17. C. bidentoìdes, Nutt. Paniculately branched; _leaves undivided, lanceolate_, coarsely toothed, tapering at both ends; heads 6--10´´ long; _achenes nearly subulate_, bearing a pair of _very slender_ upwardly roughened _awns surpassing the corolla_ (4´´ long), but shorter than the achene, often also 2 minute teeth alternate with the awns.--Shores of Delaware River, near Philad., and Delaware Bay, to Md. Hybridizes with _Bidens frondosa_. 18. C. discoídea, Torr. & Gray. Diffusely branched, 1--2° high; _leaves ternately divided_, slender-petioled; leaflets ovate-lanceolate, pointed, coarsely serrate; heads 2--3´´ long; _achenes linear-wedge-shaped_ (2--3´´ long), bearing a pair of _short and stout_ upwardly-barbed _awns of the length of the corolla_.--Wet banks and swamps, Conn. to Ohio, Ill., and southward. July. 56. BÌDENS, L. BUR-MARIGOLD. Heads many-flowered; the rays when present 3--8, neutral. Involucre double, the outer commonly large and foliaceous. Receptacle flattish; the chaff deciduous with the fruit. Achenes flattened parallel with the scales of the involucre, or slender and 4-sided, crowned with 2 or more rigid and persistent awns which are downwardly barbed.--Annual or perennial herbs, with opposite various leaves, and mostly yellow flowers. (Latin, _bidens_, two-toothed.) [*] _Achenes flat, not tapering at the summit; outer involucre foliaceous; annuals._ [+] _Heads erect, nearly rayless; leaves mostly petiolate._ 1. B. frondòsa, L. (COMMON BEGGAR-TICKS. STICK-TIGHT.) Smooth or rather hairy, tall (2--6° high), branching, _leaves 3--5-divided; leaflets mostly stalked_, lanceolate, pointed, coarsely toothed; outer involucre much longer than the head, ciliate below; _achenes wedge-obovate, 2-awned, ciliate_ (the bristles ascending except near the summit).--Moist waste places; a coarse troublesome weed, the achenes, as in the other species, adhering to clothing, etc., by their retrorsely barbed awns. Hybrids occur with _Coreopsis aristosa_ and other species. July--Oct. 2. B. connàta, Muhl. (SWAMP BEGGAR-TICKS.) Smooth (1--2° high); _leaves lanceolate_ or oblong-lanceolate, pointed, sharply serrate, tapering into margined slightly united petioles; _the lower often 3-divided, their lateral divisions united at the base and decurrent on the petiole_; outer scales longer than the head, few, mostly obtuse; _rays none; achenes narrowly wedge-form, 3- (2--4-) awned, the margins minutely retrorsely ciliate_.--E. New Eng. to Minn., and southward.--Var. COMÒSA, Gray, is stouter, the leaves commonly all simple, upper ones nearly sessile, the heads larger and with very leafy involucre. Ill., Ky., and westward. Aug.--Oct.--Var. PINNÀTA, Watson; leaves nearly all pinnately divided, the 5--7 narrow divisions sparingly incised; achenes 4-awned. Hennepin Co., Minn. (_F. L. Conillard_). [+][+] _Heads somewhat nodding, commonly radiate; leaves sessile, undivided._