8 



no further référence will be made to thi& subject (cf. 

 Kuyper, 1910, p. 136). 



In the same year 1908 yet another communication 

 appeared, by Blackman himself, namely his opening- 

 address to the botanical section of the British Association. 



In this address, which begins and ends with an attack 

 on the physiology of stimulus, Blackman explains how 

 physiology has to take cognisance of the principles of 

 physical chemistry and to apply theminsolvingitsproblems. 



Further he gives a short explanation of the principles of 

 physical chemistry of which a botanist has to take ac- 

 count, and mentions the following: 



1) the reaction-velocity, whereby he remarks that every 

 reaction is characterised by its own spécifie reaction-velocity ; 



2) the law of mass, which says that the reaction- 

 velocity is proportional to the concentration of the sub- 

 stances participating in the reaction; 



3) the influence of catalytic agents, which hâve a pre- 

 ponderating influence in the organism ; 



4) the increase of the reaction-velocity with température, 

 which is so universal, that it must be présent in the 

 living organism too. 



Metabolism in plants is consequently discussed from 

 this point of view as a catalytic reaction and Blackman 

 traces how far the facts support the conclusion that the 

 four above mentioned principles are fundamental in this 

 case also. Summarising the writer says : „Three sets of 

 phenomena we hâve observed which, though usually treated 

 in the category of stimulation, draw a clearer interprétation 

 from the conception of reaction-velocity. Thèse are : 1) the 

 relation of development to the absence or déficit of single 

 essential food constituents; 2) the occasional striking effect 

 of minute traces of added foreign substances upon the 

 whole rate of growth and metabolism ; and 3) the gênerai 



