Though gardeners generally refer to them all as bulbs, of the flowers listed, only lilies qualify as true bulbs. Glads and crocosmia come from corms (swollen underground stems, each with a growth point on the top); dahlias grow from tuberous roots (small potato-like roots); and callas and tuberoses come from rhizomes (thickened stems that grow horizontally on or beneath the soil surface).
Despite their differing underground structures, corms, tuberous roots, and rhizomes are all referred to as bulbs. They come with their own cafeterias attached ― subterranean storage organs that hold reserves of food to keep the plants alive from one growing season to the next. It's this built-in food supply that makes these types of plants more forgiving of casual care and attention.