lobe much the largest, large lobes lanceolate-elliptical, serrate, both surfaces (and 

 entire herbage) silvery gray-canescent; staminate heads in racemiform arrange- 

 ments; pistillate heads in small clusters (2 to 5) or singly in axils of lower leaves 

 and bracts; staminate involucres stalked, to 5 mm. broad, 5- to 9-lobed, the lobes 

 often somewhat irregular, shallow, the upper surface with fine dark striations along 

 the midveins obscured by pubescence; fruiting involucres to 7 mm. long and 4 mm. 

 broad, each 2-flowered; spines up to 15, scattered, narrowed to slender uncinate 

 tips. Franseria tomentosa Gray. 



Locally abundant in wet or seasonally moist swales and buffalo wallows, and in 

 moist clay soils under cult., higher parts of the Plains Country of Tex. and Okla. 

 (?), summer -early fall; Kan., Neb., Okla., Colo, and Tex. 



2. Ambrosia trifida L. Giant ragweed. Fig. 757. 



Tall erect annual taprooted herb, 1-3 (-5) m. tall; stem angled, striate and 

 scabrous; leaves opposite nearly throughout, scabrous on both sides, with petioles 

 5-15 cm. long, wingless or very narrowly winged; blades of lower leaves orbicular 

 in outline, 1-2 dm. long, usually deeply 5-cleft, the major divisions often 2- to 

 3-lobed, the upper commonly 3-cleft or simple, becoming rhombic or ovate- 

 lanceolate in outline, blades sometimes simple throughout, margin serrate; 

 staminate heads in racemiform arrangements, these often much-branched and 

 paniculate; pistillate heads in small to large clusters in axils of bracteal leaves 

 below the "racemes," subtended by (usually) lobed bracts; staminate involucres 

 saucer-shaped, 3 mm. broad, with 6 to 8 rounded lobes, the 3 outer lobes with 

 dark striations on the upper surface, hispidulous between the ribs, pales of the 

 receptacle rudimentary; fruiting involucres obovoid, about 4 mm. long, reticulate 

 and with 4 to 8 obtuse ridges and as many small or obsolete tuberculate spines, 

 the conic beak 1 mm. long or more. 



Abundant in seasonally moist stream bottoms and overflow areas throughout 

 our area, late summer-fall; throughout the midwest, and cen. U.S. to the Rocky 

 Mts. and from s. Can. to n. Mex., adv. in N.E., Fla. and the far West. 



Our plants are mostly of the form known as var. texana Scheele {A. aptera 

 DC), characterized by relatively small fruiting involucres and exceptionally 

 scabrous indument. In the central and southern United States much of the fall 

 hayfever is correctly attributed to this species. 



3. Ambrosia psilostachya DC. Western ragweed. Fig. 758. 



Erect perennial herb forming extensive colonies from runnerlike roots; stem 

 3-6 (-10 or more) dm. tall, often branched above, striate, hirsutulous with short 

 ascending hairs; leaves subsessile, lanceolate to ovate-lanceolate, pinnatifid with 

 the divisions linear-lanceolate, acute or acuminate, entire or the lower few-toothed 

 or weakly lobed, scabrous-hirsutulous and glandular-granuliferous, the hairs with 

 pustulate bases; inflorescence often much-branched; staminate heads in racemi- 

 form aggregations; pistillate heads in few- to several-headed clusters below; 

 staminate involucre oblique, broadly obconic, about 2.5 mm. broad, crenate at the 

 margins, often with 2 larger teeth at the distal side, hispidulous, the short hairs 

 with conspicuous pustulate bases; body of fruiting involucre about 2.5 mm. long, 

 obovoid, rugose, hirsutulous; beak nearly 1 mm. long, stout; tubercles 4 to 6, 

 short, acute or blunt. A. Lindheimeriana Scheele. 



Along streams, roadsides, uncultivated lands and marshy flats, very abundant 

 on the Coastal Plain and much of the cen. part of Tex., rare westw., late summer- 

 fall; through most of the U.S. and s. Can. except the Rocky Mts. and e,-cen. 

 states, s. to cen. Mex. 



1645 



