384 



AMBROSIACEAE. 



1. XANTHIUM [Tourn.] L. 



Annual coarse monoecious herbs, •uith alternate lobed or toothed leaves, 

 and rather small heads of greenish flowers, the staminate heads clustered, 

 terminal, the pistillate solitary or clustered in the upper axils. Involucre of 

 the pistillate heads closed, 1-2-celled, 1-2 -beaked, (usually 2-beaked) armed 

 with prickles, forming a bur in fruit; pistillate flowers without a corolla, the 

 style deeply 2-cleft, stamens none; achenes obovoid or oblong without pappus. 

 Involucre of the staminate heads short, of 1-3 series of bracts; staminate 

 corollas regular, 5-toothed; filaments monadelphous; style undivided. [Greek, 

 yellow, from its yielding a yellow dye.] About 25 species of wide geographic 

 distribution. Type species: Xanthium strumarium L. 



1. Xanthium longirostre Wallr. West 

 Indian Cocklebur. (Fig. 417.) Stout, 

 3° high or less, the angular stem hispidu- 

 lous. Leaves orbicular-ovate, 4'-6' long, 

 thin, scabrous on both sides, usually 5- 

 lobed, dentate, the lobes short, the base 

 cordate, the petioles 6' long or less; heads 

 short-racemose; bracts linear-lanceolate, 

 hispid; bur ellipsoid, its body 8"-10" 

 long, about 4" thick, glandular-puberulent, 

 rather densely covered with slender bristles 

 about 2" long, which are hispidulous at the 

 base, its beaks 2"-3" long, slightly in- 

 curved, hispidulous. [X. echinatiim of Le- 

 froy and of H. B. Small.] 



Frequent in waste places. Naturalized. 

 Native of the West Indies and Central 

 America. 



2. AMBROSIA [Tourn.] L. 



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. In- 

 volucre of the pistillate heads globose, ovoid or top-shaped, closed, 1-flowered, 

 usually armed with 4-8 tubercles or spines; corolla none; stamens none; style- 

 branches filiform; achenes ovoid or obovoid; pappus none. Involucre of the 

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

 flowered; receptacle nearly flat, naked, or with filiform chaff; corolla funnel- 

 form, 5-toothed; anthers scarcely coherent, mucronate-tipped ; style undivided, 

 penicillate at the summit. [The ancient classical name.] About 15 species, 

 mostly natives of America. Type species: Arribrosia maritima L, 



