344 RESEARCHES ON IRRITABILITY OF PLANTS 



definite time intervals. In this manner time interval 

 shorter than a hundredth part of a second can be measured. 

 Owing to the extreme lightness of the recorder the error 

 due to inertia is reduced to a minimum (p. 14). 



Methods of Stimulation 



The various stimuli which evoke motile response in the 

 animal are also found effective in giving rise to excitatory 

 response in the plant. Thus the plant may be excited by 

 mechanical, chemical, thermal, and electrical modes of 

 stimulation. Electric stimulation may be caused by the 

 polar action of a constant current, by the discharge of a 

 condenser, or by the induction current. The electrical 

 method allows the intensity of stimulus to be maintained 

 constant, or varied in a quantitative manner. As in the 

 skeletal muscle of animal, so also in the pulvinus of Mimosa, 

 the break induction shock is more effective than the make 

 shock. The sensitiveness of Mimosa to an electric shock 

 is very great. It often reacts to an intensity of shock 

 which is only one-tenth of the minimum perceived by a 

 human subject (p. 23). 



Time Relations of the Responsive Movement 



Different plants exhibit different characteristics of 

 response. The reactions are relatively quick in some and 

 slow in others. In a typical case of Mimosa in summer 

 the latent period was one-tenth of a second. The maximum 

 fall of the leaf was attained in three seconds, and the recovery 

 completed in 15 minutes. The rate of recovery was rapid 

 at the beginning and very slow towards the end. The 

 maximum rate of recovery was '09 mm. per second in 

 contrast to the maximum rate of fall of 24 mm. per second. 

 The movement of recovery was about three hundred times 

 slower than the movement of the excitatory fall. The 

 extent of responsive fall in Mimosa increases with the 



