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.