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Ambrosia L.

Kingdom: Plantae Rank: Genus Parent: Asteraceae Status: Valid

Common Names:

  • RAGWEED - English, United States of America

Morphological Description

Diagnosis: Annual or perennial herbs, 0.3–3+ m tall; leaves alternate, alternate above and opposite below, or opposite, nearly sessile to petioled, palmately lobed or pinnatifid, sometimes aromatic; flowers unisexual, in separate heads, these on the same plant; staminate heads in spike-like or raceme-like inflorescences; phyllaries united and cup-shaped; anthers distinct; pistillate heads 1 or few-flowered, axillary, below the staminate; phyllaries united, enclosing the achenes at maturity, forming a hard indehiscent “bur” or “fruit,” the bur often with the tips of the phyllaries projecting as spines or tubercles; corollas and pappus absent.


Notes: A cosmopolitan, but especially American genus of 43 species. The flowers of Ambrosia species are small and wind-pollinated; the abundant air-borne pollen is a problematic cause of allergic reactions during the fall and is considered the leading cause of hay fever in the U.S. (Lewis & Elvin-Lewis 1977). The allergic response is initiated when pollen grain proteins (antigens) attach to receptors on antibodies (immunoglobulin E–IgE) linked to immune system cells. This results in the immune cells releasing histamines which are the molecules directly responsible for the symptoms known as hay fever (Kuby 1997; Lim 1998). A cladistic study by Karis (1995) suggested that Xanthium is a monophyletic clade in a paraphyletic Ambrosia. (Early Greek name for aromatic plants; the mythic food of the gods) (tribe Heliantheae)