Hymenasplenium tholiformis Liang Zhang, W.B. Ju & K.W. Xu, 2022

Qiu, Yong-Lin, Xu, Ke-Wang, Ju, Wen-Bin, Zhao, Wang-Lin & Zhang, Liang, 2022, Hymenasplenium tholiformis (Aspleniaceae), a new fern species from southeastern Xizang, China based on morphological and molecular evidence, PhytoKeys 204, pp. 43-56 : 43

Hymenasplenium tholiformis Liang Zhang, W.B. Ju & K.W. Xu sp. nov.

Type.

China. Xizang: Medog County, Beibeng Xiang, Xirang , ca. 600 m from the Yarlung Zangbo River , 29°11'17.63"N, 95°03'42.27"E, 720 m elev., 28 Oct 2021, Liang Zhang & Wen-Bin Ju 4781 (holotype: KUN1543824!; isotypes: CDBI!, KUN!) .

Diagnosis.

Hymenasplenium tholiformis is morphologically most similar to H. szechuanense , but different by having larger pinnae (middle pinnae 2.5-3 cm vs. 1.5-2.3 cm), extremely ascending upper pinnae (vs. spreading or slightly ascending), curved margins of acroscopic side of pinnae (vs. truncate), and pinna-marginal teeth entire and veins terminating just below marginal teeth (vs. pinna-marginal teeth retuse to emarginate and veins terminating just below these notches).

Description.

Plants perennial, 20-36 cm. Rhizome long-creeping, ca. 2 mm in diam., apex scaly; scales dark brown, lanceolate or narrowly triangular, 0.5-1 × 0.2-0.3 mm, margins entire; roots yellowish brown when dried. Fronds remote, 10-12 mm apart, subglabrous; stipe shiny, black purple, 8-13 cm long, base ca. 2-3 mm in diam., with scales similar to those on rhizome; lamina herbaceous, once pinnate, narrowly oblong to lanceolate, 13-16 × 3-5 cm, base truncate and slightly reduced, apex acuminate to caudate; rachis 0.5-1 mm in diam., wingless, narrowly grooved adaxially, shiny, glabrous, black purple to dark purple; pinnae 15-21 pairs, trapeziform to trapeziform-lunate, basal pinnae nearly opposite, spreading or slightly ascending, upper pinnae alternate, extremely ascending, middle pinnae alternate, ascending, 2.5-3 × 0.6-1 cm, dimidiate, pinna asymmetrical, base largest, upper part of pinna enlarged, similar width as, or slightly wider than, the middle part of pinna, apex obtuse to rounded, acroscopic margins curved and irregularly toothed, teeth entire, basiscopic margins truncate or slightly curved and entire (Fig. 1 ). Veins visible on both sides of pinnae, free, forking and terminating in marginal teeth, 3-4 basal basiscopic veins lacking. Sori medial, linear or semi-elliptic, 5-8 pairs per pinna (Fig. 1 ), 2-3 mm long; indusia persistent, brown, linear or semi-elliptic, membranous, entire, opening toward costa. Spores elliptic to reniform, perispore fimbriate-alate, 43-47 μm in diam. (Fig. 1 ); 32 spores per sporangium.

Distribution and conservation assessment.

Hymenasplenium tholiformis is endemic to Medog County. Currently, only one large population with ca. 35 individuals was found. According to IUCN Red List criteria B2a or D ( IUCN 2022), this species should be listed as critically endangered (CR). More extensive fieldwork at low elevations in nearby mountains will be needed to accurately assess its conservation status.

Ecology.

Hymenasplenium tholiformis was observed in a shady place at the bottom of a large rock in the disturbed forest, at an elevation of 720 m, ca. 600 m from the Yarlung Zangbo River. High humidity and cool conditions are important for the growth of the new species.

Etymology.

The specific epithet alludes to dome shape of pinna apex.

Vernacular name.

yuan ding mo ye tie jiao jue (圆顶膜叶铁角蕨; Chinese name).

Comments.

In Medog County, ferns are highly diverse along the Yarlung Zangbo River and its tributaries. In this region, at elevations between 650 m and 4500 m, we have discovered four species in three subclades of Hymenasplenium , including H. cheilosorum (Kunze ex Mett.) Tagawa in the H. cheilosorum subclade, H. obliquissimum (Hayata) Sugim. in the H. obliquissimum subclade, and H. excisum and H. tholiformis in the H. excisum subclade. Of the four species, H. tholiformis is distributed at the lowest elevation, while H. obliquissimum is at the highest elevation between 2100 m to 2250 m.

Figures

Figure 1. Hymenasplenium tholiformis sp. nov. A habit B crozier C adaxial lamina D abaxial lamina E adaxial pinnae F abaxial pinnae, showing sori G rhizome scales H spore surface I spores. Photos credit: A-F by L. Zhang; G-I by W. - Z. Ma & Y. - L. Qiu.