Trigonostemon dipteranthus Airy Shaw

Yu, R. - Y. & Welzen, P. C. van, 2018, A taxonomic revision of Trigonostemon (Euphorbiaceae) in Malesia, Blumea 62 (3), pp. 179-229 : 194

9. Trigonostemon dipteranthus Airy Shaw — Map 3

Trigonostemon dipteranthus Airy Shaw (1966) 47. — Type: Korthals s. n. (‘ 853 ’) (holo L), Sumatra, West Coast Res., Padang Region .

Small trees, 4.5–6 m tall; flowering branches terete or wrinkled, 3–3.5 mm diam, slightly pubescent when young, glabrescent when mature. Outer bark smooth, pale brownish; inner bark c. 0.2 mm thick; wood white. Stipules falcate or subulate, 0.5–1 mm long, often pubescent. Leaves: petiole terete but sometimes slightly grooved above, 3–12.5 cm long, middle part 1–1.5 mm diam, more or less pubescent, often slightly thickened at base and apex; blade oblong to elliptic or slightly oblanceolate, 15–22 by 4.5–8 cm, membranous, base acute or narrowed rounded, margin slightly distantly crenate-serrate, teeth often small, nipple-like or falcate, apex caudate, tip 1.5–2.5 cm long, upper side glabrous, lower side pubescent and paler; midrib flat and glabrescent above, elevated and pubescent beneath, nerves 10 –12 pairs, curved and narrowed along margin, pubescent beneath, veins reticulate. Inflorescences axillary bisexual glomerules; peduncles slender, 2–6.5 cm long, finely appressed pubescent, flowers densely clustered at the top of peduncles and subtended by 2 conspicuous bracts; bracts opposite, almost sessile, widely ovate, 2–3.5 by 1.3–2.4 cm, membranous, base rounded or slightly emarginate, margin nearly entire, apex apiculate or acute, upper side glabrous, lower side appressed puberulent, especially along nerves. Staminate flowers c. 6 mm diam; pedicel c. 2 mm long, c. 0.3 mm diam, glabrescent; sepals elliptic, c. 1.2 by 0.6 mm, base connate, apex acute, sparsely puberulent outside; petals obovate, c. 3.5 by 2 mm, reddish brown when dry, base cuneate, apex rounded; disc lobes rectangular, sometimes obtrapezoid, c. 0.15 by 0.2 mm, corners obtuse, membranous, glabrous, apex truncate; stamens 5, androphore c. 1 mm long, 5-cleft at top, filaments c. 0.5 mm long, anthers separate, oblong, c. 0.7 mm long. Pistillate flowers rare, c. 6–8 mm diam; pedicel 3.5–4 mm long, apically c. 0.8 mm thick, densely pubescent; sepals oblanceolate to oblong, 6–7 by 2–3 mm, foliaceous and accrescent in fruit, apex acute to acuminate, pubescent outside, especially near base and along midrib; petals not seen, fallen; disc lobes wide semi-orbicular or rectangular, c. 0.5 by 0.7 mm, slightly folded in the middle; ovary c. 1.5 mm diam, densely pubescent; styles not seen, fallen. Fruits and seeds not seen.

Distribution — Sumatra (endemic).

Note — This species is characterized by the glomerules subtended by 2 large, leaf-like bracts. Detailed locality of the type collection, Korthals ‘ 853 ’, is based on Airy Shaw (1966).