Annuals, biennials, perennials, subshrubs, or shrubs (usually taprooted, perennating bases sometimes woody, sometimes rhizomes), mostly 3–150(–400+) cm. Leaves usually cauline, sometimes basal or basal and cauline; mostly opposite (distal sometimes alternate); petiolate or sessile; blades (palmately or pinnately nerved) deltate to linear or filiform (and intermediate shapes) sometimes 1–3+-pinnately or -ternately lobed, ultimate margins entire or toothed, faces usually hairy (scabrellous, hispidulous to sericeous, strigillose, or tomentose) and/or stipitate-glandular, or glabrous, often gland-dotted. Heads usually disciform, sometimes discoid (then usually unisexual) or radiate (Parthenium, laminae minute in Parthenice), borne singly or in (usually ebracteate, sometimes bracteate) corymbiform, glomerate, paniculiform, racemiform, or spiciform arrays. Calyculi 0. Involucres campanulate, cup-shaped, cylindric, hemispheric, obconic, rotate, saucer-shaped, turbinate, or urceolate (in pistillate heads, sometimes forming ± ovoid to fusiform, often ± spiny or winged burs). Phyllaries persistent or falling, mostly (3–)5–80+ in 1–8(–12+) series, distinct or connate, ovate or lance-ovate to linear, unequal to subequal, outer mostly herbaceous, inner membranous to scarious (sometimes indurate in fruit, accrescent and ultimately ovate to elliptic in Dicoria). Receptacles usually flat to convex, sometimes conic to columnar or hemispheric, usually paleate (at least staminate heads; paleae usually spatulate to filiform, sometimes setiform, often gland-dotted; tips sometimes dilated and strigillose or ciliate; sometimes paleae 0). Ray florets 0 or 5(–8), pistillate, fertile; corollas whitish to yellowish, laminae seldom conspicuous. Peripheral (pistillate) florets 0 or 1–10+; corollas (sometimes 0) usually ochroleucous or whitish, sometimes yellowish or pinkish, tubular to ± filiform (styles: stigmatic papillae in 2 lines, appendages essentially none). Disc (functionally staminate) florets 5–60+; corollas mostly ochroleucous or whitish, sometimes yellowish or pinkish (usually hairy and/or gland-dotted or stipitate-glandular), tubes shorter than to longer than cylindric or campanulate to funnelform throats, lobes 5, ± deltate; (staminal filaments usually connate, anthers usually distinct, ± connate in Parthenice and Parthenium) anther thecae usually pale; stigmatic papillae 0. Cypselae (sometimes shed within burlike or nutlike, spiny or winged perygynia, or shed with accessory structures) fusiform, obovoid, prismatic, or pyriform, sometimes obcompressed and plumply biconvex, sometimes flattened and cucullate (lengths usually 1–2+ times diams., sometimes finely striate or 1–5-nerved, corky-winged in Dicoria and winged margins ± toothed), usually smooth, sometimes ± tuberculate or warty, glabrous or hispidulous, scabrellous, strigillose, or villous and/or gland-dotted; pappi 0 (cypselae sometimes with apical tufts of hairs, or seeming pappi actually enations from ovary walls).

Distribution

Mostly subtropical and warm-temperate New World.

Discussion

Genera 12, species 76 (11 genera, 45 species in the flora).

In most genera of Ambrosiinae, proximal heads are erect and pistillate and distal heads are pendulous and functionally staminate. In functionally staminate florets, the staminal filaments are usually connate and the anthers are distinct or weakly coherent. Unlike nearly all other composites, such plants are wind-pollinated. Wind-borne pollen from some members of Ambrosiinae (especially ragweeds) are sources of “hay-fever” for some people.

H. Robinson (1981) affirmed inclusion of Parthenice and Parthenium in Ambrosiinae and suggested connections between Ambrosiinae and Clibadiinae and/or Ecliptinae.

Selected References

None.

Lower Taxa

Ambrosia, Chorisiva, Cyclachaena, Dicoria, Hedosyne, Iva, Leuciva, Oxytenia, Parthenice, Parthenium, Xanthium

Key

1	Pistillate and functionally staminate florets in separate heads (cypselae shed within hardened, often prickly, spiny, tuberculate, or winged perigynia, forming "burs" or nutlike structures)	> 2
1	Pistillate and functionally staminate florets usually together in same heads (sometimes some heads staminate; cypselae not enclosed within perigynia)	> 3
2	Staminate heads: phyllaries partially or wholly connate; receptacles ± flat or convex; pistillate heads: phyllaries 12–30(–80+) in 1–8+ series, outer (1–)5–8 distinct or ± connate, the rest ± connate (becoming indurate, their distinct tips forming straight or hooked spines, tubercles, or wings)	Ambrosia
2	Staminate heads: phyllaries distinct to bases; receptacles conic to columnar; pistillate heads: phyllaries 30–75+ in 6–12+ series, outer 5–8 distinct, the rest proximally connate (becoming indurate, their distinct tips usually forming hooked spines)	Xanthium
3	Cypselae shed with accessory structures (at least 2 paleae, sometimes florets and/or a phyllary as well); anthers ± connate	> 4
3	Cypselae usually shed free of accessory structures (associated with subtending phyllaries in Dicoria); anthers weakly coherent or distinct	> 5
4	Annuals, biennials, perennials, subshrubs, or shrubs; heads usually radiate (corolla laminae often inconspicuous), sometimes ± disciform (P. alpinum, a perennial); cypselae shed with subtending phyllary plus 2 contiguous disc florets and their investing paleae	Parthenium
4	Annuals; heads obscurely radiate (corolla laminae minute) or disciform; cypselae shed with 2 adjacent, ± fleshy paleae	Parthenice
5	Cypselae strongly obcompressed to obflattened (subtended by accrescent phyllaries, margins corky-winged and ± irregularly toothed)	Dicoria
5	Cypselae sometimes ± obcompressed (not subtended by accrescent phyllaries, margins not corky-winged and toothed)	> 6
6	Heads in (bracteate) racemiform or spiciform arrays (heads 1–2 per bract)	Iva
6	Heads in (± ebracteate) paniculiform arrays, or heads 3–6+ per bract, or heads borne singly (± scattered)	> 7
7	Leaves mostly opposite (distal sometimes alternate, blades 3–5-nerved, ± deltate, ovate, or rhombic, proximal 1–3 pairs often 3–5-lobed, margins usually toothed)	Cyclachaena
7	Leaves all or mostly alternate (blades, some or all, laciniately pinnately lobed or 1–3-pinnately lobed)	> 8
8	Subshrubs or shrubs (phyllaries, paleae, and cypselae ± villous)	Oxytenia
8	Annuals or perennials (rarely woody at bases; phyllaries, paleae, and cypselae glabrous or strigillose and/or hispidulous)	> 9
9	Leaf blades laciniately pinnately lobed, abaxial faces ± lanate, adaxial faces ± tomentose	Leuciva
9	Leaf blades 1–3-pinnately lobed, abaxial and adaxial faces hispid, hispidulous, and/or ± scabrellous	> 10
10	Heads usually in loose, (± bracteate or ebracteate) paniculiform arrays (sometimes 3–6+ distal to axil of each bract); herbaceous phyllaries usually 5; lobes of functionally staminate corollas soon reflexed	Hedosyne
10	Heads mostly borne singly (in leaf axils or remote from axils, ± scattered); herbaceous phyllaries usually 3+; lobes of func-tionally staminate corollas usually erect at flowering	Chorisiva