| 1 | Trunk with copious white latex from the inner bark; colleter(s) in the leaf-axils deltoid and usually more or less as wide as the leaf-scars, or narrowly triangular, 0.3-0.7 mm wide; corolla lobes sinistrorse; seeds rounded at both ends; leaves in whorls of (3-)4-9 |
| 1' | Trunk without white latex from the inner bark; colleters in the leaf-axils minute or narrowly lingulate, 0.1-0.15 mm wide; corolla lobes dextrorse; seeds acuminate or caudate; leaves consistently in whorls of 3 or 4 |
| 2 | Branches or branchlets usually with a distinctly shorter internode at the end; leaves long-acuminate; ovary syncarpous; follicles solitary |
| 2' | Branches or branchlets without a distinctly shorter internode at the end; leaf apex retuse to shortly acuminate; ovary apocarpous; follicles in pairs |
| 3 | Leaves with the tertiary venation more or less conspicuous above; petiole with a distinct narrow intrapetiolar stipule at the base; colleters narrowly triangular; corolla (sometimes partly) pubescent outside |
| 3' | Leaves with the tertiary venation mostly inconspicuous or obscure above; petioles without intrapetiolar stipule at the base; colleters deltoid; corolla glabrous outside |
| 4 | Leaves elliptic or narrowly so, rarely slightly obovate, apex usually obtuse or shortly acuminate |
| 4' | Leaves strongly obovate or spathulate, apex rounded or retuse |
| 5 | Sepals pubescent outside; leaves 1.5-7 cm wide. — Thailand, Western Malesia |
| 5' | Sepals glabrous outside; leaves 0.7-2.2 cm wide. — Papua New Guinea, Australia |
| 6 | Flowers very densely clustered, usually forming two clusters; corolla tube 1.8-2.8 times as long as the lobes which are 2.6-4 mm long; follicles glabrous |
| 6' | Flowers rather lax; corolla tube 1-1.7 times as long as the lobes which are 4-6.5 mm long; follicles pubescent |
| 7 | Leaves usually sessile, rarely petiolate; flowers many and densely clustered, usually into two clusters; sepals pubescent outside; corolla lobes 3-4.3(-5) by 2-3.2 mm; follicles pubescent |
| 7' | Leaves petiolate; inflorescences few-flowered, lax; sepals glabrous outside; corolla lobes 6.7-11 by 3.5-6 mm, follicles glabrous |
| 8 | Sepals grey- or rusty-pubescent outside |
| 8' | Sepals glabrous or laxly puberulous outside |
| 9 | Corolla glabrous or very sparsely hairy outside (usually around the apex), dark brown or blackish when dried |
| 9' | Corolla pubescent outside, sometimes puberulous only around the apex, usually pale or rusty brown due to the hairs |
| 10 | Leaves glabrous on both sides, secondary veins 12-17 pairs; sepals pubescent (except on the fused part) inside |
| 10' | Leaves glabrous or sparsely hairy above, densely velutinous beneath, secondary veins 23-26 pairs; sepals glabrous inside |
| 11 | Sepals 2.5-3.7 mm long, usually ovate; corolla tube 3.8 mm long; corolla lobes 3.5-5.8 mm long, ciliate or not. — Philippines |
| 11' | Sepals c. 1.7 mm long, turbinate; corolla tube c. 2 mm long; corolla lobes c. 2 mm long, ciliate. — Waigeo Island, Papua |
| 12 | Corolla lobes suborbicular, ovate, or sometimes shortly lingulate, 1-2.1 times as long as wide, apex rounded or obtuse |
| 12' | Corolla lobes narrowly ovate, rarely ovate, more than (2.6-)3.1 times as long as wide, apex acute or obtuse |
| 13 | Sepals pubescent inside on the lobed part. — Peninsular Malaysia, Sumatra, Borneo |
| 13' | Sepals glabrous or laxly puberulous only around the apex inside. — Java, Sulawesi, Philippines to Australia and Solomon Islands |
| 14 | Sepals with a thick margin, usually reflexed, obovate, connate at the base at least 0.5 of the length; corolla tube 3-4.5 mm long; corolla lobes 1-2.3 by 1-1.8 mm; seeds 1.2-1.6 mm wide |
| 14' | Sepals with a thin margin, not reflexed, ovate, connate at the base less than 0.5 of the length; corolla tube 5.4-5.6 mm long; corolla lobes 3-4.5 by 1.8-2.3 mm; seeds 1.8-2 mm wide |
| 15 | Corolla tube 2-3 times as long as the lobes which are ovate or subtriangular, 1-1.3 times as long as wide, not ciliate; seeds 7-9 by 3-3.3 mm; leaves thickly coriaceous, usually abruptly acuminate |
| 15' | Corolla tube 0.7-1.5 times as long as the lobes which are ovate, narrowly ovate or oblong, 2.2-5 times as long as wide, ciliate or not; seeds 5-12 by 1.6-2.5 mm; leaves chartaceous or coriaceous |
