398 Compositaceae 
AMBROSIA. RAGWEED 
Branched. Leaves alternate or opposite, entire to pinnately- or palmately-divided. 
Heads small, green, monoicous (ours). Staminate heads in spikes or racemes, in the 
upper axils and terminal; involucre mostly hemispheric or saucer-shaped, 5—12-lobed, 
open, many-flowered; receptacle nearly flat: corolla funnelform, 5-toothed. Pistillate 
heads solitary or clustered, in the upper axils; involucre globose-ovoid or top-shaped, closed, 
1-flowered, usually with 4—8 tubercles or spines: corolla none: stamens none. Akenes 
ovoid or obovoid. Pappus none. (Gk. ambrosia—food for the gods; quite inappropriate 
for ours. ) 
A. Leaves all opposite, entire to coarsely and palmately 3—5-lobed or -cleft; receptacle 
naked ; involucre of the staminate heads 3—4-ribbed; annual. E. 
A. trifida L. (Great Ragweed) 
AA. Some of the leaves often alternate, entire to |—3-pinnatifid; receptacle chaffy ; 
involucre of the staminate heads not ribbed. 
B. Leaves of the upper 1/3 of the plant mostly entire, ovate to lanceolate, acute, 
3-veined. E. A. artemisiaefolia diversifolia Pip. 
BB. Leaves pinnately lobed or pinnatifid to the inflorescence. 
C. Annual; leaves thin, 2-pinnatifid; fruit with acute teeth. E. 
A. artemisiaefolia L. (Bitter-weed) 
CC. Perennial; leaves thick, |-pinnatifid; fruit with blunt teeth or unarmed. E. 
A. psilostachya DC. (Western Ragweed) 
FRANSERIA (Gaertneria) SAND-BUR 
Sometimes woody at base, hispid or tomentose, branched. Leaves mostly alternate, 
serrate to much pinnatifid. Heads small, greenish, monoicous. Staminate heads in ter- 
minal spikes or racemes, sessile or short-peduncled; involucre broadly hemispheric, open, 
5—12-lobed: corolla 5-lobed. Pistillate heads solitary or clustered, in the upper axils; 
involucre ovoid or globose, closed, 1—4-celled, 1—4-beaked, 1—4-flowered, with sev- 
eral rows of spines and thus forming a bur in fruit: corolla none or rudimentary: stamens 
none. Akenes obovoid, thick. Pappus none. (Honor of A. Franseri, a Spanish bot- 
anist. ) 
A. Seashore plants; perennial, prostrate; spines of the burs thick but flat. 
B. Leaves cuneate-obovate, serrate or laciniate. W. F, chamissonis Less. 
BB. Leaves twice or thrice pinnately parted. W. F. bipinnatifida Nutt. 
- Not seashore plants; annual, erect or spreading; spines of the burs thin and flat. 
F. acanthicarpa Cov. 
XANTHIUM COCKLEBUR 
Annual, branched, coarse or rough or spiny. Leaves alternate, lobed or dentate, 
pinnately veined. Heads rayless, small, greenish, monoicous. Staminate heads in head- 
like clusters at the ends of the branches; involucre-bracts in 13 series: receptacle cy- 
lindric: corolla tubular, 5-toothed: filaments monadelphous. Pistillate heads in the leaf- 
axils, bur-like, ovoid or oblong; involucre the spiny covering enclosing the 2 flowers and 
later the akenes, 1|—2-beaked, 2-celled; spines hooked: corolla slender, thread-like, ap- 
parently none. Akenes 2, oblong, flat. Pappus none. (Gk. xanthos—yellow; the 
Greeks secured a yellow hair-dye from one of them. ) 
A. Leaves attenuate at both ends, with a 3-forked spine where petiole joins stems. E. 
X. spinosum L. (Spiny Cocklebur) 
