Polyalthia khaosokense Bunchalee, Leerat. & Sinbumroong, 2025

Polyalthia khaosokense Bunchalee, Leerat. & Sinbumroong View in CoL , sp. nov. ( Figs 1 View FIGURE 1 , 2 View FIGURE 2 ; Table 1)

Type: ― THAILAND. Surat Thani: Phanom District, Khlong Sok Subdistrict , 250 m elevation., 15 June 2020 (fl), Sinbumroong 14052020 (holotype: PSU; isotype: BKF) .

Polyalthia khaosokense View in CoL closely resembles Polyalthia kanchanaburiana Khumchompoo & Thongpukdee (2005: 35) View in CoL and P. khaoyaiensis Bunchalee & Chantaranothai (2019: 172) View in CoL , in leaf size, shape of the leaf base, number of lateral veins, inflorescence position, outer petal length and inner petal shape and length ( Table 1).

Trees 1.5‒4.0 m high with smooth, greyish-black bark and primary branching distichous. Young branches appressedpubescent, 0.2 mm long, mature branches glabrous, undistinguished lenticels. Petioles 1.0‒2.0 mm long, 1.5‒2.0 mm in diameter, groove above, puberulous; lamina symmetrically oblong to narrow elliptic, 7.5‒13.5 × 2.0‒4.0 cm, base cordate, asymmetrically cordate or slightly oblique, auricle 0.1‒0.4 mm long, apex cuspidate with an acumen 8.0‒12.0 mm long, glabrous on both sides except for the sparsely puberulous midrib; midrib grooved above; lateral veins 8‒13 per side, arched to the apex, at midpoint diverging 60‒70° from the midrib, brochidodromous venation 2.0‒4.0 mm from the margins; intersecondary veins present; tertiary veins reticulate. Inflorescences 1-flowered, terminal on young branches; pedicels 8.0‒10.0 mm long, 1.5–1.8 mm in diameter, puberulous; bracts 1, attached at the pedicel base, triangular, 1.0‒1.2 × 0.8‒1.0 mm, apex acute, puberulous outside and glabrous inside. Sepals valvate, green, ovate, 1.5‒2.0 × 2.5‒3.0 mm, apex cuspidate with acumen ca. 0.5 mm long, puberulous outside, glabrous inside. Petals greenish yellow outside, light green inside, erect or slightly spreading, pubescent outside, glabrous inside; outer petals ovate, 3.0–3.5 × 2.5–3.0 mm, apex acuminate to cuspidate with acumen 0.8–1 mm long; inner petals oblong, 10.0‒14.0 × 5.0‒ 7.5 mm, apex oblique acute and recurved. Stamens pale yellow, cuneate, 2.2‒2.5 × 0.8‒1.2 mm, filament ca. 0.2 mm long; anther 1.6‒1.8 mm long, anther connective apex truncate, androecium 5.0‒6.0 mm in diameter. Carpels 18‒20 per flower, 2.2‒2.5 × 0.3–0.4 mm, pubescent, style subsessile, stigmas pale yellow, clavate, 0.6‒0.8 mm long, pubescent, exceeding the height of the anther connective apices; ovules 1 per carpel, attached laterally. Torus cushion shape, slightly concave at the apex, 2.2‒2.4 mm in diameter, 0.9‒1.0 mm thick, glabrous. Fruits unknown.

Distribution and ecology:― Scattered in shady tropical rain forest, 250 m elevation. Known only from the type collection.

Phenology:― Flowering June.

Local name:― Tong laeng khao sok (Thai).

Etymology:― Referring to the type locality in Khao Sok National Park.

Conservation status:― Known from only the type locality with no information regarding populations and population size. Therefore, the species is data deficient ( DD; IUCN, 2019) .

Notes:― Cuspidate sepal and outer petal apices are the distinguishing features of P. khaosokense . This is absent in the other species of Polyalthia s.str., as reported in Flora of Thailand ( Bunchalee, 2022) versus cuspidate in the leaf apices of P. khaosokense and P. debilis ( Pierre, 1881: 29) Finet & Gagnepain (1906: 96) in the P. evecta complex and P. heliopetala Leeratiwong & Bunchalee (2021: 240) in the Polyalthia s.s. complex.