4. Oligotrichum Lamarck & de Candolle, Fl. Franç. ed. 3. 2: 491. 1805.
[Greek oligo-, few, and trichos, hair, alluding to calyptra]
Gary L. Smith Merrill
Plants small to medium-sized, not polytrichoid, gregarious, in loose tufts. Stems simple or sparingly branched by innovation. Leaves various, weakly sheathing at base, ± plane to distinctly channeled or subtubulose, erect when dry and sometimes crisped, erect spreading when moist (in O. falcatum falcate-secund wet or dry), ovate-lanceolate to lanceolate to oblong-elliptic; apex flat or cucullate, acute, obtuse, or apiculate, the margins subentire, denticulate or coarsely serrate, the margins not bordered by linear cells; costa percurrent or short-excurrent, toothed towards apex or with low abaxial lamellae; lamina broad, 1-stratose, the abaxial surface with low projecting teeth, and often with abaxial lamellae; adaxial lamellae confined to the costa, straight or more often transversely undulate, margins entire to sharply serrate, the marginal cells in cross-section not differentiated, smooth. Sexual condition dioicous; perigonia often disproportionately large, the bracts broadly ovate, at times colored and petaloid, overlapping, forming a conspicuous rosette; perichaetial leaves longer and narrower than the vegetative leaves. Seta typically solitary, smooth. Capsule cylindric, usually broadest near the base, terete, often rugose or sometimes with 4 or more indistinct angles or ridges, hypophysis not differentiated, tapering, with stomata at the base; exothecial cells not papillate or pitted; operculum rostrate; peristome teeth 32, pale, subacute to obtuse, compound or sporadically simple. Calyptra sparsely hairy. Spores finely papillose.
Species 24 (4 in the flora): almost worldwide.
The North American species of Oligotrichum show wide variation in habit, leaf form, and development of abaxial lamellae. As in Pogonatum, the peristome teeth are compound (double), but in Oligotrichum the teeth are pale and not intensely pigmented. The indistinctly angled or ridged capsules of some species also resemble those of Pogonatum, but in that genus the capsules lack stomata and the exothecial cells are papillate. Oligotrichum parallelum is a lax plant, similar to Atrichum in appearance, but without a differentiated leaf border. All four species produce abaxial as well as adaxial lamellae, most notably O. aligerum.