5. Rorippa sinuata (Nutt. ex Torr. & A. Gray) Hitchc. (spreading yellow cress)

Pl. 325 g, h; Map 1385

Plants perennial herbs, with rhizomes. Stems 10–40(–45) cm long, spreading to ascending, sometimes rooting at the lower nodes, sparsely to densely pubescent with minute, hemispherical hairs, these appearing scalelike after collapsing upon drying. Leaves basal and alternate, 2–8(–10) cm long, the lowermost petiolate, the base mostly clasping the stem with small, rounded auricles, simple and wavy-margined to pinnately lobed or divided with 3–13 blunt lobes or divisions, the lobes linear to irregularly ovate, the margins entire, wavy, or with few, shallow, blunt teeth, the upper surface glabrous, the undersurface sparsely to densely pubescent with minute, hemispherical hairs, these appearing scalelike after collapsing upon drying. Sepals 2–4 mm long. Petals 4–6(–7) mm long, yellow. Styles 1–2 mm long, the stigma no wider than the style. Fruits 5–12 mm long, 1.5–2.0 mm wide, oblong, straight or slightly arched upward. Seeds mostly 25–80 per fruit, in 2 rows in each locule, 0.9–1.1 mm long, angular, ovate, or nearly circular in outline, the surface with a fine, netlike or honeycomb-like pattern of ridges and pits, light yellow. 2n=16. April–September.

Scattered mostly along the floodplains of the Mississippi and Missouri Rivers (western U.S. and adjacent Canada east to Illinois and Arkansas). Banks of rivers; also edges of crop fields, railroads, roadsides, and open, disturbed areas.

The odd hemispherical hairs, which are visible only with magnification, are a distinguishing feature of this species. The stems frequently form loose mats. Stuckey (1972) hypothesized that the species originated in the Rocky Mountains and subsequently migrated down major river drainages eastward through the Great Plains.

A small number of historical collections from St. Louis and Jefferson Counties appear somewhat intermediate between R. sinuata and R. palustris. Because these have relatively large petals and appear rhizomatous, they would key to the former species, but they are atypical in their pubescence and leaf division pattern. The lack of extant sites for plants with this morphology has hampered a more detailed study of their putative hybrid origin.