The outermost cells of the primary plant body, usually consisting of a single layer but sometimes several layers thick (multiple or multiseriate epidermis). In either case the cells differentiate from the *protoderm
. In stems and roots exhibiting secondary growth, the epidermis is usually replaced by the *periderm
. The term epiblem (rhizodermis) is sometimes used in place of epidermis for the outermost layer of cells in the root. The main function of the epidermis is to protect the underlying tissues from excessive water loss and, to some extent, from physical injury and attack from pathogens.