Dermal tissue system is protective in function and based on its origin, it is classified into two types :
This is the primary surface tissue of the entire plant. It can be thought of as the plant’s “skin”. The epidermis is generally a single layer of closely packed cells. The cells of epidermis are parenchymatous having protoplasm and nucleus without intercellular spaces. Epidermal cells are compactly and continuously arranged; the continuity is lost by the presence of stomatal pores or breaks in the tissue. Covering the aerial epidermis, cutin (fatty substance) is present as an impregnation on cell wall. The cuticle can be separated from epidermis.
The epidermis may produce unicellular or multicellular hairy outgrowths (trichomes) and other appendages. Epidermis provides mechanical protection, allows gaseous exchange through stomata, restricts transpiration with cuticle, and is also involved in storage, photosynthesis, secretion, absorption and perception to stimuli. Stomata are Pores, each guarded by two guard cells, which control the size of the pore. Cells surrounding guard cells, but differing from other epidermal cells, are called subsidiary cells. Guard cells are kidney shaped; their cell walls are thick on the inner surfaces. Guard cells contain many chloroplasts. Sclereids (sclerenchyma cells) in epidermis provide the protection and support to the plant.
The periderm, also called bark, replaces the epidermis in plants that undergo secondary growth. The periderm is multilayered as opposed to the single layered epidermis. It consists of cork cells (phellem), phelloderm, and phellogen (cork cambium). Cork cells are nonliving cells that cover the outside of stems and roots to protect and provide insulation for the plant. The periderm protects the plant from pathogens, injury, prevents excessive water loss, and insulates the plant.