68 



TFX'HXICAL PROPERTIES OF WOOD. 



lu conifers, also, such dead knots become saturated with 

 turpentine from the wound which surrounds them ; this harden- 

 ing into rosin causes the horny knots found in hirch, pines, 

 and spruce, when grown in the open. 



Wherever the lower branches of conifers have been pruned 

 close to the stem, either to improve the timber, or for litter, this 

 being generally done when the trees are about twenty-five years 

 old — the stem appears outwardly to be free from branches, but 

 their remains will be found on a section as shown in fig. 12, and 

 defective planks will result. Hence, to be effective, pruning 



Fig. 12. Kig. 13. 



must be done early in the life of a tree, but the natural pruning 

 of the branches due to crowded growth of the trees from the first 

 gives far better results. 



Where green branches are allowed to remain on the trees they 

 certainly alter the direction of the fibres and form snags 

 (tig. 13), but tlie resulting knots are firmly connected with the 

 stem-fibres, and do not fall-out, and they prejudice the value of 

 the planks much less than the dead knots already referred to. 

 In fact, spruce, silver-fir, and beech, grown more or less in the 

 open and branched deeply down towards the ground, and 

 especially the Cembran pine, often supply finely marked timber, 

 which is prized by the cabinet-maker. 



The worst knots are those running right across the breadth of 

 a plank, which greatly reduce its strength. In the case of larch, 



