<Tr> <Th> show Cross-eye view () </Th> </Tr> <Tr> <Td> Onion seeds have a distinct shape . </Td> </Tr> <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> In the autumn, the leaves die back and the outer scales of the bulb become dry and brittle, so the crop is then normally harvested . If left in the soil over winter, the growing point in the middle of the bulb begins to develop in the spring . New leaves appear and a long, stout, hollow stem expands, topped by a bract protecting a developing inflorescence . The inflorescence takes the form of a globular umbel of white flowers with parts in sixes . The seeds are glossy black and triangular in cross section . The average pH of an onion is around 5.5 </P>

Where is food stored in the onion bulb