Carbon Assimilation. 209 
ance between the values obtained by this method of estimation and 
that of the reducing power. 
The problem of determining the different sugars in the leaf is 
one of extraordinary difficulty owing to the large number of members 
of the group and the similarity of their properties. At present we 
cannot regard as settled even the question of what sugars are 
definitely absent from the leaf. This question is nevertheless of 
much importance in the quantitative estimation of sugars in the 
leaf and as the results of such analyses are likely to be used in 
connection with theories of assimilation, the exact identity of the 
leaf sugars may be of fundamental importance in obtaining an 
understanding of the assimilatory process. 
It seems to us, therefore, that before forming a final judgment 
as regards the carbohydrates of the leaf and before accepting in all 
their details the results of quantitative analyses already made, there 
is required a thorough investigation that will settle which carbo¬ 
hydrates are present and which are not, as definitely as Willstatter 
has settled the question of the leaf pigments. 
In the following sections of this chapter we summarise the 
analytical methods employed for quantitative carbohydrate analysis 
of the leaf and the results obtained by their means, but it should 
be understood that some of these results may have to be modified 
when fuller knowledge is obtained of this important but extremely 
difficult subject. 
D. Quantitative Estimation of the Carbohydrates 
of the Leaf, 
The quantitative estimation of the carbohydrates of the leaf 
was first seriously undertaken by Brown and Morris for Tropceolum ; 
their results are given in their well-known paper in the Journal of 
the Chemical Society for 1893. Since then the most noteworthy 
contributions to the subject are those of Parkin (1911) on the 
snowdrop (Galanthus nivalis L.) which embodies the results of a 
careful series of observations extending over several years, and the 
recent work at Rothamsted of Davis, Daish and Sawyer who have 
called attention to several sources of error in the methods of earlier 
workers. With the results obtained by these different investigators 
we shall deal in the next section of this chapter. We shall here 
devote a little space to the description of the methods evolved by 
these various workers for this extremely difficult analysis. 
