Description from Flora of China
Asplenium ruta-muraria var. subtenuifolium Christ; A. subtenuifolium (Christ) Ching & S. H. Wu; Tarachia ruta-muraria (Linnaeus) C. Presl.
Plants 2-10(-15) cm tall. Rhizome erect to shortly creeping, scaly; scales blackish brown, narrowly triangular, occasionally with marginal glands, glands also on apex, subentire. Fronds caespitose; stipe (0.5-)2-5(-6) cm, green or grayish green to stramineous when dry, base castaneous to dark purplish or blackish brown, with numerous short (4-cellular) glandular hairs and hairlike scales, subglabrous toward rachis; lamina deltoid-triangular to ovate, (1-)2-5(-8) × 1-3(-5) cm, apex obtuse to subacute, 2-pinnate or pinnate-pinnatifid; pinnae 1-4 pairs, subopposite or alternate, basal pinnae largest, (4-)8-12 × (4-)8-10 mm, stalked, pinnae triangular, apex obtuse, imparipinnate or ternate with up to 3 lateral segments, segments often similarly divided; ultimate segments broadly flabellate to rhombic-ovate or trapeziform, (3-)4-6 × 2-5 mm, base broadly cuneate and decurrent on costa, margin irregularly dentate, apex obtuse. Veins obscure, flabellately anadromous, and almost parallel. Fronds subleathery, grayish green when dry; rachis and costa green, with (partly) deciduous glandular hairs, subglabrous when old, adaxially sulcate, when dry often flat. Sori 5-12 per pinna, median on subtending vein, confluent at maturity, subelliptic to linear, 1-5 mm; indusia gray-glaucous to brown, linear, thinly membranous, margin with long hairs, opening toward major veins or costa, concealed by sporangia at maturity. Perispore lophate (costate-cristate), average exospore length 41-49 µm. Plants sexual, autotetraploid: 2n = 144.
Asplenium ruta-muraria is an autotetraploid species, very common and widespread in Europe, where it often grows on buildings and walls. It has originated by chromosome doubling in its diploid ancestor A. dolomiticum (see above), from which is genetically isolated, and which has smaller spores and half the number of chromosomes. Their triploid sterile hybrid, A. ×baldense Sleep et al., is usually found where both species grow together. There is considerable variation in frond shape, and size and degree of dissection, best known and mainly described from Europe (e.g., von Heufler, Verh. Zool.-Bot. Vereins Wien 6: 235-354. 1856; Luerssen in Rabenhorst, Krypt.-Fl., 218-228. 1889; Christ, Farnkr. Schweiz, 75-79. 1900; Hedwigia 42: 153-177. 1903; Rosenstock, Allg. Bot. Z. Syst. 8: 116-120. 1902; Fiori, Fl. Ital. Crypt. 5: 193-209. 1943) and illustrated in Christ (loc. cit. 1903), Ogata (Icon. Fil. Jap. 3: 108. 1930), Khullar (Ill. Fern Fl. W. Himalaya 1: 432-435. 1994), and Wu (FRPS 4(2): 84. 1999). A hexaploid is known from Japan (Reichstein, Bot. Helv. 91: 89-139. 1981).
On limestone rocks; 800-3300 m. Gansu, Guizhou, Hunan, Liaoning, Nei Mongol, Shaanxi, Shanxi, Sichuan, Taiwan, Xinjiang, Yunnan [Afghanistan, India, Japan, Kashmir, Kazakhstan, Korea, Kyrgyzstan, Nepal, Pakistan, Russia, Tajikistan; NW Africa, SW Asia, Europe, North America].