32. Orthotrichum pusillum Mitten, J. Linn. Soc., Bot. 8: 25. 1864.
Orthotrichum psilocarpum Sullivant
Plants 0.2-0.4 cm. Stem leaves loosely appressed when dry, ovate-lanceolate to oblong-lanceolate, 1-2.2 mm; margins revolute nearly to apex, entire except for denticulate apex on some leaves; apex narrowly obtuse to broadly acute, sometimes apiculate; basal laminal cells broadly rectangular, walls thin, not nodose; distal cells 9-14 µm, 1-stratose, papillae 2-4 per cell, conic, small. Specialized asexual reproduction by gemmae on abaxial surface of leaves. Sexual condition autoicous. Seta to 1 mm. Capsule immersed to 1/2 emergent, oblong to ovate-oblong, 0.8-1.4 mm, smooth when dry, wrinkled to slightly 8-plicate when old, constricted below mouth when dry; stomata immersed; peristome single; prostome absent; exostome teeth 16, erect or recurved with age, finely papillose, rarely striate at apices; endostome segments usually absent, or 8, rudimentary, of 1 row of cells, smooth. Calyptra oblong, smooth, naked. Spores 13-20 µm.
Trunks of deciduous trees in open, dry woods, roadsides, on Juniperus along streams; low to moderate elevations (10-1000 m); Ala., Ark., Conn., D.C., Fla., Ga., Ill., Ind., Kans., Ky., La., Md., Mass., Mich., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., S.C., Tenn., Tex., Va., W. Va.
Orthotrichum pusillum is distinguished by its smooth, oblong capsules sharply contracted to the seta, 16 exostome teeth erect or recurved when old, some leaves of each plant having 1-4 small denticulations at the apex, and distal laminal cells 9-14 µm. The laminal cells are smaller than those of O. pumilum and in longitudinal rows, but they are larger, with thinner walls, than those of O. ohioense. Orthotrichum pusillum has a denticulate leaf apex that may be narrowly obtuse or acute, and laminal cells 9-14 µm with thin walls; O. pumilum has an apiculate leaf apex that is normally acute and entire, and laminal cells 14-20 µm with thin walls; and O. ohioense has an obtuse leaf apex that is never apiculate or denticulate, and laminal cells 8-10 µm with thick walls. Capsules of O. pusillum have stomata at mid capsule or beyond.