78 



INTKODUCTION TO BOTANY 



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 chiefly the leaves 

 inside the crown of the tree, which have such scanty light 

 that they evidently cannot accomplish much photosynthesis. 



FIG. 62. The pond lily, an aquatic with floating leaves and submerged steins 



Leaves, twigs, and even larger branches which are not get- 

 ting an adequate supply 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 mechanically 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 that due to the shade of neighboring trees that makes 

 the trunks free from knots and most valuable for timber in 



