Three ways of estimating the fraction of cycling cells in a meristematic population have been examined region by region throughout the root meristem of Zea mays. Only in the quiescent centre and in the nearby proliferating regions did the three methods produce similar results. The estimate based on the ratio of the average cycle duration of cycling cells and the cell-doubling time is the most reliable. The two methods that use labelling index either immediately after a short pulse of 3H-thymidine or after a long labelling period exaggerate the growth fraction by counting endoreduplicating cells which become prominent even in the proximal half of the meristem and in the second and subsequent tiers of the cap meristem. The nature of the non-cycling cells is discussed.
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F. A. L. Clowes (1976) studied this question.