Carbon A ssimilation. 
2 5 
of material produced in assimilation and the conclusions which 
have been drawn as to the energy relations of the green leaf from 
such determinations, before passing on to a discussion of work in 
which quantitative measurements of both radiant energy and heats 
of combustion have been made. Finally, we shall briefly deal with 
work on the assimilatory power of light of different wave-lengths. 
B. Quantitative Estimation of Carbon Assimilation 
BY MEANS OF THE PRODUCTS. 
In an earlier chapter we have dealt with Blackman’s and 
Willstatter’s estimation of carbon assimilation. Both these 
workers employed a method based on that of Kreusler, in which 
the intake of carbon dioxide by the leaf is used as a measure of 
carbon assimilation. 1 The assimilation could also be measured by 
estimating the increase of carbon content of the leaf, but what is 
more usually done is to measure the increase of dry weight of the 
leaf and assume that this is proportional to the increase in carbon 
content. 
Brown and Escombe (1905) in their attempt to determine the 
energy relations of the leaf, compared the increase of dry matter 
with the intake of carbon dioxide, but as their results obtained by 
the two methods were not concordant they came to the conclusion 
that the dry weight method is untrustworthy. Therefore they only 
determined the intake of carbon dioxide and estimated the increase 
in dry matter by calculation. We shall only deal briefly with the 
extensive researches of Brown and Escombe on this subject, for 
although they are the first to make quantitative measurements of 
energy in regard to assimilation and although they clearly indicate 
the complexity of the energy relations of the leaf, yet the values 
actually determined by experiment are few and those obtained by 
calculation are of doubtful value and do not agree with values 
obtained by direct measurement by other investigators. We have 
already referred to the divergence between the estimations of the 
internal leaf temperature made by Brown and Escombe, and the 
direct temperature measurements made by Blackman and Matthaei. 
In order to estimate the dry matter formed, Brown and Escombe 
multiply the weight of carbon dioxide absorbed by the leaf by a 
1 The same method has been employed by Brown and Escombe (1902). 
We have not dealt with the very interesting results recorded in this paper in 
regard to carbon dioxide as a limiting factor, as the results are confirmed by 
F. F. Blackman’s later and more complete work on limiting factors. In order 
to avoid unnecesary length of this review we have in this matter, as elsewhere, 
confined our remarks to the more complete account where two researches run 
parallel. 
