65 



theory of limiting factors; in s 15 this point will be 



discussed in détail. 



The figures of Czapek and Bach, quoted in § 2, do 



not allow of any conclusions about the validity of 



van 't Hoff's law. The séries of observations is not 



sufficiently complète nor is the accuracy sufficient for 



this purpose. Nevertheless there are some indications that 



the presentatlon-time is 2 to 3 times shorter for each 



rise of température of 10° C. Thus we can calculate from 



K 45' 



C z a p e k's figures : Qio = j^' -== 'V ^ ^■~'^' '"^"^ ^^'^^ ^'^^ ^^^ 



figures: Qio ^^ k'° ^= g^ ^ ^•^■^- Thèse values are not 

 necessarily opposed to van 't Hoffs law, when we 

 consider the way in which they are obtained. 



Haberlandt's experiments ') with objects grown at 

 very low températures, and those of F r a n c i s Darwin*) 

 with plants exposed to high températures aimed more 

 especially at gathering facts in support of the statolith- 

 theory. As I did not occupy myself with this theory, 

 there are a few points only, where there can be question 

 about agreement or disagreement with the results of 

 thèse investigators. Agreement is to be seen in so far as 

 in the case of Avenu the presentation-time is also lengthened 

 by a stay at exceedingly high or low températures. 

 Disagree.ment appears in the fact.. that hère even after a 

 stay of 15 h ours at 0° C. (experiment B 24, table I A) 

 the faculty of geotropic curvation was not lost, as was 

 found by Haberlandt in a similar case. 



A comparison of the results relating to the reaction- 

 time has already been made in ^ 10. 



1) G. Haberlandt, 1902 and 1903. 



2) F. Darwin, 1003. 



Recueil des trav. bot. Xéerl. Vul. IX. 1912. 



