Studies were made on the organization and mitotic activity of the cap meristem of the primary root of Avena sativa, 40–96 h after germination. The meristem comprises two sub-populations differing in mitotic index, in planes of division and in cell size. In the initial cells, both longitudinal and transverse divisions occur, whereas in the peripheral part of the meristem only transverse divisions were observed. Isotope labelling experiments indicated an average cycle time of the cap initials of 11 h. Calculations of radial and longitudinal transit times based on the average cycle time and the ratio between longitudinal and transverse divisions showed that, on an average, it takes c. 365 h in the radial direction and c. 134.4 h in the longitudinal direction before descendants of the most centrally situated initial cell are sloughed off. The displacement of cells in the root cap was followed by labelling with tritiated thymidine; the results agreed with the positions predicted from the calculated transit times. Assuming growth to be symplastic, a number of predictions were made about the behaviour of a putative quiescent centre in the meristem of the root proper.