If a subleaflet be burnt, instead of being cut or pinched, the phenomena above described occur more rapidly: and after they have taken place, the adjoining leaves upon the same branch are bent down in succession, their leaflets brought together, and their subleaflets folded. If the plant be very vigorous and lively, an impression [p079] made upon one leaf affects the rest in succession. It is well known that the stem, branches, flowers, and roots of the sensitive plant have no motion. But M. Desfontaines observed that, on touching the roots with sulphuric acid, the leaves become folded; and M. Dutrochet obtained a similar result on burning either the flower or the stem.
Fig. 3.
Fig. 4.
Fig. 5.
If the plant be shaken, all the leaves are simultaneously thrown down, and their leaflets folded. Mr. Lindsay attempted to elucidate the action of the intumescence in raising and depressing the petiole, in the following manner. He cut out a portion from the upper or solar surface of the intumescence; after which he found that the petiole, upon recovering, rose higher than before, (Fig. 3.) From another leaf he removed the inferior portion of the intumescence: he found, upon this injury, that the leaf declined more than before, and did not again rise, (Fig. 4.) He noticed that a thin slice, pared from either surface of the intumescence, has a like effect, but in a less degree than a deep excision: and he found that when similar experiments are made upon the intumescence of the subpetiole, there is no essential difference in the result.
Thus Mr. Lindsay discovered, that the force which raises the petiole exists in the lower part of the intumescence, and that which depresses it, in the upper. He seems to have considered that the temporary excess of force in either part is produced by an impulsion of the sap from the vessels of the yielding portion into those of the opposite portion. [p080]
Dr. Dutrochet viewed these phenomena in some respects more justly. He remarked, in addition to what Lindsay had observed, that if, instead of the upper and under surface, the lateral part of the intumescence be removed, the petiole becomes not raised or deflected, but inclined towards the side on which it is injured (Fig. 5); and that if longitudinal slices of the upper, or under, or lateral portions of the intumescence are immersed in water, these separate slices immediately become incurvated, that edge being concave which looks towards the axis of the intumescence. From these facts Dutrochet inferred that the texture of the intumescence possesses some modification of irritability; that, when excited, each length of the intumescence (to use a very imperfect expression) forcibly assumes an incurvated figure, like a curved spring returning from a state of temporary extension; that the petiole is raised, when the action of the lower part of the intumescence predominates; is depressed, when the upper portion acts with increased energy.
Mr. Burnett and myself had arrived at very similar conclusions respecting the agency of the intumescence, before we became acquainted with the inquiries of Lindsay and Dutrochet.
In Dutrochet’s able researches, a more exact analysis, however, was obtained of the functions of this part. He discovered that the cortex of the intumescence is the seat of its irritability: for upon wholly removing the bark, so as to expose the ligneous substance, the petiole was found to have been rendered motionless. Nevertheless, the intumescence, thus mutilated, remains capable of transmitting an impression made upon its leaflets to the leaves adjoining, Dutrochet further ascertained, that the ligneous substance alone is fitted to convey the peculiar stimulus, which spreads, from a point of the plant that has been irritated, to the adjoining leaves.