1 82 Flora of the Palouse Region 



H. canadense Michx. Perennial somewhat scabrous throughout: stems 

 stout, 30-120 cm. high: leaves numerous, lanceolate, entire or incisely ser- 

 rate, sessile and somewhat clasping at base, gradually smaller upwards, 1-10 

 cm. long, none clustered at base: heads corymbed, rarely solitary, on stout 

 peduncles: involucre hemispheric, puberulent or glabrous, sometimes gland- 

 lar, 1-2 cm. broad, the bracts in 2-3 series, the uppermost loose: corolla 

 yellow: akenes columnar: pappus brownish. Low ground, rare. 



302. IVA. 



Herbs: leaves simple, at least some of the lower opposite: heads 

 small, nodding, in the axils of the leaves or in terminal spikes or 

 panicles: flowers monoecious, in the same head; a few marginal 

 ones pistillate, the others staminate and more numerous: bracts of 

 the involucre few, rounded, commonly united into a cup: recep- 

 tacle chaffy, with scales subtending the sterile flowers: akenes 

 obovate, thick. 



I. xanthiifolia Nutt. Annual, tall and stout, 1-2 m. high, pubescent or 

 puberulent: leaves opposite or the uppermost alternate: broadly ovate, acum- 

 inate, incisely serrate, scabrous above, 3-ribbed, 6-12 cm. long; petiole stout, 

 with a few scattered stout hairs: panicles terminal and in the axils of the 

 upper leaves, bractless, large and dense: heads sessile or nearly so, about 2 

 mm. broad, hemispherical; bracts in two series of 5 each, the outer broadly 

 ovate, the inner obovate or oblong and truncate: akenes usually 5. Spar- 

 ingly introduced along railways. 



303. AMBROSIA. 



Annual or perennial herbs: leaves piunately-lobed, at least the 

 lower opposite: heads small, greenish, monoecious: pistillate heads 

 1 -flowered, in terminal bractless racemes or spikes: involucral- 

 bracts of staminate heads united: pistillate flowers enclosed in an 

 akene-like 1 -celled involucre, usually armed with a single series of 

 tubercles or prickles: pappus none: corollas of staminate flowers 

 broad, 5-toothed; of the pistillate none: akenes ovoid or obovoid, 

 thick. * • 



Annual: involucre spin; in fruit. A. artemisiaefolia. 



Perennial: involucre tubercled or naked in fruit. A. psilostachya. 



A. artemisiaefolia L. Annual, with many slender branches, pubescent, 

 30-90 cm. high; upper leaves alternate, the lower opposite, ovate or lanceo- 

 late, the uppermost sessile, often entire and acute, the olhers variously pin- 

 nately lobed. mostly petioled; blades 1-3 cm. long: staminate heads numer- 

 ous, 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 sandy banks of Snake River. 



A. psilostachya DC. Perennial, with numerous rootstocks, hirsutely 

 and strigosely pubescent: stems stout, erect, 60-120 cm. high: leaves thick, 

 the lower petioled, the upper sessile, once or twice pinnately cleft: lobes 



