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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]
1 Leaves basal, either elongate with parallel sides, or petiolate with a leaf blade.
..........6 Leaves differentiated into petiole and blade, the blade ovate to orbicular.
............ 7 Plant submersed; [subfamily Anacharoideae]
............ 7 Plant emergent or floating; [subfamily Hydrocharitoideae]
............ ..8 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
............ ..8 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
..........6 Leaves straplike, elongate, linear, the sides parallel and not differentiated into petiole and blade.
............ ....9 Leaves to 35 cm long; [saltwater]; [subfamily Hydrilloideae]
............ ....9 Leaves usually > 40 cm long; [freshwater].
............ ......10 Leaves lacking lacunae on each side of the midvein; leaves acuminate; seeds echinate; flowers bisexual; [subfamily Anacharoideae]
............ ......10 Leaves with longitudinal rows of lacunae on each side of the midvein; leaves rounded at apex; seeds smooth; flowers unisexual; [subfamily Hydrilloideae]

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