IV] BRANCH-CASTING 43 



Now if we accept these results as facts to be discussed 

 and their substantial accuracy is easily proved by 

 observation it is clear that either some wholesale de- 

 struction of branches must have occurred during the 

 development of the tree, or some agency must have been 

 at work to prevent the formation of the expected branches. 



Investigation shows that both causes are at work 

 ruthlessly weeding out of the shoots either at their 

 inception in the bud-stages or at later periods in life, and 

 the principal purpose of this chapter is to show how this 

 comes about, and what are its immediate consequences to 

 the tree. 



Numerous trees cast branches periodically either in 

 the stage of young shoots or twigs, or even later in life. 

 This process of branch-casting is exactly comparable to 

 that of leaf-casting, and is brought about by a similar 

 succession of normal phenomena. That is to say a layer 

 of thin-walled cells, a true absciss-layer, is formed across 

 the twig or branch, and the middle plane of this layer 

 constitutes a plane of weakness at which the upper part 

 of the twig breaks away from the lower, the surface of 

 separation being covered by a layer of cork, which at once 

 distinguishes the former from a mere wound. It is the 

 early inception of such a layer which makes the twigs of 

 Salix fragilis so brittle at the articulations. 



It is a common experience in summer and autumn 

 to find the ground beneath such trees as Pines, Oaks, 

 Elms, Walnut, Ash, Maples, Poplars, Willows &c., littered 

 with twigs or even long shoots which appear to the un- 

 initiated to have been blown off by wind or cut off by 

 squirrels or other animals: careful examination of the 

 surfaces of separation, however, shows that the process has 

 been the natural one of branch-casting, due primarily to 

 internal agencies, though of course the phenomenon may 



