Trigonostemon reidioides (Kurz) Craib

Yu, R. - Y. & Welzen, P. C. van, 2020, A taxonomic revision of Trigonostemon (Euphorbiaceae) outside Malesia, Blumea 65 (1), pp. 25-52 : 42-43

25. Trigonostemon reidioides (Kurz) Craib — sect. Spinipollen — Map 13

Trigonostemon reidioides (Kurz) Craib (1911) 464;Pax & K.Hoffm.(1912) 128; Gagnep. (1925b) 316; Airy Shaw (1972) 348; Chantar. (2005) 28; (2007) 582. — Baliospermum reidioides Kurz (1875) 32; (1877b) 411; Hook.f. (1887) 461;F.N.Williams (1905) 32; Pax & K.Hoffm.(1912) 29. — Lectotype (designated here): Teijsmann HB 5981 (K, barcode K000959313), Thailand, Kanburi [= Kanchanaburi]. — Other syntype: Teijsmann s. n. (CAL,barcode CAL0000023659), Thailand.

Trigonostemon hybridus Gagnep. (1922) 750, syn. nov.; (1925b) 318. — Lectotype (designated here): Pierre 6282 (P,barcode P00717109 ), Vietnam, Phu Quoc (other duplicates are syntypes cited under the next name).

Trigonostemon rubescens Gagnep. (1922) 754; (1925b) 317, syn. nov. — Lectotype (designated here): Thorel 2290 (P,barcode P00717140;iso NY*, barcode NY00273346,P, barcodes P00717141,P00717142, US *, barcode US 00096544), Laos, Kong et île de Khon. — Other syntypes: Pierre 6281 (P, barcodes P00717139, P00648668, P00648669, P00648670), Cambo- dia, Kompong Spen,Mont Ramcon; Pierre 6282 (K, barcode K000959312, P, barcodes P00717143, P00717144, P00717145), Vietnam, Phu-Quoc.

Shrubs, 0.5–1.5 m tall; flowering branches 1.2–3.2 mm diam, slightly to densely pubescent. Indumentum of simple and stellately bundled hairs. Outer bark c. 0.1 mm thick, greyish to dark brown; inner bark 0.1–0.2 mm thick, dark reddish, sap clear, watery, black or dark red in dry material; wood yellow or brown. Stipules acicular to subulate, 0.6–2.3 mm long, caducous, pubescent at base. Leaves: petiole terete, slightly grooved above, 0.5–2.5 cm long, 0.6–1.9 mm diam, densely pubescent to glabrescent; blade oblong to elliptic, rarely obovate or (ob)lanceolate, 3.5–15 by 0.5–6.3 cm, chartaceous to coriaceous, base acute to rounded, adaxial glands 2, falcate, blackish, margin entire, apex acute to acuminate, upper surface dull dark green, lower surface dull light green, both surfaces densely to slightly pubescent, when slightly pubescent then often only simple hairs present; venation triplinerved, pubescent on both sides, midrib and basal secondary veins flat above and elevated below, other secondary veins 2–8 pairs, bow-shaped and connected along margin, tertiary veins and veinlets reticulate. Inflorescences bisexual, axillary or terminal large panicles; main axis terete, 3–29 cm long, 0.4–1.2 mm diam, dull red, often pubescent, rarely glabrescent; bracts lanceolate to linear, 0.5–17 by 0.1–1.9 mm, pubescent. Staminate flowers 4.1–6.6 mm diam; pedicel 2.5–7 mm long, 0.15–0.5 mm diam, pale light green to dull reddish, pubescent to glabrescent; sepals elliptic, 1–2.6 by 0.5–1.6 mm, pale light green to dull light yellow or reddish, base connate, margin entire, sometimes ciliate, apex acute to rounded, pubescent to glabrescent outside; petals obovate to spathulate, 3.3–6.4 by 1.6–4.2 mm, white, dark red, deep maroon or dark purple, lower part sometimes claw-like, apex rounded, glabrous; disc annular, somewhat plicate, light orange, glabrous, margin undulate, 0.3–0.45 mm wide; stamens 3, androphore 0.35–0.7 mm long, 0.2–0.5 mm diam, cream, anthers ellipsoid, 0.5–0.8 mm long, cream. Pistillate flowers 5.3–8.9(–13) mm diam; pedicel slightly thickening toward apex, 8–20 mm long, apically 0.4–1 mm diam when flowering, elongating in fruit to (0.8–) 1.6–3.4 cm long, apically 0.7–1.5 mm diam, pubescent, glabrescent; sepals elliptic to oblong, 3–6 by 1.1–2.2 mm, base connate, margin entire, sometimes ciliate, apex acute to acuminate, pubescent outside; petals as staminate flowers but larger, 5.9–9 by 2.6–6 mm; disc as staminate flowers; ovary 1.3–1.6 mm diam, pubescent, light yellow or green, style absent, stigmas thickening toward apex, 0.7–0.8 mm long, apically 0.5–0.6 mm wide, white, pale light yellow or cream, furrowed above, apically not bifid. Fruits 1–1.3 cm diam, green, pubescent; sepals persistent but not accrescent; wall 0.4–0.6 mm thick, exocarp partly detach- ing; columella 4.2–5.8 mm long. Seeds 4.6–6.3 mm diam, often marbled; hilum oblong to elliptic to triangular, 0.7–1.8 by 0.5–1 mm.

Distribution — Thailand, Laos, Cambodia, Vietnam.

Habitat & Ecology — Often in open areas in dry dipterocarp forests, or in seasonal deciduous pine or Melaleuca forests, occasionally in waste lands or swampy forests. Often growing in sandy or rocky soil, sometimes along roads or rivers. Eleva- tion: 20– 300 m. Flowering: all year round; fruiting: January to June, August, October.

Notes — 1. This is a common species in Thailand and Indochina . The species displays a continuous variation in the indumentum: from very dense stellately bundled hairs (as represented by the type collection of T. reidioides ) to relatively sparse and mostly simple hairs (but stellately bundled hairs are still present; as in the type collections of T. rubescens ). Trigonostemon hybridus represents an intermediate form between the above mentioned two, and Gagnepain (1922) even considered it as a hybrid form. As morphological discontinuities are lacking, T. hybridus and T. rubescens are placed in synonymy. Only two species of Trigonostemon (the other species is T. balgooyi R.Y.Yu & Welzen from Johor, Malaysia) have an indumentum of both simple and stellately bundled hairs, making it a useful character in identification.

2. Several specimens (probably ‘duplicates’) are present under the collection number Pierre 6282. Of these specimens, one was identified as T. hybridus and the others as T. rubescens . In order to avoid confusion, we select Pierre 6282 (P, barcode P00717109) as the lectotype of T. hybridus (based on the specimen label) and the other specimens under this number are cited as syntypes of T. rubescens . Thorel 2290 (P, barcode P00717140) is selected as the lectotype of T. rubescens .

Figures

Map 13 Distribution of Trigonostemon reidioides (Kurz) Craib.