34. Bidens L. Beggar-ticks. Bur-marigold 



Annual taprooted herbs or less commonly perennial herbs with taproots or 

 fibrous roots; leaves opposite, entire or dentate or incised, or 1-, 2- or 3-ternately 

 or -pinnately dissected; upper leaves sometimes alternate; involucre usually cam- 

 panulate or subhemispheric; phyllaries commonly biseriate, at the base often very 

 shortly connate; outer phyllaries usually herbaceous in texture, sometimes short, 

 sometimes expanded into leaflike members; inner phyllaries usually membranous, 

 usually hyaline- or yellow-margined; receptacle flat or convex, chaffy throughout, 

 the pales narrow and flatfish; ray flowers present or absent, when present neuter 

 or less commonly pistillate but infertile; rays when present whitish or yellow or 

 less commonly rosy or a shade of red, usually shallowly 3-lobed apically; disk 

 flowers numerous or less commonly few, perfect, fertile; corolla equally 5-toothed, 

 usually yellow; style branches bearded above, tipped with short and acute or 

 longer and subulate appendages; achenes dorsiventrally compressed or less com- 

 monly triangular or rhombic in transection; pappus awns absent or 1 to 8 in 

 number and commonly persistent, usually more or less antrorsely or retrorsely 

 barbed. 



A genus of possibly 150 species in the warmer parts of the world, often 

 separated on the most technical sort of characters, thus difficult to key out unless 

 with patience and a good lens; heads both in anthesis and in fruit. The kinds 

 with retrorsely barbed pappus awns are eflSciently dispersed, becoming attached 

 easily in fur and clothing. 



1. Rays yellow, at least 10 mm. long (sometimes absent in B. cernua and B. 

 laevis) (2) 



1. Rays white or rosaceous to red (if yellow less than 10 mm. long) or absent (7) 



2(1). Pappus of 2 to 4 retrorsely barbed awns (3) 



2. Pappus awns smooth, antrorsely setose or absent (5) 



3(2). Achenes flattened or 3- or 4-angled but the median keels not developed.... 

 1. B. laevis. 



3. Achenes cuneate, 4-angled in cross section, both margins and median keels 



prominent or callous thickened (4) 



4(3). Ray flowers 5 or 6; achenes black or dark-gray, more or less papillose 

 toward the base; awns 1.5-2.7 mm. long 2. B. aurea. 



4. Ray flowers (when present) 6 to 8; achenes purplish, usually strongly callous- 



thickened on the angles; awns 3-4 mm. long 3. B. cernua. 



5(2). Achenes black, the body 2.5-4.5 mm. long; outer phyllaries 7 to 10 



4. B. mitis. 



5. Achenes brown or blackish, the body 5-7.5 mm. long (6) 



6(5). Outer phyllaries mostly 8 to 12, glabrous or moderately ciliate, usually 

 shorter than the inner ones 5. B. aristosa. 



6. Outer phyllaries 12 to 20, very ciliate or coarsely hispid, commonly longer 



than the inner ones 6. B. polylepis. 



7(1). Achenes broadly or narrowly cuneate, not narrowed below the apex (8) 



7. Achenes linear or clavate but never manifestly cuneate, often attenuate above 



(11) 



8(7). Achene bodies striate; leaves simple or deeply incised (9) 



8. Achene bodies not striate; leaves at least 1- or 2-pinnate, the terminal leaflet 



commonly petiolulate (10) 



9(8). Leaves sessile, undivided; achene body 6-9 mm. long, retrorsely barbed 

 on the angles 1. B. laevis. 



1663 



