6o 



TIMBER AND TIMBER TREES. 



[chap. 



and even from frosts ; violent and sudden gusts of wind 

 and storms may also help to produce it. It is un- 

 questionably, in some cases, due to the destructive 

 action of fungi — e.g. Pines attacked by Trameies Pint — 

 those parts of the wood injured by the fungus losing 

 and taking up water more rapidly than the sound 

 portions, from which they consequently contract away 

 as shrinkage goes on. 



Wood, as is well known, is a bad conductor of heat, 

 and standing timber usually takes a considerable time to 

 accommodate its temperature to that of the air. 



FIG. Tza. FIG. 12*. 



Suppose a tree, with abundance of water in its wood, 

 suddenly exposed to a prolonged and severe dry frost. 

 The cortex and outer layers of wood freeze first, and, 

 neglecting an initial expansion, the icy mass contracts 

 and exerts considerable pressure on the inner parts of 

 the wood ; these latter gradually freeze in turn, and at a 

 certain period in the process the stresses and strains 

 may be so arranged that the outer parts of the woody 

 mass have contracted so much, especially owing to 



