Page:Researches on Irritability of Plants.djvu/58



already stated, when the pulvinus of Mimosa is subjected to an instantaneous stimulus, say that caused by an electric shock, a responsive movement is initiated after the lapse of a very short interval. After the completion of the fall of the leaf, the contracted pulvinus slowly recovers its original expanded condition, with consequent re-erection of the leaf. The movement of the leaf is thus a visible indication of the responsive reaction and recovery of the pulvinus under stimulus. In this entire process, we may conveniently distinguish three separate phases:—

First, there is a brief period between the incidence of stimulus and beginning of the responsive movement: the contraction has not yet manifested itself. This lost time is called the Latent Period.

Secondly, after the lapse of the latent period, the leaf begins to fall, at first with increasing rapidity, which then again diminishes, till it comes to a stop. The curve described attains its maximum amplitude, corresponding to the maximum fall of the leaf. The period required, up to this Rh