Populations of cells that make up organ tissue grow and contract. A traditional approach to modeling organ size restriction to an observed 'normal' level is to postulate a physical carrying capacity: effectively a limit on the physical region that can be occupied by the organ. The purpose of this note is to provide a very simple model for a cell population that grows under the control of positive and negative growth factors. It will be seen that such a model can result in logistic growth without the necessity of postulating a physical carrying capacity.