88 



function. When allowing for this, the results obtained do 

 not contradict the theory that the optimum is the resiilt 

 of secondary influences. 



Blackman's view that the optimum is not the expres- 

 sion of a primary, gênerai rehition betvveen température 

 and physiological processes, is confirmed by this investigation. 



The way in which the presentation-time changes accor- 

 ding to the température shows that we hâve hère to do 

 with the influence of température on a chemical process. 

 Therefore we may say that perception of geotropic sti- 

 mulation is connected with a chemical process. 



This investigation was carried on in the Botanical l^a- 

 boratory of the University of Utrecht. I wish to express 

 my most hearty thanks to Professer W e n t, to whom I 

 am indebted for drawing my attention to the subject, 

 for a kindly interest taken in ail my experiments, and 

 for many valuable suggestions. 



