2.
Rorippa amphibia
(Linnaeus) Besser, Enum. Pl. 27. 1821. (as Roripa).
Sisymbrium amphibium
Linnaeus, Sp. Pl. 2: 657. 1753;
Armoracia amphibia
(Linnaeus) Petermann;
Brachiolobos amphibius
(Linnaeus) Allioni;
Cochlearia amphibia
(Linnaeus) Ledebour;
Myagrum amphibium
(Linnaeus) Loiseleur;
Nasturtium amphibium
(Linnaeus) W. T. Aiton;
Radicula amphibia
(Linnaeus) Druce
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.
2
n
= 16, 32.
Flowering Jun-Aug. Lakeshores, marshes, ponds, streams; introduced; Que.; Maine, N.Y.; Europe; Asia.
