An onion is a bulb because it has a reduced stem and thick fleshy leaves. The leaves of the onion plant store food in the form of carbohydrates in a bulb, which is a specialized underground stem. This bulb is covered by thin, papery layers that protect the bulb and keep it from drying out. When the bulb sprouts, the stored food provides energy for the plant to grow and produce leaves, flowers, and seeds.