16 NORTH AMERICAN FLORA [VOLUME 33 
Leaves finely strigoSe, soft, scarcely glandular, 
not strongly veined. 9. A. artemisitfolia. 
Leaves firm, scabrous and glandular-granuli- 
ferous or hirsutulous above, hirsutulous or 
strigose beneath, strongly veined. 
Leaves scabrous and glandular-granuliferous 
above, green; staminate heads 4-5 mm. 
broad, hirsute. 10. A. media. 
Leaves hirsutulous on both sides, dark-green; 
staminate heads 3—3.5 mm. broad. 11. A. Rugelit. 
Staminate involucre, stem, and leaves hispidulous, the 
hairs with strongly pustulate bases. 12. A. longistylis. 
Leaves subsessile, merely pinnatifid; fruit unarmed or with small 
tubercles; plant perennial, with a creeping rootstock (or 
rarely a tap-root). 
Stem and inflorescence not conspicuously hirsute. 
Hairs on the leaves and especially on the staminate involu- 
cres very short, with pustulate bases; leaf-segments linear- 
lanceolate, acute or acuminate; fruit with 5 or 6 short tu- 
bercles. 13. A. psilostachya, 
Hairs not with pustulate bases, longer; leaf-segments broader, 
those of the lower leaves obtuse; fruit mostly unarmed, 
rarely with | or 2 blunt tubercles. 14. A. coronopifolia. 
Stem conspicuously hirsute with spreading hairs. 15. A. californica. 
Leaves canescent; all perennials. 
Stems low, 1-2 dm. high; fruit unarmed. 16. A. pumila. 
Stem elongate; fruit spiny. : : ; 
Stem decumbent or prostrate, rooting; lower leaves 3-pinnatifid 
with short minute divisions; staminate heads 4-5 mm. 
broad. 17. A. hispida. 
Stem erect; lower leaves 3—pinnatifid with ampler segments; 
staminate heads about 3 mm. broad. 18. A. velutina. 
Leaves entire or 3—5—cleft. ; f 
Plants annual, tall, green; leaves at least toothed; staminate involucre 
with 3 ribs. 
Petioles more or less winged; fruit slightly if at all pitted between 
the ridges, which end in short conic spines. 19. A. trifida. 
Petioles wingless; fruit pitted between the ridges, with small tuber- 
cles or unarmed. 20. A. aptera. 
Plants perennial, low, canescent; leaves entire-margined; involucre 
without ribs. 21. A. cheiranthifolia. | 
1. Ambrosia bidentata Michx. Fl. Bor. Am. 2: 182. 1803. 
An annual; stem 3-10 dm. high, rough-hirsute, branched; leaves mostly alternate, sessile, 
lanceolate, acuminate, 3-5 cm. long, often with one lanceolate lobe or tooth on each side, hir- 
sute on both sides, the hairs with pustulate bases; staminate heads numerous, sessile, in dense 
terminal spikes; involucre very oblique, glandular-granuliferous, turbinate, 2 mm. broad, the 
upper margin produced into a lanceolate, hispid incurved lobe; corolla glabrous; pistillate 
heads 1 or 2 in the axils of the leaves; fruit elongate, obovoid, 4-angled, hirsute; body 6-7 mm. 
long; beak subulate, fully 2 mm. long; spines very sharp, subulate, directed forward, 1 mm. 
long or more. 
TYPE LOCALITY: In the Illinois region. 
DISTRIBUTION: Prairies, Illinois and Kentucky to Louisiana, Texas, and Nebraska. 
ILLUSTRATIONS: Britt. & Brown, Ill. Fl. f. 3591; ed. 2, f. 4125. 
Ambrosia bidentata > trifida A. Gray, Bot. Gaz. 11: 338. 1886. This has the leaf-form and 
stout habit of A. trifida, the small heads and pubescence of A. bidentata. ‘The prolongation of one 
lobe of the involucre is very small or none. Open prairies, St. Clair, Illinois, 1886, Eggert. 
2. Ambrosia tenuifolia Spreng. Syst. 3: 851. 1826. 
An annual; stem 3-6 dm. high, terete, striate, hirsute; leaves bipinnatifid, hirsute-canes- 
cent with ascending hairs; petioles short; blades ovate or lance-ovate in outline; divisions 
narrowly linear, acute; staminate heads numerous in terminal racemes; involucre broadly 
obconic, hispidulous, crenate, 2.5 mm. broad; paleae of the receptacle filiform, as long as the 
corollas; corolla puberulent; pistillate heads in the upper axils, mostly solitary; body 2-2.5 
mm. long, obovoid, hispidulous; beak more than 1 mm. long; tubercles 4 or 5, conic, more or 
less spreading, 0.5 mm. long. 
TYPE LOCALITY: Montevideo, Uruguay. 
DIstRIBUTION: Adventive or naturalized in I,ouisiana and Porto Rico, and in France; native of 
Argentina and Uruguay. 
