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Keyed in multiple places:

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Key to Hydrocharitaceae

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1 Leaves along the stem or at its summit.
  2 Leaves at only 2-3 closely spaced nodes at the summit of the stem, appearing verticillate or whorled; leaves to 10 cm long; [saltwater]; [subfamily Hydrilloideae]
  2 Leaves at many nodes along the stem, opposite or in whorls of 2-8, < 4 cm long; [freshwater].
    3 Leaves opposite or in whorls of 3 (-4) (no whorls with > 4 leaves).
      4 Leaves slightly narrowed or straight-sided to base, sessile; perianth present; [subfamily Anacharoideae]
      4 Leaves broadened and sheathing at base, narrowing upward via “shoulders”; perianth absent; [subfamily Hydrilloideae]
    3 Leaves in whorls of (3-) 4-8 (some or most whorls with 4 or more leaves).
        5 Leaves mostly 2-3 cm long, finely toothed with slender, weak teeth on the margins and rarely also the midrib beneath; fresh leaves not noticeably rough to the touch; leaf whorls generally crowded on all stems; petals white, 9-11 mm long; [subfamily Anacharoideae]
        5 Leaves mostly 1-2 cm long, toothed with stout, sharp teeth on the margins and also on conical bases along the midrib beneath; fresh leaves noticeably rough to the touch; leaf whorls crowded on terminal portions of stems, remote on older stems; petals translucent, 2-5 mm long; [subfamily Hydrilloideae]
          6 Leaves < 5× as long as wide; lower side of the midvein with 0-6 prickles.
          6 Leaves > 5× as long as wide; lower side of the midvein with 0 prickles
1 Leaves basal, either elongate with parallel sides, or petiolate with a leaf blade.
             7 Leaves differentiated into petiole and blade, the blade ovate to orbicular.
               8 Plant submersed; [subfamily Anacharoideae]
               8 Plant emergent or floating; [subfamily Hydrocharitoideae]
                 9 Petals broadly ovate, > 1.5× as long as the sepals; filaments fused for < 1/2 their length; larger aerenchyma (as measured at 1 mm from the midvein on the underside of the leaf) 0.1-0.5 mm across; stranded plants with deeply cordate leaves similar in shape to the floating leaves, with strongly arcing primary veins
                 9 Petals linear, < 1.5× as long as the sepals; filaments fused for > 1/2 their length; larger aerenchyma (as measured at 1 mm from the midvein on the underside of the leaf) 0.4-1.6 mm across; stranded plants with leaves with obtuse, truncate, or subcordate bases and all major veins gently curving and parallel
             7 Leaves straplike, elongate, linear, the sides parallel and not differentiated into petiole and blade.
                   10 Leaves to 35 cm long; [saltwater]; [subfamily Hydrilloideae]
                   10 Leaves usually > 40 cm long; [freshwater].
                     11 Leaves lacking lacunae on each side of the midvein; leaves acuminate; seeds echinate; flowers bisexual; [subfamily Anacharoideae]
                     11 Leaves with longitudinal rows of lacunae on each side of the midvein; leaves rounded at apex; seeds smooth; flowers unisexual; [subfamily Hydrilloideae]

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
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