The pathogen enters through the periderm or lenticels. Suberin is found in the phellem layer of the periderm (or cork). Phellogen is defined as the meristematic cell layer responsible for the development of the periderm. These can eventually rupture within the tuber periderm. Within the periderm are lenticels, which form during the production of the first periderm layer. Curing is a technique done to toughen up the outer layer of skin (periderm). The newly formed periderm is a purplish red. The outermost cortex of oldest stems developed into the bark-like lycopodiopsid periderm. Those three tissues form the periderm, which replaces the epidermis in function. The former forms secondary xylem and secondary phloem, while the latter forms the periderm. The periderm forms from the phellogen which serves as a lateral meristem. After that, the pathogen colonizes the periderm cells in the tuber. The periderm replaces the epidermis, and acts as a protective covering like the epidermis. Together, the phellem (cork), phellogen (cork cambium) and phelloderm constitute the periderm.