1. Ambrosia acanthicarpa Hook. (annual bursage)

Franseria acanthicarpa (Hook.) Coville

Pl. 271 f–h; Map 1142

Plants annual, with taproots. Stems 10–80(–120) cm long, variously with short and/or longer hairs. Leaves opposite toward the stem base, alternate toward the stem tip, mostly short- to long-petiolate (the uppermost ones sessile or nearly so). Leaf blades 2–8 cm long, ovate to lanceolate in outline, irregularly 1 or 2 times pinnately lobed (at least the larger leaves with 7 or 9 primary lobes), the lobes linear to ovate, entire or few-toothed, the surfaces sparsely to moderately pubescent with more or less appressed, white hairs, the undersurface sometimes appearing somewhat pale or whitened. Staminate heads in spikelike racemes, these frequently in paniculate clusters, the staminate involucre 3–12 mm wide, with 3–9 shallow to relatively deep lobes, glabrous but usually with 1 or few black lines or streaks. Pistillate heads at the base of the staminate racemes (or less commonly the racemes mostly pistillate), the involucre enclosing 1 floret and with 1 beak, 5–10 mm long at fruiting, more or less ovoid, with several series of relatively long, strongly flattened spines scattered across the surface, otherwise glabrous. 2n=36. August–October.

Introduced, uncommon, known thus far only from historical collections from Jackson and Wayne Counties (western U.S. east to North Dakota and Texas; Canada; introduced sporadically farther east). Glades; also railroads and disturbed areas.