Bryol. Eur. fasc. 48. 1851. -- Leskea pilifera Sw. ex Hartm., Skand. Fl. 419. 1820. -- Isopterygium piliferum (Hartm.) Loeske, Stud. Morph. Syst. Laubm. 169. 1910. -- Plagiotheciella pilifera (Hartm.) Fleisch. ex Broth. in Engl. & Prantl, Nat. Pfl. ed. 2. 11: 466. 1925. -- Dolichotheca pilifera (Hartm.) Fleisch. ex Podp., Consp. 683. 1954. -- Hypnum denticulatum (Hedw.) B.S.G. var. piliferum (Hartm.) Wahlenb., Fl. Suec. 2: 710. 1826.
Plants prostrate to ascending, mostly in very dense mats, glossy light green to whitish green, mostly rather sparingly branched. Leafy axes to 8 cm long, little or not at all tapering near the branch tips, complanate to almost terete foliate and julaceous. Leaves imbricate, often strongly so, to 1.8 mm, 3–4: 1, concave and symmetrical, ovate to oblong-ovate, abruptly contracted into a filiform and flexuose awn. Margins nearly entire but mostly with a few light serrulations near apex, narrowly recurved nearly throughout. Decurrency distally 2–3 cells wide but only one cell wide over most of its length. Median cells smooth, to 7 µm wide, linear, up to 20: 1, thin walled, not pitted. Alar region rather well demarcated with its cells thinner-walled and shorter, somewhat inflated relative to the cells of the median lamina. Basal cells only gradually demarcated, often pitted, shorter and thicker-walled than median cells. Costa short and double with the two branches unequal. Pseudoparaphyllia deltoid to lanceolate, often strongly dentate to lobed, spreading when well-developed. Cortical cells of stem broad in surface view thin-walled, rectangular, 3–8: 1; to 20 µm wide. Axillary hairs 4-celled, to 150 µm with a single short basal brown cell, not offset from leaf insertion. Stem cross-section with an obscure central strand, with a markedly enlarged hyaloderm relative to the adjacent several layers of small somewhat pachydermous cells, and with inner cortical cells enlarged with thin and hyaline walls. Rhizoids sparingly produced from a cluster of short rhizogons immediately below and medial to the leaf insertion, red-brown, mostly unbranched, to 15 µm in basal diameter. Gemmae and propagules seemingly absent from California specimens.
Autoicous. Perichaetia in axils of stolons, somewhat shorter than the subtending leaves. Seta to 1.5 cm long, erect, light brown to red-brown when mature. Capsule brown, erect to inclined, somewhat arcuate, smooth to lightly wrinkled, markedly strangulate at mouth. Urn 2–3 mm long, 2.5–3: 1. Exothecial cells at mid urn short rectangular and rather thin-walled, 1.5–2.5: 1, in regular rows, not collenchymatous, to 20 µm broad. Exothecial cells near mouth of capsule not strongly differentiated, with walls thicker and more pigmented than those of median cells. Stomata phaneroporous, often abortive or poorly developed, restricted to neck. Annulus present, not revoluble. Operculum short-rostrate, about 1/4 as long as urn. Exostome to 400 µm long, narrowly linear with a broad median commisural line, smooth or with a few inconspicuous horizontally striae below, papillose above, light-brown. Endostome segments lightly papillose, narrowly perforate, about as long as exostome with a low basal membrane occupying about 1/5 of total length. Cilia absent to rudimentary. Spores smooth, to 15 µm.
This is a plant largely of mountainous regions of western North America and Eurasia. Throughout its range it is exclusively epipetric and is of restricted occurrence. In western North America, P. piliferum ranges from southern Alaska to northern California, and it extends inland in favorable places to Idaho and Alberta. In California, this moss occurs mostly on granitic rocks in medium to high elevation forests where it especially occupies the ceilings of overhangs, sites which receive no direct rainfall.
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