Plagiotheciella pilifera (Hartman) Brotherus
Plants light- to yellowish-green, glossy, usually in dense mats. Stems to 60 × 1--1.5 mm, prostrate, complanate-foliate to subjulaceous. Leaves imbricate, smooth, usually concave, symmetric, 0.8--2.0 × 0.4--0.8 mm, oblong-ovate, abruptly contracted to a long-filiform, flexuose acumen, sometimes 1/3 length of leaf, margins usually narrowly recurved nearly to apex, entire or minutely serrulate at apex; costa short and double, ending a short distance above leaf base, sometimes lacking; leaf cells smooth, walls of basal cells pitted; median cells 36--96 × 3--5 μm; decurrent alar region triangular in outline, consisting of 2--4 vertical rows of rectangular cells, 16--75 × 7--20 μm, terminating at the base in a single cell. Specialized asexual reproduction unknown. Sexual condition autoicous, often fruiting. Seta yellow to red, straight, 0.8--1.5 cm. Capsule light brown to yellowish or reddish brown when mature, erect or sometimes slightly inclined, straight, 1--4 × 0.5--1 mm, smooth or slightly wrinkled when dry, strongly wrinkled at neck, usually contracted below mouth; operculum conic, 0.4--0.7 mm; cilia usually lacking, or 1--2 rudimentary, fragile cilia present. Spores 9--13 μm.
Capsules mature spring--summer. Common in coniferous or alder-maple woods, trees (usually alder, rarely maple), rotten logs, noncalcareous cliffs and boulders; sometimes on wood in swampy areas; 30--1500 m; Greenland; B.C., N.W.T., Que.; Alaska, Calif., Idaho, Oreg., Mont., Wash.; Europe; Asia.
Plagiothecium piliferum is very distinctive. Although its main center of distribution is in the Pacific Northwest, it has disjunctive occurrences in northern Quebec (James Bay region) and southwestern Greenland. The concave, symmetric leaves with abruptly contracted, long-filiform, flexuose acumens, sometimes 1/3 the leaf length, and the erect, straight capsules make identification easy. A scanning electron microscope study of the spores of Plagiothecium (R. R. Ireland 1987) revealed that the sculpturing can be used to distinguish P. piliferum from the other North American species. Spores of P. piliferum have branched-baculate ornamentation, the elements are covered with conic-papillae, while the other species have gemmate ornamentation, the elements covered with high-conic to low, rounded papillae.