DISEASES CAUSED BY WOUND FUNGI. 51 



margin of the newer layer is grayish white and very soft and velvety. 

 The sporophores usually occur singly, although two or more may 

 grow into an irregular mass where they grow out from the trunk 

 in close proximity to one another. A diseased trunk bears from 

 one to twenty or more of the sporophores scattered over the surface. 

 The decay induced by Fomes fomentarius starts in the outer sap- 

 wood immediately under the bark, and proceeds inward until it 

 reaches the center of the tree. (PI. VIII, fig. 2.) The decayed wood 

 is characterized by numerous irregular black lines, bounding areas 

 of wood not yet completely decayed. Wholly rotted wood is very 

 soft and spongy, light yellowish in color, and crumbles between 

 the fingers into numerous separate wood fibers. 



Beech and birch trees are usually decayed in the tops, and the 

 fungus gradually spreads down toward the base of the trunk. It is 

 no unusual sight to find a large tract of birches weakened by forest 

 fires in which almost every tree has from two to ten sporophores 

 of this fungus growing at various heights from the ground. In 

 northern New England, New York, Michigan, Wisconsin, and Minne- 

 sota in particular, the tinder fungus (Fomes fomentanus] is one of 

 the commonest wood-destroying forms found in deciduous forests. 

 It grows with great rapidity in dead wood. Beech or birch trees 

 which have been felled are rapidly destroyed by the mycelium of 

 this fungus. The mycelium will develop in large masses from cut 

 surfaces of trunks infected therewith when placed in moist surround- 

 ings. This power on the part of the mycelium to flourish away 

 from the standing tree is very characteristic of this species and 

 also of Polyporus betulinus. 



DECAY CAUSED BY POLYPORUS BETULINCS. 



So far as known Polyporus betulinus (Bull.) Fr. occurs only on 

 species of birch. It is widely distributed in Europe and Asia and 

 all over the northern part of North America, including the northern 

 United States. In this country it occurs on the yellow birch (Bc- 

 tula lutea Michx. f.), the paper birch (B. papyri/era Marsh.), and 

 the white birch (B. populifolia Marsh.). The fruiting bodies form 

 half-rounded, conspicuous brackets, which start as small rounded 

 knobs, usually growing out through a lenticel. These knobs rapidly 

 expand until they form a hemispherical papery sporophore. (PI. IX, 

 fig. 1.) The upper surface is very smooth, usually dirty white in 

 color, with no signs of marks or cracks; it resembles a very thick, 

 stiff piece of pasteboard. The outer margin is round and is usually 

 more or less white wherever the thin, dark outer layer has peeled 

 off. The rounded margin extends down on to the lower surface, 

 forming a decided ridge around it. The lower surface is yellowish 



149 



