The new species is similar to Begonia crystinilla Y.M. Shui & W. H. Chen in the thick texture and shape of leaves, but differs in the flat adaxial surface of leaves with sparse setae (vs. the uneven with dense setae), pubescent abaxial surface of leaves (vs. lane), triangulate major wing of fruit (vs. semi-lunar).

CHINA. Guangxi Zhuang Autonomous Region, Jingxi county, Banliang community, 22°53'44"N, 106°26'22"E, 260 m a.s.l., at the roadside in bushes on the shady slope in the limestone hills, in fruits, 13 May 2017, Y. M. Shui et al. B2017-300 (holotype, KUN).

Rhizome: stout, 7–10 cm long, 1–1.5 cm in diam. Stipule reddish, triangular, adaxially glabrous, abaxially villous, 1–1.5 × 0.5–0.7 cm. Leaves: petiole terete, dark red or brown, densely reddish expanding villous or strigose, 5–20 cm long, 0.5–0.7 cm in diam.; blade asymmetric, widely suborbicular to reniform, 7.5–14 × 6–11 cm; base cordate, apex subacute to obtuse; margin long ciliate; adaxially greenish, extremely sparsely short setulose; abaxially dark reddish, velutinous, densely strigose on reticulate veins; venation palmate, 5–6 primary veins, secondary veins brunching dichotomous, tertiary veins netted and obviously reticulate. Inflorescence: dichasial cyme, peduncle 13–16 cm long, villous; bracts caducous, broadly ovate to orbicular, margin serrate-ciliate, apex obtuse to rounded. Staminate flower: pedicel 0.6–1.0 cm long; tepals 3–4, white to pink, adaxially glabrous, abaxially hirsute-villous; outer tepals 2, broadly ovate, 1.2–1.5 × 1–1.3 cm, abaxially white to pale pink, glabrous to sparsely red setulose, base cuneate to rounded, apex obtuse to rounded, margin entire; inner tepals 2, white to pale pink, oblanceolate, 1–1.5 × 0.2–0.5 cm, base cuneate to rounded, apex acute to obtuse, margin entire; androecium actinomorphic, stamens numerous; filaments longer than anthers, slightly fused at base; anthers yellow, obovate, 0.5–1 mm long, apex truncate, shorter than filaments, with longitudinal slits. Pistillate flower: pedicel 1.5–2 cm long; tepals 3, pinkish or white; outer tepals 2, widely obovate to orbicular, 1.5–1.7 × 1.2–1.5 cm, base cuneate to rounded, apex rounded, margin entire; inner tepal 1, elliptic to oblanceolate, 1–1.3 × 0.2–0.4 cm, base cuneate to rounded, apex acute, margin entire; styles 3, fused at base; stigmas spirally twisted; ovary pinkish, trigonous ellipsoid, 0.6–1.2 cm long, 0.4–0.7 cm in diam. (wings excluded), red villous, 1-loculed; placentation parietal upper and axile at base. Capsule pinkish when fresh, brownish when dried, trigonous-ellipsoid, 1–1.5 mm long, 0.5–0.7 mm in diam. (wings excluded), 3-winged unequally; abaxial wing lunate, 0.6–1 ×0.3–0.6 cm; lateral wings narrowly lunate, 0.6–1 × 0.2–0.4 cm; Seeds oblong, ca. 0.2 mm long.

Flowering in August–November, fruiting in December–March next year.

The epithet refers to the reticulate pattern of nerves on the abaxial surface of leaves.

The species only grows on rocks at the entrance to caves or on the shady slope in limestone forests.

The species distributes to the border region between China and Vietnam, e.g. Jingxi county of Guangxi in China and Cao Bang and Tuyen Quang Province in Vietnam.

CHINA. Guangxi Zhuang Autonomous Region, Jingxi county, Renzhuang community, 13 May 2017, Y. M. Shui et al. B2017-301 (KUN!). VIETNAM. Cao Bang Province, Tra Linh county, Quoc Toan committee, 22°45'56"N, 106°17'32"E, 11 April 2016, Y.M. Shui, W.H. Chen, C. Liu, H.Q. Nguyen,H.T. Nguyen, N.Q. Chuong CK0953 (KUN!, CPC!); the same locality, Thang Moons mountain, 12 Apr 2016, Y.M. Shui, W.H. Chen, C. Liu, H.Q. Nguyen,H.T. Nguyen, N.Q. Chuong CK1005 (KUN!, CPC!). Tuyen Quang province, Na Hang district, Sinh Long, 22°32'36"N, 105°23'42"E, 300 m a.s.l., 28 November 2017, flower pinkish, Y.M. Shui, W.H. Chen, S.W. Guo, H.Q. Nguyen,H.T. Nguyen, N.Q. Chuong, K.S. Nguyen CK1453 (KUN! CPC!).

In Begonia sect. Coelocentrum Irmsch., the new species is unique in the dense reticulate nerves on the abaxial surface of leaves (Shui et al. 2002; Averyanov and Nguyen 2012). As to the morphology of leaves (thick texture, the even abaxial surface and broadly ovate lamina) and flowers (glabrous pink flowers and glandular hairs outside petals), the new species is similar to B. crystallina, B. lanternaria Irmsch. and B. longgangensis C.-I Peng & Yan Liu (Shui and Chen 2017). However, the latter three species are obviously different from the new species in their slightly pubescent abaxial surface of leaves. In Cao Bang Province, Vietnam, there already are the other three new species described recently (Peng et al. 2015a), viz. B. caobangensis C.-I Peng & C. W. Lin, B. circularis C.-I Peng & C. W. Lin and B. melanobullata C.-I Peng & C. W. Lin. The new proposed species is different from the above three species. During the survey in North Vietnam, B. caobangensis has been firstly discovered in the field in Tuyen Quang Province instead of Caobang Province. B. circularis seems to be similar as B. lanternaria Irmsch. but slightly different in the morphology of leaves and variable wings of fruit, while B. melanobullata is also similar to B. nahangensis Aver. & H.Q.Nguyen in Vietnam instead of B. ferox C.-I Peng & Yan Liu in China, but still slightly different with black hairs on the abaxial surface of leaves.