Ambrosia. COMPOSITE. 249 



§ 1. Cercomeris, Torr. & Gray, 1, c. Sterile heads densely spicate, closely 

 sessile ; the involucre turbinate and half-truncate, the inner margin bearing a 

 large lanceolate-acuminate hispid lobe, which by the deflexion of the head is 

 strqngly recurved and partly covers the orifice of the involucre, the bractless 

 spike thus appearing as if retrorsely bracteate ; fertile heads commonly solitary 

 in axils below : leaves closely sessile by partly clasping base. 



A. bidentata, Micnx. Roughish-hirsute animal, 1 to 3 feet high, fastigiately hrane-hed 

 above, very leafy up to the stout (span long) spikes : leaves mostly alternate, lanceolate, 

 commonly with au acute lobe or tooth on each side near the broad base, thence tapering 

 gradually to a point, usually entire : fertile involucre in fruit oblong, somewhat prismatic, 

 the 4 strong angles or ribs terminating in acute strong spines of half the length of the spine- 

 like beak: sterile heads about 10-flowered. — Fl. ii. 182; Tursh, Fl. ii. 581; Torr. & Gray, 

 Fl. ii. 292. — Prairies and alluvial ground, Illinois and Missouri to Te^as. (Adj. Mex.) 



§ 2. Ambrosia ^iroper. Sterile heads racemose or spicate : sterile involucre 

 commonly saucer-shaped or open-campaniilate, with a several -toothed or truncate 

 border : fertile flowers usually glomerate in axils below. 



* Involucre of sterile heads unilaterally 3-ribbed : no chaff on the receptacle : leaves palmately 

 cleft, ample, petioled. 



A. trifida, L. Tall and stout annual, 3 to 12 feet high, or even higher, roughish-hispidu- 

 lous, or partly hispid or hirsute, sometimes almost glabrous : leaves all opposite, very deeply 

 3-lobed or the lower 5-lobed ; the lobes ovate-lauceolate, acuminate, serrate (in the larger 

 leaves a span or more in length); petioles of the upper commonly wing-margined : sterile 

 racemes long and dense : fertile heads clustei-ed and as if involucrate by short bracts : fruit 

 (matured fertile involucre) very thick and indurated, 4 or 5 lines long, obovoid-turbinate or 

 obpyramidal, with 5 or sometimes 6 or 7 strong ribs or angles terminating above in spinous 

 tubercles around the base of the conical beak. — Spec. ii. 987 (Moris. Hist. iii. sect. 6, t. 1, 

 f. 4) ; Michx. 1. c. ; DC. Trodr. v. 527 ; Torr. & Gray, 1. c — Moist alluvial banks of streams, 

 Canada and Saskatchewan to Florida, Missouri, Nebraska, &c. 



Var. integrifolia, Torr. & Gray, 1. c. A depauperate form, with oblong or oval- 

 lanceolate undivided leaves, and mostly solitary sterile racemes : spinous tubercles of the 

 fruit less developed. — A. integrifolia, Muhl. in Willd. Spec. iv. 375. — Xew York to Illinois 

 and Virginia. 



A. aptera, DC. Very like the preceding, equally tall : petioles not margined ; larger leaves 

 commonly 5-lol)ed, and the middle lobe often 3-cleft : sterile racemes more numerous and 

 paniculate : fruit smaller, 2 or 3 lines long, more obovoid, 4-8-ribbed, and with 4 to 6 short 

 or obsolete tul)ercles. — Prodr. v. 527; Gray, PI. Lindh. ii. 226. A. tr!jida,\a.v. Texana, 

 Scheele in Linn. xxii. 156. — Low grounds, Texas to New Mexico and S. W. Arizona; first 

 coll. by Berlandier. 



* * Involucre of sterile heads not costate, indistinctly radiate-veined : receptacle with some fili- 

 form or sometimes more dilated chaff: leaves opposite and alternate (in the adjacent Mexican 

 A cheiranthi/ulia, Gray, Bot. Mex. Bound. 87, entire and canescent), mostly 1-3-pinnatifid or 

 dissected. 



A. artemisiaefolia, L. (Koman Wormwood, Eagweed, Bitter Weed.) Annual, 

 variously puljescent or liirsute, paniculately branched, a foot or two high, sometimes taller : 

 leaves thinnish, bipinnatifid or pinnately parted with the divisions irregularly pinnatifid or 

 sometimes nearly entire, on the flowering branches often undivided : sterile heads more or 

 less pedicelled : fruit not 2 lines long, short-beaked, armed Avith 4 to 6 short acute teeth 

 or spines. (Varies much, occasionally the sterile inflorescence abnormally fertile.) — Torr. 

 & Gray, Fl. ii. 291. A. artemisiafolia & A. elatior, L. Spec. 987, 988. A. absi/nfhifolia & 

 A. panicnlata, Michx. Fl. ii. 183. A. heterophi/lla, Muhl. in Willd. Spec. iv. 378. Ira mono- 

 phi/lla, Walt. Car. 232. — Dry ground, a weed of cultivated and waste groumls, Nova Scotia 

 to Saskatchewan, Texas, California, and Washington Terr. (W. Ind. & Mex. to Brazil.) 



A. longistylis, Nutt. Trans. Am. Phil. Soc. 1. c. 344, — known only from Nuttall's speci- 

 men from " Rocky Mountains," described as having pinnatifid leaves, and conglomerate fer- 

 tile flowers with styles about an inch long, — needs verification. 



