Ambrosia. COMPOSITE. 249 



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

 sessile; the involucre turbiuatu 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, ilicnx. Roughi^ll-hirsute anijual, 1 to 3 feet high, fastigiatelv hraiiehed 

 above, very leafy up to the stout (span long) spikes : leaves m.j<tly alternate, lanceolate, 

 commonly with au acute lobe or tooth on each side near the Ijroad 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 strung spines of half the length of the spiue- 

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

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



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

 commonly saucer-shaped or opeu-campanulate, with a several -toothed or truncate 

 border : fertile flowers usually glomerate in axils below. 



# Involucre of sterile heads unilaterally 3-ribbed: no chail 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 aLniust glabrous : leaves all opposite, verv deeplv 

 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 lieads 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 

 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, Jlissouri, Nebraska, &c. 



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

 lanceolate undivided lea\"es, and mostly solitary sterile racemes : spinous tubercles of the 

 fruit less developed. — A. integrifolia, Muhl. in WiUd. 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-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. Lindh. ii. 226. ^4. trijida, var. Texana, 

 Scheele in Linn. xxii. 156. — Low grounds, Texas to New ile.xico and S. AV. Arizona; first 

 coll. by Berlandier. 



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

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

 A clteirnnthifolia, Gray, Bot. 3Iex. Bound. 87, entire and canescent), mostly 1-3-pinnatitid or 

 dissected. 



A. artemisigefolia, L. (I?om.\n- AA'ormwood, Ragweed, Bi-fxER "Weed.) Annual, 

 variously pubescent or hirsute, 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 with 4 to 6 short acute teeth 

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

 & Gray, Fl. ii. 291. A. artemisicefolia & A. elatior, L. Spec. 987, 988. ^1. ahsyrdhifulia & 

 A. paniculata, Michx. Fl. ii. 183. A. heterophijlla, 31uhl. in "Willd. Spec. iv. 378. Iia mono- 

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

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



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

 men from " Piucky ^fountains," described as hai ing pinnatifid leaves, and conglomerate fer- 

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



