ABSORPTION OF CARBON 21 



hydrogen is replaced by magnesium, zinc, copper, or iron. When 

 acted upon by stronger acids, the saponification of the other groups 

 and the further breaking down of the complex molecule of chloro- 

 phyll take place. 



It is interesting and important to note that having investi- 

 gated over 200 species of plants, Willstatter has found in all of 

 them the same type of chlorophyll. Hence, this most important 

 substance seems to be uniformly the same in all plants. From 

 this the conclusion may be drawn that the fundamental chemistry 

 of the process of carbon assimilation is also similar in all chloro- 

 phyll-bearing organisms. 



6. Pigments Accompanying Chlorophyll.— Besides chlorophyll, 

 the green plastids of the leaf contain two other pigments — 

 carotin and xanthophyll. Both of these are yellow in color. 

 They may be extracted, together with chlorophyll, by treating 

 green leaves with alcohol. Their presence in the alcohol extract 

 is readily detected by shaking the solution with benzine. The 

 chlorophyll, which is more soluble in benzine than in alcohol, 

 accumulates in the upper benzine layer, while the xanthophyll 

 remains in the alcohol, coloring it yellow (Kraus' reaction). A 

 complete separation of the pigments, however, is not obtained, 

 as the carotin passes over into the benzine along with the chloro- 

 phyll and must be isolated in a much more complicated way. 



Carotin is an unsaturated hydrocarbon of the formula C40H56. 

 It is readily oxidized in air. Xanthophyll contains 2 atoms of 

 oxygen and has the formula C40H56O2. In its composition it is 

 similar to carotin, although it is not simply a product of its 

 oxidation. 



These pigments, as well as the related carotins and xantho- 

 phylls, give color to the chromoplasts in 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 formula. The color 

 of the tomato is produced by lycopin, an isomer of carotin. 



The physiological importance of the yellow pigments of the 

 chloroplasts is not yet understood, but, according to present 

 information, 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 



