THE duration of the mitotic cycle in comparable tissues of different plant species is closely correlated with the amount of nuclear DNA1,2. In a survey of seven species Van't Hof2 showed that the average duration of the mitotic cycle in root tip meristems increases at the rate of 0.3 h/pg of DNA. Our own data agree in general with these results, but we have found certain exceptions. For example, where the increase in DNA is brought about by the addition of B chromosomes the duration of mitotis is increased to a disproportionate extent3. We have also found an unexpected difference in the relation between nuclear DNA and the duration of mitosis in monocotyledons as compared with dicotyledons.