Description

Key Characters:

Growth Form: Aquatic or semiaquatic, somewhat fleshy, glabrous perennial herbs.

Stems: Stems floating, prostrate or ascending, hollow, 1–6 dm long, usually 2–6 mm in diameter.

Roots: Stems rooting at the nodes.

Leaves: Leaves simple. Alternate or rarely opposite. Blades pinnately compound with (3–)6–9 leaflets, these ovate to elliptic, 1.5–4 cm long, 0.5–2 cm wide. Margins subentire to sinuate–dentate, the terminal one often larger than the lateral ones. Petiolate, petioles not winged. Stipules absent.

Flowers: Flowers in usually bractless, terminal racemes, occasionally solitary. Flowers bisexual (perfect), actinomorphic or rarely slightly irregular. Calyx of 4 sepals, deciduous, ascending, the inner pair saccate at base,usually oblong, sometimes the inner 2 with gibbous bases that hold the nectar. Corolla of 4 petals, white, entire to emarginate, rarely lobed or fimbriate, usually with an elongate claw. ca. 4 mm long. Stamens (2–4)6(–16), short, with a pair of reniform glands at base of filament, tetradynamous, the inner 4 usually in pairs, sometimes connate at base in pairs; anthers dithecal, opening by longitudinal slits. Ovary superior, 2(4?)-carpellate, usually 2-celled by means of a false, but usually complete septum, rarely 1-celled, sessile or rarely stipitate; ovules 1 to numerous, borne on parietal placentas on replum margin at periphery of ovary wall, campylotropous or occasionally anatropous; style 1 stout; stigma capitate, 2-lobed.

Fruit: Capsules divided into 2 cells by the usually thin and membranous septum; elongate (at least 3 times as long as wide) and referred to as a silique; linear; rounded in cross section; spreading; terete (cylindrical); with a weak median nerve; the valves extending to margins of silique; not elastic nor rolling up after dehiscence; midrib conspicuous; 1.6–2.5 cm long. Seeds in 1 row per cell; with ca. 100 polygonal depressions on each face; often becoming mucilaginous when wet; endosperm essentially absent.

Ploidy: 2n = 64

Habitat: naturalized in running water or seasonally wet areas.

Elevation Range: