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1 Leaves reduced to hollow, linear, nodose-septate quills, consisting of the petiole and leaflet-less rachis, undivided
....3 Fruits 2.7-4.2 mm long; umbellets usually 5-7 per umbel; flowers usually 5-7 (-8) per umbellet; bracts subtending the umbels and umbellets with 1 (-3) linear segments; leaf segments of mid-stem leaves 15-30 (40), capillary to linear, 0.5-1.9 mm wide; flowering May-early Jun; fruiting late May-Jul; [plants of tidal marshes]
....3 Fruits 1.4-2.0 mm long; umbellets usually 10 or more per umbel; flowers usually 10 or more per umbellet; bracts subtending the umbel and umbellets with (1-) 3-5 linear segments; leaf segments of mid-stem leaves 50 or more, capillary, usually < 0.5 mm wide (except in submersed leaves); flowering Jun-Aug, fruiting Jul-Sep; [plants of a wide variety of wet habitats, including tidal marshes]
......4 Midstem leaves with 2-4 (-5) nodes along the rachis, the primary leaf segments usually alternate or opposite at the nodes (not including the node at the apex of the rachis); individual leaf segments often much longer than the rachis; involucral bract segments 1 (-3); stem sometimes slightly thickened at the base but never forming a corm; styles (0.3-) 0.4-0.6 mm long; fruits 1.0-1.9 mm long; dorsal ribs of the fruit thick, rounded; flowering Apr-Jul; fruiting late May-early Aug
......4 Midstem leaves with (6-) 7-16 nodes along the rachis, the primary leaf segments whorled or verticillate at the major nodes; individual leaf segments shorter than the rachis (or rarely as long as in P. texense); involucral bract segments (1-) 3 (-7); stem thickened and rounded at base, forming a globose or slightly elongate corm; styles (0.3-) 0.5-2.0 mm long; fruits 2.2-4.0 mm long; dorsal ribs of the fruit narrow, sharp-edged to blunt; flowering Jul-Oct; fruiting mid Jul-Nov.
........5 Leaf segments flat (at least near the nodes, but usually throughout); midvein visible; mid-stem leaves with (8-) 10-16 nodes per rachis; longest primary segments with 9-22 secondary segments; leaf shape in outline deltoid or trowel-shaped, the leaf segments becoming progressively and noticeably shorter towards the leaf apex; styles (0.8-) 1.0-2.0 mm long, slightly to distinctly spreading; flowering Jul-Oct; fruiting mid Jul-Oct; [of bottomlands, swamps, pond margins]
........5 Leaf segments 3-4-angled to subterete; midvein not visible; mid-stem leaves with (6-) 7-10 nodes per rachis; longest primary segments with 3-7 (-8) secondary segments; leaf shape in outline oblong or oval, the leaf segments not becoming progressively and noticeably shorter towards the leaf apex; styles (0.3-) 0.5-0.8 (-1.0) mm long, spreading to strongly recurved; flowering Aug-Oct; fruiting Sep-Nov; [of wet pine savannas, prairies, seeps]