A plant cuticle is the waxy film or membrane that covers leaves and other dermal tissue on plant sections above the ground. Waxes and polymers such as cutin and cutan, which contain omega hydroxy acids, ester, epoxides, and hydrophobic aliphatic compounds, make up the cuticle. Cutin is a polyester polymer, while cutan is a hydrocarbon polymer; both contribute to a plant's ability to thrive in an aerial environment. Cuticular membranes protect plants primarily from water loss, but also assist with other dermal tissue functions such as infection prevention.
In effect, a plant cuticle functions much like human skin, in that it protects the plant from losing too much water, as well as serving as a barrier against certain bacteria, fungi, and other damage-causing organisms. Its film covers both the top and bottom of leaves and other dermal areas of the plant, encapsulating the uppermost epidermal layer of plant tissue. Leaf tops tend to have a thicker cuticle than shoots or the under side of leaves, since the top of a leaf is exposed to more sun, wind, and pests than other dermal tissue.
Underneath the protective layer of the cuticle resides the plant's upper and lower epidermis, as well as the mesophyll, where plant cells convert light to energy during photosynthesis. Without a plant cuticle, the water absorption process necessary to complete photosynthesis would require a far larger water supply to compensate for evaporation. Fewer tender plants and young shoots would form, and even fewer would survive without the cuticle offering further protection from bacterial or other microscopic infections.
As part a plant's shoot system — those plant components that appear above the soil line — dermal tissue is comprised of dense cells known as epidermal cells. Epidermal cells are responsible for secreting the waxy polymers and other substances that make up the plant cuticle and help the plant retain water. Soluble waxes and polymers secreted by epidermal cells spread all along leaf and shoot surfaces to form the protective cuticle membrane as these plant parts develop and grow.
Not all plants produce a cuticle. Plants with periderm, the system of epidermal layers on woody plants most commonly referenced as bark, do not have a plant cuticle. Instead, these plants have living inner periderm layers, such as the phelloderm and the cortex, as well as dead outer periderm layers, known as rhytidome or cork. Woody plants such as trees, certain types of vines, and shrubs have periderm layers rather than a cuticular membranes.