The willow-leaf pattern commonly found on Chinese plates and cookware is descended from a series of signs and emblems used by ancient Chinese secret societies. The original pattern, designed in the fifteenth century and used as a means of communication among members of these societies, was discovered by the Manchu government, which ordered all the plates destroyed. The pattern turned up again in Europe in the eighteenth century. It had been copied by a western merchant who had managed to smuggle a few of the original plates out of China.
Source: 2201 Fascinating Facts by David Louis, Greenwich House, New York, 1983.