Epidermis: a single layer of living parenchyma cells. Outer tangential walls are encrusted with cutin, which is a fatty substance that makes the wall impermeable to water. In most plants, cutin builds up into a more or less pure layer called the cuticle. Epidermis contains pairs of cells called guard cells, with a hole, or a stomatal pore between them. Together the guard cells and the stomatal pore constitute a stoma. Most plants will have some epidermal cells elongate outward and become trichomes, or hairs, and they make it difficult for an animal to land on, walk on, or chew into a leaf.