. CHAPTER XVI 



RADIO-THERMOTROPISM 



Tropic curvature induced by different rays of light has 

 already been studied. It was found that while the more 

 refrangible rays of the spectrum were most effective, the 

 less refrangible rays were ineffective. Below the red there 

 are the thermal rays, the effect of which is complicated 

 by that of rise of temperature. The effects of these two 

 factors are antagonistic, and to this must be ascribed the 

 contradictory results that have been obtained by different 

 observers, of which Pfeffer ^ gives the following summary : 



* In addition to the action of ultra-red rays which are 

 associated with the visible part of the spectrum, dark 

 heat-rays of still greater wave-length, as well as differences 

 of temperature, may produce a thermotropic curvature 

 in certain cases. Wortmann observed that seedlings of 

 Lepidivim sativum and Zea Mays, as w^ell as sporangiophores 

 of Phycomyces, curved towards a hot iron plate emitting 

 dark heat-rays. Steyer has, however, shown that the 

 sporangiophore of Phycomyces has no power of thermotropic 

 reaction. Wortmann observed that the seedling-shoot of 

 Zea Mays was positively, but that of Lepidium negatively, 

 thermotropic. . . . Steyer, however, found that both plants 

 were positively thermotropic. Wortmann has also investi- 

 gated the radicles of seedlings by growing them in boxes of 

 sawdust, one side being kept hot, the other cold.' 



It will be noted that in the investigations described 

 above, thermotropic reaction has been assumed to be the 



1 Pfeffer, ihid. vol. iii. p. 776. 



