Annual herb to 40 cm tall; stems erect, to 4 mm diam., sparsely villous with hairs to 1.5 mm long. Leaves alternate, pinnately lobed to deeply divided, 6-8.5 cm long, 2.5-4 cm wide, acute-acuminate, the base attenuate, both surfaces dotted with tiny sessile glands and strigose with hairs 0.3 mm long (approaching villous on the midrib of the undersurface), the margin deeply lobed or divided. Inflorescence of staminate and carpellate heads separate but on the same plant; staminate heads 30-70 with peduncles 3 mm long, in terminal elongate racemose clusters to 20 cm long; carpellate heads subsessile in axillary clusters. Heads unisexual; staminate heads with involucral bracts connate, forming a shallow cuplike involucre with short, irregular lobes, gland-dotted, the receptacle paleaceous with filiform paleas 1 mm long, the corollas 5-lobed, campanulate, 1.5 mm long, the throat 0.9 mm long, the stamens 5, the appendages deltoid with a mucronate tip, the styles short, truncate, the ovary rudimentary; carpellate heads with outer involucral bracts 3-5, lanceolate to narrowly ovate, to 9 mm long, gland-dotted, the inner bracts 1-3, each tightly enclosing and fused with a single ovary, the outer surface with a crown of 3-5 small spines on the upper 1/3 of the fruit, a tapering beak 0.8 mm long at the apex, corollas and androecium absent, the styles bifid with stigmatic branches to 1 mm long. Fruits beaked, spiny, 3-4 mm long (including beak), terete, gland-dotted. Chromosome number n 18.
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