Similar to L. wallichii, L. erianthus and L. funingensis, but differs mainly by its anisophyllous leaves (vs. isophyllous), stipules usually 2-lobed (vs. entire or 3-lobed), 1-flowered inflorescence (vs. several-flowered), homostylous flowers (vs. heterostylous), and corolla tubular-campanulate, 3.2–4 cm long (vs. tubular, tubular-infundibulariform or tubular-salverform, 1.4–1.6(–3) cm).

Vietnam. Son La province: Muong La district, Ngoc Chien commune, Muong La Nature Reserve, around point 21.61032°N, 104.10576°E, elevation 1320–1350 m a.s.l., remnants of primary evergreen broad-leaved forest along streams at base and on steep slope of sandstone mountain, lithophytic perennial herb 30–45 cm tall, in wet places, locally common, 2 March 2019, Nguyen Sinh Khang, Mai Van Duc and Lo Van Chieu, NSK 1153 (holotype CSFI [CSFI069614]; isotypes CSFI, HN, LE [LE01058686, LE01058688, photo: LE01061374]).

Figure 1. 

Leptomischus hiepii A habit B leaf, upper portion, adaxial surface; lower portion, abaxial surface C capsule, view from above D dissected flower showing stamens and pistil. Drawn by Bi-Shan Li from the holotype in CFSI.

Figure 2. 

Leptomischus hiepii A typical habitat B, C habit D unequal leaf pair and stipules E inflorescence F flowers, view of the throat G dissected flower H calyx lobes and disc, seen from above. Photos and design by K.S. Nguyen and L. Wu.

Figure 3. 

Leptomischus hiepii A longitudinally dissected ovary B transversely dissected ovary C mature fruits and viviparous seeds D longitudinally dissected capsule E Transversely dissected capsule F seeds. Photos and design by K.S. Nguyen.

Perennial herb, lithophytic and occasionally terrestrial, 30–45 cm tall. Stem ascending or drooping, somewhat straight, simple or branched, terete, glabrous, green, with internodes of 6–30 mm long, 2–3 mm in diameter. Stipules persistent, glabrous, green, slightly equal to unequal, broadly to narrowly ovate, (4–)8–10(–16) × (3–)4–8(–10) mm, distally often divided up to 2/3^th of their length into 2 narrowly ovate to broadly lanceolate, 3–5(–9) mm long obtuse lobes, usually bearing 2–3 longitudinal veins united at base and 3–5 lateral veins on each side. Leaves anisophyllous, glabrous on both surfaces, dark green above, paler green below, subsessile to shortly petiolate; petioles terete, 0.5–2.5 mm long, glabrous; leaf blades lanceolate to narrowly lanceolate, cuneate at base, gradually attenuate to caudate at apex, margin entire, often irregularly wavy; veins prominent on both surfaces; of a pair of leaves, the smaller one (8–)10–20(–30) × (2.5–)3–5(–8) mm, with (3–)5–7(–9) secondary veins on each side of midrib, the larger one (3–)5–9(–11) × (0.4–)0.8–1.2(–1.5) cm, with (9–)11–15(–17) secondary veins on each side of midrib. Inflorescence terminal, single-flowered; peduncle ca. 1 mm long, densely puberulent; bracts 2, subulate, 5–6 mm long, ca. 1 mm wide at base, acute, puberulent outside, glabrous inside. Flowers solitary, sessile, 6-merous, sometimes 5-merous, bisexual, homostylous. Calyx campanulate, densely puberulent outside; hypanthium obconic, 2.5–3.2 mm long, lobes triangular, acute, 2.2–4 × 1.6–2.5 mm, almost glabrous inside, ciliate along margin, somewhat recurved at anthesis. Corolla tubular-campanulate, (3.2–)3.4–3.8(–4) cm long, (1.2–)1.3–1.5(–1.7) cm in diameter at the throat, pure white, shortly densely villous outside, almost glabrous inside; corolla lobes (5–)6, triangular ovate, straight spreading or slightly recurved at anthesis, 5–8 mm × 4–7 mm, tips acute. Stamens (5–)6, filaments white, glabrous, connate with corolla tube from base to 5–6 mm below the throat, free parts terete, 1.8–2.6 mm, slightly incurved; anthers pale yellowish, oblong elliptic, 2.2–2.5 × 0.6–0.8 mm, dorsifixed, introrse. Ovary inferior, bilocular; disk glabrous, marginally convex and concave at the center; style erect, filiform, 2.5–3.2 cm long, white, glabrous; stigma dull brownish, finely papillose, 2-lobed; lobes narrowly oblong or linear, 2.5–3.5 mm long, recurved at a straight angle from the style axis, positioned 1.4–1.8 mm below the throat, slightly above anther apices. Fruit capsular, subglobose, ca. 3–4 mm in diameter, crowned by persistent calyx lobes, dehiscent through apical portion, pericarp and septum membranous, brown; placenta fleshy, brown, mushroom-shaped during anthesis then turning dark brown, woody, and broadly conoid (when dried), attached to middle of septum, distally bearing numerous seeds; seeds angled, 0.3–0.5 mm; testa reticulate or verrucous, black.

The specific epithet honors Dr. Nguyen Tien Hiep, a famous botanist who made significant contributions to the plant taxonomy and nature conservation in Vietnam.

(paratypes). Vietnam. Son La province: Muong La district, Ngoc Chien commune, Muong La Nature Reserve, same location as type specimen, lithophytic perennial herbs bearing fleshy capsules, fruit opened by an apical operculum, 22 August 2020, Nguyen Sinh Khang & Lo Van Chieu, NSK 1347 (HN).

Lithophytic or terrestrial herb growing on rocks in streams and on moist cliffs under primary and secondary evergreen broad-leaved submontane forest on sandstone at elevations of 1300–1400 m a.s.l.. The plants flower in February–March, and bear fruits in July–August. The species was observed as being very common on waterfall cliffs and in humid places; around 300–500 mature individuals occur in Muong La Nature Reserve, spread over a very limited area of approximately 2500 m². In Muong La Nature Reserve, agricultural activities and exploitation for timber and non-timber forest products are prohibited. According to indigenous people, this species is not used as medicinal or ornamental plant, and disturbances to its existence so far have not been recorded. Numerous localities in the mountainous areas of the Hoang Lien Son range, spreading over the provinces of Lai Chau (Phong Tho, Tam Duong and Than Uyen districts) and Lao Cai (Bat Xat, Sa Pa and Van Ban districts) to Yen Bai (Tram Tau, Nghia Lo, Van Yen and Mu Cang Chai districts) and Son La (Muong La and Bac Yen districts), in north-western Vietnam and the south-western parts of Yunnan province in south-western China, fit the habitat characteristics of this new species. It is therefore expected that more populations of the species will be discovered soon if extensive field surveys are carried out in this region. At present, however, L. hiepii can be considered as an endemic species to Son La province, and is tentatively assessed as “data deficient” (DD) in accordance with the IUCN Red list categories and criteria (2017).

Endemic to north-western Vietnam (Son La province, Muong La district, Muong La Nature Reserve).

Leptomischus hiepii is a very special member of the genus in several ways, and is inconsistent with most of its congeners. Typical characters of L. hiepii are anisophyllous leaves, 1-flowered inflorescences, homostylous (5–)6-merous flowers and anthers and stigma positioned at the level of the throat of the corolla tube, while all currently known species of the genus are reported as having isophyllous leaves, several-flowered inflorescences, heterostylous 5-merous flowers, and anthers positioned much lower than the stigma or vice versa. Anisophyllous and isophyllous leaves are commonly seen in Mouretia, Mycetia and Argostemma Wall. within the tribe Argostemmateae (Chen and Taylor 2011). The 1-flowered inflorescence has not been recorded in Leptomischus but is reported in Argostemma, e.g. in A. bachmaense T.V.Do (Do et al. 2020). Homostylous flowers have hitherto not been recorded in the tribe Argostemmateae but the rare presence of homostylous flowers in otherwise heterostylous genera is not rare in Rubiaceae (Chen and Taylor 2011), e.g., homostylous flowers are reported in Mussaenda campanulata T.T.Duan & D.X.Zhang (Duan et al. 2016) and Ophiorrhiza longifloriformis Schanzer (Schanzer 2005). Flower merosity is variable in the genera Mycetia, Argostemma (Chen and Taylor 2011) and Leptomischus as well, or even within the same inflorescence of a species such as L. erianthus bearing 4–7-merous flowers (Fig. 4B, E). Anthers and stigma positioned at the level of the throat of the corolla tube are a common feature in homostylous flowers of Rubiaceae and also occurs in Neohymenopogon Bennet of the tribe Argostemmateae (Chen and Taylor 2011).

Figure 4. 

Leptomischus erianthus A habit B inflorescence C flowers D Dissected longistylous flower E dissected brevistylous flower F capsules, seen from above. L. funingensis. G infructescence, seen from above H transversely dissected ovary I Longitudinally dissected ovary. Scale bars: 1 cm (A–G); 2 mm (H, I). Photos and design by L. Wu.