| 1 | Leaves with conspicuously winged rachis — New Guinea |
| 1' | Leaf rachis unwinged, rarely weakly so |
| 2 | Leaves opposite |
| 2' | Leaves spirally arranged |
| 3 | Leaves pubescent, at least abaxially |
| 3' | Leaves glabrous |
| 4 | Venation obscure |
| 4' | Costae and intercostae prominent |
| 5 | Terminal leaflet present |
| 5' | Terminal leaflet absent; small ‘spike’ or scar ± present |
| 6 | Inflorescences axillary |
| 6' | Inflorescences ramiflorous to cauliflorous |
| 7 | Calyx splitting into lobes at anthesis |
| 7' | Calyx of discrete sepals |
| 8 | Leaves up to 4-jugate; petals 5; fruit smooth |
| 8' | Leaves up to 8-jugate; petals 5; fruit cerebriform, tomentose |
| 8'' | Leaves 5–8-jugate; petals 4; fruits smooth |
| 9 | Indumentum of simple and stellate hairs; sepaloid bracteoles absent |
| 9' | Indumentum of simple hairs, sepaloid bracteoles present |
| 10 | Leaves up to 3- (or 4-)jugate, treelet |
| 10' | Leaves with more leaflets |
| 11 | Costae c. 16 on each side; inflorescence and fruit strigose-irritant; pachycaul treelet to 6 m |
| 11' | Costae c. 10–12 on each side; inflorescences not strigose |
| 12 | Calyx 7–15 mm long |
| 12' | Calyx less than 4 mm long 13 |
| 13 | Leaves up to 8-jugate, leaflets strongly asymmetric; inflorescences ramiflorous; fruits silky hairy |
| 13' | Leaves 3–5-jugate; inflorescences ramiflorous to cauliflorous; fruits ± glabrous |
| 14 | Staminal tube ± hairy. — Thailand to S Philippines |
| 14' | Staminal tube glabrous |
| 15 | Inflorescences 3–8(–12) mm long; treelet. — New Guinea |
| 15' | Inflorescences much longer; trees. |
| 16 | Inflorescences finely adpressed hairy. — E Malesia |
| 16' | Inflorescences densely strigose hairy. — C Malesia . |
| 17 | Apical bud not spike-like, with young leaves like clenched fists |
| 17' | Apical bud spike-like or stiletto-shaped (sect. Cyrtochiton) . |
| 18 | Leaves strictly paripinnate with terminal pair of leaflets . |
| 18' | Leaves not so |
| 19 | Leaves 10–14-jugate |
| 19' | Leaves up to 7-jugate |
| 20 | Costae c. 5–7 on each side, obscure; petals 4. — Malay Peninsula |
| 20' | Costae c. 6–9 on each side, obscure; petals 5. — Borneo . |
| 20'' | Costae more, conspicuous. — Philippines, New Guinea |
| 21 | Staminal tube unceolate (to subcylindrical); pollen shed in tetrads. — New Guinea |
| 21' | Staminal tube cylindrical, pollen grains shed singly |
| 22 | Petals at least 8 mm long |
| 22' | Petals up to 5 mm long. — New Guinea |
| 23 | Petals 4 — Philippines |
| 23' | Petals 5; capsule smooth. — New Guinea |
| 23'' | Petals 5; capsule lenticellate-pustular. — New Guinea . |
| 24 | Leaves 4–6-jugate; inflorescences 4–13 cm long; petals up to 2 mm long |
| 24' | Leaves 5–7-jugate; inflorescences to 35 cm long; petals c. 4.5 mm long |
| 24'' | Leaves 3- or 4-jugate; inflorescences 4–9 cm long; petals at least 4 mm long. |
| 25 | Leaves glabrous, venation markedly scalariform; petals 5 . |
| 25' | Leaves glabrous or not, venation different |
| 26 | Leaflets narrowly elliptic (rheophyte). — Malay Peninsula |
| 26' | Leaflets not so |
| 27 | Shoots and abaxial surface of leaflets densely reddish velutinous |
| 27' | Leaflets different |
| 28 | Leaflets yellowish pilose abaxially; inflorescence spicate.. |
| 28' | Leaflets and inflorescence different |
| 29 | Costae up to 12 on each side |
| 29' | Costae (14) 15 on each side. — New Guinea |
| 30 | Leaflets asymmetric at base, rarely subequal, shiny adaxially; bracteoles inconspicuous; calyx and corolla glabrous; infructescence ± branched; whole plant, or at least fruit, smelling of onions |
| 30' | Leaflets ± symmetric, not markedly shiny adaxially; bracteoles conspicuous; calyx and corolla pubescent; infructescence ± unbranched; whole plant not smelling of onions |
| 31 | Bracts 5–10 by 1–2.5 mm; treelet with flagelliform inflorescences |
| 31' | Bracts up to 3.5 mm long; inflorescences to 20 cm long |
| 32 | Petals up to 9 mm long |
| 32' | Petals more than 13 mm long |
| 33 | Abaxial surface of leaflets brown-tomentose |
| 33' | Abaxial surface of leaflets glabrous to subglabrous |
| 34 | Leaves 6–9-jugate; leaflets with 23–25 costae |
| 34' | Leaves 2–4-jugate; leaflets with 15–18 costae |
| 35 | Intercostal venation scarcely distinguishable from costae |
| 35' | Costae distinct; intercostal venation often obscure |
| 36 | Leaves paripinnate, 2–4(–5)-jugate, glabrous to puberulous |
| 36' | Leaves ± imparipinnate with ± alternate leaflets |
| 37 | Petals 4–5 mm long; staminal tube crenulate. — New Guinea |
| 37' | Petals 8 mm long; staminal tube crenate to toothed |
| 37'' | Petals 6 mm long; staminal tube truncate. — Borneo |
| 38 | Leaflets to 30 by 10 cm; thyrses to 25 cm, with spreading branches |
| 38' | Leaflets smaller; thyrses subspiciform |
| 39 | Leaflets minutely rugulose; costae 12–15(–18). — New Guinea |
| 39' | Leaflets not rugulose; costae 7–13 |
| 40 | Calyx c. 4 mm long. — West of Wallace’s Line |
| 40' | Calyx c. 1.5 mm long. — East of Wallace’s Line |
