Why leaves change colors If you are lucky, you live in one of those parts of the world where Nature has one last fun before settling down into winter's sleep. In those lucky places, as days shorten and temperatures become crisp, the quiet green palette(调色板) of summer leaves is transformed into the vivid autumn palette of reds, oranges, golds, and browns before the leaves fall off the trees. On special years, the colors are truly breathtaking, How does autumn color happen? For years, scientists have worked to understand the changes that happen to trees and shrubs in the autumn. Although we don't know all the details, we do know enough to explain the basics and help you to enjoy more fully Nature's multicolored autumn farewell. Three factors influence autumn leaf color-leaf pigments(色素), length of night, and weather, but not quite in the way we think. The timing of color change and leaf fall is primarily regulated by the calendar. That is, the increasing length of night. None of the other environmental in flounces -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. Where do autumn colors come from? -A color palette needs pigments, and there are three types that are involved in autumn color. Chlorophyll(叶绿素),which gives leaves their basic green color. It is necessary for photosynthesis(光合作用), the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. -Carotenoids(类胡萝卜素), which produce yellow, orange, and brown colors in such things as corn, carrots, and daffodils. -Anthocyanins(花青素), which give color to such familiar things as red apples, grapes, blue berries, cherries, strawberries, and plums. They are water soluble and appear in the watery liquid of leaf cells. Both chlorophyll and carotenoids are present in the chloroplasts of leaf cells throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells. During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As night length increases in the autunm, chlorophyll production slows down and then stops and eventually all the chlorophyll is destroyed. The carotenoids and anthocyanins that are present in the leaf are then unmasked and show their colors. Certain colors are characteristic of particular species. Oaks turn red, brown, or russet hickories, golden bronze aspen and yellow-poplar, golden yellow dogwood, purplish red. Maples differ species by species: red maple turns brilliant scarlet sugar maple, orange-red and black maple, glowing yellow. Leaves of some species such as the elms simply shrink and fall, exhibiting little color. The timing of the color change also varies by species. Sourwood in southern forests can become vividly colorful in late summer while all other species are still vigorously green. Oaks put on their colors long after other species have already shed their leaves. These differences in timing among species seem to be genetically inherited, for a particular species at the same latitude will show the same coloration in the cool temperatures of high mountains at about the same time as it does in warmer lowlands. How does weather affect autumn color? The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is decreasing. Temperature and moisture are the main influences. A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most specta