8 RESEARCHES ON IRRITABILITY OF PLANTS 



of this curve gives the ampHtude of the movement, and 

 the horizontal distance measures the corresponding time. 

 The up-hne here, a b, indicates the responsive fall, and the 

 descending line, b c, the gradual erection, due to recovery. 

 The responsive movement was initiated within an exceedingly- 

 short time after the application of stimulus — in this case an 

 electrical shock — and the fall was completed also within 

 a relatively short period. In a record which will be given 

 later, taken on a faster-moving plate, these characteristics 

 will be seen better. The recovery, however, is a slow process, 

 the earlier part being comparatively quick and becoming 

 slower towards the end. The entire recovery is here seen to 

 require 12 minutes. 



If we were able to apply a stimulus of exactly identical 

 intensity at regular and suitable intervals, and if the physio- 

 logical condition of the responding tissue remained constant, 

 then we should obtain a series of responsive twitches which 

 would be practically identical. But if the physiological 

 condition were to undergo any change, under environmental 

 conditions, then the record would give us indications of that 

 internal change, otherwise entirely beyond our power of 

 scrutiny. Thus if the plant were to become depressed, the 

 amplitude of the pulse would undergo a diminution. If on 

 the other hand its excitability should be enhanced, that fact 

 would be indicated by an increase in the amplitude of the 

 response. 



The mere amplitude of the twitch, however, affords 

 only a broad indication of the physiological condition of 

 the tissue. There are many factors the effects of which find 

 expression in subtler changes of the response-curve. One 

 agency, for instance, will make the plant more alert. This 

 is at once reflected at that part of the curve which corre- 

 sponds to the Latent Period. This becomes shorter. The 

 ascent of the curve will also be more abrupt. Another 

 agency will induce, let us say, a contrary change. Different 

 agencies, similarly, will bring about definite changes in 

 the contracting and relaxing portions of the curve. The 



