Procumbent or floating and ascending, aquatic biennial to perennial, with fibrous roots. Stems hollow, striate, grooved, up to 150 cm long, usually rooting at nodes. Mature stem lvs usually 2-pinnate, sometimes 3-pinnate at base, petiolate; ultimate segments ovate, lobed or not, serrate, 10-35 mm long; lvs associated with infl.-branches reduced and 1-pinnate with smaller segments; submerged lvs similar. Umbels terminal or lf-opposed, shortly pedunculate or sessile, 2-5 cm diam.; rays 10-20; rays and pedicels somewhat thickened at fruiting; bracts 0-1, linear; bracteoles numerous, linear. Fls numerous, white, c. 2 mm diam. Fr. cylindric, 2.5-3.5 mm long; ribs prominent, broad and corky; styles slender, c. = length of fr.