Most people are familiar with tulips, daffodils and other bulbous plants. But what exactly are bulbs? Horticulturally speaking, the term bulb is often used loosely to refer to corms, tubers, rhizomes and tuberous roots. Although structurally different, they all have one thing in common: they are all specialized storage organs. Surviving adverse seasons of drought, extreme heat or cold underground, these food storage organs enable the plant to grow rapidly during favorable conditions.
Botanically speaking, a true bulb is a (usually) subterranean modified stem with leaves complete, complete with flowers in embryonic form. This can be seen by slicing through the bulb, such as an onion, vertically. Bulbs consist of a basal plate from which the roots will grow and a thick, shortened stem surrounded by fleshy scale leaves. These scales contain the food necessary to sustain the bulb during dormancy and early growth. The basal plate serves to hold these scales together.
There are two types of true bulbs: tunicate and imbricate (sometimes referred to as scaly). Tunicate bulbs have scale which are tightly wrapped around the bud and covered in a thin, dry, papery skin called a tunic. Tulips, daffodils, squill, glory-of-the-snow (Chionodoxa), grape hyacinth, iris reticulata and Danford irises are examples of tunicate bulbs. Imbricate bulbs have thick scales which are loosely arranged and may have no covering. True lilies are examples of imbricate bulbs.
During the growing season, small new bulbs, called bulblets, are produced from lateral buds on the basal plate. Some types, such as lilies, can also produce small bulbils, above ground, in the axils of their leaves.