Rorippa aquatica (Eaton) E.J. Palmer & Steyerm.

Lake Cress

Rorippa aquatica plant

Family - Brassicaceae

Habit - Perennial herbs, lacking rhizomes.

Stems - Spreading to erect, with bases often rooting at nodes, to 85 cm, usually unbranched below the inflorescence, glabrous.

Rorippa_aquatica_stem.jpgStem and leaves.

Leaves - Alternate, to 7 cm, glabrous. Submerged or lowermost leaves irregularly pinnately dissected into numerous threadlike to linear segments. Upper leaves simple, entire or toothed, lanceolate.

Rorippa_aquatica_leaves.jpgUpper stem leaves.

Rorippa_aquatica_leaf2.jpgLeaf abaxial.

Inflorescences - Terminal racemes or panicles.

Rorippa_aquatica_inflorescence.jpgInflorescence.

Flowers - 4-merous in typical Brassicaceae pattern. Sepals 2-4 mm long, ascending, elliptic to obovate, glabrous. Petals 4-8 mm long, unlobed, white. Styles 2-4 mm long.

Rorippa_aquatica_calyces.jpgCalyces.

Rorippa_aquatica_corollas.jpgCorollas.

Fruits - Spreading, often aborting prior to maturity, 3.5-7.0 mm long, less than 2 times as long as wide, elliptic to narrowly obovate in outline, circular or slightly flattened in cross-section, the stalks 6-12 mm long. Seeds 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, brownish orange

Rorippa_aquatica_fruits.jpgFruits.

Flowering - May - August.

Habitat - Swamps, sloughs, spring branches, ditches.

Origin - Native to the U.S.

Other info. - This species is uncommon in Missouri, and indeed has become uncommon across most of its range, which comprises sparse locations throughout much of the Midwest and Canada. Its white flowers are unusual, since other members of the Rorippa genus have yellow flowers. It is also unusual in that leaves which drop from the plants can root and develop into vegetative rosettes, perhaps compensating to some extent for the poor seed set. Nonconformities such as these have rendered taxonomic placement difficult, and the assignment remains controversial. Other authors have assigned these plants to the Armoracia or Neobeckia genera.

At a quick glance, this plant could be mistaken for Cardamine bulbosa, but a more careful examination of the leaves and fruits will serve for differentiation.

Photographs taken at Duck Creek Conservation Area, Stoddard County, MO, 5-13-2016 (SRTurner).



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