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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 C6: rooted aquatics with simple, cauline, alternate leaves

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1 Leaves broad, < 4× as long as wide.
..2 Floating leaves peltate
..2 Floating or emersed leaves cuneate to rounded at base.
....3 Floating leaves diamond-shaped, prominently serrate
....3 Floating or emersed leaves elliptic or suborbicular.
......4 Leaves emersed, lanceolate to narrowly elliptic; flowers 5-merous; [Eudicots]
......4 Leaves emersed or floating, suborbicular or elliptic (if emersed, then suborbicular; flowers 3-merous; [Monocots].
........5 Leaves emersed, suborbicular
........5 Leaves floating, elliptic
1 Leaves narrow, > 4× as long as wide.
..........6 Leaves 0.3-1.4 cm long, very numerous and tightly spaced
..........6 Leaves 2-35 cm long, fewer and scattered along the stem.
............ 7 Leaf divided into a sheath and blade, with a ligule 0.5-8 mm long at the juncture; inflorescence a spike, raceme, or panicle of spikelets
............ 7 Leaf not divided into a sheath and blade, but if basally sheathing lacking a ligule (though sometimes with 1-2 conspicuous stipules); inflorescence various, but not as above.
............ ..8 Leaves terete, to 35 cm long, 0.3-2 mm wide; plants of marine habitats, growing submersed in salt water; [FL and the Gulf Coast of AL, MS, and LA]
............ ..8 Leaves flat or terete, to 200 cm long; plants of fresh or brackish waters (if of marine waters, Zostera, the leaves obviously flat, 1.5-6 mm wide); [collectively widespread].
............ ....9 Leaves filiform, terete or nearly so; stipules present, adnate to the leaf base and forming a sheath around the stem > 10 mm long.
............ ......10 Stipule free at its tip, the sheathing portion not appearing inflated; flowers > 2, in an interrupted spike
............ ......10 Stipule adnate its entire length to the leaf base, appearing inflated; flowers usually 2, on a flexous, elongate peduncle
............ ....9 Leaves flat; stipules absent, or if present, either free or adnate to the leaf base and forming a sheath for < 10 mm.
............ ........11 Leaves lacking a midvein; perianth parts 6, yellow
............ ........11 Leaves with a midvein; perianth parts 0 or 4, variously colored (not yellow).
............ ..........12 Plants pubescent (at least on the upper stem); leaves pinnately veined; [Eudicots]
............ ..........12 Plants nearly or entirely glabrous; leaves with parallel venation; [Monocots].
............ ............ 13 Inflorescences of flowers solitary or in 2-4 flowered racemes, axillary; spathe lacking; perianth conspicuous with 3 pink to purple petals
............ ............ 13 Inflorescence a spike, terminal or axillary; with or without a spathe; perianth lacking.
............ ............ ..14 Inflorescence a cylindrical, interrupted spike, lacking a spathe; leaves either parallel-margined or variously with a narrow blade differentiated from a petiole; [fresh to brackish waters]
............ ............ ..14 Inflorescence either a flattened spike sheathed by a spathe-like bract, or solitary; leaves parallel-margined, to 20 dm long; [saline (marine) to brackish waters].
............ ............ ....15 Leaves with a notched or tricuspidate apex; flowers solitary; [e. NC southward]
............ ............ ....15 Leaves with a rounded apex; flowers in a flattened spike sheathed by a spathe-like bract; [e. NC northward]
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