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Key to Ludwigia

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1 Leaves opposite; plants creeping (rooting at the nodes); [section Isnardia]
1 Leaves alternate; plants erect or ascending (not rooting at the nodes), or creeping (rooting at the nodes).
..2 Stamens 8-14; sepals 4-7; petals 4-7; [of various habits, including annual and perennial herbs and shrubs, variously erect, ascending, creeping, or forming floating mats]
..2 Stamens 4; sepals 4; petals 0-4; [perennial herbs, with erect ascending flowering stems]

Key to Ludwigia, Key A: Ludwigia with opposite leaves (section Isnardia)

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1 Pedicels of flowers and fruits 5-35 mm long.
..2 Petals 7-11 mm long; pedicels of capsules 15-35 mm long, longer than the leaves
..2 Petals 4-5 mm long; pedicels of capsules 5-16 mm long, shorter than to equaling the leaves
1 Pedicels of flowers and fruits 0-3 mm long.
....3 Stems, leaves, capsules, and calyx densely hirsute; seeds dark reddish-brown, 0.3-0.4 mm long
....3 Stems, leaves, capsules, and calyx glabrous to sparsely puberulent; seeds tan, 0.4-0.8 mm long.
......4 Petals 0; floral tubes and capsules with 4 longitudinal dark green bands; bractlets (borne at or near the base of the floral tube) absent or 0-1 mm long
......4 Petals 4; floral tubes and capsules lacking green banding; bractlets (borne at or near the base of the floral tube) present, 2-4 mm long

Key to Ludwigia, Key B: Ludwigia with alternate leaves, 8-14 stamens, 4-7 sepals, and 4-7 petals
(sections Pterocaulon, Macrocarpon, Seminuda, and Jussiaea)

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1 Sepals 4 (or rarely 5); stamens 8 (or rarely 10); seeds in 2-several vertical series in each locule, free of endocarp tissue; annual or perennial herbs or a woody shrub below.
..2 Capsule obconic, gradually broadening from base to apex, weakly angled; young stems and both surfaces of leaves shaggy-pubescent with tawny hairs; [section Myrtocarpus]
..2 Capsule narrowly cylindrical, nearly isodiametric, or, if obconic, sharply 4-angled and usually winged on the angles; young stems and both surfaces of leaves glabrous, sparsely short-pubescent, or sparsely villous.
....3 Internodes of the stem not winged, weakly angled, usually villous or strigillose at least proximally (sometimes inconspicuous); capsules nearly terete, narrowly cylindrical, 1.7-5.0 cm long, often curved.
......4 Petals (1.5-) 3-5 cm long; sepals ca. 10 mm wide at base
......4 Petals 1-2 cm long; sepals 3-5 mm wide at base
....3 Internodes of the stem sharply 4-angled or 4-winged, glabrous; capsule sharply 4-angled, obconic, often winged on the angles, 1-2 cm long (rarely longer in L. longifolia); [section Pterocaulon].
........5 Leaves linear-lanceolate, 5-35 cm long, 0.4-2.5 cm wide; petals 2-3 cm long; sepals 10-18 mm long; capsules 1.0-4.2 cm long; [rare waif, FL]
........5 Leaves lanceolate to elliptic, 2-20 cm long, 0.2-5 cm wide; petals 0.4-2 cm long; sepals 3-12 mm long; capsules 1.0-2.0 cm long; [widespread].
image of plant
Show caption*© Richard & Teresa Ware
..........6 Leaves sessile; petals 1-2 cm long; sepals 7-12 mm long; stems strongly 4-winged (from decurrent leaf bases)
..........6 Leaves petiolate, petioles 0.2-2.2 cm; petals 0.4-0.5 cm long; sepals 3-6 mm long; stems sharply 4-angled but usually not winged
1 Sepals 5 (-7); stamens 10 (-14); seeds in 1 vertical series in each locule, loosely embraced or embedded in endocarp tissue.
............ 7 Stems erect; floral tube much longer than the pedicel; seeds loosely embraced by a corky, horseshoe-shaped segment of endocarp; [section Seminudae]
............ 7 Stems (at least the lower portions) decumbent, creeping, or floating in mats (the flowering stems more-or-less erect in L. grandiflora and L. hexapetala); floral tube much shorter than the pedicel; seeds embedded in the woody endocarp; [section Jussiaea].
............ ..8 Flowering stems more-or-less erect; emergent leaves chartaceous, light to medium green, dull, mostly lanceolate or oblanceolate, petioles 0.1-2 (-2.5) cm; stems, sepals, ovaries, and emergent leaves (at least along veins) villous, hairs often viscid; bracteoles obovate, 1-1.8 mm long.
............ ....9 Sepals (6-) 8-11 (-14) mm long; emergent leaves 5-8.5 cm long, 7-11 mm wide, mostly linear-lanceolate, usually widest below the middle; petals (1.2-) 1.6-2.0 (-2.6) cm long; style 4.7-6.7 (-8.2) mm long; stems densely villous
............ ....9 Sepals (8-) 12-19 mm long; emergent leaves 5.5-13 cm long, 9-18 mm wide, mostly oblanceolate, usually widest above the middle; petals (1.5-) 2.0-2.9 (-3) cm long; style (5.8-) 6-10 mm long; stems sparsely to densely villous (rarely glabrous)
............ ..8 Flowering stems decumbent, floating, or creeping; stem and leaves glabrous or glabrescent; petals mostly 1-1.5 cm long; anthers 1-1.7 mm long.
............ ......10 Stems glabrous; emergent leaves with petioles 0.6-6 cm, blades (2-) 4-10cm, eglandular-mucronate, alternate; stamens subequal, anthers all 1.2-1.4 mm, stigmas well-exserted at anthesis; pedicels 3.5-9 cm, ovaries 14-20 mm; seeds 16-18 per locule; [widespread]
............ ......10 Stems glabrous or sparsely to densely villous; emergent leaves with petioles 0.3-2.8 cm, blades (0.4-) 0.8-6 (-9.5) cm, often glandular-mucronate, alternate or fascicled; stamens unequal, filaments of 2 different lengths and anthers of 2 different sizes, 0.5-2.2 mm, surrounding the stigmas at anthesis; pedicels 0.7-3.8 cm, ovaries 6-14 mm; seeds 7-15 per locule; [s. MS westward and southward].
............ ........11 Stems densely, rarely sparsely villous; ovaries and sepals densely hirtellous; capsules (2-) 2.4-3.2 cm long; [native of South America, rare waif in LA]
............ ........11 Stems sparsely villous to glabrous; ovaries and sepals glabrous or with a few scattered hairs; capsules 1.0-1.7 (-2.5) cm long; [native, more widespread]

Key to Ludwigia, Key C: Ludwigia with alternate leaves, 4 stamens, 4 sepals, and 0-4 petals (sections Ludwigia and Microcarpium)

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1 Pedicels 2-15 mm long; capsules subglobose to spheric or cubic, about as long as wide, box-like, 4-angled, dehiscence by an apical pore (later sometimes also irregularly loculicidal); petals present, 4-15 mm long, persistent or caducous; roots fascicled, fusiform, tuberous; plants lacking basal, stoloniform shoots; [section Ludwigia].
..2 Leaves cuneate at base; pedicels 2-5 mm long; nectary discs at base of style flattish, inconspicuous; [widespread in our area, in a wide variety of habitats]
..2 Leaves rounded or truncate at base; pedicels 4-15 mm long; nectary discs at base of style domed, prominent; [nearly restricted to the Coastal Plain, primarily of pinelands].
....3 Styles 6-10 mm long; plants glabrous, glabrescent, or pubescent with very short hairs; sepals strongly reflexed in fruit
....3 Styles 1.5-3 mm long; plants glabrescent or pubescent with short to long, spreading to shaggy hairs; sepals strongly reflexed, spreading, or ascending in fruit.
......4 Sepals narrowly deltoid, broadest at or near the base, 3-4× as long as wide, ascending or spreading in fruit; plants glabrescent to hirtellous with long spreading hairs
......4 Sepals ovate, broadest near the middle, ca. 2× as long as wide, strongly reflexed in fruit; plants pubescent with relatively short, appressed to spreading hairs
1 Pedicels 0-1 (-5) mm long; capsules subglobose, obconic, or obpyramidal, about as long as wide or longer than wide, circular to quadrangular in cross-section, dehiscence irregularly loculicidal; petals absent or present, if present (L. linearis, L. linifolia) then 0-6 mm long and caducous; roots fibrous or rhizomatous; plants frequently with basal, stoloniform shoots; [section Isnardia].
........5 Capsules cylindrical, narrowly obconical, or narrowly obpyramidal, at least 2.5-5× as long as broad; petals present or absent.
..........6 Primary leaves of the flowering stems narrowly elliptical, 6-12 (-20) mm wide; petals absent.
............ 7 Capsules 2-4 mm long; seed surface cells elongate transversely to seed length
............ 7 Capsules (4-) 5-8 (-9) mm long; seed surface cells elongate parallel to seed length
..........6 Primary leaves of the flowering stems linear, 1.5-5 mm wide; petals present.
............ ..8 Sepals (3.3-) 4-7 mm long; lateral and marginal veins obscure on lower leaf surface; seeds reddish brown; capsules cylindric, parallel-sided through most of their length, not grooved; anthers 0.5-1.1 mm long
............ ..8 Sepals 2.3-5 (-5.6) mm long; lateral and marginal veins distinct on lower leaf surface; seeds yellowish; capsules elongate obpyramidal, tapering through most or all of their length, with a shallow longitudinal groove on each face; anthers 1.1-2 mm long.
............ ....9 Sepals 2.3-4 mm long, acuminate, the surfaces densely and minutely papillose, the papillae 0.02-0.05 mm long and appressed; capsules 5-8.5 (-10) mm long, 2-4 (-5) mm in diameter; pedicels 0-0.4 mm long; seed surface cells elongate parallel to the seed length (as seen at 20× or more); anthers 1.1-1.6 mm long
............ ....9 Sepals 3-5 (-5.6) mm long, elongate-acuminate to cuspidate, the surfaces densely minutely strigillose, the hairs 0.06-0.10 mm long and appressed to ascending; capsules 5-10 (-12) mm long, 3-5.5 mm in diameter; pedicels 0-3.5 (-5) mm long; seed surface cells elongate transverse to the seed length, or irregular (as seen at 20× or more); anthers (1.1-) 1.3-2 mm long
........5 Capsules subglobose, obovoid, or broadly obpyramidal, 1-1.5 × as long as broad; petals absent.
............ ......10 Flowers in compact, headlike or elongate spikes, the inflorescence lacking well-developed leaves; stems rarely branched; rhizomes often present
............ ......10 Flowers axillary in the axils of well-developed leaves; stems usually much branched; rhizomes absent.
............ ........11 Plants densely pubescent throughout.
............ ..........12 Sepal apex elongate-acuminate or subcuspidate, reflexed; pubescence of stems and leaves hirtellous (the hairs spreading); seed surface cells suborbicular (as seen at 20× or more); anthers 0.6-0.9 (-1.3) mm long; style 1-2 mm long
............ ..........12 Sepal apex acuminate, ascending; pubescence of stems and leaves strigillose (the hairs appressed) or hirtellous (the hairs spreading); seed surface cells elongate; anthers 0.3-0.8 mm long; style 0.25-1 (-1.25) mm long.
............ ............ 13 Plants hirtellous; capsules oblong-obovoid; sepals greenish on the upper surface; bracteoles (1.5-) 2-4.3 mm long, borne at or near the base of the capsule; seed surface cells elongate transverse to the seed length; anthers 0.3-0.35 mm long; style 0.25-0.5 mm long
............ ............ 13 Plants strigillose; capsules subglobose; sepals yellowish on the upper surface; bracteoles 0.5-1.5 mm long, usually borne on the short pedicel; seed surface cells in patches, some patches with cells elongate parallel to seed length, others with cells transverse to seed length, others with cells diagonal (rather resembling a badly laid-out parque floor); anthers 0.5-0.8 mm long; style 0.55-1 (-1.25) mm long
............ ........11 Plants glabrous or subglabrous throughout.
............ ............ ..14 Primary leaves of the flowering stems 4-17 mm long, 1.5-10 mm wide, mostly obovate-spatulate and 1.5-3× as long as wide; capsules 1-1.5 (-2) mm long, containing 10-20 dark reddish-brown seeds; plants typically 1-4 dm tall; nectary disc nearly flat on ovary apex
............ ............ ..14 Primary leaves of the flowering stems (18-) 30-110 mm long, 2-10 (-20) mm wide, mostly elliptic, lanceolate, oblanceolate, or linear and 4-20× as long as wide; capsules 1.8- 2-7 mm long, containing 40-500 light brown, yellowish, or tan seeds; plants typically 3-10 dm tall; nectary disc distinctly raised on ovary apex.
............ ............ ....15 Capsules obpyramidal, the corners narrowly winged with wings 0.3-0.9 mm wide.
............ ............ ......16 Stems often distinctly ridged or winged; sepals creamy-white, nearly as long as the capsule; capsule wall bulging out longitudinally between the wings; seed surface cells elongate parallel to the seed length
............ ............ ......16 Stems nearly smooth or slightly ridged; sepals greenish, about 1/2 as long as the capsule; capsule wall flat between the wings; seed surface cells suborbicular
............ ............ ....15 Capsules oblong-ovoid or subglobose, the corners not winged.
............ ............ ........17 Cauline leaves lanceolate, oblong-elliptic to narrowly so, 30-110 mm long, usually about 10 × as long as wide; capsules dehiscing by irregular rupture of the outer walls; stolons present; [collectively widespread].
............ ............ ..........18 Bracteoles 3.5-6.5 (-8) mm long; sepals green, the apex long-acuminate, reflexed; capsules oblong-obovoid; seed surface cells elongate parallel to the seed length
............ ............ ..........18 Bracteoles 0.5-1.5 mm long; sepals yellowish, the apex acuminate, ascending capsules subglobose; seed surface cells in patches, some patches with cells elongate parallel to seed length, others with cells transverse to seed length, others with cells diagonal (rather resembling a badly laid-out parquet floor)
............ ............ ........17 Cauline leaves obovate-spatulate to oblanceolate, 16-30 mm long, usually < 5× as long as wide; capsules dehiscing by 4 longitudinally lenticular slits on the wall opposite the locules; stolons not common; [FL and MS].
............ ............ ............ 19 Capsules 2.5-4.7 mm long; sepals 1.5-3 mm long; vestigial petals occasionally present; lower or seedling leaves always alternate
............ ............ ............ 19 Capsules 1.5-2.5 mm long; sepals 1.2-1.8 mm long; vestigial petals rare; lower or seedling leaves tending to be opposite or subopposite
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