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Bidens
See list of 17 species in this genusSeveral species of Bidens have been divided into varieties on the basis of differences in awn morphology (e.g., number, length, presence and orientation of barbs). Studies have revealed that variation in awn morphology can be observed within populations or even on a given plant (Weedon 1974, Roberts 1982). Therefore, taxa based solely on these differences are not recognized here. Recent phylogenetic work showed that Bidens is not monophyletic and that it is nested within Coreopsis (Kimball and Crawford 2004). This indicates the need for taxonomic realignment and potential merging (in some fashion) of these two genera. References: Weedon (1974), Haines (2003b).
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1a. Plants aquatic, with flaccid stems; leaves whorled, finely dissected into narrow, flaccid segments (except the emersed ones); capitulescence commonly a solitary capitulum [Fig. 380]; cypselas nearly terete to weakly 4-angled
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1b. Plants terrestrial or of wetlands, with erect stems; leaves opposite, entire or divided into broad, firmer segments; capitulescence commonly composed of 2 or more capitula; cypselas compressed to quadrangular
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2a. Principal leaf blades 1- to 3-times divided into distinct leaflets, at least the terminal leaflet with a thin petiolule
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3a. Ultimate segments of leaf blades linear to narrow-lanceolate, (0.5–) 2–8 (–12) mm wide; marginal flowers of capitulum with well-developed rays 10–30 mm long (except B. tenuisecta, with rays absent or short)
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4a. Marginal flowers of capitulum lacking rays or with short rays up to 6 mm long; inner cypselas quadrangular in cross-section, 8–15 mm long
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4b. Marginal flowers of capitulum with well-developed rays 10–30 mm long; inner cypselas ± flat, (4–) 5–9 mm long
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5a. Cypsela body narrow-oblanceolate to cuneate-oblong in outline, 2.5–4 times as long as wide, 0.9–2.5 mm wide, merely antrorsely ciliate on the margin; peduncles glabrous; pappus of 2 setose, very stout awns or awn-scales (rarely absent)
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5b. Cypsela body obovate to elliptic-obovate, 1.5–2 (–2.5) times as long as wide, 2.5–5.2 mm wide, with a thin, antrorsely ciliate and often erose-notched wing margin; peduncles pubescent; pappus of 2 (–4) antrorsely or restrorsely barbed awns (rarely absent)
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6a. Outer, herbaceous series of involucral bracts numbering 8–12 (–16), (4–) 5–7 (–12) mm long, usually shorter than the inner, non-herbaceous series of bracts, eciliate or ciliate on the margin
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6b. Outer, herbaceous series of involucral bracts numbering 12–21, (6–) 8–12 (–20) mm long, usually longer than the inner, non-herbaceous series of bracts, coarsely ciliate on the margin
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3b. Ultimate segments of leaf blades lanceolate or oblanceolate to ovate, (5–) 7–40 mm wide; marginal flowers of capitulum lacking rays or with short rays up to 5 mm long (except B. alba, with well-developed, white rays)
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7b. Leaf blades once pinnately divided; cypsela bodies at maturity not or only slightly exceeding (i.e., up to 1.5 times as long as) the inner, non-herbaceous series of involucral bracts of intact capitula; pappus of 2 (–5) awns; rays, when present, white (rarely short yellow rays present)
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8a. Cypsela body quadrangular in cross-section, linear in outline, 5–9 times as long as wide; marginal flowers of the capitulum with or without well-developed rays longer than 5 mm
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9a. Capitulum with 5–8 ray flowers with well-developed white rays 5–16 mm long; outer involucral bracts (8–) 12 (–16); pappus consisting of 2 awns
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9b. Capitulum lacking ray flowers or these present and with short rays up to 3 mm long; outer involucral bracts 7–10; pappus consisting of 2 or 3 (–5) awns
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8b. Cypsela body definitely compressed, oblanceolate to obovate, 2–3.5 times as long as wide; marginal flowers of capitulum lacking rays or with short rays up to 5 mm long
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10a. Capitula with 3–5 outer, herbaceous involucral bracts; involucral bracts eciliate or only sparsely ciliate; cypsela body 3–6.5 mm long, with awns 0–2.4 mm long
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10b. Capitula with 5–21 outer, herbaceous involucral bracts; involucral bracts ciliate-margined; cypsela body 5.3–11.3 mm long, with awns 2–9.5 mm long
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11a. Capitula with 5–10 outer, herbaceous involucral bracts; disk corollas abruptly ampliate above the herbaceous base, the limb delicate and the lobes prominently pigmented with yellow to yellow-orange, after anthesis collapsing around the exserted anthers and weakly attached to the summit of ovary [Fig. 382]
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11b. Capitula with 10–21 outer, herbaceous involucral bracts; disk corollas gradually expanded apically, more herbaceous throughout and the lobes only weakly pigmented with yellow, after anthesis becoming chartaceous, scarcely collapsing, with included anthers, and firmly attached to the summit of ovary [Fig. 384]
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2b. Leaf blades simple or lobed into 3–7 segments, the segments not divided completely to the midrib and, when lobed, with broadly winged petiolules
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12a. Leaf blades sessile, or sometimes the lowermost with a short, narrowed base; cypselas either with a convex, cartilaginous apex or with a truncate apex in B. tripartita
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13a. Disk corollas gradually expanded apically, more herbaceous throughout and the lobes only weakly pigmented with yellow, after anthesis becoming chartaceous, scarcely collapsing, with included anthers, and firmly attached to the summit of ovary [Fig. 384]; anthers pale brown; summit of the fruiting ovary with a truncate apex (in part)
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13b. Disk corollas abruptly ampliate above the stramineous or herbaceous base, the limb delicate and the lobes prominently pigmented with yellow to yellow-orange, after anthesis collapsing around the exserted anthers and weakly attached to the summit of ovary [Fig. 382]; anthers dark brown to black; summit of fruiting ovary with a convex, cartilaginous apex
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14a. Outer, herbaceous involucral bracts ascending to erect; disk corollas usually 4-lobed; capitula erect; body of cypsela conspicuously marked with 7–15 longitudinal striations on each surface
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14b. Outer, herbaceous involucral bracts spreading; disk corollas 5-lobed; capitula arched or nodding in age; body of cypsela not or only inconspicuously marked with longitudinal striae
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15a. Rays absent or 2–15 (–18) mm long; chaff yellow near apex; body of inner cypselas with a pale, thickened, cartilaginous margin, those of the outer ones (3–) 5–6 mm long, those of the inner ones 4–8 mm long
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15b. Rays 15–30 mm long; chaff yellow-orange to orange-red near apex; body of cypselas lacking a pale, thickened margin, those of the outer ones 6–8 mm long, those of the inner ones 8–10 mm long
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12b. Leaves with distinct petioles 1–4 cm long; cypselas with a truncate apex
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16a. Disk corollas gradually expanded apically, more herbaceous throughout and the lobes only weakly pigmented with yellow, after anthesis becoming chartaceous, scarcely collapsing, with included anthers, and firmly attached to the summit of ovary; anthers pale brown (in part)
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16b. Disk corollas abruptly ampliate above the herbaceous base, the limb delicate and the lobes prominently pigmented with yellow to yellow-orange, after anthesis collapsing around the exserted anthers and weakly attached to the summit of ovary; anthers dark brown to black
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17a. Capitula cylindric to narrow-funnelform, each with (7–) 12–25 (–35) flowers; body of cypsela ± flat, usually striate on each face, lacking tuberculae [Fig. 383]; plants restricted to tidal river shores
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17b. Capitula funnelform to hemispheric, each with 20–60 (–150) flowers; body of cypsela compressed-quadrangular (varying to almost flat), commonly with few or no striations (immature fruits with more evident striations), usually with minute tubercles [Fig. 382]; plants of varied habitats, including tidal river shores, but primarily of inland wetlands and shores
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Each photo represents one species in this genus.