| | 1. Sepals persistent; hypanthium not prolonged beyond ovary; petals generally easily deciduous; moist habitats |
| LUDWIGIA |
| | 1' Sepals deciduous; hypanthium generally conspicuously (except Gayophytum) prolonged beyond ovary; petals not generally easily deciduous; various habitats |
| |
| | 2. Petals and sepals 2; fruit bur-like, covered with hooked hairs |
| CIRCAEA (no key) |
| | 2' Petals and sepals (3)4; fruit not bur-like, not covered with hooked hairs |
| |
| | 3. Fruit indehiscent, nut-like, short |
| |
| | 4. Flower radial; hypanthium thread-like; stamens opposite petals sterile; filament teeth 0 |
| Clarkia heterandra |
| | 4' Flower bilateral; hypanthium obconic; stamens all fertile; filaments with paired teeth at base |
| GAURA |
| | 3' Fruit dehiscent, not nut-like, long |
| |
| | 5. Ovary chambers 2; hypanthium barely prolonged beyond ovary; stem branches hair-like |
| GAYOPHYTUM |
| | 5' Ovary chambers 4; hypanthium well developed or not; stem branches generally not hair-like |
| |
| | 6. Seeds hair-tufted at 1 end or sepals erect at flowering; pollen generally shed in groups of 4 |
| EPILOBIUM |
| | 6' Seeds not hair-tufted; sepals reflexed singly, in pairs, or 3 adherent and reflexed to 1 side; pollen generally shed singly |
| |
| | 7. Stigma ± head-like or hemispheric |
| CAMISSONIA |
| |
| | 8. Anthers attached at base; petal tip often lobed or petal base clawed |
| CLARKIA |
| | 8' Anthers attached at middle; petal tip notched or toothed, petal base not clawed |
| OENOTHERA
|