IV] BRANCHING 51 



here concerned, even if we had no further evidence. But 

 we have further evidence, for exactly such erections of 

 axes normally horizontal, with all the ensuing changes in 

 position and direction of leaves, buds, branches, &c. are 

 evoked in shoots of Silver Firs, &c. by the irritating action 

 of certain parasitic fungi. 



We are therefore driven to the conclusion that, while 

 gravitation undoubtedly plays an important part in the 

 arrangement and direction of growth of the shoots and 

 branches of the tree, its effects are always contingent on 

 other actions due to causes hidden in the organization of 

 the tree, and which, for want of a better phrase, we term 

 internal causes. 



That there are other factors of the environment which 

 affect the shaping of trees will be obvious to all who have 

 understood the significance of normal branching, and the 

 possibilities it offers to interfering agents. When one 

 reflects that every time a boy, intent on bird's-nesting, 

 crushes a twig or bud with his feet or breaks a bough in 

 his climbing, he destroys not merely the bud or shoots 

 directly concerned, but also all the buds and shoots 

 incipient in them : that every time a bullfinch picks out 

 the heart of a black-currant bud, or an insect lays her egg 

 in a bud, or a squirrel nibbles off a Spruce-shoot, and so 

 on, the damage done is irreparable so far as the exact 

 symmetry of the tree is concerned. 



The shaping action of pruning operations, whether 

 they take the form of mere shearing of a hedge, or care- 

 fully devised cuttings of valuable fruit trees, teach the 

 same lesson ; and here I may say a few words anent 

 Nature's great pruning agents, wind, frost, snow, &c. 



Every visitor to the seaside, or to wind-swept districts 

 of any kind, is familiar with the peculiarly shorn appear- 

 ance presented by hedges and trees of all sorts exposed to 



4—2 



