Single to gregarios near the base of trees in decidious or mixed broad-leaved and coniferous woods or on moss-covered fallen logs, typically with Fagus but also recorded with Pinus, mainly in damp places; also recorded in grassland. Late summer to autumn. Widespread but rare, north to southern Sweden. Not yet found in Norway.
Pileus 4-20(-30) mm across, conical to campanulate, expanding to parabolical or convex, +/- umbonate or papillate, delicately pruinose, glabrescent, hardly or shallowly sulcate, translucent-striate, somewhat hygrophnous, ochraceous orange-yellow, vivid orange, orange-brown, paler at the margin, darker, deep orange to brown at the centre. Odour indistinctive. Lamellae 20-26 reaching the stipe, ascending, narrowly adnate, sometimes decurrent with a short tooth, becoming dorsally intervenose, cream to pale ochraceous or with an incarnate flush, the edge paler to whitish. Stipe (9-)20-83 x (0.5-)1-2(-3) mm, hollow, straight to curved, equal, terete, entirely pruinose to minutely puberulous, at first watery white, turning greyish brown to fairly dark brown from the base, which mostly is extended into a long, dark, root, densely covered with whitish fibrils.
Basidia 24-30 x 7-9 um, clavate, generally 4-spored (but also 2- 4- spored on the same lamella), with sterigmata up to 5 um long. Spores 6.8-9.5(-10.5) x 5.5-7.2 um, Q 1.1-1.4, Qav 1.3, broadly pip-shaped to citriform to subglobose, with strongly projecting apiculus, smooth, non-amyloid. Cheilocystidia 16-48 x 4-14.5 um, forming a sterile band, fusiform, clavate, subutriform, obovoid, or somewhat irregularly shaped, apically broadly rounded to mucronate. Pleurocystidia similar if present. Lamellar trama not dextrinoid. Hyphae of the pileipellis 1-4.5 um wide, covered with simple to branched, straight to curved, cylindrical excrescences up to 14.5 x 2 um which tend to form dense masses. Hyphae of the cortical layer of the stipe 1.5-4 um wide, smooth. Caulocystidia 15-50 x 3.5-8 um, clavate to fusiform, smooth. Clamp connections present in all tissues.
The macroscopic description has been taken from Maas Geesteranus (1990), complemented with details from two Swedish collections. The microscopic details are based on reexamination of five Danish collections. Thanks are due to the curator of the herbarium at Copenhagen (C) for loan of material.
A. leptophylla was formerly classified as a a member of Mycena sect. adonideae, where it could be separated from the other members of the section on account of the orange colour of the pileus and the rooting stipe, which is darkening from the base. The subglobose to broadly pip-shaped spores, coming from both 2- and 4-spored basidia at the same lamella, are good characters as well. Gminder (2016) transferred it to the new genus Atheniella.
A. adonis has a coral red pileus; the stipe is not rooting nor turning dark brown from the base at age. In addition the spores are pip-shaped, and the basidia are mostly 2-spored. In M. acicula the stipe is yellow and neither rooting nor darkening with age..