Description from Flora of China
Asplenium bipinnatum Roxburgh var. prolongatum (Hooker) Bonaparte; A. rutifolium (Bergius) Kunze var. prolongatum (Hooker) Christ.
Plants 20-40 cm tall. Rhizome erect, short, apex scaly; scales dark brown to black, narrowly triangular, with narrow pale brown edges, entire or denticulate. Fronds caespitose; stipe green, 8-18 cm, sulcate adaxially, sparsely covered with small dark brown fimbriate scales when young, later subglabrous; lamina linear-ovate, 10-25 × 3-4.5 cm, bipinnate, apex caudate; pinnae 20-24 pairs, basal pinnae opposite or subopposite, upper alternate, subsessile, lower pinnae usually not reduced, middle pinnae narrowly elliptic to oblong, 1-3 × 0.8-1.5 cm, base nearly symmetrical, cuneate-truncate, pinnate, apex obtuse; pinnules alternate, anadromous, 2-5 pairs, linear, 4-10 × 1-1.5 mm, base adnate to costa, entire, apex obtuse; first acroscopic pinnule 2- or 3-fid. Veins and costa raised adaxially, 1 vein per pinnule or segment, with terminal hydathode, not reaching margin. Fronds subfleshy but thin when dried, green to yellow-green when dry; rachis green, usually prolonged into flagelliform and gemmiferous apex, flat or shallowly sulcate with raised supravascular ridge on adaxial side, abaxially flat. Sori 1 per pinnule or segment, median on acroscopic side of subtending vein, linear, 2.5-5 mm; indusia grayish green, linear, membranous, entire, opening toward costa and margin, persistent.
Asplenium prolongatum was confused with A. rutifolium (Bergius) Kunze by Franchet and Savatier (Enum. Pl. Jap. 222. 1876), Makino (Phan. Pter. Jap. Icon. t. 65. 1900), and others, and with A. achilleifolium (Lamarck) C. Christensen by Ogata (Icon. Fil. Jap. 2: t. 55. 1929) and Y. C. Wu et al. (Bull. Dept. Biol. Sun Yatsen Univ. 3: 202. 1932).
Though Japanese plants of Asplenium prolongatum were reported to be tetraploid with 2n = 144 (Iwatsuki, Fl. Jap. 1: 103. 1995), all Chinese plants (Guangdong, Guizhou, Hainan) checked by flow cytometry are hexaploid and may have to be ascribed to a new taxon. Asplenium ×kenzoi Kurata, the hybrid with A. wrightii, may also occur in China where its parents often grow together.
On tree trunks in forests or on wet rocks; 100-2000 m. Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Hainan, Henan, Hubei, SW Hunan, Jiangxi, Sichuan, Taiwan, Xizang, SW Yunnan, Zhejiang [India, Japan, S Korea, Malaysia, Myanmar, Sri Lanka, Vietnam; Pacific islands (Fiji)].