Page:Researches on Irritability of Plants.djvu/58

CHAPTER IV

TIME-RELATIONS OF THE RESPONSIVE MOVEMENT AND STANDARDISATION OF STIMULUS

Latent period of Mimosa—Apex-time—Rate of responsive movement of leaf—Effect of intensity of stimulus, fatigue, and temperature—Periodic dot marker—Time-relations of response and recovery—Effect of season—Response of Biophytum—Response of Neptunia—Arbitrary distinction between sensitive and ordinary plants—Differential response in Mimosa—Response of ordinary plants—Universal sensitiveness of plants—Standardisation of stimulus—Maximal and Minimal Stimuli—Extreme sensitiveness of Mimosa

As 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