COMPOSITAE (COMPOSITE FAMILY) 839 



short-ovate or oblong, the inner connate above the middle ; rays short or usually 

 none; achenes less roughened. Neb. and Kan., southw. and westw.; adv. 

 in Mo. 



56. BIDENS L. BUR MARIGOLD 



Heads many-flowered ; the rays when present 3-8, neutral. Involucre 

 double, the outer commonly large and foliaceous. Keceptacle flattish ; chaff 

 deciduous with the fruit. Achenes flattened parallel to the bracts of the invo- 

 lucre, or slender and 4-sided (rarely terete), crowned with awns or short teeth 

 (these rarely naked). Annual or perennial herbs, with opposite various leaves, 

 and mostly yellow flowers. (Latin, bidens, two-toothed.) 



\ . B. In this genus the measurements of the fruit relate to the inner 

 mature achenes. The outer are often shorter and uncharacteristic. The figure* 

 of the heads are on a scale of f , those of the achenes are life size. 



a. Achenes flat (or at most with a strong rib on either face) b. 

 b. Rays small or wanting c, 

 c. Inner achenes less than 2 mm. broad. 



Heads cylindric or ellipsoid, much longer than broad ; disk-florets 



3-4 mm. long; leaves simple. 



Inner achenes 1 cm. or more long ; awns 6-8 mm. long . 1. B. birt tntoides. 



Inner achenes 7-9 mm. long; awns 3-4.5 min. long ... 2. B. Eatoni. 

 Heads hemispherical, nearly or quite as-broad as long; disk-florets 

 1-1.5 mm. long ; achenes 5.5-8.5 mm. long ; awns 1.5-2.5 min. 



long ; leaves ternately compound 8. B. discoidea. 



C. Inner achenes 2 mm. or more broad d. 



d. Leaves simple ; heads subtended by long thick entire leafy bracts ; 

 achenes 8-10 mm. long, 2-3 mm. broad, retrorsely barbed, 

 the 3 (rarely 2 or 4) awns unequal, at most 6 mm. long . 6. B. comosa. 



d. Leaves pinnate ; achenes upwardly barbed at least at base ; 



awns 2. 

 Outer involucre of 5-8 leafy bracts; inner bracts oblong, 



equaling the disk ; achene's 2-3.3 mm. broad . . . 4. B. frondosa. 

 Outer involucre of 10-16 leafy bracts ; inner bracts ovate-trian- 

 gular, shorter than the disk ; achenes 3.3-4 mm. broad . 5. B. -culgata. 

 b. Rays conspicuously exceeding the disk e. 



e. Leaves mostly pinnate ; awns (if present) upwardly barbed /. 

 f. Inner achenes 2 mm. or less broad, cuneate. 



Inner achenes 3-4.5 mm. long 11. B. coronata. 



Inner achenes 5-7 mm. long 12. B. tr-ic/iosperma. 



f. Inner achenes more than 2 mm. broad, elliptic-obovate, with thin 



scarious margins. 

 Outer foliaceous bracts 8-10, smooth or merely ciliate, shorter 



than the inner 18. B. aristota. 



Outer foliaceous bracts 12-20, coarsely hispid, mostly longer 



. than the inner 14. B. involucrata. 



e. Leaves simple or slightly divided ; achenes obovate, 3-4.5 mm. long, 



with short teeth 11. B. coronata. 



a. Achenes angled or terete g. 



g. Achenes distinctly angled ; terrestrial or marsh plants A. 

 h. Leaves pinnate. 



Heads few-flowered, slender; achenes linear, 4-angled, the inner 



1.2-1.8 cm. long, about 1 mm. broad 10. B. bipinnata. 



Heads hemispherical, many-flowered ; achenes cuneate, 3-4 angled, 



4-6 mm. long (7) B. connala, v. pinnata. 



h. Leaves simple ; heads hemispherical, many-flowered ; achenes 



cuneate or obovate. 

 Leaves petioled or with conspicuously narrowed bases ; fruiting 



heads erect " 1. B. connata. 



Leaves sessile or connate ; fruiting heads mostly nodding. 

 Outer bracts unequal, mostly exceeding the disk ; rays at most 

 twice the length of the disk ; achenes dilated above ; leaves 



mostly connate 8. B. ctmua. 



Outer bracts subequal, rarely exceeding the disk ; rays 24 times 

 s long as the disk ; achenes not dilated above ; leaves 



sessile, but rarely connate 9. B. laevit. 



g. Achenes terete, truncate at both ends, with the 3-6 very long awns 



smooth below ; aquatic, with the immersed leaves finely divided . 15. B. Beckii. 



1. B. bidentoides (Nutt.) Britton. Glabrous, paniculately branched, 2-8 dm. 

 high; leaves lanceolate, coarsely toothed, tapering at both ends; heads 1.5-2 

 cm. long ; the outer involucre of 4-6 bracts ; rays usually wanting ; achenes 



