Parthenium

Annual herbs. Stem simple below, branching above into large open panicle. Leaves alternate, sessile, simple, elliptic or ovate in outline, deeply bipinnatisect, margins entire. Capitula radiate, small, in terminal panicles. Involucre campanulate or hemisherical; bracts in 2 or 3 rows, imbricate, outer somewhat smaller than inner. Receptacle convex, paleate. Ray florets female, fertile; small; corolla white, suborbicular. Style bifid; branches linear-lanceolate, obtuse. Cypselas compressed, keeled, crowned with persistent corolla, surrounded by inner involucral bract and 2 lateral, concave, receptacular paleae; pappus of 2 recurved awns, forming apparent margin to cypselas, but free from them, hidden by the 2 receptacular paleae to which they are attached at base. Disc florets bisexual, functionally male; small; corolla white, funnel-shaped, 5-toothed. Anthers with apical appendage, obtuse at base. Style undivided. Ovary of disc florets abortive. x = 9, 17 (6, 8, 29) (aneuploids, high polyploidy, B-chromosomes).