November 24, 2012 3:04 am

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Leaves are a plant’s food factories. Plants make their own food in a process called photosynthesis. Photosynthesis requires sunlight, air, and water. Leaves help plants collect light from the Sun. Energy from sunlight is used inside the leaves to create food for the plant.
The job of collecting sunlight is done by a chemical in leaves called chlorophyll. Chlorophyll makes leaves look green. Many leaves turn yellow and red in the fall because they stop making chlorophyll.
A typical leaf has two basic parts: a blade and a petiole. The blade is the thin, flat surface of the leaf. Photosynthesis mostly takes place in the blade. The petiole is a stalklike structure that supports the leaf blade. It also serves as a passageway for water and nutrients to reach the leaf.
Look at a leaf carefully and you will notice that the blade is patterned with veins. Many leaves have one large vein down the center with small veins branching off. These veins contain plant tissues called xylem and phloem. The xylem brings water from the plant to the leaf. The phloem carries the food made in the leaf to feed the rest of the plant.
There are tiny openings called stomata all over the surface of leaves. These openings allow gases in the air to pass in and out of the plant during photosynthesis. Plants also lose water by evaporation through the stomata. This loss of water is called transpiration. It helps keep plants cool, just like perspiration (sweat) helps keep you cool.
Leaves come in many shapes and sizes. The shape of a plant’s leaves can help you identify the plant. Plants such as oak or maple trees have a single blade. These are called simple leaves.
Other plants, such as clover, have blades that are divided into separate leaflets attached to a central stalk. These are called compound leaves. A four-leaf clover, therefore, doesn’t really have four leaves—it has one leaf with four leaflets.
Most leaves have broad, flat blades 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. Different leaf shapes help the plant adapt to a particular environment. Pine trees, for example, have needle-like leaves. These narrow leaves expose only a small surface area to dry winds that blow in cold, northern areas where most pine trees grow. Pine trees lose little moisture as a result.
It rarely rains in deserts. Many desert plants have thick, fleshy leaves that can store water. In tropical rain forests, however, there is no need to store water. The air is humid, and it rains almost every day. The leaves of rain forest plants often have pointed tips that let water easily slide off.
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 such as lettuce or spinach, we are taking in food the plant has made using the Sun’s energy.
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