The sizes and numbers of plastids were determined in the cells of the upper epidermal and upper palisade layers during development of the primary leaf of Phaseolus vulgaris. Plastids of both epidermal and palisade cells divide at all stages of plastid development, but division ceases soon after the plastids become mature. The process of plastid division involves both extension growth and fission. During growth of the primary leaf within a seed developing on the parent plant, plastids in both the epidermal and the palisade cells show little overall growth in size. During germination epidermal cell plastids remain small but plastids in palisade cells increase considerably in size as the leaf expands. Populations of potentially dividing plastids can be identified by the characteristic distribution of their size classes which, when set out graphically, give curves of distinctive shape.

Cell division takes place during development of the primary leaf both prior to and following dormancy. Plastid division also occurs during these two periods. However, within each cell the plastids do not divide in synchrony. Nor are the periods of cell and plastid division synchronized, for the plastids continue to divide during the phase of maximum leaf expansion, when cell division has ceased but while cell extension continues. In the primary leaf of Phaseolus the overall number of cycles of plastid division exceeds that of cell division by between two and three cycles.