On the inner side of the epidermis is the cortex and in many plant types it can be subdivided into an exodermis, a central cortex and endodermis. The cell walls are thickened and contain suberin and lignin. The cortex allows for the diffusion of water, mineral salts and oxygen from the root hairs inwards. The cortex stores foods reserve, especially starch. It also serves to transport water and salts from the root hairs to the center of the root.