212 



In an oxygen-free environment the perception of a 

 stimulus cannot therefore take place, provided that the 

 condition of a sufïîciently long fore-period has been satisfied. 



If, after perception of a geotropic or phototropic stimulus, 

 the seedlings are left in the oxygen-free atmosphère, they 

 do not react, so that the présence of oxygen is àlso 

 necessary for the occurrence of the reaction. 



In an atmosphère with low oxygen-content the seedlings 

 remain for a long time able to perceive normally, but a 

 prolonged stay in such an atmosphère weakens the power 

 of perception. 



There are no indications that, on complète or partial 

 withdrawal of oxygen, the reaction of seedlings to a 

 geotropic stimulus difFers from their reaction to a photo- 

 tropic one. 



