| 1 | Plants floating; vegetative stem condensed; petioles commonly swollen. |
| 1' | Plants rooted; vegetative stem short or long; petioles never swollen. |
| 2 | Vegetative stem short; leaves cordate and more than 6 cm long; inflorescences a panicle. |
| 2' | Vegetative stem elongate; leaves oblong or cordate and less than 4 cm long; inflorescence a spike. |
| 3 | Leaf base cordate; inflorescence with 2-3 flowers; individual flowers less than 2 cm across. |
| 3' | Leaf base truncate to obtuse (very rarely subcordate); inflorescence with 4 or more flowers; individual flowers more than 2.5 cm across. |
| 4 | Inflorescence moderately to densely pubescent; upper middle tepal with yellow spot; lower stamens 15-29 mm long. |
| 4' | Inflorescence glabrous or nearly so; upper middle tepal without yellow spot; lower stamens 11-15 mm long. |
