TOWARDS the latter part of this summer, Mr. Gilbert Burnett and myself made several experiments with a view to ascertain the nature of the movements exhibited by the sensitive plant. We afterwards found that the greater part of the facts which we had observed, had been previously described by Mr. Lindsay [p077] and Dr. Dutrochet. Mr. Lindsay’s observations are to be met with in a MS. preserved in the library of the Royal Society, which is dated July 1790: this essay is alluded to by Dr. Smith in his “Introduction to Botany.” Dr. Dutrochet’s experiments were published in his “Recherches anatomiques et physiologiques sur la Structure intime des Animaux et des Végétaux,” which appeared in 1824. With the latter author the reputation of originality is likely to rest: not undeservedly, indeed, as there is no reason to suppose that his experiments were suggested by a knowledge of those performed by Lindsay. It is, however, an act of literary justice to secure to Mr. Lindsay the credit of undoubted priority in describing the phenomena which he noticed in common with Dutrochet. I have drawn up the following remarks partly for this purpose—partly to have an opportunity of mentioning some circumstances which escaped the observation of both experimentalists.
The leaves of the Mimosa Pudica consist either of one or two or three pairs of leaflets, and occasionally terminate by an odd one. Each leaflet bears from twenty to sixty subleaflets, which are disposed in pairs. The petiole or stalk of each leaf, at the extremity which is attached to the branch or stem of the peant, swells into an intumescence varying from three to five in length. A similar intumescence, of proportionate dimensions, is seen upon each subpetiole, where it is articulated with the petiole, and upon the base of the stalk of each subleaflet: the intumescence is the part in which motion takes place.
During the day-time the petioles are observed to have a direction upwards, or rather to form an acute angle with the upper part of the stem or branch, to which they are attached: the subpetioles are divergent: the subleaflets are spread out, so as to lie nearly in one plane. (Fig. 1.)
During the night the petioles are found to be depressed; the subpetioles to be drawn together, the subleaflets folded, the upper or solar surfaces of each pair being brought into contact. (Fig. 2.)
The leaves rise, the leaflets diverge, and open by throwing down their subleaflets, at daybreak: the opposite changes occur about sunset. The experiments that are to be described, are supposed to be performed in the day-time. [p078]
Fig. 1.
Fig. 2.
If a terminal subleaflet be pinched with forceps, or cut with scissors, it rises, together with its fellow; then the next pair rise; then the next; and so on in succession, till all the pairs of subleaflets upon the same subpetiole are folded. In a little time afterwards, the petiole is bent downwards at its intumescence; and in a few seconds more the remaining leaflets upon the same petiole fold their subleaflets in pairs, from the base towards the point of the leaflet.