To accord sensibility to plants, is to transgress the classic laws of natural history, so that the considerations and facts which appear to us to justify this proposition ought to be most carefully stated.
1. The plant feels the sensations of pleasure and pain. Cold, for instance, impresses it painfully; it may be seen to contract itself, as if shivering, under the influence of a sudden or excessive fall of the temperature. An abnormal excess of temperature evidently causes it to suffer; when the heat is very great, leaves may be observed to hang down on the stems, curl up, and appear to wither; when the cool of the evening comes, the leaves rise up again, and the plant resumes its appearance of placid health. Drought also occasions manifest suffering to plants. Those who study nature with loving attention know that when, after a long period of drought, a plant is watered it exhibits signs of pleasure. On the other hand, a wounded plant, one from which a branch has been cut, appears to experience pain. A pathological liquid exudes from the wound, like the blood from a hurt animal; the plant is sick, and will die, if it do not receive the necessary succour. Thus persons who love plants will not cut flowers off their stems, they prefer to inhale their perfume, and contemplate their brilliant colours, on the stalk, without inflicting a painful mutilation upon the beautiful creatures which they admire.
The sensitive plant, if touched by the fingers, or even struck by a current of harsh air, folds up its leaves, and contracts itself. The botanist, Desfontaines, saw a sensitive plant, which he was bringing home in a carriage, contract its leaves while the vehicle was in motion, and expand them when it stopped, thus affording a proof that the movement distressed the plant. A drop of acid, or acrid liquid, placed on a leaf of a sensitive plant, will occasion a similar constriction. All vegetables present an analogous phenomenon. Their tissues contract when brought in contact with irritant substances. By rubbing the tips of a lettuce, the juice may be made to exude.
Vegetable sensibility exists by the same right as animal sensibility, since electricity kills plants as it kills animals, since narcotic poisons kill or stupefy plants as they kill and stupefy animals. One can narcotize a plant by watering it with opium dissolved in water, and MM. Gopport and Macaire have discovered that hydrocyanic acid kills plants as rapidly as animals.
2. Plants sleep at night. During the day they develop their vital activity, and when the night comes, or when they are in darkness, their leaves assume another position, that of repose; they fold themselves up. In the day-time, the upper surface of the leaf is turned to the sky, and the under surface towards the earth; this under surface, pierced with holes, or stomata, is the part through which absorption and exhalation take place, while the upper surface, in which there are no such openings, is only a sort of screen for the protection of the absorbing surface. It is therefore easy to understand that the horizontal attitude of the leaves is a position of vital activity, and that the refolding of those leaves during the night indicates a state of repose. It is precisely the same case with ourselves, when during the night we indulge our muscles, kept on the stretch during the day, with complete relaxation.
The sleep of plants, said to have been discovered by Linnæus, and which was certainly described for the first time in one of Upsal's Thèses de Botanique, and thoroughly elucidated by Linnæus, is not a phenomenon limited to certain families of plants. There are very few vegetables which, during the night, or in darkness, do not fold their leaves, and which do not present a different appearance by day and by night. The Sensitive is the classic plant selected for its exhibition of this phenomenon in all its intensity; but this small leguminous creature only presents us with an exaggeration of a fact which exists in a lesser degree among almost all vegetables with light leaves. We may quote the following passage from a former work on the subject of this phenomenon.
"The sleep of plants vaguely resembles that of animals. It is a remarkable circumstance that the slumbering leaf appears to wish to return to the epoch of its infancy. It folds itself almost as it was when in the bud, before it burst out, as it was in the lethargic sleep of winter, sheltered beneath its strong scales, or wrapped up in its warm down. One would think that the plant was trying every night to resume the position which it occupied in its early time, just as the sleeping animal gathers himself together, and folds his limbs as they were folded in his mother's womb."[15]
Is it possible to deny the possession of sensibility to creatures which give us alternate sign of repose and of activity, and who have the power of accommodating themselves to various external impressions? Fatigue cannot possibly be anything but the consequence of the experience of an impression.
3. Numerous physiological functions are fulfilled by plants as well as by animals; and when we consider the number and variety of these functions, it is difficult to understand how, if animals be, as the common consent of mankind declares them, possessed of sensibility, plants can be destitute of it. An ancient philosopher defined plants as animals with roots. We shall see, on examining the variety of functions performed by vegetables, whether this philosopher was not a far-seeing, wise man.
It would be difficult to name any function with which the animal is invested that the vegetable does not possess in a less degree. Respiration, for instance, is equally a property of plants and of animals. Among the latter, respiration consists in the absorption of the oxygen of the air and the emission of carbonic acid gas and watery vapour; among plants it consists in the emission of carbonic acid gas and watery vapour during the night, and during the day, under the influence of sunlight, of the emission of oxygen proceeding from the decomposition of carbonic acid. The function is evidently of the same nature in both the natural kingdoms.