Yam is the common name for some plant species in the genus Dioscorea (family Dioscoreaceae) that form edible tubers. Although some varieties of sweet potato (Ipomoea batatas) are also called yam in parts of the United States and Canada, it is not part of the family Dioscoreaceae but belongs in the unrelated morning glory family Convolvulaceae.
Differences between true yam and sweet potato "yam"
Yams are a monocot (a plant having one embryonic seed leaf) and from the Dioscoreaceae family. Sweet Potatoes are a dicot (a plant having two embryonic seed leaves) and are from the Convolvulacea family. They are therefore about as distant as two flowering plants can be. Culinarily, yams are starchier and drier than sweet potatoes. The table below lists some differences between yam and sweet potato.
|Plant Family||Morning glory||Yam|
|Origin||Tropical America (Peru, Ecuador)||West Africa, Asia|
|Edible part||Storage root||Tuber|
|Appearance||Smooth, with thin skin||Rough, scaly|
|Shape||Short, blocky, tapered ends||Long, cylindrical, some with "toes"|
|Beta carotene||Usually high||Usually very low|
|Propagation||Transplants/vine cuttings||Tuber pieces|
Yam tubers can grow up to 1.5 meters (4.9 feet) in length and weigh up to 70 kilograms (154 lb) and 3 to 6 inches high. The vegetable has a rough skin which is difficult to peel, but which softens after heating. The skins vary in color from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as the "meat". This substance ranges in color from white or yellow to purple or pink in mature yams.