208 



As Correns himself admits, he did not succeed in 

 studying separately the influence of oxygen on perception 

 and reaction. On this point, however, Arpâd Paâl 

 was successful, but he only performed geotropic experiments. 

 In order to investigate how the sensory phase is dépen- 

 dent on the oxygen content, he determined the présentation- 

 times, by which, in accordance with the old conception, 

 he understood the shortest possible times during which 

 the material must be stimulated in order to see with the 

 naked eye as after-efFect a curvature in at least 50 7ri of 

 the seedhngs. He found that on diminishing oxygen-content 

 the presentation-time became longer. This is readely 

 harmonized with my results. I always worked with the 

 same strength of stimulus, but I hâve shown that when 

 the seedlings hâve for long enough time been deprived 

 whoUy or partly of oxygen and perception takes place, a 

 weaker curvature results. Now if I had applied a weaker 

 stimulus, which did not give, as in my case, a maximal 

 curvature of 2 m. m., but a reaction which was just 

 visible, various seedlings would then hâve remained straight, 

 anyhow to the naked eye. In order in this same atmosphère 

 to obtain a curvature in a greater number, I should hâve 

 had to strengthen the stimulus, and this therefore entirely 

 agrées with Arpâd Paâl's contention. 



Further there exists an apparent disagreement between 

 my conclusion that the reaction-time was not shortened 

 by withdrawal of oxygen and that of Arpâd Paâl that 

 on diminution of the oxygen-content the reaction-time 

 becomes longer. I hâve always taken as reaction-time 

 Àrisz's conception of curvature-time '), namely, the time 

 which elapses from the beginning of stimulation till definite 

 maximal reaction is reached, whilst Arpâd Paâl employs 

 the old conception and takes the time which elapses 



') Arisz, 1. c. p. 60. 



