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THE ANSWERS TO MORE QUESTIONS YOU NEVER ASKED

Why do leaves change colors in the fall?
To fully understand the answer to this question, you have to know a bit of science. Plants get their energy from a process known as photosynthesis. In this process, molecules within a plant’s leaves (called chlorophylls) capture light from the sun and combine it with water (drawn up through the plant’s roots) and carbon dioxide (from the air) to produce oxygen and glucose (sugar). The oxygen is given off as a waste product and the glucose is transferred from the leaf to the plant, where it is stored until needed.
               Leaves get their green color from the tiny chlorophyll molecules, which are contained inside larger chloroplasts (about 500,000 per square millimeter). The reason chlorophylls appear green is because they absorb both violet-blue and red light, but not green light. The colors you see appear as such because they reflect certain wavelengths of light, not because they absorb them. This is why white—which reflects all light—isn’t really a color, while black—which absorbs all light—is really all the colors put together.
               While chlorophyll is the major player in photosynthesis and does most of the light-absorption, there are other photosynthetic pigments in the leaf which aren’t visible most of the year; they are “overpowered” by the more numerous chlorophyll pigments. When the chlorophylls die off in the fall, these other pigments start to show through: carotene (found in carrots) gives leaves an orange color, xanthophyll (found in bananas) gives leaves a yellow color, and anthocyanin (found in apples, cherries, and cranberries) gives leaves a reddish or purplish color. Each pigment captures light more efficiently at different wavelengths, so by having several pigments, a leaf can absorb the most possible light from the sun.
               What causes the chlorophylls to die in the fall? As the days grow shorter and the supply of light and water diminishes, the plant’s biological clock starts ticking, and it prepares to hibernate for the winter. As the plant begins its hibernation process, abscission layers at the base of each leaf begin to swell. This swelling cuts off the “veins” between the leaf and the plant, trapping sugars in the leaf and cutting off water flow to the chlorophylls. The chlorophylls, deprived of water, die off. The abscission layer swells until the leaf is entirely cut off from the plant; the leaf either falls from its own weight or is blown by the wind.

Sources: www.na.fs.fed.us, sciencemadesimple.com, Biology, 4th Ed. (Raven & Johnson, 1996)