A true bulb is an underground stem base that contains an embryonic plant complete with leaves, stems, and flower buds, ready to grow when conditions are right. Surrounding this embryonic plant are scales--modified leaves that overlap each other in a scalelike manner. At the bottom of the bulb is the basal plate, which holds the scales together and produces roots. Many true bulbs, including daffodil (Narcissus) and tulip (Tulipa), have a protective outer covering called a tunic. Lily (Lilium) bulbs lack a tunic; they're susceptible to drying and must be handled with care.
An individual bulb may live for many years. New, smaller bulbs, known as increases or offsets, grow from buds on the basal plate.