Description from Flora of China
Asplenium adiantum-nigrum subsp. yuanum (Ching) Reichstein et al.; A. adiantum-nigrum var. yuanum (Ching) Ching; A. yuanum Ching.
Plants 15-40 cm tall. Rhizome ascending, apex scaly; scales dark brown, narrowly triangular, up to 6 mm, hairlike in upper half, without black middle stripe, entire. Fronds caespitose; stipe 8-15(-20) cm, base conspicuously swollen, thickened, semiterete, abaxially shiny castaneous to blackish purple, with scattered brownish black hairlike scales or subglabrous, adaxially sulcate; lamina triangular-ovate, 9-25 × 4-6(-10) cm, apex acute to acuminate, tripinnate; pinnae 8-13 pairs, alternate, stalked, basal pair largest, triangular, 4-6 × 1.8-2.4 cm, base truncate, bipinnate, apex acute to acuminate; pinnules 5-8 pairs, anadromous, basal pairs largest, triangular, 1.5-3.2 × 0.6-1 cm, base cuneate-truncate, shortly stalked or subsessile, pinnate, apex acute; ultimate pinnules 4-6 pairs, alternate, anadromous, basal pair largest, elliptic to oblong or linear, 5-7 × 2.5-3.5 mm, margin serrate, apex obtuse. Costa obvious, sulcate adaxially, veins obscure, anadromously branching, simple or 2-forked, not reaching margin. Fronds firmly herbaceous to papery, green but brown-green when dry, lamina with 4- or 5-celled, uniseriate hairs, average stomatal guard cell length 54-60 µm; rachis semiterete, lower half abaxially usually shiny castaneous to dark brown, apical part green or stramineous, adaxially sulcate. Sori 1-3(or 4) pairs per pinnule or segment, median on subtending veinlet, confluent at maturity, linear, 1-2.5(-5) mm; indusia white or brownish, linear, membranous, repand to entire, opening toward costa or costule. Spores brown to dark brown, perispore lophate (cristate-alate), with average exospore length 32-37 µm. Plants sexual allotetraploids: 2n = 144.
Asplenium adiantum-nigrum, an allotetraploid species that originated via chromosome doubling in a hybrid (A. ×ligusticum Bernardello et al.) between A. cuneifolium Viviani and A. onopteris Linnaeus, is unrelated to the distantly similar A. interjectum. Asplenium adiantum-nigrum differs by the thickened stipe base (trophopod) and the less deltoid lamina.
On rocks or on forest floor near streams; 2000-3000 m. Shaanxi, Taiwan, E Xizang, NW Yunnan [Afghanistan, N India, Pakistan; Europe (Mediterranean to SW Asia), W North America].