This thick, tropical-vine tuber is popular in South and Central America, the West Indies and parts of Asia and Africa. Although sweet potatoes and yams are similar in many ways and therefore often confused with one another, they are from different plant species. In the southern United States, sweet potatoes are often called yams and to add to the confusion, canned sweet potatoes are frequently labeled yams. True yams, however, are not widely marketed and are seldom grown in the United States. Though they can be similar in size and shape to sweet potatoes, yams contain more natural sugar and have a higher moisture content. On the downside, they're not as rich in vitamins A and C as sweet potatoes. There are over 150 species of yam grown throughout the world. They can range in size from that of a small potato to behemoths over 7½ feet long and 120 pounds. Depending on the variety, a yam's flesh may be various shades of off-white, yellow, purple or pink, and the skin from off-white to dark brown. The texture of this vegetable can range from moist and tender to coarse, dry and mealy. Yams can be found in most Latin American markets, often in chunks, sold by weight. When buying yams, select unblemished specimens with tight, unwrinkled skins. Store in a place that's cool, dark and dry for up to 2 weeks. Do not refrigerate. Yams may be substituted for sweet potatoes in most recipes.