Roots of Vicia faba were treated with H3-Thymidine (1 μC/ml) for one hour, either before or after a three hour treatment with a 10-5 M solution of kinetin. The durations of the mitotic cycle and its constituent phases were then derived from the curves recording the rhythmic entry and exit of labeled cells in prophase, both for the kinetin treated roots and the controls. The rate of DNA synthesis was also determined for the control roots and for roots exposed to H3-Thymidine immediately following treatment with kinetin. - Control values for the durations of the mitotic cycle, G1, S, G2 and mitosis were in agreement with most of the results reported in the literature. Kinetin treatment resulted in an increase in the rate of DNA synthesis, but did not affect the number of cells undergoing DNA synthesis, i.e. kinetin, or a similar compound, may be involved in the control of the rate of DNA synthesis in plant root meristems, but it does not appear to be involved in the control of the initiation of the S period of interphase. - The durations of G1 and G2 are shorter and longer respectively in the kinetin treated roots as compared with the control values. Changes in the durations of these phases of interphase, S, cycle time and the rate of cell proliferation have been discussed with respect to time after kinetin treatment and their possible relationship to carbohydrate metabolism. © 1968 Springer-Verlag.
