10. Euphorbia humistrata Engelm.

Chamaesyce humistrata (Engelm.) Small

Map 1672, Pl. 382 g–i

Plants annual, with taproots (frequently also with adventitious roots at some of the nodes). Stems 5–45 cm long, usually prostrate, occasionally with ascending tips, several- to many-branched, the branches often overlapping (plants mat-forming), not flattened toward the tip, usually pale green to yellowish green, moderately to densely pubescent with short, appressed or incurved hairs, especially toward the branch tips, occasionally nearly glabrous toward the stem base, the hairs often in 2 bands along opposite sides of the stem. Leaves opposite, sessile or very short-petiolate. Stipules small scales 1.0–1.5 mm long, those from the adjacent leaf in each pair often fused into a single, small, scalelike structure on each side of the stem positioned between the leaf bases, this irregularly fringed or lobed. Leaf blades 5–17 mm long, oblong-obovate to obovate or oblong-ovate, occasionally some of the leaves narrowly oblong, asymmetrical at the base with the side toward the stem tip usually angled and the other side more or less truncate and expanded into a small, rounded auricle, rounded or less commonly broadly and bluntly pointed at the tip, the margins minutely few-toothed (best observed with magnification), the upper surface glabrous or nearly so and occasionally with a faint reddish spot, the undersurface glabrous or sparsely pubescent with loosely appressed to somewhat spreading, slender hairs and usually silvery to pale grayish green. Inflorescences axillary, of solitary cyathia or appearing as small clusters on short axillary branches. Involucre 0.6–1.0 mm long, sparsely hairy on the outer surface, the rim shallowly 4-lobed or 4-toothed, the marginal glands 4, 0.1–0.5 mm long and usually more or less equal in size, the body narrowly oblong to nearly linear, reddish purple to dark purple, with a relatively inconspicuous petaloid appendage 0.1–1.0 mm long, this white to strongly pinkish- or reddish-tinged. Staminate flowers 3–8 per cyathium. Ovaries hairy, the styles 0.5–0.8 mm long, each divided about 1/2 of the way from the tip into 2 slender lobes. Fruits 1.0–1.5 mm long, sparsely to moderately and relatively evenly pubescent with appressed or strongly incurved hairs. Seeds 0.8–1.2 mm long, more or less oblong-ovate in outline, bluntly angular in cross-section, flat to slightly convex at the base, the surface smooth or appearing finely roughened, lacking cross-ridges, white to reddish brown, lacking a caruncle. June–October.

Scattered in the state but absent from portions of the Glaciated Plains and Ozark Divisions (Kansas to Texas east to Ohio and Florida; introduced sporadically east to New Jersey). Banks of streams and rivers, sloughs, saline seeps, and margins of ponds and lakes; also levees, railroads, roadsides, and open, disturbed areas.

Some plants of E. humistrata can be very difficult to distinguish from E. maculata. Steyermark (1963) discussed some of the differences between them, and Richardson (1968) tabulated a number of characters to separate the two taxa. These include the greater tendency of E. humistrata stems to root at the nodes and the deeper lobing of its involucres. E. humistrata also tends to have paler green stems and broader leaves that are less densely hairy on the undersurface and tend to lack a reddish spot on the upper surface. It also often has the involucre split about halfway down one side (in E. maculata, the involucre is entire or with a shallow split), with the stalk of the pistillate flower arched or recurved through the gap at maturity.