Plants erect and essentially unbranched, green to rather deep green, mostly with reddish stems, scattered among other mosses or in tufts, to 4 cm high. Lower leaves scale-like, loosely appressed and distant; upper leaves spatulate-elliptic to obovate with rounded but often mucronate apices, to 6 mm long, about 2: 1, broadly spreading when moist, only slightly contorted when dry. Median laminal cells smooth, to 35 µm wide near the costa but gradually smaller toward the margins, isodiametric with angular lumens, with walls rather thick and pitted. Marginal cells differentiated into a strongly defined limbidium of 4–6 layers of linear and thick-walled cells. Basal and alar cells more elongate than median cells but not otherwise differentiated. Margins plane, entire throughout, short decurrent. Costa percurrent in a short mucro or ending near the apex. Costa cross-section with ad- and abaxial cells stereidal to substereidal and with internal cells large and leptodermous except for a group of central cells which resemble a central strand. Axillary hairs to 5 cells long, to 150 µm, with one basal brown cell, not offset from leaf insertion. Rhizoids dense at least near base of plant, smooth, red-brown, repeatedly branched, surrounding a macronematal area which is only 1.5–2 times as long as broad. Micronemata absent. Stem cross-section with a well-defined central strand, leptodermous inner corticals and several layers of strongly pachydermous, red brown outer cortical cells.
Dioicous with male plants somewhat smaller than females, bearing terminal perigonia. Perichaetia terminal with bracts somewhat smaller than vegetative leaves. Seta yellow to yellow brown, smooth, erect, to 4 cm long. Urn to 4 mm long, about 3: 1, yellow brown but dark brown near base, inclined to pendent, not at all sulcate or strumose. Operculum conic. Annulus well-defined, revoluble. Exothecial cells near capsule mouth to 15 µm broad, isodiametric with rounded lumens in up to 4 rows. Exothecial cells at mid capsule hexagonal, to 15 µm broad, to 2: 1, with walls very thick (lumen: wall ratio 0.25–1: 1). Stomates primarily on neck, cryptoporous. Exostome pale yellow to yellow brown, to 600 µm long, undivided, finely papillose with papillae in close set horizontal rows. Endostome about as long as exostome with basal membrane comprising about 1/2 its length and with segments remotely papillose, open along keel and with 2–3, nodulose cilia. Calyptra cucullate, smooth. Spores to 40 µm, smooth or nearly so.
This is one of the commonest mosses on moist, rotting logs in the humid forests of northern California. It is the only Rhizomnium known from near sea level elevations in the state. It is especially abundant in the northern Coast Ranges, but it also occurs, though sparingly, in the Sierra. It is endemic to western North America with distribution from the Alaskan archipelago to central California. It is easily distinguished from other species of Rhizomnium by the very strong border which is 4–6 cells thick near mid-leaf, and by the strong costa which is percurrent in a mucro on at least some leaves in a clone.
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