Rev. Bryol. 34: 100, figs. 1–9. 1907 (as nicholsoni),.
Plants green to dark green, usually with a reddish cast. Stems to 2 cm, central strand present. Stem-leaves appressed to weakly spreading when dry, spreading and not keeled when moist, monomorphic, long-ovate or occasionally ovatelanceolate, rarely lanceolate, grooved adaxially along the costa, especially near leaf apex, to 3.5 mm, base scarcely differentiated to ovate or oblong in shape, margins narrowly recurved in proximal 3/4 of leaf, entire, apex acute to blunt, not fragile; costa percurrent or ending before the apex, not strongly spurred, not much widened or tapering, lacking an adaxial pad of cells, adaxial costal cells quadrate to shortrectangular, ca. 6 cells wide at mid leaf, guide cells in 1 (–2) layers; basal laminal cells very weakly differentiated medially, walls thin to weakly thickened, quadrate to shortrectangular, not perforated; distal laminal cells 7–9 µm wide, 1: 1, nearly smooth or papillae simple or 2-fid, 2–3 per lumen, lumens roundedquadrate, walls thickened, weakly convex on both sides of lamina, 2-stratose in distal leaf half or occasionally only in patches or very rarely 1-stratose, cells of distal leaf margins 2-stratose or occasionally entire leaf 2-stratose distally. Specialized asexual reproduction absent. Seta ca. 1.2 cm. Capsule 1.5–2.1 mm; peristome teeth 32, linear, twisted 1/2 times to once, to 1000 µm. Spores 11–13 µm. Distal laminal KOH reaction light to dark redbrown, occasionally deep red-orange.
Phenology: Capsules mature spring–summer.
Habitat: Wet rocks, quartzite, wet silty sand, stream bank, canyon walls, streamside, chaparral
Elevation: low to high elevations (50-1900 m)
B.C., Calif., Mont., Nev., Oreg., Utah, Europe, Asia (Afghanistan)
Didymodon nicholsonii intergrades somewhat with D. vinealis but the ovate-lanceolate leaves are distinctive as is the tendency to a partially or completely 2-stratose distal lamina. It may be confused with D. rigidulus but has a broader leaf apex, the costa commonly ending before the apex, a deep apical groove over the costa, and 2-stratose distal marginal cells often in a narrow band. The western species Grimmia cinclidontea Müller Hal. is similar and grows in similar habitats, but is autoicous, has smooth leaf cells and a homogeneous costal section. S. Flowers’s (1973) illustration of D. rigidulus is actually of D. nicholsonii.
"um" is not declared as a valid unit of measurement for this property."um" is not declared as a valid unit of measurement for this property.