Ludwigia sect. Jussiaea

Herbs, perennial, subshrubs, or emergent aquatics, creeping, floating, or emergent and ascend­ing, rooting at nodes, when floating often forming spongy, white pneumatophores at nodes, when erect with spongy base. Stems decumbent to erect or ascending, terete, sometimes angled distally. Leaves alternate or fascicled. Flowers 5(or 6)-merous; petals present, yellow [white]; stamens 2 times as many as sepals, [rarely as many as]; pollen shed as monads. Capsules cylindric, subcylindric, or subclavate, terete, subterete, or obscurely angled, often up-curved, with thick, woody walls, irregularly and tardily dehiscent. Seeds in 1 row per locule, pendulous and firmly embedded in woody, coherent segment of endocarp, raphe inconspicuous. 2n = 16, 32, 48, 80, 96.

Distribution

United States, Mexico, West Indies, Central America, South America, s Asia, Africa, introduced in Europe, Pacific Islands, Australia.

Discussion

Species 9 (3 in the flora).

This cosmopolitan polyploid section of nine variable species (11 taxa) includes three diploid species (n = 8), four tetraploids (n = 16), one hexaploid [Ludwigia grandiflora, n = 24; P. H. Raven and W. Tai (1979) reported one anomalous count of n = 48], and one decaploid (L. hexapetala, n = 40; see also G. L. Nesom and J. T. Kartesz 2000). One persistent triploid (2n = 24) hybrid, described as L. ×taiwanensis C. I. Peng [L. adscendens (n = 16) × L. peploides subsp. stipulacea (n = 8); Peng 1990] is widespread in southern China and Taiwan. E. Zardini et al. (1991) reported several other natural hybrids.

Most species of sect. Jussiaea have non-naturalized distributions restricted to the New World. Section Jussiaea differs from most diplostemonous sections by releasing its pollen as monads and having woody, subcylindric capsules with uniseriate, firmly embedded seeds. Most species in sect. Jussiaea are vigorously aquatic, and several (including L. peploides, L. hexapetala, and L. grandiflora) can be invasive weeds in wetlands and wet agricultural areas; the latter two, or all three, species have recently become major invasive species in California, particularly in the Russian and Sacramento river drainages and in the San Diego region, in Arizona (especially Gila and Salt rivers), and in Washington (M. Wood 2006; P. C. Hoch and B. J. Grewell 2012).

Selected References

None.

Lower Taxa

Ludwigia grandiflora, Ludwigia hexapetala, Ludwigia peploides