Towards an understanding of the behaviour of root meristems

The behaviour of cells within the root apex and its meristem is considered within the framework of the following propositions. 1. (1) The quiescent centre is the site of a founder cell population from which all other cells of the root can be derived. During steady-state growth of the root the founder cells progress through the mitotic cycle slowly and have an indeterminate reproductive life-span. 2. (2) Surrounding the quiescent centre are initial cells and their derivatives; both types of cells progress through the mitotic cycle quickly and the derivative cells have a determinate reproductive life-span. The number of founder cells remains approximately constant during root growth, so when such a cell divides one of its daughters, or another founder cell, must displace an initial and take on its function. 3. (3) It is supposed that in the root apex there are gradients of substances that regulate aspects of cellular and nuclear behaviour. Founder cells in the quiescent centre may be the source of a gradient of a substance (perhaps a cytokinin) which, as long as its concentration is at an appropriate level, triggers mitosis and cytokinesis. A second substance (perhaps an auxin) moves towards the quiescent centre from a source in the maturing cells of the root apex; it is supposed that its concentration is appropriate to maintain nuclear DNA synthesis in cells of the meristem and elsewhere in the apex. The relative concentrations of the two substances may regulate the rates of mitosis, cell growth and the switch from mitosis to endomitosis. 4. (4) Besides their role in maintaining cell division in the meristem, the founder cells, together with the cells immediately surrounding them, may constitute a pattern of cells that is self-perpetuating. 5. (5) Founder cells may constitute a pool of cells whose proliferative potential is not impaired by the passage of time. The propositions are predictive and possible experimental tests of, as well as evidence for, their veracity are presented.