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.