GENERAL SURVEY 345 



increasing intensity of stimulus. The rate of movement is 

 also increased under the action of stronger stimulus and 

 higher temperature ; it is decreased under fatigue. A 

 stronger stimulus, generally speaking, requires a longer 

 period for recovery. Under the physiological depression 

 induced by winter the responsive reactions are modified, 

 the latent period prolonged, and the amplitude reduced. 



Biophytum sensitivum may be taken as typical of a 

 quickly reacting plant. The leaflets undergo closure within 

 a second after receiving the excitatory shock ; recovery is 

 accomplished in the course of three minutes. In marked 

 contrast with this is the extremely sluggish reaction of the 

 leaf of Neptunia oleracea, where the apex time is reached 

 only after an interval of 180 seconds, and recovery com- 

 pleted in 60 minutes (p. 44). 



Additive Effect of Stimulus 



In the response of animal tissue it is found that a single 

 stimulus by itself ineffective becomes effective on repetition. 

 The same is found to be the case with plant tissues. Thus, 

 with a given specimen of Mimosa, it was found that, while 

 an electrical stimulus of intensity 'i was singly ineffective, 

 it became effective after being repeated twenty times. It is 

 found, moreover, that this additive effect is, within limits, 

 strictly quantitative. Thus with the identical specimen of 

 Mimosa, when the intensity of individual stimulus was 

 increased from "i to '5, the number of repetitions necessary 

 to cause effective excitation was reduced from 20 to 4. Here 

 •I X 20 = '5 X 4. The effective stimulation is thus found 

 constant, being equal to individual intensity multiplied by 

 the number of repetitions (p. 54). 



Effect of Temperature and of Intensity of Stimulus 



The response of Mimosa is abolished at a low tempera- 

 ture. With the rising temperature the amplitude of re- 

 sponse is increased and the period of recovery shortened. 



