Scientific name: Begonia caobangensis C.-I Peng & C. W. LinEnglish name: Vietnamese name: Thu hải đường cao bằngOther name:
Growing as solitary plants on soil slope of 30° in forest with light to medium shade, plant not situated in proximity to the water but in drier area up the bank. Originally collected by Ms. Mary Sizemore on 19 May 2005 and distributed to American Begonia Society as Begonia U555. Type specimens pressed from plants cultivated in the experimental greenhouse in Academia Sinica, Taiwan, 9 June 2014, Peng 23895 (holotype: HAST; isotypes: A, E, HN).
Herbs, monoecious, perennial, rhizomatous. Rhizome creeping, 1–2 cm thick, internodes 0.2–0.5 cm long, minutely appressed tomentose. Stipules deciduous, reddish, ovate, 2–2.5 cm long, 0.7–1 cm wide, keeled, abaxially minutely appressed tomentose, sometimes puberulous along midrib, margin denticulate, apex aristate, arista ca. 1 mm long. Petioles reddish to crimson, flat front, rounded back (D-shaped in cross section), 3.5–8 (−10) cm long, 0.5–0.7 cm thick, minutely appressed tomentose to glabrous, narrowly winged on both sides, wings 1–2.5 mm wide. Leaves alternate, adaxially lime green, abaxially reddish to crimson, juvenile blade folding inward, mature blade flat, symmetric, narrowly elliptic to elliptic or slightly rhomboid, (8−) 11–21.5 cm long, 3.9–7.3 cm wide, apex acuminate, base attenuate, margin repand to sparsely denticulate, chartaceous to thinly coriaceous, glabrous (minutely appressed tomentose on young leaves), venation 6–8-pinnate on each side of the midrib, secondary veins branching dichotomously or nearly so, tertiary veins weakly percurrent. Inflorescences terminal or axillary on upper nodes, cymes dichasial, branched 3–4 times; peduncle 8–12 cm long, tomentose or subglabrous; bracts and bracteoles caducous, reddish, bracts ovate, ca. 1.8 cm long, 0.6–0.9 cm wide, boat-shaped, veins crimson, margin denticulate or ciliate, bracteoles similar but smaller. Staminate flower: pedicel 1.3–2.1 cm long, tepals 4, outer 2 broadly ovate, 12–18 mm long, 10–17 mm wide, abaxially reddish, sparsely setulose, inner 2 pinkish, obovate, 13–16 mm long, 6–8 mm wide; androecium actinomorphic, spheric, 6–8 mm across; stamens 85–120; anthers yellow, slightly compressed, lanceolate to oblanceolate, apex obtuse to retuse, ca. 2.5 mm long; filaments shorter than anthers, slightly fused at base. Pistillate flower: pedicel 1.2–1.8 cm long, tepals 6, pinkish-white, outer 3 obovate to elliptic, 8–14 mm long, 6–9 mm wide, inner 3 oblong to narrowly elliptic,9–18 mm long, 4–6.5 mm wide; ovary reddish, trigonous-ellipsoid, 12–15 mm long, 4–5 mm thick (wings excluded), 3-winged, wings unequal, reddish-yellow, each tipped with several succulent bristles 2–4 mm long; the 2 lateral wings 5.5–8 mm wide, shallowly triangular, verrucose, abaxial wing crescent-shaped, 10–13 mm wide; styles 2, fused at base, yellow, ca. 5 mm long, stigma strongly undulate and spirally twisted. Capsule trigonous-ellipsoid, 18–25 mm long, 4–7 mm thick (wings excluded), greenish or reddish when fresh; wings unequal, lateral wings 7–15 mm wide, abaxial wing crescent-shaped, 20–25 mm wide. Somatic chromosome number, 2n = 22.
Distribution and ecology
Northern Vietnam, around Thang Hen Lake, Tra Linh District, Cao Bang Province (Figure 3). Situated among limestone mountains, near the border of Vietnam and China. Growing on soil slope in evergreen broad-leaved forest, in light shade, at ca.1,000 m in elevation (Mary Sizemore, pers. comm.). Begonia caobangensis occurs also in Tuyen Quang province (Trinh Ngoc Bon, pers. comm.), which is adjacent to the southwest border of Cao Bang.
Begonia caobangensis resembles B. aequilateralis Irmsch., also a member of sect. Platycentrum, in having symmetric and elliptic leaves. However, B. caobangensis is markedly distinct in many other features such as blade base decurrent (vs. obtuse to cuneate), narrowly winged (vs. not winged) petiole, presence (vs. lacking) of the succulent bristles at tips of ovary wings and pistillate tepals 6 (vs. 5). A comparison of the two species is presented in Table 1. The combination of symmetric leaves, 6-tepalled pistillate flowers and the succulent bristles at tips of ovary wings make it a unique species in sect. Platycentrum.