Ambrosia. COMPOSITE. 375 



19. AMBROSIA. Tourn. ; DC. prodr. 5. p. 524 ; Endl. gen. 2482. rag-weed. 



[Ambrosia was the food of the gods. The coarse weeds of this genus were hardly entitled to so poetical a name.] 



Sterile heads in racemes or spikes ; the fertile at the base of the racemes, or in the axils of 

 the upper leaves. Sterile Fl. Involucre hemispherical or somewhat turbinate ; the scales 

 few and united. Receptacle naked, or with filiform chaff among the flowers. Anthers 

 tipped with a bristle-like iiiflexed appendage. Ovary none, or only rudimentary : abortive 

 style included, fimbriate or radiate at the summit. Fertile Fl. Involucre one-flowered, 

 closed, pointed, usually armed near the summit with several tubercles or horns in a single 

 series. Corolla and stamens none. Branches of the style filiform. Achenia ovoid. — 

 Herbaceous weeds, destitute of beauty. Leaves mostly lobed. Heads without bracts ; 

 the sterile ones in simple racemes or spikes which terminate the branches : fertile heads 

 bracteate, clustered at the base of the sterile spikes, or sessile in the axils of the upper 

 leaves. _ i 



1. Ambrosia trifida, Linn. Three-lobed Ragweed. 



Hairy and rough ; lerves deeply 3- (or often 5-) lobed ; the lobes oval-lanceolate, acuminate, 

 serrate, the petioles narrowly winged ; sterile heads pedicellate ; tlie involucre regular, with 

 the margin crenate or nearly entire ; fruit (fertile involucre) turbinate-obovoid, 6-ribbed, armed 

 with 6 spines below the summit — Linn. sp. 2. p. 987 ; Michx. Jl. 2. p. 183 ; Pursh, fi. 2. 

 p. 581 ; Ell. sk. 2. p. 476 ; Bigel. fi. Bost. p. 343 ; Beck, hot. p. 209 ; DarUngt. fl. Cest. 

 p. 479 ; DC. prodr. 5. p. 479 ; Hook. fl. Bor.-Am. 2. p. 26 ; Torr. <^ Gr. fl. N. Am. 2. 

 p. 290. 



var. integrifolia : leaves ovate-oblong, acuminate, entire or the lower ones 3-lobed. — Torr. 

 4- Gr. I. c. A. integrifolia, Mulil. in Willd. sp. 2. p. 375 ; Pursh, fl. 2. p. 580 ; DC. I. c. ; 

 Beck, hot. I. c. 



Annual. Stem 4 - 8 feet high, obtusely 4-anglcd, branching above. Leaves 4-6 inches 

 or more in length and nearly as broad, very rough on both sides. Sterile racemes 4-8 inches 

 long, somewhat paniculate ; the heads reflexed. Sterile involucre hairy and marked with 3 

 strong purple ribs on the outside, lobed or crenate. Flowers greenish-white. Receptacle 

 naked. Fruit glomerate at the base of sterile racemes ; when mature, nearly iialf an inch 

 long, with a short conical point, around the base of which are 5 or 6 short erect spines or teetli. 



Along fences and in low moist grounds ; common. August - September. 



