Leaves are a plant’s food factories. Plants make their own food in a process called photosynthesis, using sunlight, air, and water. Leaves are special organs that help plants collect light from the Sun. Sunlight is a form of energy and is changed to chemical energy inside the leaves.
The job of collecting sunlight is done by a chemical in leaves called chlorophyll. This is what makes most leaves look green. Many leaves turn yellow and red as their chlorophyll disappears in the autumn and other colours show through.
WHAT ARE THE DIFFERENT PARTS OF A LEAF?
There are tiny openings called stomata all over the surface of leaves. These let out oxygen and take in carbon dioxide from the air during photosynthesis. Plants also lose water by evaporation through the stomata. This is called transpiration. The rest of the leaf surface has a waxy outer skin to stop water loss. Plants control the amount of water they lose by opening and closing the stomata. This is done by guard cells on either side of each stoma that can change shape.
Look at a leaf carefully and you will notice that it is patterned with veins. Many leaves have one large vein down the centre with small veins branching off it. These veins contain plant tissues called xylem and phloem. The xylem brings water from the plant to the leaf, and the phloem carries the food made in the leaf to the rest of the plant.
ARE ALL LEAVES THE SAME?
Leaves come in many shapes and sizes. Looking at leaf shapes can help you to identify plants. Trees such as beech or oak have simple leaves. Others, such as ash, have a row of leaflets attached to both sides of a central stalk. These are called compound leaves. The horse chestnut has a fan-like compound leaf.
IS IT BETTER TO BE DIFFERENT?
Most leaves are broad and flat so that they expose as much surface as possible to the Sun. But leaf shape and size can vary depending on the climate in which the plant grows. Conifers have needle-like leaves with only a small surface area exposed to the drying winds of their cold northern homes.
Many plants that live in hot deserts have thick, fleshy leaves that can store water. Cacti store water in their stems and many have left so thin that they have become spines. That way they lose as little water as possible.
In tropical rainforests, there is no need to store water. The climate is hot and humid and it rains every day. The leaves of plants that grow here often have pointed tips to allow rainwater to slide off them easily. All these leaf forms are adaptations to the plant’s environment.
FOOD FOR ALL
Leaves are an important and nutritious food for many animals. We use many different leaves in cooking and salads. Every time we eat leafy vegetables, we are taking in food the plant has made using the Sun’s energy.