92 A TEXTBOOK OF BOTANY [Ch. Ill, 11 



cidentally, as a purely chemical reaction, when certain sub- 

 stances, of which sugar is certainly one, and tannin is prob- 

 ably another, happen to be present, and, under the conditions 

 prevailing in the dying leaf cells, are struck by bright light. 

 It is the fading away of the chlorophyll which admits 

 into the leaf a sufficient intensity of light to produce the 

 chemical reaction. That the light is essential to the process 

 is suggested by the extra brilhance of the colors in specially 

 bright climates and seasons, and is proven by the fact that 

 any leaf which would ordinarily turn red does not do so 

 if closely covered by another, as may be tested by experiment. 

 Thus red in these leaves does not replace yellow, which is also 

 present, but simply outshines it. The reason why some 

 kinds of leaves turn red, and others only yellow, appears 

 to be simply this, that some kinds contain the necessary 

 substances and others do not. It is highly significant in 

 this connection that the leaves which turn most brilliantly 

 red, e.g. Maples, Oaks, and Sumachs, are noted either for 

 their abundance of sugar, or of tannin, or of both. 



Next in importance of autumn colors is brown, which 

 has several origins. In some leaves it is apparently an oxi- 

 chzed product of yellow sap substances called flavone deriva- 

 tives ; in others it results from an oxidation of tannins in cell- 

 walls when exposed to the light and the air, — precisely the 

 same kind of photochemical process which turns wood or bark 

 brown with time. In these cases the color has obviously 

 no functional utility, but represents a purely incidental 

 result of the chemical and physical conditions which pre- 

 vail in the dying or dead tissues. When the lirowning 

 takes place not too rapidly, it sometimes combines with 

 the yellow of xanthophyll into a lieautiful golden bronze, 

 as in some Oaks, though it may later become so intense as 

 to mask the xanthophyll, which fades slowly, as in Beech. 

 With the brown, as with other colors, the exact shade is 

 often determined by the simultaneous presence of other 

 substances, such as resins, or even by renmants of uiifaded 



