910 FLORA. 



series of 5, the outer ovate, the inner obovate or truncate, concave, subtending the 

 usually 5 achenes; corollas of the fertile flowers rudimentary or none; staminate 

 flowers 10-15, their corollas obconic. In moist soil, or waste places. Ont. toN. W. 

 Terr., south to Wis., Neb., N. Mex. and Utah. July-Sept. 



2. AMBROSIA L. 



Monoecious (rarely dioecious) branching herbs, with alternate or opposite, most- 

 ly lobed or divided leaves, and small heads of green flowers, the staminate spicate 

 or racemose, the pistillate solitary or clustered in the upper axils. Involucre of the 

 pistillate heads globose, ovoid or top-shaped, closed, i-flowered, usually armed with 

 4-8 tubercles or spines; corolla none; stamens none; style-branches filiform; 

 achenes ovoid or obovoid ; pappus none. Involucre of the staminate heads mostly 

 hemispheric or saucer-shaped, 5-i2-lobed, open, many-flowered; receptacle nearly 

 flat, naked, or with filiform chaff; corolla funnelform. 5-toothed; anthers scarcely 

 coherent, mucronate-tipped ; style undivided, penicillate at the summit. [The 

 ancient classical name.] About 12 species, mostly natives of America. Besides 

 the following, some 5 others occur in the southern and western U. S. 



Sterile heads sessile ; a lanceolate hispid lobe on inner border of involucre. 



1. A. bidentata. 

 Sterile heads short-pedicelled; involucre depressed-hemispheric. 



Leaves opposite, palmately 3~5-lobed, or undivided ; receptacle naked. 



2. A. trifida. 

 Leaves opposite and alternate, i-2-pinnatifid ; receptacle chaffy. 



Annual ; leaves thin ; fruiting involucre spiny. 3. A. artemisiafolia. 



Perennial ; leaves thick ; fruiting involucre naked or tubercled. 



4. A psilostachya. 



1. Ambrosia bidentata Michx. LANCE-LEAVED RAGWEED. (I. F. f. 3591.) 

 Annual, hirsute, usually much branched, very leafy, 3-9 dm. high. Leaves 

 lanceolate, mainly alternate, sessile and somewhat cordate-clasping at the base, 

 acuminate at the apex, i -nerved, 3-7 cm. long, 4-8 mm. wide, usually with i or 2 

 sharp lobes at the base and a few minute sharp teeth above, or the upper ones 

 quite entire; spikes of staminate heads dense, their receptacles chaffy; fertile heads 

 solitary, or clustered, oblong. 4-angled, 6-8 mm. long, bearing 4 sharp spines. 

 Prairies, 111. to Kans.. La. and Tex. July-Sept. 



2. Ambrosia trifida L. HORSE-CANE. BITTER-WEED. GREAT RAGWEED. 

 (I. F. f. 3592.) Annual, scabrous or hispid, or nearly glabrous, branched, 1-5 m. 

 high. Leaves all opposite, petioled, 3-nerved, deeply 3~5-lobed, the lobes lanceo- 

 late or ovate, sharply serrate, acute or acuminate ; the upper sometimes ovate and 

 undivided; racemes of sterile heads 7-25 cm. long, their involucres saucer-shaped, 

 3-ribbed on the outer side; fertile heads usually clustered in the axils of the upper 

 bract-like leaves, turbinate to obovoid, 5-7-ribbed. conic-beaked, 6-8 mm. long, 

 each rib bearing a tubercle near the summit. In moist soil, Quebec to Fla., the 

 N. W. Terr., Neb., Colo, and Ark. July- Oct. 



Ambrosia trffida integrifolia (Muhl.) T. & G. Leaves ovate or oblong-lanceolate, 

 not lobed; plant usually lower. Frequent with the type. 



3. Ambrosia artemisiaefolia L. RAGWEED. ROMAN WORMWOOD. HOG- 

 WEED. WILD TANSY. (I. F. f. 3593.) Annual, pubescent, puberulent or hir- 

 sute, paniculately branched, 3-18 dm. high. Leaves thin, i-2-pinnatifid, petioled, 

 5-10 cm. long, the upper alternate, the lower mostly opposite, pale or canescent 

 beneath, the lobes oblong or lanceolate, obtuse or acute; racemes of sterile heads 

 very numerous, 2-15 cm. long, the involucres hemispheric, crenate; fertile heads 

 obovoid or subglobose, mostly clustered, 34 mm. long, short-beaked, 4 6-spined 

 near the summit, sparingly pubescent. In dry soil, often a pernicious weed in 

 cultivated fields. N. S. to Fla., west to Br. Col. and Mex. Also in the W. Ind. 

 and S. Am., and introduced into Europe as a weed. July-Oct. 



4. Ambrosia psilostachya DC. WESTERN RAGWEED. (I. F. f. 3594.) 

 Similar to the preceding species, but perennial by running roots, the leaves thick, 

 the pubescence strigose or hispid. Stems usually much branched, 6-12 dm. high, 

 rather stout; leaves 1-2 pinnatifid, 5-12 cm. long, the lobes acutish; racemes of 

 sterile heads several or numerous, 5-15 cm. long, the involucres campanulate, the 



