Bryol. Eur. fasc. 52–54. 1853. -- Hypnum populeum Hedw., Spec. Musc. 270. 1801.
Brachythecium nanopes C. Müll. & Kindb. in Mac., Cat. Canad. Pl. 6: 201. 1892.
Plants closely matted, green to brownish-green or olivaceous, irregularly and sparingly branched with the branches prostrate to ascending. Primary stems to 10 cm long with leaves not much different from those on the branches. Leaves to 1.5 mm, 2–2.5: 1, symmetric and straight without a twisted acumen, lightly or not at all concave, submarginally once plicate near leaf base, ovate-lanceolate with a rather well-demarcated acumen, closely imbricated. Margins entire at immediate base, but serrulate at shoulders and on acumen, somewhat recurved at base but plane above, very narrowly decurrent by a 1–2 cell wide strip which extends nearly to insertion of next lower leaf. Median cells smooth, spinulose and thin-walled, to 8 µm wide, mostly 8–15: 1, not pitted. Basal juxtacostal cells short-rectangular, thin-walled with little pitting. Alar region well-demarcated, present in a large patch about 10 cells in each dimension with cells to 10 µm wide, isodiametric, rather thin-walled but dense. Marginal cells of alar region short rectangular, to 12 µm wide, to 4: 1. Costa extending beyond the base of the acumen, tapering to apex, smooth to obscurely spinose distally. Juvenile leaves strongly incurved, to 200 µm long, broadly ovate, about 1.5: 1, serrulate across the acuminate apex. Axillary hairs persistent, to 25 µm, 2-celled with a single pale brown basal cell, thin-walled throughout with some diameter increase in the distal cells. Rhizoids abundantly produced on prostrate stems, elongate and planar branched, brown to reddish-brown, smooth, to 12 µm in diameter, clustered on the abaxial leaf base. Stem cross-section with central strand, a stereome 3–5 cells thick, but without an hyaloderm.
Autoicous. Perigonia bud-like, produced near the perichaetia on prostrate stems and large branches. Perichaetial leaves to 1 mm long, with costa flattened and extending nearly to leaf middle, sheathing at base but reflexed to squarrose in the abruptly contracted acumen, serrulate on margin, without alar differentiation. Seta papillose distally but sometimes almost smooth near the base, to 15 mm long, distally geniculate. Capsule dark brown to reddish brown, inclined to horizontal, strongly arcuate, not distorted or sulcate when dry. Urn to 1.2 mm, about 2: 1. Exothecial cells isodiametric to short rectangular or elongate-hexagonal, in regular rows, 15–20 µm wide, 1–2: 1, thick-walled with lumen: wall ratio 3–4: 1. Stomata present on slightly raised pustules at base of capsule, short and vertical pored. Operculum conic, about 1/4 as long as urn. Annulus present, of two rows of thick-walled cells. Exostome teeth to 250 µm long, narrowly lanceolate and obscurely shouldered, about 6: 1, tapering to a very narrow tip, horizontally striate, yellow to yellow-brown. Endostome segments lightly papillose, very narrow, about as long as exostome, with basal membrane about as long as the segments. Cilia 1–3, lightly papillose and smooth or nodose. Spores finely papillose, 12–15 µm.