CAEDUACEAE. 283 



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

 bur. Fruit-producing 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-like. 



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. CLOTBUE. 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. americanum. 



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. Rare, 

 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. RAGWEED. 



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. elatior. 



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 

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

 Common, in fields and waste places. RAGWEED. ROMAN-WORMWOOD. HOG- 

 WEED. 



FAMILY 2. CARDUACEAE. THISTLE FAMILY. 



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

 ginal ones commonly differing 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. VERXONIEAE. 



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

 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. 



