^4 /ngvar Jorgensen and Walter Stiles. 
CARBON ASSIMILATION. 
A Review of Recent Work on the Pigments of the 
Green Leaf and the Processes connected with them. 
By Ingvar Jorgensen and Walter Stiles. 
{Continued from Vol. XV, p. 232). 
Chapter VI. 
Energy Relations in Carbon Assimilation. 
A. General Remarks. 
In the introductory chapter we have referred to the fundamental 
fact that radiant energy is utilised in carbon assimilation so that 
compounds of higher energy content are produced from the simpler 
ones of the surroundings. Beyond this the energy relations of the 
green leaf are only very imperfectly known, although physical and 
chemical methods for investigating such energy relations have 
reached a high degree of development. This aspect of carbon 
assimilation exhibits perhaps more than any other an unfortunate 
isolation of effort in research, the various workers on the subject 
having generally neglected the results obtained by others, both 
along their own and related lines of investigation. 
As in those aspects of the subject we have already dealt with, 
so here also the complexity of the processes is again evident, and 
it is difficult to draw definite conclusions. It is possible to measure 
quantitatively the radiant energy incident on the leaf, and also to 
measure the amount transmitted. It is, however, by no means easy 
to determine in what way the energy absorbed is utilised, because 
we do not know with any approach to completeness how this 
energy is expended into chemical or electrical energy or heat. 
It is generally assumed that the increase in the heat of 
combustion of the leaf represents that part of the absorbed energy 
transformed into chemical energy, but it should be pointed out that 
in so doing, carbon assimilation is taken in its widest possible sense 
(not carbohydrate assimilation merely) so as to include all the 
substances formed in the leaf as a result of the photochemical and 
possible contemporary processes. Thus it is by no means settled 
as to what extent proteins are formed in the green leaf by photo¬ 
chemical or other chemical actions. In any case an error is 
introduced if proteins are formed, as their products of combustion 
cannot be identical with the substances in the leaf from which they 
are produced, so that their heat of combustion cannot have the same 
value as the radiant energy used in the leaf in their formation. 
We shall first discuss the methods for estimating the amount 
