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 
from 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 saying 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 
