Ludwigia sect. Myrtocarpus
alternate.
4(or 5)[–7]-merous;
petals present, yellow;
stamens 2 times as many as sepals;
pollen shed in polyads.
usually obpyramidal to obconic, rarely subglobose, prominently 4+-angled or subterete, with thin walls, irregularly dehiscent.
in several rows per locule, free, raphe inconspicuous.
= 16, 32, 48, 64, 80, 96, 128.
Ludwigia sect. Myrtocarpus
Species 19 (1 in the flora).
Section Myrtocarpus, the second largest in the genus, has a center of distribution in southern Brazil, Argentina, and Paraguay; 14 species are restricted to that area and five extend to northern South America [Ludwigia elegans (Cambessèdes) H. Hara], to Central America and southern Mexico [L. caparosa (Cambessèdes) H. Hara, L. nervosa (Poiret) H. Hara, and L. rigida (Miquel) Sandwith], or to the Caribbean and the southern United States [L. peruviana (Linnaeus) H. Hara]. Ludwigia peruviana is naturalized in scattered localities in Asia and Australia (P. H. Raven 1963[1964]; T. P. Ramamoorthy and E. Zardini 1987). South America has been proposed to be the center of origin for Onagraceae and for Ludwigia (Raven 1963[1964], 1988). R. H. Eyde (1977, 1979, 1981) and Raven (1963[1964]) suggested that sect. Myrtocarpus includes species with the most generalized (plesiomorphic) morphology in Ludwigia; the section also includes at least five self-incompatible species (Ramamoorthy and Zardini). Most species outcross; only two are autogamous (L. hassleriana, L. tomentosa). Section Myrtocarpus comprises a polyploid complex consisting of one diploid (L. nervosa,n = 8), four tetraploids (n = 16), five hexaploids (n = 24), five octoploids (n = 32), one decaploid (L. caparosa, n = 40), and one multiploid (L. peruviana, n = 32, 40, 48, 64; Ramamoorthy and Zardini). The chromosome numbers of L. burchellii (Micheli) H. Hara and L. rigida are unknown.
P. A. Munz (1965) included nine species from sect. Myrtocarpus in his treatment for the North American Flora, but the circumscription of the section has changed (Munz 1944; T. P. Ramamoorthy and E. Zardini 1987) and his geographical definition included the West Indies and parts of Mexico/Central America; only Ludwigia peruviana is included in this treatment. Of the other species he included, L. foliobracteolata (Munz) H. Hara, L. nervosa, and L. rigida [as L. lithospermifolia (Kunth) H. Hara] remain in sect. Myrtocarpus and have not been reported in the flora area. The other five species are now placed in other sections: L. latifolia (Bentham) H. Hara (sect. Tectiflora Ramamoorthy), L. erecta (Linnaeus) H. Hara and L. decurrens Walter (sect. Pterocaulon Ramamoorthy), L. inclinata (Linnaeus f.) M. Gómez (sect. Heterophylla Ramamoorthy), and L. sedoides (Humboldt & Bonpland) H. Hara (sect. Humboldtia Ramamoorthy); of those, only L. decurrens and L. erecta occur in the flora area. The classification of this large Myrtocarpus complex was not supported by molecular analysis (Liu S. H. et al. 2017), but any revision must await more complete taxon sampling and better resolution.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
