Page:An introduction to physiological and systematical botany (1st edition).djvu/241

 Rh Kew v. 3, t. 13. An impression made even in the most gentle manner, upon one of their leaflets, is communicated in succession to all of them, evincing an exquisite irritability, for it is in vain to attempt any mechanical solution of this phenomenon. One of this tribe, Hedysarum gyrans, has a spontaneous motion in its leaves, independent of any external stimulus, even of light, and only requiring a very warm still atmosphere to be performed in perfection. Each leaf is ternate, and the small lateral leaflets are frequently moving up and down, either equably or by jerks, without any uniformity or cooperation among themselves. It is difficult to guess at the purpose which this singular action is designed to answer to the plant itself; its effect on a rational beholder cannot be indifferent.

The chemical actions of light, heat, and the component parts of the atmospheric air, upon leaves, and, where the latter are wanting, on the green stems of plants, are now, as far as concerns all plants in common, tolerably well understood. The observations and experiments of Priestley and Ingenhousz have