Gregarious on bark of decidious trees (Ulmus, Salix, Tilia, Fraxinus). It seems to be a very rare species in Europe, reported from France, Spain, Italy and Norway.

Cap 3-6 mm across, covered with a separable gelatinous pellicle, hemispherical, conical or campanulate, flattening with age to broadly convex, (sometimes somewhat depressed at the centre?), shallowly sulcate, translucent-striate, minutely pruinose to puberulous, glabrescent, pale grey or pale brown-grey with darker centre and striae, turning whitish. Smell none or somewhat nitrous. Gills 8-14 reaching the stem, ascending, adnate to broadly adnate, white. Stem 5-9 x 0.5 mm, more or less curved, cylindrical, equal to somewhat widened at the base, minutely puberulous, the base villose-furfuraceous, white to greyish, not springing from a basal disc but often somewhat widened or bulbous at the base.

Basidia17-32 x 7-14 µm, clavate, 4-spored, with sterigmata up to 7 µm long. Spores 6-9(-11) x 6-8 µm, broadly ellipsoid to globose, more rarely somewhat triangular, Q 1.0-1.4, Qav 1.15-1.24 (n=35), smooth, amyloid.
Cheilocystidia (7-)12-20(-26) x (4.5-)7-12(-14) µm, forming at least in part a sterile band, clavate, obpyriform, obovoid to more irregularly shaped, covered apically with rather few, straight to somewhat curved or flexuous, simple to furcate, fairly coarse, somewhat conical to cylindrical, finger-like excrescences (1-)4.5-21(-30) x (0.5-)1.5-4.5 µm. Pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis 1-3(-5) µm wide, embedded in gelatinous matter, smooth to diverticulate, branched and intertwined, pileocystidia up to 60 µm long, “hair-like”, springing from a corralloid, diverticulate base. Hyphae of the stipitipellis smooth, caulocystidia abundant, with a swollen base 5-6 µm wide and a long, pointed, simple to furcate apex. Clamps present in all tissues.

The macroscopic description is based on Kühner (1938) and Maas Geesteranus (1980). The microscopic details are based on re-examination of the three Norwegian collections.

Maas Geesteranus (1980) placed Mycena pachyderma in Sect Viscipelles Kühner together with M. amicta, M. cyanorhiza and M. subcaerulea. This seems to have been in accordance with Kühner’s (1938) opinion. He placed it in Cyanescentes, which was not a validly published name and later synonym of section Viscipelles (Maas Geesteranus 1991). In his Conspectus of the Mycenas of the Northern Hemisphere (Maas Geesteranus 1984) he also treated it as member of Sect. Viscipelles, now only together with M. cyanorrhiza. Later (Maas geesteranus 1991), he emphasised that there are several major differences between M. pachyderma and the type species of the section, M. cyanorhiza, and pointed out that “there appears to be a much closer connection between this species and Mycena clavularis (Batsch: Fr.) Sacc., the single important difference being the basal disc of the latter from which the stipe arises.” In fact there are more differences: M. pachyderma possesses clamps while M. clavularis is clampless, the cheilocystidia are somewhat different, the spore size is different, and M.pachyderma possesses pileocystidia, while they are lacking in M. clavularis. Phylogeny supports the placement in sect. Viscipelles.

I am deeply grateful to my friend Carlo Zovadelli for the loan of his marvellous photos.

More photos

More images on the web:

http://herve.cochard.free.fr/Mycena/Mycena%20pachyderma.htm