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Key to Eleocharis

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1 Plants spreading primarily via proliferous spikelets and tip-rooting culms OR growing as submersed or floating aquatics with only vegetative culms present (these either simple and flaccid or proliferous); plants sometimes with a few fertile culms that bear spikelets but otherwise primarily vegetative; culms usually sprawling; tip-rooting culms dark green and firm, OR wiry, weak and light green colored; plants growing submersed in water or sprawling on moist exposed soil in areas with seasonal drawdown
1 Plants primarily with fertile spikelets born at the apex of culms, occasionally with a few proliferous spikelets, but rarely spreading primarily via vegetative means; culms erect, decumbent, or reclining, robust or diminutive; plants cespitose OR rhizomatous and mat-forming; plants growing as terrestrial or emergent wetland plants.
..2 Culms transversely nodose-septate, these cross partitions complete and evident through the outside of culms (fresh culms appear ribbed); achenes biconvex; [subgenus Limnochloa, in part]
..2 Culms not transversely nodose-septate OR if so, septae incomplete, internal, and not evident on the outside of fresh culms (only evident upon sectioning culm); achenes biconvex, terete, trigonous, or compressed-trigonous.
....3 Spikelets distinctly long and cylindric (> 4× as long as wide); the width of the spikelets the same as the culm width, thus not conspicuously expanded from the culms below; the base of the spikelets narrowly cuneate; achenes biconvex; [subgenus Limnochloa , in part]
....3 Spikelets typically ovoid, but also terete, lanceoloid, or ellipsoid and rarely cylindric (e.g. E. palustris), < 4× as long as wide; spikelets distinctly wider than the culms, thus obviously differentiated and expanded from the culms below; the base of the spikelets broadly cuneate, rounded, or truncate; achenes biconvex, terete, trigonous, or compressed-trigonous.
......4 Achenes bearing several (6-13) longitudinal ribs with very narrow horizontally elongate cells between, these ribs conspicuously raised from the achene surface; achenes diminutive, < 1.1 mm long; [subgenus Scirpidium]
......4 Achenes not bearing 6 or more longitudinal ribs; if bearing indistinct ribs, then achenes do not have thin horizontal intervening cells (as in E. tortilis and E. tuberculosa), or if bearing 3 distinct keels, these only along the margins of each achene face (as in E. tricostata); achenes diminutive to large (0.5-2.5 mm long), their surfaces smooth, finely reticulate or veined, rugulose, or honeycomb-reticulate.
........5 Achenes primarily lenticular or biconvex (therefore achene with only 2 prominent faces); styles mostly 2-branched (occasionally with some 3-branched).
..........6 Plants primarily consisting of vegetative proliferations (rarely with reproductive culms), the vegetative culms branched and terminating in numerous whorl-like arrangements; growing aquatically or submerged in standing water; spikelets one-flowered with 2 scales, these reproductive culms exserted just above the water surface at anthesis; achenes terminated by a subulate mucro, lacking a tubercle; [Eleocharis ser. Websteria]
..........6 Plants without whorled vegetative proliferations; emergent in a variety of wetlands; spikelets with a few to many flowers or scales (usually 8 or more); achenes terminated by a tubercle; [subgenus Eleocharis, in part].
........5 Achenes primarily trigonous (sometimes compressed trigonous or plano-trigonous), therefore with 3 prominent to obscured faces OR achenes nearly terete; achenes bearing more than only 2 obvious sides, these marked with obvious keels or rounded edges, OR almost entirely rounded and thus without any prominent faces; styles mostly 3-branched (occasionally with some 2-branched); [subgenus Eleocharis, in part, except for E. quinqueflora].
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