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Bryol. Eur. vol. 6. fasc. 52/54. 1853. -- Hypnum plumosum Hedw., Spec. Musc. 257. 1801.
    Brachythecium rutabuliforme Kindb. in Mac., Cat. Can. Pl. 6: 198. 1892.

Plants closely matted, dark green to brownish-green but usually with a strong russet wash, irregularly 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 2 mm, about 2.5: 1, symmetric and straight without a twisted acumen, strongly concave, not plicate, ovate-lanceolate with a rather well-demarcated acumen, closely imbricated. Margins entire at immediate base, but often serrulate above the middle, somewhat recurved at base but plane above, very narrowly decurrent by a 1–2 cell wide strip which extends only a few cells down stem. Median cells smooth, spinulose and thin-walled, to 8 µm wide, mostly 6–8: 1, not pitted. Basal juxtacostal cells short-rectangular, 2–4: 1,with lateral walls usually pitted and with lumen: wall ratio 2–4: 1. Alar region well-demarcated with cells rather thick-walled and somewhat pitted, to 10 µm wide, isodiametric to short-rectangular, opaque and often brownish. Marginal cells of alar more narrow and elongate than more interior cells. Costa extending beyond the leaf middle, strongly tapering to apex, smooth to obscurely spinose distally. Juvenile leaves strongly incurved, to 100 µm long, broadly ovate, about 1.5: 1, serrulate across the acuminate apex. Axillary hairs fugaceous, to 35 µm, 2–3-celled with a single pale brown basal cell, thin-walled and of constant diameter throughout. 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 2–4 cells thick, but without an hyaloderm.
   Autoicous. Perigonia elongate, bud-like, produced near the perichaetia on prostrate stems and large branches. Perichaetial leaves to 1.5 mm long, sheathing at base but reflexed to squarrose in the rather broad acumen, with costa flattened and extending nearly to leaf middle, and with margin serrulate, 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, strangulate but not distorted or sulcate when dry. Urn to 1 mm, about 1.5: 1. Exothecial cells isodiametric, quadrate to hexagonal, not in regular rows, about 20 µm wide, 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 1/3 as long as urn. Annulus present, of two rows of thick-walled cells. Exostome teeth to 400 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, keeled but usually not perforate, about as long as exostome, with basal membrane about as long as the segments. Cilia 1–3, lightly papillose and smooth or nodose. Spores smooth or nearly so, 14–18 µm.

With B. leibergii, B. plumosum is relatively easy to recognise even in gross morphology. The leaves, moist or dry, at their insertion diverge at about a 60 degree angle from the stem and then they arc upward almost parallel to the stems. The leaves have a greater rigidity than most other members of the genus -- a function of the rather stout costa and somewhat thick-walled cells. These two species can then be distinguished by the constant presence of submarginal plicae in B. leibergii.

Elevation by latitude plot for Brachythecium plumosum
   in California

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