Rev. Bryol. 34: 100. 1907. -- Didymodon vinealis var. nicholsonii (Culm.) Zander, Cryptogamie 2: 416. 1981.
Plants to 2 cm high, in dense cushions, green to brownish green, sometimes reddish in exposed sites. Dry leaves mostly appressed with flexuose apices. Moist leaves ascending to spreading, to 3 mm long, about 3–4: 1, grooved along distal portion of the costa. Leaves ovate lanceolate, gradually contracted from the well-defined ovate base to the acute to obtuse or cucullate apex. Median cells unistratose throughout, mostly with a few low and inconspicuous papillae and only occasionally smooth, to 8 µm broad, in straight rows, lumen/wall ratio 2–4: 1, quadrate with rounded lumens due to strong corner thickenings. Cells of margin somewhat smaller and thicker walled. Differentiated elongate basal cells markedly few in number, often present only near costa at extreme leaf base, arranged in regular rows, moderately thick-walled with the lumen/wall ratio about 2–4: 1, to 10 µm broad, 2–4: 1, with straight and not pitted lateral walls. Ad- and ab-axial surface of costa clothed with isodiametric cells like those of the lamina. Costa at base about 8–10 cells wide and occupying about 1/6 of leaf, not tapering, subpercurrent to percurrent in the blunt apex. Costa cross-section semicircular bulging dorsally, with a single layer of guide cells and with a strong dorsal stereid band but with the ventral area composed primarily of substereidal cells, with well-differentiated epidermal cells on both surfaces. Margin not or very shortly decurrent, entire, usually bistratose distally, recurved at base with recurvature usually extending above leaf middle. Gemmae absent. Axillary hairs to 100 µm, 4–5 celled with one basal brown cell. Stem rounded pentagonal in cross-section, with central strand, and with thin-walled inner cortical cells grading to 1–3 layers of thicker walled, smaller outer cortical cells. Rhizoids pale brown to brown, smooth, sparingly branched, occurring on plant bases and occasionally arising from apical cells of the leaves, to 12 µm wide at base.
Dioicous. ACCORDING TO SMITH- SPOROPHYTES UNKNOWN. Perichaetial leaves not significantly different from vegetative leaves. Seta to 10 mm, brown to yellow brown or reddish brown, straight, smooth. Capsule erect and symmetrical with the urn cylindrical, to 2 mm long, about 3–4: 1. Operculum long conic- rostrate, to 3/4 of urn length. Exothecial cells with walls to 5 µm thick, quadrate to short-rectangular, in regular rows, to 20 µm wide, 4–6: 1, gradually smaller and thicker-walled near mouth. Stomates few, restricted to base of urn. Annulus persistent or falling in fragments. Peristome yellow- to reddish-brown, to 2 mm long, divided to base, without basal membrane, with the segments papillose-spiculose throughout, straight to several times spirally twisted. Spores spherical, to about 12 µm, nearly smooth.
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