Bryol. Eur. 3 (fasc. 2/3). Orthotrichum Suppl. 1: 1. 1849.
Orthotrichum watsoni James in Watson, Bot. U.S. Geol. Expl. 40th Parall. Prof. Pap. Eng. Dept. U. S. Army 18: Vol. 5: 401. 1871. -- Orthotrichum alpestre var. watsoni ( James in Watson) Grout, Moss Fl. N. Am. 2: 125. 1935.
Orthotrichum occidentale James in Watson, Bot. U.S. Geol. Expl. 40th Parall. Prof. Pap. Eng. Dept. U. S. Army 18: Vol. 5: 402. 1871. -- Orthotrichum alpestre var. occidentale ( James in Wats.) Grout, Moss Fl. N. Am. 2: 124. 1935.
Orthotrichum alpestre var. majus Lesq. & James, Man. N. Am. Moss. 169. 1884.
Plants erect in olive-green to glaucous green tufts, to 2.5 cm high. Leaves spreading to somewhat reflexed when moist but, when dry, erect-spreading and irregularly twisted. Leaves to 3.5 mm long, 3–4: 1, ovate-lanceolate to lanceolate with acute to acuminate apices, occasionally somewhat apiculate distally. Median laminal cells isodiametric with rounded lumens, thick-walled with strong corner thickening, to 12 µm wide, with 2–3 very high and mostly bifid papillae on both surfaces. Basal juxtacostal cells pellucid and rectangular, 1.5–4: 1, to 14 µm wide with lateral walls straight but unevenly thickened to pitted or nodulose. Basal marginal cells isodiametric, similar to the median laminal cells. Margins recurved but not revolute from base to near the apex, mostly rather strongly papillose crenate above. Costa ending within a few cells of apex, distally situated in a keel. Costa cross-section at mid-leaf flattened, about 3–4:1, homogeneous, about 3 cells thick with only 2 cells in the adaxial layer, but 6–10 in the abaxial layer. Median axillary hairs with constituent cells of constant diameter throughout,to 10 cells and 300 µm long, with one basal brown cell, not offset from leaf insertion; laterally inserted axillary hairs markedly shorter and fewer-celled. Rhizoids present and sometimes abundant near base of plant but mostly sparse on distal portions of plant, smooth, pale to red-brown, to 20 µm in diameter at base, sparingly branched. Stem cross-section rounded pentagonal without a central strand, with hyaline inner corticals and with 2–5 layers of moderately pachydermous, red brown outer cortical cells. Gemmae occasional, inserted primarily on adaxial surfaces of leaf lamina, uniseriate, to 25 µm wide, composed of 4–6 mostly red-brown and isodiametric cells.
Autoicous with perigonia in leaf axils near the perichaetia or on the apices of separate short branches. Perichaetial bracts similar to the vegetative leaves. Capsule emergent or slightly exserted on a yellow to yellow-brown, smooth and erect seta of about 1.5 mm in length. Urn well-differentiated from the short neck, yellow-brown when fresh, to 2 mm long, 2: 1, erect, strongly sulcate with eight ribs, strongly strangulate. Operculum short rostrate-apiculate. Annulus well-defined, falling in fragments. Exothecial cells at capsule mouth differentiated in up to 4 rows, to 15 µm broad, isodiametric to transversely elongate with rounded lumens.
Exothecial cells at middle of urn quadrate to rectangular, 2.5–4: 1, to 35 µm broad, thick-walled on ridges of capsule with lumen: wall ratio about 1: 1, thinner walled and more elongate in valleys of the sulcate capsule. Stomata present on proximal 1/2 of urn, cryptoporous with about 1/2 of each guard cell obscured by the well differentiated subsidiary cells. Exostome teeth reflexed against capsule wall when dry, fused in pairs and appearing to be only 8 in number, pale-brown, to 300 µm long, horizontally striate below but reticulate striate above. Endostome segments present, 8 or occasionally 16, about as long as the exostome, lightly papillose, formed of 2 rows of cells near their base. Calyptra less than twice as long as wide, campanulate and lightly plicate, with cells lacking papillae or prorae, sparsely to rather densely clothed with multiseriate, very high papillose hairs. Spores to 14 µm, rather coarsely papillose.
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