Lanthanum in combination with electron microscopy was used to study the functional development of the Casparian strip in the primary root of corn (Zea mays L. WF9 X M14). The Casparian strip was first visible between 13 and 16 mm from the root tip, and was first an effective barrier to lanthanum movement through the apoplast from cortex to stele between 14 and 18 mm from the apex. Until the Casparian strip was formed, lanthanum ion freely penetrated the stelar apoplast. By 18 mm from the apex lanthanum was completely restricted to the extracellular spaces of the cortex. Treatment with 0.02 or 0.03% (v/v) Triton X-100 increased the permeability of the plasma membrane, and allowed lanthanum ion to penetrate the cortical symplast. After the detergent treatment, the endodermis no longer functioned as a barrier to La3+ movement from cortex to stele. This confirms that an intact plasma membrane is necessary for the primary endodermis to control the passage of solutes into the vascular tissues of the stele.