This rather ugly, knobby, brown vegetable is actually the root of a special celery cultivated specifically for its root. It's also called celery root and celery knob. Celeriac tastes like a cross between a strong celery and parsley. It's available from September through May and can range anywhere from the size of an apple to that of a small cantaloupe. Choose a relatively small, firm celeriac with a minimum of rootlets and knobs. Avoid those with soft spots, which signal decay. The inedible green leaves are usually detached by the time you buy celeriac. Refrigerate the root in a plastic bag for 7 to 10 days. Celeriac can be eaten raw or cooked. Before using, peel and soak briefly in acidulated water to prevent discoloration. To eat raw, grate or shred celeriac and use in salads. Cooked, it's wonderful in soups, stews and purees. It can also be boiled, braised, sautéed and baked. Celeriac contains small amounts of vitamin B, calcium and iron.