Ethylene pretreatment of intact Avena seedlings or of excised coleoptile sections results in an increased response of the cells to auxin. It is suggested that ethylene brings about the acceleration of hydrolytic reactions controlling the physical properties of cell walls and hence increases their capacity for growth. Coleoptile elongation of intact seedlings is inhibited by ethylene; this inhibition is concurrent with a lateral expansion of the entire coleoptile. It is suggested that under a given set of conditions coleoptile cells are capable of attaining a finite volume and that the preferential lateral expansion induced by ethylene is accomplished at the expense of longitudinal extension. Experiments with intact and deseeded plants indicate that lateral expansion depends on the supply of some factor from the endosperm.