THE CAROTINOIDS 3i5 



those already discussed, such as lack of certain other mineral elements (potas- 

 sium, phosphorus, calcium, etc.), deficient aeration of the roots, attacks of 

 insects, bacterial, fungous, or virus diseases, etc., may induce, directly or in- 

 directly, partial or complete chlorosis of the leaves. The failure of chlorophyll 

 synthesis to take place normally is often one of the first observable symptoms 

 of almost any derangement in the metabolic conditions within a plant. 



The Carotinoids. — Carotene is found in all green tissues, in the roots of 

 carrots, and in many flowers, fruits and seeds. This pigment is a hydrocarbon 

 with the formula C40H56. In recent years the study of this compound has 

 received new impetus because of its relation to vitamin A. Several isomers 

 of carotene are known to exist and the probable structural formula of at least 

 one of these {/S carotene) has been determined. This isomer of carotene is 

 apparently the most abundant one in leaves. Some authorities consider the 

 molecular formula of vitamin A to be C00H09OH and that this compound is 

 produced in the animal body by the hydrolysis of carotene according to the 

 following equation: 



C40H56 + 2 H2O -> 2 C20H29OH 



Lycopene, a red pigment found in the fruits of tomato, red peppers, roses, 

 and other species, and in some flowers, is isomeric with carotene, but ap- 

 parently has a very different structural formula. 



It was formerly supposed that xanthophyll was a single compound but 

 it is now considered that there is a group of these compounds, similar but 

 not identical in chemical formula, so it is more accurate to speak of the 

 xanthophylls. Lutein seems to be the principal leaf xanthophyll, often con- 

 stituting half or more of the xanthophyll extracted from leaves. Another 

 common leaf xanthophyll is zeaxanthin which, however, is usually present only 

 in relatively small amounts. The molecular formula of both of these xantho- 

 phylls is C4oH5,302. Chemically the xanthophylls are closely related to 

 carotene and undoubtedly are derived from this compound by various types 

 of chemical transformations. 



Carotene and the xanthophylls can both be extracted quantitatively from 

 green leaves by the use of suitable solvents. Neither pigment is soluble in 

 water. Carotene is soluble in ether, chloroform, and carbon bisulfide. The 

 xanthophylls will dissolve in chloroform and alcohol, but are only slightly 

 soluble in carbon bisulfide, and insoluble in petroleum ether. 



As shown by its absorption spectrum (Fig. 82), carotene /3 absorbs only 

 wave lengths in the blue-violet portion of the spectrum. The absorption 

 spectra of other carotinoids are very similar to that of carotene. 



IVlost of the carotinoid pigments are readily synthesized by plants in the 



