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Key to Eleocharis, Key E: spikerushes with achenes lenticular or biconvex and styles mostly 2-branched (2-fid)

Cyperaceae

Eleocharis

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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 (except E. macrostachya); [primarily west of MS river, with very scattered locations eastward]
               8 Culms terete or slightly compressed; distal leaf sheath apex often obtuse, rarely with a tooth present (if so to 0.1 mm); spikelets broadly lanceoloid to ovoid, the scales 3.5-4.0 (-4.5) long; achenes 1.3-1.5 (-1.8) mm long
               8 Culms markedly compressed, 3x wider than thick; distal leaf-sheath apex subtruncate, often with a tooth .6-1 mm long; spikelets narrowly lanceoloid, the scales 2.5-4 mm long, lanceolate and carinate; achenes 1.1-1.5 mm long, 0.8-1.2 mm wide
             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).
                 9 Achenes prominently reticulate-pitted or finely rugulose (at 10-20×); [of the outer Coastal Plain]
                 9 Achenes smooth to faintly reticulate; [of the Mountains, rarely Piedmont, or outer Coastal Plain].
                   10 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]
                   10 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.
                     11 Mature achenes reddish-black to black; tubercles subconic to umbonate (with a knob-like protuberance); leaf sheath apex not bearing a tooth; [series Maculosae].
                       12 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
                       12 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)
                     11 Mature achenes white, brown, yellow, or occasionally greenish (never black); tubercles deltoid; leaf sheath apex present on some or all culms; [series Ovatae].
                          13 Tubercle < 0.5 mm wide, < 2/3 the width of the achene; achenes 0.75-1 mm long; spikelets ovoid.
                            14 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]
                            14 Perianth bristles 5-7, retrorsely barbed, exceeding tubercle; tubercles 0.3-0.5 mm high; [widespread, VA w. to MO northward]
                          13 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.
                              15 Bristles much shorter than the achene, rudimentary, or absent
                              15 Bristles as long as or exceeding the tubercle, present.
                                16 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]
                                16 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]