Estimation of Growth Fractions in Meristems of Zea mays L.

Three ways of estimating the fraction of cycling cells in ameristematic population have been examined region by regionthroughout the root meristem of Zea mays. Only in the quiescentcentre and in the nearby proliferating regions did the threemethods produce similar results. The estimate based on the ratioof the average cycle duration of cycling cells and the cell-doublingtime is the most reliable. The two methods that use labellingindex either immediately after a short pulse of ³H-thymidineor after a long labelling period exaggerate the growth fractionby counting endoreduplicating cells which become prominent evenin the proximal half of the meristem and in the second and subsequenttiers of the cap meristem. The nature of the non-cycling cellsis discussed.