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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 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
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