5. Euphorbia davidii Subils

E. dentata Michx. var. gracillima Millsp.

E. dentata var. lancifolia Farw.

Map 1667

Plants annual, with taproots. Stems 20–70 cm long, erect or ascending, unbranched or few- to several-branched, the branches not flattened toward the tip, usually green to yellowish green, occasionally reddish- to purplish-tinged, densely pubescent with minute, downward-curved or downward-angled hairs, also with sparse to moderate longer, multicellular hairs. Leaves opposite (occasionally alternate at 1 or 2 of the uppermost nodes), short- to less commonly long-petiolate. Stipules absent or a pair of minute, light brown, convex, sessile glands. Leaf blades 10–100 mm long, highly variable in shape, linear to elliptic or sometimes lanceolate or ovate, not lobed, more or less symmetrically angled or tapered at the base, angled to tapered to a usually bluntly pointed tip, the margins irregularly and often relatively coarsely toothed or scalloped, the upper surface sparsely to moderately roughened with short, stiff, conical hairs having minutely pustular bases, green to dull grayish green and sometimes reddish- to purplish-tinged toward the margins or base, the undersurface moderately pubescent with relatively stout hairs, these often with a minute, persistent pustular base, and paler green than the upper surface. Inflorescences terminal, often a small, umbellate panicle with a whorl of leaves at the base, but this frequently reduced to 1–3 small clusters of cyathia. Involucre 2.5–3.0 mm long, glabrous, the rim irregularly lobed and fringed, the marginal glands 1 or less commonly 2, 0.7–1.2 mm long, appearing strongly concave and more or less 2-lipped, yellowish green to yellowish brown, lacking a petaloid appendage. Staminate flowers 25–40 per cyathium. Ovaries glabrous or with sparse, appressed hairs, the styles 1.0–1.5 mm long, each divided 1/2–3/4 of the way from the tip into 2 slightly slender or club-shaped lobes. Fruits 3–5 mm long (somewhat broader), usually glabrous at maturity. Seeds 2.5–3.0 mm long, ovate to triangular-ovate in outline, bluntly angular in cross-section (both the oblique apical portion surrounding the caruncle and the longitudinal inner faces appearing angular), more or less flattened at the base, the surface appearing relatively coarsely wrinkled or with poorly differentiated low, broad warts (appearing lumpy or irregularly swollen), these sometimes denser toward the angles, mostly dark brown or nearly black, often appearing somewhat mottled, usually with a small but well-developed, pale caruncle. 2n=56. July–October.

Scattered mostly in the western and northern portions of the state (Arizona to Texas north to Wyoming, Wisconsin, and Ohio; Mexico, Canada; introduced elsewhere in the U.S., South America, Australia). Banks of streams and rivers, bottomland forests, bottomland prairies, upland prairies, and sand prairies; also ditches, railroads, roadsides, and open, disturbed areas.

The name E. davidii was first applied to North American plants by Mayfield (1997), who elevated some of the several varieties described within E. dentata to species status and refined the set of morphological characters to distinguish them. Mayfield also first reported the taxon for Missouri. Although the apparently tetraploid (2n=56) E. davidii and apparently diploid (2n=28) E. dentata sometimes grow in mixed populations, they apparently do not hybridize readily. Crosses between plants of different ploidies would yield sterile offspring. It should be noted that Mayfield (1997) regarded a single early mitotic chromosome count of 2n=14 for E. dentata as having doubtful validity.