BUDS AND BKANCHES 



93 



run out from the trunk for long distances, remaining much 

 larger than any of the branchlets, as they do in the spruces 

 and in many pmes (Fig. 329). Why can they not? Such 

 trees are round-topped, with many forking branches. 



85. Competition among buds and branches. Of all the buds 

 yearly produced by a medium-sized tree only a small propor- 

 tion can survive even for a year or two, and a much smaller 

 proportion still can grow into 

 branches. The killmg-off proc- 

 ess is mainly one of light- 

 starvation. Looking up into 

 the crown of a tree along a 

 line nearly parallel to its 

 trunk, one is able readily to 

 see that the tree top is not 

 a rather dense mass of leaf- 

 covered twigs, as it appears 

 to be when looked at from 

 without. It is more nearly a 

 hollow cone or (in the case 

 of very round-topped trees) 

 a hemisphere, lilie an open 

 umbrella, the main branches 

 answering to the ribs of the 

 umbrella. The interior por- 

 tion of the tree top is too much shaded for rapid growth of 

 buds or young twigs, and parts of it are dark enough to kill 

 them outright, since their growth depends upon the plant 

 food which they can make by photosynthesis. 



Some simple counts and calculations may serve to make 

 clearer the fact of competition and consequent death of the 

 interior members of the tree top. On a well-grown box-elder 

 tree, perhaps twenty -five years old, the condition of the lateral 

 twigs springing from six-year-old portions of the smaller 

 branches was carefully noted in March. On the branches 

 most fully exposed to the sun, on the south side of the tree, 



A B 



Fig. 75. Accessory buds of box elder 

 (Acer Negundo). Magnified 



A, front view of group; B, two groups 

 seen in profile 



