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Keyed in multiple places:

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Key to Alismataceae

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1 Pistils in a single whorl, borne on a flat receptacle; stamens 6; inflorescence compound, many of the primary nodes bearing whorled branches which in turn bear whorled branches or whorled flowers
1 Pistils spiraled in several to many whorls, borne on a globose receptacle; stamens 6-many; inflorescence either racemose (or in some species of both Echinodorus and Sagittaria somewhat compound, with the lowermost node or two bearing branches which in turn bear whorled flowers) or umbellate (Hydrocleys).
  2 Scape septate; fruits follicles
  2 Scape not septate; fruits achenes.
    3 Achenes flattened, with winged margins and often also with irregular corky ornamentations on the faces; flower whorls subtended by 3 bracts, with no additional bracteoles
    3 Achenes turgid, with ribs or ridges; flower whorls subtended by 3 bracts and additional bracteoles.
      4 Leaf blades 5-20 cm long, 3-15 cm wide; achenes (pistils) 45-250 per head; stamens ca. 21; petals 6-12 mm long, scapes 20-120 cm tall, erect or arching/reclining
      4 Leaf blades 1-3 cm long, 0.2-2 cm wide; achenes (pistils) 10-20 per head; stamens 6 or 9; petals 1-3 mm long; scapes 5-10 cm tall, erect

Key C3: rooted aquatics with basal and simple, broad leaves

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1 Leaves peltate.
  2 Leaf blades oval in shape, ca. 1.5-2× as long as wide, at maturity floating on the water’s surface; underwater portions of fresh plant coated in transparent mucilage
  2 Leaf blades orbicular in shape, ca. 1× as long as wide, at maturity floating on the water’s surface, emersed, or submersed; underwater portions of fresh plant not mucilaginous (though possibly with green algae, etc.).
    3 Leaves small, < 8 cm in diameter, at maturity emersed or submersed
    3 Leaves large, > 20 cm in diameter, at maturity floating on the water’s surface or emersed
1 Leaves not peltate, the petiole attached at a cuneate, cordate, or sagittate base.
      4 Leaf blades cuneate, rounded, or truncate at the base.
        5 Basal leaves lanceolate, 3-6 mm long
        5 Basal leaves variously shaped, > 10 mm long.
          6 Main veins palmate from the leaf base and also diverging from the midvein, the secondary and tertiary veins then reticulating; inflorescence a spike; [Eudicots]
          6 Main veins either parallel or palmate from the leaf base with cross-veins at nearly right angles to the main veins; inflorescence either a diffuse raceme or panicle of white flowers, or a linear spadix of tightly packed golden-yellow flowers; [Monocots].
             7 Inflorescence diffuse, a raceme or panicle with whorled branches or pedicels, the flowers widely spaced and white; leaves green, “wettable”
             7 Inflorescence tightly packed with flowers, an elongate, golden yellow spadix; leaves blue-green, “unwettable”
      4 Leaf blades cordate or sagittate at the base.
               8 Leaf blades sagittate, the two lobes distinctly acute; leaf apex acute; leaf blade 1.3-3× as long as wide
               8 Leaf blades cordate, the two lobes rounded or sub-acute; leaf apex rounded or apiculate; leaf blade 0.8-8× as long as wide.
                 9 Leaf blade margins serrate, dentate, and/or incised
                 9 Leaf blade margins entire or obscurely crenate.
                   10 Flowers 4-5-merous (sepals 4-5, petals 4-5, stamens 4-5); [Eudicots]
                     11 Leaves emersed or submersed, on stout, stiff petioles (the submersed and winter leaves cuneate-based and lanceolate); flowers 4-merous (sepals 4, petals 4, stamens 4); inflorescence a spike
                     11 Leaves floating, on slender, flexuous petioles (all leaves cordate-based); flowers 5-merous (sepals 5, petals 5, stamens 5); inflorescence an umbel
                   10 Flowers 3-merous or many (>5-) -merous; [Basal Angiosperms or Monocots].
                       12 Perianth parts numerous (usually showing differentiation into sepals and petals, though often with some intergradation), borne in a spiral; stamens numerous; leaves usually > 10 cm long or > 10 cm wide, or both (a few northern species of Nymphaea with leaves as small as 2.5 cm × 2.5 cm); [Basal Angiosperms]
                       12 Perianth parts 3-6 (either differentiated or not into sepals and petals); stamens either 3, or 9-12 (-18), or >20; leaves 1-10 cm long, 1-10 cm wide; [Monocots].
                          13 Perianth strongly differentiated, the 3 sepals green, leathery, and persistent, the 3 petals yellow or white, delicate, deciduous; stamens 20 or more
                          13 Perianth not strongly differentiated, the sepals petaloid and similar in texture and color to the petals; stamens 10 or fewer.
                            14 Flowers unisexual, white, the sepals and petals separate; stamens 3, 6, 9, 12, 15, or 18; leaves either with a central area of spongiform cells (most easily seen on the lower leaf surface) (Limnobium), or without spongiform cells (Ottelia)
                            14 Flowers bisexual, white to blue, the perianth segments united below into a perianth tube 3-45 mm long; stamens 3; leaves lacking a central area of spongiform cells

Key C4: rooted aquatics with basal and simple, linear leaves

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1 Leaves thread-like or quill-like, about as thick as wide.
  2 Plants bulbous at base, and with the leaf bases expanded and containing sporangia; plant tufted or with very short rhizomes; [Lycophytes]
  2 Plants either somewhat bulbous or not at the base, the leaf-bases not containing sporangia; plant rhizomatous; [Pteridophytes, Eudicots, Monocots].
    3 New leaves unfurling with circinate vernation (a fiddlehead); plants reproducing by spores, from sporocarps on short stalks from the rhizome; [Pteridophytes]
    3 New leaves lacking circinate vernation; plants reproducing by flowers and seeds.
      4 Perianth differentiated, with either 3 sepals and 3 petals or 5 sepals and 5 petals; stamens either 7-many or stamens 4.
        5 Sepals 3; petals 3; stamens 7-many; [Monocots]
        5 Sepals 5; petals 5; stamens 4; [Eudicots]
      4 Perianth undifferentiated, with 0, 3, or 6 tepals; stamens 1, 2, or 3; [Monocots].
          6 Gynoecium of 2 or more pistils, each pistil with 1 carpel and with 1 stigma
          6 Gynoecium of 1 pistil, each pistil with (2-) 3 carpels and (2-) 3 stigmas.
             7 Fruit an achene; perianth absent
             7 Fruit a capsule; perianth of 6 tepals
1 Leaves ribbon-like or strap-like, distinctly flattened (sometimes only near the tip of the leaf).
               8 Subterranean portions of plant bearing bladder-traps; flowers yellow or purple, bilaterally symmetrical
               8 Subterranean portions of plant lacking bladder traps; flowers white, green, gray, radially symmetrical (except bilaterally symmetrical in Glossostigma in PHRYMACEAE).
                 9 Leaves broadened towards the tip; [Monocots, Eudicots].
                   10 Flowers 3-merous; [Monocots]
                   10 Flowers 4- or 5-merous; [Eudicots].
                     11 Leaves phyllodial, with obvious cross-partitions (septa); leaves alternate
                     11 Leaves not phyllodial; leaves opposite
                 9 Leaves parallel-margined or tapering towards the apex over much of their length; [Monocots].
                       12 Leaves tapering towards the apex over much of their length; plant either tufted and not rhizomatous, or short rhizomatous;.
                          13 Plant tufted, not rhizomatous; leaves spreading radially; inflorescence a tightly button-like head of very numerous small flowers, white, gray, tan, yellowish, or blackish; roots thickened, septate (not requiring magnification), unbranched
                          13 Planted short-rhizomatous; leaves distichous, equitant; inflorescence either a subglobular, ovoid, or cylindrical head, of spirally imbricate scales, or a diffuse corymb; roots not thickened, not septate, branched.
                            14 Inflorescence a diffuse corymb; rhizomes and roots bright red
                            14 Inflorescence a subglobular, ovoid, or cylindrical head of spirally imbricate scales subtending individual flowers; rhizomes and roots not bright red
                       12 Leaves parallel-margined; plant usually rhizomatous.
                              15 Plants of marine habitats, growing submersed in salt water; [FL and the Gulf Coast of AL, MS, and LA]
                              15 Plants of freshwater or slightly to somewhat brackish habitats; [collectively widespread].
                                16 Leaves lacking any midvein; flowers and fruits in globose heads
                                16 Leaves with a midvein; flowers and fruits solitary or in diffuse inflorescences.
                                  17 Leaves with a distinct, broad lacunar band along the midvein
                                  17 Leaves lacking a distinct lacunar band along the midvein.
                                    18 Stamens 3
                                    18 Stamens >6