Ragweed

Annual herb (in ours). Leaves alternate, petiolate, 2-pinnatisect. Capitula small, unisexual. Male capitula: racemose, many-flowered; phyllaries connate into a funnel-shaped involucre; receptacular scales filiform or 0; corolla 5-lobed, whitish. Female capitula: sessile or clustered in axils of leaves below the male; 1-flowered; corolla 0. Achenes tightly enclosed in the female involucre which bears spines or tubercles. Pappus 0. Another genus, which is atypical for Asteraceae.