8. Bidens laevis (L.) Britton, Sterns & Poggenb. (showy bur marigold)
Pl. 273 c, d;
(sometimes perennial farther south), terrestrial or occasionally emergent
aquatics, sometimes with taproots. Stems 20–80(–150) cm, erect
to loosely ascending, sometimes from a spreading base, occasionally rooting at
the lower nodes, glabrous. Leaves all more or less similar, sessile, opposite
or rarely the lowermost leaves in whorls of 3, the blade 2–15 cm long,
oblong-lanceolate to oblanceolate in outline, less commonly obovate or linear,
unlobed, narrowed or tapered at the base, tapered to a sharply pointed tip, the
margins usually sharply and finely to coarsely toothed, rarely nearly entire,
also minutely roughened, the surfaces glabrous. Inflorescences of solitary
terminal heads or appearing in loose, open clusters, the heads radiate (rarely
discoid elsewhere), usually nodding at fruiting. Involucre with the outer
series of 5–9 bracts 6–16 mm long, spreading to more commonly
reflexed, somewhat leaflike, narrowly lanceolate to narrowly oblong or linear,
the margins entire but usually minutely roughened, the outer surface glabrous
or sparsely short-hairy toward the base; the inner series of 6–9 bracts
4–10 mm long, lanceolate to narrowly ovate or ovate, glabrous. Chaffy
bracts narrowly oblong, reddish brown to orangish brown at the tip. Ray florets
6–8, the corolla showy, (10–)15–30 mm long, yellow.
Disc florets 40–100(–150), the corollas 3–4 mm long,
yellow to orangish yellow. Pappus of (2–)4 awns mostly 2–4 mm
long, these with downward-pointed barbs, erect at fruiting. Fruits 4–8
mm long, narrowly wedge-shaped, more or less flattened and somewhat 4-angled in
cross-section, the angles not pale and not thickened or winged, the faces
sometimes with several faint longitudinal lines, dark brown to purplish black,
glabrous or with sparse, short, pustular-based hairs. 2n=22, 24.
thus far only from a single historical collection from Dunklin County (southern U.S. north to California, Missouri, and Maine; Mexico, Central America, South
America; introduced in Hawaii). Habitat unknown, but presumably banks of
streams and rivers or bottomland forests.
(1963) noted that a number of additional specimens originally determined as B.
laevis had been redetermined during his studies as the closely related B.
cernua. He also discussed that the two species tend to intergrade and
perhaps should be considered varieties of a single species.