Orthotrichum inflexum J. K. A. Müller
Plants 3--14 mm. Stem leaves erect-appressed when dry, ovate to ovate-lanceolate, concave, 1.4--2 mm, obtuse to blunt; margins erect-incurved, entire; distal laminal cells 13--18(--20) µm wide, 1-stratose, unipapillose on both surfaces; basal laminal cells ± rectangular. Sexual condition dioicous. Seta ± 0.5 mm. Capsule ± 1/2 emergent; urn oblong-cylindric, 1.6--2 mm, strongly 8-ribbed to base; stomates superficial at or above middle of capsule; peristome double; endostome segments 8, composed of 2 rows of cells; exostome teeth 8, reflexed, evenly papillose, sometimes irregularly striate above. Calyptra short-conic, naked. Spores 20--25 µm.
Deciduous trees in open areas; in the Northeast, common on the trunks of Populus; in the West, sporadic on trunks of Juniperus, base of scrub-oaks in dry cut-over areas, and trunks of Populus balsamifera in river bottoms; in the southern boreal forest found commonly on aspen trunks; a pronounced xerophyte growing only in pioneer situations; 10--2000 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Sask., Que., Yukon; Alaska, Ariz., Calif., Colo., Idaho, Iowa, Maine, Md., Mich., Minn., Mont., Neb., N.H., N.Mex. N.Y., N.C., N.Dak., Oreg., Pa., S.Dak., Utah, Vt., Wash., Wis., Wyo.; Europe; Asia.
Species that might be confused with this are O. flowersii, O. exiguum, and O. gymnostomum. None of these species has the distal leaf cells with one papilla per cell as does O. obtusifolium. In addition, O. flowersii and O. exiguum have reflexed to revolute leaf margins and both are autoicous. Orthotrichum gymnostomum has distinctly involute leaf margins and like O. obtusifolium is dioicous.