Annuals or perennials, 10-30 cm (aquatic or subaquatic). Stems decumbent to erect, branched from bases or ± throughout (sometimes rooting at proximal nodes). Leaves cauline; mostly opposite (distal sometimes alternate); sessile; blades usually 1-3-nerved from near bases, oblong to elliptic or lanceolate, margins usually ± dentate (at least distally), faces glabrous or sparsely pilose to pilosulous and glabrescent, sometimes minutely gland-dotted. Heads discoid, borne singly or in loose or open, corymbiform arrays. Involucres mostly hemispheric or broader, 3-4(-5) mm diam. Phyllaries persistent, 18-30 in 2-3 series, obscurely 3-4-nerved, lanceolate, ± equal (herbaceous). Receptacles convex to conic, epaleate (warty). Florets 75-125; corollas whitish or pinkish to purplish, throats tubular-funnelform (lengths ca. 2 times diams.); styles: bases not enlarged, glabrous, branches linear-filiform (distally densely papillose). Cypselae weakly fusiform, 4-5-ribbed, scabrellous on ribs; pappi persistent, of 2-6+ laciniate or fimbriate to setiform scales or coarsely barbellate bristles. x = 15.