Yam (Dioscorea spp.)
DIOSCOREACEAE, Yam Family
Yams, or shall we call them "true yams," are members of the genus Dioscorea; these are monocotyledons and not at all related to the dicotyledonous yams (the orange ones; Ipomoea batatas) sold in this country. Dioscorea is a vine with a very large, white-fleshed tuber, which can grow to up to 120 pounds in weight.
Twelve species of Dioscorea are consumed as human food, and 7 species are used for making drugs. Among the edible ones, actually 5 are fairly common. Humans learned to eat native species in Africa, South America, and tropical Asia in prehistoric times, hence it appears that yams were domesticated several different times in several different places. Wild forms were used at least 50,000 years ago, judging from West African artifacts. Only one species, D. rotundata, is now grown around the world. This is probably what is sold around Los Angeles in produce departments as "yamo imo." The yams tend to be grown in regions where daily temperatures are 77-86 degrees F, and never where the temperature dips below 68 degrees. Yams are killed by frost.
A piece of the large tuber can be broken off and planted, because new shoots arise from axillary buds on the surface. The shoots often are supported on poles or wires, because this vine is a twiner. Tubers are harvested at the end of the rainy season or early dry season. They are stored after drying several hours in a well ventilated yam barn, and keep best at cool temperatures, especially 54-61 degrees F.
These yams must be cooked to remove bitter substances, including dioscorine, an alkaloid, and diosgenin, one of many triterpenes. They are generally peeled, then boiled, baked, or fried, sometimes roasted. They yield about 15% starch but have little or no protein.
If you want to see a yam plant, there is a plant growing against the Botany Building wall (west side) next to the parking lot. They are easily recognized by their heart-shaped (cordate) leaves with arc-like veins.