Morphologically similar to several Begonia having conically bullate leaves and others with a rugulose leaf surface with white maculation and a ciliolate tepal margin, but can be easily distinguished from them by the dendritic hairs on the petioles, abaxial veins and the keeled mid-rib of the stipules, glabrous peduncles, pistillate flowers with 2 tepals and glabrescent ovaries with verrucose wings.

Figure 1. 

Begonia catbensis A plants growing on rocks in the wild B habit C apical shoot bearing stipules D young leaf, abaxial view E part of adaxial leaf surface showing bullae, setae and palmate veins F dendritic hairs on petiole and abaxial veins G portion of leaf showing reddish setae on bullae and along margin. Photos and layout by K.S. Nguyen & L.N. Dong.

Vietnam. Hai Phong City, Cat Hai District, Cat Ba National Park, remnants of primary broad-leaved evergreen forest in lowland of limestone hills, around point 20.803333°N, 106.999167°E, 50–70 m a.s.l., flowers white to greenish, fruits green, rare, 24 August 2019, W.B. Xu, K.S. Nguyen, C.R. Lin, L.N. Dong, H.Q. Nguyen & X.K. Nguyen 14002 (Holotype: HN!; Isotypes: IBK00421271!, HN!).

Figure 2. 

Begonia catbensis A apical part of inflorescence showing peduncle, pedicel, bracts, bracteoles, staminate and pistillate flowers B staminate and pistillate flowers C staminate flower (upper) in front view D stamens E pistillate flower-top view F pistillate flower, lateral view G dissected styles and stigmas H fruits and their cross-cut. Photos and layout by K.S. Nguyen & L.N. Dong.

Herb perennial, monoecious, epipetric, rhizomatous, rooting at nodes, about 25–35 cm tall. Rhizome succulent, elongate, 10–30 cm long, 4–7 mm in diameter, creeping and slightly suberect apically, pale greenish to purplish-green or brownish-red, internodes ca. 1.5 cm long, covered by sparse dendritic hairs. Stipules often persistent, ovate-triangular, herbaceous, brownish-red or purplish when young, later turning pale greenish speckled with purplish patches, 12–16 × 6–12 mm, adaxially glabrous, abaxially keeled, mid-rib with sparse dendritic hairs; margin entire or somewhat undulate and recurved; apex aristate, arista 2–3 mm long. Petiole cylindrical, succulent, 7–16 cm long, 3–4 mm in diameter, densely covered with reddish long-stalked dendritic hairs. Leaves 9–27, basal, alternate, asymmetric, unlobed, broadly ovate, 10–13 × 5–7 cm, papery, adaxially glossy, dark green or rarely brown, with slightly silvery green patches along the veins, surface densely bullate, bullae conical and tipped with a red seta 1.5–2 mm long, abaxially brownish-red to red-brown (maroon), with pale green along major veins, long-stalked reddish dendritic hairs along the veins, base strongly oblique-cordate, margin repand and serrulate with red setae 1.5–2 mm long, apex acuminate; venation palmate with 5–7 primary veins, mid-rib distinct, with 2–4 secondary veins on each side, tertiary veins reticulate or percurrent, minor veins reticulate. Inflorescences axillary, dichasial cymes branched 2–3 times, arising directly from rhizome, pedunculate; peduncle terete, 13–22 cm long, 2.5–3.5 mm thick, glabrous, pale greenish-red to red; bracts and bracteoles not caducous, oblong or oval to ovate, slightly concave at the base and distally bent outwards during flowering, pale green with several longitudinal reddish veins, margin serrate-fimbriate with cilia 1.5–3 mm long, bracts 8–10 × 4–6 mm, slightly larger than bracteoles (6–8 × 2.5–3.5 mm). Staminate flower: pedicel glabrous, 12–19 mm; tepals 4, pure white, outer 2 broadly ovate to suborbicular, 9–14 × 8–12 mm, sparsely puberulent at the proximal margin, inner 2 glabrous, elliptic to oblanceolate, 6–8 × 2.8–3.8 mm; androecium actinomorphic, spherical, 5–6 mm in diameter; stamens 27–32; filaments glabrous, 1.2–1.9 mm long, fused at base, yellowish dull white; anthers somewhat greenish-yellow, narrowly obdeltoid, 1.2–1.5 mm long, widest at apex, 0.8–1 mm wide, apex obtuse, base cuneate, opened by two longitudinal slits with orange margins. Pistillate flowers: pedicel glabrous, 7–14 mm long; tepals 2, suborbicular, 8–10 mm in diameter, greenish-white when young, later turning to dull white or pure white, glabrous, margin entire or slightly undulate with sparsely puberulent at base; ovary green, glabrescent, with verrucose wings, trigonous-ellipsoid, 1-loculed; placentation parietal, with 3 placentae, each 2 branched; styles 3, fused at base or nearly free, glabrous, glossy, yellow, 3–5 mm long, apically C-shaped, stigmatic band twisted. Capsule nodding on a stipe 9–16 mm long, trigonous-ellipsoid, 13–18 mm long, 6–8 mm thick (wings excluded), fleshy, greenish when fresh, 3-winged; wings densely verrucose, unequal, abaxial crescent shaped, 7–9 mm wide, lateral 2.5–3.5 mm wide. Seeds numerous, ellipsoid, brown.

Figure 3. 

Distribution map of Begonia catbensis and its closest related species (The map is modified from https://en.wikipedia.org/wiki/Mainland_Southeast_Asia).

The species is named after the type locality, Cat Ba National Park.

Flowering in August; fruiting in August – reported earlier.

Cat Ba National Park is composed by many islets. All islets here are limestone mountains. We have had several days to survey in Cat Ba National Park and asked guides working in this National Park for the existence of this species, but we only found and recorded a single population here. Perhaps more populations will be found if more fieldwork is done in the future in this National Park. So far, Begonia catbensis is only known from the Cat Ba National Park, Cat Hai District, Hai Phong City, in northern Vietnam. Within its distribution area, the new species grows on semi-shady tops of small rocks and on steep slopes under the broad-leaved evergreen forest in lowlands of limestone hills.

Begonia catbensis is considered as a rare species because a single population with less than 200 mature individuals clustered into 10 clumps within an area of ca. 1 km² has been recorded in Cat Ba National Park. Field observation shows that the single population is strictly managed and protected by the staff of the Cat Ba National Park and the number of individuals has been stable for at least two decades. Following the guidelines of the IUCN Red List Categories and Criteria version 14 (IUCN 2019), its conservation status is currently assessed as “Endangered” (D).

Within Begonia sect. Coelocentrum, B. catbensis is apparently closest to B. melanobullata and B. montaniformis from Vietnam (Peng et al. 2015) considering the traits of the stipules and leaves, but strikingly different from them by having dendritic (vs. unbranched) hairs on the keeled mid-rib of the stipules, petioles and abaxial veins of the lamina, 5–7 (vs. 7–9) palmate veins, 2–3-branched inflorescence with 4–10 flowers (vs. 4–6 or 3–8 branched times, with above 20 flowers), glabrous (vs. tomentose or subglabrous) peduncles, glabrous (vs. setulose or velutilous) staminate flower tepals, 2 (vs. 3)-tepalled pistilate flowers and a glabrescent (vs. sessile glandular) ovary. It resembles B. fimbribracteata (Shui and Chen 2005) from China in having broadly ovate leaves with adaxial conical bullae on the lamina and serrulate-ciliolate lamina margin, 2–3-branched inflorescence and glabrous bracts with fimbriate margins: however, B. catbensis is clearly distinguished by its larger leaves, 10–13 × 5–7 cm (vs. 6–7 × 5–6 cm), acuminate (vs. rotundate) leaf apex and glabrescent (vs. hairy) peduncle, staminate flower tepals and ovary. Begonia catbensis is easily distinguishable from B. nahangensis (Averyanov and Nguyen 2012) by its papery, asymmetric, broadly ovate leaves (vs. leathery, round to slightly asymmetric broadly oblique-ovate or oblique-reniform leaves) with an acuminate apex (vs. round to rarely broadly obtuse apex) and red setae 1.5–2 mm long along the margin (vs. white soft hairs, 3–5 mm long) and dendritic hairs (vs. villous or woolly villous hairs) on the petiole and abaxial veins and from B. variegata (Shui and Chen 2005) in having dark green or rarely brown leaves, with slightly silvery green patches along the veins (vs. dark-purple stripe near the margin of the leaves and dark brown wide bands along main veins), dendritic hairy petioles (vs. hirsute-villous), glabrous (vs. glandular hairy) peduncles, outer tepals and ovary and greenish-white to white (vs. greenish or greenish-yellow) flowers. A detailed comparison of the new species with its most morphologically similar species is listed in Table 1. To help quickly identify Begonia catbensis from its congers, a key to Begonia sect. Coelocentrum with conical bullae on their leaves is provided.

Amongst Begonia sect. Coelocentrum with a rugulose leaf surface, Begonia catbensis somewhat resembles B. ningmingensis D.Fang, Y.G.Wei & C.I.Peng and B. retinervia D.Fang, D.H.Qin & C.I.Peng from China (Fang et al. 2006) in leaf shape, with white maculation of the adaxial leaf surface and tepals with ciliolate margins, but it is strikingly differentiated from them in having a conically bullate (vs. rugulose) leaf surface, adaxially glabrous (vs. subsessile glandular) stipules with dendritic hairs on the mid-rib (vs. villous or glabrous), dendritic (vs. villous) petioles and abaxial veins, glabrous (vs. sparsely minute subsessile glandular) abaxial surface of staminate flower tepals and ovary and 2 (vs. 3) tepals of pistillate flower.