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Rhizomnium nudum (Britton & R. S. Williams) T. Koponen [Mniaceae]

Plants erect and essentially unbranched, green to rather deep green, mostly with reddish stems, mostly in dense turfs, to 5 cm high. Lower leaves scale-like, loosely appressed and distant; upper leaves obovate to orbicular with rounded or rarely apiculate apices, to 8 mm long, 1.2–1.8: 1, broadly spreading when moist, somewhat contorted when dry. Median laminal cells smooth, to 75 µm wide near the costa but gradually smaller outwardly and becoming only 25 µm near the margins isodiametric with angular lumens, with walls rather thick but not pitted. Marginal cells differentiated into a strongly defined limbidium of 3–5 layers of linear, thick-walled and pitted cells. Basal and alar cells more elongate and rectangular than median cells but not otherwise differentiated. Margins plane, entire throughout, narrowly decurrent. Costa percurrent in a short mucro or ending at the apex. Costa cross-section with central cells stereidal but surrounded on both sides by large and leptodermous outer cells in several layers. Axillary hairs to 5 cells long, to 300 µm, with three heavily pigmented basal brown cells of narrow diameter but with more distal cells vesicular and elongate. Rhizoids to 30 µm wide at insertion, essentially restricted to base of plant, papillose, red-brown, repeatedly branched, surrounding a macronematal area placed in the axils of lower leaves and only 1.5–2 times as long as broad. Micronemata absent. Stem cross-section ridged-pentagonal with a well-defined central strand, leptodermous inner corticals and mostly a single layer 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 red 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 700 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 papillose, open along keel and with 2–3, nodulose cilia. Calyptra cucullate, smooth. Spores to 35 µm, smooth or nearly so.

Mail a correction to Paul Wilson ·
LiteratureChristy and Wagner 1996.
NotesDistribution map in Christy and Wagner (1996) displays range into California, however, no collections of this species have been located even after extensive surveys. Not yet found in CA but known from OR.
   

Elevation by latitude plot for Rhizomnium nudum
   in California

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