290 COMPOSITE. Ambrosia. 



cles or horns in a single series, 1-flowered. Corolla none. Stamens none. 

 Branches of the style filiform, elongated. Achenium ovoid or obovoid. — 

 Herbaceous or rarely sufTrutescent weed-like plants (chiefly American), with 

 opposite or alternate mostly lobed leaves. Branches terminating in racemes 

 or spikes, which are simple and sterile above. Sterile heads ebracteate. 

 Fertile heads clustered at the base of the sterile spikes and bracteate, or ses- 

 sile in the axils of the upper leaves. Corolla whitish. 



Lessing, De Caiidolle &;c. distinguish Franseria from Ambrosia by the chaffy re- 

 ceptacle of the sterile flowers, the presence of an abortive style, and the echinate fer- 

 tile involucre as in Xanthium, which moreover in Franseria proper (§ Xanthiopsis, 

 DC.^ is said to be 2— 4-celled: but that genus must depend upon the latter characters 

 alone (at least so far as the section Centrolsena is concerned, of which only we have 

 seen specimens); for all the species of Ambrosia we have examined have a sterile 

 style, and all but A. trifida and A. artemisisefolia exhibit the same chaffy receptacle 

 as in Franseria. In the Mexican A. fruticosa tvjr. canesceas, Bcnth.pl. Harhc, the 

 chaffy scales ai'e conspicuously dilated at the summit. No one appears to have re- 

 marked tlie setiform inflexed appendage of the anthei-s in these jilants. — A. bidentata, 

 Michx. might perhaps be properly separated from the genus. 



§ 1. Sterile heads more or less pedicellate; the involucre regular, with Oie 

 margin crenate or nearly entire : fruit {fertile involucre) subglobose or obo- 

 void. E UAiMBROSIA. 



* Involucre of the sterile heads 3-ribbed : receptade naked : fertile heads glomerate ai, 

 the base of the sterile spiciform racemes; the clusters someiohat involucraie ; leaves op- 

 posite, ujulivided or pahnately 2-b-hbed. 



1. A. trifida (Linn.) : stem tall and stout, hairy, rough ; leaves scabrous 

 and hairy, deeply 3-lobed ; the lobes oval-lanceolate, acuminate, serrate; 

 the lower leaves often 5-lobed ; petioles narrowly winged, ciliate ; racemes 

 often paniculate; fruit (fertile involucre) turbinate-obovoid, with a short 

 conical pointed apex, 6-ribbed, the ribs terminating in as many cristate tuber- 

 cles.— Li?(«. / sijcc. 2. p. 987 ; Michx. ! fl. 2. p.\H3; Willd. ! spec. 4. p. 

 375 ; Pursh, fl. 2. p. 581 ; Ell. ! sTc. 2. p. 476 ; DC. ! prodr. 5. p. 527 ; 

 Darlingt.fi. Cest. p. 479 ; Hook.fl. Bor.-Am. 2. p. 26. A. Virginiana &:c., 

 Moris, hist. 3. sect. 6. t. 1, /". 4. 



/3. integrifolia : leaves ovate or oval-oblong, acuminate, sometimes the up- 

 per, and often the lower ones 3-lobed. — A. integrifolia, Muhl. in Willd. ! I. 

 c. ; Pursh, fl. 2. p. 580 ; DC. I. c. " 



Low grounds and along streams, Canada ! to Georgia ! and west to Louisi- 

 ana and Arkansas ! Aug.-Sept. — (I) Stem 4-sided, 4-9 feet high (some- 

 times 20 feet or more ! according to Dr. Boykin), branching above. Leaves 

 large. Involucre of the sterile heads with 3 .strong dark purple radiate ribs, 

 occupying the side farthest from the axis of the raceme. Corolla greenish- 

 white, cyathiform-campanulate, scarcely attenuate at the base. Fruit 3-4 

 lines long. — A coarse unsightly plant. 



* * hwolucre of the sterile heads not ribbed: receptacle itsualli/ chaffy ; the chaff filiform , 

 or rarely dilated at the summit, rather shorter than the corolla: fertile heads solitary 

 or glomerate towards the base of the sterile spikes or racemes, or in the axils of the up- 

 per leaves ; the clusters somewhat involucrate : leaves 1-2-pinnately cleft or parted, al- 

 ternate, tlie lower opposite. 



2. A. crithmifolia (DC.) : suffruticose and prostrate at the base; the flow- 



