Onion

Onion
A display of commercially-grown bulbs, including red and yellow cultivars.
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Order:	Asparagales
Family:	Amaryllidaceae
Subfamily:	Allioideae
Genus:	Allium
Subgenus:	A. subg. Cepa
Species:	A. cepa
Binomial name
Allium cepa L.
Synonyms[1]
Species synonymy Allium angolense Baker Allium aobanum Araki Allium ascalonicum auct. Allium ascalonicum var. condensum Millán Allium ascalonicum var. fertile Millán Allium ascalonicum f. rotterianum Voss ex J.Becker Allium ascalonicum var. sterile Millán Allium cepa var. aggregatum G.Don Allium cepa var. anglicum Alef. Allium cepa var. argenteum Alef. Allium cepa var. bifolium Alef. Allium cepa var. crinides Alef. Allium cepa var. flandricum Alef. Allium cepa var. globosum Alef. Allium cepa var. hispanicum Alef. Allium cepa var. jamesii Alef. Allium cepa var. lisboanum Alef. Allium cepa var. luteum Alef. Allium cepa var. multiplicans L.H.Bailey Allium cepa var. portanum Alef. Allium cepa var. praecox Alef. Allium cepa var. rosum Alef. Allium cepa var. sanguineum Alef. Allium cepa var. solaninum Alef. Allium cepa var. tripolitanum Alef. Allium cepa var. viviparum (Metzg.) Alef. Allium cepaeum St.-Lag. Allium commune Noronha Allium cumaria Buch.-Ham. ex Wall. Allium esculentum Salisb. Allium napus Pall. ex Kunth Allium nigritanum A.Chev. Allium pauciflorum Willd. ex Ledeb. Allium salota Dostál Ascalonicum sativum P.Renault Cepa alba P.Renault Cepa esculenta Gray Cepa pallens P.Renault Cepa rubra P.Renault Cepa vulgaris Garsault Kepa esculenta Raf. Porrum cepa (L.) Rchb.

An onion (Allium cepa L., from Latin cepa meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus Allium. The shallot is a botanical variety of the onion which was classified as a separate species until 2011. The onion's close relatives include garlic, scallion, leek, and chives.

The genus contains several other species variously called onions and cultivated for food, such as the Japanese bunching onion Allium fistulosum, the tree onion Allium × proliferum, and the Canada onion Allium canadense. The name wild onion is applied to a number of Allium species, but A. cepa is exclusively known from cultivation. Its ancestral wild original form is not known, although escapes from cultivation have become established in some regions. The onion is most frequently a biennial or a perennial plant, but is usually treated as an annual and harvested in its first growing season.

The onion plant has a fan of hollow, bluish-green leaves, and its bulb at the base of the plant begins to swell when a certain day-length is reached. The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem. In the autumn (or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become more dry, and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly the onion fly, the onion eelworm, and various fungi which can cause rotting. Some varieties of A. cepa, such as shallots and potato onions, produce multiple bulbs.

Onions are cultivated and used around the world. As a food item, they are often served raw as a vegetable or part of a prepared savoury dish, but can be eaten cooked or used to make pickles or chutneys. They are pungent when chopped and contain certain chemical substances which may irritate the eyes.