For the Co uptake experiment,water containing 500-ppm Co with pH=6 was prepared from pure water,solid CoCl,and a small amount of fertilizer. A 2.0 ml of this contaminated water was put into an acrylic vessel with a diameter of 1.5 cm and a depth of 1.8 cm. This vessel was mouned on a xyz stage for adjusting the position of the duckweed leaf to the focal plane of the QMXR microbeam.