1. Culms 0.5-9 mm wide, transverse septae conspicuous and complete or obscure and incomplete; spikelets (6)9-78 mm long, most 0.6-1.1 times as wide as the culm or 2-4 times as wide | Key A |
1. Culms 0.2-1 mm wide, transverse septae incomplete and obscure; spikelets 2-12(23) mm long, 2-8 times as wide as the culm | 2 |
2. Culms with branches in successive false whorls, which terminate in clusters of leaves essentially indistinguishable from stems, the spikelets borne singly on branches arising from among leaves; spikelets consisting of 2 scales, the single flower subtended by the distal scale (Websteria) | E. confervoides |
2. Culms unbranhced, with 1 terminal spikelets; spikelets with 4-160 scales and numerous flowers | 3 |
3. Styles 2-fid; achene biconvex or lenticular (Maculosae group) | Key B |
3. Styles 3-fid (or sometimes 2-fid in E. ambigens, E. minima, E. obtusa); achene trigonous to subterete or biconvex | Key C |
1. (Key A) Spikes often subulate, generally dark brown to reddish in color, with 100-500+ densely arranged floral scales, 15-40 scales per mm; floral scales 1-3 mm long, 1-1.5 mm wide, with 1 vein (rarely 10 widely spaced veins) (Truncatae group) | 2 |
1. Spikes mostly rounded, obutse, or abruptly acute at the tip, generally pale in color, with 4-220 floral scales, 0.5-3 scales per mm; floral scales 3-8 mm long, 2-5 mm wide, with 15+ obscure to prominent veins (subg. Limnochloa) | 3 |
2. Culms (2.8)3-6(10) mm wide, subequal to the spikelet width; achene tubercle 2-4 times longer than wide, nearly as long as the achene body | E. elegans |
2. Culms 0.5-3(4) mm wide, the spikelet 2-4 times wider than the culm; achene tubercle wider than or as wide as long, <1/2 as long as achene body | E. montana |
3. Culms 0.5-1.5 mm wide; spikelets 1.4–3 mm wide, with 4–26 floral scales; submersed culms sometimes flaccid and lacking spikelets | 4 |
3. Culms 2.5-9 mm wide; spikelets 3–8 mm wide, with 30–220 floral scales; submersed culms not becoming flaccid and sterile | 5 |
4. Floral scales 3.5-4.5 mm long, usually with a conspicuous brown to blackish submarginal band; achenes 0.6–1.3 mm long, 0.5–0.8 mm wide | E. elongata |
4. Floral scales 5-8 mm long, usually without a conspicuous submarginal band; achenes 1.9–2.6 mm long, 1–1.4 mm wide | E. robbinsii |
5. Culms distinctly septate (septae complete), internally hollow between septae | 6 |
5. Culms not septate or only obscurely so (septae incomplete), internally spongy | 7 |
6. Perianth bristles filiform, smooth to weakly toothed, shorter than to subequal to achene body; achene surface with linear-oblong cells medially and apically (rectangular towards the base) | E. equisetoides |
6. Perianth bristles flattened linear-lanceolate, coarsely retrorsely spinulose, longer than achene body; achene surface with rectangular cells | E. interstincta |
7. Achene apex gradually narrowed and spongy-thickened, apex similar in texture (reticulate) to achene body except for the darkened tip (tubercle), tubercle scarcely differentiated and confluent, narrower than achene apex | 8 |
7. Achene apex constricted, sharply differentiated from the flared base of the tubercle | 9 |
8. Culm terete to scarcely trigonous; perianth bristles usually smooth; achene (including tubercle) 2.3-3.3 mm long | E. cellulosa |
8. Culm sharply 3-angled; perianth bristles retrorsely spinulose; achene (including tubercle) 1.8-2.2 mm long | E. mutata |
9. Culm 3-angled; perianth bristles usually retrorsely spinulose; achene 1.4-2 mm long, 1.2-1.6 mm wide, tubercle 0.3-0.8 mm long | E. acutangula subsp. breviseta |
9. Culm 4-angled; perianth bristles usually smooth to sparsely retrorsely spinulose; achene 1.8-3 mm long, 1.3-2 mm wide, tubercle 0.7-1.5 mm long | E. quadrangulata |
1. (Key B) Leaf sheath thin, delicate, hyaline, the margin sometimes tearing; achenes greenish to reddish or brown | 2 |
1. Leaf sheath firm, opaque, the margin intact; achenes black | 3 |
2. Achenes red-brown to dark brown, 0.4–0.8(1.1) mm long, 0.3–0.6 mm wide, apex usually not strongly constricted proximal to tubercle, perianth bristles usually shorter than to subequal to achene | E. flavescens |
2. Achenes green to golden-brown, 0.5–1.1 mm long, 0.4–0.8 mm wide, usually strongly constricted proximal to tubercle, perianth bristles usually longer than the achene | E. olivacea |
3. Perianth bristles colorless or white; achenes 0.3–0.5 mm long, 0.3–0.4 mm wide, apex often constricted to a neck below tubercle | E. atropurpurea |
3. Perianth bristles red-brown, rarely whitish; achenes 0.5–1.1 mm long, 0.3–0.7 mm wide, apex rounded below tubercle | E. geniculata |
1. (Key C) Achene surface with prominent longitudinal ribs, horizontal ridges much finer (Scirpidium group) | 2 |
1. Achene surface smooth, honeycomb-reticulate, finely reticulate, or cancellate, the ridges all fairly equal in prominence, lacking more strongly prominent longitidinal ribs | 3 |
2. Culms firm to flaccid, not very spongy, 1–45 cm long; anthers 0.5–1.5 mm long | E. acicularis |
2. Culms soft, very spongy, 1–12 cm long; anthers 0.3–0.5 mm | E. radicans |
3. Achenes apex gradually narrowed to a rudimentary or scarcely distinct tubercle, similar in texture and color to the achene body or only subtly changing | 4 |
3. Achenes apex generally narrowed below a distinct tubercle, the tubercle sharply differentiated from the achene body, the tubercle base usually flared | 6 |
4. Culms to 100 cm long, with 8 ridges; anthers 2-2.4 mm long; achene (including indistinct tubercle) 1.5-2.5 mm long | E. rostellata |
4. Culms to 55 cm long, with or without ridges; anthers 0.7-1.5 mm long; achene (including indistinct tubercle) 0.9-1.4 mm long | 5 |
5. Rhizomes with curved tubers; culms to 15(20) cm long, lacking ridges; perianth bristles 6 | E. parvula |
5. Rhizomes lacking curved tubers; culms to 55 cm long, with 8-12 ridges; perianth bristles absent | E. tricostata |
6. Achenes 0.7-1.7 mm long; spikelets or culm tips not proliferous (or if proliferous, achene nearly black) | 7 |
6. Achenes 0.4-0.8(0.9) mm long, smooth, finely pitted, or cancellate; spikes or culm tips sometimes proliferous | 13 |
7. Achene coarsely honey-comb reticulate (Tuberculosae group) | 8 |
7. Achene smoothish to finely rugulose or cancellate | 9 |
8. Culm usually trigonous; tubercle much narrower and shorter than the achene, much smaller than the achene | E. tortilis |
8. Culm terete; tubercle as long and as wide as achene, often larger than the achene | E. tuberculosa |
9. Achenes dark purplish brown to black, smooth, widest near the apex, the tubercle 0.7-1.1 mm wide, as wide as the achene; some culms often arching or decumbent and with tips rooting | E. melanocarpa |
9. Achenes stramineous, orangish, to brown, widest near the middle, the tubercle 0.2-0.6 mm wide and usually narrower than the achene; culms not rooting at tips | 10 |
10. Achenes (0.9)1-1.5 mm long, (0.7)0.8-1.3 mm wide, the tubercle 0.3-0.5 mm long, 0.4-0.8 mm wide | 11 |
10. Achenes 0.7-1 mm long, 0.6-0.8 mm wide, the tubercle 0.1-0.3 mm long, 0.2-0.4 mm wide | 12 |
11. Spikelets 2-3 mm wide, acute at the tip, floral scales 3-5 per mm | E. ambigens |
11. Spikelets (2)3-4 mm wide, obtuse or rounded at the tip; floral scales 8-20 per mm (Ovatae group) | E. obtusa |
12. Culm base pale brownish (scarcely any red color if at all); leaf sheath apex acute to lanceolate or disintegrating; perianth bristles sometimes exceeding the tubercle | E. albida |
12. Culm base distinctly reddish; leaf sheath apex subtruncate to obtuse; perianth bristles not exceeding the tubercle | E. montevidensis |
13. Scales distichous; spikes with 2-4 flowers | E. baldwinii |
13. Scales spirally arranged (or spirodistichous); spike mostly with 4-50(75) flowers | 14 |
14. Achenes 0.45–0.9 mm, finely reticulate or cancellate | E. vivipara |
14. Achene smooth or very finely pitted | 15 |
15. Stolons and rhizomes absent; floral scales 1–1.2 mm long; perianth bristles absent; spikelets not proliferating | E. nigrescens |
15. Stolons or rhizomes present; floral scales 1.3–2.2 mm long; perianth bristles present or absent; spikelets sometimes proliferating | 16 |
16. Spikelet proximal scale deciduous or sometimes persistent, similar to floral scales or smaller; perianth bristles present | 17 |
16. Spikelet proximal scale persistent, differentiated from floral scales, usually longer and resembling involucral bract, often with excurrent midrib; perianth bristles present or absent | 18 |
17. Rhizomes absent, stolons present; spikelets often proliferating; perianth bristles colorless to stramineous, slightly shorter than to equaling tubercle; achenes sometimes spotted olive or brown | E. minima |
17. Rhizomes present; spikelets not proliferating; perianth bristles red-brown, equaling to exceeding tubercle; achenes not spotted | E. nana |
18. Proximal scale apex rounded | 19 |
18. Proximal scale apex subacute to acute | 20 |
19. Plants usually robust; floral scales pale brown and papery; perianth bristles well developed; achenes to 0.8 mm long, minutely pitted | E. brittonii |
19. Plants usually small; floral scales whitish with red-brown markings; perianth bristles absent or poorly developed; achenes not more than 0.6 mm long, smooth | E. brittonii variant |
20. Spikelet proximal scale 0.8–1 mm wide; perianth bristles 0.6–1 mm long | E. microcarpa var. filiculmis |
20. Spikelet proximal scale 0.4–0.5 mm wide; perianth bristles 0.2–0.4 mm long | E. microcarpa var. microcarpa |