4. Hymenasplenium retusulum (Ching) Viane & S. Y. Dong
微凹膜叶铁角蕨 wei ao mo ye tie jiao jue
Basionym: Asplenium retusulum Ching, Acta Phytotax. Sin. 10: 185. 1965.
Plants 20-30 cm tall. Rhizome long creeping, 1.5-3 mm in diam., apex scaly; scales narrowly triangular to triangular. Fronds remote, up to 12 mm apart, green when dry, membranous-herbaceous, subglabrous; stipe shiny, dark purplish black, 4-12 cm, base with yellowish brown indument, some scales similar to those on rhizome, or subglabrous; lamina 1-pinnate, narrowly oblong-triangular, 15-20 × 3-4.5 cm, truncate at base, apex acuminate; rachis shiny, purplish; pinnae 18-25 pairs, almost sessile, rectangular to trapeziform, middle pinnae 2.2-2.6 × 0.5-0.7 cm, dimidiate, apex obtuse, base asymmetrical, acroscopic side truncate to cuneate and parallel to rachis, basiscopic side cut away and narrowly cuneate and entire, acroscopic margin with deep crenate incisions (1-2 × ca. 1 mm) with lobes minutely retuse; pinnae spreading, basal pinnae slightly reduced. Veins distinct, forking and free, 1 vein per marginal lobe, vein ending below marginal notch, basiscopic side with 3 or 4 veins lacking. Sori inframedial, elliptic, 3-4 mm; indusia persistent, yellowish brown to brown, semi-elliptic, membranous, entire, opening toward costa. Plants sexual.
● On rocks in mixed forests; ca. 2000 m. SE Yunnan.
Hymenasplenium retusulum is similar to the following taxa from which it differs in its more deeply cut acroscopic margins with relatively long and obtuse teeth, which are only minutely retuse at their tips. Due to confusion with similar taxa, the relationships and the distribution of this species are not well known.