Plants green, often blackish. Stems to 2 cm, central strand present. Stem leaves appressed-incurved to weakly spreading when dry, spreading or not and not keeled when moist, monomorphic, long-ligulate, ovate-lanceolate, elliptic-lanceolate or long-lanceolate, broadly channeled across leaf or occasionally somewhat channeled along costa, 0.8-1.7(-3) mm, base scarcely differentiated to ovate, margins recurved in proximal 1/4-3/4, entire, apex acute to subulate, occasionally fragile; costa percurrent to excurrent as a long, thick, blunt subula, not much widened or tapering through the leaf, not strongly spurred, lacking a bulging adaxial pad of cells, adaxial costal cells quadrate, in (2-)3-4(-5) rows, guide cells in a single layer; basal laminal cells weakly differentiated medially, rectangular, walls thin to evenly thickened, proximal maginal cells little differentiated; distal laminal cells mostly 8-11 µm wide, 1:1, papillae absent or simple or occasionally 2-fid, lumens oval to rounded-quadrate, walls evenly thickened, moderately bulging on both sides or only abaxially, sometimes 2-stratose marginally or at apex of leaf, or throughout lamina. Specialized asexual reproduction by axillary, ovate to elliptic, multicellular gemmae. Seta 0.7-1.7 cm. Capsule 1-2 mm; peristome teeth 32 or 16 cleft to base or rudimentary or occasionally absent, filamentous or long-triangular, straight or weakly twisted, to 740 µm. Spores 9-12 µm. Distal laminal KOH reaction yellow- or red-orange.
Varieties 6 (5 in the flora): North America, Mexico, s South America, Eurasia, n Africa.
Didymodon rigidulus in the broad sense, as emended by R. H. Zander (1981b) is polymorphic, with several varieties distinguished by fairly good correlations of combinations of characters. Specimens of intermediate morphology that are not clearly assignable to any one variety may be identified as D. rigidulus in the broad sense. Although some authors use the presence of axillary gemmae as diagnostic of the typical variety, other varieties, notably var. gracilis, may occasionally have them. Such gemmae are also found in other species, especially those of the D. vinealis complex. Didymodon vinealis may have 2-stratose distal laminal cells, and should be carefully distinguished.
Stem leaves appressed to weakly spreading when dry, costa usually percurrent to short-excurrent as a rigid subula, short-lanceolate, seldom-lanceolate, base evenly broadened, ovate to elliptic or oblong, proximal cells short-rectangular, distal laminal cells usually papillose, quadrate, with quadrate lumens, occasionally 2-stratose on the margins in patches; gemmae often present in distal leaf axils. Peristome teeth short and straight to long and twisted.
Capsules mature spring-winter. Basalt, calcareous outcrops and ledges, gravel, soil, silt, frostboil, tundra, along roads and paths; low to high elevations (0-3000 m); Greenland; Alta., B.C., Man., Nfdl. and Labr., N.W.T., Nunavut, Ont., Que., Sask., Yukon; Alaska, Ariz., Colo., Iowa, Kans., Minn., Mo., Mont., Nev., N.Mex., S.Dak., Tenn., Tex., Utah, Wis., Wyo.; Mexico; Central America; South America; Europe; Asia (Siberia); Africa; Atlantic Islands (Iceland).
Variety gracilis may sometimes have gemmae and the distal lamina is occasionally 2-stratose in patches, but it differs from var. rigidulus most clearly in its the short- to long-lanceolate leaves. The distal laminal cells are commonly papillose, and their lumens are oval or rounded-quadrate. Because of intergradation, some collections must be assigned to this variety only on the basis of a majority of the characters given in the key. Although leaves in this variety are short in dry habitats, associated with fragile stems, leaf length in collections from moist environments is generally longer. The small distal laminal cells, broad costa to 6 cells in width at mid leaf, and the strong stem central strand serve to distinguish this taxon from Didymodon asperifolius.
Stem leaves appressed to weakly spreading when dry, spreading when moist, oblong-lanceolate, elliptic-lanceolate or long-triangular, base not or weakly broadened, oval; costa usually short-excurrent and blunt; proximal cells short-rectangular or quadrate; distal laminal cells smooth to weakly papillose, quadrate, with quadrate lumens, distal lamina 2-stratose at apex or along margins; gemmae often present in distal leaf axils. Peristome teeth short and straight to very weakly twisted.
Variety rigidulus is relatively uniform in eastern North America, with slightly spreading, oblong-lanceolate to long-triangular leaves with thickened distal margins, percurrent or short-excurrent costa, thick-walled cells, gemmae usually present, and the peristome straight or only weakly twisted. When the distal margins are not 2-stratose or only slightly so, it can be difficult to identify, especially as it may occasionally have the costal groove of D. vinealis, and large specimens may similarly have guide cells in two layers. It intergrades in the West with the other varieties, and propagula are commonly absent. The typical variety, with distinctive oblong-lanceolate leaf shape and propagula, is rare in the Arctic, and though generally hygrophilic may be found on merely moist substates in southern states. From D. vinealis in the narrow sense, this variety may be distinguished by the combination of stiff, long-triangular or oblong-lanceolate leaves, usual presence of many gemmae in the leaf axils and the commonly yellow-green color in water (sometimes blackish green, rarely reddish below) and yellow or yellow-orange color in KOH. Commonly misidentified as this variety is the western D. tectorum, which also has axillary gemmae, but differs by the costa narrowly excurrent from a blunt leaf apex, and a rectangular (not elliptical) leaf base.