268 RESEARCHES ON IRRITABILITY OF PLANTS 



descent. Independent of or concomitant with this will be 

 the changes of excitability, more or less obscure, which are 

 brought about by increasing age. Before subjecting this 

 inference to the test, I will describe an experiment which 

 shows that the after-effect of moderate stimulation on 

 a sub-tonic tissue is to raise its susceptibility to polar 

 excitation. 



After-Effect of Moderate Stimulation 



For this purpose I used a specimen which appeared to 

 be in a pre-optimum condition, and took its records under 



Fig. 127. — Transformation of Type II. to Type III. as after-effect of 

 previous stimulation. First two responses taken before, and last 

 three after, tetanisation. 



polar excitation by an identical current, before and after 

 the application of tetanising shocks of moderate intensity 

 and duration. 



Under an e.m.f. of 16 volts, the records obtained show 

 excitation (fig. 127) at kathode-make and anode-break, 

 characteristic of Type II. The record was then stopped 

 and tetanising electric shocks of moderate intensity applied 

 for 2 minutes. After a period of rest of 15 minutes, the 

 record of polar excitation t was taken once more, the applied 



