Trigonostemon pachyphyllus Airy Shaw

Yu, R. - Y. & Welzen, P. C. van, 2020, A taxonomic revision of Trigonostemon (Euphorbiaceae) outside Malesia, Blumea 65 (1), pp. 25-52 : 40-41

22. Trigonostemon pachyphyllus Airy Shaw — sect. Trigonostemon — Map 3

Trigonostemon pachyphyllus Airy Shaw (1971) 546; (1972) 347; Chantar. (2005) 27; (2007) 581. — Type: Kerr 19429 (holo K, barcode K000959302 ), Thailand, Trang, Khao Soi Dao .

Shrubs, c. 1.5 m tall; flowering branches 3.2–4 mm diam, glabrescent. Outer bark c. 0.1 mm thick, brown; inner bark 0.1–0.2 mm thick, reddish brown; wood yellow. Stipules subulate, 0.6– 0.8 mm long, caducous, pubescent. Leaves: petiole wrinkled, flat or slightly grooved above, 0.3–1 cm long, 2–2.2 mm diam, pubescent to glabrous; blade oblong, 12–19 by 2.1–3.9 cm, coriaceous, base acute, adaxial glands not seen, margin entire, occasionally with very sparse glandular teeth, apex acuminate, upper surface glabrous, dull green or puplish brown when dry, lower surface sparsely pubescent, yellow when dry; venation pinnate, midrib flat above and elevated beneath, often pubescent beneath, secondary veins 12–15 pairs, curved, bifurcate and connected along margin, tertiary veins reticulate. Inflorescences seemingly unisexual; staminate ones axillary, thyrses, axis 1–1.4 cm long, 0.8–1.1 mm diam, pubescent, bracts triangular, 0.15–1.3 by 0.2–0.6 mm, pubescent; pistillate ones terminal or axillary, racemes, axis up to 1.5 cm long, 0.6–1.1 mm diam, pubescent, bracts lanceolate, 0.6–2.5 by 0.3–0.8 mm, pubescent outside. Staminate flowers at least 3.5 mm diam; pedicel c. 1.2 mm long, c. 0.35 mm diam, pubescent; sepals elliptic, 1–1.7 by 0.3–0.8 mm, base connate, margin entire, apex acute, pubescent or hirsute outside; petals obovate, 3.4–4.5 by 1.4–2.5 mm, dark red-purple, glabrous; disc lobes rectangular, 0.35–0.5 by 0.35–0.45 mm, apex truncate to slightly rounded; stamens 5, anthers ellipsoid, 0.8–1 mm long, apically divaricate. Pistillate flowers (fruiting) 6–10.5 mm diam; pedicel thickening toward apex, 3.6–4.5 mm long, apically 1.5–2 mm diam, pubescent; sepals triangular to lanceolate, 8.5–10 by 3.5–4.6 mm, outer surface pubescent, sometimes marbled, mixed patches in dark red and yellow, inner surface glabrous, dark purplish red when dry, base connate, margin distantly serrate, teeth glandular, apex acuminate, ending in a gland; petals fallen, not seen; disc lobes semi-orbicular, 0.7–0.9 by 0.45–0.6 mm, apex rounded; ovary c. 1.5 mm diam, densely pubescent; style c. 0.2 mm long; stigmas apically deeply bifid, free arms 0.5–0.6 mm long. Fruits c. 1 cm diam, green; sepals persistent, green. Seeds not seen.

Distribution — Thailand (Trang and Phatthalung, endemic).

Habitat & Ecology — Evergreen forests, understorey, near streams. Flowering: March to April; fruiting: April.

Note — A rare species endemic to Thailand. It resembles T. capillipes ( Milne 1995) in the 5 stamens and relatively large pistillate sepals, and was treated as a synonym in our previous revision (Yu & Van Welzen 2018). However, after examining more collections, we reinstate the species here. The main spotting characters of T. pachyphyllus includes the coriaceous leaves (thick leaves as indicated by the specific epithet) and the discolorous surfaces when dry. In addition, the pistillate sepals sometimes display a marbled pattern on the outer surface and the inflorescences are considerably smaller.

Figures

Map 3 Distribution of Trigonostemon bonianus Gagnep.(X), T. lanceolatus (S.Moore) Pax (▲), T. pachyphyllus Airy Shaw (★) and T. tuberculatus F.Du & Ju He (●).