Ambrosia. COMPOSITiE. 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 

 strongly 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, Miciix. Eoughisli-hirsute annual, 1 to 3 feet high, fastigiately hranched 

 ahove, 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 lengtli of the spine- 

 like beak: sterile heads about 10-flowered. — Fl. ii. 182; Pursh, ¥1. ii. 581; Torr. & Gra}', 

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



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

 commonl}^ saucer-shaped or open-campanulate, witlx a several -toothed or truncate 

 border : fertile flowers usually glomerate in axils below. 



* Iiivdlucre of sterile heads unilatcrMlly 3-tibbcd: no chaff on the receptacle: leaves pabnately 

 cleft, ample, petioled. 



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

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

 3-lobed or the lower 5-lobed ; ihe lol)es 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 sliort 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 

 tul)crcles 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, Toitn. & 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, INIuhl. 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 

 coirimonly 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 ol)S(>lete tubercles. — Prodr. v. 527; Gray, PI. Lindh. ii. 226. ^1. trijida, var. Texana, 

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

 coll. by Derlandier. 



* * Involucre of sterile bonds 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 folia, Gray, Bot. Mox. Bound. 87, entire and cancscent), mostly 1-3-pinnatifid or 

 dissected. 



A. artemisiaefolia, L. (Koman Wormwood, Ragweed, Bitter Wef.d.) Annual, 

 variously pubescent or hirsute, ])aiiiculatcly branched, a foot or two high, sometimes taller : 

 leaves thinnish, bipinnatifid or pinnatcly 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 lijies long, short-beaked, armed witii 4 to 6 short acute teeth 

 or spines. (Varies much, occasionally the sterile inflorescence a])normally fertile.) — Torr. 

 & Gray, Fl. ii. 291. A. artemisiafoUa & A. elatior, L. Spec. 987, 988. A. ahsijnthi folia & 

 A. panicidcita, JMichx. Fl. ii. 183. A. hctcrophi/lln, Muhl. in Willd. Spec. iv. 378. Ii'a mono- 

 phi/lla, Walt. Car. 232. — Dry ground, a weed of cultivated and waste grounfLs, Nova Scoti:i 

 to Saskatchewan, Texas, Csilifornia, and Washington Terr. (W. Ind. & I\Iex. to Brazjl.) 



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

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

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



