The new species is morphologically similar to Microchirita hamosa, but it differs from the latter in its corolla tube 5–6 mm long, with four yellow patches, each one originating at the base of each filament and reaching the throat, the throat-base diameter ratio of the corolla tube 1–1.2 (vs. corolla tube 8–15 mm long, with one yellow patch ventrally, the throat-base diameter ratio of the corolla tube 2–3); stamens 4, 2–2.5 mm long, ca. 0.3 mm in diameter (vs. stamens 2, ca. 1.5 mm long, ca. 0.1 mm in diameter); pistil 6–8 mm long (vs. ca. 16 mm long); ovary 2–3 mm long (vs. ca. 14 mm long).

Figure 1. 

Microchirita minor sp. nov. A habit B flower C opened corolla with stamens and staminodes; D abaxial surface of calyx lobes E pistil and calyx. Drawn by Rui-Feng Li.

Vietnam. Son La Province: Moc Chau District, Xuan Nha Nature Reserve, in moist crevices of the limestone cliff and mouth of limestone caves in a subtropical evergreen seasonal rainforest, 20°43'32"N, 104°40'50"E, elev. ca. 539 m, 1 November 2019, F. Wen, T.V. Do, Z.B. Xin & S. Maciejewski VMN-CN 1231 (holotype VNMN!; isotypes IBK!, VNMN!).

Annual herb, up to 7 cm tall. Stems maroon green, erect or sub-erect, sparsely eglandular pubescent; unbranched. Leaves 1, rarely 3 to 5, lowermost solitary, leaves towards apex opposite; petioles 1–10 mm long, densely and shortly pubescent; blades mid-green adaxially, paler abaxially, ovate to elliptic, 1–10 cm long, 0.5–8 cm wide, base cordate broadly attenuate to obtuse, apex acute to acuminate, eglandular pubescent adaxially and abaxially, margin near entire, mid-rib impressed adaxially, prominent abaxially, lateral veins 5–7 pairs, sparsely eglandular pubescent. Inflorescences cristate, epiphyllous, 5–15-flowered; peduncles extremely short, ca. 1 mm long; bracts absent; pedicels pale green, 4–6 mm long, densely glandular and eglandular pubescent. Calyx 5, segments unequal, upper lobes 3-parted to near the base, lower lobes 2-parted to the base, the central upper lobe (alternate to the upper corolla lobes) shorter and thinner than the other lobes, pale green, lobes lanceolate, larger lobes 5–7× ca. 1 mm, smaller lobes ca. 4 × 0.6 mm, apex acuminate, margin entire, densely glandular and eglandular pubescent outside, glabrous inside. Corolla 8–10 mm long, tube white, with four yellow patches inside the tube, each one of them originating at the base of each filament and reaching the throat, corolla tube tubular, 5–6 mm long, 1.7–2.2 mm in diameter, eglandular hairy outside, glabrous inside; lobes elliptic, upper lobes 1–1.2 × 1.2–1.5 mm, lower lobes 1.2–1.5 × 1.5–2.0 mm. Stamens 4, inserted ca. 3 mm above the corolla base; filaments straight, white, glabrous, 2–2.5 mm long, ca. 0.3 mm in diameter; anthers white, papilionaceous, ca. 0.8 × 0.6 mm; staminode 1, adnate to ca. 2.5 mm above the corolla base, 0.3–0.5 mm long. Disc annular, margin entire. Pistil 6–8 mm long, densely glandular and eglandular pubescent from the base, more sparsely towards the stigma; ovary 2–3 mm long; style 3–4 mm long, eglandular pubescent, stigma elliptic, ca. 1 × 0.8 mm. Capsule green, 1.4–1.6 cm long, 1.5–2.0 mm in diameter, eglandular pubescent, straight.

Figure 2. 

A–H Microchirita minor sp. nov. I–J M. hamosa A habitat B plants with flowers C abaxial surface of leaf blade D adaxial surface of leaf blades E calyx lobes (adaxial surface above, abaxial surface below) F lateral view of the flower G pistil and calyx H opened corolla with stamens and staminodes I habitat J plant with flowers. (Photos C–H from type material).

Latin minor, smaller, alluding to size of plants and flowers.

Flowering was observed from October to November. Fruiting from November to December.

The new species is currently only known from one population within Xuan Nha Nature Reserve, Moc Chau District, Son La Province, north-western Vietnam. The new species grows in moist crevices of the limestone cliff and mouth of limestone caves in a subtropical evergreen seasonal rainforest, at elevations of 530–545 m.

The new species is only known from a single population in Xuan Nha Nature Reserve, Moc Chau District, Son La Province, north-western Vietnam. This single population has no more than 1000 mature individuals, all growing on moist and shaded rocky surfaces on the cliff. They are easily disturbed by human activities as the known habitat is located in the buffer zone of the Nature Reserve and near the sugar-cane field. The species is provisionally assessed as data deficient (DD), following the IUCN Red List Categories and Criteria (IUCN 2022), because more surveys are needed.

The most striking character of Microchirita minor is the tiny, white corolla with four stamens. It is most easily confused with M. hamosa, from which it differs in the much smaller corolla, shorter corolla tube (5–6 mm long vs. 8–15 mm long), with four yellow patches inside the tube, each one of them originating at the base of each filament and reaching the throat, four larger and sturdier stamens (2–2.5 mm long, ca. 0.3 mm in diameter vs. ca. 1.5 mm long, ca. 0.1 mm in diameter) and shorter pistil (6–8 mm long vs. ca. 16 mm long). Detailed morphological comparisons of the new species with M. hamosa are shown in Table 1. The floral measurements of M. hamosa were mainly derived from Wang et al. (1990, 1998), Puglisi and Middleton (2017) and our own observations and dissection.