Description from Flora of China
Asplenium fontanum (Linnaeus) Bernhardi var. exiguum Beddome; A. fontanum var. yunnanense (Franchet) Beddome; A. glenniei Baker; A. kangdingense Ching & H. S. Kung; A. loherianum Christ; A. moupinense Franchet; A. moupinense var. dareiforme Franchet ["dareaeformis"]; A. woodsioides Christ; A. yunnanense Franchet; A. yunnanense var. dareiforme (Franchet) H. S. Kung ["dareaeforme"].
Plants 5-25 cm tall. Rhizome erect, scaly; scales dark brown to black, narrowly triangular, margin fimbriate at base. Fronds caespitose; stipe 1-5 cm, abaxially purplish black to reddish brown and semi-shiny, adaxially sulcate, with blackish brown narrowly triangular scales mixed with filiform scales; lamina lanceolate, 3-15 × 0.8-2.5(-4) cm, attenuate to both ends, apex pinnatipartite or shortly flagelliform and rooting, 1- or 2-pinnate, up to 3-pinnatifid in large specimens; pinnae 10-20 pairs, shortly stalked, basal pinnae opposite and often strongly reduced or flabellate, upward becoming alternate; middle pinnae largest, narrowly triangular to ovate-elliptic or oblong, (2-)10-15(-20) × 2-7 mm, base almost symmetrical to asymmetrical, acroscopic side truncate, basiscopic side cuneate, pinnatisect to pinnatifid, apex obtuse and with 1 gemma in apical sinus; segments 2-4 pairs, apex obtuse and dentate-serrate with 2-4 teeth; teeth obtuse, mucronate or acute. Veins not prominent, anadromously forking, not reaching margin. Fronds firmly herbaceous, green, lamina with uniseriate gland-tipped hairs or subglabrous, average guard cell length 43-50 µm; rachis with purplish brown to castaneous color extending from basal part up to 2/3 its length on abaxial side, apical part green or stramineous, with purplish black hairlike scales, adaxially green or stramineous, flat or slightly sulcate, green. Sori usually 2-4 on basal acroscopic segment, on others 1 per segment, often confluent at maturity, basal to submedial on subtending veinlets, close to costa, subelliptic to elongate, ca. 1 mm; indusia grayish green to grayish brown, narrowly elliptic, membranous, repand to entire, opening toward costa or major vein. Spores with lophate (costate-cristate) perispore, average exospore length 31-35 µm. Plants sexual, autotetraploid: 2n = 144.
Large specimens of Asplenium exiguum are similar to well-developed plants of A. nesii and can best be distinguished by the presence of gemmae in the apical notch of the pinnae.
Asplenium exiguum is an autotetraploid taxon that most probably originated via chromosome doubling in diploid A. lushanense. It is a variable species with an easily overlooked, tiny bud in the sinus at the pinna apex, well illustrated in Ching (Icon. Filic. Sin. 4: pl. 174, 2b. 1937). The morphological variability, both within China and throughout the Himalaya, is relatively large, though mainly restricted to size differences. Growing conditions may strongly influence frond length and the shape of pinnae (from almost oblong to triangular). In some populations (S India, China, Philippines), the rachis is flagelliform and terminates in a slender tail with a terminal budlet. This variation led to much confusion and the description of several (local) species, e.g., A. glenniei, A. loherianum, A. moupinense, A. woodsioides, and A. yunnanense. Asplenium moupinense var. dareiforme represents a luxurious, well-developed plant that does not differ in any essential character from other members of this group. Since there are no fundamental morphological, nor cytological, differences between these taxa, we treat them as synonyms of tetraploid A. exiguum following Hope (Bull. Torrey Bot. Club 26: 58-62. 1899; J. Bombay Nat. Hist. Soc. 13: 657-671. 1901), Ching (Icon. Filic. Sin. 4: pl. 174. 1937), Maxon (Amer. Fern J. 28: 141. 1938), Tagawa (Acta Phytotax. Geobot. 8: 91-100. 1939), Tagawa and Iwatsuki (in Smitinand et al., Fl. Thailand 3(2): 261-291. 1985), and Mickel and Smith (Pterid. Mexico, 94. 2004). The diploid and tetraploid species within this complex can be separated by microcharacters (Viane & Reichstein, Pterid. New Millennium, 91-94. 2003): Asplenium lushanense (diploid) with an average exospore length of 24-28 µm and stomata 32-40 µm, and tetraploid A. exiguum with average exospore length of 30-36 µm and stomata 43-52 µm. Where both species grow together (China, Nepal, Vietnam), their sterile triploid hybrid, with aborted spores, an intermediate morphology, and often showing hybrid vigor, is common. Asplenium ×mickelii Viane & Reichstein, nothosp. nov. Type: China. Yunnan: Kunming, Xishan, Longman, ca. 2120 m, on steep limestone cliff, under forest, 29 Sep 1986, Z. R. Wang & M. Chu C806-b1 (=TR-6605-b1) (holotype, PE). Planta hybrida, inter parentes A. exiguum et A. lushanense quoad divisionem laminae atque dimensiones cellularum accessoriarum stomatum intermedia, ab eis sporis abortivis necnon chromosomatum numero triploideo (2n = 108, meiose trivalentibus 0-4, bivalentibus 32-36 et univalentibus 29-36) differt. This hybrid is named after J. Mickel, who collected it in Mexico (Oaxaca: Destrito Ixtlán, 16 Sep 1972, J. T. Mickel & L. Pardue 6488-A [NY]) and sent us living plants for our research on the A. exiguum complex.
From their changing concept of this complex, it is not clear if Fraser-Jenkins, Pangtey and Khullars Asplenium exiguum nothosubsp. luyunense Z. R. Wang ex Fraser-Jenkins, Pangtey & Khullar (Indian Fern J. 27(1-2): 198. 2011), a validation at the subspecific level of "A. ×luyunense" (Wang, Acta Bot. Sin. 45: 11. 2003, nom. nud.), also refers to this hybrid.
In crevices of (limestone) rocks in forests; 1100-3300 m. Guangxi, Guizhou, Hebei, Henan, Hunan, Shanxi, Sichuan, Taiwan, Xizang, Yunnan [India, Mongolia, N Myanmar, Nepal, Philippines, Russia (SW Siberia), Thailand, N Vietnam; North America (Mexico, United States)].