Although yams and sweet potatoes are both angiosperms (flowering plants), they are not related botanically. Yams are a monocot (a plant having one embryonic seed leaf) and from the Dioscoreaceae or Yam family. Sweet Potatoes, often called ‘yams’, are a dicot (a plant having two embryonic seed leaves) and are from the Convolvulacea or morning glory family.

Yams
Yams are closely related to lilies and grasses. Native to Africa and Asia, yams vary in size from that of a small potato to a record 130 pounds (as of 1999). There are over 600 varieties of yams and 95% of these crops are grown in Africa. Compared to sweet potatoes, yams are starchier and drier.

Sweet Potatoes
The many varieties of sweet potatoes (Ipomoea batatas) are members of the morning glory family, Convolvulacea. The skin color can range from white to yellow, red, purple or brown. The flesh also ranges in color from white to yellow, orange, or orange-red. Sweet potato varieties are classified as either ‘firm’ or ‘soft’. When cooked, those in the ‘firm’ category remain firm, while ‘soft’ varieties become soft and moist. It is the ‘soft’ varieties that are often labeled as yams in the United States.

Why the confusion?
In the United States, firm varieties of sweet potatoes were produced before soft varieties. When soft varieties were first grown commercially, there was a need to differentiate between the two. African slaves had already been calling the ‘soft’ sweet potatoes ‘yams’ because they resembled the yams in Africa. Thus, ‘soft’ sweet potatoes were referred to as ‘yams’ to distinguish them from the ‘firm’ varieties.

Today the U.S. Department of Agriculture requires labels with the term ‘yam’ to be accompanied by the term ‘sweet potato.’ Unless you specifically search for yams, which are usually found in an international market, you are probably eating sweet potatoes!
Source(s):
http://www.loc.gov/rr/scitech/mysteries/sweetpotato.html


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