Final Remarks on the Leaf Pigments. 47 



3. Variations in the Proportions of the Two Yellow Pigments. 

 The mean value of the ratio of carotin to xanthophyll was 



found by Willstatter to be 0-546 ± 0-15 to 0-2. 



In the case of the yellow pigments, shade leaves show a wider 

 divergence from the normal than they do in regard to chlorophyll. 



Thus normal leaves give an average ratio of — '^^'° '" — of 0-603 



xanthophyll 



+ 0-1 corresponding to a molecular ratio of 1 : 1-5 to 2. In shade 

 leaves on the other hand, the average ratio of carotin to xantho- 

 phyll was as low as 0-421 ±0-1. 



4. Relation between quantities of Green and Yellow Pigments. 

 The average molecular ratio of the total amount of green to 



the total amount of yellow pigment is 3'56, varying from 3-07 in the 

 case of sun leaves to 4-68 in the case of shade leaves. 



It will be observed that in the case of shade leaves the ratio of 

 quantity of green to quantity of yellow pigments is raised. An 

 exception to this rule was found in the Plantain. On the other 

 hand in leaves well suited for growth in the shade values as high as 

 6 have been obtained for the ratio of green to yellow pigments. 



It has been shewn above that in shade leaves the amount of 

 chlorophyll a is raised in relation to that of chlorophyll b, while of 

 the yellow pigments it is the xanthophyll which is relatively more 

 abundant in these leaves: that is, of the green pigments the one 

 poorer in oxygen is increased in amount, of the yellow pigments, 

 the one richer in oxygen. 



No simple relation could, however, be found between the ratios 

 chlorophyll a . carotin 

 chlorophyll b xanthophyll. 



G. Final Remarks. 



In the preceding pages we have endeavoured to give the 

 outlines of the work of Willstatter and his co-workers, and we 

 have emphasised the value of this work as one of the most brilliant 

 researches in organic chemistry. There has been in the past, 

 and in spite of Willstatter's work there will probably be in the 

 future, much loosely performed work on chlorophyll and involving 

 the use of chlorophyll, so that it may be well to emphasise the 

 criteria of purity of this substance that are now available. 



These criteria are as follows : 



1. The ash should consist of pure magnesium oxide, and 

 should weigh 4-5% of the weight of chlorophyll used. 



