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| 1 Distal leaf sheath apex delicate, membranous, or disintegrating (thus, sometimes not readily apparent); usually inflated or wrinkled, often with a torn edge; culms 0.1-0.6 (to 1 mm on fresh specimens) mm wide. |
| ..2 Culms 0.1-0.3 mm in diameter; achenes whitish to pale brown; leaf sheaths of the upper culm closely sheathing the stem, not wrinkled, the apex acute |
| ..2 Culms 0.3-0.6 mm in diameter; achenes rufous- or olivaceous-brown to black; leaf sheaths of the upper culm usually prominently wrinkled, inflated, membranous, and disintegrating. |
| ....3 Achenes rufous-brown to reddish-purple to black, (0.3-) 0.4-0.6 mm wide; longer bristles retrorsely barbed, shorter than to equaling achene body |
| ....3 Achenes olivaceous-brown to black (often olivaceous), 0.5-0.7 (-0.8) mm wide; longer bristles either retrorsely barbed and equaling to exceeding the tubercle (E. olivacea var. olivacea), or smooth and shorter than the tubercle (E. olivacea var. reductiseta). |
| ......4 Bristles retrorsely barbed, the longest bristles equaling to far exceeding the tubercle; [wet sandy or peaty habitats, NS to FL, w. to MS and TX] |
| ......4 Bristles smooth, the longest bristles shorter than the tubercle; [tidal rivers, in s. NJ] |
| 1 Distal leaf sheath apex firm, opaque, somewhat thickened, with an acute to obtuse apex, and sometimes bearing a small tooth; rarely disintegrating, not inflated or wrinkled; culms 0.3-5 mm wide. |
| ........5 Rhizomatous perennials growing from evident horizontal rhizomes; achenes not persistent in the spikelet after scales have fallen, 0.9-1.9 mm long; tubercles pyramidal or mammillate (nipple-like); [E. palustris complex]. |
| ..........6 Basal (proximal) scale of spikelet encircling 2/3-3/4 of the culm; culms terete to markedly compressed; (rarely amplexicaulous in E. macrostachya, if so then culms also usually markedly compressed). |
| ............ 7 Basal 2-3 scales empty or fertile; spikelets 5-40 mm long, 2-5 mm wide; achene yellow, yellow-brown, or dark-brown; leaf sheath apex often with tooth; [primarily west of MS river, with very scattered locations eastward] |
| ............ 7 Basal 2-3 scales sterile; spikelets 5-25 mm long, 3-7 mm wide; achene stramineous to dark brown; leaf sheath apex without tooth; [of the Mountains, rarely the Piedmont and scattered southwestward, widespread northward; e. and w. of MS river] |
| ..........6 Basal (proximal) scale of spikelet amplexicaulous (encircling the culm). |
............ ..8 Achenes prominently reticulate-pitted or finely rugulose (at 10-20×); [of the outer Coastal Plain] |
| ............ ..8 Achenes smooth to faintly reticulate; [of the Mountains, rarely Piedmont, or outer Coastal Plain]. |
| ............ ....9 Culms slender to filiform; scales obtuse, 30-40 per spikelet; scales 2-3.5 mm long; achenes 0.9-1.6 mm long; leaf sheath apex with tooth (0.1 mm long) sometimes present; [of basic soils, southward in the Mountains and rarely Piedmont, widespread northward] |
| ............ ....9 Culms thicker, somewhat inflated; scales acute, 5-30 per spikelet; scales 3-4 mm long; achenes 1.3-1.8 mm long; leaf sheath apex without tooth; [of brackish habitats of the outer Coastal Plain, NC northward] |
| ........5 Tufted or cespitose annuals without thick horizontal rhizomes; achenes persistent in spikelets, 0.3-1.2(-1.5) mm long; tubercles deltoid, or umbonate to subconic. |
| ............ ......10 Mature achenes reddish-black to black; tubercles subconic to umbonate (with a knob-like protuberance); leaf sheath apex not bearing a tooth; [series Maculosae]. |
| ............ ........11 Spikes lance-ovoid to subcylindric; achene 0.3-0.5(-0.6) mm long, apex often constricted at junction with tubercle (achene with thin neck); bristles colorless or white |
| ............ ........11 Spikes ovoid to subglobose; achene 0.7-1.0 mm long, apex rarely constricted at junction with tubercle (achene without thin neck); bristles red-brown (sometimes whitish) |
| ............ ......10 Mature achenes white, brown, yellow, or occasionally greenish (never black); tubercles deltoid; leaf sheath apex present on some or all culms; [series Ovatae]. |
| ............ ..........12 Tubercle < 0.5 mm wide, < 2/3 the width of the achene; achenes 0.75-1 mm long; spikelets ovoid. |
| ............ ............ 13 Perianth bristles 2–4, or absent, if present then highly reduced or rudimentary (or rarely, equaling achene), never exceeding tubercle; tubercles 0.2–0.3 mm high; [fresh tidal river shores, rare; DE, NJ, and PA northward] |
| ............ ............ 13 Perianth bristles 5-7, retrorsely barbed, exceeding tubercle; tubercles 0.3–0.5 mm high; [widespread, VA w. to MO northward] |
| ............ ..........12 Tubercle > 0.5 mm wide, > 2/3 the width of the achene (often nearly as wide as achene); achenes 0.9-1.2 mm long; spikelets broadly ovoid to narrowly lanceoloid. |
| ............ ............ ..14 Bristles much shorter than the achene, rudimentary, or absent |
| ............ ............ ..14 Bristles as long as or exceeding the tubercle, present. |
| ............ ............ ....15 Spikelet narrowly lanceoloid, apex acute; scales ovate with acute apices; styles 2-fid only; tubercle 0.5-0.7 mm wide; culms typically 20-35 cm long; [west of MS river, disjunct and rare in TN] |
| ............ ............ ....15 Spikelet mostly broadly ovoid (sometimes ellipsoid/lanceoloid), apex usually rounded (sometimes acute); scales elliptic with rounded apices; tubercle 0.7-0.9 mm wide; styles 2-fid AND 3-fid; culms 3–50(–90) cm long; [widespread e. and w. of MS river] |