Ludwigia sect. Pterocaulon
Onagraceae subfam. ludwigioideae
alternate.
stipules present.
4[or 5]-merous;
petals present, yellow [white];
stamens 2 times as many as sepals;
pollen shed in polyads [rarely tetrads].
floral tube absent;
sepals (3 or)4 or 5(–7), persistent at ovary apex after dehiscence of other floral parts;
petals yellow or white, rarely absent.
oblong-linear to subcylindric, subclavate to oblong-obovoid, or narrowly obpyramidal, prominently 4(or 5)-ribbed or -winged, with thin walls, irregularly dehiscent.
in several rows per locule, free, raphe inconspicuous.
= 16.
Ludwigia sect. Pterocaulon
Onagraceae subfam. ludwigioideae
Species 5 (2 in the flora).
Section Pterocaulon consists of five erect, mainly annual, diploid species (T. P. Ramamoorthy 1979; Ramamoorthy and E. Zardini 1987); three occur only in Argentina, Brazil, Paraguay, and Uruguay. Ludwigia decurrens and L. erecta occur more widely in South America and extend to the southeastern United States, Mexico, and the Caribbean; both also occur in Africa, and L. decurrens is naturalized in Asia and Europe (P. H. Raven 1963[1964]; Ramamoorthy and Zardini). Four species are autogamous, with the remaining one (L. longifolia) self-compatible but primarily outcrossing. Unlike species of sect. Myrtocarpus, in which section these species were treated prior to 1979, all are diploid annuals with mostly winged stems and very narrow, sublinear leaves. Sect. Pterocaulon appears to be polyphyletic in analyses by Liu S. H. et al. (2017), but not all species were sampled, and no change in classification is proposed here.
This section was delimited and named earlier as Diplandra Rafinesque 1840, but that name was pre-occupied by Diplandra Bertero 1830 and Diplandra Hooker & Arnott 1838, name conserved.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Genus 1, species 82 (31 in the flora): North America, Mexico, West Indies, Bermuda, Central America, South America, se Asia, Africa, Indian Ocean Islands, Pacific Islands, Australasia; introduced in Europe, w Asia.
Ludwigioideae were segregated as a distinct subfamily (W. L. Wagner et al. 2007) to reflect the phylogenetic relationship of Ludwigia as sister to other genera of Onagraceae in morphological and molecular analyses (see R. A. Levin et al. 2003, 2004). Ludwigia is distinguished by the absence of a floral tube, persistence of sepals on capsules after other floral parts dehisce, pollen shed in tetrads or polyads (or as monads in some sections, tetrads sometimes found elsewhere in Onagraceae), double ovule vascular supply, uniquely including a central supply (R. H. Eyde 1981), single-celled ovule archesporium (H. Tobe and P. H. Raven 1996), and a base chromosome number of x = 8.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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