A bulb is a part of a plant that is usually below the ground, or partly below the ground. Bulbs are usually quite spherical but pointed at the top like a teardrop. Bulbs grow roots from the bottom and leaves from the top. Some types of plants that grow from bulbs are onions, lilies and tulips. Anything that has a similar shape to a bulb may also be called a bulb, such as the bulb at the bottom of an old fashioned thermometer.