XXXVII.— RADIO-THERMOTROPISM 



By 



Sir J. C. BosE, 



Assisted by 



GURUPRASANNA DaS. 



We have studied the tropic curvature induced by 

 different rays of light. We saw that while the more refran- 

 gible rays of the spectrum were most effective, the less 

 refrangible rays were ineffective. Below the red, there are 

 the thermal rays about whose tropic effect very little is 

 definitely known. 



The intricacies of the problem are very great owing to 

 the difficulty of discriminating the effect of temperature 

 from that of radiation ; to this must be ascribed the contra- 

 dictory results that have been obtained by different obser- 

 Vcirs, 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 Le2)i- 

 dium sativum and Zea Mays, as well as sporangiphores of 

 Phycomyces curved towards a hot iron plate emitting dark 

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

 phore of Phycomyces has no power of thermotropic re- 

 action . . . Wortmann observed that the seedling shoot of 



* Pfeffer— /6trf— Vol. Ill, p. 776, 



