Linnaea 30: 458. 1860. -- Orthotrichum lyellii var. papillosum (Hampe) Sull., Musci Bor.-Amer. ed. 2: 33. 1864.
Orthotrichum pringlei C. Muell., Bull. Torr. Bot. Club 13: 120. 1886. -- Orthotrichum lyellii var. pringlei (C. Muell.) Mac. & Kindb., Cat. Can. Pl. 6: 92. 1892.
Orthotrichum lyellioides Kindb., Rev. Bryol. 23: 21. 1896.
Plants spreading in dark green to yellow-green tufts, often prostrate with only the branch tips ascending, and with the main axes to 10 cm long. Leaves spreading to strongly spreading when moist but appressed when dry, ovate-lanceolate to lanceolate, broadest near the base, with acute to narrowly obtuse apices, to 6 mm long, about 8–10: 1. Median laminal cells to 12 µm wide, arranged in regular rows, isodiametric to slightly elongate along an oblique axis. Lumens of the median cells rounded due to the strong corner thickenings. Papillae arranged over the lumens on each surface, 1–2, often bifid. Basal marginal cells to 12 µm wide, qwuadrate to short rectangular with large trigones, pellucid and smooth or papillose, usually clustered in a small rounded and leptodermous auricle on at least one margin. Basal juxtacostal cells at least in part papillose, linear and highly incrassate with lumen: wall ratio 0.2–0.6: 1, to 12 µm wide, 4–8: 1, pitted, pale yellow to hyaline. Cells on adaxial surface of costa elongate and pitted in 1–2 rows. Margins markedly papillose crenate, usually recurved on at least a portion of one side. Costa ending within a few cells of apex, not lying in a keel. Costa cross section homogeneous, about 2: 1, mostly 3 cells thick, and with 8–10 cells along the abaxial perimeter. Median axillary hairs with one basal brown cell, not offset from leaf insertion, to 5 cells and 150 µm long with the laterally inserted ones 2-celled and 60 µm long. Rhizoids densely produced at base of plant, to 25 µm in diameter at insertion, smooth and pachydermous, brown to pale-brown, sparingly to rather closely branched. Stem cross-section rounded pentagonal, without a central strand, mostly with leptodermous and hyaline inner corticals and with 3–4 layers of strongly pachydermous, red brown outer cortical cells.
Dioicous with male plants markedly smaller than the females. Perichaetial bracts not differentiated from vegetative leaves. Capsule somewhat immersed to emergent on a yellow to yellow brown, smooth and erect seta to 2.5 mm long. rn pale-brown, to 3 mm long, 2.5–3.5: 1, erect, strongly or lightly sulcate with eight ribs, usually not strangulate. Operculum conic-apiculate to short rostrate. Annulus well-defined, falling in fragments. Exothecial cells in cross-section single-layered, with outer cell wall very thick but with inner wall thin and collapsing. Exothecial cells at capsule mouth to 25 µm wide, quadrate and highly incrassate with angular lumens in 1–4 rows. Exothecial cells at middle of urn short rectangular, 2–4: 1, to 35 µm broad, differentiated from ridge to valley of the sulcate capsule. Stomata phaneroporous, scattered on proximal 1/2 of urn. Exostome teeth narrowly triangular, reflexed when dry, paired but separating with age, pale to light yellow, to 500 µm long, densely papillose. Endostome segments 16, as long as exostome, densely papillose, of 2 rows of cells at base. Calyptra campanulate with constituent cells smooth, sparsely covered with papillose-prorate and papillose hairs. Spores bimodal in size with largest ones to 35 µm but smallest to 20 µm, green and papillose.
Orthotrichum papillosum in many ways bridges the differences between Ulota and Orthotrichum. Like the former genus, O. papillosum has long and narrow, somewhat crispate leaves with highly incrassate interior basal cells. Those leaves are, however, not shouldered. Another Ulota character is the tendency of the plants to spread by means of densely rhizoidal, somewhat microphyllous axes.
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