iv.J FROST-CRACKS. 6i 



the drying of the wood-cell walls as their water freezes 

 out in ice-crystals in the cavities, that any expansion of 

 the inner mass may result in violent and sudden rupture. 

 Such an expansion may result at a certain stage of 

 the freezing of the inner woody cylinder, and the con- 

 sequence is 2l frost-crack. 



So far as such cracks are radial only, they come 

 under the category of " star-shakes," as we have seen ; 

 but it is obvious that the sudden and violent rupture of 

 these outer layers of wood may lead to separation of 

 the whole mass of outer shells of wood from the inner 

 ones, and such separations constitute " cup-shakes." 



These latter may also result, however, especially 

 in soft-wooded trees like Poplars, by the shearing 

 action of violent winds, for in bending beneath the 

 gusts the cylindrical shells of woody layers tend to be 

 alternately compressed and extended on opposite sides, 

 a process obviously calculated to cause the layers to 

 separate at the softer parts and shell off. 



It will sometimes happen that only a portion of a 

 layer is detached, making the segment of a cup ; at 

 other times, a small part of several layers ; and again, 

 in some instances, we find that the disjunction is not 

 complete, owing to there being a few fibres remaining to 

 connect the two layers. When, however, it assumes its 

 worst shape, that is, when the ring or cup is perfect, it 

 will in all probability be found to pervade the greater 

 part, if not the whole tree, evidence of it being frequently 

 traceable in the remotest branches. 



Experience has shown that with only a segmental 

 cavity open, there is not much to fear, as it seldom 

 extends far up from the root ; any log, therefore, not 

 having more cup- defect than this, may without hesi- 

 tation be converted into plank, board, or scantling ; 



