1. Rorippa aquatica (Eaton) E.J. Palmer & Steyerm. (lake cress)

Armoracia aquatica (Eaton) Wiegand

A. lacustris (A. Gray) Al-Shehbaz & V.M. Bates

Neobeckia aquatica (Eaton) Greene

Pl. 313 h, i; Map 1381

Plants perennial herbs, with slender roots, lacking rhizomes, but the stem bases often becoming horizontal and buried, rooting at most nodes. Stems 30–85 cm long, spreading to ascending, unbranched below the inflorescence or branched with age, glabrous. Leaves alternate (and basal when young), 2–7 cm long, glabrous, the submerged or lowermost leaves irregularly pinnately dissected into numerous linear lobes or threadlike segments, oblong in outline, the emergent leaves unlobed, entire or shallowly toothed, lanceolate. Sepals 2–4 mm long, ascending, elliptic to obovate, glabrous. Petals 4–8 mm long, unlobed, white. Styles 2–4 mm long. Fruits spreading, often aborting prior to maturity, 3.5–7.0 mm long, about as long as wide or less than 2 times as long as wide, elliptic to narrowly obovate in outline, circular in cross-section or slightly flattened at a right angle to the septum, the stalks 6–12 mm long. Seeds (when rarely present) 12–30 per fruit, in 2 rows in each locule, 0.7–1.4 mm long, ovoid to subglobose, the surface with a netlike or honeycomb-like pattern of ridges and pits or minutely roughened, brownish orange. 2n=24. May–August.

Uncommon and widely scattered, mostly south of the Missouri River (eastern U.S. and adjacent Canada west to Minnesota and Texas). Swamps, sloughs, streams, and spring branches; also ditches, often emergent aquatics in open areas with still or slow-moving water, rarely on mud.

This unusual species has become uncommon throughout much of its range. The highly dissected, submerged leaves may sometimes be mistaken for those of Myriophyllum (or occasionally those of other genera of submerged aquatics). Some of the leaves become detached during the summer and fall, and these leaves are capable of forming plantlets that root and develop into rosettes that presumably overwinter and flower the following year (reviewed by Al-Shehbaz, 1988b; Les, 1994). The species also is unusual in that the septum between the carpels is incomplete (perforated), and the fruits thus are functionally 1-locular.

The generic placement of lake cress remains controversial, and the species has been classified in six different genera. Les (1994) presented morphological and molecular evidence that it is more closely related to Rorippa than to Armoracia, although he concluded that it should be treated in a separate genus as Neobeckia aquatica.