Summary:
Features include 1) growth in thin flat waxy to membranous patches on wood, 2) whitish to light ocher color, 3) a smooth to sparsely tuberculate surface, 4) elliptic, verrucose, amyloid spores, 5) sulfovanillin positive gloeocystidia with granular contents, and 6) a monomitic hyphal system with clamp connections.

Microscopic:
SPORES (4.4)4.8-5.2(6.0) x (2.6)2.8-3.2(3.4) microns, elliptic to nearly cylindric, verrucose to minutely verrucose (some appearing smooth), amyloid, colorless, thin-walled, with a small, blunt apiculus; BASIDIA 4-spored, 17.0-28.0 x (3.6)4.0(4.4) microns, narrowly clavate to almost cylindric, sterigmata up to 4.0 microns long; GLOEOCYSTIDIA numerous, 50-90(170) x 8-12(14) microns, "arising either in the hymenium or from the basal hyphal layer", tubular, fusoid, or flask-shaped, "some with an elongated neck, some with a long narrow base, thin-walled, the contents yellowish green in KOH, granular", sulfo-positive but the occasional collection sulfo-negative; HYPHAE monomitic, subiculum a basal layer up to 100 microns thick "of woven but essentially parallel and horizontally arranged hyphae"; hyphae 2.0-3.5 microns wide, colorless, inamyloid, thin-walled, with clamp connections, (Ginns(24)), SPORES 4.5-5.5 x 2.5-3 microns, elliptic, sometimes somewhat flattened on one side, finely verrucose, amyloid, colorless, with droplets; BASIDIA 4-spored, 20-25 x 3.5-4 microns, narrowly clavate, with basal clamp connection; GLOEOCYSTIDIA 60-80 x 9-20 microns (according to literature up to 150 microns long), cylindrical to fusiform, "sometimes sinuous, narrowing abruptly into a thin hypha at the base", sulfovanillin-positive; HYPHAE monomitic: hyphae 2-3 microns wide, thin-walled, with clamp connections, (Breitenbach), SPORES 4.5-6(8) x 2.5-3.5 microns, elliptic, subovate [somewhat oval] to subcylindric [somewhat cylindric], finely verrucose (best seen on empty spore coats in Melzer''s), amyloid; BASIDIA 4-spored, 18-22 x 3-4 microns, narrowly clavate; GLOEOCYSTIDIA (= pseudocystidia) numerous, 60-150 x 8-15 microns, tubular, more or less sinuous, often coming laterally from the bearing hypha, "contents in KOH granular, oily, yellowish, in cotton blue with a number of oildrops of varying sizes, with positive reaction to sulfovanilline"; HYPHAE monomitic: hyphae 2-3 microns wide, thin-walled, with clamp connections, thin basal layer (25-50 microns) next to substrate of hyphae that are mainly parallel, "subhymenial trama of densely interwoven, vertical hyphae", (Eriksson), SPORES (4.5)5-6.5 x 3-3.5 microns, cylindric to elliptic, asperulate, amyloid, colorless, acyanophilic, thin-walled; BASIDIA 4-spored, 22-26 x 5-6 microns, clavate, colorless, thin-walled, with basal clamp connection, sterigmata 3-4 microns long; PSEUDOCYSTIDIA "numerous, arising throughout subiculum and subhymenium", embedded, 80-160 x (9)12-15 microns, obclavate to tubular, sinuose, thin-walled to slightly thick-walled, with basal clamp connection, turning blue-black in sulfobenzaldehyde; HYPHAE monomitic, SUBICULAR HYPHAE 3-4.5 microns wide, "loosely interwoven but forming a compact layer of parallel hyphae next to the substrate", colorless, thin-walled, "nodose septate, moderately branched", smooth or moderately encrusted with colorless crystals, (Nakasone)

Notes:
Collections of Gloeocystidiellum porosum were examined from BC, ON, PQ, AZ, CT, IL, MI, MN, NH, NM, NY, NC, OH, and the United Kingdom, and it was also reported from WA, OR, AB, MB, NB, AK, CA, CO, GA, IA, MA, MD, ME, MT, NJ, PA, TN, VT, and WI, (Ginns). It occurs in Europe including "all parts of Scandinavia" (Eriksson). It also occurs in Switzerland, and Asia, (Breitenbach).