Carbon Assimilation 
185 
It will be observed that the assimilation numbers are very high, 
indicating that it is the chlorophyll which is limiting assimilation. 
Chlorotic Leaves. 
Willstatter has also examined chlorotic leaves and finds, in 
spite of the low chlorophyll content, comparatively low assimilation 
numbers showing that the chlorophyll is only partially utilised. 
Of course one would not expect if one of the essential elements 
(iron) were wanting, that the assimilatory apparatus should be 
properly developed. Willstatter considers that the fact that in 
chlorotic leaves even a small amount of assimilation takes place, 
makes the assumption of B. Moore (1914), that iron plays an 
important part in the assimilatory process, even more improbable 
than ever. 
Although it is premature to attempt to summarise the results 
of Willstatter’s plant physiological work, it seems reasonable to 
conclude that under certain circumstances when no other factor 
is limiting, the amount of chlorophyll determines the intake of 
carbon dioxide by the leaf. Further it is clearly brought out by 
Willstatter’s experiments with leaves in different states of develop¬ 
ment, yellow varieties, etiolated leaves, etc., that besides chlorophyll 
other internal factors are operative. This idea is not novel, as 
it has been expressed for instance by Pfeffer and Blackman, but 
Willstatter, for the first time, determines the relation between 
quantity of chlorophyll and assimilatory activity. The novelty 
which Willstatter claims for his researches lies in the exposition of 
carbon assimilation as consisting of two different processes, one 
photochemical and one enzymatic, the complete experimental proof 
of which Willstatter has not yet brought forward. For plant 
physiologists there should be nothing new in this view, as from 
Blackman’s experiments it was seen that carbon assimilation had a 
temperature coefficient between 2 and 2-5 and consequently the 
photochemical reaction must be coupled with chemical reactions. 
But it would indeed be noteworthy if the famous German chemist 
should succeed in convincing plant physiologists that in what 
Blackman terms the “katalytic honeycomb of the cell” only two 
processes are concerned in assimilation. 
Willstatter’s theories of carbon assimilation, both those 
expressed in his book and in his more recent papers, we shall 
refer to later. 
