Treatments with tritiated thymidine (TdR-3H) have revealed the existence of two populations of mitotically active cells in meristems of lateral roots of Vicia faba. A rapidly dividing population, with a cycle time of 14 hr, constitutes about half the cells in the meristem. A second population of cells, with a cycle time in excess of 30 hr, is also present. Estimates of the relative size of this slowly dividing population are more difficult to make, but we calculate that this population includes 27–43% of meristem cells. The remaining fraction of the meristem is made up of cells that divide rarely or not at all. Since, at all times, both populations contribute to the mitotic index, the curve of the percentage of labeled mitoses that can be determined after a pulse label with TdR-3H differs from the curve expected of an ideal population in an important way: the peak value of the curve of the percentage of labeled mitoses is always less than 100%, usually between 75 and 80%. This heterogeneity within a meristem must be borne in mind in terms of the response of meristems to disruptive treatments, the mechanisms controlling mitotic cycle duration, and the spatial organization of a heterogeneous population in an organ that shows polarized growth.

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