Monolophus unifolius Y. H. Tan & H. B. Ding 2025, sp. nov.

Monolophus unifolius Y.H.Tan & H.B.Ding sp. nov. (Fig. 2) Diagnosis: This species is distinct from all presently known congeneric species because it only has one leaf blade per leafy shoot and white labellum tinged with a yellow spot at the throat. Type: — Myanmar. Sagaing Region, Hkamti District, Hkamti Township, Htamanthi Wildlife Sanctuary, near Nam E Zu Camp 1, 95º25ʹ16ʺ E, 25º29ʹ41ʺ N., elev. 180 m, 01 June 2019, Yun-Hong Tan, Min Deng, Bin Yang, Hong-Bo Ding and Xiao-Dong Zeng M 5728 (holotype HITBC [HITBC0125275]; isotype RAF). Perennial rhizomatous herbs, 25–50 cm tall, forming a loose clump with 4–18 leafy shoots. Rhizome 3–6 mm in diam. (when fresh), horizontally elongated and creeping. Leafy shoot with only 1(–2) laminae per shoot, leafless sheaths 2–3, 2–8 cm long, pale straw yellow to pale green when young, weakly striate, glabrous, turning papery with age (striae more prominent); leaf sheaths pale yellow-green, smooth, glabrous, ligule small, hardly visible, hidden below sheathing bracts; leaves erect, petiole 0.5–2 cm long (excluding leaf sheaths), leaf blade oblanceolate to lanceolate with sides unequal in width, 7.0–20 × 2.0–5.0 cm, glabrous on both sides, base attenuate, apex attenuate, adaxial surface dark green, abaxially light green. Inflorescences breaking through leaf sheaths 4–5 cm long, pluriflorous, rachis glabrous; flowers white, bracts 6–7, 2 flowers each bract, along the rachis, bract linear, 3.5–4.0 × 0.2–0.3 cm, pale straw, apex cuspidate to 0.8 cm; bracteoles lanceolate, ca. 1.5 × 0.3 cm, pale straw yellow, apex cuspidate to 0.5 cm; calyx tubular, 0.9–1.1 × 0.2–0.3 cm, glabrous, pale straw yellow tinged with pink at base, split 6–7 mm down one side, apex acute; floral tube 1–1.2 cm, ca. 3 mm wide at mouth, white, glabrous; dorsal corolla lobe ovate, 1.2–1.3 × 0.4–0.5 cm, white, glabrous, hooded at apex, apical cusp 2–3 mm long; lateral corolla lobes lanceolate, 1.0–2.1 × 0.3–0.4 cm, white, glabrous, apex acute; lateral staminodes obovate, 1.0–1.2 × 0.5–0.6 cm, white, apex rounded; labellum obovate, 1.8–2.0 × 1.2–1.4 cm, white tinged with a yellow spot at the throat. Filament very short, ca. 1 mm long, anther 4–5 mm long (excluding anther crest); anther crest flabellate with irregular apex, 2–3 × 5–6 mm, white, apex reflexed; stigma funnel-shaped, inserted between anther sac, margin raised on both ends; ovary oblong, 2–3 mm long, purplish red, epigynous glands 2, ca. 1.2 mm long. Phenology: —Flowering May to June. Distribution and Conservation status: — Monolophus unifolius is endemic to Sagaing Region, Northern Myanmar, and is currently only known from its type locality, the Chindwin River Basin. It grows on the surface of stones along the damp streams in the tropical rainforest understory at 150–200 m elevations. Although there is a specific population size, based on insufficient knowledge and data, conservation status is best assessed as Data Deficient (DD; IUCN 2022). Etymology: —The specific epithet ‘ unifolius ’ refers to its characteristic of only one leaf blade per leafy shoot. Specimens examined (paratypes):— Myanmar. Sagaing Region, Hkamti District, Hkamti Township, Htamanthi Wildlife Sanctuary, near Nam E Zu Camp 1, 95º25ʹ16ʺ E, 25º29ʹ41ʺ N., elev. 183 m, 23 May 2019, Yun-Hong Tan, Min Deng, Bin Yang, Hong-Bo Ding and Xiao-Dong Zeng M 5315 (HITBC); Htamanthi Wildlife Sanctuary, near Nam E Zu Camp 2, 95º32ʹ01ʺ E, 25º30ʹ56ʺ N., elev. 190 m, 28 May 2019, Yun-Hong Tan, Min Deng, Bin Yang, Hong-Bo Ding and Xiao-Dong Zeng M 5551 (HITBC). Notes: —Morphologically, Monolophus unifolius is significantly distinct from other Monolophus species, featuring only one leaf blade per leafy shoot and white labellum tinged with a yellow spot at the throat. So far, there are four species of Monolophus in Myanmar, namely M. kayinensis (Picheans. & Sangnark in Sangnark et al. 2016: 135) Barbhuiya, S.Dey & D.Verma in Barbhuiya et al. (2018: 175), M. monensis (Picheans. & Sangnark in Sangnark et al. 2016: 136) D.Verma, Barbhuiya & S.Dey in Barbhuiya et al. (2018: 175), M. odontochilus Y.H.Tan & H.B.Ding in Ding et al. (2020: 156) and this new species. A key to the taxa of Monolophus presently recorded in Myanmar is provided below.