26 FOREST TREE DISEASES. 



a wood-destroying fungus in a large tree. At the 

 earliest stage of development the mycelium of a wood- 

 destroying fungus, starting, for instance, from a large 

 burn in the butt of the tree, lives on a few wood cells ; 

 in other words, the volume of decayed wood is still 

 very small; in growing the mycelium permeates a 

 larger volume of wood, and when it is sufficiently ma- 

 ture to produce fruiting bodies it must have drawn its 

 substance from a considerable volume of wood, which, 

 in doing so. it has reduced to what we call decay. The 

 first appearance of a fruiting body on the bole of 

 the affected tree, therefore, gives us an indication of 

 a certain development of mycelium in the heartwood 

 and, consequently, of the extent of decay. Annual 

 fruiting bodies grow very fast, and are either de- 

 stroyed during summer by insect larvae and squirrels or 

 they die off in fall and rot. The mycelium in the 

 tree, however, continues to grow by attacking hitherto 

 sound wood, and in the next year a new fruiting body 

 indicates the progress of the mycelium in the wood and, 

 consequently, of the decay. Perennial fruiting bodies 

 grow in size from year to year; therefore, their dimen- 

 sions will constitute an equally valuable indication as 

 to the corresponding extent of the mycelium and the 

 decay in the heartwood. Practical knowledge of this 

 relation becomes particularly valuable in reconnais- 

 sance and in estimating standing timber. 



The extent of decay varies, of course, very much, 

 not only with the species of fungus and the species of 

 tree, but also with the development of heartwood and 

 its chemical and physical qualities. Evidently there 



