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Keyed in multiple places:

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Key to Hydrilla

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1 Leaves < 5× as long as wide; lower side of the midvein with 0-6 prickles.
1 Leaves > 5× as long as wide; lower side of the midvein with 0 prickles

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]
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