44 



STEMS 



it springs. The angle may be quite acute, as in the birch 

 (Fig. 38); or more nearly a right angle, as in the ash (Fig. 31). 

 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, which, however, in the shoot, is 

 exactly opposite to that in the root. 



This is really only a brief way of saying that the growing tip 

 of the main stem of the tree, or of any branch, is made to take 



/ 



and keep its proper direc- 

 tion, whether vertically 

 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 FIG. 35. 



it is elastic enough to 



thorn of honey locust 



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. 

 58. Thorns as branches. In many trees some 

 branches show a tendency to remain dwarfish 

 iind incompletely developed. Such imperfect 

 branches may form thorns, as in the familiar wild 



crab-apple trees and in the pear trees which occur in old pas- 

 i in the northeastern states. In the honey locust very for- 

 midable branching thorns spring from adventitious or dormant 

 buds on the trunk or limbs. They sometimes show their true 

 nature as branches by bearing leaves (Fig. 35). 



