CHAPTER V. 



Effect of thc complète withdrawal of 

 free oxygen on the reaction. 



The influence of oxygen on the perception was deter- 

 mined, by stimulation in nitrogen, after a long fore-period 

 in the same gas, and allowing the reaction to take place 

 in atmospheric air. This suggests a considération of the 

 influence of oxygen on the reaction-process in the reverse 

 direction, by allowing the seedlings to perceive in ordinary 

 air and to rotate them on the clinostat in nitrogen during 

 the reaction-time. But this method is quite impracticable, 

 because the replacement of atmospheric air by nitrogen 

 requires IV2 to 2 hours and it is therefore impossible to 

 place the material in nitrogen at once after perception. 



I had therefore to proceed in a wholly différent manner, 

 I began by giving the seedlings a fore-period of 3 hours 

 in nitrogen, then stimulated them in nitrogen and left them 

 in it for the whole of the reaction-time. The différence 

 between thèse experiments and those under A, B, C and D of 

 chapter IV lies therefore in the fact that there a normal 

 reaction occurred in atmospheric air, after the fore-period 

 in nitrogen had lasted three hours, and the stimulus had 

 been applied in this gas, whilst in thèse experiments the 

 whole stimulation-process took place in nitrogen. 



If there is an unequivocal différence between the two 

 sets of experiments, then it can only be attributed to the 

 influence on the reaction of the deprivation of oxygen. 



