AMBROSIACEAE. 431 



united bracts, the pistillate heads sometimes larger and nut-like or bur-like. 

 Staminate and pistillate flowers in the same, or in separate heads. Re- 

 ceptacle chaffy. Pistillate flowers with no corolla, or this reduced to a 

 short tube or ring; calyx adnate to the 1-celled ovary, its limb none, or a 

 mere border; style 2-cleft. Staminate flowers with a fhinnelform tubular 

 or obconic 4-5-lobed corolla; stamens mostly 5, separate, or their anthers 

 merely comment, not truly syngenesious, with short indexed appendages; 

 ovary rudimentary; summit of the style often hairy or penicillate. Eight 

 genera and about 60 species, mostly natives of America. 



Staminate and pistillate flowers in different heads. 



Bracts of staminate involucres distinct. 1. Xanthtum. 



Bracts of staminate involucres united. -. Ambrosia. 



Staminate and pistillate flowers in the same heads. 3. Im. 



1. XANTHIUM L. Sp. PI. 9S7. 1753. 



Monoecious annual branching coarse rough or spiny herbs, with alternate 

 lobed or dentate leaves, and rather small heads of greenish discoid flowers, the 

 staminate ones capitate-clustered at the ends of the branches, the pistillate 

 axillary. Staminate heads with a short involucre of 1 to 3 series of distinct 

 bracts; receptacle eylindric, chaffy; corollas tubular, 5-toothed; anthers not 

 coherent, mucronate at the apex; filaments monadelphous; style slender, un- 

 divided. Pistillate heads of an ovoid or oblong, closed involucre, covered with 

 hooked bristles, 1-2-beaked, 2-eelled, each cavity containing one obovoid or 

 oblong achene; corolla none; pappus none; style 2-cleft, its branches exserted. 

 [Greek, yellow, from its yielding a yellow hair-dye.] About 25 species, of 

 wide geographic distribution. Type species: Xanthium strumarium Linn. 



1. Xanthium chinense Mill. Gard. Diet. ed. S, no. 4. 1768. 



Xanthium longirostre Wallr. Beitr. Bot. I 2 : 237. 1S44. 



Hispidulous, 1 m. high or less. Leaves broadly ovate-reniform in outline, 

 slender-petioled, 8-15 cm. long and about as wide as long, acute or acuminate 

 at the apex, rather thin, coarsely irregularly toothed and usually 3 5-lobed, 

 scabrous on both sides, the base triangular-cuneate ; staminate heads numer- 

 ous, about 5 mm. in diameter; fruit oblong, its body 2 cm. long or less, 5.5-S 

 mm. thick, hispidulous and glandular or sometimes glabrous; bristles rather 

 slender, 3-4.5 mm. long, hispidulous toward the base; beaks stout, 4-6 mm. 

 long, somewhat incurved. 



Waste places in New Frovidenee : Bermuda: Cuba to Tortola and Martinique: 

 Jamaica; United States and Central America. Reported by Dolley and by Hitchcock 

 as X. strumarium L. West Indian Cocklebur. 



2. AMBROSIA L. Sp. PL 987. 1753. 



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

 mostly lobed or divided leaves, and small heads of green flowers, the Btamini 

 Bpicate or racemose, the pistillate solitary or clustered in the upper axils. 

 Involucre of the pistillate heads globose, ovoid or top-shaped, closed, 1-flow- 

 ered, usually armed with 48 tubercles or spines; corolla none; stamens none; 

 style-branches filiform; achenes ovoi.l or obovoid; pappus none. Involucre of 

 the staminate heads mostly hemispheric or Baucer-shaped, 5 L2-lobed, open, 

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



