Perennials; (submerged aquatic, rooting from proximal nodes, forming clumps); glabrous or pubescent proximally on young leaves. Stems prostrate basally (hollow), branched, (2-)4-12(-13) dm. Basal leaves not rosulate; blade margins pinnatifid. Cauline leaves (proximal) petiolate, (distal) sessile; blade pectinate or pinnatisect to pinnatifid (especially when submerged), oblanceolate to elliptic, 4-12 cm × 5-30 mm, base auriculate or not, margins entire or denticulate to serrate (emergent ones usually undivided, rarely lobed). Racemes considerably elongated. Fruiting pedicels horizontal to reflexed, straight or recurved, (5-)7-15 (-18) mm. Flowers: sepals erect, oblong, 2.5-3.5 × 1-1.7 mm; petals yellow, obovate, (3.3-)3.8-5.5 (-6.2) × 1.5-2.5 mm; median filaments 3-4 mm; anthers ovate, 0.7-1 mm. Fruits silicles, straight, ovoid-oblong, (2.5-)3-5.5(-7) × 1.7-2.7(-3) mm; valves glabrous; ovules 36-62 per ovary; style (0.5-)1-2.2(-2.5) mm. Seeds biseriate, reddish brown, ovoid, 0.7-1 mm (0.5-0.6 mm diam.), colliculate. 2n = 16, 32. Flowering Jun-Aug. Lakeshores, marshes, ponds, streams; introduced; Que.; Maine, N.Y.; Europe; Asia.
Erect or ascending perennial to 1 dm; stems hollow, often rooting below; lvs lanceolate to oblanceolate, to 12 cm, the lower or submersed ones commonly laciniate to pectinately parted, the upper narrower and merely dentate; pet about twice as long as the sep; mature pedicels 8-15 mm, horizontally spreading or somewhat deflexed; frs ellipsoid, straight, the body 3-6 mm, the stylar beak 1-2 mm; 2n=16, 32. Native of Eurasia, established in swamps and shallow water from Que. to Me. and Mass. and perhaps elsewhere. June, July.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
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