THE CAROTENOIDS 



415 



The carotenes are isomeric hydrocarbons. The most common one 

 in leaves is jS-carotene. Mackinney (1935) found it in all of the 59 species 

 he investigated; in 40 of them he also found a-carotene, in concentrations 

 up to one-half that of /3-carotene. 



Table 15.IV 

 Concentration of Carotenols in Relative Units 



Table 15. IV shows the composition of a typical carotenol mixture 

 from green leaves, as analyzed by Strain (1938). Several bands in 

 Strain's chromatogram remained unidentified, so that the mixture 

 probably contained additional components, in quantities similar to those 

 of violaxanthol. Luteol, although undoubtedly the most common of 

 the leaf carotenols, does not form more than one-half the mixture. In 

 autumnal leaves, the carotenoids undergo changes whose nature is not 

 yet well understood. Shortly before the leaves are shed, zeaxanthol 

 (the pigment of yellow corn) becomes the main component of the carotenol 

 mixture. According to Strain, it is not formed from other carotenoids, 

 but merely survives them because of its greater stability. 



The algae contain the same ubiquitous carotenes (a and jS) as the 

 higher plants, but often a different assortment of carotenols and other 

 carotene derivatives. After the first survey of the field by Kylin (1927) 

 and Boresch (1932), the analysis of algal carotenoids was carried out 

 by Tischer (1936, 1937, 1938) and Heilbron (cf. Heilbron and Phipers 

 1935; Heilbron and Lythgoe 1936; and Carter, Heilbron, and Lythgoe 

 1940). The last-named authors investigated algae from seven of the 

 eleven main algal classes. Table 15. V is a condensation of their results. 



Fucoxanthol, the carotenol pigment most characteristic of Phaeo- 

 phyceae, Chrysophyceae and Bacillariophyceae (diatoms) also was found 

 in a few green algae (e. g., Zygnema pectinata) and red algae (e. g., Poli- 

 syphonia negrescens), but it has never been discovered in land plants. 

 With these exceptions, the carotenoids of green and red algae are similar 

 to those of the higher plants (that is, they consist mainly of carotene 

 and luteol), while blue-green algae contain at least two carotenoids not 



