24 CAROTINOIDS AND RELATED PIGMENTS 



in showing only one wide absorption band at F, and in forming, 

 according to the statement of certain of their investigators, true com- 

 pounds with sodium, calcium and barium. These points of divergence 

 from the carotinoids should be examined in the light of our present 

 knowledge of carotin and xanthophylls before it can be stated with 

 assurance that these pigments are distinct substances. They all cor- 

 respond completely to the class characteristics of the older termi- 

 nology of lipochromes. 



Summary 



Red, orange and yellow pigments which have certain simple prop- 

 erties in common arc found in almost all forms of plants and animals. 



These pigments have been variously classified as luteins, lipo- 

 chromes, lipoxanthins and chromolipoids. 



These classifications have been based on general properties rather 

 than on composition and are accordingly subject to both error and 

 criticism. 



As a general class term the name chromolipoid seems to conform 

 more nearly to present conceptions of these pigments as well as to 

 more common usage in connections with substances with fat-like 

 properties. 



Investigations regarding the composition of the chromolipoids show 

 that a large majority of them are apparently chemically and gener- 

 ically related to carotin, a specific chromolipoid widely distributed 

 in plant and animal tissues. 



It seems reasonable to believe, therefore, that a great many chromo- 

 lipoids can be classified more specifically as carotinoids, a name pro- 

 posed for them by Tswett (1911a). 



Two classes of carotinoids are recognized in Tswett 's definition; 

 carotins, whose constitution and properties show them to be hydro- 

 carbons identical or isomeric with carotin; and xanthophylls, whose 

 constitution and properties show them to be oxy-hydrocarbons and 

 which are chemically, as well as generically, related to carotin. 



Carotinoids are not the only yellow to red colored pigments occur- 

 ring in plants and animals. Many of these non-carotinoids resemble 

 the true carotinoids in one or more properties and some even in com- 

 position. The reader is referred to the text for the detailed discussion 

 of the non-carotinoids and the properties which they have in common 

 with the carotinoids as well as their distinguishing characteristics. 



