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§ 12. Van 't Hoffs law. 



According to Blackman's theory, stated in § 1, chemical 

 réactions in the living organism foUow van 't Hoffs 

 law, which says that the reaction-velocitj^ increases 2 to 

 3 fold for each ri se of température of 10° C. 



The behaviour of the presentation-time in this respect 

 could not be predicted theoretically. In the case ofphoto- 

 tropism one could hâve expected van 't Hoffs law to 

 apply, for Blaauw'j has pointed out, that probably 

 phototropic perception occurs through a photo-chemical 

 reaction. With regard to geotropism however, perception 

 by means of a chemical reaction was not to be anticipated. 

 Only a séries of exact figures could settle the question 

 as to the application of van 't Hoffs law to the field of 

 geotropism. 



When we ask ourselves, what is the process, which is 

 influenced by température and of which we détermine 

 the reaction velocity by means of the presentation-time, 

 it is at once clear, that this process is not „reaction" as 

 contrasted generally with perception. The length of the 

 presentation-time will dépend upon the perception or upon 

 any other process, which makes its influence felt upon 

 the threshold of stimulation. 



The presentation-time was found to be longer when the 

 température was higher; the chemical reaction-velocity 

 must be greater when température is rising; therefore 

 I hâve regarded the reaction-velocity of the process deter- 

 mining the presentation-time as inversely proportional to 

 the presentation-time. If this process be the perception, 

 this assumption is surely quite valid, as the presentation- 

 time is shorter when the perception proceeds faster. The 



1) A. H. Blaauw, 1909. 



