4. Rorippa sessiliflora (Nutt. ex Torr. & A. Gray) Hitchc. (sessile-flowered cress, marsh cress, yellow cress)

Nasturtium limosum Nutt.

Pl. 325 a–c; Map 1384

Plants annual, with taproots. Stems (10–)20–50 cm long, erect or ascending, not rooting at the nodes or rooting sporadically only at the lowermost few nodes, glabrous. Leaves basal and alternate, 1.5–10.0 cm long, glabrous, the basal and lowermost stem leaves petiolate, the base not or only slightly clasping the stem, simple and entire or wavy-margined to pinnately lobed with 3–15 blunt, irregular lobes, the lobes linear to irregularly ovate, the margins entire, wavy, or with few, shallow, blunt teeth. Sepals 1–2 mm long. Petals absent. Styles absent or less than 0.5 mm long. Fruits 5–10 mm long, 1.8–3.0 mm wide, oblong, straight or slightly arched upward, the surface smooth or slightly roughened with minute, light-colored ridges, the stalk up to 2.5 mm long. Seeds mostly 100–200 per fruit, in 2 rows in each locule, 0.4–0.5 mm long, circular or somewhat cordate in outline, the surface with a fine, netlike or honeycomb-like pattern of ridges and pits, tan to light yellowish brown. 2n=16. April–October.

Scattered nearly throughout Missouri but absent or very uncommon in most of the Ozark Division (eastern U.S. west to South Dakota and Texas). Bottomland forests, banks of streams and rivers, and sloughs; also levees, railroads, and roadsides.

Rorippa sessiliflora is distinguished from other Missouri species of Rorippa by its relatively large, broad fruits that contain large numbers of very small seeds and the absence of petals. The stalks of the fruits are also quite short in this species, usually less than 3 mm long, which distinguishes it from the vegetatively similar R. palustris, which has stalks 3–7 mm long.

This species is mainly distributed from the Atlantic and Gulf Coastal Plains north through the Mississippi Embayment. Stuckey (1972) noted that it is not common north of the southern limits of Pleistocene glaciation. Its absence from much of the Ozarks may be a result of its preference for muddy sites, and this may change over time if plants are able to colonize the disturbed margins of livestock ponds.