This large edible root belongs to the morning-glory family and is native to tropical areas of the Americas. There are many varieties of sweet potato but the two that are widely grown commercially are a pale sweet potato and the darker-skinned variety Americans erroneously call "yam" (the true yam is not related to the sweet potato). The pale sweet potato has a thin, light yellow skin and a pale yellow flesh. Its flavor is not sweet and after being cooked, the pale sweet potato is dry and crumbly, much like a white baking potato. The darker variety has a thicker, dark orange skin and a vivid orange, sweet flesh that cooks to a much moister texture. Fresh sweet potatoes are available sporadically throughout the year, though not as readily during the summer months. Canned and frozen sweet potatoes are available year-round and are sometimes labeled as yams. When buying fresh sweet potatoes choose those that are small- to medium-sized with smooth, unbruised skins. Sweet potatoes don't store well unless the environment is just right, which is dry, dark and around 55°F. Under perfect conditions they can be stored for 3 to 4 weeks. Otherwise, store in a cool, dark place and use within a week of purchase. Do not refrigerate. Sweet potatoes — particularly the pale variety — can be substituted for regular potatoes in most recipes. They can be prepared in a variety of ways including baking, boiling and sautéing. Sweet-potato chips can now be found on some restaurant menus. Sweet potatoes are high in vitamins A and C. See also potato.