/ngvar Jorgensen and Walter Stiles. \ i 7 
CARBON ASSIMILATION. 
A Review of Recent Work on the Pigments of the 
Green Leaf and the Processes connected with them. 
Bv Ingvar Jorgensen and Walter Stiles. 
(Continued from p. 96). 
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 all, that which is of the greatest interest to 
us, the photo-chemistry 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, as 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. P. Blackman on the intake of carbon 
dioxide by the leaf. 
