830 AMBROSIACEAE 



Involucres of the staminate heads 3-ribbed; leaves palmately 3-5-cleft or entire. 



1. A. trifida. 

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

 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. 

 Perennialwithrootstock;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 outhne, 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, wdth as many conical tubercles. Moist and waste places: Que. 

 —Fla.— Colo.— B.C. Plain, Jl-0. 



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, 'nith lanceolate, acute divisions; staminate heads 



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



ciliate-pilose; fruit obovoid; 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 — 

 SubmonL Au-0. 



3* A. media Rydb. Annual; stem 4^6 dm. high, hispid and strigose; leaves 

 pinnatifld, 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 ram. 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-0. 



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



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

 sides, thick and strongly veined, pinnatifld; 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 imarmed, sometimes with 

 obtuse tubercles. A. coronopifoUa T. & G. Prairies and plains: 111.— Sask.— 

 La.— Calif.— Ida. Plain—SuhmonL Jl-0. 



7. FRANSERIA Cav. 



Annual 



or perennial herbs, rarely shrubby, resembling Amhrosia. Leaves 

 mostly alternate, lobed or pinnatifld. 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 stammate or rarely mked with them. Involucres closed, glo- 

 bose or ovoid, beaked, enclosing \^ pistils, with several processes in more than 

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

 [(jaerinerxa Medic] 



Fruit not \^nous. 



Spines of the fruit flat, never hooked. 



^^^bla!^^^^^* ^^^^^ sHvery white; pistiUate heads mostly 2-flowered and 2- 

 ^^^ T P^^.^'hi ■ n^trf.t'^^t-^H''^^ ^^^^^' Pistm^e heads 1-flow^ed and'lSked. 



Leaves bi- or tn-pinnatifid. ^— op nranthocarva. 



. Leaves pinnately 3-o-cleft. * S* i' montana 



Spines terete. usuaUy more or less curved " ^'^"'^""• 



Bur 1-beaked, 1-2-flowered; leaves not tomentose beneath 



Annual; di^asions of the leaves oblong or linear-oblong, acute. 



Perennial with a suffrutescent base; leaf-segments linear, obtiSe.'^""'^^'*'"' 



^"^ ^ro^Sk^"^'''^^'^'^ • ^^^""^ tomentose beneath; plants^ plreS^^th a 

 Leaves interruptedly pinnatmd. with lobed or coarsely toothed divisions. 



6. F, discolor. 



