During plant development, the dermal tissue system develops in a plant’s outermost layer of cells, called its epidermis. The stems and roots of woody plants develop a dermal tissue called periderm.
The epidermis grows to cover the expanding plant body. The cells of the epidermis are initially small and round and usually have a small central vacuole or none at all. Once cell division ceases in the epidermis of an organ, the epidermal cells expand. Some epidermal cells differentiate to form one of three specialized structures, about which you’ll learn more later:
Stomatal guard cells, which form stomata (pores) for gas exchange in leaves
Trichomes, or leaf hairs, which provide protection against insects and damaging solar radiation, as well as secrete oils (such as those produced by mints)
Root hairs, which greatly increase root surface area, thus providing more surface for the uptake of water and mineral nutrients
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The aboveground epidermal cells secrete a protective extracellular layer called a cuticle. The cuticle is made up of cutin (a polymer composed of long chains of fatty acids), a complex mixture of waxes, and cell wall polysaccharides. The cuticle limits water loss, reflects potentially damaging solar radiation, and serves as a barrier against pathogens.