How does the endodermis control the entry of substances into the xylem?

In the roots, the endodermis cells are impregnated with suberin; a waxy substance which is water resistant, to form distinctive Casparian strips and prevent movement of water via the apoplast pathway. Water carrying mineral salts can only enter the root through the interior of cells (the symplast) as opposed to passing between cells (the apoplast). This means that the plant can directly regulate what enters the root proper. The endodermis cells contain starch granules as energy store. Mineral ions are actively secreted into the xylem vessels to maintain root pressure.