146 PLANT RESPONSE 



of sensitiveness, which often appears as the after-effect of a 

 preceding cyclic variation of temperature. That is to say, if 

 we take a series of responses while the temperature is rising, 

 and afterwards a similar series while the temperature is falling, 

 it is found that during the process of cooling, the responses 

 are markedly enhanced in amplitude, as compared with those 

 given at the corresponding temperatures during heating. 

 This is seen in a very clear manner in fig. 80. 



(b) On longitudinal mechanical response. — I have ob- 

 served a very similar phenomenon in the longitudinal 

 mechanical response of, for example, the coronal filaments of 

 Passiflora. An ascending series of responses was taken at 



temperatures of 25 , 35 , and 45 C. 

 They exhibited a regular decrease, 

 as has already been explained. On 

 now, however, taking records of re- 

 sponses during cooling, it was found 

 that the response at 35° C. was 50 per 



Fig. 81. Effect of Cyclic ,« v u j u u .l 



Rise and Fall of Tempe- cent, greater than it had been when the 

 rature on Longitudinal temperature was ascending (fig. 8 1 ). 



Mechanical Response in 



Plant I have already explained that 



response is found to be abolished 

 when the plant is killed, by raising the temperature above 

 a certain maximum point. The exact determination of 

 this point has hitherto been a matter of great uncertainty. 

 I shall in the next two chapters, however, explain several 

 methods by which this investigation may be carried out with 

 great precision. 



Summary 



The electrical and mechanical responses of plants undergo 

 diminution under the influence of cold. 



The latent period of response is prolonged by lowering of 

 temperature. 



When the temperature is raised above an optimum, the 



