The mean rates of mitosis for all cycling cells and the cell-doubling times have been measured independently by pulse-labelling and by blocking the cell cycle and then used to measure the fraction of cycling cells along the meristem of primary roots of Zea mays. The attenuation of mitotic activity that occurs towards the proximal and distal margins of the meristem is due solely to a reduction in growth fraction and not at all to a change in the rate of cycling. This is contrasted with the situation in the quiescent centre of the meristem where the low rate of cell formation results from both a reduction in growth fraction and prolongation of cell cycles. DNA synthesis outside a normal mitotic cycle begins to occur in regions containing cycling cells on both sides of the quiescent centre.
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