| 1 | Fertile flowers with 10 stamens (Section Neptunia) |
| 1' | Fertile flowers with 5 stamens (Section Pentanthera Windler) |
| 2 | Spike globoid in bud, consisting of less than 25 flowers; peduncle 3-4.5(-5.9) cm long, bracts absent. Stipules less than 5 mm long. Lateral veins of leaflets raised and reticulate |
| 2' | Spike obovoid in bud, consisting of c. 30 flowers; peduncles usually more than 4.5 cm long, provided with bracts. Stipules more than 5 mm long. Lateral veins of leaflets obscure |
| 3 | Stems branched, usually erect or ascending. Leaves with a reduced gland between or just below the lowest pair of pinnae. Seeds 8-20 per pod. Leaflets frequently more than 20 pairs per pinna. Tropical and subtropical America, introduced to the Malay Peninsula |
| 3' | Stems rarely branched, usually free floating. Leaves with glands. Seeds 4-8 per pod. Leaflets 20 pairs or less per pinna |
| 4 | Pods 1- (or 2-)seeded. Peduncles less than 4.5 cm long; the spikes 35-55-flowered, bracts persistent. Ovary usually white haired |
| 4' | Pods 1-seeded. Peduncles usually longer than 4.5 cm; the spikes with less than 35 flowers (frequently less than 25), bracts often deciduous before flowering. Ovary glabrous |
