This new species is similar to H. succosum Kerr, as both species have shortly pedunculate inflorescences, on which the flowers open in gradual succession (with generally a single flower open) and have relatively large rotate flowers (generally > 14 mm diam.). They are separated by the presence of a distinct rachis that develops in the inflorescence (which is absent in H. succosum); the margins of the corolla lobes are revolute (vs. flat in H. succosum), the pedicels are shorter (5–10 mm, vs. 15–30 mm in H. succosum) and by the colour of the adaxial surface of the corolla (red with white-yellow spots vs. yellow-orange with reddish-brown spots in H. succosum).

Figure 1. 

Heterostemma cucphuongense T.B.Tran & Rodda A flowering branch B flower, view from above (adaxial) C flower, from underneath D corona, from above E ovary F pollinarium. (Photographs by N.Q. Dat, T.B. Tran, Thuong V.A., photo edit by M. Rodda)

Vietnam. Ninh Binh province, Nho Quan district, Cuc Phuong commune, Cuc Phuong National Park, 5 June 2019, Le Ngoc Han et al. VB 809 (HN, holotype; HN, isotype).

Liane , at least 1 m in length. Stem and branches pubescent, longitudinally grooved, 1.5–2 mm diam.; internodes 2.5–10 cm long. Leaves: petiole 5–25 × 0.6–2 mm, glabrous or sparely pubescent; lamina herbaceous, ovate to oblong, 4.5–8.5 × 2–4.5 cm, glabrous above, pubescent below, apex acuminate, base rounded to obtuse, venation pinnate with 3–4 basal secondary veins and 3–6 secondary veins departing from main vein, anastomosing near the edge of the lamina; basal colleters 6–8 at base of lamina; Inflorescences 1(–8) flowered cymes; peduncle (0)3–5.5 × 2–2.5 mm, older peduncles developing a rachis to 7 mm long. Pedicel 5–10 × 1–1.35 mm, brown-red, glabrous or sparsely pubescent. Flower 1.4–1.9 cm diam. Flower buds 5-angled, just before anthesis 0.9–1 cm diam. Sepals deltate, 1.5–2.2 × 0.8–1.2 mm, brownish-green, sparely pubescent on both surfaces; with colleters at sinus. Corolla rotate, 1.4–1.9 cm diam., red with fine white-yellow spots (becoming more concentrated towards the centre) and glabrous or sparsely pubescent adaxially; bright purple-red and glabrous abaxially; tube 3.4–4.2 × 6.8–8.4 mm; lobes deltate, 5.5–6.7 × 4.5–5.2 mm, margins recurved. Corona staminal, 4.9–5.2 mm diam., ca. 0.9 mm high, upper surface orange with red centre, lower surface dull red and shiny, glabrous or sparely pubescent; lobes broadly ovate, 1.58–1.66 × 1.10–1.14 mm, with 2 lateral ledges at the junction between adjoining lobes, inner apex acute, outer apex rounded. Pollinarium: pollinia erect, broad elliptic, yellow, 0.2–0.21 × ± 0.16 mm; corpusculum linear lanceolate, brown, ca. 0.18 × 0.04 mm; caudicles ± 0.05 × 0.04 mm; crests translucent, ± 0.12 × 0.02 mm. Ovary ca. 1.42 × 1.18 mm, greenish-white, sparsely pubescent. Fruits and seeds not observed.

The species is named after the type locality, Cuc Phuong National Park, in Ninh Binh Province, northern Vietnam.

Heterostemma cucphuongense was only collected once, near one of the main trails in the Cuc Phuong National Park. It was found in primary evergreen forest on soils derived from degraded limestone. It was collected in flower in June.

Heterostemma cucphuongense is endemic to the Cuc Phuong National Park. Since it is known from a single collection, its conservation status is Data Deficient (DD; IUCN 2012).

Heterostemma cucphuongense is similar to H. succosum Kerr (1939), a species found in Thailand and Laos. Both have shortly pedunculate inflorescences, in which the flowers generally open one at a time and are relatively large and rotate (mostly > 14 mm diam.). The two species can be easily separated because, in H. cucphuongense, the inflorescences form a rachis to 7 mm long with age while the inflorescences of H. succosum do not develop any rachis. Furthermore, the pedicels of H. cucphuongense are 5–10 mm long, while H. succosum has pedicels 15–30 mm long. Further distinguishing characters (that, however, are less obvious in dried material) are the margins of the corolla lobes that are recurved in H. cucphuongense (vs. flat in H. succosum). The two species also differ in that the colour of the adaxial surface of the corolla is red with white-yellow spots in H. cucphuongense (vs. yellow-orange with reddish-brown spots in H. succosum). These and additional diagnostic characters separating the two species are listed in Table 1.