Types of Bulbs:
True Bulbs: have a complete miniature plant encased in a fleshy set of modified leaves called "scales". These usually have a papery covering called a tunic (and easily seen on tulip bulbs). They have a basal plate where the bottoms of the scales join together, (that part of the onion you cut out). The roots grow from this basal plate. Tulips, daffodils, lilies and hyacinths are examples of true bulbs.
Corms: are the bases of stems that become swollen and solid. there are no scales. They are often covered by tunics like the true bulbs, and also have a basil plate. Corms completely expend themselves during the growth cycles, and a new one develops from buds that appear on top of or beside the old one. Freesia, gladiolas, crocus and acidanthera are examples of corms.
Tubers: have no tunic or basal plate, but do have a tough skin that generates roots from many parts of its surface. Corms usually have a knobby surface with the growth buds or eyes from which the shoots of the plants emerge. Some tubers grow larger each growing season (i.e. begonia), and others produce new ones from the sides of the original ones (i.e. caladium). Begonia, gloxinia, caladium and anemone are examples of tubers.
Tuberous Root: is a fleshy root. The food supply is in the root, not the stem or leaf as in other bulbs. The roots do not take up water themselves, they send up a system of fibrous roots that take in moisture and nutrients. They produce buds from which new plants grow, and most buds are restricted to the neck of the old stem. This area is called the crown. Ranunculus and daylilies are examples of tuberous roots.
Rhizome: is a thickened stem that grows horizontally along or below the surface of the soil, sending stems up at intervals. They contain buds with small scale-like leaves that appear on the top or sides of the rhizome. Some like lily of the valley send up small upright detachable growths called PIPS which have their own roots. Canna, lily of the valley and calla are examples of rhizomes.