Orthotrichum tenellum var. pumilum (Swartz) Boulin
Plants to 5 mm. Stem leaves loosely-erect when dry, ovate-lanceolate to lanceolate, 1.8--2.8 mm, sharply to slenderly acute, most leaves ending in 1--2 celled apiculus, rarely obtuse; margins revolute nearly to apex, entire; distal laminal cells 14--20 µm wide, 1-stratose, smooth or with 1--2 small, conical papillae; basal laminal cells rectangular, grading to quadrate at margin, non-nodose. Sexual condition autoicous. Seta less than 0.5 mm. Capsule immersed to 3/4 emergent; urn oblong to oblong-cylindric, 1.2--1.5 mm, strongly 8-ribbed; stomates immersed, at middle of capsule, 1/2 or less covered by subsidiary cells; peristome double; endostome segments 8, of 1--2 rows of cells, smooth; exostome teeth 8, rarely splitting to 16, reflexed, finely and evenly papillose. Calyptra oblong, sparsely hairy or rarely naked. Spores 14--21 µm.
Trunks, lower branches, and bases of deciduous trees, rarely on coniferous trees or in crevices of rocks; common in open, hardwood forests and on trees along roadsides; 10--1000 m; Alta., Man., Ont., Sask.; Ariz., Calif., Colo., D.C., Idaho, Ill., Iowa, Kans., Md., Mass., Mich., Minn., Mis., Mo., Mont., Neb., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Okla., Pa., S.C., S.Dak., Tenn., Texas, Utah, Vt., Va., Wash., W.Va., Wisc.; Europe; n Africa.
Orthotrichum pumilum can usually be distinguished by its strongly ribbed capsules, reflexed peristome teeth, apiculate leaves with cells larger than 14 µm, and papillae that are very small or absent. In eastern North America this is a uniform, non-variable species, but in the West the taxon is much more variable. Many western specimens have capsules that are oblong-cylindric, often strangulate, over 1/2 emergent, and gradually contracted to the seta.