4.S 



2. Method. 



The assiin ilalioii apparalus. A great many methods ha ve 

 beeii worked oul for llie (juanlitalive eslimalion of Ihe assimilation. 

 Whiie the half-leaf method given by Sachs and more recently (1910) 

 improved by Thoday uses the gain of dry weight as the criterium 

 of the intensily of assimilation, the gasometric methods employed 

 by the majoritj^ of scientists have as their object the estimation of 

 the amount of carbon dioxide absorbed by the leaf. In theso latler 

 melhods the assimilation must take place in a closed space. This 

 fact involves a drawback for ecological ])urposes, which however is 

 in my opinion counterbalanced by the greater sensitiveness and 

 accuracy of the method. 



In the assimilation-chamber necessary for the gasometric methods 

 one can either include, in addition to the object of exj)eriment, a 

 certain quantity of air, or else an air-current of known velocity may 

 be passed through. The intensity of assimilation is derived in either 

 case from the diminulion \vhich appears in the carbon dioxide 

 percenlage of the air, the amount of carbon dioxide being measured 

 in known fashion by absorption in alkali and tilralion or weighing. 

 It is especially in experiments undertaken during the last few de- 

 cades that the principle of passing air through bas been preferably 

 employed, chielly with the arrangement given by Kreusler (1iS<S5). 



From an ecological point of view, the air-current method involves 

 the objection that the ajjparatus is complicated and awkward lo 

 manage, so that it is difficult to use in field-work. And the com- 

 monly used method of COg absorption wilh baryta in a Pkttenkofkr 

 lube can seldom, in my opinion, give a guarantee of complete ab- 

 sorptio^n, especially with such a low COo percentage as that of the 

 air, and in experiments of short duration. The modification of the 

 absorption-vessel proposed b\' Bovsen-Jensen (1918) involves, so 

 far as my exhaustive tests have been able to convince me, a de- 

 terioralion in several respecis. The air-current method can only be 

 reliable when carried out in the exact hut complicated fashion adopted 

 by Brown and ?2scombe (1902, 1905), Blackman and bis collaborators, 

 and WiLLSTÄTTER and Stoll (1918). 



For ecological use however there is recjuired a method that shall 

 be at once easy to manipulate and also as exact as possible, and 

 the principle of the closed assimilation-chamber is in my opinion 



