50 Carbon Assimilation. 



It is well to remember that Willstatter's work has not 

 exhausted chemical investigation on the subject of leaf pigments. 

 There is a great deal yet which is not clear, and much which is 

 very hypothetical, as for instance, the relation between the two green 

 pigments, the reactions occurring in the changes from chlorophyll 

 to chlorophyllin salts, the oxidations and reductions of chlorophyll 

 derivatives, and above ail, that which is of the greatest interest to 

 us, the photo-cheniistry of chlorophyll. On this last subject we have 

 so far had no publication from Willstatter, although it is evident 

 from his papers that he has been working at it and has realised that 

 the phenomena of carbon assimilation such as we know them in 

 living plants cannot be imitated by experiments with the four 

 pigments " in vitro." Knowledge of the photo-chemistry of chloro- 

 phyll will probably help us to estimate the true significance of many 

 of the observations which have already been made on chlorophyll 

 outside the plant. 



We have so far mainly dealt with Willstatter's work in organic 

 chemistry, and in a later chapter we shall discuss Willstatter's 

 plant physiological work ; before concluding this chapter it must, 

 however, be mentioned that Willstatter's physico-chemical work, 

 that on the state of aggregation of chlorophyll, for example, does 

 not appear so brilliant and convincing as his work in organic 

 chemistry. Although the extension of our knowledge of the 

 colloidal state of chlorophyll must be regarded as a great advance, 

 yet Willstatter's arguments and experiments on this point are not 

 very complete, and he seems intentionally to avoid any detailed 

 discussion of the question. The reason for this may be found in 

 the fact that before the subject is properly attacked, an investiga- 

 tion of the colourless substances which accompany the pigments in 

 the chloroplasts, £)s thorough and as detailed as that of the pigments 

 themselves, is necessary. It is to be hoped that Willstatter or 

 some other equally capable organic chemist will direct his attention 

 to this subject which so much needs investigation. 



Willstatter's work is one of those monumental pieces of 

 research which are of permanent value. In the following chapters we 

 shall deal with another piece of work which will always retain its 

 value — the work of P. F. Blackman on the intake of carbon 

 dioxide by the leaf. 



