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
. Until 2010, the shallot was classified as a separate species.
Onions have cylindrical, hollow leaves and an enlarged bulb that develops at ground level. The roots come off the bottom of the bulb. The flowers are produced in the second growing season (following a required "rest" period) in a rounded umbel (cluster with all flower stems originating from the same point) on a stalk 2-4 ft tall. The umbels, about 2 in in diameter and consisting of many small purplish flowers, are quite showy. There are hundreds of onion cultivars, differing in day-length requirement, skin color (white, brown, yellow, red, or purple), size (1-6 in or 2.5-15.2 cm in diameter), shape (globe-shaped, flattened or spindle-shaped), pungency and sweetness. Both pungency and sweetness (which are not mutually exclusive) are determined to a considerable extent by the chemical characteristics of the soil in which the onion is grown.The onion is known only in cultivation, but probably was developed from a wild ancestor that grows in western Asia. Onion seeds have been found in Egyptian tombs dated to 3200 BC, and some authorities believe the onion may have been one of the first vegetables domesticated by humans.
The onion plant has been grown and selectively bred in cultivation for at least 7,000 years. It is a biennial plant, but is usually grown as an annual. Modern varieties typically grow to a height of 15 to 45 cm (6 to 18 in). The leaves are yellowish- to bluish green and grow alternately in a flattened, fan-shaped swathe. They are fleshy, hollow, and cylindrical, with one flattened side. They are at their broadest about a quarter of the way up, beyond which they taper towards a blunt tip. The base of each leaf is a flattened, usually white sheath that grows out of the basal plate of a bulb. From the underside of the plate, a bundle of fibrous roots extends for a short way into the soil. As the onion matures, food reserves begin to accumulate in the leaf bases and the bulb of the onion swells.
mature onion bulb is most often eaten, onions can be eaten at immature stages. Young plants may be harvested before bulbing occurs and used whole as spring onions or scallions. When an onion is harvested after bulbing has begun, but the onion is not yet mature, the plants are sometimes referred to as "summer" onions.
Onions contain antioxidants and compounds that fight inflammation, decrease triglycerides and reduce cholesterol levels — all of which may lower heart disease risk. Their potent anti-inflammatory properties may also help reduce high blood pressure and protect against blood clots.