Psilopilum Bridel, Bryol. Univ. 2: 95. 1827.
[Greek psilos, naked, and pilon, hair, referring to the naked calyptra]
Gary L. Smith Merrill
Plants small, not polytrichoid, gregarious, often in extensive colonies on soil. Stems unbranched. Leaves only slightly sheathing at base, distinctly concave to channeled, cucullate, the apex obtuse or apiculate; lamina broad and 1-stratose, margins entire or bluntly and irregularly toothed distally by projecting ends of rhomboidal marginal cells, forming an indistinct border; costa smooth at back or roughened near apex, rarely with a few low adaxial lamellae; lamellae restricted to costa, transversely undulate, margins entire or incised and irregularly dentate, marginal cells in section undifferentiated, smooth. Sexual condition dioicous; male plants smaller, perigonia conspicuous, cupulate, bracts broadly ovate, flaring at apex; perichaetial leaves erect, lanceolate, often much longer than stem leaves. Capsule moderately to strongly curved and cylindric to gibbous and asymmetric, wrinkled when dry, not angled, tapering to base; exothecium smooth, exothecial cells short to longitudinally elongate, often with thickened radial longitudinal walls, no thin spots or pits; stomata superficial, at base of capsule; operculum rostrate; peristome deeply divided to below capsule rim, the teeth 32, linear, crowded, at times poorly formed and irregular in shape. Calyptra smooth or roughened at tip. Spores large (20--25 µm), finely papillose.
Species 2 (2 in the flora).
This is a small genus of pioneer mosses on soil in disturbed or otherwise unstable, non-calcareous sites, in Arctic and subarctic North America and Eurasia. The closest affinities of Psilopilum are with Atrichum, as indicated by the tendency towards bordered leaves (P. laevigatum), finely longitudinally striate exothecium, and a similar peristome of 32 linear, crowded teeth. Sporophytes are commonly produced in both species. The best recent treatment is that of D. G. Long (1985), who examined an extensive series of collections from Arctic America.
Long, D. G. 1985. Psilopilum. In: Mogensen, G. S. (ed.), Illustrated Moss Flora of Arctic North America and Greenland. Fasc. 1. Polytrichaceae. Meddel. Grönland, Biosci. 17: 10--14.