172 PLANT PHYSIOLOGY 



are yellow in color. They may be extracted, together with 

 chlorophylls, by treating green leaves with alcohol. Their pres- 

 ence in the alcohol extract is readily detected after shaking the 

 solution with benzine. The chlorophylls, which are more soluble 

 in benzine than in alcohol, accumulate in the upper benzine 

 layer, while the xanthophylls remain in the alcohol, coloring it 

 yellow (Kraus's reaction). A complete separation of the pig- 

 ments, however, is not obtained, as the carotenes pass over into 

 the benzine along with the chlorophyll and must be isolated in 

 a much more complicated way. 



Carotene exists in several forms, all of which are unsaturated 

 hydrocarbons of the formula C40H56. These are readily oxidized 

 on exposure to air. Xanthophyll contains 2 atoms of oxygen 

 and has the formula C40H56O2. In its composition, it is similar 

 to carotenes, although it is not merely a product of their oxida- 

 tion and not easily convertible into carotenes. 



Carotenes and xanthophylls, as well as related carotenoid 

 pigments, give a yellow color to the chromoplasts in many fruits 

 and in flower petals. Lutein, the pigment of the yolk of the 

 hen's egg, is closely related to xanthophyll and is isomeric with 

 it in its empirical formula. The color of the tomato fruit is 

 produced by lycopene, an isomer of carotene. 



The physiological importance of the yellow pigments of the 

 chloroplasts is not understood, but according to present views, 

 they do not take a direct part in carbon assimilation. Some 

 authors assume that the yellow pigments, which are always 

 present in the chloroplasts, act as screens or light filters, thus 

 protecting the chlorophyll against rapid destruction by light. 

 Others believe that the yellow pigments are the carriers of the 

 oxygen formed in assimilation, taking it from the peroxide-like 

 combination of chlorophyll with carbon dioxide (see Art. 43), 

 which forms during the early stages, and afterward giving it up 

 to the atmosphere, thus promoting photosynthesis. These 

 hypotheses, however, have not been sufficiently established. 

 There is a possit^le relationship between carotene and sexual 

 reproduction in plants. 



Of great interest is the close connection of the chemical com- 

 position of carotene and vitamin A. The long hydrocarbon 

 chain of carotene ruptures at the center linkage into two equal 

 halves; with the addition of oxygen, each half forms a molecule 



