Vol. III.] 



RAGWEED FAMILY. 



295 



2. Ambrosia trifida L. Horse-cane. Bit- 

 ter-weed. Great Ragweed. (Fig. 3592.) 



Ambrosia Irifida L. Sp. PI. 987. 1753. 



Annual, scabrous or hispid, or nearly glabrous, 

 branched, 3°-i7° high. Leaves all opposite, peli- 

 oled, 3-nerved, deeply 3-5-lobed, the lobes lanceo- 

 late or ovate, sharply serrate, acute or acuminate; 

 lower leaves often 1° wide, the upper sometimes 

 ovate and undivided; racemes of sterile heads 3'- 

 10' long, their involucres saucer-shaped, 3-ribbed 

 on the outer side, crenate-margined or truncate, 

 their receptacles naked; fertile heads usually clus- 

 tered in the axils of the upper bract-like leaves, tur- 

 binate to obovoid, 5-7-ribbed, conic-beaked, 3"-4" 

 long, each rib bearing a tubercle near the summit. 



In moist soil, Quebec to Florida, west to the North- 

 west Territory. Nebraska, Colorado and Arkansas. 

 Also called Tall Ambrosia, Richweed, Wild Hemp. 

 July-Oct. 

 Ambrosia trifida integrifolia (Muhl.) T. & G. FI. N. A. 



2: 290. 1S41. 

 A. integii/olia Muhl. ; Willd. Sp. PI. 4: .VS- 1805. 



Leaves ovate or oblong- lanceolate, not lobed; plant 

 usually lower. Frequent with the type. 



3. Ambrosia artemisiaefolia L,. Rag- 

 weed. Roman Wormwood. Hogweed. 

 Wild Tansy. (Fig. 3593-) 



Ambrosia artemisiaefolia L. Sp. PI. 987. 1753. 



Annual, pubescent, puberulent or hirsute, pan- 

 iculately branched, i°-6° high. Leaves thin, i- 

 2-pinnatifid, petioled, 2'-4' long, the upper alter- 

 nate, the lower mostly opposite, pale or canescent 

 beneath, the lobes oblong or lanceolate, obtuse or 

 acute; uppermost leaves of the branches some- 

 times linear-lanceolate and entire; racemes of 

 sterile heads very numerous, i'-6' long, the 

 involucres hemispheric, crenate, the receptacle 

 chaffy; fertile heads obovoid or subglobose, mostly 

 clustered, i^"-2" long, short- beaked, 4-5-spined 

 near the summit, sparingly pubescent. 



In drj' soil, often a pernicious weed in cultivated 

 fields, Nova Scotia to Florida, west to British Colum- 

 bia and Mexico. Also in the West Indies and South 

 America, and introduced into Europe as a weed. 

 Also called Bitterweed, Stickweed, Stammerwort, 

 Carrot-weed. ]uly-0ct. 



4. Ambrosia psilostachya DC. Western 

 Ragweed. (Fig. 3594-) 



Ambrosia psilostachya DC. Prodr. 5: 526. 1836. 



Similar to the preceding species, but perennial, 

 the leaves thick, the pubescence strigose or hispid. 

 Stems usually much branched, 2°-6° high, rather 

 stout; leaves 1-2 pinnatifid, 2'-5' long, the lobes 

 acutish; racemes of sterile heads several or numer- 

 ous, 2'-6' long, the involucres campanulate, the 

 receptacles chaffy; fertile heads mostly solitary, 

 ovoid or obovoid, reticulated, short-pointed, un- 

 armed, or with about 4 short tubercles, pubescent, 

 \Yz"-2" long. 



In moist open soil, Illinois to the Northwest Terri- 

 tory, south to Texas, Mexico and California. July-Oct. 





