Hydrocharitaceae

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  Najas

  1a. Leaves opposite; flowers without a perianth; androecium with 1 stamen; gynoecium composed of a single, unilocular, unicarpellate ovary; fruit an achene
• See list of 6 genera in 1b
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  1b. Leaves whorled (at least in part) or all basal; flowers with a perianth composed of 3 sepals and 3 petals; androecium with usually 2 or 9 stamens; gynoecium composed of 3–6 carpels; fruit usually ± capsule-like or berry-like
  • See list of 2 genera in 2a
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    2a. Leaves differentiated into blades and petioles, the blades broad-ovate to cordate-orbicular, floating on the surface or becoming emergent in dense vegetation, with evidently non-parallel primary lateral veins [Fig. 130]
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      Hydrocharis

      3a. Primary lateral veins of leaf blade forming a 75- to 90-degree (or greater) angle with the midvein [Fig. 130]; filaments distinct most of their length; petals broad-obovate, 
1.5 times or more or long as the sepals; anthers oval in outline; stipules 2, distinct from the petiole; seeds without blunt, cylindrical hairs
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      Limnobium

      3b. Primary lateral veins of leaf blade forming a 30- to 80-degree angle with the midvein; filaments connate more than ½ their length into a column; petals linear to narrow-oblong, up to 1.5 times as long as the sepals; anthers linear in outline; stipule 1, basally connate to the petiole; seeds covered with blunt, cylindrical hairs
  • See list of 4 genera in 2b
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    2b. Leaves not differentiated into blades and petioles, the linear to narrow-ovate blades submersed and with ± parallel veins [Figs. 131,132]
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      Vallisneria

      4a. Leaves all basal, 10–110 cm long, with a prominent, broad, central lacunar band 
 [Fig. 132]; staminate flowers with 2 stamens
    • See list of 3 genera in 4b
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      4b. Leaves borne on an elongate stem, mostly in whorls of 3–8, 0.4–4 cm long, without a prominent, central lacunar band; staminate flowers with 3–9 stamens
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        Hydrilla

        5a. Leaves with prickles on the abaxial midrib; intravaginal squamules (i.e., small, stipule-like scales in the leaf axils) fringed with orange-brown hairs; staminate flowers with 3 stamens
      • See list of 2 genera in 5b
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        5b. Leaves without prickles; intravaginal squamules entire or fringed with clear hairs; staminate flowers with 7–9 stamens
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          Elodea

          6a. Leaves numbering 2–4 per node, usually in whorls of 3 [Fig. 129]; petals inconspicuous, up to 4.5 mm long, scarcely, if at all, longer than the sepals
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          Egeria

          6b. Leaves numbering 4–8 per node, usually in whorls of 5 or more; petals more showy, 9–11 mm long, much longer than the sepals

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