DEFECTS IN SOUND WOOD. 



139 



lever of the bole between it and the crown of the tree. They 

 occur chiefly on the eastern side of the stem, are less frequent 

 on its western side, but hardly ever appear on its northern 

 or southern sides, as storms rarely come from these directions. 

 In very exposed localities, besides the tangential cracks, there 

 may be a radial crack running through the pith (Fig. 57 b). 



Wind-shakes are more frequent when the centre of an 

 affected stem has very narrow-grained wood. Trees in 

 selection forests, which have stood long in the shade as poles 

 with very narrow zones of wood, begin 

 abruptly to produce broad zones when 

 the shade is removed, so that at the 

 margin of the narrow and broad zones 

 the circumferential cohesion of the fibres 

 is weak, and the resistance to bending 

 strains differs in the two kinds of wood. 



Cup-shakes in the heartwood of trees 

 resemble wind-shakes in appearance, but 

 originate in a different way, bring due 

 to injuries received when the tree was 

 young. A portion of the bark of the stem 

 or root-stock of a sapling or pole may 

 be abraded by deer or s juirrcls (larch) 

 by resin-tapping, contact with wheels or 

 with falling trees, etc., or owing to the 

 death of part of the cambium and bark 

 by forest fire; this produces an occlusion, 

 which, if the injury is slight, gives rise 

 merely to abnormal direction of the fibres, 

 but if a larger part of the wood is exposed, to decay of the 

 subjacent wood. The decayed wood is not then connected 

 with the overlying occluding tissue (Fig. 58). 



Even ants starting a nest at the base of a tree affected by 

 root-rot may, by gnawing the soft spring-wood of conifers, 

 produce cup-shake, as in silver-fir attacked by Fomes tinnosn* 

 (cf. Vol. IV.). 



Neither wind-shake nor cup-shake extend further than 

 over a certain part of the circumference of an annual 

 zone ; if the damage should extend all round the zone, 



Fiir. ">8. (" n ) Sui face 

 ]Mvlrd by deer. (1) i) 

 Wood occluding the 

 woundand subsequent 



layers. 



