PHYSIOLOGY OF PLANTS.
The corroborations I am now about to bring forward of the phenomenon of the will in plants, proceed chiefly from French sources, from a nation whose tendencies are decidedly empirical and which is reluctant to go a step beyond what is immediately given. The informant moreover is Cuvier, whose rigid adherence to the purely empirical gave rise to the famous dispute between him and Geoffroy de St. Hilaire. So we must not be astonished if the language we meet with here is less decided than in the preceding German corroborations and if we find each concession made with cautious reserve.
In his "Histoire des Progrès des Sciences Naturelles depuis 1789 jusqu'á ce jour,"[247] Cuvier says: "Plants have certain apparently spontaneous movements, which they show under certain circumstances and which at times so closely resemble those of animals, that a sort of feeling and will might almost be attributed to plants on this account, especially by those who think they can perceive something of the same kind in the movements of the inward parts of animals. Thus the tops of trees always have a vertical tendency, excepting when they incline towards the light. Their roots seek out good earth and moisture and, in order to attain these, deviate from the straight course. Yet these different tendencies cannot be explained by the influence of external causes, unless we also assume the existence of an inner natural disposition, susceptible of being roused, which differs from the mere mechanical force in inorganic bodies.... Decandolle made some remarkable experiments that proved to him the existence of a sort of habit in plants which may be overcome by artificial light, but only after a certain time. Plants that had been shut up in a cellar which was continually lit by lamps, did not on this account leave off closing in the evening and opening again in the morning for several days. And there are other habits besides which plants are able to adopt and to abandon. Flowers that habitually close in wet weather, finish by remaining open if the wet weather lasts too long. When M. Desfontaines took a sensitive plant with him in his carriage, the jolting movement at first caused it to contract, but at last it expanded again as when in complete repose. Therefore even in these cases, light, moisture, &c., &c., only act in virtue of an inner disposition, which may be neutralized or modified by the continuation of that very activity itself; and the vital energy of plants, like that of animals, is subject to fatigue and exhaustion. The hedysarum gyrans is singularly characterized by the movements of its leaves which continue day and night without needing any sort of stimulus. Surely, if any phenomenon can cause illusion and remind us of the voluntary movements of animals, it is this. Broussonet, Silvestre, Cels and Halle have fully described it, and have shown that the plant's action depends entirely upon its own healthy condition."
Again, in the third volume of the same work, p. 166 (1828), Cuvier says: "M. Dutrochet adds some physiological considerations to which his own experiments had led him, and which in his opinion prove that the movements of plants are spontaneous, i.e. that they depend upon an inner principle which immediately receives the influence of outer agencies. As he is however reluctant to admit that plants have feeling, he makes use of the word 'nervimotilité.'"—Here I must observe, that when we come to examine it closely, what we think to ourselves in the conception of spontaneity, is in the end always the same thing as manifestation of will, with which spontaneity would therefore be simply synonymous. The only difference between them consists in the conception of spontaneity being derived from outer perception, while that of manifestation of will is drawn from our own consciousness.—I find a remarkable instance of the impetuous violence of this spontaneity, even in plants, in the following communication contained in the "Cheltenham Examiner:"[248] "Last Thursday four enormous mushrooms performed a heroic feat of a new kind, in one of our most crowded streets, by lifting up a huge block of stone in their strenuous effort to make their way into the visible world."
In the "Mém. de l'Acad. d. Sciences de l'année" (1821), Cuvier says[249]:—"For centuries botanists have been searching for the reason why in a seed which is germinating the root invariably grows downwards, while the stalk as invariably grows upwards, no matter what be the position in which the seed is placed. M. Dutrochet put some seeds into holes bored in the bottom of a vessel filled with damp mould, which he hung up to a beam in his room. Now, in this case, the stem might have been expected to grow downwards. Not at all: the roots found their way to the air below, and the stems were prolonged so as to traverse the damp mould until they reached its upper surface. According to M. Dutrochet, the direction in which plants grow, is determined by an inner principle and not at all by the attraction of the bodies towards which they direct themselves. A mistletoe seed that was fastened to the point of a perfectly moveable needle fixed on a peg, with a small plank placed near it, was induced to germinate. It soon began to send out shoots towards the plank, which it reached in five days without having communicated the slightest movement to the needle. The stems of onions and leeks with their bulbs, deposited in dark places, grow upwards, although more slowly than in light ones; they grow upwards even if placed in water: a fact which suffices to prove that neither light nor moisture determines the direction of their growth."—Still C. H. Schultz asserts[250] that he made seeds germinate in a dark box with holes bored in the bottom, and succeeded in inducing the plants to grow upside down, by means of a mirror fastened to the box, which reflected the sunlight.
In the "Dictionnaire des Sciences Naturelles" (article Animal) we find: "If, on the one hand, animals show avidity in their search after nourishment as well as power of discrimination in the selection of it, roots of plants may, on the other hand, be observed to direct themselves towards the side where the soil contains most nourishment, nay, even to seek out the smallest crevices in rocks which may contain any food. If we twist a bough so as to make the upper surface of its leaves the under one, these leaves even will twist their stems in order to regain the position best suited for the exercise of their functions (i.e. so as to have the smooth side uppermost). Is it quite certain that this takes place unconsciously?"
F. J. Meyen has devoted a chapter, entitled "Of the movements and sensations of plants," to a full investigation of the subject now before us. In this he says[251]: "Not unfrequently potatoes, stored in deep, dark cellars, may be observed towards summer to shoot forth stems which invariably grow in the direction of the chinks through which the light comes into the cellar, and to continue thus growing until they at last reach the aperture which receives the light directly. In such cases potato-stalks have been known to reach a length of twenty feet; whereas under ordinary circumstances, even such as are most favourable to the growth of the potato, the stalk is seldom longer than from three to four feet. It is interesting to watch closely the course taken by a potato-stalk thus growing in darkness, in its endeavours to reach the light. It tries to do so by the shortest road, but not being firm enough to grow straight across through the air without support, it lets itself drop on to the floor, and thus creeps along the ground till it reaches the nearest wall, up which it then climbs." Even this botanist too is led by his facts to the following assertion (p. 576): "On observing the freedom of movement of oscillatoria and other inferior plants, we may perhaps have no alternative but to attribute a species of will to these beings."
Creepers bear distinct evidence as to manifestation of will in plants; for, when they find no support near enough for their tendrils to cling to, they invariably direct their growth towards the shadiest place, or even towards a piece of dark-coloured paper, wherever it may be placed; whereas they avoid glass, on account of its glitter. In the "Philosophical Transactions" of 1812, Th. Andrew Knight relates some very pleasing experiments on this subject (especially with ampelopsis quinquefolia,)[252] although he strives hard to explain the matter mechanically, and will not admit that it is a manifestation of will. I appeal to his experiments, not to the conclusions he draws from them. A good test might be, to plant several free creepers in a circle round a tree-trunk and to observe whether they all crept towards the trunk centripetally. On the 6th Nov. 1843, Dutrochet read a treatise on this subject in the "Acad. de Sciences" called "Sur les Mouvements Révolutifs spontanés chez les Végétaux," which, notwithstanding its great length, is well worth reading, and is published among the "Comptes rendus des Séances de l'Académie des Sciences" for Nov. 1843. The result is, that in pisum sativum (green pea), in bryonia alba (wild bryony) and in cucumis sativus (cucumber) the stems of those leaves which bear the tendrils, describe a very slow circular movement in the air, the time in which they complete an ellipsis varying from one to three hours according to temperature. By this movement they seek at random for solid bodies round which, when found, they twine their tendrils; these then support the plant, it being unable to stand by itself without help. That is, they do the same thing as the eyeless caterpillar, which when seeking a leaf describes circles in the air with the upper part of its body. Dutrochet contributes a good deal of information too concerning other movements in plants in this treatise: for instance, that stylidium graminifolium in New Holland, has a column in the middle of its corolla which bears the anthers and stigma and alternately folds up and unfolds again. What Treviranus adduces is to the same effect:[253] "In parnassia palustris and in ruta graveolens, the stamina incline one after the other, in saxifraga tridactylites in pairs, towards the stigma, and erect themselves again in the same order."—Shortly before however, we read in Treviranus with reference to this subject: "Of all apparently voluntary movements of plants, the direction of their boughs and of the upper surface of their leaves towards the light and towards moist heat, and the twining movements of creepers round their supports, are the most universal. In this last phenomenon especially there is something which resembles animal movements. While growing, creepers, it is true, if left to themselves, describe circles with their tips and by this means reach an object near at hand. But it is no merely mechanical cause that induces them to adapt their growth to the form of the object they have thus reached. The cuscuta does not twine round every kind of support: for instance, limbs of animals, dead vegetable matter, metals and inorganic substances are not used for this purpose, but only living plants, and not even all kinds—not mosses, for instance—only those from which it can extract nourishment by its papillæ; and these attract it from a considerable distance."[254] The following special observation, communicated to the "Farmer's Magazine," and reproduced by the "Times" (13th July 1848) under the title "Vegetable Instinct," is however still more to the point: "If a basin of water be placed within six inches of a young pumpkin-stalk, or of a stem of the large garden pea, no matter on what side, the stalk will approach the basin during the night and it will be found next morning with one of its leaves floating on the water. This experiment may be renewed every night till the plant begins to fructify.—Even if its position be changed every day, a stick fixed upright within six inches of a young convolvulus is sure to be found by the plant. If, after having wound itself for a certain distance round the stick, it is unwound and wound round again in the opposite direction, it will return to its original position or lose its life in the endeavour to do so. Nevertheless, if two such plants grow close to one another without having any stick near enough for them to cling to it, one of them will change the direction of its winding and they will twine round each other. Duhamel placed some Italian beans in a cylinder filled with moist earth; after a little while they began to germinate and naturally sent their plumula upwards in the direction of the light and their radicula downwards into the mould. After a few days the cylinder was turned round to the extent of a quarter of its circumference and the same process was repeated until it had been turned completely round. The beans were then removed from the earth, when it was found that both plumula and radicula had twisted at each turn that had been given, in order to adapt themselves to it, the one endeavouring to rise perpendicularly, the other to descend, so that they had formed a complete spiral. Yet, notwithstanding this natural tendency to descend, when the soil below is too dry, roots will grow upwards in order to reach any moist substance which may be lying higher than themselves."
In Froriep's "Memoranda" for 1833 (No. 832) there is a short article upon the locomotivity of plants: in poor soil, where good mould lies near at hand, many plants will send out a shoot into the good mould; after a time the original plant then withers, but the offshoot prospers and itself becomes the plant. By means of this process, a plant has been known to climb down from a wall.
In the same periodical (1835, No. 981) is to be found a communication from Professor Daubeny, of Oxford (taken from the "Edinburgh New Philosophical Journal," April-July, 1835), in which he shows with certainty, by means of new and very careful experiments, that roots of plants have, at any rate to a certain degree, the power to make choice from those substances in the soil which present themselves to their surface.[255]