There are four types of simple permanent plant tissues: parenchyma, collenchyma, sclerenchyma, and epidermis. Parenchyma cells have thin primary walls and form the "filler" tissue in soft plant parts. Collenchyma cells have some areas of secondary thickening and are found under the epidermis of young stems and in leaf veins. Mature sclerenchyma cells are dead with thick secondary walls containing cellulose and lignin, making them rigid. The epidermis is a single outer layer of relatively flat cells that forms a continuous protective sheet covering the entire surface of the plant.