390 COMPARATIVE ELECTRO-PHYSIOLOGY 



limit, after which further stimulation would produce little or 

 no effect 



Thus we see that if a tissue from any cause be sub-tonic, 

 even moderately strong stimulus will induce positive or 

 absorptive response. If it, on the other hand, be highly 

 excitable, the response may be expected to be negative or 

 expulsive. Between these two, in the intermediate state of 

 excitability, the effect will be neither one nor the other, that 

 is to say, zero. These results concern the application of 

 somewhat strong stimulus, such as that of electrical shocks. 

 Feeble stimulus will generally evoke response by absorption. 

 We can also clearly see that the tonic condition, and there- 

 fore the excitability, of a tissue will vary with the seasons, 

 being low at the end of winter, and high in spring or summer. 

 Thus the same strong stimulus which in the one season will 

 induce absorption, might be expected in the other to provoke 

 expulsion. In experimenting on suction during the period 

 of seasonal variation, I obtained results which verified these 

 inferences. 



These experiments, as will be remembered, were begun 

 in February, when the spring had scarcely commenced, and 

 the plants were in a sub-tonic condition. Under these 

 circumstances, we saw that the terminal application ot 

 stimulus uniformly evoked positive response, by enhance- 

 ment of suction. By the end of February, however, when 

 warmer weather prevailed, and the vigour of the plants was 

 evidently greater, I was surprised to find that the same 

 stimulus, applied in the same way, to similar specimens of 

 Croton .evoked little or no response. A week later that is 

 to say, in the beginning of March, when the Indian spring 

 was well advanced, and the physiological activity of the 

 plants high I found that the response which had thus seemed 

 to disappear was renewed, but had become reversed in sign. 

 Strong terminal stimulation now, as a rule, evoked responsive 

 expulsion. 



From the considerations enumerated at the beginning of 

 these investigations, it was seen that the various physical 



