P. ulma´rius Bull.—ulmus, an elm. Pileus 3–5 in. and more broad, becoming pale-livid, often marbled with round spots, fleshy, compact, horizontal, moderately regular although more or less excentric, convex then plane, disk-shaped, even, smooth. Flesh white, tough. Stem 2–3 in. long, 1 in. thick, solid, firm, elastic, somewhat excentric, curved-ascending, thickened and tomentose at the base, not rarely villous throughout, white. Gills horizontal, emarginate or rounded behind, slightly adnexed, broad (broader in the middle), somewhat crowded, whitish.
The pileus is sometimes cracked in a tessellated manner. Stevenson.
Spores nearly globose, 5µ long Morgan; 5–6.5µ broad Peck; 6µ W.G.S.
Var. aceri´cola—acer, maple; colo, to inhabit. Plant smaller, cespitose.
Trunks and roots of maple trees. Adirondack mountains. September.
Var. populi´cola—populus, poplar; colo, to inhabit. Plant subcespitose, stem wholly tomentose. West Albany. Peck, Monograph, N.Y. Species of Pleurotus, Rep. 39.
The gills are sometimes torn across like those of Lentinus.
The historic elms of Boston Common have borne copious crops of this well-known and easily distinguished species from time immemorial. Every fall, about the first of September, if the season is favorable, later if not, copious crops appear decorating the trunks, and branches, sometimes at a height of thirty or forty feet. Growth takes place where branches have broken off or the trees have been wounded from other causes. They occur very generally on elms in the outlying districts of the city, but are rare in the country, seeming to be distinctly urban in their tastes. No damage is apparent from their growth.
Immediately in the rear of Independence Hall, Philadelphia, a fine cluster appears with equal autumnal regularity.
Though the elm tree is the chosen habitat of this fungus, it is little less select in its choice than other members of its genus.