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 — Differ- 

 ential 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 sub- 

 jected 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 



35 D2 



