PIGMENTS ACCOMPANYING CHLOROPHYLL 227 



ordinarily obtained, and to which they had originally assigned 

 the formula C i)5 H 72 O 6 N 4 Mg, is in reality a mixture of two 

 substances : 



Chlorophyll a C 55 H 72 O 5 M 4 Mg * 

 and Chlorophyll b C 55 H 70 O 6 N 4 Mg. 



Accompanying chlorophyll are three yellow or reddish- 

 brown pigments, Carotin, Xanthophyll, and Fucoxanthin (the 

 latter occurring only in brown algae), which are known col- 

 lectively as the Carotinoids. Owing to the similarity in 

 solubilities between these substances and chlorophyll, their 

 complete separation is a matter of some difficulty; it was 

 first effected by Willstatter and Hug.f 



The average proportions in which these various constituents 

 occur in different plants have been determined by Willstatter, 

 and are approximately as follows : 



In Land Brown Algaj J Green Algae t 



Plants.! (Fucus). (Ulva). 



Chlorophyll a . . . -62 -16 -093 



,, b . -22 'oi *o66 



Carotin .... -055 -0312 '014 



Xanthophyll . . . -093 '0305 -036 



Fucoxanthin . . . -059 



From these figures the following interesting deductions 

 may be made : 



1. The molecular proportions between chlorophylls and 

 carotinoids are as 3*5 to I J in terrestrial plants, but only I to I 

 in the case of algae. 



2. In the brown algae chlorophyll a predominates, only 

 about 5 per cent of the mixture being chlorophyll b ; in ter- 

 restrial plants, on the other hand, the proportion is pretty 

 constantly about 3:1. 



3. In the green algae there is relatively more of chloro- 

 phyll b. 



Concerning the physiological significance of these sub- 



* For the physical characteristics of these two substances see page 231. 



f Willstatter and Hug: "Annalen," 1911, 380, 177. 



J These figures are percentages calculated on the dry material. 



With regard to this ratio, it has been stated by Willstatter that it is re- 

 markably constant, and that there is a greater variation between different leaves 

 of the same plant than between corresponding leaves of different plants. This 

 view is, however, contested by Borowskaand Marchlewski (" Biochem. Zeitschr.," 

 1913, 57, 423), who hold that it is entirely dependent on external circumstances, 

 such as soil, stage of growth, etc. 



15* 



