8. Bidens laevis (L.) Britton, Sterns & Poggenb. (showy bur marigold)

Pl. 273 c, d; Map 1156

Plants annual (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. August–October.

Uncommon, known 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.

Steyermark (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.