1 Leaf blades sagittate or cordate (at least some of the leaves on a plant with sagittate or cordate basal lobes); some species are keyed both here and below).

  2 Sepals appressed in fruit; lower flowers bisexual, the stamens either functional or not; stamen filaments roughened with minute scales (except glabrous in S. spatulata); [subgenus Lophotocarpus].

    3 Leaves primarily phyllodial, lanceolate or spatulate (sagittate leaves rare in the population and few on a given plant); flowers in 1-2 (-3) whorls; stamen filaments glabrous (use 10×); [native, tidal marshes]

    3 Leaves primarily sagittate (phyllodial leaves rare in the population and few on a given plant); flowers in 3-12 whorls; stamen filaments roughened with minute scales (use 10×); [either introduced exotics, sometimes in tidal marshes, or native, in inland alkaline sites].

      4 Plants with floating, sagittate blades; inflorescences floating; achene faces tuberculate

      4 Plants with emersed, sagittate blades; inflorescences emersed; achene faces smooth.

        5 Petals white, immaculate; stamens of pistillate flowers functional; [inland sites, native or introduced at a given locality]

        5 Petals white, with a purple spot at the base; stamens of pistillate flowers generally nonfunctional; [exotic, introduced around coastal ports]

  2 Sepals reflexed or at least widely spreading in fruit; lower flowers pistillate; stamens glabrous (except roughened with minute scales in S. rigida); [subgenus Sagittaria].

          6 Leaves cordate basally, floating; stalks of the pistillate flowers stout, reflexed in fruit; stamens mostly fewer than 15

          6 Leaves sagittate basally, emersed; stalks of the pistillate not notably stout, ascending in fruit; stamens 15 or more.

             7 Beak of the achene lateral (at a right angle to the long axis of the achene); bracts of the inflorescence 2-15 mm long, boat-shaped, obtuse or broadly acute.

               8 Lowermost (pistillate) flowers sessile or on short pedicels (to 5 mm or rarely 10 mm long), the pedicels of the lowermost flowers notably shorter than those in whorls above; inflorescence normally bent at the lowest whorl of flowers; stamen filaments minutely roughened with minute scales

               8 Lowermost (pistillate) flowers on long pedicels (at least 20 mm), the pedicels of the lowermost flowers longer than those in whorls above; inflorescence normally not bent; stamen filaments glabrous.

                 9 Basal lobes of the leaf > 2× as long as the blade body (the terminal lobe); basal lobes of the leaf narrow, < 2.5 cm wide even at their bases; achene beak 1-2 mm long, lateral and horizontal; achene faces tuberculate; [OK and TX westwards]

                 9 Basal lobes of the leaf < 1.5× as long as the blade body (the terminal lobe); basal lobes of the leaf broad or less commonly narrow, usually > 2.5 cm wide at their bases; achene beak 0.1-0.6 mm long, lateral and ascending; achene faces smooth; [widespread in our region].

                   10 Leaf blades glabrous; achene faces with resin ducts

                   10 Leaf blades rather densely fine-pubescent; achene faces lacking resin ducts

             7 Beak of the achene terminal (extending along the long axis of the achene); bracts of the inflorescence 5-40 mm long, either blunt or acuminate, not boat-shaped.

                     11 Bracts of the inflorescence thick and herbaceous, 5-25 mm long, rounded at the tip; flowers in 2-4 whorls; achenes with facial resin-ducts; [acidic, blackwater habitats of the Coastal Plain]

                     11 Bracts of the inflorescence papery and tan, 7-40 mm long, acuminate at the tip; flowers in 5-12 whorls; achenes without resin-ducts; [primarily other habitats, collectively widespread].

                       12 Bracts connate at least ¼ total length, leaves emersed but also often floating or submerged.

                       12 Bracts distinct or connate much less than ¼ total length; leaves emersed only.

                          13 Petiole sharply 5-wing-angled in cross-section; inflorescence unbranched; fruiting heads 1.0-1.5 cm in diameter, globular

                          13 Petiole corrugated but not wing-angled in cross-section; inflorescence often branched at the base; fruiting heads (1.2-) 1.7-2.2 cm in diameter, often globular-depressed

1 Leaf blades linear, lanceolate, or ovate, or modified as linear, bladeless phyllodia, these often of spongy texture.

                            14 Stalks of the pistillate flowers reflexed in fruit, often stout; stamen filaments glabrous (except roughened with minute scales in S. platyphylla and S. calycina).

                              15 Sepals appressed in fruit; lower flowers bisexual, the stamens either functional or not; [subgenus Lophotocarpus].

                                16 Leaves generally primarily sagittate (phyllodial leaves generally rare in the population); flowers in 3-12 whorls; stamen filaments roughened with minute scales (use 10× magnification); [inland alkaline sites]

                                16 Leaves primarily phyllodial, lanceolate or spatulate (sagittate leaves rare in the population and few on a given plant); flowers in 1-2 (-3) whorls; stamen filaments glabrous (use 10× magnification); [tidal marshes]

                              15 Sepals reflexed or at least widely spreading in fruit; lower flowers pistillate; [subgenus Sagittaria].

                                  17 Plant generally with erect, emersed leaves with well-developed blades with firm texture, the blades 2-8 cm wide.

                                    18 Fruiting pedicels ascending, filaments not dilated

                                    18 Fruiting pedicels spreading to recurved, filaments dilated

                                  17 Plant with all leaves phyllodial, if expanded at the summit, the expanded blade of weak texture, floating.

                                       19 Leaves 2-10 (-30) cm long, 3-8 mm wide (sometimes with dilated tip to 20 mm wide); [tidal, fresh to brackish waters]

                                       19 Leaves 30-300 (or more) cm long, either 1-3 or 7-14 mm wide; [nontidal waters].

                                         20 Leaves very variable from population to population, in swiftly flowing black water typically about 100 cm long and 1-3 mm wide, in more stagnant water (or when emersed by dropping water levels, typically with lax petioles and floating blades, the blades lanceolate, or elliptic, the base cuneate, rounded, or cordate; [blackwater streams and ponds, MA south to FL, west to s. AL]

                                         20 Leaves 100-300 (or more) cm long, 7-14 mm wide; [springs and spring-runs, endemic to FL]

                            14 Stalks of the pistillate flowers ascending or spreading in fruit, not notably stout; stamen filaments roughened with minute scales (except glabrous in S. engelmanniana, S. papillosa, and S. ambigua).

                                           21 Stamen filaments linear, less thick than the anther, changing little in diameter from near base to near summit.

                                             22 Leaves all phyllodial, without flattened blades; bracts of the inflorescence strongly papillose; [s. MS westward]

                                             22 Leaves with flattened blades; bracts of the inflorescence smooth, papillose, or longitudinally striate; [collectively widespread]

                                               23 Bracts of the inflorescence firm or delicate in texture, smooth; stamen filaments glabrous; [of inland wetlands]

                                                 24 Achenes 1.5-2.1 mm long, 0.8-1.5 mm wide; [inland circumneutral wetlands, west of the Mississippi River]

                                                 24 Achenes 2.4-4 mm long, 1.5-3.8 mm wide; [inland acidic wetlands, east of the Mississippi River]

                                               23 Bracts of the inflorescence either papillose or longitudinally striate-ribbed; stamen filaments roughened with minute scales; [estuarine areas and associated nontidal wetlands].

                                                    25 Bracts and sepals striate-ribbed; stamen filaments 2-5 mm long; leaves linear to ovate

                                           21 Stamen filaments either distinctly dilated toward the base (often broadly conic) or thickened throughout, the filament (at least basally) as thick or thicker than the anther.

                                                      26 Lowermost (pistillate) flowers sessile or on short pedicels (to 5 mm or rarely 10 mm long); inflorescence normally bent at the lowest whorl of flowers

                                                      26 Lowermost (pistillate) flowers on longer pedicels; inflorescence normally not bent.

                                                        27 Leaves all phyllodia, the phyllodia terete or nearly so.

                                                          28 Phyllodia of emersed flowering plants elongate (1/2-1× as long as scape), slender, emersed or laxly ascending and submersed in water; phyllodia of stranded flowering plants elongate (1/3-1× as long as scape, but may be shorter), relatively stiff; [se. NC and southward]

                                                          28 Phyllodia of emersed flowering plants short (2-8 cm), very thick, deeply submersed; when plants are stranded, phyllodia of stranded flowering plants elongate (1/2-1× as long as scape), slender, stiffly erect; [DE-MD and northward]

                                                        27 Leaves with blades and petioles, or if all phyllodia, the phyllodia flattened on upper surface or triangular in cross-section; [collectively widespread].

                                                            29 Plants with corms and/or stolons, lacking coarse rhizomes.

                                                              30 Blades of emersed leaves lanceolate, narrowly spatulate, > 5 mm wide; [Mountain and upper Piedmont bogs, swamp forests, and adjacent ditches]

                                                              30 Blades of emersed leaves linear (< 3 mm wide, rarely to 4 mm) or phyllodial; [Coastal Plain depression ponds and impoundments]

                                                                 31 Achenes 1.5-2.0 (-2.5) mm long; achene faces with 3 or more keels and 2 or more resin ducts; inflorescence bracts connate for > 50% of length; [se. NC and southward]

                                                                 31 Achenes (2.2-) 2.4-3.0 mm long; achene faces with 2-3 keels and 1-2 resin ducts; inflorescence bracts connate for < 40% of length; [restricted to Sandhills region of NC and SC, likely in e GA]

                                                            29 Plants with coarse rhizomes, lacking corms and stolons.

                                                                   32 Abaxial wing of fruit scalloped or toothed; [n. AL and nw. GA]

                                                                   32 Abaxial wing of fruit entire; [collectively widespread].

                                                                     33 Larger phyllodes 0.8-2.5 cm wide, the apices blunt (rarely acute); longer pistillate pedicels 2-5 (-6.5) cm long; median resin duct of mature achene linear, about as wide as the posterior duct (or ducts absent)

                                                                     33 Larger phyllodes to 1 cm wide (except sometimes wider in S. chapmanii), the apices acute; longer pistillate pedicels 1-4 cm long; median resin duct of mature achene club-shaped and 2× the diameter of the posterior duct.

                                                                       34 Inflorescence branched at the base (in at least some plants of a population); bracts of the inflorescence only slightly connate, the free tips narrowly triangular, 6-15 mm long

                                                                       34 Inflorescence unbranched at the base; bracts of the inflorescence slightly to almost fully connate