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SUMMARY

The duration of each phase of the mitotic cycle in four regions of the root meristem of Zea mays has been determined by pulse labelling with 3H thymidine. S, G2 and mitosis are similar in length in all the regions, but G1 differs considerably and is largely responsible for the diversity in rates of mitosis. G1 is non-existent in the cap initials, 151 hours in the quiescent centre, and 2–4 hours in the stele.

These data, together with information on sensitivity to ionizing radiation, suggest that it is the ratio of the lengths of G1 to the rest of the mitotic cycle that decides how resistant a tissue will be to radiation and they lead to a general explanation of the law that rapidly dividing cells are more susceptible than slowly dividing cells. The extreme sensitivity of the cap initials and the high resistance of the quiescent centre are correlated with the brevity of G1, in the former and the prolongation of G1 in the latter. The fraction of the mitotic cycle spent in G1 (when each nucleus has only the 2C amount of DNA) therefore probably determines both radio-sensitivity and rates of division.