Leaf

A leaf is a flat, usually green part of a plant that grows out from the stem. Leaves are the food factories of most plants. They use sunlight, air, and water to make sugar through a process called photosynthesis. Almost every plant on land has leaves of some kind, from tiny moss leaves to giant palm fronds longer than a school bus.

Most leaves have the same basic parts. The wide, flat part is called the blade. A small stalk called the petiole connects the blade to the stem. If you hold a leaf up to the light, you can see thin lines running through it. These lines are veins. Veins carry water from the roots into the leaf and carry sugar back out to the rest of the plant.

The flat shape of a leaf is not an accident. A wide, thin blade catches as much sunlight as possible. Inside the leaf are millions of tiny green parts called chloroplasts. Chloroplasts hold a chemical called chlorophyll, which catches the energy in sunlight. Chlorophyll is also what makes leaves green.

Leaves also breathe, in a way. The bottom of a leaf is covered with tiny holes called stomata. The stomata open to let in carbon dioxide from the air. They also let out oxygen and water vapor. A single tomato leaf can have more than a million stomata, all too small to see without a microscope.

In places with cold winters, many trees lose their leaves every fall. These trees are called deciduous. Before the leaves drop, the tree pulls the green chlorophyll back into its branches. Other colors that were hidden underneath, like yellow, orange, and red, suddenly show through. That is why fall leaves change color. Trees that keep their leaves all year, like pine and holly, are called evergreen.

Leaves come in an amazing range of shapes and jobs. Cactus spines are actually leaves shaped into needles to save water. The Venus flytrap has leaves that snap shut to catch insects. The leaves of the giant Amazon water lily can grow over nine feet across, big enough for a small child to sit on. Onions are made of layered leaves that grow underground.

Without leaves, the air would be very different. Almost all the oxygen people and animals breathe was made by leaves and other green living things, slowly turning sunlight into life.