The purpose of the waxy outer layer of a leaf, which is called a cuticle, is to keep the leaf from losing water inside the leaf. Plants that live in the water do not have this problem, so they have no cuticle.
The outer layer of a leaf is called the epidermis (from a Greek word meaning "over-skin"). Its function is to protect the insides of the leaf, which carry out the chemical reaction known as photosynthesis.
The surface area between the tip and the bottom of a leaf is called the lamina. The lamina is attached to the plant stem by the petiole. The lamina is supported by veins, which carry nutrients to the leaf tissue.