Cleisostoma furcatum Aver & Vuong, 2022

Averyanov, Leonid V., Thai, Tran Huy, Truong, Ba Vuong, Nguyen, Van Canh, Nguyen, Tuan Hoang, Maisak, Tatiana V., Nguyen, Khang Sinh & Nguyen, Van Khuong, 2022, New orchids in the flora of Vietnam (Orchidaceae, Aeridinae), Phytotaxa 555 (2), pp. 113-135 : 118-119

Cleisostoma furcatum Aver & Vuong , sp. nov.

( Figs. 3 , 4 )

Type: — VIETNAM. N Vietnam, Lang Son Province, Binh Gia District, Thien Long Commune , forest on karstic limestone mountain near Lan Luong Village , 18 April 2019, Truong Ba Vuong , Hoang Minh Tuan, BV 355a (holotype VNM, isotype LE01066647 https://en.herbariumle.ru/?t=occ&id=133883, authentic photos LE01073485 http:// en.herbariumle.ru/?t=occ&id=17628, drawings LE01123082 https://en.herbariumle.ru/?t=occ&id=133321) .

Etymology. The species name refers to the furcate apices of the lip side-lobes.

Description. Herb perennial monopodial epiphytic. Stem simple or basally few branched, semi-woody, pendulous, (10)12–18(20) cm long, terete or slightly flattened, (3.8)4–5(5.2) mm in diameter, with (3)4–6(8) leaves near the apex, in basal part with few wiry, straight or flexuose dull gray roots branching distally; internodes (0.6)0.8–1.2(1.4) cm long. Leaves subdistichous, sessile, dorsiventral, leathery, straight or slightly recurved and twisted, lanceolate, (5)7– 10(12) cm long, (0.8)0.9–1(1.2) cm wide, obtuse and unequally shortly bilobed at apex. Inflorescence lateral raceme or few branching panicle, (8)10–20(25) cm long, scape and rachis green, speckled with purple to dirty purple; scape (6)8–15(18) cm long, pendulous, slightly flexuose to almost straight, with 2–4 short, tubular, brownish gray sterile bracts, simple or with 2–3(4) branches; rachis (4)6–14(16) cm long, straight to distinctly zig-zag, with (3)5–15(18) spirally arranged, lax flowers distant on (4)5–14(16) mm. Floral bracts dull brown, minute, triangular, acute, 1.2–1.5 mm long, 1–1.2 mm wide. Pedicel and ovary terete, longitudinally grooved, (5)5.5–7(8) mm long, (0.9)1(1.2) mm in diameter, glabrous, dull purple, slightly broadening at the base. Flowers widely opening, (7)8–10(11) mm across; sepals and petals somewhat fleshy, glabrous, spreading, pale yellow with purple- or brownish-violet margin; lip white, bright yellow at base, with violet median lobe and purple-violet apices of side-lobes, spur white; column white with yellow base; anther cap white; pollinia yellow. Sepals obovate, concave, blunt to rounded at apex, (5)5.5–6(6.5) mm long, (3.3)3.5–4(4.2) mm wide, median sepal cucullate, lateral sepals slightly oblique. Petals narrowly obovate, almost flat, blunt to round at apex, (3.6)3.8–4.2(4.4) mm long, 2.4–2.6 mm wide, slightly oblique. Lip spurred, (5.8)6– 6.2(6.4) mm long (from spur apex to the apex of epichile), 3-lobed; side-lobes narrowly conoid, twisted on 90º and furcate at apex, parallel and forward protruding, (1.7)1.8–2(2.1) mm long, (1)1.2(1.4) mm wide at the base, outside finely verruculose; median lobe very fleshy, almost half-globular in outline, (2.8)3(3.2) mm long, 2.8–3 mm wide, forward directed or slightly upright, apiculate, with short uprising mucro, disc at base with fleshy longitudinal keel continuing into the spur in form of spur septum; spur saccate, almost parallel to ovary, (1.7)1.8–2(2.1) mm long, (1.8)2(2.2) wide, slightly dorso-ventrally flattened, with entire, round apex and incomplete longitudinal septum inside. Back-wall callus in form of large, erect, T-shaped, flattened, papillose outgrowth about 1.2–1.4 mm tall, at apex with two recurved conoid teeth or horns, at base connected with fleshy side-lobes by fleshy papillose ridges. Column short, broad, indistinctly cubic, (2.4)2.5–3(3.2) mm high, (1.8)2(2.2) mm wide, at front with small rostellum lobes supporting viscidium horns; stigma in form of transversely oblong hollow. Anther cap helmet shaped, thin-walled, skin-like, 1.6–1.8 mm tall and wide, without beak. Pollinia 2, each deeply split into 2 half-globular bodies. Stipe (tegula) in form of rhomboid scarious plate 1.5–1.8 mm long and wide (being flattened), enveloping pollinia in form of narrow funnel after removing anther cap, viscidium in form of two narrow subulate forward directed acute horns 0.8–0.9 mm long. Fruits unknown.

Habitat, phenology and conservation status. Epiphyte. Broad-leaved evergreen forests on karstic limestone. Fl. April. Very rare. Suggested IUCN Red List status – CR.

Distribution. Northern Vietnam (Lang Son Province, Binh Gia District). Endemic.

Note. This species belongs to Cleisostoma sect. Paniculata Seidenf. (Seidenfaden, 1975, 1992), which is represented in eastern Indochina with five species, namely C. chapaense , C. duplicilobum , C. equestre , C. inflatum and C. paniculatum ( Averyanov et al. 2015b) . Among them the new species is closest to C. equestre sharing such specific characters as the shape of lip lobes; similar T-shaped back-wall callus; skin-like anther cap tightly fitting the abaxial surface of the pollinarium stipe; flat, broad, plate-like pollinarium stipe covering the pollinia from above (like an umbrella); and prominent subulate forward directed horns of viscidium. This combination of characters has never been observed in other species of the genus. At the same time, the new species differs from C. equestre in: stem to 20 cm long (vs. stem about 5 cm long), leaves lanceolate 0.8–1.2 cm wide (vs. leaves broadly lanceolate 1.5–2 cm wide), inflorescence to 27 cm long, sometimes branching (vs. inflorescence to 25 cm long, simple), background of sepals and petals pale yellow, lip median lobe violet (background of tepals white, lip median lobe white), lip side-lobes narrowly conoid, twisted on 90º, furcate at apex (vs. side-lobes broadly triangular, not twisted, entire at apex), median lip lobe at the base without callus, at apex apiculate, with short ascending mucro (vs. median lobe with prominent conoid callus at the base, apex almost flat, without ascending mucro), column 2.4–3.2 mm high with anther cap 1.6–1.8 mm tall (vs. column 3.6–4.2 mm high, anther cap 2.2–2.4 mm tall), and flattened stipe rhomboid, as long as wide (vs. flattened stipe circular).

The new species is probably a strict endemic of the central part of the South Chinese floristic province as circumscribed by Averyanov et al. (2003). It was discovered in Vietnam near the Chinese border, hence it may be also found in similar limestone areas of Yunnan and Guangxi.

Up to now, the species has found in only one locality, where less than 50 mature plants were observed. No more samples of this species were detected in the area during additional floristic explorations. Such conditions meets IUCN criteria (2019) that formally identify conservation status of new species as Critically Endangered – CR D.

Several plants in the studied population with pure white flowers represent a true albino form: Cleisostoma furcatum Aver & Vuong forma albiflora Aver & Vuong , forma nov. Type: — VIETNAM. Lang Son Province, Binh Gia District, Thien Long Commune, forest on karstic limestone mountain near Lan Luong Village, 18 April 2019, Truong Ba Vuong, Hoang Minh Tuan, BV 355b (holotype VNM), photos LE01061302 http://en.herbariumle.ru/?t=occ&id=12496.

Diagnosis: —new form differs from the type in pure white flowers except bright yellow base of the lip and column (vs. sepals, petals and lip heavily marked with dark violet).

Figures

FIGURE 3. Cleisostoma furcatum. Details of flower. A. Separated lip, view from above. B. Lip, tangential section, view from above (part with side-lobes is removed). C. Lip, tangential section, view from below (part with median lobe is removed). D. Half-flattened lip, tangential section, view from below (part with median lobe is removed), in the center seen back-wall callus (adaxial side). E. Separated, lip, side view. F. Separated, lip, sagittal section. G. Flattened pollinarium stipe (tegula), abaxial side. H. Flattened pollinarium stipe (tegula), adaxial side. I. Anther cap, frontal view and view from above. J. Pollinarium, side and half-side views. K. Pollinia. All drawn from the type specimen (BV 355a) by L. Averyanov and T. Maisak).

FIGURE 4. Cleisostoma furcatum. Flowering plants, inflorescences and details of flower. All photos made by Truong Ba Vuong from the plants used for the preparation of type voucher specimens (BV 355a), design and correction by Truong Ba Vuong, L. Averyanov and T. Maisak.