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Darwin and Modern Science (1909)

Edited by A.C. Seward


XIX. DARWIN'S WORK ON THE MOVEMENTS OF PLANTS.

By FRANCIS DARWIN,
Honorary Fellow of Christ's College, Cambridge.

M
y father's interest in plants was of two kinds, which may be roughly distinguished as evolutionary and physiological. Thus in his purely evolutionary work, for instance in "The Origin of Species" and in his book on "Variation under Domestication", plants as well as animals served as material for his generalisations. He was largely dependent on the work of others for the facts used in the evolutionary work, and despised himself for belonging to the "blessed gang" of compilers. And he correspondingly rejoiced in the employment of his wonderful power of observation in the physiological problems which occupied so much of his later life. But inasmuch as he felt evolution to be his life's work, he regarded himself as something of an idler in observing climbing plants, insectivorous plants, orchids, etc. In this physiological work he was to a large extent urged on by his passionate desire to understand the machinery of all living things. But though it is true that he worked at physiological problems in the naturalist's spirit of curiosity, yet there was always present to him the bearing of his facts on the problem of evolution. His interests, physiological and evolutionary, were indeed so interwoven that they cannot be sharply separated. Thus his original interest in the fertilisation of flowers was evolutionary. "I was led" ("Life and Letters", I. page 90.), he says, "to attend to the cross-fertilisation of flowers by the aid of insects, from having come to the conclusion in my speculations on the origin of species, that crossing played an important part in keeping specific forms constant." In the same way the value of his experimental work on heterostyled plants crystalised out in his mind into the conclusion that the product of illegitimate unions are equivalent to hybrids--a conclusion of the greatest interest from an evolutionary point of view. And again his work "Cross and Self Fertilisation" may be condensed to a point of view of great importance in reference to the meaning and origin of sexual reproduction. (See Professor Goebel's article in the present volume.)

The whole of his physiological work may be looked at as an illustration of the potency of his theory as an "instrument for the extension of the realm of natural knowledge." (Huxley in Darwin's "Life and Letters." II. page 204.)

His doctrine of natural selection gave, as is well known, an impulse to the investigation of the use of organs--and thus created the great school of what is known in Germany as Biology--a department of science for which no English word exists except the rather vague term Natural History. This was especially the case in floral biology, and it is interesting to see with what hesitation he at first expressed the value of his book on Orchids ("Life and Letters", III. page 254.), "It will perhaps serve to illustrate how Natural History may be worked under the belief of the modification of species" (1861). And in 1862 he speaks (Loc. cit.) more definitely of the relation of his work to natural selection: "I can show the meaning of some of the apparently meaningless ridges (and) horns; who will now venture to say that this or that structure is useless?" It is the fashion now to minimise the value of this class of work, and we even find it said by a modern writer that to inquire into the ends subserved by organs is not a scientific problem. Those who take this view surely forget that the structure of all living things is, as a whole, adaptive, and that a knowledge of how the present forms come to be what they are includes a knowledge of why they survived. They forget that the SUMMATION of variations on which divergence depends is under the rule of the environment considered as a selective force. They forget that the scientific study of the interdependence of organisms is only possible through a knowledge of the machinery of the units. And that, therefore, the investigation of such widely interesting subjects as extinction and distribution must include a knowledge of function. It is only those who follow this line of work who get to see the importance of minute points of structure and understand as my father did even in 1842, as shown in his sketch of the "Origin" (Now being prepared for publication.), that every grain of sand counts for something in the balance. Much that is confidently stated about the uselessness of different organs would never have been written if the naturalist spirit were commoner nowadays. This spirit is strikingly shown in my father's work on the movements of plants. The circumstance that botanists had not, as a class, realised the interest of the subject accounts for the fact that he was able to gather such a rich harvest of results from such a familiar object as a twining plant. The subject had been investigated by H. von Mohl, Palm, and Dutrochet, but they failed not only to master the problem but (which here concerns us) to give the absorbing interest of Darwin's book to what they discovered.

His work on climbing plants was his first sustained piece of work on the physiology of movement, and he remarks in 1864: "This has been new sort of work for me." ("Life and Letters", III. page 315. He had, however, made a beginning on the movements of Drosera.) He goes on to remark with something of surprise, "I have been pleased to find what a capital guide for observations a full conviction of the change of species is."

It was this point of view that enabled him to develop a broad conception of the power of climbing as an adaptation by means of which plants are enabled to reach the light. Instead of being compelled to construct a stem of sufficient strength to stand alone, they succeed in the struggle by making use of other plants as supports. He showed that the great class of tendril- and root-climbers which do not depend on twining round a pole, like a scarlet-runner, but on attaching themselves as they grow upwards, effect an economy. Thus a Phaseolus has to manufacture a stem three feet in length to reach a height of two feet above the ground, whereas a pea "which had ascended to the same height by the aid of its tendrils, was but little longer than the height reached." ("Climbing Plants" (2nd edition 1875), page 193.)

Thus he was led on to the belief that TWINING is the more ancient form of climbing, and that tendril-climbers have been developed from twiners. In accordance with this view we find LEAF-CLIMBERS, which may be looked on as incipient tendril-bearers, occurring in the same genera with simple twiners. (Loc. cit. page 195.) He called attention to the case of Maurandia semperflorens in which the young flower-stalks revolve spontaneously and are sensitive to a touch, but neither of these qualities is of any perceptible value to the species. This forced him to believe that in other young plants the rudiments of the faculty needed for twining would be found--a prophecy which he made good in his "Power of Movement" many years later.

In "Climbing Plants" he did little more than point out the remarkable fact that the habit of climbing is widely scattered through the vegetable kingdom. Thus climbers are to be found in 35 out of the 59 Phanerogamic Alliances of Lindley, so that "the conclusion is forced on our minds that the capacity of revolving (If a twining plant, e.g. a hop, is observed before it has begun to ascend a pole, it will be noticed that, owing to the curvature of the stem, the tip is not vertical but hangs over in a roughly horizontal position. If such a shoot is watched it will be found that if, for instance, it points to the north at a given hour, it will be found after a short interval pointing north-east, then east, and after about two hours it will once more be looking northward. The curvature of the stem depends on one side growing quicker than the opposite side, and the revolving movement, i.e. circumnutation, depends on the region of quickest growth creeping gradually round the stem from south through west to south again. Other plants, e.g. Phaseolus, revolve in the opposite direction.), on which most climbers depend, is inherent, though undeveloped, in almost every plant in the vegetable kingdom." ("Climbing Plants", page 205.)

In the "Origin" (Edition I. page 427, Edition VI. page 374.) Darwin speaks of the "apparent paradox, that the very same characters are analogical when one class or order is compared with another, but give true affinities when the members of the same class or order are compared one with another." In this way we might perhaps say that the climbing of an ivy and a hop are analogical; the resemblance depending on the adaptive result rather than on community of blood; whereas the relation between a leaf-climber and a true tendril-bearer reveals descent. This particular resemblance was one in which my father took especial delight. He has described an interesting case occurring in the Fumariaceae. ("Climbing Plants", page 195.) "The terminal leaflets of the leaf-climbing Fumaria officinalis are not smaller than the other leaflets; those of the leaf-climbing Adlumia cirrhosa are greatly reduced; those of Corydalis claviculata (a plant which may be indifferently called a leaf-climber or a tendril-bearer) are either reduced to microscopical dimensions or have their blades wholly aborted, so that this plant is actually in a state of transition; and finally in the Dicentra the tendrils are perfectly characterized."

It is a remarkable fact that the quality which, broadly speaking, forms the basis of the climbing habit (namely revolving nutation, otherwise known as circumnutation) subserves two distinct ends. One of these is the finding of a support, and this is common to twiners and tendrils. Here the value ends as far as tendril-climbers are concerned, but in twiners Darwin believed that the act of climbing round a support is a continuation of the revolving movement (circumnutation). If we imagine a man swinging a rope round his head and if we suppose the rope to strike a vertical post, the free end will twine round it. This may serve as a rough model of twining as explained in the "Movements and Habits of Climbing Plants". It is on these points--the nature of revolving nutation and the mechanism of twining--that modern physiologists differ from Darwin. (See the discussion in Pfeffer's "The Physiology of Plants" Eng. Tr. (Oxford, 1906), III. page 34, where the literature is given. Also Jost, "Vorlesungen uber Pflanzenphysiologie", page 562, Jena, 1904.)

Their criticism originated in observations made on a revolving shoot which is removed from the action of gravity by keeping the plant slowly rotating about a horizontal axis by means of the instrument known as a klinostat. Under these conditions circumnutation becomes irregular or ceases altogether. When the same experiment is made with a plant which has twined spirally up a stick, the process of climbing is checked and the last few turns become loosened or actually untwisted. From this it has been argued that Darwin was wrong in his description of circumnutation as an automatic change in the region of quickest growth. When the free end of a revolving shoot points towards the north there is no doubt that the south side has been elongating more than the north; after a time it is plain from the shoot hanging over to the east that the west side of the plant has grown most, and so on. This rhythmic change of the position of the region of greatest growth Darwin ascribes to an unknown internal regulating power. Some modern physiologists, however, attempt to explain the revolving movement as due to a particular form of sensitiveness to gravitation which it is not necessary to discuss in detail in this place. It is sufficient for my purpose to point out that Darwin's explanation of circumnutation is not universally accepted. Personally I believe that circumnutation is automatic--is primarily due to internal stimuli. It is however in some way connected with gravitational sensitiveness, since the movement normally occurs round a vertical line. It is not unnatural that, when the plant has no external stimulus by which the vertical can be recognised, the revolving movement should be upset.

Very much the same may be said of the act of twining, namely that most physiologists refuse to accept Darwin's view (above referred to) that twining is the direct result of circumnutation. Everyone must allow that the two phenomena are in some way connected, since a plant which circumnutates clockwise, i.e. with the sun, twines in the same direction, and vice versa. It must also be granted that geotropism has a bearing on the problem, since all plants twine upwards, and cannot twine along a horizontal support. But how these two factors are combined, and whether any (and if so what) other factors contribute, we cannot say. If we give up Darwin's explanation, we must at the same time say with Pfeffer that "the causes of twining are...unknown." ("The Physiology of Plants", Eng. Tr. (Oxford, 1906), III. page 37.)

Let us leave this difficult question and consider some other points made out in the progress of the work on climbing plants. One result of what he called his "niggling" ("Life and Letters", III. page 312.) work on tendrils was the discovery of the delicacy of their sense of touch, and the rapidity of their movement. Thus in a passion-flower tendril, a bit of platinum wire weighing 1.2 mg. produced curvature ("Climbing Plants", page 171.), as did a loop of cotton weighing 2 mg. Pfeffer ("Untersuchungen a.d. Bot. Inst. z. Tubingen", Bd. I. 1881-85, page 506.), however, subsequently found much greater sensitiveness: thus the tendril of Sicyos angulatus reacted to 0.00025 mg., but this only occurred when the delicate rider of cottonwool fibre was disturbed by the wind. The same author expanded and explained in a most interesting way the meaning of Darwin's observation that tendrils are not stimulated to movement by drops of water resting on them. Pfeffer showed that DIRTY water containing minute particles of clay in suspension acts as a stimulus. He also showed that gelatine acts like pure water; if a smooth glass rod is coated with a 10 per cent solution of gelatine and is then applied to a tendril, no movement occurs in spite of the fact that the gelatine is solid when cold. Pfeffer ("Physiology", Eng. Tr. III. page 52. Pfeffer has pointed out the resemblance between the contact irritability of plants and the human sense of touch. Our skin is not sensitive to uniform pressure such as is produced when the finger is dipped into mercury (Tubingen "Untersuchungen", I. page 504.) generalises the result in the statement that the tendril has a special form of irritability and only reacts to "differences of pressure or variations of pressure in contiguous...regions." Darwin was especially interested in such cases of specialised irritability. For instance in May, 1864, he wrote to Asa Gray ("Life and Letters", III. page 314.) describing the tendrils of Bignonia capreolata, which "abhor a simple stick, do not much relish rough bark, but delight in wool or moss." He received, from Gray, information as to the natural habitat of the species, and finally concluded that the tendrils "are specially adapted to climb trees clothed with lichens, mosses, or other such productions." ("Climbing Plants", page 102.)

Tendrils were not the only instance discovered by Darwin of delicacy of touch in plants. In 1860 he had already begun to observe Sundew (Drosera), and was full of astonishment at its behaviour. He wrote to Sir Joseph Hooker ("Life and Letters", III. page 319.): "I have been working like a madman at Drosera. Here is a fact for you which is certain as you stand where you are, though you won't believe it, that a bit of hair 1/78000 of one grain in weight placed on gland, will cause ONE of the gland-bearing hairs of Drosera to curve inwards." Here again Pfeffer (Pfeffer in "Untersuchungen a. d. Bot. Inst. z. Tubingen", I. page 491.) has, as in so many cases, added important facts to my father's observations. He showed that if the leaf of Drosera is entirely freed from such vibrations as would reach it if observed on an ordinary table, it does not react to small weights, so that in fact it was the vibration of the minute fragment of hair on the gland that produced movement. We may fancifully see an adaptation to the capture of insects--to the dancing of a gnat's foot on the sensitive surface.

Darwin was fond of telling how when he demonstrated the sensitiveness of Drosera to Mr Huxley and (I think) to Sir John Burdon Sanderson, he could perceive (in spite of their courtesy) that they thought the whole thing a delusion. And the story ended with his triumph when Mr Huxley cried out, "It IS moving."

Darwin's work on tendrils has led to some interesting investigations on the mechanisms by which plants perceive stimuli. Thus Pfeffer (Tubingen "Untersuchungen" I. page 524.) showed that certain epidermic cells occurring in tendrils are probably organs of touch. In these cells the protoplasm burrows as it were into cavities in the thickness of the external cell-walls and thus comes close to the surface, being separated from an object touching the tendril merely by a very thin layer of cell- wall substance. Haberlandt ("Physiologische Pflanzenanatomie", Edition III. Leipzig, 1904. "Sinnesorgane im Pflanzenreich", Leipzig, 1901, and other publications.) has greatly extended our knowledge of vegetable structure in relation to mechanical stimulation. He defines a sense-organ as a contrivance by which the DEFORMATION or forcible change of form in the protoplasm--on which mechanical stimulation depends--is rendered rapid and considerable in amplitude ("Sinnesorgane", page 10). He has shown that in certain papillose and bristle-like contrivances, plants possess such sense- organs; and moreover that these contrivances show a remarkable similarity to corresponding sense-organs in animals.

Haberlandt and Nemec ("Ber. d. Deutschen bot. Gesellschaft", XVIII. 1900. See F. Darwin, Presidential Address to Section K, British Association, 1904.) published independently and simultaneously a theory of the mechanism by which plants are orientated in relation to gravitation. And here again we find an arrangement identical in principle with that by which certain animals recognise the vertical, namely the pressure of free particles on the irritable wall of a cavity. In the higher plants, Nemec and Haberlandt believe that special loose and freely movable starch-grains play the part of the otoliths or statoliths of the crustacea, while the protoplasm lining the cells in which they are contained corresponds to the sensitive membrane lining the otocyst of the animal. What is of special interest in our present connection is that according to this ingenious theory (The original conception was due to Noll ("Heterogene Induction", Leipzig, 1892), but his view differed in essential points from those here given.) the sense of verticality in a plant is a form of contact-irritability. The vertical position is distinguished from the horizontal by the fact that, in the latter case, the loose starch-grains rest on the lateral walls of the cells instead of on the terminal walls as occurs in the normal upright position. It should be added that the statolith theory is still sub judice; personally I cannot doubt that it is in the main a satisfactory explanation of the facts.

With regard to the RAPIDITY of the reaction of tendrils, Darwin records ("Climbing Plants", page 155. Others have observed movement after about 6".) that a Passion-Flower tendril moved distinctly within 25 seconds of stimulation. It was this fact, more than any other, that made him doubt the current explanation, viz. that the movement is due to unequal growth on the two sides of the tendril. The interesting work of Fitting (Pringsheim's "Jahrb." XXXVIII. 1903, page 545.) has shown, however, that the primary cause is not (as Darwin supposed) contraction on the concave, but an astonishingly rapid increase in growth-rate on the convex side.

On the last page of "Climbing Plants" Darwin wrote: "It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them."

He gradually came to realise the vividness and variety of vegetable life, and that a plant like an animal has capacities of behaving in different ways under different circumstances, in a manner that may be compared to the instinctive movements of animals. This point of view is expressed in well- known passages in the "Power of Movement". ("The Power of Movement in Plants", 1880, pages 571-3.) "It is impossible not to be struck with the resemblance between the...movements of plants and many of the actions performed unconsciously by the lower animals." And again, "It is hardly an exaggeration to say that the tip of the radicle...having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements."

The conception of a region of perception distinct from a region of movement is perhaps the most fruitful outcome of his work on the movements of plants. But many years before its publication, viz. in 1861, he had made out the wonderful fact that in the Orchid Catasetum ("Life and Letters", III. page 268.) the projecting organs or antennae are sensitive to a touch, and transmit an influence "for more than one inch INSTANTANEOUSLY," which leads to the explosion or violent ejection of the pollinia. And as we have already seen a similar transmission of a stimulus was discovered by him in Sundew in 1860, so that in 1862 he could write to Hooker ("Life and Letters", III. page 321.): "I cannot avoid the conclusion, that Drosera possesses matter at least in some degree analogous in constitution and function to nervous matter." I propose in what follows to give some account of the observations on the transmission of stimuli given in the "Power of Movement". It is impossible within the space at my command to give anything like a complete account of the matter, and I must necessarily omit all mention of much interesting work. One well-known experiment consisted in putting opaque caps on the tips of seedling grasses (e.g. oat and canary-grass) and then exposing them to light from one side. The difference, in the amount of curvature towards the light, between the blinded and unblinded specimens, was so great that it was concluded that the light-sensitiveness resided exclusively in the tip. The experiment undoubtedly proves that the sensitiveness is much greater in the tip than elsewhere, and that there is a transmission of stimulus from the tip to the region of curvature. But Rothert (Rothert, Cohn's "Beitrage", VII. 1894.) has conclusively proved that the basal part where the curvature occurs is also DIRECTLY sensitive to light. He has shown, however, that in other grasses (Setaria, Panicum) the cotyledon is the only part which is sensitive, while the hypocotyl, where the movement occurs, is not directly sensitive.

It was however the question of the localisation of the gravitational sense in the tip of the seedling root or radicle that aroused most attention, and it was on this question that a controversy arose which has continued to the present day.

The experiment on which Darwin's conclusion was based consisted simply in cutting off the tip, and then comparing the behaviour of roots so treated with that of normal specimens. An uninjured root when placed horizontally regains the vertical by means of a sharp downward curve; not so a decapitated root which continues to grow more or less horizontally. It was argued that this depends on the loss of an organ specialised for the perception of gravity, and residing in the tip of the root; and the experiment (together with certain important variants) was claimed as evidence of the existence of such an organ.

It was at once objected that the amputation of the tip might check curvature by interfering with longitudinal growth, on the distribution of which curvature depends. This objection was met by showing that an injury, e.g. splitting the root longitudinally (See F. Darwin, "Linnean Soc. Journal (Bot)." XIX. 1882, page 218.), which does not remove the tip, but seriously checks growth, does not prevent geotropism. This was of some interest in another and more general way, in showing that curvature and longitudinal growth must be placed in different categories as regards the conditions on which they depend.

Another objection of a much more serious kind was that the amputation of the tip acts as a shock. It was shown by Rothert (See his excellent summary of the subject in "Flora" 1894 (Erganzungsband), page 199.) that the removal of a small part of the cotyledon of Setaria prevents the plant curving towards the light, and here there is no question of removing the sense-organ since the greater part of the sensitive cotyledon is intact. In view of this result it was impossible to rely on the amputations performed on roots as above described.

At this juncture a new and brilliant method originated in Pfeffer's laboratory. (See Pfeffer, "Annals of Botany", VIII. 1894, page 317, and Czapek, Pringsheim's "Jahrb." XXVII. 1895, page 243.) Pfeffer and Czapek showed that it is possible to bend the root of a lupine so that, for instance, the supposed sense-organ at the tip is vertical while the motile region is horizontal. If the motile region is directly sensitive to gravity the root ought to curve downwards, but this did not occur: on the contrary it continued to grow horizontally. This is precisely what should happen if Darwin's theory is the right one: for if the tip is kept vertical, the sense-organ is in its normal position and receives no stimulus from gravitation, and therefore can obviously transmit none to the region of curvature. Unfortunately this method did not convince the botanical world because some of those who repeated Czapek's experiment failed to get his results.

Czapek ("Berichte d. Deutsch. bot. Ges." XV. 1897, page 516, and numerous subsequent papers. English readers should consult Czapek in the "Annals of Botany", XIX. 1905, page 75.) has devised another interesting method which throws light on the problem. He shows that roots, which have been placed in a horizontal position and have therefore been geotropically stimulated, can be distinguished by a chemical test from vertical, i.e. unstimulated roots. The chemical change in the root can be detected before any curvature has occurred and must therefore be a symptom of stimulation, not of movement. It is particularly interesting to find that the change in the root, on which Czapek's test depends, takes place in the tip, i.e. in the region which Darwin held to be the centre for gravitational sensitiveness.

In 1899 I devised a method (F. Darwin, "Annals of Botany", XIII. 1899, page 567.) by which I sought to prove that the cotyledon of Setaria is not only the organ for light-perception, but also for gravitation. If a seedling is supported horizontally by pushing the apical part (cotyledon) into a horizontal tube, the cotyledon will, according to my supposition, be stimulated gravitationally and a stimulus will be transmitted to the basal part of the stem (hypocotyl) causing it to bend. But this curvature merely raises the basal end of the seedling, the sensitive cotyledon remains horizontal, imprisoned in its tube; it will therefore be continually stimulated and will continue to transmit influences to the bending region, which should therefore curl up into a helix or corkscrew-like form,—and this is precisely what occurred.

I have referred to this work principally because the same method was applied to roots by Massart (Massart, "Mem. Couronnes Acad. R. Belg." LXII. 1902.) and myself (F. Darwin, "Linnean Soc. Journ." XXXV. 1902, page 266.) with a similar though less striking result. Although these researches confirmed Darwin's work on roots, much stress cannot be laid on them as there are several objections to them, and they are not easily repeated.

The method which--as far as we can judge at present—seems likely to solve the problem of the root-tip is most ingenious and is due to Piccard. (Pringsheim's "Jahrb." XL. 1904, page 94.)

Andrew Knight's celebrated experiment showed that roots react to centrifugal force precisely as they do to gravity. So that if a bean root is fixed to a wheel revolving rapidly on a horizontal axis, it tends to curve away from the centre in the line of a radius of the wheel. In ordinary demonstrations of Knight's experiment the seed is generally fixed so that the root is at right angles to a radius, and as far as convenient from the centre of rotation. Piccard's experiment is arranged differently. (A seed is depicted below a horizontal dotted line AA, projecting a root upwards.) The root is oblique to the axis of rotation, and the extreme tip projects beyond that axis. Line AA represents the axis of rotation, T is the tip of the root just above the line AA, and B is the region just below line AA in which curvature takes place. If the motile region B is directly sensitive to gravitation (and is the only part which is sensitive) the root will curve (down and away from the vertical) away from the axis of rotation, just as in Knight's experiment. But if the tip T is alone sensitive to gravitation the result will be exactly reversed, the stimulus originating in T and conveyed to B will produce curvature (up towards the vertical). We may think of the line AA as a plane dividing two worlds. In the lower one gravity is of the earthly type and is shown by bodies falling and roots curving downwards: in the upper world bodies fall upwards and roots curve in the same direction. The seedling is in the lower world, but its tip containing the supposed sense-organ is in the strange world where roots curve upwards. By observing whether the root bends up or down we can decide whether the impulse to bend originates in the tip or in the motile region.

Piccard's results showed that both curvatures occurred and he concluded that the sensitive region is not confined to the tip. (Czapek (Pringsheim's "Jahrb." XXXV. 1900, page 362) had previously given reasons for believing that, in the root, there is no sharp line of separation between the regions of perception and movement.)

Haberlandt (Pringsheim's "Jahrb." XLV. 1908, page 575.) has recently repeated the experiment with the advantage of better apparatus and more experience in dealing with plants, and has found as Piccard did that both the tip and the curving region are sensitive to gravity, but with the important addition that the sensitiveness of the tip is much greater than that of the motile region. The case is in fact similar to that of the oat and canary-grass. In both instances my father and I were wrong in assuming that the sensitiveness is confined to the tip, yet there is a concentration of irritability in that region and transmission of stimulus is as true for geotropism as it is for heliotropism. Thus after nearly thirty years the controversy of the root-tip has apparently ended somewhat after the fashion of the quarrels at the "Rainbow" in "Silas Marner"--"you're both right and you're both wrong." But the "brain-function" of the root-tip at which eminent people laughed in early days turns out to be an important part of the truth. (By using Piccard's method I have succeeded in showing that the gravitational sensitiveness of the cotyledon of Sorghum is certainly much greater than the sensitiveness of the hypocotyl--if indeed any such sensitiveness exists. See Wiesner's "Festschrift", Vienna, 1908.)

Another observation of Darwin's has given rise to much controversy. ("Power of Movement", page 133.) If a minute piece of card is fixed obliquely to the tip of a root some influence is transmitted to the region of curvature and the root bends away from the side to which the card was attached. It was thought at the time that this proved the root-tip to be sensitive to contact, but this is not necessarily the case. It seems possible that the curvature is a reaction to the injury caused by the alcoholic solution of shellac with which the cards were cemented to the tip. This agrees with the fact given in the "Power of Movement" that injuring the root-tip on one side, by cutting or burning it, induced a similar curvature. On the other hand it was shown that curvature could be produced in roots by cementing cards, not to the naked surface of the root- tip, but to pieces of gold-beaters skin applied to the root; gold-beaters skin being by itself almost without effect. But it must be allowed that, as regards touch, it is not clear how the addition of shellac and card can increase the degree of contact. There is however some evidence that very close contact from a solid body, such as a curved fragment of glass, produces curvature: and this may conceivably be the explanation of the effect of gold-beaters skin covered with shellac. But on the whole it is perhaps safer to classify the shellac experiments with the results of undoubted injury rather than with those of contact.

Another subject on which a good deal of labour was expended is the sleep of leaves, or as Darwin called it their NYCTITROPIC movement. He showed for the first time how widely spread this phenomenon is, and attempted to give an explanation of the use to the plant of the power of sleeping. His theory was that by becoming more or less vertical at night the leaves escape the chilling effect of radiation. Our method of testing this view was to fix some of the leaves of a sleeping plant so that they remained horizontal at night and therefore fully exposed to radiation, while their fellows were partly protected by assuming the nocturnal position. The experiments showed clearly that the horizontal leaves were more injured than the sleeping, i.e. more or less vertical, ones. It may be objected that the danger from cold is very slight in warm countries where sleeping plants abound. But it is quite possible that a lowering of the temperature which produces no visible injury may nevertheless be hurtful by checking the nutritive processes (e.g. translocation of carbohydrates), which go on at night. Stahl ("Bot. Zeitung", 1897, page 81.) however has ingeniously suggested that the exposure of the leaves to radiation is not DIRECTLY hurtful because it lowers the temperature of the leaf, but INDIRECTLY because it leads to the deposition of dew on the leaf-surface. He gives reasons for believing that dew-covered leaves are unable to transpire efficiently, and that the absorption of mineral food-material is correspondingly checked. Stahl's theory is in no way destructive of Darwin's, and it is possible that nyctitropic leaves are adapted to avoid the indirect as well as the direct results of cooling by radiation.

In what has been said I have attempted to give an idea of some of the discoveries brought before the world in the "Power of Movement" (In 1881 Professor Wiesner published his "Das Bewegungsvermogen der Pflanzen", a book devoted to the criticism of "The Power of Movement in Plants". A letter to Wiesner, published in "Life and Letters", III. page 336, shows Darwin's warm appreciation of his critic's work, and of the spirit in which it is written.) and of the subsequent history of the problems. We must now pass on to a consideration of the central thesis of the book,—the relation of circumnutation to the adaptive curvatures of plants.

Darwin's view is plainly stated on pages 3-4 of the "Power of Movement". Speaking of circumnutation he says, "In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements." He then points out that curvatures such as those towards the light or towards the centre of the earth can be shown to be exaggerations of circumnutation in the given directions. He finally points out that the difficulty of conceiving how the capacities of bending in definite directions were acquired is diminished by his conception. "We know that there is always movement in progress, and its amplitude, or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli."

It may at once be allowed that the view here given has not been accepted by physiologists. The bare fact that circumnutation is a general property of plants (other than climbing species) is not generally rejected. But the botanical world is no nearer to believing in the theory of reaction built on it.

If we compare the movements of plants with those of the lower animals we find a certain resemblance between the two. According to Jennings (H.S. Jennings, "The Behavior of the Lower Animals". Columbia U. Press, N.Y. 1906.) a Paramoecium constantly tends to swerve towards the aboral side of its body owing to certain peculiarities in the set and power of its cilia. But the tendency to swim in a circle, thus produced, is neutralised by the rotation of the creature about its longitudinal axis. Thus the direction of the swerves IN RELATION TO THE PATH of the organism is always changing, with the result that the creature moves in what approximates to a straight line, being however actually a spiral about the general line of progress. This method of motion is strikingly like the circumnutation of a plant, the apex of which also describes a spiral about the general line of growth. A rooted plant obviously cannot rotate on its axis, but the regular series of curvatures of which its growth consists correspond to the aberrations of Paramoecium distributed regularly about its course by means of rotation. (In my address to the Biological Section of the British Association at Cardiff (1891) I have attempted to show the connection between circumnutation and RECTIPETALITY, i.e. the innate capacity of growing in a straight line.) Just as a plant changes its direction of growth by an exaggeration of one of the curvature-elements of which circumnutation consists, so does a Paramoecium change its course by the accentuation of one of the deviations of which its path is built. Jennings has shown that the infusoria, etc., react to stimuli by what is known as the "method of trial." If an organism swims into a region where the temperature is too high or where an injurious substance is present, it changes its course. It then moves forward again, and if it is fortunate enough to escape the influence, it continues to swim in the given direction. If however its change of direction leads it further into the heated or poisonous region it repeats the movement until it emerges from its difficulties. Jennings finds in the movements of the lower organisms an analogue with what is known as pain in conscious organisms. There is certainly this much resemblance that a number of quite different sub-injurious agencies produce in the lower organisms a form of reaction by the help of which they, in a partly fortuitous way, escape from the threatening element in their environment. The higher animals are stimulated in a parallel manner to vague and originally purposeless movements, one of which removes the discomfort under which they suffer, and the organism finally learns to perform the appropriate movement without going through the tentative series of actions.

I am tempted to recognise in circumnutation a similar groundwork of tentative movements out of which the adaptive ones were originally selected by a process rudely representative of learning by experience.

It is, however, simpler to confine ourselves to the assumption that those plants have survived which have acquired through unknown causes the power of reacting in appropriate ways to the external stimuli of light, gravity, etc. It is quite possible to conceive this occurring in plants which have no power of circumnutating--and, as already pointed out, physiologists do as a fact neglect circumnutation as a factor in the evolution of movements. Whatever may be the fate of Darwin's theory of circumnutation there is no doubt that the research he carried out in support of, and by the light of, this hypothesis has had a powerful influence in guiding the modern theories of the behaviour of plants. Pfeffer ("The Physiology of Plants", Eng. Tr. III. page 11.), who more than any one man has impressed on the world a rational view of the reactions of plants, has acknowledged in generous words the great value of Darwin's work in the same direction. The older view was that, for instance, curvature towards the light is the direct mechanical result of the difference of illumination on the lighted and shaded surfaces of the plant. This has been proved to be an incorrect explanation of the fact, and Darwin by his work on the transmission of stimuli has greatly contributed to the current belief that stimuli act indirectly. Thus we now believe that in a root and a stem the mechanism for the perception of gravitation is identical, but the resulting movements are different because the motor-irritabilities are dissimilar in the two cases. We must come back, in fact, to Darwin's comparison of plants to animals. In both there is perceptive machinery by which they are made delicately alive to their environment, in both the existing survivors are those whose internal constitution has enabled them to respond in a beneficial way to the disturbance originating in their sense-organs.


[ Francis Darwin, "Darwin's work on the movements of plants." In A. C. Seward Darwin and Modern Science. Cambridge: Cambridge University Press, 1909, pp. 385-400. ]


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