Fomitopsis betulina
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Ecology: Saprobic on the deadwood of birch trees, including European white birch, paper birch, and yellow birch; also sometimes reported as parasitic on living trees; causing a brown cubical rot; growing alone or gregariously; annual; originally described from France; widespread and common in Europe, western Asia, and North America, wherever the host trees occur; also occasionally reported from South America. The illustrated and described collections are from Michigan.
Cap: At first lump-like and nearly spherical; at maturity 5-25 cm across and 4-10 cm deep; more or less semicircular in outline; convex or broadly convex; smooth or slightly roughened; whitish at first, becoming brownish and, eventually, dull brown; with a thick margin that is rolled over smoothly to form a rim around the pore surface.
Pore Surface: White, aging grayish to brownish; not bruising; with 2-4 pores per mm; tubes to 1 cm long.
Stem: Absent or rudimentary and stubby.
Flesh: White; unchanging when sliced; thick; corky.
Odor and Taste: Odor strong and fragrant when fresh; taste usually a little bitter.
Microscopic Features: Spores 4-6 x 1-1.5 m; allantoid; smooth; hyaline in KOH; inamyloid. Cystidia not found. Setae not found. Hyphal system dimitic; skeletal hyphae 3-6 m wide, smooth, thick-walled, aseptate, hyaline to slightly yellowish in KOH; generative hyphae 2-3 wide, smooth, thin-walled, hyaline in KOH, clamped.
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