294 



AMBROSIACEAE. 



Iva xanthiifolia (Fresen.) Nutt. 

 (Fig. 3590.) 



Fresen. Ind. Sem. 



[Vol. III. 



Burweed Marsh Elder. 



Am. Phil. Soe. 



Cyclachaena xanthiifolia 



Hort. Franc. 4. 1836. 

 Iva xanthiifolia Nutt. Trans 



(11)7:347- 1841. 



Annual; stem much branched, pubescent 

 or puberulent above, glabrous below, 3-6 

 high. Leaves nearly all opposite, broadly 

 ovate, long-petioled, acuminate at the apex, 

 abruptly or gradually contracted at the base, 

 coarsely and irregularly dentate, pale and 

 canescent or puberulent beneath, roughish 

 above, 3-ribbed, the lower often 6' long and 

 wide; inflorescence spicate-paniculate, termi- 

 nal and axillary, ample, naked; heads sessile 

 or minutely peduncled, less than \" broad; 

 bracts of the involucre in 2 series of 5, the 

 outer ovate, the inner obovate or truncate, con- 

 cave, subtending the usually 5 achenes; corol- 

 las of the fertile flowers rudimentary or none; 

 staminate flowers io-i5,their corollas obconic. 



In moist soil, or sometimes in waste places, 

 Michigan to Northwest Territory, south to Wis- 

 consin, Nebraska, New Mexico and Utah. Plant 

 with the aspect of a Chenopodium. July-Sept. 



2. AMBROSIA L. Sp. PI. 9S7. 1753. 



Monoecious (rarely dioecious) branching herbs, with alternate or opposite, mostly lobed 

 or divided leaves, and small heads of green flowers, the staminate spicate or racemose, the 

 pistillate solitary or clustered in the upper axils. Involucre of the pistillate heads globose- 

 ovoid or top shaped, closed, i-flowered, usually armed with 4-8 tubercles or spines; corolla 

 none; stamens none; style-branches filiform; achenes ovoid or obovoid; pappus none. In- 

 volucre of the staminate heads mostly hemispheric or saucer-shaped, 5-12-lobed, open, many- 

 flowered; receptacle nearly flat, naked or with filiform chaff; corolla funnelform, 5-toothed; 

 anthers scarcely coherent, mucronate-tipped; style undivided, penicillate at the summit. 

 [The ancient classical name. ] 



About 12 species, mostly natives of America. Besides the following-, some 5 others occur in 

 the southern and western United States. 



Sterile heads sessile; a lanceolate hispid lobe on inner border of involucre. 

 Sterile heads short-pedicelled, involucre depressed-hemispheric. 



Leaves opposite, palmately 3-5-lobed, or undivided; receptacle naked. 



Leaves opposite and alternate, 1-2-pinnatifid; receptacle chaffy. 



Annual; leaves thin; fruiting involucre spiny. 3. A 



Perennial; leaves thick; fruiting involucre naked or tubercled. 4. A. 



1. A. bidentata. 



2. A. trifida. 



a rtemisiaefolia . 

 psilostachya. 



i. Ambrosia bidentata Michx. 

 L,ance-leaved Ragweed. (Fig. 3591.) 



Ambrosia bidentata Michx. Fl. Bor. Am. 2: 182. 



1803. 



Annual, hirsute, usually much branched, very 

 leafy, i-3 high. Leaves lanceolate, mainly 

 alternate, sessile and somewhat cordate-clasping 

 at the base, acuminate at the apex, 1 -nerved, 

 I/-3' long, 2 // ~4 // wide, usually with 1 or 2 sharp 

 lobes at the base and a few minute sharp teeth 

 above, or the upper ones quite entire, rough and 

 hirsute or ciliate; spikes of staminate heads 

 dense, $'-7' long, their involucres turbinate, 

 bearing a long lanceolate hispid reflexed lobe 

 appearing like a bract on the inner border, 

 their receptacles chaffy; fertile heads solitary, 

 or clustered, oblong, 4-angled, 3 // -4 // long, bear- 

 ing 4 sharp spines. 



Prairies, Illinois to Missouri, south to Louisiana 

 and Texas. July-Sept. 



