What is the waxy covering on plant leaves?

The waxy covering on plant leaves is the cuticle. Produced by epidermal cells, the plant cuticle is a thin, hydrophobic barrier made chiefly from the lipid polymer cutin and associated waxes. It coats the outer surface of leaves and other above-ground organs.

Its most important job is limiting water loss through evaporation. The cuticle also helps prevent liquid water, dissolved substances and many pathogens from entering the leaf surface. In some plants, a layer of epicuticular wax sits on top of the cuticle, creating a powdery or bluish bloom and making the surface strongly water-repellent.

The cuticle was a major adaptation in the transition from aquatic environments to life on land more than 450 million years ago. It works alongside stomata: the cuticle provides a relatively sealed surface, while stomata open and close to regulate carbon-dioxide intake and water-vapor loss. Desert plants such as cacti generally have thick cuticles. The famous self-cleaning lotus effect also depends on microscopic surface structure combined with wax, not simply on a smooth coating.