Description from Flora of China
Asplenium densum Brackenridge, p.p.; A. melanocaulon Willdenow; A. melanolepis Colenso, p.p. (1888), not Franchet & Savatier (1879).
Plants 10-30 cm tall. Rhizome erect, short, scaly; scales narrowly triangular, 3(-4) × ca. 0.5 mm, with opaque, red to dark brown central stripe and paler narrow clathrate borders, entire. Fronds caespitose; stipe shiny castaneous-brown, 2-8 cm, base scaly, upward subglabrous, abaxially semiterete but adaxially grooved, with brown, membranous and subentire narrow wings, texture papery, stipe and rachis usually persisting after shedding of pinnae; lamina linear, 10-25 × 0.9-1.6 cm, base slightly reduced, 1-pinnate, apex acute and 2-4 mm wide; pinnae 20-30 pairs, usually obliquely inserted, sessile, middle pinnae elliptic or ovate to orbicular, 2.5-7.5 × 2-4 mm, base nearly symmetrical, cuneate, margin crenate, apex obtuse; lower pinnae gradually reduced. Veins pinnate, costa obscure, veins obliquely simple or up to 2-forked, basal acroscopic vein usually 2-forked. Fronds papery, green or brown when dry, average stomatal guard cell length 35-42 µm; rachis castaneous, shiny, subglabrous, abaxially terete, adaxially grooved and with a relatively low lateral brown membranous wing. Sori 4-8 per pinna, oval to linear, 1-3.5 mm, usually on acroscopic vein; indusium white or brown after drying, oval to linear, membranous, free margin repand to entire, opening toward costa, persistent. Spores with lophate perispore, average exospore length 27-31 µm. Plants sexual diploid: 2n = 72.
Asplenium trichomanes is often circumscribed as a collective species consisting of various cytotypes (six taxa are known from C and S Europe). These taxa, for convenience often recognized at the subspecific level, are reproductively isolated (producing sterile hybrids when growing together) and are treated as species in this flora. The species can be distinguished by microcharacters, by flow cytometry, by counting chromosomes, and by their edaphic preference. When the complex is subdivided into species or subspecies, the diploid silicolous taxon has been designated (since Lovis, Brit. Fern Gaz. 9: 147-160. 1964) as A. trichomanes s.s. or as subsp. trichomanes; the common tetraploid as A. quadrivalens or as A. trichomanes subsp. quadrivalens D. E. Meyer. A lectotype was selected in this sense by Viane (in Jonsell & Jarvis, Nordic J. Bot. 14: 145-164. 1994), but as a result of the Linnean typification project this selection was overruled by Grubovs (Novosti Sist. Vyssh. Rast. 21: 5-21. 1984), who "selected" the tetraploid specimen in the Linnaean Herbarium at LINN as type without any argument or fundamental study. However, pending a proposal to conserve the name A. trichomanes with a conserved type, based on a diploid plant, we continue to use the name A. trichomanes s.s. in its traditional ("2x") sense in compliance with Art. 57.1 of the Melbourne Code. However, if such a proposal is not accepted, the correct specific name for the current diploid species will not only have to change to A. melanocaulon Willdenow, but the name A. trichomanes will have to be given to the following tetraploid taxon, A. quadrivalens.
Asplenium ×lusaticum D. E. Meyer (the sterile hybrid between A. trichomanes and A. quadrivalens) is not rare where both parents grow together. It can be easily recognized by its aborted spores. In China, this hybrid is cytologically confirmed for Sichuan ("Longchi").
In crevices of non-calcareous (sandstone, granite) rocks, in open or half-shaded situations; 400-3400 m. Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Shaanxi, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan, Zhejiang [worldwide in all temperate zones, in tropics on high mountains].