The anatomy of the developing root of Arabidopsis is described using conventional histological techniques, scanning and transmission electron microscopy. The root meristem is derived from cells of the hypophysis and adjacent cells of the embryo proper. The postembryonic organization of the root is apparent in the mature embryo and is maintained in the growing primary root after germination. Cell number and location is relatively invariant in the primary root, with 8 cortical and endodermal cell files but more variable numbers of pericycle and epidermal cells. The organisation of cells in lateral roots is similar to that of the primary root but with more variability in the numbers of cell files in each layer. [3H]thymidine labeling of actively growing roots indicates that a quiescent centre of four central cells (derived from the hypophysis) is located between the root cap columella and the stele. This plate of four cells is surrounded by three groups of cells in, proximal, distal and lateral positions. The labeling patterns of these cells suggest that they are the initials for the files of cells that comprise the root. They give rise to four sets of cell files: the stele, the cortex and endodermis, the epidermis and lateral root-cap and the columella. A model of meristem activity is proposed based on these data. This description of Arabidopsis root structure underpins future work on the developmental genetics of root morphogenesis.