| | 1. Plant 10–26 dm, generally emergent; inflorescence axis generally branched; stigmas 1–2; fruit sessile, obconic, 6–10 mm, top truncate or dome-like, beaked |
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| | 2. Fruit top dome-like; fruit 4–7 mm, 2.5–5 mm wide; many flowers with 1 stigma; many ovaries 1-chambered |
| S. erectum subsp. stoloniferum |
| | 2' Fruit top truncate to slightly rounded; fruit 6–10 mm, 6–8 mm wide; many flowers with 2 stigmas; many ovaries 2-chambered |
| S. eurycarpum subsp. eurycarpum |
| | 1' Plant 1–8 dm, submersed or emergent; inflorescence axis simple; stigma 1; fruit stalked, slender, fusiform to elliptic or oblanceolate, sometimes narrowed at middle, 3–5.5 mm; beak 0 |
| |
| | 3. Plant generally emergent; stem ± stiff, erect; leaves and lower inflorescence bracts 3–12 mm wide, keeled or triangular below middle; inflorescence 12–25 mm diam in fruit |
| S. emersum subsp. emersum |
| | 3' Plant generally submersed; stem ± flexible, floating above; leaves and lower inflorescence bracts 1–5 mm wide, flat or sides convex, not keeled or triangular; inflorescence 5–15 mm diam in fruit |
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| | 4. Lowest inflorescence bract < 20(60) cm, >> inflorescence; staminate inflorescence generally 2+; pistillate inflorescence generally 7–15 mm diam in fruit, some pistillate inflorescences on separate peduncle; fruit 3.5–5 mm, 2–2.5 mm wide |
| S. angustifolium |
| | 4' Lowest inflorescence bract < 2 cm, ± = inflorescence; staminate inflorescence generally 1; pistillate inflorescence 7–10 mm diam in fruit; fruit 2–4 mm, 1–2.5 mm wide |
| S. natans
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