AUTUMNAL LEAF COLORATION 385 



a limiting factor in the natural formation of anthocyanin pigments. Con- 

 trariwise the destruction of anthocyanins in plant tissues is accompanied by 

 the release of considerable quantities of oxygen. 



The Anthoxanthins. — Exposure of the petals of almost any white flower 

 to ammonia vapor will cause them to turn yellow. This is due to the pres- 

 ence in such tissues of what may be regarded as a colorless form of one of 

 the anthoxanthins. Like the anthocyanins these compounds are chemically 

 related to flavone and usually occur in plants in the form of glycosides. 

 IVlost of the anthoxanthins are colorless or nearly so as they occur in the 

 plant but upon extraction and treatment in various ways their typical yellow 

 or orange color develops. Like the anthocyanins they are water-soluble, 

 and are usually found in the cell sap. In some plant tissues, the anthoxan- 

 thins present are yellow in color. The yellow pigment in the inner bark 

 of the black oak {Quercus velutina) is due to an anthoxanthin called querci- 

 trin. Similar pigments occur in the wood of various other species (osage 

 orange, sumac, etc.), and in certain fruits (oranges). Some flowers, as for 

 example yellow snapdragons, owe their yellow color to anthoxanthins. The 

 color of most yellow flowers is due, however, to the plastid pigments carotene 

 and the xanthophylls. 



Autumnal Leaf Coloration. — The most spectacular display of pigmenta- 

 tion in the plants of temperate regions is the annual autumnal coloration 

 of leaves, especially of woody species. The "turning" of leaves in the 

 autumn is not due, as is commonly believed, to the effects of frost. In fact 

 early frosts will greatly reduce the abundance and brilliance of the autumn 

 leaf colors by killing or severely injuring the leaves before the pigments reach 

 their maximum development. The sequence of events leading to the coloration 

 of leaves in the autumn seems to be about as follows : In late summer or 

 early autumn chlorophyll synthesis in the leaves ceases, while the destruction 

 of the chlorophyll already present apparently proceeds at an accelerated rate. 

 As the chlorophyll disappears the residual yellow plastid pigments — carotene 

 and xanthophylls — become apparent. The yellow color of the leaves of many 

 species at this season, as for example, the tulip polar, sycamore, and birch, is 

 due to the disappearance of the chlorophyll which has masked the presence of 

 the yellow pigments during the summer season. The golden yellow effect 

 produced in some leaves, such as those of beeches, is due to the presence in 

 the cells of a brownish pigment, probably a tannin, in addition to the yellow 

 pigment. 



The more prominent colors in most autumn landscapes, however, are the 

 various shades of red and purplish red which develop in the leaves of such 

 species as the red maple, many oaks, sumac, dogwood and black gum. These 



