Will the real Yam please stand up ...

Do you know a yam when you see one?

Here we have three candidates, all claiming to be yams.  At the top is “Mr. Yam”, wrapped and ready to microwave, courtesy Walmart.  Below left is an organic yam from the Big Hollow Food Coop; its shapely form and beauty-marks appeal to the coop's clientele.  Finally, the little guy on the lower right is an Asian yam -- keladi in Malaysia and yùtou in China.  But wait! ... these are all impostors!!  Not one of them is a true yam.

These are true yams (left), starchy tubers from plants of the genus Dioscorea.  Yams are grown worldwide, but more than 95% of the world’s crop is produced in Africa.  The drawbacks of yams include low nutritional value and large amounts of labor needed to grow them.  But with only low tech cultivation required and good storability (up to six months without refrigeration), the yam will remain a major food crop, at least until poverty comes to an end.  Photo by C Ford.

Purple yam, Dioscorea alata (in cross section above above courtesy Deepugn), was used to make this beautiful ube cake! (courtesy Powella)

Plants of the genus Dioscorea, the true yams, are perennial vines.  The yams themselves are root tubers -- underground structures for storing starch.  To right, Discorea bulbifera, the air potato.  Photo courtesy Forest and Kim Starr.

Most yams sold in the United States, like the sensuously-shaped organic yam above, are in reality sweet potatoes, Ipomoea batatas, a member of the morning glory family (photo to left courtesy Indiana State University).  The sweet potato also is a vine with starchy root tubers, but it is not closely related to the true yam.  In fact, the latter is a monocot while the sweet potato is a dicot -- in other words they are in different major groups of flowering plants.

According to the US Department of Agriculture, sweet potatoes sold as yams in the US must be labeled as such, but until I found Mr. Yam in Walmart, I had never seen a US yam sold with the requisite label.

Read the fine print ... Mr. Yam actually is a variety
of sweet potato, one with softer orange flesh.

Some Chinese friends cook a type of yam called yùtou, 芋头 or yùnǎi, 芋艿, which they also call taro -- as do many people around the world.  (This is the smallest yam impostor in the first photos above.) The term "taro" is used for starchy tubers from several different plants in the Araceae or Arum family, most commonly Colocasia esculenta, featured here.  Photo to right courtesy Kahuroa.

Taro is not closely related to either the sweet potato or the true yam; it is in the same family as calla lily, arum and skunk cabbage.  Taro probably is native to the Indo-Malayan region, but now is widely cultivated, especially in southeast Asia, Africa and Oceania.  The leaves are edible also.  Both the leaves and the tubers contain oxalic acid, and are toxic without cooking.

Taro plant, also called "elephant ears".  Source.

As I write, Mr. Yam has been cooking for 8 minutes on high in the microwave, popping occasionally but without breaking his wrapper.  Now he is perfectly cooked and ready to eat.  Sweet potatoes are no more "healthful" than true yams ... but they are "tasty" indeed!

Mr. Yam, "a tasty & healthful food"

Note (7/30/2012):  check out The “Ube” and Purple Filipino Food at Lola Jane's World for more on the ube (“ou-beh”), the purple yam so popular in the Philippines:

"Filipinos are accustomed to purple food from the flavor and coloring of ube — and it must be ingrained in us.  Ube flavored food varies from a light shade of lavender to a deep, dark purple.  It does not matter the shade as I think I can speak for most Filipinos and Filipino-Americans here, that when we see purple or an ube-shade of food, we immediately think…oh look, purple…yes, it’s ube…its good….get it….eat it!"