14. Schistidium frigidum H. H. Blom, Bryophyt. Biblioth. 49: 181, fig. 70. 1996.
Plants in open, occasionally flattish tufts or cushions, oliva-ceous, often in part yellow-green, rarely blackish. Stems 1-2(-4) cm, central strand distinct or absent. Leaves erect or slightly curved, usually imbricate when dry, narrowly ovate-lanceolate to ovate-lanceolate, sharply keeled distally, 1.1-2.1 mm, 1-stratose, rarely with 2-stratose striae distally; margins recurved to just before the apex, often denticulate distally, 1- or 2-stratose; apices acute or somewhat obtuse; costa excurrent as a coarsely denticulate, often slightly flexuose, usually decurrent awn, rarely percurrent (in alpine habitats), smooth; some basal cells sometimes much lighter than adjacent cells, often hyaline, sometimes with yellowish walls, marginal cells quadrate or elongate-rectangular, often strongly trigonous or with more thickened transverse walls; distal cells often of various shapes, short-rectangular or isodiametric, sometimes ovate or sub-triangular, 6-10 µm wide, usually strongly sinuose. Sexual condition autoicous. Capsule red-brown, sometimes orange-brown, cylindric, 0.7-1.1 mm; exothecial cells irregularly shaped, isodiametric or oblate, thin-walled or slightly thickened, trigonous; sto-mata present; peristome patent to squarrose, sometimes revolute, 230-300 µm, orange-red or yellowish red, densely to finely papillose, weakly perforated. Spores 10-14 µm, granulose or nearly smooth.
Capsules mature late spring to early summer. Rock in open to shaded habitats; moderate to high elevations (500-3300 m); Greenland; Alta., B.C., Nfld. and Labr. (Nfld.), N.W.T., Nunavut, Ont., Que., Yukon; Alaska, Colo., Idaho, Mich., Mont., Nev., N.Mex., Oreg., Wash., Wyo.; Eurasia.
Schistidium frigidum is rather common yet is one of the more difficult taxa to determine with confidence, as the examination of many leaves is often necessary before the elongate basal marginal cells are obvious. Often, some central basal cells are much lighter than adjacent cells, and are often hyaline, and form contrasting patches, which helps to identify the species. In contrast to S. dupretii and S. confertum, which have a large proportion of elongate exothecial cells, S. frigidum has exothecial cells that are mainly isodiametric or oblate.