IN 1875 appeared another great work from the master’s pen, “Insectivorous Plants,” which was destined to place in a yet more striking light the many-sidedness and fertility of his mind. As usual Darwin tells us that this work dated from many years back. “During the summer of 1860,” he says, “I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex. I had heard that insects were thus caught, but knew nothing further on the subject. I gathered by chance a dozen plants, bearing fifty-six fully expanded leaves, and on thirty-one of these dead insects or remnants of them adhered.” Here was the germ of something, the discoverer scarcely knew what. It was evident to him that the little sun-dew was excellently adapted for catching insects, and that the number of them thus slaughtered annually must be enormous. What bearing might this have upon the problem of the struggle for existence?

A masterly series of experiments was forthwith set on foot, with the result of proving that sun-dews and a number of other plants obtain the bulk of their nourishment by catching, killing, and digesting insects. They may be called truly carnivorous plants. What an unexpected reversal this was of the order of things hitherto believed to prevail universally. Animals live on other animals or on plants. Here were plants living on animals, and keeping down their number. Moreover, without a nervous system, the action of the parts of a sun-dew leaf was proved to be as apparently purposive as the combined action of the limbs of an animal. Without a stomach, the sun-dew poured forth a digestive fluid as effective in extracting and fitting the nutritious matter of the insect for its own purposes as that of an animal. Without sensory nerve-endings, there was a percipient power in the sun-dew which recognised instinctively and at once the non-nutritious nature of various objects, and which responded to the most delicate chemical stimuli and to the minutest weights.

We cannot describe the little sun-dew better than in Darwin’s own words: “It bears from two or three to five or six leaves, generally extended more or less horizontally, but sometimes standing vertically upwards. The leaves are commonly a little broader than long. The whole upper surface is covered with gland-bearing filaments, or tentacles as I shall call them from their manner of acting. The glands were counted on thirty-one leaves, but many of these were of unusually large size, and the average number was 192; the greatest number being 260, and the least 130. The glands are each surrounded by large drops of extremely viscid secretion, which, glittering in the sun, have given rise to the plant’s poetical name of the sun-dew.”

This secretion, when excited by nutritious matter, becomes distinctly acid, and contains a digestive ferment allied to the pepsin of the human stomach. So excited, it is found capable of dissolving boiled white of egg, muscle, fibrin, cartilage, gelatine, curd of milk, and many other substances. Further, various substances that animal gastric juice is unable to digest are not acted upon by the secretion of the sun-dew. These include all horny matter, starch, fat, and oil. It is not however prejudiced in favour of animal matter. The sun-dew can absorb nutriment from living seeds of plants, injuring or killing them, of course, in the process, while pollen and fresh green leaves yield to its influence.

The action of salts of ammonia and other chemicals was even more wonderful. “It is an astonishing fact that so inconceivably minute a quantity as the one twenty-millionth of a grain of phosphate of ammonia should induce some change in a gland of Drosera sufficient to cause a motor impulse to be sent down the whole length of the tentacle; this impulse exciting movement often through an angle of above 180°. I know not whether to be most astonished at this fact, or that the pressure of a minute bit of hair, weighing only ¹ ₇₈₇₀₀ of a grain, and largely supported by the dense secretion, should quickly cause conspicuous movement.”

These are but specimens of a multitude of profoundly interesting facts brought out in this exhaustive investigation. If this single research were his only title to fame Darwin’s name must rank high as an experimenter of rare ingenuity and success. But he concludes his summary of results by the utterly modest remark, “We see how little has been made out in comparison with what remains unexplained and unknown.”

The facts relating to Venus’ fly-trap (DionÆa muscipula) and other members of the order to which the sun-dew belongs were better known, but Darwin elicited new truths by his ingenious and varied experiments. The rapidity with which the two lobes of the leaf of dionÆa close together when anything touches the tiny spikes which stand up vertically from the upper surface of the lobes, is astonishing, and any insect which causes the closure is almost certain to be caught. Digestion is accomplished in the case of the dionÆa by a separate agency, consisting of a large number of minute reddish glands covering the surface of the lobes. These secrete a digestive fluid when stimulated by the contact of any nitrogenous matter, and of course this takes place when any insect is caught. In fact, essentially the same process of digestion and absorption takes place as in the sun-dew. The insect is held firmly for days, until its juices have been absorbed, and then the leaf slowly reopens, not being able to close again for many subsequent days.

It is interesting to note the extreme caution with which the great naturalist speculates upon the mode by which the varied members of the sun-dew order became modified from an ordinary plant-form to such a remarkable degree. The details are too special for quotation here. He suggests, but he does not in the slightest degree dogmatise. For many years to come Darwin’s suggestions and comments must be the pregnant soil out of which fruitful research will spring, and his caution will remain the model, to depart from which will but sow hindrances in the path of scientific progress.

The order to which the butterwort and the bladderworts belong also afforded valuable results. The leaf of the butterwort bears glandular hairs, and its margins curve inwards when excited by contact of various bodies, especially living insects, and, at the same time, these are caught in the viscid secretion of the glands, and their juices absorbed by the plant. The bladderworts are even more remarkably constructed, for they have a portion of their leaves developed into subaqueous bladders, with a narrow entrance beneath, defended by a complex valve, which facilitates the entrance of water insects or crustaceans, but prevents their exit. The whole interior of the bladder is lined with transparent four-branched protoplasmic hairs, but nevertheless the bladderwort is unlike the preceding plants in having no power of digesting its prey, however long it may remain in captivity. Yet there is no doubt that the imprisoned creatures do decay in their watery cell, and that the hairs just described absorb the products of their decay.

Such is a brief account of Darwin’s work on “Insectivorous Plants.” With his characteristic expressions he acknowledges the valuable aid given him by Professor Burdon-Sanderson, and by his son, Mr. Francis Darwin. The former was enabled to give the first brief account of the process of digestion in these plants, as observed by Darwin, in a lecture before the Royal Institution, in June, 1874, and Dr. (now Sir Joseph) Hooker called general public notice to the subject of Carnivorous Plants in his lecture before the British Association at Belfast in the same year: so that a thoroughly awakened attention was given to this new work from Darwin’s pen. The public and the scientific world learnt to appreciate yet more keenly his varied talent, his long patience, his reserve of power; and thence dated very definitely a general appreciation of the fundamental unity of the animal and plant kingdoms, seeing that the salient faculties of digestion, of purposive locomotion, of rapid communication and consentaneous action were no longer restricted to animals, but were possessed in a high degree by plants also. Eager followers soon brought forward further proofs of unity of functions in the two kingdoms, and of reciprocal combinations between them, and now no one in the slightest degree acquainted with modern biology doubts that life is at bottom one phenomenon, shared equally and manifested in essentially the same modes by the living substance of plant and animal alike.

Following “Insectivorous Plants” came “The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom,” in 1876. Darwin had led the way in the study of this subject by his book on Orchids, and his lead had been excellently followed by Hildebrand, Hermann MÜller, Sir John Lubbock, and others. The path having been indicated, it had appeared comparatively easy for botanists to follow it up. But there yet remained a region of experimental inquiry which it required Darwin’s patience and ingenuity to master and to expound conclusively. Although it might be practically granted that natural selection developed a process because advantage was gained by it, was it possible to demonstrate that flowers cross-fertilised bear more and larger seeds, which produce healthier offspring than those fertilised from their own pollen? This Darwin set himself exhaustively to do. For more than a dozen years after his book on orchids appeared, unwearied experiments on plants were progressing, and nature was being questioned acutely, untiringly. Competitive germination was carried on. The two classes of seeds were placed on damp sand in a warm room. As often as a pair germinated at the same time, they were planted on opposite sides of the same pot, with a partition between. Besides these pairs of competitors, others were planted in beds, so that the descendants of the crossed and self-fertilised flowers might compete. The resulting seeds were carefully compared, and their produce again compared. Species were selected from widely distinct families, inhabiting various countries. From a large number of plants, when insects were quite excluded by a thin net covering the plant, few or no seeds were produced. The extent of transport of pollen by insects was unveiled, and the relation between the structure, odour, and conspicuousness of flowers, the visits of insects, and the advantages of cross-fertilisation was shown. “We certainly,” says Darwin, “owe the beauty and odour of our flowers, and the storage of a large supply of honey, to the existence of insects.” The multitude of facts gathered about insects could only have been discovered and rightly appreciated by one who was a true entomologist as well as a botanist.

In the last chapter of the book the author discusses with remarkable power the causes of the phenomena he has discovered. He believes that the favourable effects of crossing are due to the parents having been subjected to diverse conditions; but what the precise benefit is, or how it can operate so as to render the offspring more healthy and vigorous, he cannot discern. “And so it is,” he observes, “with many other facts, which are so obscure that we stand in awe before the mystery of life.” So it is. The man who probably understood nature better than any man who has ever lived, who had not only asked her multitudinous questions, but to whom very many answers had been undoubtedly vouchsafed in response to his persevering, humble, diligent, acute questioning, acknowledges that he knows little; that much remains a mystery. But from all we know of him, from his books, his letters, his friends, his was the joy of a soul in sympathy with the master power of the universe. He marched continually on the confines of the unknown, and to him was granted the felicity of largely extending the boundaries of the known.

Again, in 1877, a new work proceeded from Darwin’s pen, “The Different Forms of Flowers in Plants of the same Species,” dedicated to Professor Asa Gray. It gathered up the contents of numerous papers read before the Linnean Society, with later additions, and showed conclusively how many plants possess distinctive forms of flowers in the same species, adapted to, and in some cases absolutely necessitating, reciprocal fertilisation through the visits of insects. It gave evidence of all the well-known Darwinian characteristics of long-continued labour, thought, and experiment.

In 1880 “The Power of Movement in Plants” was exemplified in a fresh volume, in which the veteran was materially assisted by his son, Mr. Francis Darwin. Its object was to describe and connect together several large classes of movements, common to almost all plants. The surprising fact was established, that all the parts or organs of plants, whilst they continue to grow, are continually revolving, or circumnutating as Darwin called it. This movement commences even before the young seedling has broken through the ground. The combination of this with the effects of gravity and light explains countless phenomena in the life of plants. The tip of the rootlet is thus enabled to penetrate the ground, and it is proved to be more sensitive than the most delicate tendril. Movement goes on through all stages of life. Every growing shoot of a great tree is continually describing small ellipses; the tip of every rootlet endeavours to do the same. The changes of position of leaves and of climbing plants, and the sleep of leaves are all brought under this great principle of circumnutation. It is impossible in reading the book not to be struck with the great resemblance between the movements of plants and many of the actions performed unconsciously by the lower animals. “With plants an astonishingly small stimulus suffices, and, even with allied plants, one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals, and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves. Yet plants do not of course possess nerves or a central nervous system; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts.”

Here we see how much light may be thrown on animal structures and functions by vegetable physiology. We learn to limit our ideas of the superiority of animals by discovering how much of what we consider peculiar to them is found in plants. We appreciate the unity of biology, indivisible without injury to our knowledge of its parts. No structure in plants appears more wonderful, as Darwin describes it, than the tip of the rootlet of a seedling. It is impressed by and transmits influences of pressure, injury, moisture, light, and gravity to other parts, and determines the course pursued by the rootlet in penetrating the ground. “It is hardly an exaggeration 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;” and the brain of Charles Darwin, in working out this acquisition of knowledge for mankind, has added a new department to vegetable physiology and to biology.