Analysis of Heterogeneity of Relative Division Rates in Root Apical Meristems

Lengths of cells in mitosis in different regions of root apical meristems of Pisum and Hyacinthus were used to compare relative division rates in the cortex and stele regions of the meristem at different distances from the root apex. The results suggest that division rates in the stelar meristem are lower than those at corresponding positions in the cortical meristem and that they become progressively more so with distance from the tip Mitotic divisions also cease closer to the tip in the stele than in the cortex. Together, these differences are responsible for determining differences in cell lengths in the mature tissues of the root and indicate that tissue-specific differentiation begins within the meristem itself very close to the tip of the root. Other evidence is interpreted as showing that, in at least one region of the meristem, cell proliferation obeys the laws of exponential growth.