Lengths of cells in mitosis in different regions of root apical meristems of Pisum sativum cv. Alaska and Hyacinthus orientalis were used to compare relative division rates in the cortex and the stele regions of the meristem at different distances from the root apex. Division rates in the stelar meristem may be lower than those at corresponding positions in the cortical meristem and become progressively more so with distance from the tip. Mitotic divisions cease closer to the tip in the stele than in the cortex. 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. In at least 1 region of the meristem, cell proliferation may obey the laws of exponential growth.