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Ragweeds are flowering plants in the genus ''Ambrosia'' in the aster family, Asteraceae. They are distributed in the tropical and subtropical regions of the New World, especially North America,〔(''Ambrosia'' ). Flora of North America.〕 where the origin and center of diversity of the genus are in the southwestern United States and northwestern Mexico. Several species have been introduced to the Old World and some have naturalized.〔 Other common names include bursages〔(''Ambrosia'' ). Integrated Taxonomic Information System (ITIS).〕 and burrobrushes.〔(''Ambrosia'' ). The Jepson eFlora 2013.〕 The genus name is from the Greek ''ambrosia'', the "food of the gods".〔 Ragweed pollen is notorious for causing allergic reactions in humans, specifically allergic rhinitis. Up to half of all cases of pollen-related allergic rhinitis in North America are caused by ragweeds. ==Description and ecology== Ragweeds are annual and perennial herbs and shrubs. Species may grow just a few centimeters tall or well exceed four meters in height. The stems are erect, decumbent or prostrate, and many grow from rhizomes. The leaves may be arranged alternately, oppositely, or both. The leaf blades come in many shapes, sometimes divided pinnately or palmately into lobes. The edges are smooth or toothed. Some are hairy, and most are glandular.〔 Ragweeds are monoecious, most producing inflorescences that contain both staminate and pistillate flowers. Inflorescences are often in the form of a spike or raceme made up mostly of staminate flowers with some pistillate clusters around the base. Staminate flower heads have stamens surrounded by whitish or purplish florets. Pistillate flower heads have fruit-yielding ovules surrounded by many phyllaries and fewer, smaller florets.〔 The pistillate flowers are wind pollinated,〔(Genus ''Ambrosia'' ). Arizona-Sonora Desert Museum.〕 and the fruits develop. They are burs, sometimes adorned with knobs, wings, or spines.〔 Many ''Ambrosia'' species occur in desert and semi-desert areas, and many are ruderal species that grow in disturbed habitat types.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ragweed」の詳細全文を読む スポンサード リンク
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