17. Hymenasplenium obliquissimum (Hayata) Sugimoto, Keys Herb Pl. Jap. Pterid. 356, 406. 1966.
荫湿膜叶铁角蕨 yin shi mo ye tie jiao jue
Asplenium unilaterale Lamarck var. obliquissimum Hayata, Icon. Pl. Formosan. 4: 230. 1914; A. obliquissimum (Hayata) Sugimoto & Sa. Kurata; A. resectum Smith var. obliquissimum (Hayata) Hayata; A. unilaterale Lamarck var. udum Atkinson ex C. B. Clarke; Hymenasplenium unilaterale (Lamarck) Hayata var. obliquissimum (Hayata) Hayata ex Sasaki.
Plants 20-30 cm tall. Rhizomes shortly creeping, 1.5-2.5 mm in diam., apex densely scaly; scales grayish brown, broadly ovate-triangular; phyllopodia distinct, ca. 2 mm tall, up to 10 mm apart. Fronds very thin to translucent, brown-green when dry; lamina 1-pinnate, narrowly triangular to lanceolate, 15-24 × 2.5-5 cm, widest in basal 1/3, apex gradually narrowing, subglabrous; stipe shiny, dark castaneous-purple, 10-20 cm, subglabrous, base sparsely scaly; rachis shiny, dark castaneous-purple abaxially, adaxial side grooved and with 2 grayish green narrow wings; pinnae strongly dimidiate, shortly stalked and purplish rachis color extending via stipicel onto costa abaxially, 15-25 pairs, alternate, ascending, trapeziform to falcate, 1.5-3.5 × 0.4-0.7 cm, base asymmetrical, acroscopic side truncate, basiscopic side straight with more than 1/2 cut away, acroscopic margin (bi)crenate-sinuate, teeth obtuse and not retuse, apex subacute to obtuse. Veins forking and terminating in marginal teeth, (2 or)3 or 4 basal basiscopic veins lacking. Sori linear, 3-4 mm, basal on subtending vein and appearing subcostal; indusia brownish, linear, membranous, entire, opening toward costa. Plants sexual or agamosporous.
On rocks near or in streams, in forests; 800-2800 m. Guangdong, Guangxi, Guizhou, Hunan, Jiangxi, Sichuan, Taiwan, Yunnan [India, Indonesia, Japan, Nepal, Vietnam].
Hymenasplenium obliquissimum is an aggregate of species with different chromosome number and life strategy; three cytotypes, based on x = 39, are known. A diploid sexual (2n = 78), a tetraploid sexual (2n = 156), and a triploid agamosporous type (2n = 117) which is probably a hybrid between the diploid and the tetraploid (Murakami, Pl. Spec. Biol. 13: 51-56. 1998). A thin lamina (only 2 epidermal layers) is characteristic for this taxon but occurs elsewhere in the genus and is influenced by growing conditions; using it as a diagnostic character may lead to wrong identifications.
In the literature (Iwatsuki, Ferns Japan, 151. 1992; Hasebe et al., Amer. Fern J. 85: 134-181. 1995; Iwatsuki et al., Fl. Japan 1: 109 1995), this species has sometimes been put into synonymy with Hymenasplenium filipes (Philippines), but considering cytological and morphological differences, we prefer to keep these taxa distinct. Hymenasplenium obliquissimum differs from H. filipes by its thinner lamina, thicker rhizome (up to 0.15 cm in H. filipes), shinier stipe and rachis (almost dull in H. filipes), rachis color extending onto the costa, larger number of pinnae (up to 15 in H. filipes), larger number of reduced basal basiscopic veins (1 or 2 in H. filipes), and sorus position along the subtending vein (median in H. filipes).
Asplenium unilaterale Lamarck var. decurrens (Beddome) H. S. Kung (Fl. Sichuan. 6: 357. 1988; A. resectum Smith var. decurrens Beddome, Suppl. Ferns S. Ind. 10. 1876) was treated as a synonym of A. unilaterale var. udum in FRPS (4(2): 38-40. 1999).