| 1. Spikelets generally 1(3), ± sessile; stem and leaf blade << 1.5 mm wide |
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| 2. Plant submersed except inflorescence, leaf tip generally floating; spikelet 613 mm |
| S. subterminalis |
| 2' Plant terrestrial or emersed, leaf tip not floating; spikelet 27 mm |
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| 3. Largest spikelets generally < 6-flowered; fruit smooth; High Sierra Nevada, > 2400 m |
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| 4. Rhizome 0; fruit sharply 3-angled; inflorescence bract > spikelet, tip with awn generally > 1 mm |
| S. clementis |
| 4' Rhizome slender; fruit compressed, 23-angled; inflorescence bract < spikelet, tip abruptly pointed |
| S. pumilus |
| 3' Largest spikelets generally > 8-flowered (except some S. cernuus); fruit generally papillate, pitted, or ribbed; w of High Sierra Nevada, < 1500 m |
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| 5. Flower bract strongly keeled throughout, ± folded in fruit; fruit sides concave to ± flat, fine-papillate |
| S. koilolepis |
| 5' Flower bract keeled above middle, ± flat in fruit; fruit sides convex, ribbed or fine-papillate to -pitted |
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| 6. Fruit sharply 3-angled, sides fine-papillate or -pitted; rhizome 0; spikelet 1 |
| S. cernuus |
| 6' Fruit obscurely 3-angled, sides ribbed; rhizome thread-like; spikelets 13 |
| S. setaceus |
| 1' Spikelets generally 2many, sessile or stalked; stem and leaf blade (0.6)215 mm wide (stem generally < 1 mm wide in S. nevadensis and S. saximontanus) |
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| 7. Annual; rhizome 0; stem generally < 1 mm wide; fruit sharply transverse-ribbed |
| S. saximontanus |
| 7' Perennial; rhizome short to long; stem (0.6)215 mm wide; fruit smooth (low-ridged in S. mucronatus) |
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| 8. Well developed inflorescence bracts generally 2+, thin, leaf-like, generally spreading; leaves ± cauline, blades generally > sheaths, thin |
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| 9. Spikelet 13.5 mm wide; fruit ± 0.52 mm; flower bract glabrous, tip acute to abruptly pointed; tubers 0 |
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| 10. Spikelets not clearly clustered, ± stalked |
| S. pendulus |
| 10' Spikelets mostly in head-like clusters, sessile |
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| 11. Fruit generally 2-angled; stigmas 2; perianth bristles straight in fruit, fine-toothed throughout |
| S. microcarpus |
| 11' Fruit 3-angled; stigmas generally 3; perianth bristles ± contorted to straight in fruit, fine-toothed above middle |
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| 12. Flower bract dark brown to blackish in fruit; perianth bristles < 4 mm, teeth sparse, ascending |
| S. congdonii |
| 12' Flower bract brown in fruit; perianth bristles generally < 2.5 mm, teeth dense, reflexed |
| S. diffusus |
| 9' Spikelet 410 mm wide; fruit 24.5 mm; flower bract scabrous or sparse-puberulent, tip awned; tubers generally present |
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| 13. Perianth bristles persistent on fruit; fruit sharply 3-angled, sides ± concave; longest inflorescence bract generally 415 mm wide when dry; leaf blade generally 620 mm wide when dry; stigmas 3 |
| S. fluviatilis |
| 13' Perianth bristles generally not persistent on fruit; fruit ± weakly 23-angled, sides convex; longest inflorescence bract generally 26 mm wide when dry; leaf blade generally 212 mm wide when dry; stigmas 23 |
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| 14. Spikelet 45 mm wide; fruit 22.5(3) mm; stigmas 3 |
| S. tuberosus |
| 14' Spikelet 510 mm wide; fruit (2.5)33.5 mm; stigmas 23 |
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| 15. Flower bract not strongly appressed, ± pale grayish to orange-brown; spikelets often in 1 dense cluster; leaf sheath top opposite blade with a triangular, veinless area |
| S. maritimus |
| 15' Flower bract strongly appressed, ± orange-brown; spikelets generally on open, elongate branches; leaf sheath top opposite blade clearly veined |
| S. robustus |
| 8' Well developed inflorescence bract generally 1, thick, ± stem-like at least above, erect or bent to side; leaves ± basal; blades 0 or generally < sheaths (except S. nevadensis, S. pungens), thin to stem-like |
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| 16. Stem 3-angled, at least above |
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| 17. Inflorescence panicle-like, dense to open, spikelets 20many; perianth bristles on fruit thick, flat, fringed with soft hairs; stem often cylindric below, < 4 m |
| S. californicus |
| 17' Inflorescence head-like, or rarely with 12 short branches, spikelets 112(20); perianth bristles on fruit slender, minutely spiny; stem 3-angled throughout, generally < 2.2 m |
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| 18. Leaf blade vestigial or 0; inflorescence bract generally bent to side; flower bract tip abruptly pointed, notch 0; fruit low-ridged; rhizome short, obscured by crowded stems |
| S. mucronatus |
| 18' Leaf blade < to > sheath; inflorescence bract generally erect; flower bract tip awned from notch; fruit smooth; rhizome long, not obscured |
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| 19. Flower bract tip with awn < 0.5 mm; uppermost leaf blade generally < sheath; stem sides deep-concave |
| S. americanus |
| 19' Flower bract tip with awn 0.51.5 mm; uppermost leaf blade > sheath; stem sides ± flat to shallow-concave |
| S. pungens |
| 16' Stem cylindric throughout |
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| 20. Inflorescence head-like; spikelets 110; flower bract tip acute to obtuse, notch 0 |
| S. nevadensis |
| 20' Inflorescence panicle-like; spikelets 3100+; flower bract tip short-awned, notched (tip sometimes worn off in age) |
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| 21. Spikelets solitary at branch tips; fruit 3-angled, 2.53 mm, beak 0.31 mm; stigmas 3 |
| S. heterochaetus |
| 21' Spikelets many, mostly clustered at branch tips; fruit ± 23-angled, ± 22.5 mm, beak 0.10.4 mm; stigmas 23 |
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| 22. Flower bract generally prominently spotted, awn generally contorted, 0.51 mm; spikelets generally 17 per cluster |
| S. acutus var. occidentalis |
| 22' Flower bract faintly spotted or not, awn ± straight, generally < 0.5 mm; spikelets generally solitary or 23 per cluster |
| S. tabernaemontani
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