Diversity of cell lengths in terminal portions of roots: location of the proliferative cell population

Terminal meristems are responsible for all primary growth of roots. It has been asserted that all cells of root meristems are actively dividing and th at the stem cell (proliferative) population expands exponentially. Lengths of cells in roots just proximal to the root cap/root initial boundary were used to determine the numbers of cortex and stele cells in the meristem. Me ristem cells were defined as cells that did not have significantly differen t cell lengths from initial cells at the boundary. Data show that, for five of the six species (Allium cepa, Pisum sativum, Pyrus communis. Triticum a estivum, Vicia faba, and Zea mays) tested, only the first 15 stele and the first 10-35 cortex cells in median longitudinal sections would be in the me ristem. For T. aestivum, no discrete meristem was found because all cells p roximal to initial cells were longer than initial cells. In addition to thi s subject area, distributions of lengths of cells in the root meristem usin g this definition. for the six species were compared with a theoretical cel l-age distribution for exponentially dividing cells, to determine if distri butions of cell lengths were similar to a theoretical distribution of expon entially dividing cells. For all species tested, distributions of cell leng ths were not similar to a theoretical cell-age distribution. From the data of this study with six plant species, we conclude that either contiguous pr oliferative cell populations of root meristems are very small or the prolif erative cell population is not continuous. In addition, such populations do not resemble a theoretical exponential cell-age distribution. Moreover, it seems that the proliferative capacities of cells within terminal root segm ents differ markedly among species and are not easily characterized. (C) 20 01 Elsevier Science B.V. All rights reserved.