Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into oxygen and glucose(葡萄糖). Oxygen is a gas in the air that we need to breathe. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis(光合作用).

　　A chemical called chlorophyll (叶绿素)helps make photosynthesis happen. Chlorophyll is what gives plants their green color.

　　As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter.

　　During winter, there is not enough light or water for photosynthesis. The trees will rest, and live on the food they stored during summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green dies away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just can't see them in summer, because they are covered up by the green chlorophyll.

　　The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.

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