The structure of leaves has evolved over time to make the maximum use of the plant's surface space in order to produce the nutrients that the plant needs in order to survive. Within the leaves photosynthesis takes place, which produces glucose, an energy source for the plant. From here the resulting carbohydrate is then stored in the leaves for immediate use or transferred to other areas of the plant. Leaf structure is complicated, and within its epidermal layer includes multi-functional cells surrounding internal vascular tissue consisting of a vein. The leaf is designed so that it can both ingest external resources in the production of energy, and provide a respiratory function that allows gases to escape.
The external structure of a leaf is made of a thick dermal layer that protects the internal cells. This furthest external layer also produces what is referred to as a cuticle, which further protects the leaf with a waxy coating. Within this layer stomata are formed in pairs called “stoma” individually. The stomata that form allow gases such as carbon dioxide to enter the plant cells for use in photosynthesis, as well as allowing its bi-products such as oxygen to leave.
The dermal layer protects stores of plant material in the form of parenchyma and collenchyma cells. The parenchyma cells are the most common type of plant cell and are used to store starches and water; this is also where photosynthesis occurs. The collenchyma plant cell is a specialized cell which serves a particular function in younger plants and, together with parenchyma cells, forms around the vascular system. These types of cells are differentiated by their function as well as their ability, or lack thereof, to form hardened cell walls.
Within these layers of storage and specialized cells runs the vascular system of the leaf cell. It is through this vascular network that the cell transfers necessary fluids and nutrients. The vascular system is comprised of the xylem, which conducts water through the plant, and the phloem which transmits nutrients. The xylem and the phloem exist side by side and together form the vein.
All of these layers of differentiating cell structure and function combined form an efficient production facility for glucose, as well as a by-product that all life needs, which is oxygen. Plants are so efficient at converting water and carbon dioxide into energy that they often store large amounts for future use. The leaf structure is specialized for the most efficient use of its own surface area in this endeavor. From its very apex to its axil, the leaf is a complex biological machine capable of sustaining its much larger plant structure past the point of survival and toward reproduction.