Bidens

Annuals or perennials [shrubs, vines], 5–400 cm. Stems usually 1, usually erect, (terete or 4-angled, often striate or sulcate) branched distally or ± throughout. Leaves usually cauline; usually opposite, rarely whorled, distal sometimes alternate; petiolate or sessile; blades simple, compound (leaflets petiolulate), or 1–3+-pinnatisect or -pinnately lobed (submerged leaves multifid in B. beckii, an aquatic), ultimate margins entire, dentate, laciniate, serrate, or toothed, faces usually glabrous, sometimes hirtellous, hispidulous, pilosulous, puberulent, scabrellous, or strigillose. Heads usually radiate or discoid, sometimes ± disciform, usually in corymbiform arrays, sometimes in 2s or 3s or borne singly. Calyculi of (3–)5–13(–21+) erect to spreading or reflexed, ± herbaceous (sometimes foliaceous) bractlets or bracts (sometimes surpassing phyllaries). Involucres mostly hemispheric or campanulate to cylindric, (1–)4–12(–25+) mm diam. Phyllaries persistent, mostly (4–)8–21(–30+) in ± 2 series, usually distinct, sometimes connate 0.05–0.1 their lengths, mostly oblong or ovate to lance-oblong, chartaceous to membranous or scarious (usually striate with brownish nerves, margins usually hyaline). Receptacles flat or slightly convex, paleate; paleae usually falling, (usually stramineous, sometimes yellow to orange, with darker striae) ± flat to slightly navicular. Ray florets usually 1–21+ (often 3, 5, 8, or 13), sometimes 0, usually neuter, sometimes styliferous and sterile; corollas usually yellow, sometimes white or pinkish. Disc florets (5–)12–60(–150+), bisexual, fertile; corollas usually yellow to orange, sometimes whitish [purplish], tubes shorter than throats, lobes (3–)5, ± deltate (staminal filaments glabrous; style-branch appendages deltate or lanceolate to subulate). Cypselae usually obcompressed to flat, unequally 3–4-angled, and cuneate to oblanceolate or obovate, sometimes (all or inner) ± equally 4-angled and linear-fusiform, rarely subterete, margins (± corky-winged in B. aristosa, B. cernua, and B. polylepis) usually retrorsely, sometimes patently or antrorsely, barbed or ciliate, apices sometimes attenuate, not beaked [beaked], faces smooth, striate, or ± tuberculate, glabrous or hairy, each sometimes with 2 grooves; pappi 0, or persistent, of (1–)2–4(–8) usually retrorsely, sometimes antrorsely, barbellate or ciliate, rarely smooth, awns. x = 13.

Distribution

Widespread, especially in subtropical, tropical, and warm-temperate North America and South America.

Discussion

Species 150–250+ (25 in the flora).

Within species of Bidens, leaves may be simple, 1-pinnately compound, or 1–3-pinnatisect or -pinnately or -ternately lobed. For compound leaves, leaflets are described; for leaves mostly 1-pinnately lobed, primary lobes are described; for leaves 2–3-pinnatisect or -pinnately lobed, the ultimate lobes are described. Involucres in Bidens species are subtended by more or less herbaceous (sometimes foliaceous) bractlets or bracts; collectively, they constitute calyculi and they often well surpass the phyllaries. Together, phyllaries constitute involucres. In keys and descriptions here, shapes, heights, and diameters given for involucres are for involucres at flowering; involucres are sometimes notably larger in fruit. In some Bidens species, the outer and inner cypselae are obcompressed (unequally 3–4-angled) to flat, are mostly cuneate to obovate or nearly linear (broadest at or near apices), and are more or less similar except for sizes (outer smaller); in other species, the outer cypselae are obcompressed to flat and obovate to cuneate or linear and the inner are more or less equally 4-angled (in cross section) and linear-fusiform (thickest near their middles and more or less attenuate toward their tips).

Selected References

Hall, G. W. 1967. A Biosystematic Study of the North American Complex of the Genus Bidens (Compositae). Ph.D. dissertation. Indiana University.

Sherff, E. E. 1937. The genus Bidens. Publ. Field Mus. Nat. Hist., Bot. Ser. 16.

Weedon, R. R. 1973. Taxonomy and Distribution of the Genus Bidens (Compositae) in the North-central Plains States. Ph.D. dissertation. University of Kansas.

Lower Taxa

Bidens amplissima, Bidens aristosa, Bidens aurea, Bidens beckii, Bidens bidentoides, Bidens bigelovii, Bidens bipinnata, Bidens cernua, Bidens connata, Bidens discoidea, Bidens eatonii, Bidens frondosa, Bidens heterodoxa, Bidens heterosperma, Bidens hyperborea, Bidens laevis, Bidens lemmonii, Bidens leptocephala, Bidens mitis, Bidens pilosa, Bidens polylepis, Bidens tenuisecta, Bidens trichosperma, Bidens tripartita, Bidens vulgata

Key

1	Leaves (some or most) multifid (aquatics, multifid leaves usually submersed, ultimate lobes mostly 0.1–0.3 mm diam.); pappus awns 13–25(–40) mm	Bidens beckii
1	Leaves simple, 1-pinnately compound, or 1–3-pinnatisect or -pinnately lobed (plants sometimes rooted in mud, submersed leaves usually 0; leaflets or lobes mostly 0.5–40+ mm wide); pappus awns 0 or (0.1–)2–4(–8.5+) mm	> 2
2	All or inner cypselae ± equally 4-angled, mostly linear-fusiform (usually thickest in middle 1/2 and distally ± attenuate proximally and distally, each of 4 faces usually 2-grooved)	> 3
2	All cypselae ± flattened (if 3–4-angled, 2 angles ± acute and 1–2 ± obtuse), usually cuneate, linear, oblanceolate, or obovate, seldom ± linear-fusiform (usually broadest in distal 1/3)	> 10
3	Involucres ± campanulate to cylindric, 1–3(–4) mm diam.; disc florets (3–)5–9(–13)	> 4
3	Involucres campanulate to turbinate or hemispheric or broader, (3–)4–8 mm diam.; disc florets (5–)20–80+	> 7
4	Leaves: primary lobes lance-rhombic or ovate to lanceolate, 15–30(–50+) × 5–15(–30+) mm; pappi (1–)2–4 mm	Bidens bigelovii
4	Leaves: primary or ultimate lobes ovate, oblanceolate, lanceolate, or spatulate to linear or filiform, 2–25+ × 0.5–5(–15) mm; pappi 0 or 1–3 mm	> 5
5	Calyculus bractlets spatulate to linear (or 3–5+ lobed, some ± foliaceous), 3–10(–25+) mm (usually surpassing phyllaries)	Bidens lemmonii
5	Calyculus bractlets linear to subulate or filiform (not foliaceous), 1–4 mm (seldom surpassing phyllaries)	> 6
6	Leaves: ultimate lobes 0.5–1(–2) mm wide; cypselae usually glabrous	Bidens heterosperma
6	Leaves: ultimate lobes (1–)2–3(–15) mm wide; cypselae ± hispidulous (at least distally)	Bidens leptocephala
7	Leaves: ultimate lobes ± linear, (2–)5–15+ × (0.5–)2–3+ mm	Bidens tenuisecta
7	Leaves: blades simple, or primary or ultimate lobes lance-rhombic, lance-deltate, ovate, or lanceolate, (10–)15–30(–90+) × (5–)10–15(–40+) mm	> 8
8	Leaf blades either ovate to lanceolate and 30–70(–120) × 12–18(–45) mm, or 1-pinnately lobed, primary lobes 3–7, ovate to lanceolate, (10–)25–80+ × (5–)10–40+ mm; disc florets 20–40(–80+)	Bidens pilosa
8	Leaf blades rounded-deltate to ovate or lanceolate overall, (20–)25–90 × 15–35(–60+) mm, usually 2–3-pinnatisect, ultimate lobes lanceolate, lance-rhombic, obovate, or ovate, 15–30(–50+) × 5–15(–30+) mm; disc florets 10–20(–30+)	> 9
9	Outer cypselae 6–7 mm, inner cypselae 10–14 mm; pappi of 2(–3) awns	Bidens bigelovii
9	Outer cypselae 7–15 mm, inner cypselae 12–18 mm; pappi of (2–)3–4 awns	Bidens bipinnata
10	Blades of all or most leaves either 1-pinnately compound (leaflets 3–5+, petiolulate) or 1–2-pinnately lobed (ultimate lobes lanceolate or oblanceolate to lance-linear or linear, usually 1–12 mm, sometimes to 25 mm wide); calyculus bractlets seldom foliaceous	> 11
10	Blades of all or most leaves simple, margins ± dentate to serrate or margins coarsely incised (lobes 3–7+); calyculus bractlets or bracts often foliaceous.	> 18
11	Ray florets 0, or 1–3+, or 3–5, laminae 2–3.5 mm	> 12
11	Ray florets (5–)8–13, laminae 10–30 mm	> 14
12	Calyculus bractlets (3–)4(–5+), seldom ciliate; disc florets (10–)15–20+	Bidens discoidea
12	Calyculus bractlets 5–21, usually ciliate; disc florets 20–150	> 13
13	Leaves usually 3(–5)-foliolate (leaflets lanceolate to lance-ovate); calyculus bractlets (5–)8(–10)	Bidens frondosa
13	Leaves usually laciniately 1-pinnatisect or 2–5-foliolate (leaflets or lobes ± lanceolate), rarely 2–3-pinnatisect; calyculus bractlets 10–16(–21)	Bidens vulgata
14	Phyllaries ovate to lanceolate (margins usually yellow); ray florets 5–6; margins of cypselae not barbed or ciliolate	Bidens aurea
14	Phyllaries lance-linear, lanceolate, lance-ovate or oblong (margins not notably yellow); ray florets (5–)8–13; margins of cypselae usually barbed or ciliate (except B. mitis)	> 15
15	Cypselae narrowly cuneate (lengths mostly 2.5–4 times widths)	Bidens trichosperma
15	Cypselae broadly cuneate, oblanceolate, or obovate (lengths mostly 1.5–2.5 times widths)	> 16
16	Cypselae 2.5–5 mm, margins (not winged) not barbed or ciliate, faces glabrous or hirtellous; pappi 0, or of 2 antrorsely barbed awns or± deltate scales 0.5(–1) mm	Bidens mitis
16	Cypselae (4–)5–8 mm, margins (often ± corky-winged) usually barbed or ciliate, faces glabrate, glabrous, or strigillose; pappi 0, or of 2(–4) antrorsely or retrorsely barbed awns or scales 0.1–6 mm	> 17
17	Calyculus bractlets 8–12(–16), (4–)5–7(–12) mm	Bidens aristosa
17	Calyculus bractlets 12–21+, (6–)8–12(–20+) mm	Bidens polylepis
18	Leaves sessile	> 19
18	Leaves petiolate, petioles (5–)10–25(–60) mm (sometimes ± winged)	> 24
19	Calyculus bractlets (3–)8–12(–25+) mm, or (6–)10–12(–20+) mm, usually spreading to reflexed	> 20
19	Calyculus bractlets (6–)10–30(–75+) mm, usually erect, sometimes spreading	> 21
20	Rays usually 6–8, sometimes 0, laminae 2–15(–18) mm; outer cypselae (3–)5–6+ mm, inner 4–8 mm (margins ± thickened or winged); pappi of (2–)4 awns (1–)2–4 mm (paleae stramineous to yellowish at tips)	Bidens cernua
20	Rays usually 7–8, rarely 0, laminae (10–)15–25(–30) mm; outer cypselae 6–8 mm, inner cypselae 8–10 mm (margins not notably thickened or winged); pappi of 2–4 awns 3–5 mm (paleae usually orange at tips)	Bidens laevis
21	Involucres (4–)7–10 mm diam.; calyculus bractlets erect; disc florets 15–30(–60);cypselae narrowly cuneate (faces ± 1-nerved and ± striate)	Bidens hyperborea
21	Involucres (3–)6–12(–15+) or 18–25 mm diam.; calyculus bractlets ± erect to spreading or reflexed; disc florets (5–)25–60(–150+); cypselae cuneate to oblanceolate or linear (faces usually smooth or weakly striate, sometimes tuberculate)	> 22
22	Involucres 6–10 × 18–25 mm; ray florets usually 6–11	Bidens amplissima
22	Involucres (4–)5–7(–12) × (3–)6–12(–15+) mm; ray florets usually 0, sometimes 1–5	> 23
23	Cypselae ± flattened, sometimes weakly 3(–4)-angled, faces usually smooth, seldom notably tuberculate (disc corollas gradually ampliate, anthers usually pale)	Bidens tripartita
23	Cypselae (at least inner) usually ± 4-angled, faces usually tuberculate (disc corollas abruptly ampliate, anthers usually dark, blackish)	Bidens connata
24	Ray florets 8–13 (laminae 12–25+ mm); cypselae 2.5–4.5 mm (margins not barbed or ciliate)	Bidens mitis
24	Ray florets 0 or 1–11 (laminae 2–12 mm); cypselae (3–)6–13 mm (margins sometimes barbed or ciliate)	> 25
25	Involucres ± hemispheric or broader, 6–10 × 18–25+ mm; ray florets 6–11 (laminae 4–8 mm)	Bidens amplissima
25	Involucres cylindric, campanulate, or hemispheric or broader, 4–10(–16) × (3–)5–8(–15) mm; ray florets 0 or 1–7 (laminae 2–12 mm)	> 26
26	Involucres campanulate to hemispheric or broader; disc florets (5–)20–60(–150+); cypsela faces usually smooth or tuberculate (not notably striate)	> 27
26	Involucres usually campanulate to cylindric, sometimes ± hemispheric; disc florets (6–)10–25(–60); cypsela faces usually ± striate, sometimes tuberculate	> 28
27	Cypselae ± flattened, sometimes weakly 3(–4)-angled, faces usually smooth, seldom notably tuberculate (disc corollas gradually ampliate, anthersusually pale)	Bidens tripartita
27	Cypselae (at least inner) usually ± 4-angled, faces usually tuberculate (disc corollas abruptly ampliate, anthers usually dark, blackish)	Bidens connata
28	Petioles ± winged; involucres usually ± campanulate to cylindric, sometimes ± hemispheric, 5–7(–9) × (4–)7–10 mm; disc florets 15–30(–60)	Bidens hyperborea
28	Petioles not notably winged; involucres mostly campanulate to cylindric, (4–)8–10(–16) × (4–)7–9(–12) mm; disc florets 6–20(–35+)	> 29
29	Cypselae narrowly cuneate to linear, margins evenly antrorsely strigillose, faces ± evenly antrorsely strigillose	Bidens bidentoides
29	Cypselae broadly to narrowly cuneate or oblanceolate to linear, margins usually barbed or ciliate (proximally antrorsely, distally retrorsely), faces glabrous or sparsely setulose	> 30
30	Involucres (4–)8–10(–12) × (4–)7–9 mm; cypselae: outer 5.5–10 mm, inner 6.5–11 mm	Bidens eatonii
30	Involucres (4–)6–7 × (4–)5–7+ mm; cypselae: outer 4–5(–6) mm, inner (5–)6–7(–8+) mm	Bidens heterodoxa