Trigonostemon heteranthus Wight

Yu, R. - Y. & Welzen, P. C. van, 2020, A taxonomic revision of Trigonostemon (Euphorbiaceae) outside Malesia, Blumea 65 (1), pp. 25-52 : 35

11. Trigonostemon heteranthus Wight — sect. Tylosepalum — Map 1

Trigonostemon heteranthus Wight (1852) 24,t. 1890; Müll.Arg. (1866) 1109; Kurz (1877b) 406 (‘heteranthum ’); Hook. f. (1887) 396; N.P. Balakr. & Chakrab. (1991) 615,f. 4. — Lectotype (designated here): Griffith KD 4796 (K, barcode K000246910 ; iso P, barcode P00717107 ), Myanmar, Mergui. — Other syntypes: Griffith s. n. (K,barcode K000246861 ), Myanmar, Mergui; Helfer KD 4796 (K, barcode K000246862 ), Myanmar, Mergui.

Shrubs, 1.3–5 m tall, dbh c. 6.4 cm; flowering branches 2.3–3 mm diam, often pubescent when young. Outer bark 0.1–0.2 mm thick, pale brown to slightly reddish brown; inner bark 0.1–0.2 mm thick, dark reddish brown; wood pale yellow. Stipules subulate, 0.9–1.8 mm long, pubescent at base. Leaves: petiole terete but grooved above, 1–15.8 cm long, thickened at apex and base, in middle 0.7–1.9 mm diam, slightly pubescent when young; blade elliptic to oblong or oblanceolate, 7.8–28 by 2.6–10 cm, chartaceous, base rounded to acute to acuminate, with 2 adaxial glands, margin distantly serrate, teeth glandular or falcate, apex acuminate to slightly caudate, both surfaces glabrous, but pubescent beneath when young; venation pin- nate; midrib slightly elevated above and distinctively elevated beneath, sometimes sparsely pubescent beneath, secondary veins 9–17 pairs, bifurcate and connected along margin, veinlets reticulate, often obscure. Inflorescences bisexual or unisexual, racemose thyrses, often terminal, pistillate flowers single per node, open first; axis 4–21 cm long, 0.6–1.4 mm diam, pubescent; involucral bracts as stipules, but somewhat longer, 1.1–2.4 mm long; bracts under each node and flower, lanceolate, 0.6–4.1 by 0.2–1 mm. Staminate flowers c. 2 mm diam; pedicel 4.5–7.5 mm long (2–5 mm long below articu- lation, c. 5 mm long above articulation), 0.1–0.2 mm diam, glabrous; sepals elliptic, 1–1.6 by 0.8–1.1 mm, base connate, margin serrate, apex acute to rounded, pubescent; petals el- liptic to obovate, c. 2.5 by 2 mm, white; disc glands 5; stamens 3, clustered on an erect androphore. Pistillate flowers 5–7.3 mm diam; pedicel 8–11.5 mm long and apically 0.4–0.6 mm diam when flowering, elongating to 1.9–2.6 cm long and api- cally 0.8–1.2 mm diam when fruiting, an articulation present at 2/3 of length below apex; sepals ovate to triangular, 1.8–2.5 by 0.8–1.5 mm when flowering, accrescent to 5.2–7 by 2–3 mm when fruiting, base connate, margin fringed and serrate, with capitate glands; petals obovate, 2.4–3.4 by 1.4–2.4 mm, contort and conical in bud, white; disc lobes semi-orbicular, c. 0.2 by 0.6 mm, apex rounded, glabrous; ovary 0.8–1.2 mm diam, glabrous, style 0.15–0.2 mm long, stigmas 0.5–0.6 mm long, thickened but not bifid at apex. Fruits c. 1 cm diam. Seeds not seen.

Distribution — Myanmar (Mergui and Tavoy, endemic).

Habitat & Ecology — In forests. Elevation: 65–200 m. Flow- ering: January to March.

Note — The species is similar to T. kerrii Craib in the fringed pistillate sepals with capitate glands, but differs in the white petals and the stigmas without division.

Figures

Map 1 Distribution of Trigonostemon adenocalyx Gagnep. (●),T. capillipes (Hook.f.) Airy Shaw (X), T. capitellatus Gagnep. (★), T. heteranthus Wight (▲) and T. kerrii Craib (■).