An onion is a bienniel herb related to the lily. The bulbs are used as vegetables and for flavoring other foods. A chemical, allyl sulfide, escapes when the onion is cut and affects nerve endings in the nose. The nerve endings stimulate tears to flow from the eyes.
   The onion grows to be two or three feet tall. The stem is flat, disk-like and underground. The fleshy underground leaves surrounding the stem are white. As they grow and receive sunlight, chlorophyll is produced. In the second year of growth, a flower stalk produces a flower cluster.
   Onions are propagated by seed sets (small bulbs) and by bulblets that grow at the top of the stem instead of flowers.