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Nov. Act. R. Soc. Sci. Upsal. 12: 257. 1844. -- Mnium triquetrum L. in Richter, Codex Bot. Linn. 1045. 1840.
    Meesia trifaria Crum, Steere & Anderson, Bryologist 68: 434. 1965.
    Meesia tristicha Bruch, Flora 9: 165. 1826.

Plant in small tufts or cushions mostly interspersed with other bryophytes, to 6 cm high, dark green to grass-green above, occasionally red-brown below due to dense rhizoids. Stems pale brown to yellow brown, closely foliate, with leaves keeled, spreading and laterally rolled when dry, spreading when moist, not secund but appearing crispate. Margin plane to recurved at mid-leaf but plane below and near the apex, strongly decurrent by a strip up to 4 cells wide at the top, extending to insertion of next lower leaf. Leaves entire throughout or minutely serrulate on the distal 1/2–2/3, to 4 mm long, lanceolate and broadest at about 1/3, 2.5: 1, with the apex acute to acuminate. Cells throughout lamina in regular rows, smooth with angular lumens. Median cells of lamina moderately thick-walled with rectangular lumens, to 20 µm broad, rather variable in length, mostly 1–2.5: 1. Marginal cells of leaf somewhat more narrow and proportionately longer than adjacent more interior cells. Cells across the leaf base somewhat longer and more pellucid than median cells but otherwise similar. Cells of alar region not differentiated. Macronematal initials to 120 µm broad, orbicular, median axillary in insertion. Costa filling about 1/10 of the leaf base, gradually tapering to the subpercurrent to percurrent apex. Lamina unistratose throughout. Costa cross section with a stongly differentiated ad- and abaxial epidermis surround a large cluster of stereid cells. Stem cross section rounded triangular, with a stereome of 1–2 very strongly differentiated, small, red-brown and pachydermous cells, and with the central strand well-defined. Rhizoids restricted to the axillary macronematal apparati, red-brown, densely papillose, extensively but irregularly branched, to 20 µm wide at base. Axillary hairs to 180 µm long with 3–4 rather elongate brown cells and a single rather thin-walled cylindrical terminal cell.
    Presumably dioicous but antheridial material not seen in our material. Perichaetia terminal but with perichaetial bracts similar to vegetative leaves. Seta to 60 mm long, straight and smooth, brown to yellow brown. Urn arcuate and asymmetric, to 4 mm long, oblong to short cylindric, brown to yellow brown, wrinkled when dry but not regularly sulcate. Operculum short conic. Neck of dry capsule moderately well-defined, not much wrinkled when dry, comprising up to 1/2 of capsule length. Suboral exothecial cells transversely elongate in 5–8 rows, 0.5–0.8: 1, to 15 µm broad. Median exothecial cells to 25 µm broad, hexagonal and thin-walled, 1–2: 1. Stomata not seen. Annulus poorly defined. Exostome inconspicuous, inserted far below mouth of capsule and hardly protruding from mouth, to 100 µm long, brownish to hyaline with prominent ventral trabeculae. Endostome inconspicuously keeled, smooth or nearly so with basal membrane inconspicuous, but with 1–2 cilia. Spores spherical, finely papillose, to 40 µm.

Elevation by latitude plot for Meesia triquetra
   in California

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