Plants in dense patches, dark green, brown-green, sometimes almost black. Stems 0.5--1 cm, central strand strong. Stem leaves lanceolate to ovate lanceolate, 1--1.8 × 0.2--0.6 mm, keeled, not plicate, one margin commonly recurved, sometimes both plane, awn 0.3--0.8 mm, costal transverse section prominent, terete; distal laminal cells 1-stratose, often bulging, marginal cells 2-stratose, often bulging; medial laminal cells quadrate, sinuose, thick-walled; basal juxtacostal laminal cells rectangular to elongate, straight, thin-walled; basal marginal laminal cells short- to long-rectangular, straight, thick transverse and thin lateral walls, not hyaline. Perichaetial leaves not enlarged. Sexual condition cladautoicous. Seta straight, 1--2.5 mm. Capsule usually present, exserted, yellow, ovate to elliptic, exothecial cells short-rectangular, thin-walled, stomates present, annulus of 1 row, quadrate, thick-walled, operculum conic, usually with a short obtuse beak, peristome present, fully developed, split or solid in distal half.

Exposed or sheltered, moist, acidic granite and sandstone, alpine; 1100--3900 m; Greenland; Alta., B.C., Labr., N.W.T., Quebec, Yukon; Alaska, Ariz., Calif., Col., Idaho, Mont., N.H., N.Y., Nev., Oreg, Wash., Wyo.; South America (Argentina, Chile); Eurasia; Africa (Kenya, South Africa, Uganda); Antarctic.

Grimmia sessitana is widely distributed in high elevation sites throughout western North America, principally in mountains forming the continental divide, but also in the northern Sierra Nevada and the coastal range of Oregon and Washington. It is rare in eastern North America, but occurs in the northern Appalachians of the United States, on the Gaspe Peninsula of Quebec, and in Labrador. This is the least xerophilous member of the group; specimens have been collected in runoff zones from late-lying snow patches in the alpine. It is very rare in the dry interior mountains of the American southwest. Grimmia sessitana is gametophytically variable and often difficult to identify with certainty. The presence of stomates, and the autoicous condition, separate this species from both G. montana and G. alpestris, which are the most common misidentifications. Confirming the presence or absence of stomates requires careful dissection of the capsule, including the neck, as the stomates are often just above the attachment of the seta. Sexuality can be difficult to determine, as G. sessitana is cladautoicous. If the leaf margin is recurved this will confirm the identification. However, some specimens may have plane margins. The presence of bulging-mammillose cells should separate G. sessitana from G. montana, but a significant number of specimens of G. sessitana have only weakly bulging laminal cells. Thus, specimens with capsules and antheridia are usually identifiable, while sterile specimens may be problematic. The other species that is often confused with G. sessitana is G. donniana. Both species have capsules with stomates and are autoicous. However, G. donniana very consistently has leaves with plane margins, a uniform basal areolation with hyaline, narrowly rectangular thin-walled basal cells, and its laminal cells do not bulge. Grimmia sessitana appeared in H. A. Crum and L. E. Anderson’s (1981) flora of Eastern North America as Grimmia tenerrima. E. Lawton (1971), who accepted autiocous specimens in the concept of G. alpestris, excluded G. sessitana from the flora of the Pacific Northwest. J. Muñoz (1998b) put G. sessitana in synonymy with G. reflexidens. However, E. Maier (2002) has examined type material of G. reflexidens and found it to be an unknown species of Coscinodon. The earliest valid name for this species is thus G. sessitana.