applicable to the figures given by différent investigators 

 for several physiological processes. 



Blackman finds that van 't Hoffs law is valid in 

 the field of botany for températures roughly between 

 10° C. and 27' C, but above 27° C. cjuick falling off takes 

 place, so that at higher températures the values obtained 

 do not nearly reach those which might be expected if 

 calculated by van 't Hoffs law. Blackman, in his 

 explanation of this phenomenon, lays stress on a new 

 point of View, calling attention to the time-factor which 

 hère comes into play. Sachs and Pfeffer had indeed 

 already pointed out the fact, that a short exposure to a 

 very high température is not so harmful as a prolonged 

 exposure to a slightly lower température, but up till then 

 very little attention had been bestowed upon the time-factor. 



With the aid of Miss Matthaei's figures relating to 

 carbon-assimilation, Blackman traces the infiuence of 

 time and the déviations from van 't Hoffs law, which 

 this infiuence brings about. 



He summarises his results in three laws: 



1) At high températures (30° C. and above, for cherry- 

 laurel) the initial rate of assimilation cannot be main- 

 tained, but falls off regularly. 



2) The higher the température the more rapid is the rate 

 of falling off. 



8) The falling off at any given température is faster at 

 first and subsequently becomes less rapid. 



The following diagram taken from Blackman's paper 

 represents graphically the figures found by Miss Mat- 

 thaei for the assimilation. 



The dotted line gives the values calculated according 

 to van 't Hoffs law, starting from the values found 

 experimentally at lower températures. The other curves 

 are drawn through the ascertained values at higher tem- 



