Endodermis is usually considered to be the innermost layer of the cortex. It is made up of a single layer of barrel-shaped cells which do not enclose intercellular spaces. The young endodermal cells possess a band of thickening which runs along their radial and tangential walls. This band of thickening is called Casparian strip. It is made up of both suberin and lignin. Casparian strips prevent plasmolysis of endodermal cells. Due to the presence of Casparian strips, the endodermal cells do not allow wall to wall movement of substances between cortex and pericycle. Substances must enter the cytoplasm of endodermal cells. As a result, endodermis functions as a biological check post.