Vietnam. Ha Giang Province: Vi Xuyen District, Minh Tan Village, primary evergreen broad-leaved forest, around point 23°00'14.9"N 104°54'55.9"E, altitude 533 m, 19 January 2021, N.V.Ly, NVL 20210119001 (holotype: VNF!; isotype: LE http://en.herbariumle.ru/?t=occ&id=91550).

Hemiboea chanii is similar to H. crystallina Y.M.Shui & W.H.Chen in the shape of leaf blade, leaf margin and the externally pubescent corolla, but differs in having non-inflated nodes, diamond-shaped not winged involucre, flat calyx margin, corolla red-spotted inside with a ring of hairs, and stigma slightly swollen. It also morphologically resembles H. sinovietnamica W.B.Xu & X.Y.Zhuang but differs in having 4–8 branches on main stem, repand-crenate leaf margin, involucre diamond-shaped with a cirrose apex, and pink corolla and a slightly swollen stigma (Table 1).

Perennial lithophytic herb. Stem ascending to erect, with 12–16 nodes on the main stems, not inflated, glabrous, subterete, 50–90 cm tall, 3–7 mm in diameter, with 4–8 branches. Leaves petiolate, opposite, unequal to sub-equal in a pair; petiole 1.5–4.5 cm long, about 2 mm in diameter, glabrous, green to purple; leaf blade narrowly ovate, ovate, oblong or elliptic, coriaceous when dry, 7.5–12 cm long, 2.5–5 cm wide, glabrous, adaxial surface dark green, abaxial surface pale greenish, at base cuneate, sometimes slightly oblique, apex acute or shortly acuminate, repand-crenate along the margin, median and lateral veins inconspicuous adaxially and protuberant abaxially, lateral veins in 6–10 pairs. Inflorescence subterminal, 2–3-flowered cyme; peduncle 0.8–1 cm long, about 1 mm in diameter, glabrous, green to pale green; involucre diamond-shaped, not winged, 1.6–1.8 cm in diameter, outside glabrous, with two cirrose opposite apices. Calyx white, actinomorphic, 5-lobed, dissected from the base; segments subequal, narrowly lanceolate 1.3–1.4 × 0.2–0.3 cm, glabrous, margin entire flat, 1-veined. Corolla infundibular, outside pink with numerous red spots on adaxial lip, inside red spotted, 3.5–4 cm long; tube 3.0–3.6 cm long, 1.4–1.6 cm in diameter at the orifice, 3–4 mm in diameter at the base, sparsely glandular pubescent outside; inside with a ring of hairs adnate to 3–4 mm above the corolla base; limb distinctly two-lipped; adaxial lip 3–4 mm long, 2-lobed at apex, lobes subequal, nearly semi-circular, margin recurved; abaxial lip 6–8 mm long, 3-lobed, lobes unequal, with rounded apex, median lobe larger, broadly ovate, lateral 2 smaller, slightly obliquely ovate. Stamens 2, adaxial, adnate to 1.1–1.3 cm above the corolla base; filaments filiform, coiled, 1.2–1.4 cm long, 1 mm in diameter; anthers basifixed, globular, 1.5–1.7 mm in diameter, coherent at the apex. Staminodes 3, linear, glabrous, with inflated apex, adnate to 13–15 mm above the corolla base, the middle one 2–3 mm long, lateral ones 9–10 mm long. Disc circular, lemon-yellow, 1.2–1.4 mm high, margin repand, glabrous. Pistil 24–26 mm long; ovary narrowly cylindrical, glabrous, 7–8 mm long, about 2 mm in diameter; style 17–18 mm long, terete, glabrous, about 1 mm in diameter, apex curved; stigma slightly swollen, truncate. Capsule oblique in relation to pedicel, terete to narrowly fusiform, 2.6–3 cm long, glabrous, slightly curved, dehiscing adaxially; valves 2, straight, not twisted.

Figure 1. 

Hemiboea chanii C.H.Nguyen & Aver., sp. nov. A plants in natural habitat B leaf blade, abaxial surface C flowering branch D flower, frontal view E–G flower, side and half side views H stamens and staminodes I involucre J flower inside and outside views K ripening capsule, cross section L-M pistil and ripening capsule, side view N calyx segments. Photos by Nguyen Van Ly, correction and design by C.H. Nguyen.

Figure 2. 

Hemiboea chanii C.H.Nguyen & Aver., sp. nov. isotype specimen detail. N.V.Ly, NVL 20210119001 (isotype: LE 01077046 http://en.herbariumle.ru/?t=occ&id=91550).

The new species is only known from Minh Tan Village, Ha Giang Province, growing in cracks of limestone rocks in moist shady places or occasionally in the upper part of slopes, and on rocky hilltops in primary evergreen broad-leaved forests on karstic limestone at elevations 500–700 m a.s.l. Some of main herbaceous species accompanying the new species have been recorded as Aglaonema modestum Schott ex Engl., Amorphophallus tonkinensis Engl. & Gehrm., Begonia spp., Impatiens bonii Hook.f., Laportea interrupta (L.) Chew, Primulina balansae (Drake) Mich.Möller & A.Weber, and Rhaphidophora decursiva (Roxb.) Schott

Flowering from January to February, and fruiting from February to April.

Hemiboea chanii is named in honor of the lecturer, Mr. Le Mong Chan, for his outstanding contributions to the conservation of the flora of Vietnam.

At present, only one known population with less than 100 mature individuals is confirmed in the field assessment. Its estimated area of occupancy is less than 5 km². The population and habitat are greatly susceptible to various human activities and damage, particularly forest logging, agriculture, and grazing. There is a high risk of habitat degradation in the future because it is located close to the local village. Following the IUCN Red List Categories and Criteria (IUCN 2017), the new species may be assessed tentatively as Critically Endangered [B2ab (ii, iii), CR].

Hemiboea chanii is morphologically similar to H. crystallina and H. sinovietnamica in its ecology. It has only been found in a limestone area and grows in the evergreen broad-leaved forest. The new species can be easily distinguished from H. crystallina in having a subterete stem up to 90 cm tall (vs. stem terete up to 40 cm tall), non inflated nodes (vs. nodes inflated), 1-veined calyx with flat, not revolute margin (vs. calyx 3-veined with revolute margin), corolla red spotted inside with a ring of hairs (vs. corolla inside purplish-red spotted with pale yellowish lines and absence of hair ring), and stigma slightly swollen (vs. stigma not swollen). The new species differs from H. sinovietnamica in having a subterete, 4–8 branched stem up to 90 cm tall (vs. stem subtetragonal, simple, up to 45 cm tall), repand-crenate leaf margin (vs. entire leaf margin), involucre diamond-shaped with a cirrose apex (vs. involucre triangular with acute apex), corolla pink (vs. corolla yellowish), stigma slightly swollen (vs. stigma distinctly capitate), and a flowering period lasting in January-February (vs. flowering period lasting during August-October). The comparison of the key morphological characters of H. chanii, H. crystallina, and H. sinovietnamica is presented in Table 1.