In the water fern, Azollapinnata, all the cells of the root are ultimately derived from a single apical cell. The programme of sequential cell divisions which take place as the root develops can be identified with precision. A median longitudinal thin section of this small root provides an ultrastructural transect in which successive stages of cell development are displayed. From a series of sections of roots of different ages it was established that the state of plastid development and the size of the chloroplast population within a cell depend on (1) the age of the root (2) the type of cell and (3) the distance of that cell from the single apical cell.