AbstractThe duration at 14−19°C of the mitotic cycle of the cells in the shoot apex of the mossHookeria lucenswas studied using continuous labelling with tritiated thymidine. Cells in young leaves, and in axial and lateral derivatives of the cells cut off from the apical cell, have cycles respectively of 48, 59 and 90 h. The cycle of the apical initial itself is estimated to be not less than 105 h. The duration of mitosis was 2·6−2·9 h and was not significantly different from one apical region to another. It was shown that the mitotic index could be regarded as representative of the rate of cell division inHookeria. These results were compared with those obtained withPolytrichum formosum.