IV] BRANCH-CASTING 45 



of the crown of the grey and black Poplars, and the 

 peculiar whip-like denuded character of the twigs of the 

 latter and of Willows, are due to branch-casting; and the 

 same operation plays an important part in determining 

 the knotty and gnarled appearance of the Oak. 



In some trees the process of branch-casting begins as 

 early as July but in others it prevails in August and 

 September or even into late autumn, and variations are 

 observed in both individual trees and seasons. In Taxo- 

 dium the casting of the leaf-bearing shoots has become 

 annual and replaces the phenomenon of leaf-fall charac- 

 teristic of deciduous trees. 



It is not to be assumed, however, that all trees show 

 the process of branch-casting above described : apparently 

 the Spruce and Silver Fir are cases in point. Moreover, 

 by no means all the litter of fallen twigs and branches 

 found beneath trees which do cast branches, are due to 

 this phenomenon. Squirrels, birds, and insects often play 

 their parts often causing great havoc, for instance in the 

 crown of the Scots Pine. 



But even more powerful in its effects than all these 

 agents, is the killing action of shade, a subject to which I 

 must now turn for treatment in some detail, since it is 

 the great factor in shaping the crown of the tree. 



All green leaves are organs adapted for exposure to 

 light and air, and only when properly expanded in a 

 suitably illuminated atmosphere can they supply the 

 twigs which bear them, and through these the other parts 

 of the branch-system and tree, with food- materials. Con- 

 sequently all shade beyond a certain tolerable degree is 

 fatal to the leaf-bearing shoots and results in their poor 

 nutrition, in starvation of their buds and tissues, and in 

 short in their enfeeblement in the competition with other 

 foliage-shoots in the immediate neighbourhood. 



