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Many of the plants we call bulbs aren't bulbs at all. It's become common to lump together under this term not only true bulbs, but all plants that grow from a thickened or bulbous storage organ. (Plants like daylilies, clivia, and iris are in a shady area between bulbs and perennials, so you find them discussed in books on bulbs and also in books on perennials.) Here's how bulbs differ so you can tell them apart.
True Bulb
A modified subterranean leaf bud, the true bulb has a basal plate, above which are food-storing scales (rudimentary leaves) surrounding a bud that contains the magic makings of a plant. Some bulbs, like onions, tulips, and daffodils, are tunicate — they're covered with a papery skin. Others, like lilies, are imbricate — they have overlapping scales.
Corm
A thickened subterranean stem that produces a plant. The inside is just a solid piece of tissue. The buds are on top. After bloom the old corm is used up, but new ones have grown on top or at the sides to take its place. Gladioli, sparaxis, and freesia grow from corms.
Rhizome
A thickened stem or branch that grows on the surface of the ground or horizontally underground, such as bearded irises and cala lilies.
Tuber
A thickened stem that serves as a storage chamber but is usually shorter, thicker, and rounder that a rhizome. It grows totally or partially underground. Tuberous begonias, cyclamen, and potatoes grow from tubers.
Tuberous Root
Growing underground, this differs from a tuber in that it's a swollen root rather than a thickened stem. Tuberous roots have growth buds on top in the old stem portion, from which spring the plants. Dahlias and sweet potatoes grow from tuberous roots.
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