78 INTRODUCTION TO BOTANY 
the lilacs, the syringa (Philadelphus), 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. 
Fic. 62, The pond lily, an aquatic with floating leaves and submerged stems 
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 
