In root apices of Sinapis alba mitosis is partially synchronous under certain cultural conditions, with two maxima and two minima of mitotic index each day. But the phase of the partial synchrony is different in the stele and cortex from that in the cap initials and that in the quiescent centre. The average rates of mitosis vary from once in 35 hours in the cap initials to once in 500 hours in the quiescent centre. Cells of the cortex and stele divide more slowly near the quiescent centre than at 150 microns from it. In this respect Sinapis differs from Zea and the dissimilarity in organization may be due to the possession of partial synchrony and the opportunity it gives for phasic differences in synthesis.