Bryum muehlenbeckii Bruch & Schimper, Bryol. Europ. 4: 163, plate 381. 1846

Plants small to medium-sized, dark red to red-green, rarely entirely green. Stems 0.5-2(-3) cm, julaceous, without metallic sheen, older stem sometimes densely radiculose. Leaves rigid, imbricate, crowded, dark red to red-green, sometimes dark green, ovate, distinctly concave, 1-2(-3) mm; base not or weakly decurrent; margins revolute proximally, plane distally, entire to finely serrulate distally, limbidium absent; apex rounded to broadly acute, not cucullate; costa reddish, not reaching apex or rarely percurrent, awn absent; basal laminal cells occasionally pigmented, in 1 row; proximal cells abruptly quadrate, with scattered short-rectangular cells, 1-2:1; medial and distal cells hexagonal, (40-)50-70 × (16-)18-24 µm, (2-) 3-4:1, walls firm, parallel to costa. Specialized asexual reproduction rare, by rhizoidal tubers, on rhizoids arising from leaf axils, orange-red to red-brown, 100-200 µm. Seta ± straight, red, red-brown, or purple. Capsule nutant, dark red-brown, short-pyriform, 2-3 mm, neck short. Spores 14-18 µm, papillose, yellow-brown.

Capsules mature Jun-Aug (summer). Damp shaded siliceous rock, soil in rock crevices, montane; moderate to high elevations (500-3000 m); Greenland; B.C., Nfld. and Labr., Ont.; Calif., Idaho, Maine, Mich., Mont., Nev., N.H., N.Mex., N.Y., Oreg., Pa., Tenn., Wash., Wyo.; w Europe; Asia (Caucasus); Atlantic Islands (Madeira).

Imbribryum muehlenbeckii is similar to small forms of I. alpinum, but has strongly concave, ovate leaves, short distal laminal cells, and a weak costa. Most material from western North America can be referred to an undescribed species; see discussion under 1. I. alpinum.