Polytrichum cylindricum F. Weber in F. Weber and D. M. H. Mohr, Beitr. Naturk. 2: 397. 1810; Atrichum undulatum var. attenuatum Bruch & Schimper

Plants dark green. Stems (2.5-)4-5 cm, densely leafy. Leaves 7-8 × 0.8-1 mm wide midway between base and tip, narrowly triangular-lanceolate to linear, subfalcate, plane, not undulate, with a few scattered abaxial teeth, or none, long-tapering to a narrowly acute apex, margin stoutly bordered, doubly toothed from proximal ¹/8 of leaf, often colored; costa spinose abaxially towards leaf apex; lamellae (4-)5(-6), low and strictly parallel, (1-)3-4 cells high; median leaf cells 20-25 µm wide, rounded hexagonal, with their longest dimension at right angles to the costa, with distinct trigones, smooth or occasionally with striate papillae near base of leaf. Sexual condition polygamous, most shoots apparently unisexual and female; unisexual male shoots not reported, perigonial leaves similar to the others, but with broader bases, to broadly lanceolate and unbordered in proximal ¹/2, the innermost bracts tiny, obovate. Seta reddish brown, 3-4(-5) cm, usually 1 per perichaetium, occasionally 2. Capsule 6-8(-10) × 0.8-1 mm, reddish brown, finely striate, narrowly cylindric, somewhat curved, erect to inclined. Spores 18-20 µm.

Moist soil, ravines, ditches, stream banks in mixed forest, low wet woods and bottomland (swamp forest); low elevations; Ala., Fla., Ill., Ky., La., Miss., Mo., N.C., S.C., Tenn., Tex., Va.

Atrichum cylindricum is distributed along the Atlantic and Gulf coastal plain from Virginia to Florida and eastern Texas, and in the Mississippi Embayment and Cumberland Plateau. In addition to the published distribution (Gary L. Smith 1977), the species has also been found on hummocks in a cypress swamp in southern Illinois. Atrichum crispulum is similar in size to A. cylindricum, but is dioicous, with broader, distinctly undulate leaves and well-developed teeth on the back of the lamina. Atrichum altecristatum is a more slender plant with taller lamellae, shorter, broader, undulate leaves, toothed on the back and less strongly bordered, and shorter capsules.