I] 



ASSIMILATION 



15 



measures the proportion of carbonic acirl and water 

 in the incominc; air. Thus the amount of carbonic acid 

 absorbed, and therefore decomposed, in a ^iven time 

 by a leaf, whose area can be afterwards measured, is 

 directly determined, and such factors as illumination and 

 temperature can be varied at will. The energetics of 

 the process have been worked out by Brown and 

 Escombe, from whose paper the following examples 

 have been selected : — 



Tabi K I.— Utilisation or the Energy incident on a Gicfen 

 Leaf. (I'rown and Kscomhe.) 



These experiments show that the leaf of a plant 

 is not to be regarded as a very efficient machine for 

 the decomposition of carbonic acid, since in no case was 

 more than 1-66 per cent of the total energy incident 

 on the leaf used for photo-synthesis, so that even dull, 

 diffuse daylight can amply provide a growing plant 

 with the energy it wants for assimilation. The process, 

 however, is limited by many factors, any one of which 

 may fix a minimum rate at which assimilation will take 



