201 



The earlier therefore that the perceived stimulus is 

 enabled to find expression, the greater the reaction becomes. 

 Nevertheless in this experiment the exhaustion phenomenon 

 was also well marked, so that sufficient proof has been 

 given that the reaction-time is not lengthened in consé- 

 quence of the absence of oxygen, but that a seedhng of 

 Avena cannot furnish a geotropic reaction in an atmosphère 

 without oxygen, although it can do so again as soon as 

 normal conditions are restored. The extent of the curva- 

 ture dépends in that case on the greater or lesser extent 

 to which the stimulus is already falling off. 



C. Phototropic experiments with Avena. 



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

 gas foUowed by a long period, also in nitrogen. 



TABLE 23. 



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



