CAROTINOIDS 329 



and xanthophyll, a good deal of speculation is rife in view of 

 their close association with chlorophyll and the possibility 

 that in the plant the conversion of carotin into xanthophyll 

 may be a reversible process. It has been suggested by 

 Willstatter and Stoll that chlorophyll a in taking up carbon 

 dioxide is itself oxidized to chlorophyll b in reducing the 

 carbon dioxide ; the resulting chlorophyll b is then reduced 

 back to chlorophyll a by carotin, which is thereby converted 

 into xanthophyll ; the reconversion of xanthophyll to carotin 

 is supposed to be effected by a reductase. Willstatter has, 

 however, not been able to show that carotin can be oxidized 

 to xanthophyll. The work of Tammes and Kohl * shows that 

 carotin absorbs certain rays of radiant energy which may 

 be made use of in photosynthesis. 



In those cases where a large amount of carotin occurs in 

 organs of storage, such as the roots of the carrot, it may be of 

 value as a reserve food-material. Finally, where the colours 

 of flowers are due to its presence, carotin is important in the 

 floral biology. 



Carotin is insoluble in water and very slightly soluble in 

 acetone or cold alcohol ; in hot alcohol it is more soluble ; 

 and in ether, chloroform, light petroleum, and carbon bi- 

 sulphide it is readily soluble. The colour of the solution 

 varies from yellow to red ; on crystallization fiat reddish- 

 yellow plates are formed which exhibit the phenomenon of 

 dichroism, being orange-red by transmitted light and greenish- 

 blue in reflected light. 



According to Willstatter and Mieg,t carotin may be ex- 

 tracted from stinging nettle leaves by light petroleum ; it has 

 the molecular formula C40H56, and is probably identical with 

 the substances erythrophyll and chrysophyll described by 

 Bougarel and Schunck respectively. 



It absorbs 34-3 per cent of its weight of oxygen, being 

 converted into a colourless substance. With iodine it forms 

 the compound C40H56I2, which crystallizes in dark violet 

 prisms. 



* Kohl : " Ber. deut. bot. Gesells.," 1906, 24, 222. 

 f Willstatter and Mieg : " Annalen," 1907, 355, i. 



