Ludwigia sect. Macrocarpon

Herbs, perennial, or shrubs.Stems erect or spreading, terete, subterete, or ridged. Leaves alternate. Flowers 4-merous; petals present, yellow; stamens 2 times as many as sepals; pollen shed in tetrads or polyads. Capsules cylindric to clavate-cylindric, ± angled to subterete, with thin walls, irregularly dehiscent. Seeds in several rows per locule, free, raphe enlarged, nearly equal to seed. 2n = 16, 32, 48.

Distribution

se, s United States, Mexico, West Indies, Central America, South America, Eurasia, Africa, Indian Ocean Islands, Pacific Islands, Australasia.

Discussion

Species 4 (2 in the flora).

Section Macrocarpon consists of four species, all of which occur in Argentina, Brazil, and Paraguay. Ludwigia bonariensis also occurs as a disjunct in Mexico and the southern United States; L. octovalvis is found worldwide in subtropical and tropical areas and is widely dis­tributed in the southern United States. Section Macrocarpon, which is supported as mono­phyletic by molecular data (Liu S. H. et al. 2017), differs from sect. Myrtocarpus by having strictly 4-merous (versus 4+-merous) flowers, cylindric (versus obconic) capsules, and dis­tinctive seeds with an enlarged raphe.

Ludwigia bonariensis and L. lagunae are diploid (n = 8) and self-incompatible; L. neograndiflora is tetraploid (n = 16; Liu S. H. et al. 2017), but compatibility is unknown. In contrast, most populations of the multiploid L. octovalvis are polyploid, with some reports of diploids in the New World (P. H. Raven and W. Tai 1979), and self-compatible. Ludwigia octovalvis is extremely variable in morphology and ploidy level, and the entire section is in need of taxonomic revision.

Selected References

None.

Lower Taxa

Ludwigia bonariensis, Ludwigia octovalvis