Polytrichum angustatum Bridel, Muscol. Recent., suppl. 1: 79. 1806; Atrichum angustatum var. plurilamellatum (Jennings) Frye; A. macmillanii (Holzinger) Frye; A. papillosum (Jennings) Frye; A. xanthopelma (Müller Hal.) A. Jaeger & Sauerbeck

Plants small, dull yellowish green, sometimes with a reddish cast. Stems 1-2 cm. Leaves 4-8 × 0.4-0.8 mm, narrowly ligulate to oblong-lanceolate above, concave, strongly undulate, with prominent teeth in oblique rows on abaxial surface corresponding to the undulations, margins with mostly double teeth; costa subpercurrent, toothed on the back above; lamellae 6-9(-12), with wavy margins, 5-10(-15) cells high, widely spreading and covering much of the leaf surface; median cells of lamina 8-17(-19) µm wide, ± isodiametric with firm walls, bulging-mammillose on adaxial surface, usually with prominent minute, rounded or striate papillae, especially on the abaxial surface. Sexual condition dioicous; male plants about same size or smaller than females, the perigonial bracts broad, forming conspicuous antheridial buds, often with more than one bud in sequence per plant. Seta 1-3 cm, erect, typically 1 per perichaetium. Capsule 3-7 × 0.3-0.6 mm, narrowly cylindric, usually ± straight to somewhat curved, suberect or inclined. Spores 11-16 µm.

Capsules mature spring-summer (Apr-Jul). Exposed soil banks along roads and trails in woods; low to moderate elevations; Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., Que.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; w Europe.

Atrichum angustatum is a small, dioicous species, easily distinguished by its narrow, channeled leaves with numerous, tall, wavy lamellae, and small, ± isodiametric, bulging-mammillose, papillose leaf cells. A characteristic habitat for this species is mounds of subsoil thrown up by the roots of fallen trees. All Atrichum species develop cuticular papillae to some degree, but in no case are they as evident as in this species. Plants growing in more humid situations tend to be less strongly papillose. The distinction between A. angustatum, with smooth cells, and A. xanthopelma and A. macmillanii, with papillose cells, is without merit. The leaves of the type of A. angustatum (from South Carolina) are strongly papillose.