22 TECHNICAL PKOrEKTIES OF WOOD. 



from their lower liranclR-s will yield very inferior plunks juul 

 scantling. 



It may be possible to secure cleanness of bole by means of 

 pruning ; but this should be considered only as a last resource, 

 for the soundness of the timber may be thus compromised. 

 Where pruning is undertaken in place of close planting, it must 

 be commenced very early, and continued till the trees are thirty 

 or forty years old : late pruning clears only the superficial parts 

 of the boles from knots, leaving them in its more central portion. 



(d) Cylindrical shape. — A bole is said to be cylindrical or non- 

 tapering, the more it approaches the cylindrical shape ; it is 

 tapering or conical, the more it approaches the shape of a cone. 

 It is easy to see that the more cylindrical a bole, the more useful 

 its timber, and the greater the diameter of the smaller end of 

 a log for the same length, the more valuable it will be. Length, 

 and diameter at the smaller end are therefore better measures of 

 the value of a log, than its cubic contents, or its length and 

 diameter taken midway along its axis. 



The absolute measure of the cylindricity of a stem is its 

 form-factor, i.e., the ratio of its real volume to that of the 

 ideal cylinder of the same height and diameter as the stem (the 

 diameter being measured chest-high). Thus, to take examples : 

 mature silver-fir have form-factors between 0*44 and O'ST, 

 spruce between 0-41 and 0*58, and beech between 0'4G and 0*49. 



The more or less cylindrical shape of timber depends chiefly 

 on the species, the density of growth, the height and age of the 

 trees, the nature of the locality, &c. As regards species, it 

 is evident that trees which, when crowded, produce tall boles 

 without much subdivision into branches, especially those which 

 have small branches (silver-tir, spruce, larch and Scotch pine), 

 must have boles more cylindrical than others, such as most 

 broad-leaved species, which have a greater tendency to sub- 

 divide into branches. In the case of trees growing isolated in 

 the open, the crown is largely developed and comes low down the 

 bole, so that the nourishment available for the stem from the 

 foliage of the crown increases downwards with the insertion of 

 each bough. The annual zones are therefore often broader in the 

 lower part of the bole than above, and the bole assumes a conical 

 shape. This is most n;arked in the case of low-branching, iso- 



