The underground segments of a plant are normally referred to as a root, but they're botanically known as corms. Harvested corms are usually round shaped, like turnips or radishes.
Common edible corms consist of:
Another form of corm which is harvested for ingestion that most don't know of is that of a Jack-in-the-pulpit (Arisaema triphyllum). The root of this plant is harvested during summer months, sliced crosswise, and dried. When first reaped, the root has an extremely hot flavor. After time and drying, the intensity of the heat dwindles, enabling easier edibility.
Warning: Jack-in-the-pulpits are considered an endangered species in some areas, so don't go around digging these little critters up to have your mouth burnt off by the corm!
Scientific names of the plants were obtained from www.1upinfo.com