Dioscorea mexicana, Mexican yam or cabeza de negro is a species of yam in the genus Dioscorea.Dioscorea mexicana is a caudiciform dioscorea having either a partly to completely above-ground dome-shaped caudex with a thick, woody outer layer up to 3 feet(90 cm) in diameter and 8-10 inches(20 to 25 cm) in height. The caudex of D. mexicana is divided into regular polygonal plates that become protuberant with age, and separated by deep fissures.