Plant: Perennial aquatic forb from fascicled fibrous roots, to 1 m; stems weak, usually supported by other plants, ascending or erect, branched, hollow, rooting at lower nodes Leaves: leaves simply pinnate, the segments linear to ovate, entire to variously lobed; petiole 4-12 cm, narrowly sheathing; blade 1.5 dm, oblong, much-dissected if submersed, leaflets 7-12 pairs, 1-8 cm, oblong to ovate INFLORESCENCE: umbels compound, peduncled; bracts, bractlets leaf-like, lanceolate; rays, pedicels ± unequal, spreading Flowers: many, small; calyx lobes minute, persistent; petals wide, white; tamens 5; pistil 1, ovary inferior, 2-chambered, generally with a ± conic, persistent projection or platform on top subtending 2 free styles Fruit: 2 dry, 1-seeded halves that separate from each other but generally remain attached for some time to a central axis, 1.5-2 mm, nearly round, compressed side-to-side, glabrous; ribs thread-like, inconspicuous in corky fruit wall; oil tubes apparent but deeply embedded in fruit wall; fruit axis divided to base, its branches adhering to and falling with fruit-halves Misc: Marshy areas, streams; < 1800 m; Jul-Oct