NATURAL ENEMIES OF THE FOREST. 



233 



cular direction following the annual rings. Such checks are often 

 called "wind shakes" and "cup shakes/' Fig. 41. c, p. 47. These in- 

 juries are found in regions where sudden changes of temperature 

 occur, rather than in the tropics or in very cold climates. 



VEGETABLE ENEMIES. 



Under this head may be classed., in addition to fungi, a number 

 of unrelated plants, including such as: moosewood and dogwood,, Fig. 

 80, which crowd out young trees; vines, like bitter-sweet, which wind 

 about trees and often choke them 

 by pressure, cutting thru the 

 bark and cambium ; sapro- 

 phytes, which smother the foli- 

 age of trees, of which Spanish 

 moss, Fig. 47, p. 201, is an ex- 

 ample; and finally such para- 

 sites as the mistletoes, which 

 weaken and deform the trees. 



The most important of the 

 vegetable enemies of trees are 

 fungi. It should be remembered, 

 however, that, without the decay 

 produced by them, the fallen 

 trees would soon cover the ground, 

 and prevent any new growth, thus 

 destroying the natural forest. 



Every tree, as has been noted 

 (p. 17), is composed of two parts, 

 one part, including leaves, young 

 branches, roots and sap-wood, living, and the other part, namely, the 

 heart-wood, practically dead. 



Fungi that attack the live parts of a tree are called parasites, 

 while those that live on dead trunks and branches are designated as 

 saprophytes. The line, however, between these two classes of fungi 

 is not well defined, since some parasites live on both living and dead 

 wood. The parasites are of first importance, for, since they kill many 

 wees, they control to a large extent the supply of living timber. 



Fig-. 79. Contraction Frost Check. 

 U. S. Forest Se.rnce* 



