The Hunt and Lu [Metall. Mater. Trans. A 27A (1996) 611] model for the selection of primary spacing, lambda, during cellular and dendritic growth predicts a range of spacings falling between minimum and maximum values of lambda (min) and lambda (max) respectively. Within the model lambda (max)/lambda (min) will be at least 2 and may, under certain circumstances, be significantly greater. In this paper we use a free boundary model of solidification within an array to demonstrate that interaction between the tips of the cells or dendrites leads to a transverse adjustment mechanism that will tend to equalise the spacing as growth proceeds. This transverse adjustment mechanism is shown to be rapid for the spacings characteristic of cellular growth but much more gradual for the spacings characteristic of dendritic growth. These findings are consistent with observations of the primary spacing of dendrites grown in alloys of the transparent casting analogue, succinonitrile.