306 tAF^ MOVEMENTS IN PLANTS 



(4) The law which determines the direction of respon- 

 sive movement. 



EFFECT OF RISE OF TEMPERATURE. 



As regards the effect of rise of temperatura we have 

 seen that, within normal limits, it induces expansion a ad 

 acceleration of the rate of growth. Stimulus, on the otlier 

 hand, induces precisely the opposite effect. Hence the 

 physiological reaction of steady rise of temperature is^ 

 generally speaking, antagonistic to that of stimulus. This 

 conclusion is supported by numerous experiments whicli I 

 have carried out with various plant organs. Example of 

 this will be found in the present and subsequent chapters. 



DIFFERENT THERMONASTIC ORGANS. 



The only condition requisite for the exhibition of 

 response is the differential excitability of an anisotropic 

 organ. It is therefore likely to be exhibited by a large 

 variety of plant organs, such as pulvini, petioles, leaves, and 

 flowers, and my results show that this is actually the case. 

 This particular sensibility, moreover, is not confined to 

 delicate structures, but is extended to rigid trees and their 

 branches. 



Before proceeding further, it is necessary to draw 

 attention to the confusion which arises from the use 

 of the common prefix ' thermo ' in thermonasty and 

 thermo-tropism. With regard to this Pfeffer says " It 

 is not known whether radiated and conducted heat 

 exercise a similar thermotropic reaction. "* I shall show 

 that the reactions to radiant heat, and to conducted heat 



* Pfeffer— /6/V?, Vol. Ill, p. 177. 



