506 



I'HOTKCTION ACAINST KKdST 



is a certain ratio l)etween the decree of frost and tlie girth 

 of the tree (Dulianiel, Caspar}^, M. Hartig, NorcUinger, 

 Vonhausen). 



The contraction of tlie wood, the reasons for which have 

 been explained in the preceding section, commences at freezing- 

 point and increases as the mercury falls, but the stem recovers 

 its original dimensions during a thaw. If, therefore, the 

 temperature but slowly decreases, so that the tree becomes 

 gradually colder from the exterior, inwards, the volume of the 



Fig. 239. — Transverse section of an oak with two Inotcracks. ,t and f> 



bole contracts equably, and no rupture of the wood occurs. 

 For a tree to crack there must be a rapid fall of temperature, 

 down to -18° C. (0' F.). The external layers of wood (sapwood) 

 then freeze to such a degree that much water passes from the 

 cell-walls and passes into the cell-cavities. The central zone 

 (heartwood, or imperfect heartwood) is either unafU'ected or 

 much less affected than the outer zones ; it remains, therefore, 

 either unfrozen or much less frozen than the latter. The 

 circumferential contraction is therefore chiefly confined to the 

 outer zones of the wood, and consequently they can no longer 

 enclose its unRhrnnken central portion, and the outer zones 

 therefore split radially and longitudinally, the crack thus 



