RADtO-THfiRMOtHOPlSM 413 



thermal radiation. Figure 151 gives a record of response 

 of the stem of Dregea to stimulus of short duration ; the 



Fi(i. 151. — Positive response to short exposure to thermal radiation. Sue. 

 cessive dots at intervals of 5 seconds. {Dregea volubllis.) 



induced curvature is positive or towards the source of 

 heat. On the cessation of stimulus, there is a recovery 

 which is practically complete, and which takes place at a 

 slower rate than the excitatory positive curvature. Repeti- 

 tion of stimulus gives rise to responses similar to the 

 first. Successive stimuli of moderate intensity thus give 

 rise to rejjeated resjjonses of growth curvature. An arbi- 

 trary distinction has been made between the responses of 

 pulvinated and of growing organs. The former is disting- 

 uished as a movement of variation, with its supposed 

 characteristic of repeated response. But the experiment 

 described shows that this is also met with in the response 

 by growth curvature. It is only under long continued 

 stimulation that the curvature is fixed by growth. 



DIA-RADIO-THERMOTROPISM. 



The positive curvature is induced by retardation of 

 growth at the proximal side, and enhancement of growth 



