Summary:
Subulicystidium longisporum is recognized because of the characteristic cystidia and spores. Features include 1) resupinate growth on wood, 2) soft, open membranous, whitish to grayish patches, with a smooth surface, 3) spores that are narrowly cylindric to fusiform (usually somewhat curved or sigmoid), inamyloid, acyanophilic, and colorless, 4) 4-spored basidia, often surrounded near the base by another wall thought to be the remnant of a previous basidium, 5) abundant cystidia that are projecting, cylindric, tapering to a narrow tip, bearing longitudinal rows of flat crystal-like bodies (except near the tip), and 6) a monomitic hyphal system, all hyphae with clamp connections. Subulicystidium longisporum has been found in BC, WA, ID, MB, ON, PQ, AZ, CO, FL, GA, IL, LA, ME, MI, MN, MT, NC, NY, and TX, (Ginns).

Microscopic:
SPORES 10-11 x 1.8-2.2 microns, narrowly cylindric to fusiform, smooth, inamyloid, colorless; BASIDIA 4-spored, 12-14 x 4.5-6 microns, suburniform, with basal clamp connection; CYSTIDIA 40-70 x 4 microns, slender, acuminate [gradually narrowing to a point], thick-walled toward base, midpart strongly encrusted, tip usually smooth; HYPHAE monomitic, 1.5-3 microns wide, "thick-walled, weakly incrusted", septa with clamp connections, (Breitenbach), SPORES 12-15(25) x (2)2.5-3 microns, usually slightly sigmoid (but may be narrowly fusiform), inamyloid, thin-walled, containing varying numbers of oil droplets; BASIDIA 4-spored, 10-15 x 3.5-4.5 microns, subclavate-suburniform, "often with a membrane-like incrustation in the basal part"; CYSTIDIA numerous, 40-80(100) x 3-4 microns, of subicular origin, subulate [awl-shaped], "mostly of uniform width but sometimes basally thick-walled except for the apical, acute part", +/- cyanophilic, "provided with a characteristic encrustation of unknown matter, in the microscope at low magnification in KOH visible as longitudinal ridges, at higher magnification as rectangular plates dissolving in sulfovanillin", in phase contrast "the crystalline plates appear to have concave horizontal sides"; HYPHAE monomitic, 3-4 microns wide, colorless, "thin-walled or with slightly thickened walls", somewhat cyanophilic, encrusted; SUBICULUM "little developed, composed of rather long-celled hyphae, branching at right angles"; all hyphae with clamp connections, (Eriksson), SPORES 10-16 x 1.5-3 microns, more or less cylindric, "straight or somewhat curved, slightly narrowed at both ends", inamyloid, acyanophilic, colorless, thin-walled, often with several droplets, apiculus not distinctive; BASIDIA 4-spored, 18-25 x 4-6 microns, cylindric to narrowly clavate, somewhat flexuose, with clamp connection at base, sterigmata about 2-3 x 0.5 microns, subulate, "in well developed specimens the basal half of most of the basidia is loosely surrounded by a wall (which may be slightly incrusted); there is strong evidence that these are the remaining walls of the former basidia, inside which a new basidium has developed" (= basidial repetition); CYSTIDIA abundant, 40-80 x 4-5 microns, (projecting up to 50 microns), cylindric, acuminate, brittle, with thickened walls (about 1-1.5 microns), "bearing flat circular crystal-like bodies normally arranged in two longitudinal rows" (although in phase contrast there seem to be "3-4 rows of small oblong crystals"), the basidial base mostly bifurcate; HYPHAE 2.5-4 microns wide, colorless, thin-walled to slightly thick-walled (up to about 0.8 microns), with clamp connections at all septa, (Julich(1)), "The cystidia of the genus Subulicystidium have been studied with the scanning electron microscope. Their ornamentation consists of two rows of ribbon-shaped structures, arranged cross-wise to the main axis of the cystidia. The free ends of these structures are double-refractive and have formerly been described as ''four rows of short crystals''." (Julich(7), from summary)

Notes:
It has also been found in Scandinavia, (Eriksson), in Europe including Switzerland, Asia, (Breitenbach), and in Jamaica, France, Germany, Netherlands, United Kingdom, and Sweden, (Julich(1)).