44 



STEMS 



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

 (Fig. 38) ; or more nearl) 7 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 

 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. 



58. Thorns as branches. In many trees some 

 branches show a tendency to remain dwarfish 

 and 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- 

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

 midable brandling 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). 



FIG. 35. Leaf-bearing 

 thorn of honey locust 



