Plants light green to dark brownish green, dull, in loose tufts. Stems 1-6(-18) cm, tomen-tose to scarcely tomentose with white or reddish brown rhizoids. Leaves falcate-secund, often strongly so, sometimes somewhat sparse, distal leaves curled, proximal leaves with flexuose apices, slightly to strongly curled and crisped when dry, smooth, (3-)4-7(-10) × 0.6-1 mm, concave below, keeled above, from a lanceolate base to a gradually narrowed, fine, keeled subula; margins entire proximally, serrulate to strongly serrate in the distal half, sometimes nearly entire throughout; laminae 1-stratose and usually 2-stratose above on one or both margins, rarely some 2-stratose regions near costa; costa excurrent, 1/6-1/4 the width of the leaves at base, strong, terete, papillose to spinose distally on abaxial surface, occasionally almost smooth, sometimes extending to costa, abaxial ridges absent, with a row of guide cells, 2 well-developed stereid bands extending to distal part of leaf, adaxial epidermal layer of cells not differentiated, abaxial epidermal layer differentiated; cell walls between lamina cells slightly bulging; leaf cells smooth to slightly papillose below on abaxial surface, papillose to spinose prorate above on abaxial surface; alar cells 2-stratose, strongly differentiated, sometimes extending to costa; proximal laminal cells elongate, pitted, (19-)43-62(-93) × (2-)6-8(-12) µm; distal laminal cells irregularly rounded, elliptic, short-rectangular to quadrate, not pitted, (7-)9-23(-36) × (4-)8-12(-14) µm. Sexual condition dioicous; male plants as large as females, growing intermixed, or in separate patches; interior perichaetial leaves abruptly short-acuminate, convolute-sheathing. Seta 1-3.5 cm, solitary, yellow, reddish yellow or brown. Capsule 1-3 mm, arcuate, inclined to horizontal, strumose, strongly furrowed when dry, somewhat contracted below mouth, dark brown to reddish brown; operculum 1-2 mm. Spores 14-24 µm.
Varieties ca. 7 (2 in the flora): North America, Europe, Asia.
Plants in loose tufts. Stems 8-18 cm, scarcely tomentose. Leaves strongly falcate-secund, somewhat sparse; margins nearly entire; costa almost smooth on abaxial surface; proximal laminal cells elongate, (19-)42-54(-84) × (5-)7-9(-12) µm; distal laminal cells irregularly rounded, elliptic or short-rectangular, (7-)9-19(-36) × (4-)8-9(-13) µm. Seta 2.5-3.5 cm.
Capsules mature summer. Moist humus banks, humus on coniferous forest floors, and bogs on small conifers (often black spruce); 900-1300 m; B.C., N.W.T., Que., Yukon; Europe.
Variety flexicaule is an arctic-alpine taxon that needs further collecting and study to determine its taxonomic status. J. Kucyniak (in D. Löve et al. 1958) called attention to it in the Quebec bryoflora and many European bryologists recognize it as a form (W. Mönkemeyer 1927), variety (J. Podpera 1954; L. I. Savicz-Lubitzkaya and Z. N. Smirnova 1970; V. M. Melnichuk 1970), or species (E. Nyholm 1986+, fasc. 1). It is immediately noticed because of the large size of the plants, whose stems are up to 18 cm, sparsely foliated with strongly falcate-secund leaves, and the long seta, 2.5-3.5 cm. It is further recognized microscopically by the irregular shape of the distal leaf cells, the nearly entire leaf margins, and the costa almost smooth on the abaxial surface.
Plants in loose tufts. Stems 1-6(-10) cm, tomentose with white or reddish brown rhizoids. Leaves falcate-secund, usually densely foliate; margins serrulate to strongly serrate in distal half; costa papillose to spinose distally on adaxial surface; proximal leaf cells elongate, pitted, (25-)43-62(-93) × (2-)6-8(-12) µm; distal laminal cells short-rectangular to quadrate, not pitted, (8-)18-23(-31) × (5-)8-12(-14) µm. Seta 1-3.5 cm.
Capsules mature spring. Coniferous or deciduous tree trunks and bases of trees, rotten logs, stumps, soil, boulders, rock outcrops, cliff shelves, and humus in woodlands, or sometimes bogs; 10-2300 m; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Alaska, Calif., Ga., Idaho, Ky., Maine, Mass., Mich., Minn., Mont., N.H., N.Mex., N.Y., N.C., Ohio, Oreg., Pa., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo.; Europe; Asia.
Variety fuscescens is highly variable but is best recognized by the loose tufts of green to brownish green, dull plants, the slender, falcate-secund leaves ending in a slender, keeled subula, slightly to strongly crisped when dry, the strongly serrated, often 2-stratose distal leaf margins, the excurrent costa that is conspicously rough above with papillae and spines on the abaxial surface, the nonpitted, short-rectangular, quadrate or irregularly angled distal leaf cells, and the solitary, often strumose, capsules that are inclined to horizontal.
Some plants of var. fuscescens, especially those in the northern part of Canada, may be confused with Dicranum acutifolium. The latter, however, usually has a few undulations on the leaves and the leaf cross section often reveals larger, more rounded bulging cell walls between the lamina cells and fewer 2-stratose regions on the margins than D. fuscescens. Dicranum sulcatum, considered a synonym by R. S. Williams (1913), has been recognized by W. L. Peterson (1979) as a distinct species. The diagnostic features are duller color due to a greater degree of papillosity, long-excurrent costa, wider costa at mid leaf, and presence of more rows of stereid cells, 3-5 rows compared to 2-3 rows in D. fuscescens. Dicranum sulcatum is reported to occur usually on living coniferous trees in the Pacific Northwest, from southern Alaska south to central California, inland to northern Idaho and northwestern Montana. All of the diagnostic characters are too variable to be important in maintaining that species. Furthermore, they are all quantitative characters, which makes it difficult to establish a distinct species without at least one good qualitative character. Further studies could help to establish it as a variety.
Variety fuscescens has been reported from Kentucky by J. A. Snider et al. (1988), Massachusetts by F. J. Hilferty (1960) and Ohio by Snider and B. K. Andreas (1996).