The yam and sweet potato are quite different plants, although they may look somewhat alike. In most cases, if you are eating a very sweet orange or yellow fleshed potato, you are actually eating a sweet potato and not a yam. Unless you’re shopping at an international market, or happen to be consuming your meal in Africa, chances are that you are consuming a sweet potato.
The confusion begins when Africans were brought to the Americas as slaves. The yam, which is a very important and starchy part of many African diets, looks a little bit like a sweet potato. Thus many slaves used the words yam and sweet potato interchangeably and the name stuck.
In the US, the two foods may be distinguished because the sweet potato tends to be less sweet than the yam. In reality, the real yam and sweet potato difference works in the opposite direction: African yams are less sweet, larger, and starchier. In fact, an African yam may reach as large as 100 pounds (45.35 kilograms). The largest recorded yam was 130 pounds (59 kg).
The yam tends to be softer, and herein lies the great difference in the debate. Firmer sweet potatoes were initially grown in the US. Softer varieties began to be grown and were often labeled as yams. Thus comparing a yam and sweet potato may mean the distinction between a fairly soft when cooked potato, or a fairly firm when cooked one.
There are some basic botanical differences between a real yam and sweet potato. The African yam is related to certain grasses and to lilies. It is also a monocot, exhibiting one embryonic seed leaf when it first sprouts from the ground. Conversely, the sweet potato is related to the morning glory family, and more closely related to other types of potatoes. It is a dicot and exhibits two embryonic leaves when sprouting.
The misnamed yam often gives a cook an idea of the type of sweet potato he or she is actually going to serve. For example, the garnet yam (actually a sweet potato) is often considered one of the most delightful, and is noted for its very sweet taste and deep orange color.