FROSTCRACK. 507 



formed penetrating more or less deeply towards the centre of 

 the tree. The expansion of the sap, which flows into the crack 

 and freezes, also assists in further splitting the tree and in 

 extending the crack to the centre. 



Most frostcracks occur on cold nights, between midnight 

 and 8 a.m., when the temperature is lowest, and the contraction 

 of the wood is consequently greatest ; the split is accompanied 

 by a noise like a pistol-shot. 



3. Damage done. 

 A. General Nature of Damage. 



The timber of trees cracked by frost is of reduced technical 

 value, and secondary damage by insects or fungi may 

 supervene. 



The extent of the damage depends on the depth of the 

 crack, which varies with the severity of the frost. 



Frostcracks close again with the rising temperature, after 

 the ice which has formed in the wood has thawed, and the 

 sap fills the cell-walls again. A new zone of wood forms over 

 the wound ; owing to the reduced pressure of the bark, this 

 new zone is broader than the previous ones. If the cold 

 continues to increase the crack increases in length and depth. 

 Frostcracks may attain lengths of 6 feet and more. 



In subsequent years considerably less degrees of cold in 

 winter suffice to open out the crack again, as there is only the 

 thin wood of one year's zone to be cracked. It is closed up 

 again during summer. If this process be repeated for 

 several successive cold winters, owing to the successive super- 

 position of annual callus growths, a bevel-edged projection, 

 termed a frost-rib, is eventually formed, as shown in Fig. 239. 

 If there should be a succession of mild winters, a frostcrack 

 may heal up and not extend any further. But the bark, 

 which freezes much less readily, and is therefore less liable to 

 contraction, than the wood, may exercise tension on the latter 

 and prevent it from cracking, even in hard winters. In such 

 cases internal frostcracks result, which may be either radial 

 or peripheral. In the oak, such internal frostcracks are 

 said to occur only on the root-stock. As secondary damage, 



