Leaves prevent water loss through stomata and cuticle and are the location where most photosynthesis takes place.
A leaf is the primary photosynthetic organ of vascular plants. Every day, thousands of chemical reactions take place in the chloroplasts of leaves. Leaves take in carbon dioxide and release oxygen and water vapor. Additionally, leaves are the end point for water transport through plants. Most leaves have a petiole, a stalk that attaches the leaf to a branch, and some have small branch-like extensions called stipules. The stipule is a small flap that protects new leaf growth. A blade, another name for a leaf, contains a network of veins, a central midrib (a thick, linear support structure that extends along the length of the leaf), and a petiole or stem connecting the leaf blade to a branch. A petiole contains thin, pipe-like structures. These structures are used for transporting water and minerals to the leaves as well as carbohydrates, produced by photosynthesis, from the leaves back to the stems or branches. For leaves that have stipules, these are tiny, leaf-like protuberances or flaps where the petiole connects to a branch and protects young leaf blades as they grow. Willows and some cherry trees have stipules, as do roses, mesquite, and tulip poplars.

Leaf blades may be long and thin, such a pine needles or grass, or more oval in shape, such as the leaves of rubber plants. Leaf blades carry the chloroplasts, where photosynthesis takes place, and they have distinctive edges, shapes, and veining. For example, smooth-edged leaves can be found on mulberry trees, while toothed-edge leaves appear on white birches and lobed leaves are on white oaks. Leaves also bear different vein patterns: pinnate, palmate, or parallel. Grass and irises have parallel veining that runs in long, thin lines from the branch to the leaf point. Pinnate veins run off the center vein and travel at angles from the center vein to the leaf's edge. Palmate veining is like the veins in a hand, spreading out from the base of the blade to leaf edges and having many branch veins for each primary vein.

Before photosynthesis takes place, leaves take in essential carbon dioxide used in producing carbohydrates. Once photosynthesis occurs, the plant must release plant wastes, which are water and oxygen. Those waste products are released through the stomata. Photosynthesis is a continuous process when the sun is up, with the stomata opening and closing, but stomata remain closed throughout the night based on environmental signals that trigger their movement. The mechanism that works stomata is fairly simple, operating like an automatic, sliding door. The stoma is surrounded by the thick inner walls of the two guard cells. A guard cell is one of two curved cells surrounding stoma that work jointly to open and close the stoma. When the sun shines, a drop in the amount of carbon dioxide increases levels of potassium and water in the guard cells. This causes guard cells to swell; the stoma opens, allowing intake of carbon dioxide and output of water waste. At night, potassium and water levels in guard cells decrease; the cells collapse and the stoma closes. Controlling the size and movement of the stomatal portal optimizes the efficient use of water. Of the water held in a plant, 90% moves through the plant and is lost through the stomata. Transpiration, the loss of water from the plant leaves through the stomata as a result of temperature and humidity, is faster at higher temperatures and slower in humid conditions.

Many leaves have a protective, thick, clear wax coating, called the cuticle. Cuticle is a clear, waterproof layer on leaves and stems secreted from epidermal plant cells. This coating prevents needless water loss from leaves or stems. At the same time, the cuticle is translucent, so sunlight enters surfaces coated with cuticle, and photosynthesis still takes place.
Guard cells control the opening and closing of the stoma. Sunlight triggers the guard cells to swell, which opens the stoma during the day, allowing carbon dioxide (CO2) to enter the cell and for photosynthesis (production of food from sunlight) and transpiration (release of water (H2O) and oxygen (O2)) to occur. At night when it is dark, the guard cells shrink, closing the stoma.