90 THE TREE BOOK 



summer heat. But, if we examine them at dose 

 range, we see that they are beginning to lose 

 their deep green luster. There may be 

 splotches here and there of russet, or red, or 

 yellow, or the whole leaf may be tinged a sort 

 of yellowish-green. This is caused by the sap 

 of the tree slowly absorbing the living matter 

 and food cells of the leaf. The nourishment so 

 stored will help to keep the tree alive during 

 the winter, and serve to feed the tiny buds next 

 spring until they get old enough to look after 

 themselves. 



The chlorophyll grains, the machinery of 

 "the miU," of course, contain no food value. 

 As the living substance is withdrawn and the 

 action of the leaf broken up, the waxy granules 

 slowly change to a yeUow liquid, which shows 

 through the thin leaf skin as plainly as when 

 its elements were still green. This is the yel- 

 low in the brilliant autumn foliage. The red is 

 the color which the cell sap assumes. The 

 browns are dead leaves. Other colors are 

 caused by the decomposition of the mineral sub- 

 stances. Because the brilliant colors of the 

 leaves come when warm days and frosty nights 

 are common, we have erred in thinking them 

 due to the action of the frost. It is true frost 

 hastens the breaking up of the chlorophyll, but 



