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.