The length of the mitotic cycle and the time spent in mitosis are calculated for six regions of the apical meristem of roots of Zea. The two methods used are the measurement of the rates of labelling nuclei with radioactive thymidine and the rates of accumulation of metaphases in roots grown in colchicine. The mitotic cycle lasts from 12 hours in the cap initials to about 200 hours in the quiescent centre of the same roots. Some points about the organization of apices are discussed in the light of these estimates.