A broad species concept is employed here to accommodate the wide range of variation in leaf morphology displayed by southern African plants. It may be more correct to consider this a species complex. This concept of B. alpinum is likely to include the North American Bryum miniatum Lesq. and the European B. muehlenbeckii B.S.G., B. mildeanum Jur. and B. gemmiparum De Not. Plants with lanceolate leaves, acuminate leaf apices, revolute leaf margins, linear-rhomboidal to vermicular laminal cells, thick cell walls and short excurrent costae, are frequently red to reddish green in colour. Plants with broader, oblong-lanceolate or elliptical leaves, rounded-obtuse leaf apices, generally plane leaf margins, rhomboidal to short-rhomboidal laminal cells with thinner walls and costae ending below the leaf apices, are green to yellowish green in colour. Intermediates are frequently found and variation sometimes occur among leaves of the same plant. Narrow leaved plants may resemble species of Pohlia but B. alpinum will be recognized by its mucronate to short excurrent costa and the crowded leaves which are more or less evenly spaced along the stem. Plants with narrow leaves, short-excurrent costae and long-rhomboidal or vermicular, incrassate upper laminal cells (Wager PRE-CH106, 483), resembling the east African Bryum stenophyllum Dix., are treated here provisionally.
Plants medium-sized to large, caespitose, red to reddish green or green to yellowish green above, yellowish brown or reddish brown to brown below, frequently shiny; terricolous or saxicolous. Stems 5-55 mm tall, simple or branching by innovations, occasionally tomentose below; rhizoids red-brown, coarsely papillose, tubers rare, 140-300(-1000) µm, red to red-brown. Leaves crowded or occasionally distant below, generally equidistant, about equal in size, appressed to erect when dry, erect to erect-spreading when wet, lanceolate to oblong-lanceolate, ovate-lanceolate or oblong to elliptical, frequently concave, 1-3 mm long; apex acuminate to rounded-obtuse, occasionally cucullate; margins plane to revolute, entire or crenulate to denticulate at apex; border absent. Costa ending below apex to short-excurrent, reddish or yellowish to brown; in section subround to round, laminal insertion ventral, ventral surface cells incrassate, dorsal stereid band strong, stereids in 2-4 rows, dorsal surface cells incrassate or substereids, hydroids present between stereid band and guide cells. Upper laminal cells short-rhomboidal to linear-rhomboidal or vermicular, occasionally pitted, frequently incrassate, (32-)37-63(-98) µm long, (7-)10-15(-20) µm wide; basal cells rectangular to quadrate, frequently orange or yellow to brown, frequently pitted, occasionally bulging below, walls frequently thickened. Dioicous, sporophytes rare. Perichaetia terminal; leaves lanceolate or triangular, 1.0-1.4 mm long, cells linear-rhomboidal to vermicular. Seta 13-40 mm long, red to reddish brown; capsule pendulous, clavate or pyriform, red to reddish brown, urn 1-2 mm long, neck frequently wrinkled when dry, 0.5-1.2 mm long; exothecial cells irregular in shape, incrassate, smaller towards mouth; peristome double, exostome teeth narrowly oblong-acuminate, yellowish brown below, hyaline above, minutely papillose, endostome segments tapered above, broadly perforated, cilia 2-3, nodose to appendiculate, basal membrane high, yellowish, minutely papillose; operculum mammillate; calyptra cucullate; spores 17-23 µm, minutely papillose.
Plants medium-sized to large, frequently reddish green, shiny. Stems 5-55 mm long. Leaves crowded, appressed to erect when dry, generally narrowly ovate, frequently concave, 1-3 mm long; apex mostly acuminate; margins plane to revolute, crenulate to denticulate above; border absent; costa ending below to short-excurrent. Upper laminal cells generally linear-rhomboidal, incrassate. Capsule pendulous, red to reddish brown.
Imbribryum alpinum has a cosmopolitan distribution. In southern Africa this species is frequently collected in the mountains of Lesotho, Natal and Orange Free State and occasionally in the southwestern, northwestern, central and eastern Cape, Transkei and southern, central, eastern and northern Transvaal.