180 RESEARCHES ON IRRITABILITY OF PLANTS 



different species of plants, but varies to a certain extent 

 in different individuals of the same species. Again, the 

 transmission under feeble stimulus is slower than under 

 strong stimulus. This accounts for the result frequently 

 obtained — that the propagation-time is much quicker under 

 the strong stimulus of thermal shock than under the 

 moderate stimulation by constant current. 



Another interesting fact which has attracted my 

 attention is that the velocity of transmission is, generally 



Fig. g6. — Positive followed by negative impulse in Biophytum, caused 

 by single indirect thermal stimulus. In this and in the following 

 records, ' down ' curve represents positive, and ' up ' curve negative, 

 response. Frequency of oscillation once in a second. 



speaking, slower in the stem than in the petiole. There 

 also seems to be a loss of time when excitation has to 

 pass from the stem to the leaf. Taking advantage of 

 these facts we may, when desired, obtain long periods of 

 transmission by applying stimulus of moderate intensity 

 on the stem. 



I will now describe experiments giving quantitative 

 results, which demonstrate the occurrence of two distinct 

 impulses under different forms of stimuli, the experimental 

 specimen being Biophytum sensitivum. 



Thermal stimulus. — The electro-thermic stimulator was 

 employed for the application of thermal shocks. The record 



