66. Bidens L. (beggar-ticks)

Plants annual or perennial herbs (shrubs elsewhere). Stems erect to loosely ascending (except in submerged aquatics), unbranched or more commonly few- to numerous-branched, with several fine to coarse longitudinal lines or ridges, sometimes relatively strongly 4- or 5-angled, glabrous or sparsely to moderately pubescent. Leaves opposite or less commonly in whorls of 3 and sometimes also basal, variously sessile to long-petiolate, the bases slightly expanded and wrapping around the stem. Leaf blades simple or 1–3 times pinnately lobed, dissected, or compound (repeatedly dichotomously dissected in submerged leaves of B. beckii), variously shaped, the margins otherwise usually toothed, glabrous or variously hairy, not glandular. Inflorescences of solitary terminal heads or appearing as loose, open clusters or small panicles, occasionally some of the heads appearing axillary, the heads with short to long, bractless stalks or rarely the stalk with 1 or 2 inconspicuous, minute bracts. Heads radiate or discoid. Involucre broadly to narrowly cup-shaped to slightly bell-shaped, the bracts in 2(3) dissimilar overlapping series. Involucral bracts free at the base, those of the outer series 2–25, mostly as long as or longer than the others, variously shaped, green, glabrous or sparsely to moderately hairy, not glandular, usually inconspicuously 1- or 3-nerved; those of the inner series 5–8(–12), variously shaped, yellowish brown to yellowish green, more or less scalelike with narrow to broad, lighter, thinner margins, glabrous (inconspicuously hairy in B. alba), not glandular, with several to numerous conspicuous nerves or longitudinal lines. Receptacle flat or slightly convex, not elongating as the fruits mature, with chaffy bracts subtending the disc florets, these linear to narrowly oblong or narrowly lanceolate, relatively flat to somewhat concave, sometimes slightly wrapped around the florets toward the base. Ray florets absent or 1–8(–12), when present sterile (lacking stamens and style at flowering and with an ovary that is shorter and thinner than those of the disc florets, not developing into a fruit; pistillate elsewhere), the corolla inconspicuous or showy, 2–30 mm long, relatively broad, yellow or white, not persistent at fruiting. Disc florets 10–100(–150), perfect, the corolla usually 5-lobed (sometimes 4-lobed in B. tripartita), yellow to orangish yellow, not thickened at the base, not persistent at fruiting. Style branches with the sterile tip slightly to moderately elongate and tapered to a sharply pointed tip. Pappus of the disc florets of (1)2–4(–8) short or long awn(s) (these smooth or with sparse, upward- or downward-angled barbs), rarely absent, when present usually persistent at fruiting. Fruits 3–18 mm long, sometimes dissimilar in the same head (the outer ones grading into the inner ones) narrowly oblong to linear in outline, strongly flattened or 4-angled, not appearing curved or curled, more or less truncate at the tip, not beaked, the angles lacking wings or less commonly with slender wings, the surfaces often angled or with 1 or more longitudinal grooves, glabrous or variously hairy, tan to dark brown or nearly black, sometimes mottled, the wings (if present) usually pale, sometimes slightly shiny. More than 200 species, nearly worldwide.

As noted in the treatment of the genus Coreopsis, that genus is difficult to separate from Bidens, and further research undoubtedly will result in the revision of generic circumscriptions in the subtribe Coreopsidinae. In addition, some of the species of Bidens are difficult to distinguish from one another. Barkley (1986) advised studying several specimens when attempting to identify Bidens species.