2. Hydrocotyle verticillata Thunb. (whorled pennywort)

Pl. 208 c, d; Map 864

Leaf blades 0.5–5.5 cm in diameter, peltate, circular or nearly so, the margins finely scalloped or with a few very shallow, blunt lobes. Inflorescences spikes (often appearing headlike on smaller plants) with 1 or 2(–4) whorls of 2–7 flowers, these sessile or nearly so. Fruits 1–3 mm long, depressed-circular in outline, rounded or truncate to rarely very shallowly cordate at the base, shallowly notched at the tip, strongly flattened laterally, glabrous, tan to brown, each mericarp with the lateral ribs somewhat corky-thickened, the intermediate ribs narrow and unwinged, and the dorsal ribs somewhat corky-thickened, tapered to a thin edge. 2n=Å88. May–August.

Uncommon, known thus far only from Oregon and Ozark Counties (eastern and southwestern U.S., Hawaii; Mexico, Central America, South America, Caribbean Islands, Africa). Banks of spring branches and rivers, usually in sandy soil, sometimes emergent aquatics in shallow water.

The single Oregon County specimen consists of robust nonflowering material collected at Morgan Spring, where aquatic plants formerly were raised commercially in concrete troughs, and from which materials washed into the spring branch and adjacent Eleven Point River during one or more floods. The plants have not been relocated since the original discovery by Paul Redfearn in 1970. Because H. verticillata and the superficially similar H. umbellata L. (water pennywort) cannot be distinguished reliably from vegetative samples, there is a possibility that the Oregon County specimen is actually the latter species. Hydrocotyle umbellata is widespread in North America (and elsewhere in the New World) and occurs natively as close as Fulton County, Arkansas, which is just to the south of Oregon County, Missouri. Like H. verticillata, it is sold in the horticultural trade for cultivation in ponds and wet areas. The two species differ in inflorescence type (H. umbellata has simple umbels with long-stalked flowers) and fruits (H. umbellata tends to have slightly smaller fruits with more rounded ribs and somewhat cordate bases). This species should be searched for in southern Missouri.

Mathias and Constance (1944–1945) recognized several varieties in H. verticillata, differing in minor characters of the leaves and inflorescences. Missouri plants are all referable to var. verticillata. The validity of these taxa requires further study.