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Epidermal tissue is a single layer of cells, (except in velamen,pepperomia and rubber plants) that covers the plant body

Functions:

acts as a buffer between the environment and the internal plant tissues;
absorption of water and minerals primarily in the root region,,
on the stem and leaves generally covered with a cutin which prevents evaporation, secretion of cuticle (waterproof; made of cutin= fatty substances)
can form a barrier resistant to bacteria and fungi,
can prevent leaching of materials in or out of the surface
recognition: lectin on epidermal surface can permit recognition of other types of cells- sometimes of other organism such as invading fungi

Specialized cells:

Root hairs- absorb water and the material it carries ( minerals).;they increase absorptive surface area of roots Although single celled, they can be viewed by the eye on rapidly growing radishes
Leaf hairs (trichomes)- 1-2 cells; important in boundary layer and in defense; * they reflect light to protect against overheating and excessive water loss. This is an incredibly important function for plants in dry regions where excess light may lead to photobleaching of pigments and excess absorption of light would overheat the tissue. the layer acts to hold in a layer of humidity 'trapped' between the epidermis and the tips of the trichomes. As water diffuses from a region of high to low density, less water will escape from the stomata to the outside air since the air layer outside the stomata is 'moist' due to its entrapment by the trichomes. This layer also prevent air moving directly against the stomata which would encourage water loss. Its' rough surface breaks up wind currents. When in high density they act to deter herbivory by small animals ( image being an insect walking through a forest of these spiky trichomes). Dependent on species they may have barbs or tips that give off nasty compounds when hit. Some have rigid barbs that impale insects dropping onto them, as with flying insects)
Stomata- created by guard cells; most abundant on underside of leaves; regulate diffusion of CO2 into the leaf for photosynthesis as well as regulate loss of water from the leaf; Stomata respond to turgor pressure stomata open when K⁺ and water enter, thereby increasing turgor pressure in guard cells, thus pushing them apart stomata close when K⁺and water leave the guard cells, thus leaving them flaccid and closing upon each other.
Salt glands- dump sites for the excess salt absorbed in water from the soil; help plants adapt to life in saline environments; a crust of salt forms on leaves which tastes bad and the white surfaces act to reflect light The white mangrove pictured to the right has 2 salt glands on each leaf which exude salts taken up with water.