Leaves serve plants by converting energy from the sun into chemical energy the plant can then use, exchanging gases with the atmosphere and transporting biochemical products throughout the plant. A leaf's structure consists of several distinct layers and components. The parts of the leaf are important in allowing the leaf to carry out these important functions.
The epidermis functions like a skin to the plant, providing a barrier between the outside world and the inner workings of the leaf. The epidermis is only one cell thick, making it a thin barrier. The epidermis protects the more delicate insides of the leaves. Its waxy covering, called the cuticle, prevents the loss of gases and water from the leaf. Special cells on the epidermis may grow into trichomes, small hairlike projections that make the leaves unpalatable to animals, protect the plant from the weather and secrete substances that further protect the leaf.
Under the epidermis at the top of the leaf lies the palisade layer. The palisade layer contains long cells situated at right angles to the epidermis. Within the palisade layer, the majority of photosynthesis occurs in special organelles called chloroplasts. Photosynthesis is a biochemical process that allows plants to convert energy from the sun into chemical energy in the form of sugars. Using carbon dioxide and water as the inputs, plants produce sugar and oxygen.
Beneath the palisade layer is a layer of loosely arranged cells called the spongy layer or spongy mesophyll. While some photosynthesis occurs in the spongy layer, its primary purpose is to store the gases needed for photosynthesis, making them available to the palisade layer. Because the cells in the spongy layer are loosely packed, these essential gases exist in the spaces between the cells. The spongy layer also communicates with the stomata on the lower epidermis, signaling when the leaf needs to release oxygen or take up more carbon dioxide.
Stomata and Guard Cells
On the epidermis on the undersides of leaves, plants contain pores called stomata. Stomata allow the leaf to take up the carbon dioxide needed for photosynthesis and release the oxygen produced by it while losing minimal water to evaporation by the sun. Stomata are flanked by two guard cells in the epidermis. When osmotic pressure inside the guard cells increases, the stomata open, allowing for the exchange of gases and photosynthesis to occur. When pressure decreases, the guard cells close. The most important reason stomata are able to close concerns the loss of water, called transpiration. According to Harvard professor John Kimball, a plant loses about 90 percent of its water to transpiration.
Leaf veins connect the leaf to the plant. Leaves require water for photosynthesis, and the sugars that they produce nourish the whole plant, not just the leaves in which they were produced. Leaf veins act as a transport system for the plant, delivering water and minerals to the leaves and removing sugars produced during photosynthesis. In most leaves, any single cell in the spongy layer is only a few cells from a leaf vein.