LEAVES 



5 2 9 



usually in dying foliage leaves), or in both, and the leaf may appear uniformly col- 

 ored, or colored chiefly on the veins or over the mesophyll (figs. 755, 811). The 

 same pigments occur in flowers, where other colors than red, especially blue, are 

 frequent. In alpine regions anthocyans are more abundantly developed both in 

 leaves and in flowers than they are in the lowlands. 



The factors concerned in anthocyan production. Anthocyans commonly are 

 accompanied by an excess of sugar, and premature coloration may be induced in 

 the leaves of plants by growing them in concentrated cane-sugar solutions. Low 

 temperatures often have been assigned as a cause of red coloration, and, since sugar 

 tends to accumulate at low temperatures (starch formation being relatively unusual), 

 the sugar and temperature theories of pigmentation seem to harmonize. It is prob- 

 able also that the development of the absciss layer (p. 582) tends to impede sugar 

 migration from the leaf to the stem, thus facilitating anthocyan production. Decor- 

 tication and probably all other factors that retard conduction facilitate the forma- 

 tion of anthocyan; furthermore, drought may incite brilliant coloration, even in 

 midsummer. The cause of spring coloration is less obvious, but it may be associ- 

 ated with the flow of sugar into the developing leaves. Usually those species that 

 color most in autumn also color most in spring, and among the more highly colored 

 plants are many in which tannins are produced in abundance, as oaks and sumacs. 

 A current view of the composition of the anthocyans is that they are oxidized glu- 

 cosids, formed from a tannin and a sugar. Sunlight usually facilitates anthocyan 

 production, perhaps accounting for the general predominance of color on the upper 

 leaf surface, and for the occasional high color on the morphologically under surface 

 of upturned wilted leaves; sunlight also may account for the high coloration in 

 open situations as compared with that in dense woods. Certain roots, as those of 

 the corn and the willow, color in the sunlight. Many flowers and fruits color in 

 complete darkness, if there is an adequate food supply. 



The rdle of anthocyan. Whether leaf anthocyan is of any advantage to plants is 

 very doubtful. Some investigators have regarded the red pigment as a screen that 

 absorbs those rays which are injurious to chlorophyll or which tend to inhibit the 

 migration of carbohydrates. Another aspect of the screen theory is that anthocyan 

 absorbs the violet and ultraviolet rays, which are injurious to the leaf enzyms. 

 There is slight experimental evidence for any aspect of the screen theory. A com- 

 moner view is that anthocyan absorbs certain rays which increase the leaf tempera- 

 ture and hence increase the efficiency of the chlorophyll. Red-leaved varieties of 

 certain species (e.g. the copper beech) seem to have a slightly higher leaf tempera- 

 ture than do green-leaved varieties, as is indicated by the temperature of solutions in 

 which the leaves are placed, by the rapid melting of cocoa butter, and by thermo- 

 electric measurements. These temperature differences, however, are so slight as 

 to seem inconsequential, the maximum difference observed being four degrees. 1 

 The red color of deep-water algae has been thought to be advantageous, because 

 of its power to absorb the blue rays, which penetrate deeper than do the others; 

 red rays penetrate scarcely below fourteen meters, a depth at which red algae are 

 the dominant forms. Few of the theories here mentioned are more than guesses, 



1 Red leaves, as a rule, contain less chlorophyll than do green leaves, so that on 

 the whole the former probably are the less efficient working organs. 



