Didymodon hengduanensis J. A. Jimenez, D. G. Long, Shevock & J. Guerra 2016, sp. nov.

Diagnosis: Differing from all other species of the genus Didymodon by its lanceolate to ovate-lanceolate and appressed-incurved leaves when dry, acute leaf apices, recurved to revolute margins, the yellowish color in KOH, percurrent to subpercurrent costa, and marginal basal cells forming a distinctly differentiated area of smooth and transversely thick-walled cells.

TYPE:— CHINA. YUNNAN: Lushui County, Gaoligongshan Range, southern end of the Hengduanshan, Nu Jiang (Salween River) watershed, at Pianma Fengxue Pass northwest of Liuku at km marker 58 about 38 km from Lushi, 15 October 2002, Shevock 23596 (holotype: KUN; isotypes: CAS, E, H, MO, MUB, NY, PE).

Plants 1–3.7 cm high, growing in dense turfs, dull to yellowish green above, brown to red-brown below. Stems erect, simple or branched, sometimes radiculose at base, without hyalodermis, sclerodermis undifferentiated, central strand developed; axillary hairs filiform, 2–5 cells long, with 1 brown basal cell and hyaline upper cells. Rhizoidal tubers absent. Leaves appressed-incurved when dry, erect-patent, sometimes spreading when moist, lanceolate to ovate-lanceolate, 1.8–2.6 × (0.6–) 0.7–0.95 mm, channelled ventrally in the upper part; lamina unistratose, KOH reaction yellowish; apex acute, not apiculate, not cucullate; margins recurved to revolute from base to 3/4 of the leaf, entire, unistratose, often decurrent on stem; costa 67–100 μm wide at base, percurrent to subpercurrent, not spurred, tapering to apex, ventral surface cells of the costa subquadrate, oblate or shortly rectangular, smooth, dorsal surface cells of the costa subquadrate or oblate, seldom shortly rectangular, smooth, in cross-section below midleaf semicircular, with 4–6 guide cells in 1 layer, 2 layers of ventral stereids, 2 layers of dorsal stereids, semicircular in shape, without hydroids, ventral surface cells differentiated, smooth, dorsal surface cells differentiated, smooth; upper and middle laminal cells subquadrate or oblate, rarely shortly rectangular, 3.5–9.6 × 4.8–9.6 μm, evenly thick-walled, not bulging, with 1(–2) simple low papillae centered over the lumen; juxtacostal basal cells rectangular, rarely quadrate, 7.5–45 × 5–10 μm, slightly and evenly thick-walled, smooth or ventrally with single, very low papillae; marginal basal cells oblate, rarely subquadrate, often with lumens angular, 3.2–10(–15) × 4.8–12.8 μm, transversely thick-walled, extending up to approximately midleaf, forming a distinctly differentiated area of 5–11 rows of smooth cells. Gemmae absent. Sexual condition dioicous. Perichaetial leaves differentiated, more sheathing than vegetative leaves, oblong-lanceolate, abruptly narrowed to a subulate apex, 2.2–2.8 × 0.65–0.8 mm, margins plane, costa excurrent,. Seta 1.2–2 cm long, weakly twisted to the left, orange to reddish brown. Capsule erect; theca cylindrical, 1.7–2.6 × 0.5–0.75 mm, brown to reddish brown; annulus persistent, composed of 1 row of rectangular, thick-walled cells; peristome of 32 teeth spirally twisted, 1.2–1.5 mm, orange, papillose; operculum long rostrate, 1.3–1.6 mm long, cells in twisted rows. Calyptra not seen. Spores 10–17.5 μm in diameter, weakly papillose, yellowish brown.

Paratypes: — CHINA. SICHUAN: Yajiang County, east slope of pass west of Yajiang on Highway 318, 2 September 2010, Long 39775 (E). YUNNAN: Heqing County, camino a Lijiang desde Dali, 21 September 2014, Lasso de la Vega s.n. (MUB 49677). Gongshan County, Bingzhongluo Xiang, east slope of Gaoligong Shan, Nu Jiang (Salween) catchment, moraine across valley on SW slope of Gawagapu Mountain, 21 August 2006, Long 35955 (CAS, E, KUN); Shangri-la County, Pudacuo National Park, 25 September 2014, Lasso de la Vega s.n. (MUB 49673).

Etymology: —The specific epithet, hengduanensis, refers to the Hengduan Mountains where the species occurs.

Distribution and Habitat: —At present, Didymodon hengduanensis is known from five localities in Sichuan and Yunnan provinces, southwestern China (Fig. 3). All occurrences lie within the Hengduan Mountains region between unusually wide altitudinal extremes of 2500 and 3970 m. It occurs on both calcareous and granitic rocks and boulders with accumulated soil or more rarely on disturbed soil, in a variety of habitats, both in the Abies forest zone and also above the tree-line amongst shrubby Rhododendrons and bamboo, and on an alpine moraine ridge.