1150 AMBKOSIACEAE 



3. AMBROSIA L. 



Annual or perennial often woody caulescent herbs, with monoecious or rarely dioecious 

 flowers. Leaves opposite or alternate : blades lobed or variously divided or rarely merely 

 toothed. Heads not showy, the staminate in terminal spikes or racemes, the pistillate soli- 

 tary or clustered below the staminate. Staminate involucres saucer-like to hemispheric, 

 with the 5-12 partially united bracts or wanting : receptacle nearly flat, naked or with 

 filiform chaff : corolla funnelform ; lobes 5 : anthers mucronate at the apex : style sim- 

 ple, often brush-like at the tip. Pistillate involucres turbinate to subglobose, 1-flowered, 

 with several spines or tubercles in one row : corolla wanting : stigmas filiform. Achenes 

 ovoid to obovoid. Pappus wanting. The plants flower in summer and fall. RAGWEED. 



Staminate involucres wanting. 1. A. bidentata. 



Staminate involucres peduncled. 



Leaves all opposite : blades palmately 3-5-lobed or merely toothed : 'receptacle 



naked. 



Petioles of the upper leaves margined : fruit 8-10 mm. long. 2. A. trifida. 



Petioles not margined : fruit 4-6 mm. long. 3. A. aptera. 



Leaves both alternate and opposite; blades 1-2-pinnatifid : receptacle chaffy. 



Annual plant : fruit with 4 tubercles or spine-like teeth. 4. A. artemisiifolia. 



Perennial plant : fruit with 4 tubercles, or these obsolete. 



Stems branched at the base, the branches spreading : upper leaf-blades 



2-pinnatifid. 5. A. hispida. 



Stems erect : upper leaf-blades 1-pinnatifid. 6. A. psilostachya. 



1. Ambrosia bidentata L. Annual, rough hirsute. Stems erect, 3-9 dm. tall, 

 branching, rough : leaves usually alternate ; blades lanceolate to oblong, 2-8 cm. long, 

 acute, usually with an acute tooth-like lobe on either side near the base, otherwise entire or 

 sometimes sparingly toothed : staminate heads numerous, often crowded ; involucres 3-3.5 

 mm. broad, with a conspicuous, lanceolate reflexed lobe on the inner side : pistillate heads 

 solitary or clustered : fruit nearly oblong, 6-8 mm. long, 4-angled, with 4 spines at the 

 base of the beak. 



On prairies, Illinois to Missouri, Louisiana and Texas. 



2. Ambrosia trifida L. Annual, hirsute to hispid. Stems erect, 1-5 m. tall : 

 leaves opposite ; blades deeply 3-5-lobed or sometimes merely toothed, 1-3 dm. long, lobes 

 acute or acuminate, serrate ; petioles shorter than the blades, margined : racemes of stami- 

 nate heads 5-15 cm. long ; involucres saucer-shaped, 3-ridged on the outer side, crenate : 

 pistillate heads clustered in the axils of the leaf-like bracts ; involucres turbinate or obovoid, 

 5-7-ridged : fruit pubescent, 8-10 mm. long, short-beaked, each rib terminating in a 

 tubercle. 



In moist soil or waste places, Quebec to the Northwest Territory, Florida, Arkansas and Colorado. 



3. Ambrosia aptera DC. Annual, scabro-pubescent. Stems erect, 1-4 m. tall, 

 branching, especially above : leaves opposite ; blades merely toothed, 3-5-lobed, 1-3 dm. 

 long ; lobes acuminate, serrate, the middle one often 3-5-lobed ; petioles shorter than the 

 blades, not margined : racemes of staminate heads slender, commonly panicled ; involucres 

 often sparingly pubescent on the outer side : pistillate heads clustered in the axils of leaf- 

 like bracts ; involucres 4-8-ridged : fruit nearly glabrous, 4-6 mm. long, stout-beaked, 4-6 

 of the ridges ending in tubercles or these nearly obsolete. 



In low grounds, Texas to Arizona and adjacent Mexico. 



4. Ambrosia artemisiifolia L. Annual, more or less hirsute. Stems erect, 3-18 

 dm. tall, branched : leaves opposite or alternate ; blades bipinnatifid or pinnately parted, 

 with pinnatifid segments, or the upper leaves sometimes entire or merely toothed : racemes 

 of staminate heads numerous, 2.5-15 cm. long; involucres 3-4 mm. broad, crenate; re- 

 ceptacle chaffy : pistillate heads clustered : fruit sparingly pubescent, subglobose or obovoid, 

 3-3.5 mm. long, short-beaked, with 4-6 spines or spine-like teeth near the top. 



In dry soil or waste places, Nova Scotia to British Columbia, Florida and Mexico. 



5. Ambrosia hispida Pursh. Perennial, hirsute to hispid. Stems branched at the 

 base, the branches spreading, 2-8 dm. long, sometimes simple, often copiously leafy : 

 leaves opposite ; blades 4-12 cm. long, 2-3-pinnatifid, or pinnately divided into short and 

 small segments ; petioles much shorter than the blades : racemes of staminate flowers usu- 

 ally elongated, solitary or more rarely panicled ; involucres 4-5 mm. broad, rigidly or 

 spiny-pubescent: pistillate heads clustered : fruit 2.5-3 mm. long, with a short beak and 

 usually 4 tubercles. 



On sandy sea beaches, Florida and the West Indies. 



6. Ambrosia psilostachya DC. Perennial, both strigose and hirsute. Stems erect, 

 5-20 dm. tall, usually branching, rough : leaves numerous ; blades 5-15 cm. long or shorter 



