Begonia triquetra T.P.Anh, C.W.Lin, K.S.Nguyen & B.H.Quang, sp. nov. (Figs. 1 & 2)

Type: — VIETNAM. Son La province: Quynh Nhai district, Muong Giang commune, remnants of primary evergreen broad-leaved forest on limestone hills, at base and middle of steep slopes, around point 21.63414° N 103.55421° E, elevation of 1145 m a.s.l, 6 March 2019, K. S. Nguyen & V.D. Vi, NSK 1175 (holotype HN 000078810!, isotypes HN 000078811!, HN 000078812, LE 01058690!, LE 01058691!, photographic epitype LE 01087552).

Diagnosis: —Most similar to B. xenos regarding suberect habit, subpeltate lamina, 4-tepaled staminate flowers, 2 styles, and 2-locular ovary, but differs significantly by its triangular (vs. terete) cross section of leaf petiole, un-keeled, glabrous (vs. keeled, red hairy) abaxial surface of stipules, entire (vs. slightly crenulate) staminate tepals, and zygomorphic (vs. actinomorphic) androecium with 18–24 (vs. 52–85) stamens.

Description:— Perennial, monoecious, erect or ascending herb, lithophytic. Stem slender, young stem brownish-red to olive green with sparsely white verrucose, more densely so toward nodes, becoming greyish-brown when mature, internodes 4−17 mm long, 7−11 mm thick, with sparse minute brown scale-like trichomes (by microscopic examination); young plants rooting at nodes and climbing vertically up cliffs. Stipules semi-persistent, crimson, becoming brown with age; narrowly triangular, 12–21 × 6–9 mm, herbaceous, glabrous, absent keeled, margin entire, apex attenuate and apiculate. Leaves alternate, oblique, held vertically or hanging downwards; petiole triangular, 4-ribbed (3-ribbed obviously, with 1 smaller rib on above surface); olive green tinged crimson basally, lenticels slightly prominent and creamy white, 5−14 cm long, 4−6 mm thick, with sparse minute brown scale-like trichomes (by microscopic examination); leaf blade asymmetric, ovate, 11−18.5 × 5.4–10.2 cm, broad side 3.5–7.3 cm wide, glabrous, waxy, subpeltate, apex caudate, margin subentire, sometimes obscurely denticulate, slightly undulate; leaf thinly coriaceous, succulent, adaxially olive green, sometimes tinged brownish-red along primary and secondary veins; abaxially whitishgreen to pale purplish-green, venation brownish-red; venation palmate-pinnate, midrib ca. 3 secondary veins on broad side, other primary veins branching dichotomously. Inflorescences axillary, cymose arising directly from stem, 3 or 4 orders of branching; peduncle pale green tinged red basally, 9.5–17 cm long, glabrous; bracts deciduous, ovate. Staminate flower: pedicel white to pinkish-white, ca. 15 mm long, glabrous, tepals 4, white, glabrous, margin entire; outer 2 ovate, 18–22 × 10–16 mm, apex obtuse; inner 2 oblanceolate to obovate, 7–10 × 2–4 mm, apex obtuse to acute; androecium zygomorphic, ca. 5 mm across; stamens yellow, 18–24; filaments fused at base; anthers obovate, ca. 1 mm long, 2-locular, ventrally curved, apex rounded. Pistillate flower: pedicel pale green tinged pink distally, ca. 15 mm long, glabrous, tepals 5, white to pinkish-white, ovate, 9–13 × 6–9 mm; ovary body trigonous-ellipsoid, ca. 7 mm long, 2.5 mm thick (wings excluded), white to pinkish-white, glabrous; 3-winged, wings unequal, white to pinkish-white, glabrous, ca. 9 mm long, abaxial wing crescent-shaped, rarely triangular, 4–6 mm wide, lateral wings narrowly crescent-shaped, 1.3–2 mm wide, margin entire; ovary 2-locular, placenta bilamellate; styles 2, fused at base, yellow, ca. 3 mm long, stigma spirally twisted. Capsule pedicel green, 2–3 cm long, capsule trigonous-ellipsoid, 11–14 mm long, 6 mm thick (wings excluded), greenish when fresh; wings unequal, 15–18 mm long; abaxial wing lingulate to crescent-shaped, 10–14 mm wide, lateral wings 2.5–5.5 mm wide.

Paratype: — VIETNAM. Son La province: Quynh Nhai district, Muong Giang commune, evergreen broad-leaved forest on limestone hill, around point 21.63468° N, 103.55394° E, elev. 1152 m a.s.l., 25 July 2024, T.T.P. Anh, B.H. Quang, N.T. Thanh, T.D. Binh, T.X. Thanh & L.T. Anh, TPA 502 (HN).

Distribution and ecology:— Begonia triquetra is currently known only from two close localities on the same mountain, of Muong Giang commune, Quynh Nhai district, Son La province, Northwestern Vietnam. It grows on vertical cliffs or rocks at the base or middle of a limestone hill, at elevation of 1000–1150 m a.s.l., under evergreen broad-leaved forests.

Etymology:— The specific epithet triquetra is derived from the triangular cross-section of the leaf petioles.

Conservation status:— Field observation recorded in total of 200–300 mature plants inhabiting a limestone hill of ca. 5 square km. Karst landscapes are very dominant here and estimated stretching over 300 km 2 from Tua Chua and Tuan Giao districts of Dien Bien province to Quynh Nhai district, Son La province. The vegetation of the hill or mountain in which the new plant lives is highly degraded due to unsustainable agricultural activities and deforestation in the past. Further field surveys are needed to accumulate adequate overall abundance and distribution range of Begonia triquetra. Our current data tentatively placed its conservation status here under the Data Deficient (DD) based on the guidelines of IUCN Red List Categories and Criteria version 16 (IUCN 2024).

Notes:— The new species, Begonia triquetra, is unique among the Begonia species native to Indochina due to its sub-woody stem, ribbed petioles, and subpeltate lamina. With the exception of B. xenos (see introduction), it also resembles B.quadripetiolata Aver.(2019: 135) in having 4-ribbed petioles, glabrous leaves, 4-tepaled staminate flowers, and a 2-locular ovary. A comparison of the salient characteristics of the new species with these two morphologically related species is presented in Table 1.