CONSTRUCTION OF THE SHOOT SYSTEM 57 



prominent part in the construction of the head. If the 

 first axis grows more vigorously than its branches a 

 behaviour we found to lead to the formation of a tap 

 root in the root system and if each of the branches in 

 turn is longer and stronger than those arising from it, 

 the ultimate form of the head is pyramidal, for the 

 successive branches arise nearer and nearer to the apex, 

 and so long as the growth is regular the lowest will be 

 generally the most widespreading. This is true of the 

 series of branches which each of them bears. This type 

 of branching is said to be indefinite or racemose (Fig. 19), 

 and it is illustrated by such trees as the spruce fir. 



If, on the other hand, the growth of each axis or 

 branch is soon checked and so its development becomes 

 exceeded by the growth of the daughter-axes to which 

 it gives rise, the head will be sub-globular or rounded. 

 The exact shape, however, will largely depend on the 

 number of branches springing from below the apex of 

 each in turn, for these all arise at the same node. They 

 are therefore on the same level, and do not grow in 

 what is called acropetal succession, as in the first case. 

 A very common form is that in which each branch is 

 solitary. This form of branching is called definite or 

 cymose. Examples are afforded by the elms, oaks, and 

 many other forest trees (Fig. 21). 



Another factor in the shape which the branching 

 helps to give to the shoot system is the non-develop- 

 ment of some of the buds. We have seen that a bud is 

 produced in the axil of every foliage leaf. It often 

 happens, however, that a twig cannot adequately feed 

 all the buds it bears. Hence some perish and others 

 remain dormant for some time, circumstances which 

 cause a good deal of irregularity. 



Before we study the influence of the arrangement of 

 the leaves upon the form of the shoot and the shoot 

 system we must look a little more closely at the peculi- 



