CHLOROPHYLL 313 



existence of more than one substance ; confirmatory evidence 

 was subsequently obtained by Tswett.* In 1912, however, 

 Willstatter and Isler f definitely showed that chlorophyll as 

 ordinarily obtained, and to which they had originally assigned 

 the formula C55H7206N4Mg, is in reality a mixture of two 

 substances — 



Chlorophyll a CsgH^PsNiMg J 

 and Chlorophyll b C^sHjoOsN^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.§ 



The average proportions in which these various constituents 

 occur in different plants have been determined by Willstatter, 

 and are approximately as follows :• — ■ 



From these figures the following interesting deductions 

 may be made : — 



I. The molecular proportions between chlorophylls and 

 carotinoids are as 3-5 to i ^ in terrestrial plants, but only i to i 

 in the case of algse. 



* Tswett : " Ber. deut. bot. Gesells.," 1906, 24, 326 ; 1907, 25, 137 ; 

 " Ber. deut. chem. Gesells.," 1908, 41, 1352. 



t Willstatter and Isler : " Annalen," 1912, 390, 269. 



X For the physical characteristics of these two substances, see p. 316. 



§ Willstatter and Hug : " Annalen," 191 1, 380, 177. 



II These figures are percentages calculated on the dry material. 



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

 remarkably 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 Borowska and Marchlewski 

 {" Biochem. Zeit.," 1913, 57, 423), who hold that it is entirely dependent on 

 external circumstances, such as soil, stage of growth, etc. 



