138 



PROPERTIES OF WOOD. 



clear-cuttings with subsequent planting or sowing, merchants 

 prefer wood from naturally regenerated crops. 



(c) Defects in Sound Wood. 



Heart-shake occurs when radial cracks extend from the pith 

 outwards ; they are visible on the transverse section soon after 

 the tree is felled. In conifers the heartwood of which contains 

 only sufficient water to saturate the cell-walls, heart-shake is 



seen as soon as the tree is felled, 

 for the heat caused by friction of 

 the saw dries the wood so as to 

 cause a fine crack through the 

 pith, which exposure to the air soon 

 opens more widely. (Fig. 57 a). 

 Where there are several such 

 cracks, the defect is termed star- 

 shake, and simple heart-shake, 

 when there is only one ; in the 

 latter case, by sawing parallel to 

 the crack, excellent planks may 

 be produced. 



Air-cracks are due also to 

 shrinkage ; they appear parallel 

 to the course of the fibres on the 

 surface of stems that have been 

 stripped of their bark (Fig. 57, a 

 and b) ; if the wood is rapidly 

 dried, these cracks may penetrate 

 deeply into the stem and reduce 

 the value of the wood for planks, etc. The methods of 

 combating this defect are discussed in Part V., B. 



Wind-shake occurs usually in trees that have been given an 

 open position late in life, such as mother trees destined for natural 

 regeneration. When trees suddenly isolated are blown about 

 by the wind, which the harder tissues of trees that have always 

 stood in the open can resist better, and the fibres of the wood 

 separate along an annual ring forming arc-like shakes 

 (Fig. 57 b), it is chiefly the base of the stem, up to a height 

 of 3 to 6 feet, that is exposed to those cracks, owing to the 



a 1, 



Fig. 57. (a) Straight-fibred stem 

 with air-cracks on its surface 

 and heart-shake on its trans- 

 verse section. (b~) Torse-fibred 

 stem, ditto, also with wind- 

 shake on its transverse section. 



