Plants small, medium to large, densely to loosely caespitose or scattered among other bryophytes, rarely with individual plants scattered on a persistent protonema. Stems erect, acrocarpous, from a ± developed underground rhizome, simple or rarely branched, bracteate proximally, grading gradually or abruptly to mature leaves. Leaves various, with a chartaceous, sheathing base and a divergent, firm-textured blade (polytrichoid), or the whole leaf membranous and sheath not or weakly differentiated, the blade rarely transversely undulate, crisped and contorted when dry; adaxial surface of blade with numerous closely packed longitudinal photosynthetic lamellae across most of the blade, the marginal lamina narrow, or the lamellae restricted to the costa, flanked by a broad, 1 (rarely 2)-stratose lamina, rarely with abaxial lamellae; margins 1(-3)-stratose, entire, denticulate, serrate, or toothed (in Atrichum bordered by linear, thick-walled cells); costa narrow in basal portion, in the blade abruptly broadened and diffuse, smooth or toothed adaxially, rarely with abaxial lamellae, in cross section with a prominent arc of large diameter guide cells and an abaxial stereid band; lamellae entire, finely serrulate, crenulate, or coarsely serrate, the free margin smooth or cuticular-papillose, the marginal cells in cross-section undifferentiated or sharply distinct in size and/or shape from those beneath; transition in areolation from sheath to blade gradual or abrupt, with “hinge-tissue” at the shoulders (except Atrichum and Psilopilum); cells of back of costa (or cells of the membranous lamina) typically in longitudinal rows, ± isodiametric to transversely elongate-hexagonal. Vegetative reproduction none, or by proliferation of an underground rhizome. Sexual condition dioicous or rarely monoicous; male inflorescence indeterminate, innovating from the center and continuing the growth of the stem, often several successive perigonia per shoot; female inflorescence terminal, perichaetial leaves long-sheathing or not much differentiated. Seta solitary or rarely several from the same perichaetium. Capsule obtusely to sharply (2-)4(-6)-angled, with indistinct longitudinal angles or ridges, or terete; hypophysis tapering and indistinct or delimited by a constriction at base of capsule; exothecium smooth, mammillose, or scabrous; stomata present (absent in Atrichum and Pogonatum); peristome pale or strongly pigmented, nematodontous, with a single series of [16-]32-64 rigid, unjointed teeth composed of elongate, fiber-like, sinuate cells, the teeth simple or compound, attached by their tips to the epiphragm (tympanum) covering the capsule mouth. Calyptra cucullate, with a matted felt of hairs arising from its tip and covering all or part of the capsule, or the calyptra sparsely ciliate to smooth. Spores minute and echinulate, or larger and finely papillose.
Frye, T. C. 1910. The Polytrichaceae of western North America. Proc. Wash. Acad. Sci. 12: 275-281. Long, D. G. 1985. Polytrichaceae. In: G. S. Mogensen, ed. Illustrated moss flora of arctic North America and Greenland. 1. Meddel. Grønland, Biosci. 17: 9-57. Smith, Gary L. 1971. A conspectus of the genera of Polytrichaceae. Mem. New York Bot. Gard. 21(3): 1-83.