Summary:
Gymnopilus penetrans is a member of Arora''s Gymnopilus sapineus group but (like G. flavidellus) with a smooth yellow to yellow-orange cap and a whitish evanescent veil, (Arora). Distinctive characters include a bald cap, white flesh, gills that are rusty-spotted at maturity or when old, a white veil, no pileocystidia, and caulocystidia that are gregarious or in tufts, (Hesler).

Cap:
3-5cm across, bell-shaped - convex then subplane [more or less flat]; chrome yellow to golden and fading flavescent [yellowish]; bald, not viscid, (Hesler), 2-5cm across, "bell-shaped to convex then flattened, often with a wavy margin; chrome yellow to golden then tawny" and fading yellowish when old; smooth, (Phillips), "brownish orange-yellow to reddish yellow or pale red-brown, usually with a lighter edge when expanded"; smooth to slightly scaly, (Trudell)

Flesh:
white or whitish (Hesler), whitish, (Phillips)

Gills:
adnate to sinuate, close, medium broad; "at first pale yellow or yellowish white", becoming rusty-spotted when old, (Hesler), "adnate, close, moderately broad; gold or yellowish white becoming tawny-spotted", (Phillips), yellowish, often red-spotted when old, (Trudell)

Stem:
4-6cm x 0.4-0.7cm, equal or sometimes widening downward; pallid, paler than cap, base white-tomentose, (Hesler), 4-6cm x 0.4-0.7cm, sometimes enlarged toward base; yellowish, base whitish with downy hairs, (Phillips), yellow to russet, (Courtecuisse), yellow top and bottom half red-brown to dark brown, (Schalkwijk-Barendsen), whitish to yellowish, typically becomes reddish brown in lower part; often with whitish veil fibrils on surface; white cords often extend from the stem base into substrate, (Trudell)

Veil:
white, fibrillose, fugacious [fleeting], (Hesler), fibrous, white, leaves no ring, (Phillips), veil "whitish to pale yellowish and usually disappears as the fruitbody develops, although some specimens retain a slight fibrillose ring", (Trudell)

Odor:
none or mild (Hesler, Phillips)

Taste:
bitter (Hesler, Phillips), very bitter (Trudell)

Microscopic spores:
spores (6.5)7-9(9.5) x 4-5.5 microns, elliptic in face view, somewhat almond-shaped in side view, verrucose, dextrinoid, no germ pore; basidia 4-spored, 24-31 x 6-7 microns; pleurocystidia 25-33(50) x 4-7 microns, ventricose, cheilocystidia 25-34(48) x 3-7 microns, "flask-shaped, apices capitate or non-capitate, at times expanding up the sides of the gills"; gill trama of subparallel hyphae 4-7 microns wide; cap trama radial, cap cuticle "of repent, sometimes incrusted hyphae", pileocystidia none; caulocystidia 30-50(66) x (2)6-8(13) microns, "flask-shaped to clavate, more or less capitate, gregarious or in tufts"; clamp connections present, (Hesler)

Spore deposit:
orange-brown (Phillips)

Notes:
Hesler studied material from WA, OR, ID, QC, CA, ME, MI, MO, TN, TX, WY, Jamaica, and Sweden. It was reported from BC by Schalkwijk-Barendsen and by Paul Kroeger (voucher at University of British Columbia). Collections from France, Netherlands, and Switzerland were used by Guzman-Davalos(2).

EDIBILITY

no (Phillips)

Synonyms and Alternate Names:
Flammulina penetrans (Fr. ex Fr.) Quel.
Fulvidula penetrans (Fr. ex Fr.) Singer