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Flora of Taiwan | Family List | Parthenium

Parthenium hysterophorus L., Sp. Pl. II. 988. 1753; Peng, Hu & Kao, J. Taiwan Mus. 41(2): 96, pl.1. 1988.


Parthenium hysterophorus

Credit: HAST

Herbaceous annuals from an erect tap root. Stems 30-150(-200) cm tall, erect, unbranched at base, much branched apically, puberulous, glandular. Leaves alternate, cauline, initially forming a basal rosette but this wanting at anthesis, lanceolate and smaller upward, deeply pinnately dissected or bipinnatifid and larger toward base, to 18×10 cm, puberulous and glandular on both surfaces, petiolate. Inflorescences corymbose cymes, terminal and axillary, peduncle densely yellowish puberulous. Heads 3-5 mm across, numerous. Outer involucral bracts 5, narrowly ovate, acute, 2.1-2.5×1.2-1.4 mm, puberulous; inner bracts 5, obovate, thin, transparent, 1.5-1.8×2-2.2 mm wide, margins glandular, subtending a ray floret with 2 disc florets on either side. Ray florets 5, ligules white, ca. 0.5-0.7×0.5-1 mm, truncate to emarginate; style branches linear, smooth, slightly exserted; pappus of 2 scales, whitish green, obovate, ca. 0.5 mm long; corolla whitish green, 4-lobed, lobes acute; stamens 4, white, ca. 1.1-1.3 mm long; pollen white; ovary rudimentary. Paleae oblanceolate, ca. 1.5×0.8 mm, thin, margins glandular, enclosing disc floret. Achenes black, narrowly obovate, ca. 2×1.1-1.4 mm, sparsely glandular, crowned by 1 persistent ligule and 2 pappus scales, fused at base to 2 adjacent sterile disc florets. Chromosome number, n = 17 (Peng, Hu & Kao, 1988).

TAIPEI: Nankang, Academia Sinica, Peng 11542. KAOHSIUNG: Kaohsiung City, Yang 3992; Liukuei, Yen 1745.

The southern United States, Mexico, Honduras, the West Indies and South America; widely adventive in subtropical areas of the Old World. Taiwan, widely naturalized on open seashores and in lowland waste places throughout the island.


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