94 Ingvar Jorgensen and Walter Stiles. 
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 0546 + 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 — caiotin Q f q-603 
8 8 xanthophyll 
+ OT 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 ift 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. 
