CHAPTER IV. 



Effect of the complète withdrawal of free 

 oxygen on the perception. 



In order to find out whether oxygen is necessary for 

 the perception of a stimulus, I flrst left the seedlings for 

 some time in the thermostat through which a continuous 

 current of nitrogen was passed. Àfter this they were 

 stimulated in nitrogen whilst the nitrogen current was at 

 once replaced by air, drawn through the thermostat by 

 means of an aspirator. The seedlings were therefore in 

 ordinary air during the reaction-time. 



An examination of the gas, that flows out of the 

 thermostat, showed me, that after IV2 to 2 hours ail the 

 original air had been expelled. When, for instance, a 

 fore-period of 3 hours in nitrogen is spoken of in the 

 tables, there 3 hours are reckoned from the beginning of 

 the passing through of the current, so that during the 

 first 2 hours the seedlings were not yet in an atmosphère 

 of pure nitrogen. 



A. Geotropic experiments with Avena. 



The seedlings of Avena were placed vertically in the 

 thermostat ^), whilst for a time nitrogen was passed through, 

 then they were for 15 minutes geotropically stimulated 



') For this reason henceforward the experiments were carried out with 

 one box at a time. A second box was indeed fastened in the clamp, 

 but this served exdusively to balance the other in order to secure the 

 uniform rotation of the chnostat during the reaction-time. 



