Underground tuber is the edible part of yam, and it is sometimes called rhizome because of its long shape like root. According to the shape of tuber, it can be roughly divided into long yam, flat yam and round yam, but there are intermediate types of variation in each type. This variation is mainly influenced by heredity and environment, among which soil environment has the greatest influence.

(1) The upper end of the rod-shaped long yam is very thin, and the middle and lower parts are relatively thick. Generally, the length is about 60 ~ 90cm, and the longest can reach 2m. Its diameter is generally 3 ~10 cm, and the weight of tuber per plant is 0.5 ~ 3.0 kg, and the heaviest tuber can reach more than 5 kg. Long yam with a stick shape, extremely white meat and a lot of mucus.

(2) The tuber of Dioscorea oblata is flat, narrow at the top and wide at the bottom, and has longitudinal folds, which are shaped like soles of feet.

(3) Most round yams are short cylindrical or lumpy, with a length of15cm and a diameter of10cm. There are many shapes and colors of sweet potatoes, such as long, flat and round, with five strands and eight branches, and the meat is red or purple.

There is a hidden bud and stem mark on the top of yam tuber, which is usually used as seed, called yam seed. Except this terminal bud, there are adventitious buds in other parts. Therefore, any part of yam tuber can be cut and planted to grow yam.