28 TEXTBOOK OF BOTANY 



The various chemical compounds in plants absorb certain light rays 

 and reflect others. We receive our color impressions from the reflected 

 rays. As a matter of convenience we call these substances pigments and 

 we ascribe to them the colors perceived through the eye when light is 

 reflected from them. Thus we speak of green pigments, yellow pigments, 

 and various others from red to violet. Among the most important pig- 

 ments in plants are chlorophylls (leaf green); the carotinoids, varying 

 from pale yellow to orange-red; and the anthoctjanins, varying from red 

 through violet to blue.^ 



Pigments and the colors of leaves. The colors of certain leaves — for 

 example, the purple-leaved coleus and canna — may result from a com- 

 bination of all these pigments. The anthocyanins mav occur in the outer 

 cell layers of the leaf, the chlorophylls and carotinoids in the inner cell 

 layers. When a red leaf is placed in hot water the anthocyanins disap- 

 pear from the cells into the water and the green color of the chlorophyll 

 within becomes evident. The chlorophylls and carotinoids may be dis- 

 solved from the leaves by alcohol, and then separated from each other 

 by appropriate chemical means. Carotinoids are always present in green 

 leaves, but their presence is obscured by the chlorophvlls until the latter 

 disintegrate. Surprising as it may seem, the carotinoids may be present in 

 as large amounts in the leaves of midsummer as in the yellow autumn 

 leaves. 



In many common trees and shrubs the anthocyanins are formed mainly 

 in the autumn; in others, such as certain varieties of maple, the young 

 leaves may be red also. The anthocyanin in purple varieties of beech, 

 plum, hazel, and barberry is evident throughout the growing season. 



■ ^ Pigments Apparent Color Chemical Composition 



Chlorophyll a Blue-green C56H7205X4Mg 



Chlorophyll b Yellow-green C55H7o06X4Mg 



Carotenes Y'ellow to red C40H56 



Xanthophylls Yellow C40H56O2 



Anthocyanins Red to blue Various combinations of C, H, and O 



The anthocyanins are very complex substances composed of a few fundamental com- 

 pounds ( anthocyanidins ) combined with various sugars and benzene compounds. They are 

 soluble in water and occur in the solutions in the plant cells. Some of the anthocyanins, 

 such as those in certain varieties of apples and peaches, are formed only in light especially 

 of short wave length. Others, such as those in red beets and radishes, may be formed in 

 darkness. Chlorophylls and carotinoids are not soluble in water and are formed in definite 

 bodies in the protoplasm known as plastids. The carotinoids of plants when eaten by 

 animals reappear in yellow cream, yellow body fat, egg yolks, and butter. Carotene has 

 recently been shown to be the forerunner of \'itamin A. The molecule of the vitamin is 

 just one-half of the carotene molecule. 



