FUNCTION OF CAROTINOIDS IN PLANTS, ANIMALS 265 



tion, but that this abilitj* is still lacking even in such shoots which 

 have developed a considerable amount of green color. 



A somewhat different aspect to the possible function of the caroti- 

 noids in the chloroplastids is given by another theory of Willstatter 

 and Stoll in which it is supposed that the carbon dioxide assimilation 

 is controlled by the equilibrium between the chlorophyll components 

 a and b, and that this equilibrium is in turn controlled by the caroti- 

 noids. The process, as imagined by Willstatter, is as follows. Carbon 

 dioxide is attracted by the affinity of the magnesium compounds 

 (chlorophylls) for C0 2 , and is at once reduced by chlorophyll a. 

 Chlorophyll a is thereby oxidized to chlorophyll b. Carotin then with- 

 draws the oxygen from chlorophyll b, reducing it again to chlorophyll 

 a, the carotin at the same time being oxidized to xanthophyll. The 

 reduction of the xanthophyll to carotin, in order to complete the cycle 

 of Willstatter's theory, is effected by a reductase. One difficulty with 

 this theory is that Willstatter himself, as he has pointed out, does not 

 admit that carotin can be oxidized to xanthophyll. The ability of 

 carotin to reduce chlorophyll b and thereby become oxidized is un- 

 questioned, as evidenced by its strong reducing action on ferric salts. 

 Xanthophyils, however, share this property with carotin. At the same 

 time some support is given to the theory of a functional relation be- 

 tween the chlorophylls and carotinoids by the possibility that fucoxan- 

 thin plays the part of chlorophyll b in the brown algae, which lack 

 this chlorophyll component. 



Ewart has recently (191' ^mpted to show that carotin and xan- 

 thophyll can play a part in photo-synthesis. His experiments purport 

 to show that carotin yields HCHO when submitted to photo-oxidation 

 in a stream of pure oxygen and that xanthophyll yields both HCHO 

 and sugar under similar conditions. In view of the fact that Ewart's 

 conception of xanthophyll includes the idea that it "is soluble in water 

 and in any mixture of alcohol and water," and also since there is no 

 assurance that his carotin was free from impurities, his results can not 

 be given unqualified acceptance. In the same paper Ewart claims to 

 have produced xanthophyll from chlorophyll, but Jorgensen and Kidd 

 (1917) have shown that the "xanthophyll'' which Ewart produced in 

 his experiments was probably phaeophytin. 



The question of the origin of the carotinoids in plants, which is sug- 

 gested by Ewart's attempt to produce xanthophyll from chlorophyll, 

 is closely related to the question of their function. The fact that the 

 carotinoids form in etiolated plants without chlorophyll is a strong 



