Polystichum xuansonense Li Bing Zhang, N. T. Lu & Liang Zhang 2023, sp. nov.

Polystichum xuansonense Li Bing Zhang, N.T.Lu & Liang Zhang, sp. nov. (Figs. 3 & 4). Type:— VIETNAM. Phu Tho Province: Tan Son District, Xuan Son Commune, Xuan Son National Park, limestone mountain, 21° 7’58.11” N, 104°55’41.16” E, elev. ca. 345 m, 27 November 2013, Li Bing Zhang, Liang Zhang & Ngan T . Lu 6565 (holotype VNMN!; isotypes CDBI!, MO). Diagnosis:— This species is somewhat similar to Polystichum excellens Ching (1958: 234) in having oblong-lanceolate pinnae, but the former has middle pinnae with undulate to bluntly dentate acroscopic margins, proximal margins of pinnae parallel to or overlapping the rachis, and rounded pinna apex, whereas the latter has middle pinnae with low obtuse teeth or nearly repand acroscopic margins, proximal margins of pinnae forming an angle of 15–30° with the rachis, and acute pinna apex. Plants medium-sized, evergreen, (13–) 35–45 cm tall. Rhizomes erect, ca. 1.5 cm long, ca. 1.2 cm in diam.; roots brown blackish when dried, up to 20 cm long, ca. 0.5 mm in diam. Fronds 9–14 per rhizome, 10–43 cm; stipe brown, 4.5–10 cm long, ca. 1.0 mm in diam., basal stipe scales brown, membranous, lanceolate-ovate, 0.2–1.1 × 2.2–4.1 mm, margins entire, apex short acuminate; distal petiole scales lanceolate, subulate, 0.2–0.5 × 1.0– 2.12 mm, membranous, margins subentire to irregular dentate, apex long-acuminate or caudate. Laminae 1-pinnate, linear-lanceolate, 18–35 cm long, 2.3–4.5 cm wide, apex acuminate; rachises ca. 1 mm in diam., stramineous, adaxially sulcate; abaxially and adaxially with subulate scales, membranous, light brown, margins long outward, apex long-caudate. Pinnae 17–32 pairs, alternate, attached at 70° (–90°) angles to rachis, separate from each other, 0.6–1.2 cm distant (between two adjacent points), acroscopic auriculate; basalmost pinnae rhombic-ovate, 0.5–1.2 × 2.0–6.0 mm, shortly stalked, 2 basal pairs ca. 0.5 mm apart, apices acute or obtuse, bases obviously asymmetrical, acroscopic margins blunt dentate, basiscopic margins entire; middle pinnae largest, oblong-lanceolate, 0.4–0.7 × 1.3–2.5 cm, shortly stalked, bases cuneate, proximal margins parallel to or overlapping the rachis, acroscopic margins undulate to bluntly dentate, basiscopic margins entire or slightly dentate upward, apices rounded; adaxially glabrous, abaxially sparsely covered with linear light brown microscales; frond texture papery; venation pinnate, distinct abaxially, slightly distinct adaxially, lateral veins nearly reaching pinna margin. Sori small, terminal on lateral veins of fertile pinnae, 1.1–1.4 mm in diam., closer to margins than to midrib, 1–12 on acroscopic side and 1–5 on basiscopic side, centers ca. 2 mm apart from one another; indusia present, light brown, entire. Etymology:— The species epithet is from Xuan Son, the name of the national park in northern Vietnam, where the new species was discovered. Distribution:— This species is currently known to be endemic to the type location. Notes:—Both Polystichum xuansonense and P. excellens occur in northern Vietnam and the latter is common in limestone areas in Cao Bang, Ha Giang, and Hoa Binh provinces (Lu et al. 2014a) but not found in Phu Tho Province where the former was discovered. Polystichum xuansonense is quite rare and was found only in one small location in relatively dry forest with about 30 individuals. It might occur elsewhere and more explorations are needed. Our unpublished DNA data shows that P. xuansonense might be closely related to P. dielsii Christ (1906: 238), but the two are morphologically quite different from each other: P. xuansonense has oblong-lanceolate pinnae, whereas P. dielsii has oblong pinnae. The latter occurs in China and Vietnam (Zhang & Barrington 2013).Published as part of Lu, Ngan Thi, Zhang, Liang, Thanh, Nguyen Trung, Toan, Le Chi & Zhang, Li-Bing, 2023, Three new species and one new record of the fern genus Polystichum (Dryopteridaceae) from Vietnam, pp. 85-94 in Phytotaxa 607 (1) on pages 89-90, DOI: 10.11646/phytotaxa.607.1.7, http://zenodo.org/record/821224