Annual or perennial herbs (in Mal.), occasionally somewhat woody near the base, sometimes aquatic. Leaves spiral or opposite. Stipules absent or reduced, deltoid. Flowers mostly 4-merous, rarely 5-merous (in Mal.), solitary or arranged in a terminal racemose inflorescence, subtended by (often reduced) leaves or bracts. Sepals erect, persistent. Petals caducous, contorted in aestivation, white, pink or yellow, sometimes emarginate. Stamens 4, 5, 8, or 10, in 2 whorls, rarely with an intermediate number, epipetalous ones sometimes shorter. Ovary inferior, (in Mal.) 4- or 5-celled and with ovules; Ovules with axial placentation, 1-pluriseriate. Fruit (in Mal.) a mostly long and slender loculicidal or irregularly rupturing capsule. Seeds rounded or elongate, in Ludwigia sometimes embedded in powdery or surrounded by cork-like endocarp tissue, in Epilobium with a chalazal plume of trichomes (coma);
About 17 genera and more than 600 spp. in tropical and temperate regions, with a distinct centre of diversity on the northern hemisphere in the New World, in Malesia two native genera which are both almost ubiquist.
Almost all of the Malesian species are self-pollinated, shedding pollen directly on the stigma at or before anthesis and rarely visited by insects. In Ludwigia peruviana, introduced in the Old World, the anthers are extrorse and shed pollen away from the stigma; thus outcrossing is predominant. Some outcrossing probably also occurs in the relatively large-flowered L. adscendens and L. octovalvis, which are known to be visited by insects, and in Epilobium detznerianum, in some populations of which the stigma is even held above the anthers. In our area, HEIDE () reported Melipona sp., a bee, visiting the flowers of Ludwigia peruviana (as Jussieua peruviana), and Bombus rufipes at the flowers of the locally naturalized Oenothera stricta (as O. lamarckiana) and Fuchsia magellanica (as F. coccinea). The Melipona bees were not observed to contact the anthers or stigma of the large-flowered Ludwigia, but would certainly do so in visiting smaller-flowered species. All of the Malesian species are genetically self-compatible.
Epilobium spp. are manifestly wind dispersed by virtue of their coma. Ludwigia spp. depend on dispersal by water and possibly incidental exozoic dispersal by water birds; in Ludwigia hyssopifolia there are two kinds of seed, one of which is enveloped by a corky tissue derived from the endocarp, enhancing their buoyancy.
All species of Epilobium sect. Epilobium, a taxon that includes all Malesian species, which have been examined have had a gametic chromosome number of n = 18. Species of Ludwigia have a gametic chromosome number of n = 8 and multiples. These genera differ from most others in Onagraceae in having small chromosomes that are heteropycnotic and dark-staining throughout the mitotic cycle. Naturally occurring interchange heterozygotes, abundant in the tribe Onagreae, are not known to occur in either group. The original basic chromosome number of the family is × = 11, as in Fuchsia, Circaea, and others.
Only some species of Ludwigia are mentioned to be in use for minor medicinal purposes; see under Ludwigia spp.
Raphides, needle-like crystals of calcium oxalate, are ubiquitous in the vegetative parts of Onagraceae. The few reports of alkaloids are doubtful and seem to indicate rather the presence of secondary amines. Ellagic acid occurs. Among the flavonoids reported from the family, fiavonols based quercetin are ubiquitous, whereas kaempferol and more highly oxygenated types based on myricetin are frequent. The anthocyanins include predominantly malvidin and cyanidin derivatives, with the latter predominant in the rose-purple petals of Epilobium. The yellow petals of most species of Ludwigia are colored by carotenoids, with the chalcone isosalipurposide forming a non-ultraviolet-reflective centre in many species, including L. peruviana.