66 



PRACTICAL BOTANY 



60. Self-pruning of leaves and twigs. Many trees and shrubs, 

 begin to shed some of their leaves even in the spring, very 

 soon after the leaves are well grown. Examples of this are 

 the lilacs, the syringa {Philadelplius), the cottonwood, the 

 horse-chestnut, the box elder, and some lindens. Still more 

 common is the loss of leaves during the summer, which may 

 amount to 30 per cent of the total number of leaves. This 

 leaf fall, coming long before the leaves are cast off in the autumn 

 as a preparation for winter, affects mostly the leaves inside 



the crown of the 

 tree, which have 

 such scanty light 

 that they can- 

 not accomplish 

 much photosyn- 

 thesis. 



Leaves, twigs, 

 and even larger 

 branches which 

 are not getting 

 an adequate sup- 

 ply of light or of 

 water are pruned 

 away by the tree. 

 Were it not for 

 this, the dense growth in the interior of the tree top and 

 along the trunk would soon render further branching me- 

 chanically impossible. What one sees on looking up along 

 the trunk into the top of a large tree is mainly dead or dying 

 branches, with few leaves. It is this self -pruning and pruning 

 by neighboring trees which makes the straight trunks, free 

 from knots and most valuable for timber, in trees grown in 

 woodlands, where they stand moderately close together. In 

 some instances, as the so-called snap willows, the cotton- 

 wood, and the large-toothed aspen, live twigs fall very freely 

 from the tree during wind or snowstorms, or when it is loaded 



FIG. 54. A leaf of acacia 



A, as seen by day ; B, the same leaf at night. After 

 Darwin 



