A critical assessment is presented of the literature on variability in even-aged stands of plants. The variance of weight per plant accumulates during two physiologically distinct periods of growth, pre- and post-seedling emergence, and can be expressed as a function of the variances and covariances of three physiological variables - the rate of growth, the duration of growth, and the size per plant at the start of the period. This provides a quantitative framework for analyzing the physiological sources of variability in plant stands.