Herbs, usually perennial, rarely annual, or shrubs, [rarely trees], caulescent, usually gla­brous, strigillose, villous, or hirtellous, rarely glandular-puberulent. Stems erect to spreading or pros­trate and then often rooting at nodes, sometimes floating, submerged parts, when present, sometimes swollen with spongy aerenchyma or bearing inflated, white, spongy pneumatophores, usually branched. Leaves cauline, usually alternate, rarely opposite; stipules present, often deciduous, usually dark reddish green; usually petiolate, sometimes sessile; blade usually reduced distally, usually linear to lanceolate, oblong, or obovate, rarely deltate, with 1[or 2] ± conspicuous submarginal vein[s], margins entire, serrulate, or glandular-serrulate, usually without oil cells. Inflorescences spikes, racemes, or clusters, solitary or paired in leaf axils, erect or decumbent and ascending at tip; bracteoles usually 2 and conspicuous, black or dark red, often scalelike, at or near base of ovary, sometimes deciduous early, rarely absent. Flowers bisexual, actinomorphic, pedicellate or sessile; floral tube absent; sepals persistent after anthesis or tardily caducous, (3 or)4 or 5(–7), green, sometimes yellow or cream, often becoming flushed with red post-anthesis, spreading to suberect; petals caducous, usually (3 or)4 or 5(–7), sometimes 0, usually yellow, sometimes white, when yellow, then often ultraviolet-reflecting, margins entire; sta­mens as many as sepals in 1 series, or 2 times as many as sepals in 2 subequal or unequal series; anthers versatile, on smallest flowers appearing basifixed, pollen shed singly or in polyads or tetrads, 3(–5)-aperturate; ovary usually with as many locules as sepals, rarely more, apex flat or conical, often with raised or depressed nectary lobes surrounding base of each epipet­alous stamen; style present [very rarely absent in sect. Arborescentes], usually glabrous; stigma entire or irregularly lobed, capitate or hemispherical, distal 1/2 receptive, surface wet and papillate. Fruit a capsule, spreading to erect, obconic, cylindric to clavate, turbinate, obpyramidal, or globose to cuboid, terete to sharply 4+-angled, straight to slightly curved, dehiscent irregularly or by a terminal pore, an apical ring, or flaps separating from valvelike apex, long-pedicellate to subsessile. Seeds 50–400, in 1–several rows per locule, usually free, sometimes embedded in endocarp, narrowly ovoid, smooth or finely pitted, raphe usually inconspicuous, sometimes expanded and nearly equal to seed [very rarely expanded into asymmetrical wing]. x = 8.

Distribution

North America, Mexico, West Indies, Bermuda, Central America, South America, se Asia, Africa, Indian Ocean Islands, Pacific Islands, introduced in Europe, w, s Asia, Australia.

Discussion

Species 82 (31 in the flora).

Ludwigia is a pansubtropical genus currently divided into 22 sections (P. H. Raven [1963]1964; T. P. Ramamoorthy and E. Zardini 1987; Zardini and Raven 1992; W. L. Wagner et al. 2007). The genus is well represented in North America and South America, and is frequent in Africa and south Asia. Raven provided a synopsis of Ludwigia, including in it all previously segregated genera (Isnardia, Jussiaea, Ludwigiantha, and Oocarpon), based in part on P. A. Munz (1942, 1944), H. Hara (1953), and others. H. E. Baillon (1866–1895, vol. 6) was the first author to merge Isnardia and Jussiaea under Ludwigia, and consequently Ludwigia is treated as having priority over the former two genera. Therefore, J. P. M. Brenan’s (1953) subsequent merging of Ludwigia under Jussiaea is not acceptable (vide Shenzhen Code Art. 11.5). Hara (1953) referenced both Baillon and Brenan, and followed the then-practiced botanical code to accept Ludwigia over Jussiaea. Hara, and later Raven, made most of the new combinations needed in Ludwigia, and their works firmly established Ludwigia. Raven subdivided the genus into 17 sections using a combination of characters: sepal number, stamens as many or two times as many as sepals; pollen as monads or in tetrads (polyads were not distinguished until reported by J. Praglowski et al. 1983), capsule morphology, and seed morphology. The large sect. Myrtocarpus, primarily distributed in South America, was later subdivided into a total of eight sections (Ramamoorthy 1979; Ramamoorthy and Zardini; Zardini and Raven). Wagner et al. placed sect. Oocarpon into sect. Oligospermum (= sect. Jussiaea), and the present treatment, based on recent molecular analysis (Liu S. H. et al. 2017), combines formerly recognized sect. Microcarpium with sect. Isnardia. This reduces the number of sections to 22, 14 of which are monospecific; Liu et al. did not have sufficient resolution to evaluate classifi­cation of sect. Myrtocarpus and its segregates.

Since the synopsis by P. H. Raven ([1963]1964), data have become available for Ludwigia from cytology (M. Kurabayashi et al. 1962; Raven and W. Tai 1979; E. Zardini et al. 1991), palynology (J. J. Skvarla et al. 1975, 1976, 1978; J. Praglowski et al. 1983; V. C. Patel et al. 1984), embryology (H. Tobe and Raven 1983, 1985, 1986, 1986b, 1996), and anatomy (S. Carlquist 1975, 1977, 1982b; R. H. Eyde 1977, 1979, 1981, 1982; R. C. Keating 1982), as well as several published and unpublished revisions of sections. These data provide a rich source of potential characters for phylogenetic analysis. All recent analyses, whether morpho­logical or molecular (see especially Eyde 1977, 1979; R. A. Levin et al. 2003, 2004; Liu S. H.et al. 2017), strongly support Ludwigia as monophyletic and sister to the rest of the family. Liu et al. also found strong support for a monophyletic sect. Ludwigia, a monophyletic sect. Isnardia that includes sect. Microcarpium, and a clade comprised of sects. Isnardia, Ludwigia, and Miquelia that is sister to the rest of the genus. Liu et al. found poor resolution in that second branch, due in part to inadequate sampling, but strong support for mono­phy­letic sects. Jussiaea and Macrocarpon.

Ludwigia appears to have diverged from the common ancestor of the family between 80 and 93 Ma (E. Conti et al. 1997; K. J. Sytsma et al. 2004). The genus exhibits a complex biogeographic pattern, with ten sections endemic or centered in South America (39 species), two in North America (24 species), five in Africa (seven species), two in Asia (two species), and two not clearly centered in a single continent (10 species). Ludwigia has a base chromosome number of x = 8; aneuploidy is unknown, but polyploidy is extensive (P. H. Raven and W. Tai 1979; E. Zardini et al. 1991). S. A. Graham and T. B. Cavalcanti (2001) proposed that x = 8 is the base chromosome number for Lythraceae, which is sister to Onagraceae. This suggests that x = 8 in Ludwigia is a plesiomorphy for Onagraceae, and that the chromosome number changed to x = 11 or x = 10 (in Hauya) on the branch leading to the rest of the family.

In the absence of a more thorough revision and formal phylogenetic analysis of Ludwigia, this treatment follows the most recent classification of the genus by W. L. Wagner et al. (2007), which is based primarily on P. H. Raven ([1963]1964) and supported by subsequent systematic and anatomical studies (especially Raven and W. Tai 1979; J. Praglowski et al. 1983; T. P. Ramamoorthy and E. Zardini 1987; Zardini and Raven 1992). Molecular analyses by Liu S. H. et al. (2017) support inclusion of sect. Microcarpium within sect. Isnardia, which involves more than half of the North American species. The sections are arranged using characters from Raven and elsewhere, as described in Wagner et al.

Species of Ludwigia characteristically grow in wet habitats, and some are nearly or fully aquatic. Those species often have adaptations for growing in water: aerenchyma—respiratory tissue with particularly large intercellular spaces—in the proximal stems, and/or pneumato­phores, which are spongy, white roots arising from internodes on floating stems that facilitate aeration needed for root respiration in hydrophytic plants. Species in sect. Ludwigia have fusiform, tuberous roots that may also serve an adaptive function in wet habitats.

Seventy-five species of Ludwigia are self-compatible and seven (in sects. Macrocarpon and Myrtocarpus) are self-incompatible (P. H. Raven 1979). Flowers of Ludwigia are diurnal, remaining open for several days or, sometimes, for only one day (in small-flowered autogamous species); species may be outcrossing and pollinated by bees, flower-flies, or butterflies, or autogamous (Raven).

Several species of Ludwigia are cultivated as aquarium plants (for example, L. repens); others are grown in water gardens. Several species, especially in sect. Jussiaea, are considered noxious, invasive species (M. Wood 2006).

Selected References

None.

Lower Taxa

Ludwigia sect. Isnardia, Ludwigia sect. Jussiaea, Ludwigia sect. Ludwigia, Ludwigia sect. Macrocarpon, Ludwigia sect. Myrtocarpus, Ludwigia sect. Pterocaulon, Ludwigia sect. Seminudae

Key

1	Stamens 2 times as many as sepals, in 2 series; seeds in 1 row per locule and embedded in endocarp, or 2 to several rows and free.	> 2
1	Stamens as many as sepals, in 1 series; seeds in several rows per locule, free.	> 10
2	Seeds in 1 row per locule, embedded in segment of endocarp, with inconspicuous raphe; capsules cylindric, subcylindric, or subclavate, terete, subterete, or obscurely angled.	> 3
2	Seeds in several rows per locule, free, raphe enlarged or inconspicuous; capsules ob­conic, obpyramidal, or cylindric, angled, winged, or terete.	> 6
3	Stems usually erect or ascending, rarely floating or creeping; seeds loosely embedded in horseshoe-shaped segment of endocarp, easily detached; pollen in polyads; leaves alternate [Ludwigia sect. Seminudae].	Ludwigia leptocarpa
3	Stems floating or creeping and ascending to erect; seeds firmly embedded in woody segment of endocarp; pollen as monads; leaves alternate or fascicled [Ludwigia sect. Jussiaea].	> 4
4	Bracteoles deltate, 0.5–1 × 0.5–1 mm; leaf blades mostly oblong or elliptic, petioles 0.3–6 cm; capsules 10–14 mm, pedicels 7–60(–90) mm; seeds 1–1.5 × 0.9–1.3 mm; sepals 3–12 mm.	Ludwigia peploides
4	Bracteoles narrowly or broadly obovate, 1–1.8 × 0.7–0.8 mm; leaf blades mostly lanceolate or oblanceolate, petioles 0.1–2(–2.5) cm; capsules (11–)14–25(–30) mm, pedicels (9–)13–25(–85) mm; seeds 0.8–1 × 0.8–1 mm; sepals 6–19 mm.	> 5
5	Emergent plant sparsely to densely villous; petioles 0.5–2(–2.5) cm; sepals (8–)12–19 mm; petals (15–)20–30 mm.	Ludwigia hexapetala
5	Emergent plant generally villous and viscid; petioles 0.1–1.1 cm; sepals 6–12(–16) mm; petals (12–)16–20(–26) mm.	Ludwigia grandiflora
6	Seeds with enlarged raphe ± equal to seed body; capsules cylindric to clavate, subterete to ± angled; sepals 4 [Ludwigia sect. Macrocarpon].	> 7
6	Seeds with inconspicuous raphe; capsules obconic or obpyramidal, oblong-obovoid, or subclavate to squarish cylindric, 4+-angled; sepals 4 or 5[–7].	> 8
7	Stems 60–250(–400) cm; sepals (6–)8–13 × 3–7 mm; petals (5–)10–20 × 5–20 mm; capsules (17–50 mm) usually exceeding pedicels (5–25 mm).	Ludwigia octovalvis
7	Stems 20–120 cm; sepals 10–20 × 7–12 mm; petals 20–35 × 10–30 mm; capsules (20–35 mm) rarely exceeding pedicels (10–40 mm).	Ludwigia bonariensis
8	Stems terete or angled; capsules ± 4- or 5-angled; plants perennial herbs or shrubs, usually pubescent, rarely glabrous [[Ludwigia sect. Myrtocarpus].	Ludwigia peruviana
8	Stems strongly 4-angled or -winged; capsules strongly 4+-angled or -winged; plants annual or short-lived perennial herbs, glabrous or sometimes strigillose on leaves and/or inflorescences [Ludwigia sect. Pterocaulon].	> 9
9	Leaves sessile; stems sharply 4-angled and -winged; sepals 7–12 × 1.5–4 mm; petals 10–20 × 10–18 mm.	Ludwigia decurrens
9	Leaves with petioles 0.2–2.2 cm; stems 4-angled, rarely -winged; sepals 3–6 × 1–2 mm; petals 3.5–5 × 2–2.5 mm.	Ludwigia erecta
10	Stems erect; roots often fusiform, fascicled; capsules globose, subcuboid, or ellipsoid, terete to 4-angled or 4-winged, with hard walls, dehiscing by terminal pore; petals present; leaves alternate [Ludwigia sect. Ludwigia].	> 11
10	Stems erect, ascending, prostrate, or decumbent; roots not fusiform, often with stolons or rhizomes; capsules subcylindric to clavate, oblong-obovoid, obconic, broadly obpyramidal or subglobose, terete to sharply angled, with hard or thin walls, dehiscent by apical ring or lenticular slits along locule edges or indehiscent; petals present or absent; leaves alternate or opposite [Ludwigia sect. Isnardia].	> 14
11	Leaves: petioles 0.1–0.3(–0.7) cm, blades attenuate; nectary disc slightly elevated, rounded; pedicels 2–7 mm, shorter than or equaling capsule.	Ludwigia alternifolia
11	Leaves: sessile, blades cuneate to attenuate; nectary disc elevated, domed, (prominently 4-lobed); pedicels 3–17 mm, equal to or exceeding capsule.	> 12
12	Stems usually densely erect-hirsute, sometimes glabrous, well branched distally; leaf blades lanceolate to ovate-oblong; bracteoles 2.5–7 mm; pedicels 3–10 mm.	Ludwigia hirtella
12	Stems strigillose to glabrate, simple or sparsely branched, often near base; leaf blades ovate or obovate proximally, lanceolate-linear or linear distally; bracteoles 0.7–3.2(–5) mm; pedicels 5–17 mm.	> 13
13	Petals 9–12 mm; style 1.5–3.3 mm, stigma not exserted beyond anthers; capsules 4–7 × 4–5 mm, subcuboid to squarish globose, often 4-winged.	Ludwigia maritima
13	Petals 14–19 mm; style 5–9.5 mm, stigma as long as or exserted beyond anthers; capsules 2–6.8 × 1.6–3.3 mm, subglobose to ellipsoid, not winged.	Ludwigia virgata
14	Leaves opposite; stems prostrate or decumbent, erect at tips, sometimes ascending; plants often forming mats, without stolons or rhizomes.	> 15
14	Leaves alternate, rarely opposite proximally; stems erect or ascending, rarely prostrate; plants not forming mats, often with stolons, rarely with rhizomes (in L. suffruticosa).	> 19
15	Petals 0; sepals 1–2 mm; anthers 0.2–0.4 mm; capsules (1.6–)2–5 mm, less than 2 times as long as broad, walls thin; pollen shed as monads.	> 16
15	Petals 4, sometimes caducous; sepals 1.8–10 mm; anthers 0.4–2 mm; capsules 4–10.5 mm, generally more than 2 times as long as broad, walls hard; pollen usually shed in tetrads, rarely as monads.	> 17
16	Plants nearly glabrous; petioles 0.1–2.5 cm; seeds yellowish brown.	Ludwigia palustris
16	Plants densely strigillose; petioles 0.3–0.9 cm; seeds dark reddish brown.	Ludwigia spathulata
17	Leaf blades narrowly elliptic to broadly lanceolate-elliptic or suborbiculate; petals 1.1–3 mm; sepals 1.8–5 mm, about as long as wide; mature pedicels 0.1–3 mm.	Ludwigia repens
17	Leaf blades narrowly elliptic to oblanceolate-elliptic or narrowly oblanceolate to linear; petals 4.5–11 mm; sepals (3.5–)4–10 mm, about 2–3 times as long as wide; mature pedicels (4.5–)6–45 mm.	> 18
18	Petals 7–11 mm; anthers 1.3–2 mm; mature pedicels (12–)17–45 mm, generally much longer than subtending leaves.	Ludwigia arcuata
18	Petals 4.5–5.5 mm; anthers 0.7–1 mm; mature pedicels (4.5–)6–15 (–20) mm, as long as or shorter than subtending leaves.	Ludwigia brevipes
19	Capsules subcylindric to elongate-obpyramidal, 2–10(–12) mm, usually at least 2 times as long as broad.	> 20
19	Capsules oblong-obovoid, obpyramidal, or obconic to subglobose, (1–)1.5–5(–7) mm, less than 2 times as long as broad.	> 22
20	Petals 0; sepals 1.1–2.3 mm; stem leaves usually narrowly elliptic to elliptic, rarely sublinear, petioles 0–1.5 cm, blades 3–12 cm.	Ludwigia glandulosa
20	Petals 4; sepals 2.5–7 mm; stem leaves linear to elliptic-linear or linear-oblanceolate, sessile or subsessile, blades 1.5–6(–8.5) cm.	> 21
21	Capsules subcylindric, irregularly dehiscent; seeds reddish brown; anthers 0.6–1.1 mm.	Ludwigia linifolia
21	Capsules elongate-obpyramidal, dehiscent by apical ring; seeds light brown; anthers 1–2 mm.	Ludwigia linearis
22	Flowers in densely clustered terminal racemes or spikes; stems unbranched or slightly branched; rhizomes often present.	Ludwigia suffruticosa
22	Flowers in elongate, loose, leafy axillary racemes or spikes; stems usually well branched, sometimes unbranched; rhizomes absent.	> 23
23	Plants usually densely pubescent throughout.	> 24
23	Plants glabrous or nearly so, sometimes with hairs on raised lines decurrent from leaf axils.	> 26
24	Plants densely strigillose; bracteoles 0.5–1.5 mm; mature pedicels 0.5–1.2(–2.3) mm.	Ludwigia sphaerocarpa
24	Plants densely hirtellous; bracteoles (1.5–)2–6.5(–7.2) mm; mature pedicels 0–1 mm.	> 25
25	Sepals 3.5–5.5(–6) mm, adaxial surface creamy white, tips reflexed; style 1–2 mm; seed surface cells nearly isodiametric.	Ludwigia pilosa
25	Sepals 1.5–3 mm, adaxial surface green, tips not reflexed; style 0.3–0.5 mm; seed surface cells transversely elongate.	Ludwigia ravenii
26	Capsules obpyramidal, sharply 4-angled and 4-winged, dehiscent by apical ring.	> 27
26	Capsules obconic or oblong-obovoid, with 4 rounded angles or subterete, dehiscent by loculicidal slits, apical ring, or irregularly.	> 28
27	Stems subterete or slightly ridged; sepals pale green, 1.5–2.5 mm, about 1/2 as long as capsule; capsule wall not bulging; pollen shed in tetrads; seed surface cells nearly isodiametric.	Ludwigia lanceolata
27	Stems slightly to distinctly winged; sepals creamy white adaxially, 2–4 mm, about as long as capsule; capsule wall somewhat bulging; pollen shed as monads; seed surface cells transversely elongate.	Ludwigia alata
28	Nectary disc nearly flat at ovary apex; capsules 1–1.5 mm, thin- walled, dehiscent by apical ring; seeds dark reddish brown.	Ludwigia microcarpa
28	Nectary disc raised 0.3–0.75 mm at ovary apex; capsules 1.5–7 mm, hard-walled, dehiscent irregularly or by loculicidal slits; seeds light brown to brown.	> 29
29	Leaf blades elliptic, lanceolate, oblong-elliptic to very narrowly so; stolons present; capsules irregularly dehiscent; pollen shed in tetrads.	> 30
29	Leaf blades spatulate to oblanceolate to very narrowly so; stolons rarely present; capsules dehiscent by loculicidal slits; pollen shed as monads.	> 31
30	Bracteoles 3.5–6.5(–8) mm; sepals pale green, apex elongate-acuminate, spreading or reflexed; capsules 4–7 mm, oblong-obovoid, pedicels 0.1–0.3 mm.	Ludwigia polycarpa
30	Bracteoles 0.5–1.5 mm; sepals yellow or cream adaxially, apex acuminate, ascending; capsules 2–4(–4.5) mm, subglobose, pedicels 0.5–1.2(–2.3) mm.	Ludwigia sphaerocarpa
31	Capsules 1.5–2.5 mm; sepals 1.2–1.8 mm; vestigial petals 0 or very rare; basal leaves sometimes opposite.	Ludwigia simpsonii
31	Capsules (2–)2.5–4(–4.7) mm; sepals 1.5–3 mm; 1–3 vestigial petals sometimes present; all leaves alternate.	Ludwigia curtissii