Carbon Assimilation. 
207 
as the only hexose sugars fermented by ordinary yeasts are d?-glucose, 
r/-mannose, (/-fructose and less easily rf-galactose, while numerous 
disaccharides with cupric-reducing power are not fermented by 
yeasts, as well as the pentoses, while there is always the possibility 
of the presence of substances with cupric-reducing properties other 
than sugars. 
The further evidence of the presence of pentoses is derived 
from the fact that these purified plant extracts on subjection to 
distillation with hydrochloric acid according to the Krober-Tollens 
process (see Tollens, 1914) yield a weight of phloroglucide which 
would be given by practically the same amount of pentose calculated 
as a mixture of Z-arabinose and /-xylose. It must be admitted that 
the concordance is not very striking and Kiuyver (1914) has pointed 
out that the presence of hexoses and disaccharides in such a solution 
is a source of error, as on distillation with hydrochloric acid these 
also give small quantities of furfural-like compounds which yield an 
insoluble phloroglucide, so that the method could not give any very 
accurate value for small quantities of pentose in presence of large 
quantities of other sugars. Davis and Sawyer admit the truth of 
this criticism, but point out that the error actually introduced in 
this way is small (about 18%). 
Nevertheless the evidence that the only sugars present in the 
leaf are sucrose, (/-glucose, ('/-fructose and pentoses does not carry 
complete conviction. 
Besides sugars and starch Davis, Daish and Sawyer have 
estimated the complex derivatives of the pentoses, the pentosans, 
by distillation of the leaf-matter insoluble in alcohol by the Krober- 
Tollens method. 
The table overleaf may therefore be regarded as summing up 
our knowledge in regard to the presence of carbohydrates in the 
leaf. 
The absence of rf-mannose, which is closely related to (/-glucose 
and (/-fructose in chemical constitution and in its behaviour as 
regards fermentation by yeasts appears to have been generally 
accepted without the production of any sound evidence in support 
of the opinion. It is also generally assumed that the / forms of the 
hexoses are completely absent from the leaf. Thus E. F. Armstrong 
(1913) says: “ In spite of frequent search it has never been possible 
to detect /-glucose or /-fructose in the leaves of plants, and the 
work of Brown and Morris leaves hardly any doubt that hexoses of 
the (/-series and their polysaccharides are the only products of 
