Longitudinal growth of etiolated pea stem segments has been shown to be strongly stimulated by indoleacetic acid. Lateral growth, on the other hand, was hardly stimulated either by high or low indoleacetic acid concentrations. Addition of sucrose to high indoleacetic acid concentrations (10 /ug/cc) enhanced lateral growth. Although the 80% ethanol soluble sugar content increased greatly during the growth period, it was only slightly affected by indoleacetic acid. Lateral growth is not, therefore, a result of an increased osmotic value of the cell sap, and the cells consequently do not swell. Not more than 5% of the water absorbed by stem segments during longitudinal and lateral growth is released when the segments are incubated in solutions of high osmotic value. It is concluded that lateral growth of etiolated pea stem segments in solutions of 10 fig/cc IAA and in the presence of sucrose requires, in the first instance, a weakening of the cell wall in a lateral direction and sucrose is somehow needed for this process.