with the terrible tsetse fly, which, as Livingstone pointed out, renders pasturage and animal transport impossible within its region, so condemning civili- sation to a lower type. But perhaps his best illustration is the most familiar one, which should never become trite to us. ‘‘ Plants and animals, remote in the scale of nature, are bound together by a web of complex relations. ... I have found from experiments that humble-bees are almost indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are necessary for the fertilisation of some kinds of clover—thus, 100 heads of red clover (77ifolium pratense) produced 27,000 seeds, but the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great measure on the number of field-mice, which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble-bees, believes that more than two-thirds of them are thus destroyed all over England.”! Now the number of mice is largely dependent, as every one knows, on the number of cats; and Colonel Newman says, “ Near villages and small towns I have found the nests of humble- bees more numerous than elsewhere, which I attribute to ! Origin of Species, chap. iii. Lond. 1859. 142 Chapters