Trigonostemon villosus Hook.f.

Yu, R. - Y. & Welzen, P. C. van, 2018, A taxonomic revision of Trigonostemon (Euphorbiaceae) in Malesia, Blumea 62 (3), pp. 179-229 : 215-220

31. Trigonostemon villosus Hook.f. — Fig. 16 , 17 ; Map 12, 13

Trigonostemon villosus Hook.f. (1887) 397; Pax & K.Hoffm. (1911) 88; Ridl. (1924) 265; Jabl. (1963) 158; Whitmore (1973) 135; Airy Shaw (1975) 204; (1981) 358; R.I. Milne (1995a) 37. — Type: King’s collector s. n. (holo K), Malaya, Perak.

Trigonostemon tomentellus Pax & K.Hoffm. (1911) 89. — Type: Anonymous s. n. (holo B†), Malacca.

Trigonostemon carnosulus Airy Shaw (1978) 415. — Type: CF (Yeop) 844 (holo K), Malaya.

Trigonostemon polyanthus auct. non Hook.f.: R.I. Milne (1995a) 25.

Small trees, up to 6 m tall, dbh up to 6 cm; flowering branches 2.5–6 mm diam, buds pubescent. Outer bark 0.1–0.3 mm thick, pale brownish to greyish, often pubescent when young, glabrescent lower, sometimes fissured; inner bark 0.1–0.3 mm thick, dark reddish; sapwood 0.6–1.4 mm thick, pale yellowish; heartwood 0.6–1 mm diam, brownish, soft. Stipules subulate, 0.8–1.8 mm long, pubescent. Leaves: petiole terete but often furrowed above, 0.5–25 cm long, glabrous to hirsute, sometimes slightly thickened at base and apex; blade elliptic to oblong (Malay Peninsula, Borneo, Philippines) or oblanceolate (Sumatra), 6.5–32 by 2.5–14 cm, chartaceous, base cuneate, acute, obtuse or rarely slightly cordate (East Kalimantan), 2 adaxial glands present, margin distantly serrate, teeth small, subulate or falcate, apex acuminate to caudate,

Map 12 Distribution of Trigonostemon villosus Hook.f. var. borneensis (Merr.) Airy Shaw (★) and T. villosus var. merrillianus (Airy Shaw) R.Y.Yu & Welzen (●).

Map 13 Distribution of Trigonostemon villosus Hook.f. var. villosus (◆) and T. villosus var. cordatus R.Y.Yu & Welzen (▲).

upper side glabrous, dark green, lower side paler and to a different extent villose; midrib flat or slightly raised above and distinctly elevated beneath; nerves 7–12(–15) pairs, often bow-shaped, narrowed along margin, veins reticulate, sometimes obscure. Inflorescences unisexual or bisexual, racemes (pistillate ones if unisexual) or thryses, sometimes condensed, often axillary, sometimes cauliflorous, villose; rachis up to 42 cm long, 0.3–0.9 mm diam, staminate flowers clustered into short cymes or glomerules along rachis, pistillate flowers often single per node; bracts linear to lanceolate, 0.8–3.2 by 0.2–1.1 cm, margin entire (Borneo, Philippines) or serrate (Malay Peninsula, Sumatra), glabrous to villose, bracteoles (Malay Peninsula) lanceolate to linear, up to c. 5.5 by 1 mm, villose. Staminate flowers 3.5–6 mm diam, pedicel 2–3.5 mm long, 0.2–0.3 mm diam, pinkish (Sabah), glabrous; sepals elliptic, 2–2.5 by 1–1.4 mm, imbricate, white (Sabah), margin ciliate, apex rounded, outer surface slightly pubescent, sometimes with a very faint gland near apex, inner surface glabrous or rarely slightly villose (only one collection, van Balgooy 2187, Malay Peninsula); petals elliptic to obovate to spathulate, 2–3.8 by 1–2 mm, purplish black, base often with a pinkish flame-like honey mark inside (Borneo), margin entire, apex rounded, smooth and glabrous outside, rough and papillose inside; disc lobes rectangular or obtrapezoid, c. 0.5 by 0.5 mm, yellowish (Sabah), apex truncate, sometimes reflexed; stamens 3, androphore 0.4–0.9 mm long, anthers ellipsoid, 0.5–0.6 mm long, pinkish (Sabah), connectives apically with numerous reddish droplets with secretion. Pistillate flowers 6 –8 mm diam, pedicel often slightly thickened towards apex, 3.5–10 mm long, apically 0.5–1.2 mm diam, green or red, glabrous to hairy; sepals lanceolate (Malay Peninsula, Sumatra) or elliptic to oblong (Borneo, Philippines), 2.5–6 by 0.8–1 mm when flowering, sometimes accrescent when fruiting, up to c. 9 by 2.3 mm, white in flowers, turning green in fruits (Sabah), margin serrate (teeth often glandular, Malay Peninsula, Sumatra) to entire (Borneo, Philippines), apex acuminate, outer surface sparsely to densely villose, sometimes with a faint abaxial gland near apex, inner surface glabrous, very occasionally villose (one collection from Malay Peninsula); petals as staminate flowers but larger and caducous, 3–5 by 1.5–2.3 mm; disc rectangular, 0.3–0.9 by 0.5–0.6 mm, membranous, apex rounded or truncate; ovary c. 1 mm diam, glabrous to densely villose, styles short, indistinct, stigmas 3, completely bifid, free arm 0.8–1.3 mm long. Fruits c. 0.9–1.2 cm diam, villose; wall 0.4–0.6 mm thick; columella 4–6 mm long. Seeds 5–6.5 by 4.5–6.5 mm, hilum heart-shaped, 1.8–3.2 by 1.4–2.4 mm.

Distribution — Malay Peninsula, Sumatra, Borneo, Philippines.

Notes — 1. Milne (1995a) misinterpreted T. polyanthus because he cited a wrong type specimen. The actual T. polyanthus is a very distinct species, differing from T. villosus in its penicillate or fastigiate inflorescences.

2. This species consists of four varieties spreading over a huge area (Map 12, 13) of great morphological variation. We made this treatment based on a large number of collections showing that this massive variation actually connects all single forms together into a species complex, even though the ex- treme forms often do not resemble each other (see Fig. 18). These extremes, furthermore, have often become the source of synonyms, where the species status is only valid in the type locality, but in a larger area, intermediate forms are always found.As morphological discontinuities are the only valid argu- ment for species / variety delimitation , all the forms in Fig. 18 are regarded as a single species despite the wide distribution; and T. villosus var. cordatus is described as a separate variety, regardless of its small distribution area (Sabah).

3. Generally, the hairs on leaf surfaces, the size of petioles and leaf blades and the inflorescence structure can be good characters to tell the varieties apart.

The varieties villosus and borneensis all have relatively stable and similar leaf blade and petiole sizes ( Fig. 16 ), but they are different in inflorescence and flower structures – in var. borneensis , the inflorescences are unisexual (vs bisexual in var. villosus , but staminate flowers often fall off), the pistillate inflorescences have fewer flowers (often fewer than 4) at a time (vs often more than 4 in var. villosus ), and the pistillate sepals are not much accrescent and often have an subapical gland outside (vs obviously accrescent and with a faint or no gland in var. villosus ).

The new variety cordatus has a hairy leaf blade, which forms a typical morphological discontinuity separating it from var. borneensis .

The variety merrillianus is different from var. borneensis by its much (variable) longer petioles, much larger blades, often leaf-like bracts and staminate flowers in condensed glomerules.

Key to the varieties

1. Petiole longer than 8 cm ........... d. var. merrillianus

1. Petiole shorter than 8 cm ........................ 2

2. Inflorescences bisexual. — Malay Peninsula............................................ a. var. villosus

2. Inflorescences unisexual......................... 3

3. Leaves glabrous above. — Sumatra, Sabah, Philippines)................................ b. var. borneensis

Figures

Fig. 16 Trigonostemon villosus Hook.f.var.villosus (a, c, f), Forest Research Institute Malaysia,Kepong, Malay Peninsula and Trigonostemon villosus Hook.f. var. borneensis (Merr.) Airy Shaw (b, d, e, g–m), Segaliud Lokan Forest Reserve, Sabah, Malaysia. a, b. Growing habit; c, d. top view of the tree; e. bark and reddish exude; f, g. pistillate inflorescence; h, i. staminate flower; j, k. pistillate flower; l, m. fruit. — Photos by Ren-Yong Yu.

Fig. 17 Trigonostemon villosus Hook.f. var. cordatus R.Y.Yu & Welzen, from Kabili-Sepilok Forest Reserve (a, e, f) and Tawai Forest Reserve (b–d, g), Sabah, Malaysia. a. Growing habit; b. juvenile tree, showing the leaves; c. bark and wood; d. staminate inflorescence; e. staminate flower; f. pistillate flower; g. fruit. — Photos by Ren-Yong Yu.