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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 C7: rooted aquatics with simple, cauline, opposite or whorled leaves

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1 Leaves whorled, most nodes with 3 or more leaves.
  2 Leaves strongly toothed to pectinate
  2 Leaves entire or finely toothed.
    3 Leaves tipped by a callus (visible at 10× magnification); leaf margins entire; flowers many, grouped in a terminal involucrate head; [Eudicots]
    3 Leaves not callus-tipped; leaf margins finely toothed or at least with conical protrusions remaining from the disintegration of better-developed deciduous teeth; flowers solitary on elongate, flexuous stalks; [Monocots]
1 Leaves opposite, no nodes with 3 or more leaves.
      4 Cauline leaves in 1 pair
      4 Cauline leaves in 2-many nodes.
        5 Leaves in 2-3 pairs, appearing verticillate; plants of marine waters; [of FL, MS, LA and southward]
        5 Leaves along stem at 2-many nodes; plants of fresh to brackish waters; [collectively widespread].
          6 Flowers 3-merous; [Monocots].
             7 Leaf margins (or at least leaf sheaths) serrate or minutely spiny; fruits sessile, entire; leaves 5-15× as long as wide
             7 Leaf margins (including sheaths) entire; fruit stalked, dentate on one side; leaves >20× as long as wide
          6 Flowers 4- or 5-merous; [Eudicots].
               8 Leaves 1-12 cm long; flowers borne in axillary or terminal spikes or clusters
               8 Leaves 0.5-3 (-5) cm long; flowers solitary, axillary.
                 9 Carpels 4-5, separate; fruit an aggregate of follicles; leaves succulent
                 9 Carpels 2-5, fused; fruit capsular (variously dehiscent); leaves thin in texture or somewhat succulent (e.g. Bacopa in PLANTAGINACEAE).
                   10 Leaves dimorphic, the terminal leaves usually spatulate (strongly expanded towards the apex); corolla absent; stamen 1
                   10 Leaves monomorphic, obovate, oblanceolate, or parallel-margined; corolla present (absent in Didiplis in LYTHRACEAE); stamens 2-6.
                     11 Flower radially symmetrical, 3-4-merous; petals absent or separate
                       12 Stems lacking ridges running down from leaf bases
                       12 Stems with ridges running down from leaf bases
                     11 Flower bilaterally symmetrical, 4-5-merous; petals present, fused at least basally, forming a tube.
                          13 Corolla 4-merous
                          13 Corolla 5-merous
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