As
summer ends and
autumn comes, the days get shorter and shorter. This is how the trees sense that they should begin getting ready for
winter.
During winter, there is not enough light or water for photosynthesis. The trees rest then, 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 colours 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 autumn. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red colour. The brown colour of trees like oaks is made from wastes left in the leaves.
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