TREES IN WINTER. 75 



the fall is a curious and interesting one, and it 

 begins very early in the life of the leaf. At the 

 base of each leaf-stalk a layer of cells is formed 

 which gradually cuts across the whole petiole, and 

 finally completely separates the leaf from its stalk, 

 so that a gust of wind, or the mere weight of the 

 leaf, is sufficient to cause it u> fall. The walls of 

 the cells of this separating layer generally become 

 thickened and waterproof before the leaf falls, so 

 that the scar is already healed. 



This process, then, is one that is a part of the 

 life-history of the leaf, and is not caused by 

 changes in temperature. In climates where the 

 plants are active during the entire year, the leaves 

 fall gradually. As new leaves are formed, the 

 old are dispensed with, and there is never a time 

 when the plants are leafless. But in our climate 

 most of the trees and shrubs are leafless for a 

 large portion of the year. This is a provision 

 which enables the plants to live through a long 

 period of cold. By the loss of their leaves, and 

 the withdrawal into safe places of all their food 

 materials, the plants are able to survive uninjured. 

 The fall of the leaves is hastened, although not 

 caused, by the cold. We do not understand ex- 

 actly why the leaves all fall at once ; we can only 



