1.Several methods for relating the volumes of individual nuclei to their relative interphase stage have been applied to rapidly proliferating cells of the cortex in root tips of broad bean(Vicia faba).The methods are based upon analysis of the frequency with which particular volume classes appear within a population of nuclei. Presuming random sampling of a particular homogeneous population, it is possible and convenient to relate the volume of an individual nucleus to its age in relative interphase time by using the geometric mean volume and standard deviation of the sampled population to locate a straight line on probability paper. This graph can then be used to estimate the relative interphase time for any given nuclear volume in the population.2.In cases of heterogeneous populations of nuclei, as found for cells of bean root meristems, synthetically active cells can be distinguished from inactive or quiescent cells by means of isotopic labeling. The individual quiescent cells which occur sporadically throughout the cortex of bean meristems do not incorporate H3-thymidine even after prolonged labeling periods, suggesting a possible relationship with cells of the discrete quiescent center previously described for this and other angiosperm root apices.