154 



movement, it is also intelligible that the after-efFect of 

 the geotropic stimulus can still occur so long as slight 

 growth is still possible. He also carried out experiments 

 in a hydrogen atmosphère. Shoots, which had been kept 

 in this for some time in a horizontal position, showed 

 when brought into the air, a further growth in length 

 but no trace of a geotropic after-effect. 



Correns ^) has written a long paper on the influence 

 of oxygen on ail kinds of stimulation-phenomena, including 

 the movements due to gravitational- and light-stimuli. 

 He also followed the method of placing the expérimental 

 material under reduced pressure or in a vacuum. In order 

 to expel the oxygen completely, after having exhausted 

 he let in hydrogen, then pumped the vessel emply again 

 and repeated this several times. He worked with seedlings 

 of Helianthus, Vicia Faba, Lepidium sativum and Sinapis 

 alba. The various types of stimulation-movements require 

 various concentrations of oxygen. The reason for this lies 

 partly in the différent oxygen requirements of various 

 species of plants, independently of the character of the 

 stimulation-movements, and partly in the conditions which 

 are characteristic of certain stimulation-movements. He 

 flrst discusses the geotropic experiments. The vessel in 

 which the seedlings were placed was exhausted in the 

 way described and then placed in a horizontal position. 

 The plants were examined after 6 to 12 hours. They ail 

 behaved in the same way, that is to say, so long as 

 growth could be observed, the geotropic curvature still 

 took place. After exhausting and letting in hydrogen five 

 times, Correns still obtained perceptible curvatures, 

 for example, in Helianthus on the other hand Sinapis 

 alba only reacted with an oxygen-content of 4 7o to 5 7o 



^) C Correns. Ueber die Abhàngigkeit der Reizerscheinungen hôherer 

 Pflanzen von der Gegenwart freien SauerstofFes. Flora 75, 1892, p. 87. 



