The new species differs from closest R. axillaris in serrulate leaves, purple spots and glandular hairs on abaxial surface of corolla, purple longitudinal stripes on median lobe of lower lip, and pubescent filaments stamens and pistil (Table 1).

Vietnam. Quang Nam Province, Tay Giang District, A Xan Village, primary evergreen broad-leaved forest, around point 15°48'57"N, 107°19'47"E, elevation 1270 m, 20 April 2022, C.H. Nguyen, K.S. Nguyen, H.X. Cao, CKH 2022042068 (holotype VNF; isotypes HN, LE).

Perennial herb with stem to ca. 3 m long ascending to ca.1 m tall. Stem branching, velutinous when young, glabrescent with age. Leaves opposite, equal to unequal in size; petioles 1.5–3.5 cm long, densely hirsute with appressed hairs; leaf blade symmetrically elliptic, 8–16 × 3.4–6.2 cm, 2.3–2.6 times as long as wide, base cuneate, apex acute to acuminate, hirsute with appressed hairs on both sides, more densely on veins, serrulate along the margin, eucamptodromous venation with 8–14 pairs of secondary veins, tertiary venation ramified. Inflorescences arising in axils of lower leaves and in leaf scar axils, 1- or 2-flowered, up to 3 inflorescences in an individual axil, 5–6 cm long (including flower); all axes with 0,5–1mm long glandular hairs; peduncle 10–18 mm long; bracts narrowly elliptic, 4–5 × 0.8–1 mm long, greenish, with 0,5–1mm long glandular hairs; flowers spreading almost horizontally to slightly pendulous; pedicels 10–12 mm long. Calyx of 5 lobes free from the base, lobes 6–7 × 1.3–1.5 mm long, with long glandular hairs outside, glabrous inside. Corolla infundibular, outside white with purple tint to purple, inside white with purple on adaxial lip and purple longitudinal lines on median lobe of abaxial lip, 4.2–4.8 cm long, sparsely covered with long glandular hairs outside, inside with glandular short hairs at apex of adaxial lip, with two to three prominent ridges ventrally at the base of throat, limb distinctly two-lipped; tube 3.5–3.9 cm long, 8–10 mm wide, swelling at middle and narrowing at base, the distal part broadening towards throat; adaxial lip 2-lobed, lobes subequal, half round, 5–6 mm long, 5–6 × 7–9 mm wide, sinus 4–5.5 mm deep; adaxial lip 3(4)-lobed, lobes unequal, 14–16.5 mm long, lateral lobes 6–8 × 7–9 mm wide, middle lobe 7–9.2 × 4.8–6.1 mm wide. Stamens 4(5), in 2 pairs, each pair adnate at the anthers, filaments filiform, distally shortly glandular puberulent, geniculate near the middle; adaxial pair adnate to 14–16 mm above the corolla base, 9–11 mm long, 1 mm in diameter, anthers 0.8–1 × 1.1–1.3 mm long; abaxial pair, adnate at c.14 mm above the corolla base, 6–8 mm long, 0.8 mm in diameter; staminode 1, slightly hooked, 3–4 mm long. Disc circular, light lemon yellow, 1–1.2 mm high, margin repand, glabrous. Pistil 2–2.2 cm long, puberulent throughout; ovary 12–13 mm long; style 6–7 mm long; stigma c. 2 mm, 2-lobed. Capsule green when young, bent at the base, narrowly fisiform, pubescent, dehiscing adaxially, straight, not twisted.

Figure 1. 

Raphiocarpus taygiangensis C.H.Nguyen, K.S.Nguyen & Aver., sp. nov. A natural habitat B–D flowering plant in natural habitat E leaves F leaf margin G inflorescences H flowers, views from different sides I flower, frontal views J sagittal section of corolla showing inner surface with stamens and staminodes K flower tube inside and outside L, M stamens N staminode O calyx lobes P pistil, base of pistil and its apex with stigma Q young fruit. Photos by Cuong Huu Nguyen and Khang Sinh Nguyen, correction and design by Cuong Huu Nguyen.

The new species is only known from A Xan Commune, Tay Giang District, Quang Nam Province in central Vietnam. Raphiocarpus taygiangensis usually grows in moist shady places near waterfalls, along streams and occasionally on the sandstone slopes covered by evergreen broad-leaved forests at elevations of 1200–1300 m a.s.l. As common plants in habitats of the new species have been recorded Aeschynanthus bracteatus Wall. ex A.DC., Angiopteris evecta (G. Forst.) Hoffm., Asplenium unilaterale Lam., Begonia spp., Crepidomanes auriculatum (Blume) K. Iwats., Hedyotis sp., Impatiens clavigera Hook. f., Leptochilus sp., Molineria capitulata (Lour.) Herb., Mycetia sp., Phymatosorus lucidus (Roxb.) Pic. Serm., Rhaphidophora decursiva (Roxb.) Schott, Rhynchotechum ellipticum (Wall. ex D. Dietr.) A. DC., and Symplocos banaensis Guillaumin.

Figure 2. 

Raphiocarpus taygiangensis C.H.Nguyen, K.S.Nguyen & Aver., sp. nov. holotype specimen, Cuong et al., CKH 2022042068 (VNF).

Flowers from April to May, fruiting from May to June.

The species epithet refers to the name of the district of the type location (Tay Giang District in Quang Nam Province).

The special field studies around the type location revealed no other populations outside the occupancy area which was estimated to be about 5 km². The type location consists of approximately 500 mature individuals growing in moist shady places near waterfall and along the stream. The population territory currently does not belong to any protected area, and its future protection is needed. The area has been relatively undisturbed to date due to its significance to the surrounding village, although local people continue to forage in it for firewood. Considering the small population size and fragile habitat, we propose that the new species should be preliminarily assessed as Vulnerable (VU D2) according to criteria IUCN (2019).

Figure 3. 

Raphiocarpus taygiangensis C.H.Nguyen, K.S.Nguyen & Aver., sp. nov. A flower front view B habitat C lateral view of flower and fruit; Raphiocarpus axillaris D.J.Middleton, sp. nov. D flower front view E habitat F lateral view of flower and fruit. Photos by Ly Van Nguyen (D–F), Cuong Huu Nguyen and Khang Sinh Nguyen, correction and design by Cuong Huu Nguyen.

A comparison of the most significant morphological characters of Raphiocarpus taygiangensis and its closest congeners, R. axillaris. The new species can be easily distinguished from R. axillaris in having basally prostrate and distally ascending stem; sericeous hairs on young stem, leaf petiole and adaxial mid-vein; sparsely serrulate leaf margin; axillary inflorescence; sparse glandular hairs on the peduncle, pedicel, calyx, pistil, and abaxial surface of corolla; calyx 5-lobed, dissected from the base; white or light purplish flowers with purple stripes inside corolla tube; twining filaments; and dish-shaped stigma formed by 2 semi-orbicular lobes spreading horizontally. The comparison of the key morphological characters of R. taygiangensis, and R. axillaris is presented in Table 1.