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Heads small, unisexual; staminate heads uppermost, many-fld, with poorly developed or no invol, the receptacle short, cylindric, chaffy, the filaments monadelphous, the pistil composed mainly of the undivided style; invol of the pistillate heads completely enclosing the 2 fls, forming a conspicuous 2-chambered bur with hooked prickles; pistillate fls lacking a cor, and with the styles exsert from the invol; achenes thick, solitary in the chambers of the bur; pappus none; coarse, taprooted, annual weeds with alternate lvs and solitary or clustered axillary and terminal heads. 3, originally New World.
Annual herbs. Lvs alternate, simple, entire to serrate or lobed. Capitula unisexual, sessile or shortly pedunculate, solitary, terminal and in panicles or racemes, or clustered in axils. ♂ capitula ± globose; involucral bracts in 1 row; receptacle conic; scales present; corolla tubular. ♀ capitula ovoid; involucral bracts in 2 rows, the outer small and free, the inner connate, prickly and ending in 1-2 beaks containing the 2 florets; corolla 0. Achenes all similar, ovoid, enclosed within the expanded prickly involucre; pappus 0.
Annual herbs. Leaves alternate. Capitula unisexual, monoecious, axillary or terminal, discoid. Male capitula globose, many-flowered; involucre with few phyllaries; receptacle paleate; corolla subcylindric, 5-dentate; anthers with minute apical appendage; style undivided. Female capitula beneath the male, 2-flowered; involucre connate, 2-beaked at apex and covered with hooked bristles; corolla absent; style bifid. Achenes enclosed in the hardened involucre; pappus absent.
