Ambrosia. COMPOSITE. 249 



1. CERCOM'EKIS, Torr. & Gray, 1. e. 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 

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

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

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



A. bidentata, MICHX. Eoughish-hirsute annual, 1 to 3 feet high, fastigiately branched 

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

 commonly with an 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-rlowered. Fl. ii. 182; Pursh, Fl. ii. 581 ; Torr. & Gray, 

 Fl. ii. 292. Prairies and alluvial ground, Illinois and Missouri to Texas. (Adj. Mex.) 



2. AMBROSIA proper. Sterile heads racemose or spicate : sterile involucre 

 commonly saucer-shaped or open-campanulate, 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-lanceolate, 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 clustered 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. Prodr. 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, Mulil. in Willd. Spec. iv. 375. New York to Illinois 

 and Virginia. 



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

 commonly 5-lobed, 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 tubercles. Prodr. v. 527; Gray, PI. Liudh. ii. 226. A. Irijida, var. Tc.nuiti, 

 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 cJieiranthifolia, Gray, Bot. Mex. Bound. 87, entire and canescent), mostly 1-3-pinnatifid or 

 dissected. 



A. artemisieefolia, L. ("ROMAN WORMWOOD, UAGWEED, BITTER WEED.) Annual, 

 variously pubescent or hirsute, paniculatcly 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 podicclled: fruit not 2 lines long, short-beaked, armed with 4 to 6 short acute teeth 

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

 & Gray, Fl. ii. 291. A. artemisiafolia & A. clatior, L. Spec. 987, 988. .-1. absyntkifolia & 

 A. paniculata, Michx. Fl. ii. 183. A. hetcrojihi/lla, Muhl. in Willd. Spec. iv. 378. lea mono- 

 phi/Ha, Walt. Car. 232. Dry ground, a weed of cultivated and waste grounds, 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 Nnttall's speci- 

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

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



