Ludwigia sect. Macrocarpon
alternate.
4-merous;
petals present, yellow;
stamens 2 times as many as sepals;
pollen shed in tetrads or polyads.
cylindric to clavate-cylindric, ± angled to subterete, with thin walls, irregularly dehiscent.
in several rows per locule, free, raphe enlarged, nearly equal to seed.
= 16, 32, 48.
Ludwigia sect. Macrocarpon
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.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
