CAEDUACEAE. 283 



of the pistillate heads distinct or sometimes united and accrescent into a 

 bur. Fruit-produciug flowers apetalous or with much reduced corollas. 

 Achenes subtended by or enclosed in an involucre. Pappus wanting or 

 obsolete. 



Bracts of the staminate involucres distinct : pistillate involucres bur-lilte. 



1. Xanthidm. 

 Bracts of the staminate involucres united : pistillate involucres not 



bur-like. 2. Ambrosia. 



1. XANTHIUM [Tourn.] L. Coarse, sometimes spiny herbs. Leaf- 

 blades toothed or lobed. Mature pistillate involucre copiously and evenly 

 spiny. — Sum. and fall. — Clotbur. Burweed. Cocklebur. 



Leaf-blades much longer than broad : leaf-bases accompanied by 3-pronged spines : 

 fruits usually 1-beaked or beakless. 1. X. spinosum. 



Leaf-blades as broad as long or but little longer : leaf-bases 



without spines: fruits usually 2-beaked. 2. X. americaniim. 



1. X. spinosum L. Plants spiny, 2-12 dm. tall: leaf -lobes lanceolate, mostly 

 3-lobed, white- or pale-pubescent beneath: fruits 10-13 mm. long. — S. Eare, 

 in waste grounds. Nat. of trop. Am. 



2. X. americanum Walt. Plants glabrate or finely pubescent, 2-13 dm. tall: 

 leaf-blades ovate to deltoid, 5-30 cm. long, rather coarsely toothed and more or 

 less distinctly lobed: fruits oblong, 15-20 mm. long, thickly beset with slender 

 hooked spines. — Common, in waste places and on roadsides. 



2. AMBROSIA [Tourn.] L. Weed-like herbs, ours annual. Leaf -blades 

 toothed, lobed, or divided. Mature pistillate involucre tuberculate or spiny 

 near the top. — Sum. and fall. — Eagweed. 



Leaves opposite ; blades palmately 3-5-lobed or merely toothed : receptacle naked. 



1. A. triflda. 

 Leaves alternate and opposite ; blades 1-2-pinnatifid : recep- 

 tacle chaffy. 2. A. clatior. 



1. A. trifida L. Stems hirsute or hispid, 8-15 dm. tall: leaf-blades deeply 

 3-5-lobed or sometimes merely toothed, 8-30 cm. long: pistillate heads clus- 

 tered at the base of leaf -like bracts: fruits turbinate or obovoid, 8-10 mm. 

 long, with several tubercles at the base of the beak, — Common, on roadsides 

 and in moist soil. — Great-ragweed. Horse-cane. 



2. A. elatior L. Stems hirsute, 3-18 dm. tall or diffuse: leaf -blades mainly 

 bipiunatifid or pinnately parted, 3.5-12 cm. long: fruits 3-3.5 mm. long. — 

 Common, in fields and waste places. — Eagweed. Eoman-wormwood. Hog- 

 weed. 



Family 2. CARDUACEAE. Thistle Family. 



Herbs, or rarely shrubs or trees. Flowers borne in heads, the mar- 

 ginal ones commonly dilSering from the central ones in having a 1-sided 

 more or less elongate corolla-limb. Pappus usually present, often con- 

 spicuous. 



Stigmatic lines at the base of the stigma or below the middle. 



Stigmas filiform or subulate, hispidulous. Tribe I. Vernonieae. 



Stigmas more or less clavate, papillose-puberulent. Tribe II. Eupatokieae. 

 Stigmatic lines extending to the tip of the stigma or 

 to the appendages. 

 Anthers without elongate appendages at the tip. 



Anther-sacs tailed at the base. Tribe IV. Inuleae. 



Anther-sacs not tailed at the base. 

 Receptacle naked. 



Bracts of the involucre well imbricate. 



