Plants erect in glossy red-brown tufts (sometimes rich green in shaded sites), to 6 cm high. Leaves evenly inserted along stem, not at all comose. Leaves somewhat concave and keeled, elliptic to elliptic-lanceolate, broadest near leaf middle, bluntly rounded to cucullate at apex, to 3 mm long, about 3: 1, erect when moist, not much changed when dry, not plicate. Median laminal cells 3–4: 1, elongate-hexagonal with rather thick walls (lumen: wall ratio 4–6: 1), completely smooth, not pitted, to 18 µm wide. Basal and alar cells to 25 µm broad, quadrate to short rectangular, 1–1.5: 1, in an abruptly defined patch, usually with bistratose areas. Margins not decurrent, erect throughout and nearly entire. Limbidium essentially absent but marginal cells somewhat elongate and narrowed. Costa reddened, especially at base, ending only a few cells below apex or percurrent. Costa cross-section elliptic, about 2: 1, with a well-demarcated abaxial stereid band and with two layers of rather small cells adaxial to the stereid band. Cells of axillary hairs almost uniform throughout, to 6 cells long, to 200 µm, with basal cells unpigmented and almost as long as the distal cells. Micronemata absent. Macronemata mostly smooth or with smaller branches papillose, to 30 µm wide at insertion, pale brown, closely branched, surrounding an isodiametric macronematal area. Stem cross-section rounded-pentagonal but ribbed by costa bases, with a strongly demarcated central strand, with rather pachydermous corticals and with an outer stereome only 2–3 cells thick between the ridges formed by the costa bases, never with an hyaloderm. Gemmae absent from our specimens.
Dioicous with male plants somewhat smaller than females. Perichaetial leaves similar but smaller relative to the vegetative leaves. Seta red-brown to brown, smooth, erect but geniculate distally, to 20 mm long. Urn with neck comprising about 1/4–1/3 of capsule length, to 3.5 mm long, cylindric, 3.5–4.5: 1, pendulose to pendent, neither sulcate nor strumose. Operculum short conic. Annulus inconspicuous, falling in fragments. Suboral exothecial cells to 25 µm broad, transversely elongate in several rows. Exothecial cells at middle of urn irregularly rectangular with curved lateral walls, 2–4: 1, to 20 µm broad, not in regular rows, evenly thick walled with lumen: wall ratio 2–4: 1. Stomates restricted to neck of capsule, not raised on pustules, elongate. Exostome yellow-brown, to 600 µm long, obviously shouldered, lightly and evenly punctate-papillose. Endostome with basal membrane constituting about 1/2 of total length of endostome. Segments smooth to lightly papillose, somewhat shorter than exostome, perforated as orbicular holes along keel, with 2–4 appendiculate cilia. Calyptra cucullate, mostly shed before capsule maturity. Spores to 18 µm, finely papillose.
|Literature||Bourell 1981; Crum and Anderson 1981; Flowers 1973; Harpel 1980a; Harthill et al. 1979; Holmberg 1969; Howe 1896; Jamieson 1969; Kellman 2003; Koch 1950a, 1951e; Koch and Ikenberry 1954; Lawton 1971; Lesquereux 1868; Lesquereux and James 1884; Long 1978; McGrew 1976; Mishler 1978; Showers 1982; Spence 1988a; Spjut 1971; Toren 1977; Watson 1880; Yurky 1990, 1995. As Bryum atwateriae Lesquereux and James 1884; Phelps 1878; Watson 1880.|
|Illustrations||Crum and Anderson 1981; Flowers 1973; Lawton 1971; Sharp et al. 1994.|
|Notes||Needs update to Imbribryum miniatum (Lesquereux) J. R. Spence.|
|Bioregions||CaR, CW, NW, SN, SNE, SW.|
|Vouchers||Alpine Co.: Red Lake Creek, Toiyabe National Forest, Norris 88124; Del Norte Co.: Smith River along Highway 199 about 9 miles east of junction with Highway 101, Six Rivers National Forest, Norris 85001; Mariposa Co.: base of Yosemite Falls, Yosemite National Park, Koch 1686 (UC); Napa Co.: Napa-Monticello Road, Koch 1367 (UC); Riverside Co.: Fleming Ranch Road, San Jacinto Mountains, San Bernardino National Forest, Harpel 2970 (pers. herb.); Santa Cruz Co.: Big Basin State Park, Kellman 694 (CAS) [determined by Spence]; Tulare Co.: between Redwood Crossing and Long Meadow, Wishon Fork Tule River, Golden Trout Wilderness, Sequoia National Forest, Shevock 10609.|