| 1 | Aquatic herbs with stipulate, finely dissected submerged leaves; petals white or yellow only at the base; carpels transversely wrinkled |
| 1' | Terrestrial herbs; leaves exstipulate; petals usually yellow or sometimes coloured; achenes not transversely wrinkled |
| 2 | Floating leaves absent; flowers usually large; peduncles generally much longer than leaves, seldom recurved |
| 2' | Floating leaves present, divided into 3-5 bluntly lobed segments; flowers rather small; peduncles often shorter than floating leaves, usually recurved |
| 3 | Leaves all undivided, entire, with dentate or subentire margins |
| 3' | All or some of the leaves compound or deeply lobed |
| 4 | Flowers very large, 2.5-7 cm diam.; petals often red, pink or white; anthers violet-purple |
| 4' | Flowers smaller, rarely exceeding 3.5 cm diam.; petals and anthers yellow |
| 5 | Stems creeping, rooting at the nodes; perennial |
| 5' | Stems not rooting at the nodes, but sometimes diffuse or sprawling |
| 6 | Flowers solitary, axillary or leaf-opposed; annual plants; flowers small and inconspicuous (except in R. marginatus and R. cornutus) |
| 6' | Flowers terminal; perennial plants; flowers usually conspicuous |
| 7 | Achenes echinate or strongly muricate |
| 7' | Achenes muriculate, papillose or smooth, not echinate or strongly muricate |
| 8 | Basal leaves spathulate-cuneate, dentate at apex; upper leaves with linear divisions; achenes with long slender spines; achene-margin at right angles to surface |
| 8' | Basal leaves reniform, deeply 3-7-lobed; upper leaves sometimes simple; achenes shortly echinate-muricate, with a broad flat marginal wing |
| 9 | Peduncle shorter than adjacent leaf, incrassate in fruit; achenes narrowly bordered, with a short beak |
| 9' | Peduncle as long as, or longer than adjacent leaf, slender, not incrassate |
| 10 | Achene narrowly bordered, and with a short (usually upturned) beak; upper leaves often simple or rather coarsely laciniate |
| 10' | Achene distinctly winged, with a prominent (often deflexed) beak; upper leaves laciniate with numerous linear-lanceolate lobes |
| 11 | Inflorescence scapose, very sparsely branched, usually leafless or almost so; rootstock wholly or partly tuberous (grumose); achenes more than 2 mm long |
| 11' | Inflorescence branched, often leafy; if sparsely branched, then with achenes shorter than 2 mm (R. gracilis) |
| 12 | Rootstock crowned with a "collar" of interwoven fibres (old, decayed leaf-bases); lower leaves often simple, broadly ovate-cuneate |
| 12' | Rootstock without a "collar" of fibres |
| 13 | Leaves broadly reniform, palmately divided into blunt lobes; plant glabrescent, often tinged purple; petals 5-12 or more |
| 13' | Leaves deltoid, pinnately divided into small ovate or cuneate lobes; petals normally 5 |
| 14 | Sepals patent or adpressed to petals; tubers clavate |
| 14' | Sepals reflexed; tubers elongate-fusiform |
| 15 | Achenes small, c. 1.5 mm long, with upturned beak |
| 15' | Achenes 2-4 mm long, with straight, recurved or hooked beak |
| 16 | Plant glabrous or glabrescent; radical leaves broadly reniform or almost orbicular, rather leathery |
| 16' | Plant pubescent or villose; radical leaves broadly deltoid, deeply lobed or dissected, not leathery |
| 17 | Radical leaves 2-3-pinnate, villose; peduncles divaricate, conspicuously incrassate in fruit; achenes forming a cylindrical head |
| 17' | Radical leaves broadly deltoid, 3-lobed or reniform, crenate and shallowly lobed; peduncles erect or ascending, not incrassate |
| 18 | Achenes with a long, slender, straight or slightly recurved beak; root-tubers short, about 4-10 cm long |
| 18' | Achenes with a short, triangular or hooked beak; root-tubers elongate, 4-12 cm long, or wanting |
| 19 | Radical leaves reniform, shallowly lobed; sepals not reflexed in flower |
| 19' | Radical leaves broadly deltoid, deeply 3-lobed or almost ternate; sepals reflexed in flower |
| 20 | Achenes uniformly brown, without a margin; roots all fibrous, or at most slightly thickened |
| 20' | Achenes with a pale margin; roots intermixed with elongate cylindrical tubers |
| 21 | Stock conspicuously swollen and corm-like; achenes finely punctate |
| 21' | Without corm-like stock; achenes smooth |
