362 NATURE-STUDY 



are studied it should be with some application in view, as the 

 use of these forms in identification of trees and other plants. 



The important function of the leaves of a tree being real- 

 ized, it will be seen why a tree should suffer from the destruc- 

 tion of its foliage by cankerworms, as often happens. 



In the autumn, as the soil is cooled and the roots are chilled, 

 the flow of the sap is diminished, and the leaves are thereby 

 affected. The life processes are accordingly checked. Most of 

 the still useful substances in the cells of the leaf are absorbed 

 and stored in the neighboring twigs and buds. The leaf 

 cells are thus slowly dying, and undergo chemical changes 

 that destroy the chlorophyll and reveal the reds, yellows, and 

 other colors that were there before, or are now produced 

 by the processes of decay. Thus the leaf puts on the gor- 

 geous colors of autumn. Though there is a great variety 

 of hues in the forest as a wnole, yet the individual species 

 as a rule have certain colors peculiar to them. Sometimes 

 there is a gradation of colors through a certain gamut. Elms, 

 soft maples, and poplars put on clear yellow. Hard maples 

 have yellow, orange, and red. The red oak turns a choco- 

 late brown. Conifers, on the other hand, retain their green, 

 though with subdued intensity, in the winter. 



It is commonly said that frost causes the bright autumn 

 colors. This is not strictly true. The chilling of the roots, 

 even without a frost, begins the work of the fairy painters. 

 The most brilliant colors of the forest are produced in cool, 

 moist weather, with no frost. A severe frost will turn the leaves 

 sere and brown, instead of orange, red, and yellow. 



In cold latitudes the trees shed their leaves before winter. 

 The leaves would freeze anyway and, if not, the too great 

 transpiration when the roots were furnishing so little sap 



