and it is possible that it referred to several different plants, such as sow thistle, chicory, or smartweed, including tea. In the Chronicles of Huayang, it was recorded that the Ba people in Sichuan presented tu to the Zhou king. The state of Ba and its neighbour Shu were later conquered by the Qin, and according to the 17th century scholar Gu Yanwu who wrote in Ri Zhi Lu (日知錄): "It was after the Qin had taken Shu that they learned how to drink tea." The first known reference to boiling tea came from the Han dynasty work "The Contract for a Youth" written by Wang Bao where, among the tasks listed to be undertaken by the youth, "he shall boil tea and fill the utensils" and "he shall buy tea at Wuyang". The first record of cultivation of tea also dated it to this period (Ganlu era of Emperor Xuan of Han) when tea was cultivated on Meng Mountain (蒙山) near Chengdu. From the Tang to the Qing dynasties, the first 360 leaves of tea grown here were picked each spring and presented to the emperor. Even today its green and yellow teas, such as the Mengding Ganlu tea, are still sought after. An early credible record of tea drinking dates to 220 AD, in a medical text Shi Lun (食论) by Hua Tuo, who stated, "to drink bitter t'u constantly makes one think better." Another possible early reference to tea is found in a letter written by the Qin dynasty general Liu Kun. However, before the mid-8th century Tang dynasty, tea-drinking was primarily a southern Chinese practice. It became widely popular during the Tang dynasty, when it was spread to Korea, Japan, and Vietnam. Laozi, the classical Chinese philosopher, was said to describe tea as "the froth of the liquid jade" and named it an indispensable ingredient to the elixir of life. Legend has it that master Lao was saddened by society's moral decay, and sensing that the end of the dynasty was near, he journeyed westward to the unsettled territories, never to be seen again. While passing along the nation's border, he encountered and was offered tea by a customs inspector named Yin Hsi. Yin Hsi encouraged him to compile his teachings into a single book so that future generations might benefit from his wisdom. This became known as the Dao De Jing, a collection of Laozi's