SHAPE OF TREES. 17 



considerable difference in the proportion of heartwood and 

 sapwood in the same tree, and broader zones are often formed 

 in youth and narrower zones later on, whilst the breadth of the 

 annual zone has much influence on the density of the wood — it 

 cannot be said that heartwood is always heavier, harder and 

 more durable than sapwood. 



Section III. — Shape of Trees. 



There are technical differences in the wood taken from different 

 parts of a tree, so that a distinction is made between stem- 

 timber, branchwood and rootwood. Forestry is chiefly interested 

 in the production of stem-timber, for the stem is the chief factor 

 of the timber-harvest, both as regards quantity and quality. 



1. The relation between the masses of stem-, branch- and 

 root-wood varies considerably in different trees, principally as 

 regards species, density of crop, age, and quality of locality. 



(a) Species of Tree. — Each woody species has its own peculiar 

 mode of growth, and no two are alike in shape. There are trees, 

 such as the spruce, silver-fir, larch [and Corsican pine — Tr.], 

 in which, even when grown in the open, the development of the 

 bole predominates over that of the branches. The stem of these 

 trees generally grows undivided straight through the crown to 

 the leading shoot, and the crown consists only of side-branches. 



The Scotch pine also at first produces a fine bole, but later 

 on divides into boughs, which are frequently large and numerous 

 forming a spreading crown. In the case of European broad- 

 leaved species, during middle-age and often earlier, the crown 

 gets the better of the bole ; amongst these, the alder, sessile oak, 

 ash, poplars, common elm, birch and aspen produce the longest 

 boles. 



Speaking quite generally, it may be said that when grown in 

 the open, conifers and light-demanding broad-leaved species yield 

 the best boles. 



(b) Density of Crop. — The general rule here is that the pro- 

 duction of clean boles of mature timber is greatest, and branch- 

 wood and to a certain extent rootwood, least, the denser the croj) 

 of trees. 



Broad-leaved species gain most in this respect from a dense 



VOL. v. C 



