Leaves are nature's phenomenal food factories. This is how they work. Plants take water from the ground through their roots and they take carbon dioxide from the air. They use the sunlight to turn water and carbon dioxide into oxygen and glucose. They then use the glucose as food for energy and as a building block for growing. This process of turning water and carbon dioxide into oxygen and sugar is called photosynthesis. A chemical called chlorophyll helps make photosynthesis happen and chlorophyll is what gives plants their green color.
Three factors influence autumn leaf color-leaf pigments, length of night, and weather, but not quite in the way we think. The timing of color change and leaf fall are primarily regulated by the calendar, that is, the increasing length of night. None of the other environmental influences - temperature, rainfall, food supply, and so on - are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature's autumn palette.
During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades 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 the 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 turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
It is the combination of all these things that make the beautiful fall foliage colors we enjoy each year.
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