Peliosanthes thachii Luu, X. B., sp. nov.

Peliosanthes thachii Luu, X.B. Nguyen-Le & H.C.Nguyen, sp. nov. (Figs. 1 & 2) Diagnosis: —Most morphologically similar to P. curviandra but differs from the latter by its smaller leaves (petiole <20 cm long vs> 23 cm long; lamina 10–16 cm long vs. 15.5–22 cm long), spherical and narrowly open (vs. campanulate and widely open) perianth with smaller lobes (≤ 1.6 mm long × ≤ 1.4 mm wide vs. 3.1–4.2 mm long × 2.1–2.8 mm wide) and obconical (vs. funnel-shaped) tube, completely separated (vs. half-fused) filaments in fully opened flowers, strongly (vs. slightly) tortuous style and purple (vs. white) ovules. Type:— VIETNAM. Ninh Thuan Province: Bac Ai District, Phuoc Binh National Park, 11°59’11.29”N 108°44’58.91”E, 320 m elevation, collected on 7 March 2021 by Hong Truong Luu, herbarium specimens prepared from the living cultivated plants on 23 March 2023 by Hieu Cuong Nguyen C-2023-002 (holotype SGN! SGN 005687, paratype SGN! SGN 005650). Herb 20–25 cm tall, evergreen, perennial, glabrous. Rhizome plagiotropic, about 1.5–2 cm long, about 4 mm in diameter, inside white. Roots white to yellowish, 2.5–13.5 cm long, 2–4 mm in diameter, inside white. Cataphylls papyraceous, up to 4 cm long. Leaves 8–9, green, petiolate, ascending or arching. Petiole rigid, sulcate, 11–20 cm long, 1–3 mm in diameter. Lamina elliptic, 10–16 cm long, 2.5–3.3 cm wide, glabrous, basally cuneate, distally acuminate, with margin entire and 9 longitudinal veins and numerous subtransversal secondary veinlets; surface of abaxial leaf epidermis slightly rugose with irregular prominent striations. Inflorescence a raceme, about 6–9 cm tall. Scape green, erect, glabrous, about 2–3 cm long, 3 mm in diameter, bearing 6–7 sterile bracts at base; sterile bracts green, lanceolate, 8.5–10 mm long, 2–4 mm wide. Rachis green, bearing about 20 flowers, 5–6 cm long, 1.6–2 mm in diameter. Floral bracts (bract and bracteole) 2 per flower, green, scarious, narrowly lanceolate; bracts attached at the lower position of the pedicel base 3–6 mm long, 0.9–1 mm wide, reducing in size toward the apex; bracteoles attached at the side of the pedicel base, 1–2 mm long, 0.5 mm wide. Pedicel 3–4 mm long, 0.6 mm in diameter. Flowers bisexual, solitary, sparsely arranged, nutant, spherical, hardly opening, 4–4.5 mm long, about 4 mm in diameter when fully opened; lobes 6, light yellowish, touching each other, ovate, subequal, incurved when fully opened, arranged in two whorls; outer lobes overlapping inner lobes, 1.4–1.6 mm long, 1.2–1.4 mm wide; inner lobes 1.3–1.4 mm long, 1.1–1.2 mm wide; tube 0.9–1.5 mm long, 1.2–2 mm in diameter; basal part (below ovary) ca. 0.3–0.4 mm long, slightly obconical, articulated with pedicel. Stamens 6; filaments 0.4–0.6 mm high, 0.45–0.53 mm wide, attached on the radii of tepals, green, flat, connate at base to almost completely separated, forming a somewhat higher coronal structure, with 3 filaments at the radii of inner tepals erect and 3 filaments at the radii of outer tepals incurved; anthers white to yellowish, dorsifixed, 0.25 mm high, 0.36 mm wide, introrse, yellow. Style strongly tortuous, attenuate toward apex, 0.7–0.8 mm long, 0.2 mm in diameter at base. Stigma with 3 shallow lobes, 0.1 mm in diameter. Ovary inferior, obconical, 3-locular, 0.55 mm high, 0.7 mm in diameter, concave at center, each locule bearing 2–4 ovules; ovules purple, on basal placenta, 0.4 mm high, 0.14–0.26 mm in diameter. Fruit unknown. Etymology:— The species is named to honor Dr. Le Buu Thach, Vice Director of the Southern Institute of Ecology, for his great contributions to this study and many other projects. Vernacular name:— Sâm cau Thạch (Vietnamese). Phenology:— Flowering from February to March; fruiting unknown. Distribution and ecology:— The species is known only from Phuoc Binh National Park and Nui Chua National Park (Ninh Thuan Province, Vietnam). It grows in damp soils along a stream at elevations of around 320 m in a semideciduous forest. Proposed IUCN conservation status: — Data Deficient (IUCN Standards and Petitions Subcommittee 2022). The species was found in a small population of less than 20 mature individuals but similar plants were observed in Nui Chua National Park in the same province. Further field surveys are needed to determine its natural distribution before a certain assessment can be made. Note: —In the Neolourya group, the new plant appears to be morphologically closest to P. curviandra in general appearance of leaves and inflorescence but both species differ from each other by several key characteristics as shown in the diagnosis. Peliosanthes thachii can be distinguishable from all the other species of the group by the smaller dimension of its leaves and floral parts, spherical hardly opening perianth, short and strongly tortuous style and purple ovules. In addition, P. choriandra has funnel-shaped perianth with very bright deep green lobes, meanwhile P. triandra and P. weberi have distinct corona formed by almost completely connate filaments. Detailed morphological differences among the compared species are shown in Table 1.