Both pulse and continuous labelling experiments were performed to study cap genesis in the primary root of Avena sativa L. The cap meristem comprises one tier of initial cells situated immediately distal to the root cap junction. In the periphery of this tier, the cells have lost their initial character. Compensation for this loss of initial cells is provided by anticlinal divisions of more centrally situated initial cells. Pulse labelling experiments indicated an average cycle time of the cap initials of 10 to 12 hours. From this cycle time and the number of cells in the central rows of the cap it was inferred that for Avena it takes at most 5 to 6 days before a derivative cell is sloughed off. The results are compared with data from Clowes (1971) and Phillips & Torrey (1971, 1972).