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(Last Modified On 6/14/2013)
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(Last Modified On 6/14/2013)
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Genus
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Ambrosia L.,
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PlaceOfPublication
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Sp. P1. 987. 1753
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Note
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TYPE: A. rnaritima L.
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Description
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Shrubs or subshrubs to perennial or annual herbs, glandular, aromatic, anemophilous. Leaves alternate (Panama) or opposite, unlobed to pinnately or palmately lobed or dissected, petiolate or sessile. Inflorescence of distinct staminate and carpellate heads normally on the same plant; staminate heads sessile to stalked, in terminal racemose or spicate clusters; carpellate heads in sessile or stalked clusters in axils of leaves subtending staminate racemes or spikes. Heads unisexual; staminate heads with involucral bracts laterally connate and forming a broadly funnel-shaped to cupulate or saucer-shaped involucre with the tips of the bracts more or less prominent as marginal lobes, the receptacle paleaceous, the florets several to numerous, the corollas hyaline, campanulate, 5-lobed, the stamens 5, weakly connivent, sometimes separating during pollen discharge, the appendages deltoid to long-attenuate, tails absent, the style short, truncate, penicillate, the ovary rudimentary; carpellate heads with bases of involucral bracts fused to form a hard, vase-shaped conceptacle, the bract tips forming more or less spiny processes, these straight to uncinate, flattened to terete, sometimes nearly absent, scattered over the bract surface in various patterns, the paleas absent, the florets 1-7 (if more than 1, interior of fruiting involucre compartmentalized, with each floret in a separate chamber), corolla and androecium lacking, the styles exserted through spinelike beaks at the involucral apex, elongate, linear, spreading, the ovary fertile, obovate, abruptly rounded to the short style base. Fruit an "involucral fruit" consisting of an achene tightly enclosed in the involucre. Chromosome numbers n = 12, 17, 18, 36, 54, 72.
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Habit
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Shrubs or subshrubs to perennial or annual herbs
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Distribution
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A genus of approximately 30 species distributed primarily in North America.
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Note
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Economically, Ambrosia is of great importance in the adverse effects caused by its wind-borne allergenic pollen. Fortunately, the genus has not yet spread to any large extent into Panama.
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Reference
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Payne, W. W. 1964. A re-evaluation of the genus Ambrosia (Compositae). Jour. Arn. Arbor. 45: 401-438.
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