i)(i AMBROSIACEAE. 



* * Stiimuiiiic and pixtillnie finwerx in xeparaU: hcfirfx. 



Amljrosia. Involucral b~acis of the staminate heads united. 



Xanthium. Involucral bracts or the staminate heads separate: involucre of the pistil- 

 late head* closed, covered with hooked spines. 



IVA L. Herbaceous or shrubby plants, with opposite or alternate leaves, 

 and small nodding- greenish-white discoid heads. Fertile and sterile flowers 

 in the same head. Anthers nearly separate. Involucral scales 3-9, roundish. 

 Receptacle small, chaffy. Achenes obovoid or lenticular. 



I. xanthiifolia (Fresen.) Kutt. .~\rnrsti Elder. Annual, stem tall, 4-10 feet 

 high, rougbish; leaves mostly opposite, petioled, ovate, rhombic or cordate, 

 lobed, doubly serrate or cut toothed, hoary: flowers in terminal or axillary 

 panieled spikes, bracts not obvious; fertile flowers fl. corolla rudimentary 

 or wanting. Waste grounds; July-September; common in the western coun- 

 ties but less frequent eastward. 



AMBF^OSIA L. Annual or perennial herbs, with opposite or alternate 

 lobed or pinnatifid leaves, and inconspicuous g'reenish flowers in spike-like 

 racemes. Flowers monoecious; fertile 1-3 together, axillary, near the base 

 of the raceme; the involucre oblong, closed, achene-like; pistil with 2 long 

 protruding' styles; stamens wanting; sterile flowers many, surrounded by a 

 depressed hemispherical cup formed from united scales. Anthers nearly 

 separate. Pappus wanting. Achenes ovoid. 



* Leaven (ippnaUe, pnlmtitely lobed. i>r undivMcd; receptacle linked. 



A. trifida L. Great Riuj weed. Annual; stem stout, 3-12 feet high, hairy: 

 leaves opposite, petioled, hairy, deeply 3-lobecl, lobes oval, long-pointed, ser- 

 rate; petioles margined; fruit obovate, 5-0 ribbed: and tubercled. Low 

 grounds; July-September; common. The variety integrifooa. T. & G., a 

 smaller form, has ovate undivided leaves. This and intermediate forms are 

 frequently found. 



* * Leaves opposite and alternate, plnnatijiil; receptacle cliaffy. 



A. artemisiaefolia L. Annual; stem 1-3 feet high, much branched, hairy 

 or rough-pubescent; leaves opposite or alternate, thin, mostly twice pinnati- 

 fid, smooth above, hoary beneath; petioles eiliate; fruit obovoid or globular, 

 armed with <i short teeth. Fields and waste places; July-September; com- 

 mon. 



A. psilostachya DC. Perennial with slender running rootstocks; stem 2- 

 •t feet high, paniculately branched, whitish, hispid; leaves thickish, mostly 

 once pinnatifid; fruit obovoid, tubercles small or wanting. Low grounds and 

 prairies; July-September; common in western Iowa, rare eastward; Fre- 

 mont, Decatur, and Johnson counties; reported from Pottawattamie, Story. 

 Fayette, and Muscatine counties. 



XANTHIUM L. Annuals, with alternate petioled leaves, and monoecious 

 heads of flowers. Fertile flowers in clusters, the sterile in racemes above. 

 Sterile involucres of separate scales: receptacle eylindric. Fertile involucre 

 closed, coriaceous, covered with hooked prickles, 2-cellcd, 2-flowered. Pappus 

 wanting. Corolla thread-form. Anthers nearly separate. Achenes oblong, 

 flat. 



X. canadense Mill. Cncklcbur. Stem 1-3 feet high, frequently brown 

 spotted; leaves ovate or cordate, 3-nerved, dentate, frequently lobed: fruit 

 about 1-inch long, densely prickly, beaks hooked or incurved. Fields and 

 waste places: July-September; common. 



