52 DISEASES OF DECIDUOUS FOREST TREES. 



to brown in color. The pores are irregularly jagged and toward the 

 outside assume a more or less horizontal position. The sporophores 

 grow to be large — in some cases as much as 12 inches in diameter. 

 They are annuals and are usually short lived, because they are at- 

 tacked by insects with avidity. 



The decay caused by the birch fungus is very similar to that described 

 for Fomes fomentarius. The fungus gains entrance through the bark, 

 probably through the lenticels or wounds, and starts growing in the 

 outer sapwood, progressing inward toward the center. (PI. IX, fig. 2.) 

 The decayed wood is very uniformly yellowish in color and shows 

 numerous cracks extending both radially and tangentially throughout 

 the mass. Badly decayed wood crumbles into a fine powder, and 

 differs in this respect from birch wood destroyed by Fomes fomentarius. 

 Mayr (53) discusses the various microscopic changes caused by this 

 fungus in birch wood, treating not only the morphological but also the 

 chemical changes induced by this fungus. 



Wherever birch trees are weakened either by fire or other causes, 

 the birch fungus will attack them with great rapidity, and it must be 

 considered as one of the most destructive fungi to birch wood. 



SAP-ROTS OF SPECIES OF DECIDUOUS TREES. 



The line between those fungi which are capable of growing on dead 

 wood that has died after being cut from a living tree and the fungi 

 which grow on dead wood of the still living tree can not be sharply 

 drawn. Certain of the wood fungi cause disease while the tree is still 

 standing, and where such fungi have universally been found associated 

 with a certain type of disease the evidence is strong that the fungus 

 causes the disease. 



In many recent descriptions of diseases of forest trees reference is 

 made to fungi which in the opinion of the writers must be considered 

 strictly saprophytic forms which occur only on dead wood. A striking 

 instance of this is Fomes applanatus (Pers.) Wallr. This fungus is 

 frequently found on living trees, but a careful examination always 

 shows it to be growing on wood which is actually dead, and generally 

 on the dead outer sapwood. So far as the writers have been able to 

 observe Fomes applanatus does not cause what may be called a disease 

 of living trees. The same is true of many other fungi — for instance, 

 Daedalea quercina (L.) Pers., Polystictus pergamenus Fr., P. Jiirsutus 

 Fr., Poria vapor aria Fr., Polyporus gilvus Schweinitz, and Stereum 

 frustulosum Fr. It is natural that observers should describe some 

 of these fungi as being responsible for diseases of the living tree, 

 because they occur frequently on standing trees which are but par- 

 tially alive. The writers believe, however, that most of these forms, 

 if not all, will not grow on a tree until it has already been so weakened 



149 



