282 RESEARCHES ON IRRITABILITY OF PLANTS 



which however reach a limit. The same is true of the 

 responses of a skeletal muscle of frog. 



In carrying out experiments on Biophytum, I first 

 determined the minimal intensity of induction-shock that 

 was effective in inducing response. This happened when 

 the intensity of stimulus was -i unit. The record of this 

 response under minimal stimulation was then taken. After 

 this, a second response to stimulus which was ten times as 

 strong, was recorded. It will be seen that both minimal 

 and maximal stimuli induced practically the same effect (fig. 

 133). In other words, we have here an example of what is 



Fig. 133. — Record of responses of Biophytum to stimuli 'i and 

 I unit, respectively. 



known as response on the ' all-or-none ' principle. The leaflet 

 either responds to its fullest or not at all. 



In the various characteristics which have just been 

 described, the responses of the vegetal organism bear a 

 curious resemblance to those of the cardiac tissue of the 

 animal. In the response of the animal heart the latent 

 period is relatively long, and it exhibits a similar prolonged 

 refractory period. Its responses are also on the * all-or-none ' 

 principle — it either responds to the utmost or not at all. 



Returning to Biophytum, we have seen that in order to 

 induce any response a certain minimal intensity of stimulus 

 was necessary. But when the intensity of this stimulus 

 was further increased, the outward expression, or response 

 of the leaflet, remained apparently as before. What then 

 became of the excess of energy that impinged upon it in 

 the form of stimulus ? It is not necessary to suppose that 



