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of the normal-atmosphère. When, a geotropic curvature, 

 however small, occurred in the vessel, it continuée! when, 

 on return to atmospheric air, the seedling was placed in 

 a vertical position. But if absolutely no reaction had 

 taken place, in the first instance, there was no after- 

 efFect whatsoever. 



He concluded from ail thèse experiments that oxygen 

 is necessary to the carrying out of a geotropic process. 

 He also placed seedlings for 1 to 2 hours horizontally in 

 the vessel, pumped it emply, fiUed it with hydrogen and 

 led a current of hydrogen through it. Afterwards the seed- 

 lings were placed in the dark in an upright position. 

 Even when the material had in this way been in contact 

 with hydrogen for several hours an after-effect could still 

 be observed, but the longer time hydrogen had been 

 passed the smaller the after-effect was found to be. 

 From this experiment he draws the conclusion that the 

 after-effect had been only interrupted by the deprivation 

 of oxygen, but not destroyed. 



Correns saw even less occurrence of movements in 

 his heliotropic experiments without oxygen. Illumination 

 took place by turning the uncovered side of the vessel 

 towards the day-light. In order to get a heliotropic reaction 

 about 1 7o of the original quantity of oxygen must be 

 présent, and in the case of Sinapis seedlings even 6 Vc 

 at least to get a curvature visible to the naked eye. 



The geotropic curvatures are therefore carried out by 

 the same plants with smaller quantities of oxygen than 

 the phototropic ones. 



He also carried out experiments in which he allowed 

 both stimuli to act simultaneously on the same material, 

 and in an atmosphère, containing so much oxygen that 

 growth and geotropic curvatures could take place, but in 

 which the seedlings were not able to give a heliotropic 

 reaction. Seedlings, which were placed horizontally in the 



