206 



From this also it is évident that a fore-period of 10 

 hours in 4.7 % oxygen has not the least influence on 

 perception. Àfter a fore-period of 24 hours the seedHngs 

 curve less strongly, so that in the long run they are 

 evidently unable to perceive a phototropic stimulus normally 

 in this atmosphère. The results are therefore identical with 

 those of the phototropic experiments. 



The object with which I had begun thèse experiments 

 was as foUows: to détermine that oxygen-content of air 

 at which, for geotropism as well as for phototropism the 

 seedHngs could still carry out the normal stimulation-process. 



Since, however, in order to feel the retarding influence 

 of an oxygen-content of 4 — 5 'Vo the seedlings must already 

 undergo in it a fore-period of 24 hours, I hâve not con- 

 tinued thèse experiments. It was naturally superfluous to 

 experiment with an atmosphère of smaller oxygen-content 

 than 4—5 "^ o- because the plants would undoubtedly be 

 affected adversely by a fore-period in it of some duration. 

 With regard to an atmosphère with a higher oxygen- 

 content, it is very difBcult to décide whether this has a 

 harmful effect or not. It is quite possible that the seedlings 

 in, for example, 8 7o oxygen still give a normal reaction 

 after 24 hours but not after 48 hours. For we can only 

 postulate a weakening of the power of perception in a 

 given atmosphère if the seedlings hâve remained in it 

 long enough before the application of the stimulus. Such 

 experiments are impossible on account of the rapid growth 

 of the expérimental material. 



