Basidiomata gregarious on fallen, decaying leaves in alpine shrubs of Salix glauca and S. lapponum and possibly on Betula leaves. Autumn. Widely distributed and not uncommon in alpine areas in Norway. Also found on Jan Mayen.

Pileus 1-3 mm across, hemispherical, bell-shaped to parabolical, without or with a very small umbo, sometimes more flattened or even slightly depressed at the centre, often somewhat lobed at the margin, pruinose, glabrescent, sulcate, more or less translucent-striate, dark brown when very young, then beige or pale brown, incarnate brown to pinkish, usually with dark brown centre, the margin whitish. Flesh very thin, whitish. Odour none. Lamellae 8-13 reaching the stipe, ascending, fairly broad, the edge convex, narrowly adnate, not or rarely decurrent with a tooth, beige to whitish, sometimes with a slightly incarnate tinge, the edge beige to whitish. Stipe 10-25 x 0.2 mm, fragile, hollow, terete, equal to somewhat widened or somewhat bulbous at the base, straight to flexuous, pruinose, glabrescent except for the base, which is pruinose to puberulous, pale brown when young, and then darker below, becoming entirely white or watery white, mostly insititious, but sometimes arising from a whorl of radiating, white mycelial hyphae.

Basidia 21-30 x 7-9 µm, clavate, clamped, 4-spored, with sterigmata up to 5 µm long. Spores 8.1-11.5 x 4.3-5.5 µm, Q = 1.7-2.4, Qav ~ 2.1, narrowly ellipsoid, smooth, amyloid. Cheilocystidia 13-29 x 6-12 µm, forming a sterile band, clamped, clavate, obpyriform or somewhat more irregularly shaped, covered with regularly spaced warts and simple or occasionally branched, straight to somewhat curved, cylindrical excrescences, 0.5-3 x 0.5-1 µm. Pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis 3- 9 µm wide, densely covered with narrow cylindrical excrescences, 0.5-1.5 x 0.5 µm; terminal cells globose to clavate. Hyphae of the cortical layer of the stipe 1-5 µm wide, densely covered with short cylindrical, straight to slightly curved excrescences, 1-3 x 0.5-1 µm, the terminal cells (caulocystidia) numerous, especially near the base of the stipe, 8-28 x 5-11 µm, clavate to subglobose, covered with cylindrical excrescences, 1-3.5 x 0.5 µm.

Microphotos of cheilocystidia 1

Microphotos of cheilocystidia 2

The taxon is clearly recognizable as a member of section Polyadelphia Singer ex Maas Geest., and was first tentatively identified as Mycena smithiana Kühner. It differs from M. smithiana, however, in having a somewhat more brownish pileus with a faint pink tinge, being 4-spored and clamped, and having smaller but slightly broader spores; further more it grows on Salix leaves and not Quercus leaves. Mycena guldeniana Aronsen & B.A. Perry. is another member of sect. Polyadelphia growing on fallen, decaying Salix leaves in alpine areas. This species can also be found in the same area as Mycena exilis, but can easily be separated on account of smooth cheilocystidia and a pale brown to brown-grey pileus.

In species of this small size it can be difficult to determine the exact colours. The colours tend to change from natural to artificial light, and they may often also change soon after the specimens have been collected. The pink colour that was observed in the pileus seems to be very fugitive and tended to disappear shortly after the specimens had been collected. Robich (2004) divided the section into three groups on account of the colour of the pileus. The first group has a pileus that is white or grey with a concolorous centre; in the second group the pileus has brownish to geyish brown or creamy to greyish to white colours with the centre brown to brownish, and in the third group the pileus has various colours, more or less with a tinge of purple, vinaceous or pink.

Considering Mycena exilis as a species of the group with a brownish pileus, it comes close to the following four species. Mycena lohwagii Singer can be separated on account of a larger size, larger number of lamellae (14-17) which are adnate to decurrent, a pale yellow-brown stipe with whitish apex, larger cheilocystidia, and its cespitose growth on fern rhizomes. In Mycena alniphila Robich the pileus is pale brown, hardly darker at the centre, the lamellae are adnate and pale cream, the stipe is pale greyish brown, the terminal cells of the stipitipellis are hardly inflated, and it is growing on fallen leaves of Alnus viridis (Robich 2003). Mycena pinophila M. Villarreal, Arenal & V. Rubio has a dark greyish brown pileus, the lamellae are adnate to slightly decurrent with a tooth, the cheilocystidia and the hyphae of the pileipellis and the stipitipellis are much wider, and it grows on fallen needles of Pinus sylvestris (Villarreal et al. 2002). Mycena calangianuensis Robich, collected on stems of Juncus, differs in having a whitish pileus suffused with a brown hue and with darker centre, a larger number of lamellae reaching the stipe, adnate-decurrent lamellae, and the different type of habitat (Robich 2004).

On the other hand, considering Mycena exilis as a species with incarnate or pink colours, it relates with M. juncicola (Fr.) Gillet, M. culmigena Maas Geest., and M. catalaunica Robich. Here Mycena juncicola differs, among other things, in having a vinaceous or pale pink pileus, broadly adnate lamellae, a stipe with a vinaceous apex and a patch of stiff, greyish fibrils at the base, and in differently shaped cheilocystidia. Mycena culmigena differs by a greyish vinaceous or purplish pileus, broadly adnate lamellae which are pale greyish vinaceous, and a stipe arising from a flat, vinaceous to brownish disc up to 1 mm across (Maas Geesteranus 1986). Mycena catalaunica can be distinguished by its occurrence on fallen leaves of Quercus ilex, fewer lamellae (7-9), 2-spored basidia, broadly ellipsoid to subglobose spores, presence of large, inflated terminal cells in the cortical layer of the stipe, and the absence of clamp connections.

Mycena elongata Maas Geest. & de Mejer and M. tuberifera Maas Geest. & de Mejer, described from Paraná in Brazil (Maas Geesteranus and de Mejer 1997) are further two species with pink colours in the pileus, but among other features they can be separated on account of smooth hyphae of the cortical layer of the stipe and smooth caulocystidia.

Etymology: exilis, small, thin, slender.

© A. Aronsen 2005