Chlorophyll Content. 95 



It will be observed that the assimilation numbers are very high, 

 indicating that it is the chlorophyll which is limiting assimilation. 



Chlorotic Leaves, 



Willstiitter 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. Mooie (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 ai'e 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, 



