258 COMPOSITAE. 



Leaves all opposite; receptacle not chaffy. A. trifida. 

 Leaves opposite and alternate; receptacle chaffy. 



Fruiting involucre spiny; leaves thin. A. artemisiaefolia. 



Fruiting involucre tubercled; leaves thick. A. psilostachya. 



Ambrosia trifida L. Stems stout, 1-6 m. high, rough-hairy; leaves rough- 

 hairy, deeply 3-lobed, the lobes oval-lanceolate and serrate; petioles margined; 

 fruit obovoid, tubercled. Sparingly introduced from the eastern states. 



Ambrosia artemisiaefolia diversifolia Piper. Annual, with many slender 

 branches, pubescent, 30-90 cm. high; upper leaves alternate, the lower op- 

 posite, ovate or lanceolate, the uppermost sessile, often entire and acute, the 

 others variously pinnately lobed, mostly petioled; blades 1-3 cm. long; 

 staminate heads numerous, pedicelled, drooping, bractless, the involucre 

 crenate; pistillate heads solitary or clustered, sessile in the leaf axils below 

 the staminate racemes; fruit 3 mm. long, short-beaked, armed with 4-6 

 teeth or spines. Native on the gravelly banks of Snake River at Almota. 



Ambrosia psilostachya DC. Perennial, with numerous rootstocks, hir- 

 sutely and strigosely pubescent; stems stout, erect, 60-120 cm. high; leaves 

 thick, the lower petioled, the upper sessile, once or twice pinnately cleft; 

 lobes lanceolate, acute, entire or toothed; staminate heads short-peduncled, 

 in strict racemes; pistillate heads solitary in the axils; fruit obovoid or oblong, 

 3 mm. long, short-pointed, rough-reticulated, unarmed or with a few short 

 teeth. Introduced sparingly from the eastern states. 



384. GAERTNERIA. 



Herbs (in ours) with mostly alternate leaves; sterile and fertile 



heads separate or sometimes mixed in the inflorescence; fertile 



involucre 1-4-celled, with a single pistil in each cell, armed with 



spines in more than one row, bur-like. 



Gaertneria acanthicarpa (Hook.) Britt. Annual, erect or spreading, 

 30-60 cm. high, loosely branched, hispid-pubescent; leaves bipinnatifid, or 

 the upper pinnatifid; racemes panicled; fertile involucre 1-flowered, armed 

 with flat subulate spreading straw-colored spines. Rare on the gravelly 

 banks of Snake River. 



385. COLEOSANTHUS. 



Herbs or undershrubs with opposite or alternate leaves; heads 

 whitish; involucre campanulate, the scales imbricated, lanceolate 

 or linear, the outer shorter, none herbaceous; receptacle flat, 

 naked ; pappus one row of separate capillary barbed or scabrous 

 bristles. 



Leaves cordate-triangular, coarsely toothed. C. grandiflorus. 



Leaves oblong or lanceolate, entire. C. linifolius. 



Coleosanthus grandiflorus (Nutt.) Kuntze. Stems herbaceous, branched, 

 erect or ascending, 60-90 cm. high; leaves ovate or triangular-cordate, coarsely 

 toothed, acuminate, 5-10 cm. long, puberulent, on slender petioles; heads 

 paniculate, drooping, each about 40-flowered; involucral bracts thin, the outer 

 short and ovate, the inner oblong-linear; akenes minutely hispid, not glandular, 

 Hanks of Spokane River. 



Coleosanthus linifolius (D. C. Eaton) Kuntze. Stems numerous, usually 

 simple, erect, from a branched woody base, 15-30 cm. high; leaves oblong or 



