| 1 | Stipules for a ± long part adnate to the leaf base, all leaves linear to lanceolate, submerged |
| 1' | Stipules completely free or almost so, floating leaves present or absent |
| 2 | Stipules adnate to the leaf for at least 3/4 of its length, adnate part 8-65 mm long, leaves entire, 0.5-1 mm wide, apex acute |
| 2' | Stipules adnate to the leaf for less than half of its length, adnate part 2-6 mm long, leaves strongly serrulate, 1.5-4 mm wide, apex rounded or truncately acute. |
| 3 | Submerged leaves linear, less than 5 mm wide, sessile, entire |
| 3' | Submerged leaves linear-lanceolate to roundish, usually more than 5 mm wide, rarely reduced to narrow phyllodia, sessile or petiolate, entire or dentate |
| 4 | Floating leaves often present, submerged leaves linear, less than 1 mm wide, with more than 6 rows of lacunae along the midrib, fruit with a prominent beak. |
| 4' | Floating leaves always absent, submerged leaves to 4 mm wide, lacunae not more than 3 rows, fruit with short or indistinct beak |
| 5 | Submerged leaves less than 2 mm wide, 3(-5)-veined, without additional sclerenchymatous strands, stipules often connate in the lower part, particularly when young. |
| 5' | Submerged leaves 2-4(-6) mm wide, (3-)5-7(-9)-veined, with few to many additional sclerenchymatous strands, stipules convolute |
| 6 | Leaves 3-7(-9)-veined, with 2-16 indistinct sclerenchymatous strands, stipules persistent, stem terete |
| 6' | Leaves (3-)5-veined, with 20-32 sclerenchymatous strands, stipules decaying to fibres, stem compressed |
| 7 | Stem compressed, subquadrangular, fruits adnate at base, with long curved beak, leaves serrate with teeth visible to the naked eye, margin often undulate. |
| 7' | Stem terete, achenes free, beak short, margin entire or minutely denticulate |
| 8 | Submerged leaves sessile, clasping the stem, floating leaves always absent. |
| 8' | Submerged leaves usually petiolate, floating leaves present or absent |
| 9 | Peduncles in fruit significantly thicker than uppermost part of the stem, floating leaves present or absent |
| 9' | Peduncles not or only slightly thickened, floating leaves usually present |
| 10 | Floating leaves always absent, submerged leaves shortly petiolate (-15 mm), apex with acute tip |
| 10' | Floating leaves often present, most leaves long petiolate, petioles of involucral leaves approximately as long as the peduncle, submerged leaves partly cordate or sagittate at base, apex obtuse |
| 11 | Submerged leaves linear-lanceolate or ribbon-like, less than 15 mm wide, floating leaves oblong-lanceolate, acute, peduncles often opposite to leaves. |
| 11' | Submerged leaves lanceolate to oblong-lanceolate, usually wider, peduncles regularly in the axils of subopposite leaves |
| 12 | Carpels 1-2(-3), floating leaves partly very long petiolate (-400 mm). |
| 12' | Carpels (3-)4, floating leaves, if present, shorter petiolate (-200 mm) |
| 13 | Submerged leaves 9-13-veined, apex acute to acuminate, floating leaves usually absent |
| 13' | Submerged leaves 11-21-veined, apex rounded or obtuse, floating leaves abundantly present |
