The leaf is a good deal more complex than many would assume. Their daily functions are spectacular and give us a lot of insight as to how a plant uses leaves to manufacture complex enzymes, proteins, vitamins, sugars, etc. from the nutrients it takes up through its roots. We’ll start with the epidermis. The epidermis is the outermost layer of the leaf as well as of our skin. Like our epidermis, its main purpose is to provide protection to the inner tissues of our skin and the leaf. The epidermis usually consists of a single cell layer, but some leaves will have multiple cell layering allowing them have a stronger outer layer. The epidermis consists of two surfaces. The adaxial surface (also known as the upper epidermis) and the abaxial surface (also known as the lower epidermis). Both the upper and the lower epidermis are present on either side of the leaf. The upper epidermis is responsible for the safety of the leaf from things like dehydration, intense sunlight, and being eaten by insects and livestock. In many instances dehydration will be the biggest threat to the plant. This is why the plant is covered with a cuticle, a layer of cutin.
Plant cuticles are protective, lipophilic membranes that cover the leaves and stems. The cuticle consists of a polymer make-up that is covered with epicuticular waxes and incorporates intracuticular waxes. Together these constituents provide a good but imperfect barrier against the loss of water and solutes and ingression by pests and pathogens. Below the epidermis are layers of cells known as the mesophyll, or “middle leaf.”
The mesophyll of most leaves contains the palisade parenchyma (also called the palisade mesophyll) and the spongy parenchyma. The palisade mesophyll’s structure are column-shaped, tightly packed cells. Underneath the palisade mesophyll are the loose, irregular shaped cells known as the spongy parenchyma (or spongy mesophyll). Both of the these layers of the meosphyll contain chlorplasts. But the palisade mesophyll contains the majority of all the chlorplasts. A major role of the palisade mesophyll is to contain large amounts of chloroplast to allow photosynthesis giving the plant what it needs for fundamental task of converting solar energy to chemical energy.
Kerstiens, Gerhard(Jan 2016) Plant Cuticle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002088.pub3]