156 



vessel and parallel to the window, showed a very clear 

 geotropic curvature without exhibiting the smallest helio- 

 tropic reaction towards the side of the Ught. 



He further carried out experiments in order to investigate 

 whether at that oxygen-content, in which both growth 

 and geotropic curvatures are still possible, a heliotropic 

 stimulus can be perceived and whether a stimulus perceived 

 in air progresses at this oxygen-content. In order to answer 

 the first question, the required atmosphère was obtained 

 by partial exhaustion of the vessel and admission of 

 hydrogen, and the seedlings were exposed for iVa to 2 

 hours to a unilatéral illumination of a gas-flame, without a 

 curvature appearing. 



Then the gaseous mixture was replaced by ordinary 

 air. There was no trace of any after-efïect. In order to 

 décide the second question the seedlings were first illuminated 

 from IV2 to 2 hours by the same source of light which 

 caused an obvions curvature. The oxygen was then 

 diminished to 4 7o- No further after-efFect took place, 

 but the curvature was neutralized by the counter-efFect 

 of geotropism. 



He finds it very remarkable that the after-efFect remains 

 absent in an atmosphère in which the oxygen-content 

 still allows the geotropic-reaction, because this shows 

 that the way in which the heliotropic curvature is executed 

 is différent from that of the geotropic, although both are 

 phenomena of growth. It may certainly be assumed that 

 the actual growth is in both cases the same and further 

 that the heliotropic after-effect has certainly been induced. 

 Hence between the establishment of the disposition to 

 the after-effect and its mechanical exécution there must 

 necessarily intervene a process that is specially influenced 

 by oxygen. 



In his gênerai results Correns says that he recognizes 

 in ail stimulation-movements a différence between percep- 



