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Key C6: rooted aquatics with simple, cauline, alternate leaves

Plantae

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1 Leaves broad, < 4× as long as wide.
  2 Floating leaves peltate
  2 Floating or emersed leaves cuneate to rounded at base.
    3 Floating leaves diamond-shaped, prominently serrate
    3 Floating or emersed leaves elliptic or suborbicular.
      4 Leaves emersed, lanceolate to narrowly elliptic; flowers 5-merous; [Eudicots]
      4 Leaves emersed or floating, suborbicular or elliptic (if emersed, then suborbicular; flowers 3-merous; [Monocots].
1 Leaves narrow, > 4× as long as wide.
          6 Leaves 0.3-1.4 cm long, very numerous and tightly spaced
          6 Leaves 2-35 cm long, fewer and scattered along the stem.
             7 Leaf divided into a sheath and blade, with a ligule 0.5-8 mm long at the juncture; inflorescence a spike, raceme, or panicle of spikelets
             7 Leaf not divided into a sheath and blade, but if basally sheathing lacking a ligule (though sometimes with 1-2 conspicuous stipules); inflorescence various, but not as above.
               8 Leaves terete, to 35 cm long, 0.3-2 mm wide; plants of marine habitats, growing submersed in salt water; [FL and the Gulf Coast of AL, MS, and LA]
               8 Leaves flat or terete, to 200 cm long; plants of fresh or brackish waters (if of marine waters, Zostera, the leaves obviously flat, 1.5-6 mm wide); [collectively widespread].
                 9 Leaves filiform, terete or nearly so; stipules present, adnate to the leaf base and forming a sheath around the stem > 10 mm long.
                   10 Stipule free at its tip, the sheathing portion not appearing inflated; flowers > 2, in an interrupted spike
                   10 Stipule adnate its entire length to the leaf base, appearing inflated; flowers usually 2, on a flexous, elongate peduncle
                 9 Leaves flat; stipules absent, or if present, either free or adnate to the leaf base and forming a sheath for < 10 mm.
                     11 Leaves with a midvein; perianth parts 0 or 4, variously colored (not yellow).
                       12 Plants pubescent (at least on the upper stem); leaves pinnately veined; [Eudicots]
                       12 Plants nearly or entirely glabrous; leaves with parallel venation; [Monocots].
                          13 Inflorescences of flowers solitary or in 2-4 flowered racemes, axillary; spathe lacking; perianth conspicuous with 3 pink to purple petals
                            14 Inflorescence a cylindrical, interrupted spike, lacking a spathe; leaves either parallel-margined or variously with a narrow blade differentiated from a petiole; [fresh to brackish waters]
                            14 Inflorescence either a flattened spike sheathed by a spathe-like bract, or solitary; leaves parallel-margined, to 20 dm long; [saline (marine) to brackish waters].
                              15 Leaves with a notched or tricuspidate apex; flowers solitary; [e. NC southward]
                              15 Leaves with a rounded apex; flowers in a flattened spike sheathed by a spathe-like bract; [e. NC northward]