g30 AMBROSIACEAE 



Involucres of the staminate heads 3-ribbed; leaves palmately 3-5-cleft o^ entire^ 



Involucres of the staminate heads not ribbed; leaves once to thrice pinnatifid. 

 Annual; fruit with acute teeth. 



Leaves thin, not strongly veined, bipinnatifid. 2. A. elatior. 



Leaves thick, strongly veined, pinnate with broad toothed or cleft segments. 



3. A. media. 

 Perennial with rootstock; fruit with unarmed or blunt tubercles. 4. A. psilostachya. 



1. A. trifida L. Annual; stem 1-5 m. high, more or less hispid; leaves 

 petioled; blades scabrous on both sides, ovate in outline, 3-5-cleft or entire, ser- 

 rate, acute or acuminate; sterile racemes many; heads nodding; involucres 3- 

 ribbed on the back, slightly hispidulous or glabrous; fruit 6-7 mm. long, turbin- 

 ate 3-5-ribbed, with as many conical tubercles. Moist and waste places: Que. 

 Fla. Colo. B.C. Plain. Jl-O. 



2. A. elatior L. Annual; stem 3-10 dm. high, strigose and ciliate; leaves 

 with ciliate petioles, thin, hispidulous or glabrate above, more or less grayish- 

 strigose beneath, bipinnatifid, with lanceolate, acute divisions; staminate heads 

 nodding, oblique, about 3 mm. broad, puberulent or in the western form more 

 ciliate-pilose; fruit obqvoid; body slightly pubescent or glabrate, 2.5-3 mm. 

 long, with 5-7 sharp spines about 0.5 mm. long; beak 1-1.5 mm. A. artemisiae- 

 folia A. Gray, not L. Waste places: N.S.Va. Colo. Wash. B.C. Plain 

 Submont. Au-O. 



3. A. media Rydb. Annual; stem 4r-Q dm. high, hispid and strigose; leaves 

 pinnatifid, or the upper merely cleft, 5-10 cm. long, scabrous above, hispid- 

 strigose beneath; divisions lanceolate, the lower lobed or toothed; staminate 

 heads numerous; involucre 5-lobed and crenulate, 4-5 mm. broad, hispid-strigose; 

 fruit obovoid; body 3 mm. long, puberulent; beak 1 mm. long, pubescent; spines 

 5-7, sharp, subulate, 0.5 mm, long. Dry places: S.D. Kans. N.M. Nev. 

 Wash. Plain Submont. Jl-O. 



4. A. psilostachya DC. Stem 3-8 dm. high, canescent-strigose; leaves 

 sessile or the lower with short broadly-winged petioles, grayish-strigose on both 

 sides, thick and strongly veined, pinnatifid; divisions entire, toothed or cleft, 

 lanceolate, acute; staminate heads nodding, oblique, 3 mm. wide; fruit round- 

 elliptic; body pubescent, 3-3.5 mm. long, usually unarmed, sometimes with 

 obtuse tubercles. A. coronopifolia T. & G. Prairies and plains: 111. Sask/ 

 La. Calif. Ida. Plain Submont. Jl-O. 



7. FRANSERIA Cav. 



Annual or perennial herbs, rarely shrubby, resembling Ambrosia. Leaves 

 mostly alternate, lobed or pinnatifid. Heads monoecious, the staminate ones 

 in terminal racemes or spikes. Involucres hemispheric, open, with 5-12 more or 

 less united bracts; receptacle chaffy. Corollas short, 5-lobed. Anthers scarcely 

 coherent. Styles rudimentary, simple. Pistillate heads solitary or in small 

 clusters below the staminate or rarely mixed with them. Involucres closed, glo- 

 bose or ovoid, beaked, enclosing 1-4 pistils, with several processes in more than 

 one series, becoming bur-like at maturity. Stigmas 2. Achenes obovoid. 

 [Gaertneria Medic.] 



Fruit not villous. 



Spines of the fruit flat, never hooked. 



Plant shrubby; leaves silvery white; pistillate heads mostly 2-flowered and 2- 



beaked. 1. p. albicaulis. 



Plant annual; leaves strigose, not white; pistillate heads 1-flowered and 1-beaked. 

 Leaves bi- or tri-pinnatifld. 2. F. acanthocarpa. 



_ Leaves pinnately 3-5-cleft. 3. F. montana. 



Spines terete, usually more or less curved. 



Bur 1-beaked, 1-2-fl owered ; leaves not tomentose beneath 



Annual; divisions of the leaves oblong or linear-oblong, acute. 



Perennial with a suffrutescent base; leaf-segments linear, obtuse. 



Bur 2-beaked, 2-flowered; leaves tomentose beneath; plants 'perennial with a 



rootstock. 



Leaves interruptedly pinnatifid, with lobed or coarsely toothed divisions. 



6. F. discolor. 



