Polystichum unicum Li Bing Zhang, N. T. Lu & Liang Zhang 2023, sp. nov.
Creators: name: Lu, Ngan Thi
name: Zhang, Liang
name: Thanh, Nguyen Trung
name: Toan, Le Chi
name: Zhang, Li-Bing
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Plantae
Tracheophyta
Polypodiopsida
Polypodiales
Dryopteridaceae
Polystichum
Polystichum unicum

description: Polystichum unicum Li Bing Zhang, N.T.Lu & Liang Zhang, sp. nov. (Fig. 2).
Type:— VIETNAM. Ha Giang Province: Vi Xuyen District, Tung Ba Commune, Du Gia National Park (Khau Ca Nature Reserve), Hom Mountain, 22°50’40’’N, 105°07’47’’E, elev. ca. 1150 m, at the top of a limestone mountain, 13 December 2013, Li Bing Zhang, Liang Zhang & Ngan T. Lu 6933 (holotype VNMN!; isotypes CDBI!, MO!)
Diagnosis:— This is a very distinct species and does not seem to be similar to any species known so far in having the combination of crenulate pinna margins, distal basiscopic pinna margins forming a ca. 140° angle with the rachis, acute pinna apex, and auricles overlapping the rachis.
Plants medium-sized, evergreen, 23–27 cm tall. Rhizomes erect, ca. 1 cm in diam., scales similar to those of the stipe base, but softer, crushed; roots blackish when dried, up to 12 cm long, ca. 0.3 mm in diam., densely covered with red iridescent hairs. Fronds in clusters but few, 5 or more per rhizome; stipe brown, 7–9 cm long, ca. 0.6 mm in diam., scales subulate, lanceolate or ovate-lanceolate, 0.5–3.0 × 0.4–0.8 mm, brown-blackish on abaxial side, glossy-blackish on adaxial side, thick papery, margins entire, often light brown, apex acuminate or long twisted; distal petiole scales long subulate, narrow lanceolate, 0.3–0.5 × 1.2–3.2 mm, thick membranous, margins subentire to irregular dentate, apex long-acuminate. Laminae 1-pinnate, lanceolate, 2.7–3.7 × 15–20 cm, apex long acuminate; rachises ca. 1 mm in diam., stramineous, adaxially sulcate, sparsely scaly, abaxially densely scaly; scales appressed, brown, subulate with rounded base, thick membranous, long acuminate, up to 2 mm long, margins dentate at base, irregular sparsely dentate at distal part. Pinnae ca. 22 pairs, alternate, distal basiscopic margins forming a ca. 140° angle with rachis, proximal margins attached at 75° (–90°) angles to rachis, separate, 0.6–1.1 cm distant (from attached point), acroscopic auriculate; basal pinnae obliquely oblong, 0.5–1.2 × 2.0–6.0 mm, short stalked, apices acute, sometimes obtuse, bases obviously asymmetrical, acroscopic base with deltate or semicircular auricles, and proximal margins of auricles often overlapping rachis, margins both sides toothed, teeth mostly long spinulose apically; middle pinnae largest, oblong to obliquely oblong, 0.4–0.7 × 1.3–2.5 cm, short stalked, bases acute to obtuse, margins long spinulose, apices acute; adaxially glabrous, abaxially sparsely covered with appressed, subulate, filiform, light brown microscales; frond thickly papery texture; venation distinct abaxially, slightly distinct adaxially, lateral veins simple, nearly reaching pinna margin. Sori small, often on distal part of pinnae, lower pinnae not seen sori, often broken with debris on the leaves, terminal on shorter veinlets, 0.5–0.8 mm in diam., close to pinna margin than midvein, 0–6 on acroscopic side and 0–3 on basiscopic side, centers ca. 1 mm apart from one another; indusia not seen.
Etymology:— From the Latin unicum, unique, referring the unique morphology of the new species in the genus.
Distribution:— This species is currently found only in Ha Giang Province, northern Vietnam.
Notes:— Polystichum unicum is somehow similar to P. rectum Li Bing Zhang, M.Q.Han & Yan Liu in Han et al. (2018: 163) in having crenulate acroscopic pinna margins, but the former has acute pinna apex, distal basiscopic pinna margins forming a ca. 140° angle with the rachis, and auricles overlapping the rachis, whereas the latter has rounded pinna apex, basiscopic distal pinna margins forming a (60–)70–90° angle with rachis, and auricles not overlapping the rachis. In addition, the new species occurs in an open mountain top with an elevation of ca. 1150 m, whereas the latter grows in a limestone cave in southern Yunnan, China, with an elevation of 2000 m.
Polystichum unicum has auricles that overlap the rachis, which is less common (18 out of 48 species) than auricles that parallel the rachis.
description: 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 87-89, DOI: 10.11646/phytotaxa.607.1.7, http://zenodo.org/record/8212241