XIX. DARWIN'S WORK ON THE MOVEMENTS OF PLANTS. By Francis Darwin,
Honorary Fellow of Christ's College, Cambridge.
My 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.)