Tongcao is the stem pith of Tongdemu, a plant in the family of Wuzha, which is cylindrical in shape after drying, with a length of about 0.3-0.6m and a diameter of only about 1.2-3cm. The color is white, with transverse and longitudinal grooves, light and flexible, and also easily broken. In China, Sichuan, Guizhou, Jiangsu and Yunnan and other places are distributed, has good medicinal value, cut into segments after cutting and poking out the pith dry on the line.