THE COMPOSITION OF CHLOROPHYLL 237 



tion, for these leaves were also recommended by Stokes, 

 sixty years previously, who also obtained two green and 

 two yellow pigments from leaf green, as I have already 

 told you. 



A long series of experiments were undertaken to 

 determine the variation in amount of the different con- 

 stituents of leaf green, the results of which I have briefly 

 summarised above. The time of the day at which the 

 leaves are gathered does not seem to make any difference 

 in the relative amounts of the pigments nor in the total 

 quantities present. The relative proportion of the two 

 chlorophylls also appears to be approximately constant 

 both in different plants and in sun and shade leaves, 

 but in the case of the yellow pigments the ratio between 

 them seems to be greater in shade leaves, as is also the 

 case in regard to the ratio between the total green and 

 the total yellow pigments. Such quantitative deter- 

 minations are of some importance seeing that earlier 

 writers, such as Haberlandt, held that the intensity of 

 photosynthesis varied with the amount of chlorophyll 

 present in the leaf. Willstatter has carried out preliminary 

 experiments on this subject, but as these investigations 

 are, so far as I am aware, still incomplete, I do not propose 

 to trouble you with them, beyond saying that it would 

 appear that the amount of chlorophyll present does 

 condition the amount of carbon dioxide absorbed, pro- 

 vided no other " limiting factor " intervenes to disturb 

 the relationship. 



A good deal of emphasis has been laid by many 

 writers on the close similarity in chemical composition 

 between the red colouring matter of the blood, haemo- 

 globin, and chlorophyll, but, according to Professor 

 Bayliss, Willstatter himself " does not regard the 

 similarity ... as being of any great significance." 



Finally, recent research appears to show that the 

 pigment is not diffused through the plastids, as was 

 once believed to be the case, but forms " a highly con- 



