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Keyed in multiple places:

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Key to Eichhornia

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  2 Plants rooted; stems elongate, with leaves spaced and alternate; petioles not inflated
  2 Plants floating (or stranded by dropping water levels); stems short, with leaves in a rosette with very short internodes; petioles inflated

Key to Pontederiaceae

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1 Inflorescence with > 30 flowers; fruit 1-seeded, indehiscent, a utricle; seeds smooth; leaves lanceolate to ovate, 1.5-10× as long as wide, the base cordate, truncate, or cuneate
1 Inflorescence with < 30 flowers; fruit 10-200-seeded, a capsule; seeds longitudinally winged; leaves either reniform or ovate, 0.5-1.5× as long as wide, the base cordate or rounded, or narrowly linear, 20-50× as long as wide, the base attenuate.
  2 Leaves membranous; petioles never expanded into air-filled floats; perianth lobes linear to oblanceolate, 0.4-2.0 cm long; stamens 3
  2 Leaves coriaceous; petioles either expanded into air-filled floats or not; perianth lobes ovate, 1.3-3.7 cm long; stamens 6.
    3 Herbs procumbent-emergent, stems elongate; sessile leaves late deciduous, rarely persistent in mature plants, petiolate leaves distichously-alternate, evenly distributed along the stem; perianth funnelform (the corolla limb expanding gradually from the widening corolla tube); style glabrous
    3 Herbs erect emergent or free-floating, stems inconspicuous; sessile leaves early deciduous, petiolate leaves spirally-alternate, congested at the apex of the stem; perianth salverform (the corolla limb spreading abruptly at a nearly right angle to the straight corolla tube); style glandular-pubescent.
      4 Erect emergents, never with inflated petioles; herbs never producing stolons; ligule truncate; inflorescences erect at post-anthesis, emerging from an inflated leaf-sheath, basal bract flat; flowers ca. 2-3 cm in diameter, perianth tightly enclosing the developing fruit; seeds subglobose to broadly oblong
      4 Free-floating aquatics, with inflated petioles; herbs stoloniferous; ligule flabellate, petioles generally inflated; inflorescences deflexed post-anthesis and in fruit, emerging from a non-inflated leaf-sheath, basal bract tubular; flowers ca. 4-6 cm in diameter, perianth loosely enclosing the developing fruit; seeds oblong

Key C1: floating aquatics

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1 Individual leaves > 2 cm wide.
  2 Leaves obovate, cuneate at the base, sessile, pale green; plants floating because of “unwettable” leaf surfaces
  2 Leaves orbicular, cordate or truncate at the base, petiolate, dark green; plants floating because of petioles inflated as bladders, or inflated cells centrally located on each leaf.
    3 Petiole terete, not air-filled; plants floating because of inflated cells centrally located on each leaf (most easily seen on the lower surface)
    3 Petiole conspicuously expanded into an air-filled bladder; plants floating because of petioles inflated as bladders
1 Individual leaves or “fronds” < 2 cm wide, or leaves absent.
      4 Submersed portions of the plant with small (< 4 mm in diameter) bladder-traps
      4 Submersed portions of plant lacking small bladder traps.
        5 Plants dichotomously forked, upper surface of leaves velvety or variously hairy
        5 Plants unbranched, or if branched, irregularly so; upper surface of leaves glabrous, waxy.
          6 Plant thalloid; flowers embedded in the thallus in reproductive pouches
          6 Plant with well-differentiated stems and leaves, the leaves obviously and alternately spaced along a well-developed stem; flowers axillary
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