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Allium is a genus of monocot perennial bulbous plants, informally referred to as the onion genus. The generic name Allium is the Latin word for garlic (Lyons 1900). Many Allium species are used as food plants; in most species, both bulb and leaves are edible.

The genus includes many economically important crops, garden vegetables, and herbs, including onions (A. cepa), shallots (A. cepa var. aggregatum), French shallot (A. oschaninii), leeks (A. ampeloprasum), scallions (various Allium species), Welsh onion (A. fistulosum), garlic (A. sativum), wild garlic (A. ursinum) and chives (A. schoenoprasum). Global production of onions was more than 73 million metric tons (harvested from 3.6 million hectares) in 2008, and over 22 million metric tons of garlic (from nearly 1.4 million hectares; FAOSTAT 2011).

Alliums are also planted as ornamentals. There are over 150 species and cultivars that have been developed for garden use, with colors ranging from white to blue to shades of purple, and flower heads up to the size of a softball (Davis 1993).

The Allium genus is taxonomically difficult and species boundaries are unclear. Most authorities accept about 750 species, but estimates range from 260 to 860 (Hirschegger et al. 2010, Rahn 1998). The type species for the genus is Allium sativum. The Allium genus was formerly placed in Liliaceae, and later in Alliaceae, but the 2009 classification by the Angiosperm Phylogeny Group treats Alliaceae as subfamily Allioideae of Amaryllidaceae (APG 2009).

Allium species occur in temperate climates of the northern hemisphere, except for a few species occurring in Chile (such as A. juncifolium), Brazil (A. sellovianum) or tropical Africa (A. spathaceum). They vary in height from 5 cm to 150 cm. The flowers form an umbel at the top of a leafless stalk (scape). The bulbs, which are usually tunicate (covered with a papery skin), vary in size across species, from very small (around 2–3 mm in diameter) to rather large (8–10 cm). Some species (such as Welsh onion, A. fistulosum) develop thickened leaf-bases rather than forming bulbs.

Allium species produce chemical compounds (mostly cysteine sulfoxide) that give them a characteristic onion or garlic taste and odor. Folk medicinal uses of Alliums are numerous, and considerable research has focused on identifying and examining compounds for medicinal use. Allicin, derived from garlic, is used for fighting fungal infections and parasites, lowering blood cholesterol, and promoting circulatory function (Biser 1998). Preliminary European findings suggest that a diet high in onions and garlic is correlated with reduced risk of several cancers (Galeone et al. 2006). However, some onion species are toxic to cattle, cats, dogs, sheep, and goats (Cope 2011, Merck 2011).

Some Eurasian Allium species have escaped cultivation and naturalized widely in North America. A. vineale is reported invasive in Connecticut, Maryland, North Carolina, New Jersey, Pennsylvania, Tennessee, Virginia, and West Virginia, and is classified as a noxious weed in Hawaii and California (USDA Forest Service 2006). In Arkansas, wild onions and wild garlic (Allium spp.) are classified as noxious weeds.

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