64 MANUAL OF TREE DISEASES 



bacterium (see page 304). The galls commonly found on oak 

 are probably initiated by insects (Fig. 4). Galls are usually 

 formed of abnormal wood and bark. They may frequently 

 serve as a place of entrance for wood-rotting fungi when the 

 bark is injured. 



Wood-Rots 



Caused by fungi of the order Hymeniales 



The wood in the roots, trunk and branches of trees is a 

 very stable and durable substance. While still a part of 

 the living tree, the heartwood is of but little value to the tree 

 except for support and is essentially dead tissue. During 

 the life cf the tree and after it is made into timber and its 

 various derivative products, wood remains durable and intact 

 except when fire, insects and wood-decaying fungi destroy it. 

 Were it not for these three destructive factors, the wood in the 

 living tree would remain sound until utilized and would then 

 be useful for an indefinite time, until ordinary weathering and 

 abrasion made it worthless. By eliminating the factor of decay 

 in all forms of timber now in use, the saving in the annual cut 

 for replacement would amount, for the United States as a 

 whole, to nearly a hundred million dollars. Stated in another 

 way we would need to produce less than half the timber now 

 used if the factor of decay were eliminated. In the forest, 

 under the present systems of management in this country, the 

 losses from wood-rots reduce enormously the yield of timber. 

 Likewise, with trees outside the forest, wood-rots are more 

 or less destructive, and are the factor which largely determines 

 the length of life of the tree. In the forest and outside, many 

 of the fungi which cause decay of the heartwood also extend 

 their activities into the sapwood and bark. When the latter 

 tissues are attacked, certain parts of the trees die because of 

 the interference with the necessary transportation of food 

 materials, between the roots and leaves. 



