68 FOUNDATIONS OF BOTANY 



the tree is characterized by frequent forking, and has 

 no long horizontal branches. 



If the terminal bud keeps the lead of the lateral ones, 

 but the latter are numerous and most of them grow into 

 slender twigs, the delicate spray of the elm and many 

 birches is produced (Fig. 37). 



The general effect of the branching depends much upon 

 the angle which each branch or twig forms with that one 

 froni which it springs. The angle may be quite acute, as 

 in the birch ; or more nearly a right angle, as in the ash 

 (Fig. 30). The inclination of lateral branches is due to 

 geotropism, just as is that of the branches of primary roots. 

 The vertically upward direction of the shoot which grows 

 from the terminal bud is also due to geotropism. 



This is really only a brief way of sajdng that the grow- 

 ing tip of the main stem of the tree or of any branch is 

 made to take and keep its proper direction, whether verti- 

 cally upward or at whatever angle is desirable for the tree, 

 by the steering action of gravity. After growth has ceased 

 this steering action can no longer be exerted, and so a tree 

 that has been bent over (as, for instance, by a heavy load 

 of snow) cannot right itself, unless it is elastic enough to 

 spring back when the load is removed. The tip of the 

 trunk and of each branch can grow and thus become 

 vertical, but the old wood cannot do so. 



83. Thorns as Branches. In many trees some branches 

 show a tendency to remain dwarfish and incompletely 

 developed. Such imperfect branches forming thorns are 

 familiar in wild crab-apple trees and in the pear trees 

 which occur in old pastures in the Northeastern States. In 

 the honey locust very formidable branching spines spring 



