122 CHEMISTRY OF PLANT LIFE 



response of the pigmentation of the leaves to the need for increased 

 heat-absorption. On the other hand, the red pigments of the 

 beet-root, etc., which seem to be identical in composition with the 

 other anthocyanin pigments, can have no such function as those 

 which have just been described. Furthermore, the fact that the 

 pigment often varies in color from red to yellow or brown, depend- 

 ing upon the temperature under which the tissue is ripening, 

 makes it an open question whether the pigment is the regulating 

 agency or whether its nature is the result of the environmental 

 conditions. Or, in other words, it is a question whether these 

 changes in color are a mechanism by which the plant cell adjusts 

 its absorptive powers, or whether they are only the inevitable 

 result of the changes in temperature upon a pigment material 

 which is present in the cell for an entirely different use. 



A very interesting side-light upon the color changes which 

 many species of plants undergo when the external temperature 

 falls has been shown by the investigations of the relation of the 

 sugar content of the plant tissues to their pigmentation. It is 

 a well-known fact that not only do many species of deciduous 

 plants show the characteristic reddening of their leaves after 

 frost in the autumn but also many evergreens (Ligustrum, Hedera, 

 Mahonia, etc.) exhibit a marked reddening, or purpling, of their 

 foliage during the winter months, with a return to the normal 

 green color in the spring. Earlier investigations, which have 

 been confirmed by several repetitions, showed that the red or 

 purple leaves always contain higher percentages of sugar than 

 do green ones of similar types. More recent studies have shown 

 that artificial feeding of some species of plants with abnormally 

 large portions of soluble sugars produces a reddening of the foliage 

 tissues which is apparently identical with that which these tissues 

 undergo as the result of low temperatures. Thus, the connection 

 between the natural winter reddening of foliage and the develop- 

 ment of sugar in the tissues during periods of low temperatures 

 (see page 64) seems to be clearly demonstrated. It appears 

 that at least a part of tl\e seasonal changes in color of plants is 

 either the cause of, or the effect of, variations in sugar content 

 of the tissues of the plants, accompanying the changes in external 

 temperatures. 



Oftentimes, the anthocyanin pigments seem to be associated 

 with sugar production, as contrasted with the chlorophylls, which 



