202 



This table therefore shows that a phototropic stimulus 

 which has been perceived cannot give a reaction in the 

 absence of oxygen. 



D. Phototropic experiments with Avena. 



Fore-period of 3 hours in nitrogen. Stimulation in this 

 gas; nitrogen replacée! by atmospheric air 75 minutes after 

 beginning of stimulation, i.e. after the reaction-time of a 

 maximal phototropic curvature of 2 m. m. under normal 

 conditions (PI. XIX, fig. 1). 



TABLE 24. 

 Strength of stimulus 40 M.C.S. Température 20"^ C. 



The results of the phototropic experiments therefore 

 completely correspond mutatis mutandis with those of the 

 geotropic, so that hère also it is seen that a perceived 



