232 



WOOD AND FOREST. 



than in the east, in the Rockies, for instance, where there are elec- 

 trical storms without rain. 



Fires will be considered later under man's relation to the forest. 



Snow and ice often bring serious harm to saplings by perma- 

 nently bending them over, Fig. 78, or by breaking off tops and 

 branches. 



Frost kills young plants; and sudden changes in temperature 

 seriously affect grown timber, producing "frost checks" and "wind 



Fig-. 78. Slim Trees Kent Over by Snow; Stouter Trees Unharmed. Zurich, 

 Switzerland. U. S. Forest Service. 



shakes." When there is a sudden fall in temperature, the outside 

 layers of the tree, which are full of sap, contract more rapidly than 

 the inner portions, with the result that the tree splits with a sudden 

 pistol-like report, the check running radially up and down the tree. 

 This is called a "frost check" or "star shake," Fig. 41.a, p. 47, and 

 such wounds rarely heal, Fig. 79. 



On the other hand when the temperature rapidly rises, the outside 

 layers of the tree expand so much more rapidly than the inside, that 

 they separate with a dull muffled chug, the check extending in a cir- 



