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Ambrosia
See list of 4 species in this genusReference: Strother (2006a).
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1a. Staminate capitula sessile or subsessile, in a solitary, spike-like capitulescence; staminate involucres prolonged into a long, retrorse, lanceolate to triangular-ovate, hooded, bristly tooth; leaf blades lanceolate to narrow-lanceolate, entire to lobed, when lobed with 1 or 2 pairs of pinnately arranged lobes that are confined to the base of the blade
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1b. Staminate capitula peduncled, borne in 1 or more raceme-like capitulescences; staminate involucres without a prolonged, retrorse lobe, actinomorphic or nearly so; leaf blades lanceolate or elliptic to ovate or triangular, 1- or 2-times pinnately lobed, or entire to palmately lobed in A. trifida, but that species with some broad-lanceolate to ovate, unlobed blades on the plant
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2a. Plants annual, 0.5–5 m high; leaves opposite throughout, the blades entire to palmately lobed with 3 (–5) lobes [Fig. 372]; receptacle without chaff; staminate involucre with 3 evident nerves on one side; carpellate involucre 5–10 mm long in fruit
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2b. Plants annual or perennial, 0.2–1 (–2.5) m high; leaves usually opposite below and alternate above, once- or twice-pinnatifid; receptacle with chaff; staminate involucre inconspicuously nerved (i.e., the faint nerves of ± equal prominence in all directions); carpellate involucre mostly 3–5 mm long in fruit
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3a. Plants usually perennial from a creeping rootstock; leaf blades usually once-pinnatifid, relatively thicker; carpellate involucre with 4 tubercles near the apex, these sometimes short and inconspicuous
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3b. Plants annual; leaf blades once-, or more commonly, twice-pinnatifid, relatively thinner; carpellate involucre with 4–7 sharp spines near or above the middle [Fig. 371]
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Each photo represents one species in this genus.