What gardeners generally call bulbs may actually be bulbs, but could also be corms, rhizomes, or tubers. Botanically, all of these are geophytes, a term for plants with their growing point beneath the soil. They survive by underground food-storage organs, which are essentially modified stems or roots. Yet there are clear distinctions among the growth habits and needs of these plants. They usually are more pointed on top, where the leaves and stem emerge, and flat on the bottom, where roots grow from the stem, called a basal plate.
Some bulbs have an outer layer that is thin, dry, often brown, and easily removed.
Called a tunic, it helps to keep the inner scales from drying out. Daffodils and tulips normally have brown tunics, while a hyacinth may have a purple or white tunic, depending on its bloom color. True bulbs increase by division. These offshoots, called bulblets, form on the side of the basal plate located on the bottom of the bulb. In perfect conditions, bulbs double in quantity each year.