412 LTPE MOVEMENTS IIJ PLANTS 



means of electric current. The intensity of incident radia- 

 tion could thu3 be maintained constant, or increased or 

 decreased by approach or recession of the radiating spiral. 

 The effect of unilateral stimulus of heat-rays was found 

 exactly similar to that of light ; i.e., there was at first 

 an erectile movement due to indirect stimulation, followed 

 by the fall of the leaf due to transmitted excitation. It 

 will be noticed that under the particular condition of the 

 experiment, the responding pulvinus was completely shielded 

 from temperature-variation. The reaction to thermal 

 radiation is thus similar to that of light. 



As regards the effects of rise of temperature and radia- 

 tion I have 8ho\vn that they are antagonistic to each other 

 (pp. 211, 308). Thus in positive types of thermonastic organs 

 like the flower of ZephyranUies, while rise of temperature 

 induces a movement of opening, radiation causes the oppo- 

 site movement of closure. Again, in the negative type 

 exemplified by Nymphcea, rise of temperature induces a 

 movement of closure ; radiation on the other hand, brings 

 about the opposite movement of opening. The tropic effect 

 of thermal radiation thus takes place in opposition to that 

 of rise of temperature, and the resultant effect is therefore 

 liable to undergo some modification, depending on the 

 relative sensibility of the organ to radiation and to 

 variation of temperature. 



The facts that have been given above prove that infra- 

 red radiation is as effective a mode of stimulation as the 

 more refrangible rays of the spectrum. Phototropic and 

 radio-thermotropic reactions would therefore prove to be 

 essentially similar. The following experiments fully con- 

 firm the similarity of the two reactions. 



POSITIVE RADIO-THERMOTROPISM. 

 Experiment 157. — I shall now describe the normal reac- 

 tion of a growing organ to the unilateral stimulus of 



