Plants growing in turf or cushions, light to dark or olive green distally, light to dark brown basally. Stems to 2.7 cm, branching often; pentagonal to rounded-triangular in transverse section, hyalodermis usually absent, sclerodermis absent or occasionally present, central strand usually present, weak; sparsely radiculose; axillary hairs 3-10 cells in length, the basal 1-2 usually brownish. Leaves appressed to appressed-incurved when dry, weakly spreading to spreading recurved when moist; usually long-ligulate, occasionally ovate to circular, adaxial surface flat to broadly convex, 0.3-1.1(-2) mm; base scarcely differentiated to ovate, margins of leaf base often denticulate; distal margins plane or occasionally recurved below mid leaf, entire or minutely crenulate by projecting papillae, in patches sometimes 2-stratose marginally beyond mid leaf; apex rounded-obtuse to broadly acute, sometimes apiculate; costa percurrent or ending 2-5 cells before apex, costal adaxial cells quadrate or shortly rectangular (longer in much reduced plants) in 2-4 rows, abaxial cells elongate, occasionally shortly rectangular to quadrate distally, transverse section of costa ovate to semicircular, adaxial epidermis present, adaxial stereid band weak or lacking, guide cells 2(-4) in 1 layer, hydroid strand absent, abaxial stereid band present, often weak, crescent-shaped to round in section, abaxial epidermis occasionally little differentiated; basal cells differentiated across leaf or rising higher medially, rectangular, little wider than medial leaf cells, 2-4:1, walls of basal cells thin; distal laminal cells subquadrate, 7-11 µm; 1:1; papillae simple to 2-fid, low, small but crowded, scattered, 3-5 per lumen, cell walls thin to weakly and evenly thickened, flat to convex on both sides. Specialized asexual reproduction when present by spherical to obovoid or spindle-shaped gemmae, usually of 5-10 multiseriate cells, borne on branching stalks in leaf axils. Sexual condition dioicous; perigonia gemmate, terminal or lateral; perichaetia terminal or lateral on short branches or both, inner leaves little different from the cauline though commonly broadened below mid leaf, to sheathing and ovate-lanceolate or triangular, sometimes marginally serrulate, to 2 mm, laminal cells little different from those of cauline leaves though rectangular and occasionally bulging below mid leaf to mostly rectangular or rhomboidial and smooth or weakly papillose throughout. Seta yellowish to reddish brown in color, generally 0.3-0.6 cm, twisted clockwise. Capsule yellowish to reddish brown, ovoid to elliptical, often with a weak circumstomal collar, (0.4-)1.4-1.8 mm, exothecial cells quadrate to rectangular or bulging, walls thin to somewhat thickened, annulus of 1-3 rows of smaller, transversely elongate cells; peristome teeth absent; operculum rostrate to conic-rostrate, ca. 0.4-0.5 mm, cells straight. Calyptra cucullate, 0.5-1.2 mm, smooth. Spores 11-16 µm, smooth to papillose, brownish. KOH laminal color reaction yellow to yellow-orange.
Species ca. 22 (3 in the flora): worldwide except Antarctica.
Gymnostomum is a calcicole of moist environments. It differs from the similar Gyroweisia largely by the small annulus (though commonly differentiated as a circumstomal ring), basal leaf cells differentiated less strongly) and the perichaetial leaves only slightly larger or smaller than the cauline (not distinctly much larger). Barbula convoluta is similar, but occurs in dry soil, fields, and rarely rock fissures, the adaxial surface of the leaf is keeled, and the costa has elongate adaxial cells. The genus Gymnostomum needs revision as many species worldwide are of doubtful distinction or belong in other genera. Those of the flora area are, however, well placed though closely related. Hymenostylium differs in lacking a stem central strand in the area of the flora, leaves always lanceolate to ligulate, never short-elliptical or ovate, the leaf margins commonly recurved on only one side (often strongly so), the adaxial costal cells smooth, and the laminal papillae usually simple and scattered, less obscuring of the cell lumen than those of Gymnostomum.