192 PLANT PHYSIOLOGY 



become red if kept in the dark, but the beet even when grown in 

 the dark becomes red, possibly owing to the large amount of sugars 

 present. In Elodea and other aquatic plants, Ewart (1897) found 

 that the anthocyan would appear if the plants were grown in a 

 weak sugar solution and strong sunlight. In water and diffuse 

 light no pigment appeared. Low temperatures, which favor the 

 storage of carbohydrates in the form of sugar, also promote antho- 

 cyan production, which may explain its presence in alpine and 

 arctic plants. Similarly, unfolding leaves of hickory, oak, etc., 

 are commonly red while much sugar is being sent into them from 

 the winter storage tissues; and if the midrib of an (Enothera leaf 

 is cut, the distal end becomes red as the sugars accumulate. 

 E. Overton (1899), Boodle (1903), and Combes (1909) have all 

 found that the amount of anthocyan present depends primarily 

 upon the presence of soluble carbohydrates. In reddening cherries, 

 the respiration quotient is such as to indicate that oxygen is being 

 fixed inside the fruit, and Obaton (1923) suggested that this may 

 be due to the fact that sugars are being combined with oxygen to 

 form the pigments. But, in addition to the presence of requisite 

 sugars, certain other inherited conditions, including the factors 

 for the formation of necessary enzymes, must be met, as shown 

 by Wheldale (1911). The sugar beet has even more sugar than the 

 ordinary beet and yet it is not red. 



The anthoxanthins also are glucoside derivatives of compounds 

 like anthocyanidin called flavones and xanthones. The flavones 

 are probably oxidation products of the anthocyanidins. To them 

 are due the yellow pigments found in many plants such as yellow 

 wood (Moms), osage orange (Madura), and sumac (Rhus) which 

 when extracted yield valuable dyes. They are not so common 

 in yellow flowers and leaves, where the color is chiefly due to the 

 carotinoids, but the pigments of Antirrhinum (snapdragon), sun- 

 flowers, and California poppy, are flavones as is that of yellow 

 apples. The yellow coloring matter from onion skins, used in 

 dying Easter eggs, is also a flavone derivative. Both the antho- 

 cyans and anthoxanthins are often associated with tannins, and 

 it is possible that the tannins play some part in their formation. 



The protein pigments or chromoproteins are found especially 

 in the blue-green, brown, and red algae, where they give the char- 

 acteristic color to these groups. They are very complex compounds 

 of high molecular weight (100,000-200,000) and seem to be com- 



