Description from Flora of China
Isnardia Linnaeus; Jussiaea Linnaeus; Oocarpon Micheli.
Herbs slender, erect to prostrate and rooting at nodes, or shrubs or rarely small trees; underwater parts often swollen and spongy or with inflated white spongy pneumatophores. Leaves alternate [or opposite], usually entire; stipules present, reduced and/or deciduous; bracteoles 2, at or near base of ovary, or absent. Flowers perfect, actinomorphic, in upper leaf axils or in spikes, racemes, or clusters; floral tube not prolonged beyond ovary. Sepals (3 or)4 or 5(-7), green, persistent after anthesis. Petals as many as sepals or absent, yellow or white, caducous. Stamens as many as or 2 × as many as sepals; anthers versatile or sometimes basifixed; pollen shed singly or in tetrads or polyads. Ovary with as many locules as sepals, rarely more, apex flat or conic, often with a depressed nectary surrounding base of each epipetalous stamen; stigma capitate or hemispheric, entire or lobed, upper 1/2-2/3 receptive. Fruit an obovoid to cylindric capsule, dehiscent irregularly or by a terminal pore or by flaps separating from valvelike apex. Seeds numerous, in one to several rows per locule, free or embedded in powdery or woody endocarp, raphe small or conspicuous, sometimes equal in size to body of seed. 2n = 16, 32, 48, 64, 80, 96, 128.
Ludwigia is distinctive within the family, and morphological, anatomical, and molecular evidence indicates that it is the sister group to the remainder of the family. Historically, plants of this affinity with stamen number equal to sepal number were Ludwigia, and those with stamens twice as many as sepals were Jussiaea, but Raven and others demonstrated reticulate variation in this character, and treated the two groups as a single genus. Polyploidy and autogamy are important evolutionary phenomena within the genus.
Eighty-two species: cosmopolitan, on all continents except Antarctica; nine species (one endemic) in China.
(Authors: Chen Jiarui (陈家瑞 Chen Chia-jui); Peter C. Hoch, Peter H. Raven)