Monoecious (rarely dioecious) branching herbs, with alternate or opposite, mostly 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, 1-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, 5-12-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 15 species, mostly natives of America. Besides the following, some 5 others occur in the southern and western United States. Type species: Ambrosia maritima L.

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, 1-2-pinnatihd; receptacle chaffy. Annual; leaves thin: fruiting involucre spiny.

3. A. elatior.

Perennial; leaves thick; fruiting involucre naked or tubercled.

4. A. psilostachya.

1. Ambrosia Bidentata Michx. Lance-Leaved Ragweed

Fig. 4125. Ambrosia Bidentata Michx. Fl. Bor. Am. 2: 182. 1803

Annual, hirsute, usually much branched, very leafy, 1°-3° high. Leaves lanceolate, mainly alternate, sessile and somewhat cordate-clasping at the base, acuminate at the apex, 1-nerved, 1' - 3 long, 2"-4" wide, usually with 1 or 2 sharp lobes at the base and a few minute sharp teeth above, or the upper ones quite entire, rough and hirsute or ciliate; spikes of staminate heads dense, 3'-7' long, their involucres turbinate, bearing a long lanceolate hispid reflexed lobe appearing like a bract on the inner border, their receptacles chaffy; fertile heads solitary, or clustered, oblong, 4-angled, 3"-4" long, bearing 4 sharp spines.

Prairies, Illinois to Missouri, Kansas, Louisiana and Texas. July-Sept.

1 Ambrosia Bidentata Michx Lance Leaved Ragweed 7961 Ambrosia Bidentata Michx Lance Leaved Ragweed 797

2. Ambrosia Trifida L. Horse-Cane. Bitter-Weed. Great Ragweed

Fig. 4126

Ambrosia trifida L. Sp. Pl. 987. 1753. A. integrifolia Muhl.; Willd. Sp. Pl. 4: 375. 1805. Ambrosia trifida integrifolia T. & G. Fl. N. A. 2: 290. 1841.

Annual, scabrous or hispid, or nearly glabrous, branched, 3°-17° high. Leaves all opposite, petioled, 3-nerved, deeply 3-5-lobed, or undivided, the lobes lanceolate or ovate, serrate, acute or acuminate; lower leaves often 1° wide; racemes of sterile heads 3'-10' long, their involucres saucer-shaped, 3-ribbed on the outer side, crenate-margined or truncate, their receptacles naked; fertile heads usually clustered in the axils of the upper bract-like leaves, turbinate to obovoid, 5-7-ribbed, conic-beaked, 3"-4" long, each rib bearing a tubercle near the summit.

In moist soil, Quebec to Florida, west to Manitoba, Nebraska, Colorado and New Mexico. Tall ambrosia. Richweed. Wild hemp. Horse-weed. Buffalo-weed. Hay-fever weed. July-Oct.

3. Ambrosia Elatior L. Ragweed. Roman Wormwood. Hogweed. Wild Tansy

Fig. 4127

Ambrosia elatior L. Sp. Pl. 987. 1753. Ambrosia artemisiaefolia L. Sp. Pl. 988. 1753.

Annual, pubescent, puberulent or hirsute, panicu-lately branched, 1°-6° high. Leaves thin, 1-2-pin-natifid, petioled, 2'-4' long, the upper alternate, the lower mostly opposite, pale or canescent beneath, the lobes oblong or lanceolate, obtuse or acute; uppermost leaves of the branches sometimes linear-lanceolate and entire; racemes of sterile heads very numerous, 1'-6' long, the involucres hemispheric, crenate, the receptacle chaffy; fertile heads obovoid or subglobose, mostly clustered, 1 1/2"-2" long, short-beaked, 4-6-spined near the summit, sparingly pubescent.

In dry soil, often a pernicious weed in cultivated fields, Nova Scotia to Florida, west to British Columbia and Mexico. Bermuda. Introduced into Europe as a weed. Consists of several slightly differing races. Also called bitterweed, stickweed, stammerwort, carrot-weed, black, or tassel-weed, hay-fever weed. July-Oct.

3 Ambrosia Elatior L Ragweed Roman Wormwood Hogwee 7983 Ambrosia Elatior L Ragweed Roman Wormwood Hogwee 799

4. Ambrosia Psilostachya Dc. Western Ragweed

Fig. 4128

Ambrosia psilostachya DC. Prodr. 5: 526. 1836.

Similar to the preceding species, but perennial by long rootstocks, the leaves thick, the pubescence stri-gose or hispid. Stems usually much branched, 2°-6° high, rather stout; leaves 1-2-pinnatifid, 2'-5' long, the lobes acutish; racemes of sterile heads several or numerous, 2'-6' long, the involucres campanulate, the receptacles chaffy; fertile heads mostly solitary, ovoid or obovoid, reticulated, short-pointed, unarmed, or with about 4 short tubercles, pubescent, 11/2"-2" long.

In moist open soil, Illinois to Saskatchewan, Texas, Mexico and California. July-Oct.