BODY AND BRANCH DISEASES AND INJURIES 65 



The fungi which destroy wood are mostly of one general type 

 and are commonly known as the bracket-fungi or polypores. 

 Some of the toadstools also are wood-destroyers. Several 

 species of these two types of fungi are known which enter the 

 wood of living trees and cause its decay. A few of these are 

 active parasites, which advance into and kill living tissue in 

 the sapwood and bark. The others attack only the heart- 

 wood. Numerous other closely related bracket-fungi and toad- 

 stools never enter the wood of living trees but exist as sapro- 

 phytes, destroying wood after the tree is dead either in the 

 forest or while it is in use as a timber product. Some of these 

 latter forms often appear to be parasitic, when they are found 

 growing on a living tree which is severely injured. The para- 

 sitism is only apparent, however, since the wood is exposed 

 to weathering and is essentially in the same condition as if 

 the tree were dead. The various important wood-rots are 

 described under the kind of tree commonly affected, but since 

 the life history of all of the causal fungi is essentially the same, 

 a general discussion of the mode of infection, nature of the 

 process of wood-decay, production of fruiting-bodies, dissemina- 

 tion of the spores and methods of control is given here to 

 avoid repetition or lack of detail concerning these points under 

 the individual discussions. 



Mode of infection. 



Normally the tree is protected from invasion of wood-de- 

 stroying fungi by the bark. But whenever the bark is injured 

 to the extent that the sapwood or heartwood is exposed for 

 any considerable length of time, infection may occur by the 

 lodgment and germination of the spores on the exposed wood. 

 Conifers and some deciduous trees are capable of producing 

 resinous and gummy substances, where the sapwood and 

 bark are wounded. These substances when exuded in quantity 

 cover the wounded area and protect it from infection. For 



