206 Rhodora [October 



Bidens cernua in eastern America. Bidens cernua L. is a 

 highly variable species with several well defined varieties in north- 

 eastern America. It belongs to a group of three species with simple 

 leaves and achenes with a convex cartilaginous summit. These three 

 species may be distinguished as follows. 



Mature disk (except in depauperate extremes) 1.3-2.8 cm. 

 broad: fruiting heads often nodding: outer involucre 

 reflexed, spreading or merely subascending: disk- 

 corollas 4-5 mm. long, 5-toothed: anthers exserted, 

 purple-black: achenes not conspicuously striate between 

 the margins and midribs or keels; the central 1.8-2.5 

 mm. broad. 

 Achenes straight and flat, not winged nor strongly keeled, 

 deep-brown or purplish; the outer 6-8 mm. long, with 

 marginal awns 2.8-4.5 mm. long; the central 8-9.5 

 mm. long, with marginal awns 3.5-5 mm. long: stem 

 firm and usually smooth; its rooting base up to 6 dm. 

 long: outer involucre rarely longer than the inner: 



chaff reddish-tipped: rays 1.5-3 cm. long B. laevis. 



Achenes curved, with almost wing-like pale margins and 

 keels, olivaceous; the outer 3.3-6.3 mm. long, with 

 marginal awns 2-2.8 mm. long; the central 4.2-7.8 

 mm. long, with marginal awns 2.6-4 mm. long: stem 

 soft and usually somewhat hispid; its rooting base 

 rarely 1 dm. long: outer involucre mostly longer than 

 the inner: chaff yellow-tipped: rays wanting or at 



most 1.7 cm. long B. cernua. 



Mature disk rarely 1.5 cm. broad: fruiting heads erect: outer 

 involucre ascending: disk-corollas 3.5-4 mm. long, 4- 

 toothed: anthers included, pale : achenes distinctly 7-15- 

 striate on each face; the central 1.4—1.9 mm. broad, fiat, 

 olive-brown or drab B. hyperborea. 



Bidens laevis is not specially variable with us; the variations of 



B. hyperborea have recently been discussed; 1 and to round out the 



treatment of this group the northeastern varieties of B. cernua are 



here considered. Our variations of this species are as follows. 



Stems stoutish, 0.25-1 cm. in diameter at base, commonly 

 branching, 0.5-1.8 m. high: leaves sessile or at most 

 narrowed at base, thickish, 0.2-2 dm. long; heads com- 

 monly numerous, broadly hemispherical, many-flowered; 

 the primary ones with disks 1-2.7 cm. broad, nodding 

 in fruit: outer involucre of 5-10 bracts; inner of about 

 8 bracts 6-12 mm. long. 

 Leaves tapering to long acuminate-attenuate tips; the 

 primary with 4-24 pairs of sharp serrations: bracts 

 of outer involucre linear to lanceolate, acute or 

 acutish. 

 Leaves with broad connate or subconnate bases, scarcely 

 narrowed below the middle. 

 Leaves linear to oblanceolate, with 4-13 pairs of 



coarse teeth 1-5 mm. high B. cernua (typical) . 



i Rhodoba, xx. 146-150 (1918). 



