1. Fruiting pedicels recurved or rarely spreading; pistillate sepals mostly erect and closely enclosing flower or fruiting head, occasionally spreading to recurved. | → 2 |
1. Fruiting pedicels spreading to ascending or absent; pistillate sepals mostly spreading to recurved, not enclosing flower. | → 9 |
2. Leaves, at least some, emersed. | → 3 |
2. Leaves submersed, floating, rarely emersed, or plants stranded on shore. | → 5 |
3. Fruiting heads 1.2–2.1 cm diam.; leaf blade hastate to sagittate | S. montevidensis |
3. Fruiting heads 0.7–1.2 cm diam.; leaf blade linear-ovate to lance-elliptic. | → 4 |
4. Achenes abaxially keeled, mostly more than 2 mm; faces not tuberculate, often with glands; California. | S. sanfordii |
4. Achenes not abaxially keeled, mostly less than 2 mm, faces tuberculate, without glands; e of Rocky Mountains. | S. platyphylla |
5. Achene faces tuberculate; floating leaves with sagittate blades present at least on some plants of population. | S. guayanensis |
5. Achene faces not tuberculate; floating leaves absent or, if present, with unlobed or hastate blades. | → 6 |
6. Fruiting pedicels 0.2–1.1 cm; phyllodia lenticular in cross section; tidal muds; mostly brackish waters. | S. subulata |
6. Fruiting pedicels 1.5–6.5 cm; phyllodia flattened in cross section; rarely tidal muds; mostly fresh waters. | → 7 |
7. Leaves with blades and petioles usually present on some plants; some plants often stranded along shore, these usually with expanded leaf blades. | S. filiformis |
7. Leaves all phyllodia; plants almost always submersed, rarely stranded, these without expanded leaf blades. | → 8 |
8. Phyllodia 50–250 cm; Florida springs. | S. kurziana |
8. Phyllodia 12–53 cm; streams and lakes, New Mexico. | S. demersa |
9. Filaments pubescent to tomentulose (except S. fasciculata). | → 10 |
| → 17 |
10. Leaves all phyllodia, nearly terete; ne United States. | S. teres |
10. Leaves with blades and petioles or if phyllodia, then flattened to triangular in cross section; mostly widespread. | → 11 |
11. Pistillate flowers sessile to subsessile. | S. rigida |
11. Pistillate flowers obviously pedicellate. | → 12 |
| S. lancifolia |
| → 13 |
13. Rhizomes present, coarse; stolons and corms absent. | → 14 |
13. Rhizomes absent or if present, then not coarse; corms and/or stolons present. | → 15 |
14. Abaxial wing of fruit ± entire; plants widespread | S. graminea |
14. Abaxial wing of fruit scalloped or toothed; ne Alabama | S. secundifolia |
15. Filaments exceeding anthers in length. | S. cristata |
15. Filaments shorter than or equaling anthers in length. | → 16 |
16. Blades of emersed leaves 0.5 cm or more wide; w Carolinas. | S. fasciculata |
16. Blades of emersed leaves, if present, 0.4(–0.5) cm or less wide; se coastal plain. | S. isoetiformis |
17. Emersed leaf blades linear to ovate. | → 18 |
17. Emersed leaf blades cordate, sagittate, or hastate. | → 20 |
| S. papillosa |
18. Bracts not papillose. | → 19 |
19. Emersed plants with erect to ascending petioles; leaf blades lanceolate to ovate. | S. ambigua |
19. Emersed plants with recurved petioles; leaf blades linear to sagittate. | S. cuneata |
20. Bracts distinct or connate much less than ¼ total length. | → 21 |
20. Bracts connate at least ¼ total length. | → 23 |
21. Flowers in 2–4 whorls; achenes with facial glands. | S. engelmanniana |
21. Flowers in 5–12 whorls; achenes without facial glands. | → 22 |
22. Petiole winged in cross section; achene beak strongly recurved. | S. australis |
22. Petiole ridged in cross section; achene beak ascending apically. | S. brevirostra |
23. Achene beak 1–2 mm, horizontal. | S. latifolia |
23. Achene beak 0.1–0.6 mm, erect or incurved. | → 24 |
24. Emersed plants with recurved petioles and linear to sagittate blades; basal lobes equal to or shorter than remainder of blade; submersed leaves phyllodial, floating leaves cordate to sagittate | S. cuneata |
24. Emersed plants with ascending to erect petioles and sagittate blades; basal lobes longer than remainder of blade; submersed and floating leaves absent. | S. longiloba |