<P> 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 a basal disc . From the underside of the disc , 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 . </P>
<P> 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 a basal disc . From the underside of the disc , 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 . </P>
where is food stored in the onion bulb
where is food stored in the onion bulb
leaf bases
leaf bases