50 DISEASES OF DECIDUOUS FOREST TREES. 



groups— one including those fungi which gain entrance through 

 wounds or branch stubs and grow in the heartwood of the living trees; 

 the other, those fungi which grow on dead wood, including both wood 

 removed from the living tree and in some cases dead heartwood or 

 sapwood actually exposed to the air, but still forming a part of the 

 living and actively growing tree. While the first class is not dis- 

 tinctly parasitic in the ordinary sense that a rust is parasitic on a living 

 leaf, the fungi are nevertheless always associated with living trees, 

 and their mode of life may be called parasitic. 



Fomes fomentarius and Polyporus bctulinus rarely, if ever, cause a 

 heart-rot; that is, they rarely gain entrance through a branch stub 

 into the center of the tree and cause a type of decay starting at the 

 center and extending outward. Both of them are found as frequently 

 (and probably in the greater number of cases) on dead trees or logs 

 as they are on living trees. The writers have found both of these 

 forms growing on living and on dying trees, but they are unable to 

 say whether these trees were weakened by the fungi or whether the 

 fungi were able to grow upon the trees because they were already 

 weakened by other causes. The sporophores of both of these fungi 

 grow in large numbers on standing dead trees, and they have 

 frequently been held responsible for the death of trees. The most 

 exhaustive investigation of one of these fungi was made by Mayr (53), 

 who transplanted wood infected by the mycelium of Polyporus 

 betulinus into the sapwood of healthy trees and found that in a- 

 period from August to November the mycelium had grown 2\ 

 centimeters into the sound wood around the point of infection. 

 He regards this as evidence of the parasitic nature of the fungus, 

 but the final results should be known before passing judgment upon 

 his experiments. 



In view of the common occurrence of these fungi, they are de- 

 scribed with the expectation that further investigation may decide 

 their true nature. 



DECAY CAUSED BY FOMES FOMENTARIUS. 



Fomes fomentarius (L.) Fr. occurs in the United States mainly 

 on the beech ( Fagus atropunicea (Marsh.) Sud worth) and the yellow 

 birch (Betula luteaMichx. f.), but it also occurs on other deciduous 

 species. The sporophores are distinctly hoof shaped. They appear 

 as small rounded knobs on the surface of the trunk; that is, they 

 are not confined to branch stubs, but occur also at other wounds. 

 Their upper surface is smooth and more or less definitely marked 

 by concentric ridges. The older sporophores are uniformly gray 

 and have a somewhat powdered appearance. The lower surface is 

 red-brown and shows numerous regular small round pores. The 



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