Plants: elliptic to globose, ca. 2 mm. Stem: leaves cuspidate, broadly channeled; laminal cells smooth. Seta: short, about 0.3 the diameter of the capsule. Spores: spheric to elliptic, 30–50 µm, papillose or nearly smooth. Temperate areas of the Northern Hemisphere, Africa.
Varieties 2 (2 in the flora). As H. A. Crum (1969) and A. J. Grout (1939) pointed out, American specimens reported as Acaulon rubrum are A. muticum var. rufescens. These plants are often small, rather three-sided and may be confused with A. triquetrum. The leaves of both varieties stain red, at least in blotches, in KOH, and they intergrade to some extent.
Plants: yellow-brown in nature. Spores: densely low spiculose-papillose. Phenology: Capsules mature late fall to spring.
Soil, among grasses, pastures. low to moderate elevations. Calif., Iowa, Kans., Mass., N.Y., N.C., Okla., Tenn., Tex., Europe, Asia, Africa.
It may be suspected that young plants of both var. muticum and var. rufescens sometimes have smooth, somewhat elliptic spores. A collection from Texas (Bastrop County, Bastrop, F. McAllister, Feb. 1934, A. J. Grout, North American Musci Perfecti 258, UBC) has spheric, heavily papillose spores, 40–45 µm. Specimens with partially smooth spores but a few large granules adherent or scattered through the spore sac (e.g., Oregon, Lane County, Eugene, Alton Baker Park, D. Wagner 1834, Mar. 9, 1978, UBC) are here placed tentatively with the typical variety. A specimen from Iowa (Poweshiek County, Conard & Peck v.11.35, MO) has leaves blotched red in KOH and, variably among capsules, weakly papillose to distinctly crowded-spiculose spores.
A specimen of var. rufescens from New Jersey (Austin 51, UBC) is reddish and has small (ca. 20 µm), round, weakly papillose spores; generally, spore size is not a good character. Another specimen, with large (38–40 µm), round, punctulate spores, is piebald with adherent granules similar to the papillae of var. muticum. One specimen placed here (California, Santa Clara County, Stanford University, W. Schofield 5724, Feb. 23, 1955, UBC), is rather immature, has a greenish cast and smooth, elliptic spores about 28-30 µm.