Orthodontium gracile Schwaegr. ex B.S.G.
Bryol. Eur. 4: 70. 330. 1844. -- Stableria gracilis (B.S.G.) Lindb. in Braithw., Brit. Moss Flora 2: 141. 66. 1890.
Plants erect in green to pale green tufts, to 5 cm high. Leaves closely imbricated, irregularly appressed and wrinkled when dry but erect spreading when moist, keeled. Leaves broadest near the base and gradually narrowed to the apex, very narrowly lanceolate to linear-lanceolate with acute apices, to 3 mm long, about 12–18: 1, not plicate. Median laminal cells rectangular with oblique end walls and with those walls rather thick (lumen: wall ratio about 5–6: 1) 7–10: 1, smooth, not or lightly pitted, to 12 µm wide. Leaf lamina unistratose throughout but markedly thicker near the costa compared with the margin. Basal and alar cells mostly yellowed, shorter and broader (to 30 µm broad) than median cells. Cells near leaf apex somewhat more narrow than median cells, 5–8: 1, smooth. Margins not decurrent, plane throughout, remotely serrulate with the teeth formed of thickened cell ends or sometimes with a single discrete very small cell. Limbidium not well-differentiated but with more narrow cells, unistratose throughout. Costa cross-section elliptic, 1.5–2: 1, filling about 1/5 of leaf base, tapering above, green throughout, percurrent in the narrowly acute leaf apex. Costa cross-section 3–4 cells thick at mid-leaf, with much enlarged cells in the adaxial epidermis, without distinguishable guide cells, without stereids. Axilary hairs to 4 cells long, to 150 µm, without basal brown cells, broad at base and attenuating distally. Macronemata deep red-brown, to 30 µm wide at insertion, smooth to very lightly papillose, sparingly branched, surrounding an isodiametric macronematal area to 100 µm wide. Micronemata to 15 µm wide at insertion, pale to hyaline, abundantly branched. Stem cross-section round, without a central strand, with the hyaline and leptodermous inner corticals abruptly transitting to a strongly enlarged hyaloderm. Gemmae occasional as fragmented distal portions of the larger, ascending rhizoids.
Paroicous. Perichaetial leaves undifferentiated or somewhat smaller and more narrow than the adjacent vegetative leaves. Seta pale-brown, smooth, erect, to 5 mm long. Calyptra cucullate and smooth. Urn to 1.5 mm long, ovoid to short cylindric, 1.5–2.2: 1, erect to slightly inclined, neither sulcate nor strumose. Operculum short rostrate, to 1/3 of urn length. Suboral exothecial cells to 15 µm broad, rounded isodiametric and trigonous in 2–3 rows. Exothecial cells at mdddle of urn isodiametric to short rectangular, 1–1.5: 1, mostly with irregularly thickened walls and with obvious trigones, to 30 µm broad, in somewhat regular rows, with lumen: wall ratio about 6: 1. Stomates phaneroporous, restricted to neck of capsule, almost as long as broad. Annulus not differentiated. Exostome pale to yellow-brown, to 400 µm long, inconspicuously shouldered, horizontally striate nearly throughout with dorsal lamellae low. Endostome with basal membrane constituting less than 1/4 of total length. Segments smooth, about as long as exostome, broadly perforated along center line and obviously keeled. Cilia absent. Calyptra cucullate and smooth. Spores to 18 µm, smooth to very finely papillose.