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  3. 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

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

admin2009-04-23  72
问题                                     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 spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditlons—lots of sugar and lots of light—spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimsoil. Because carotenoids are always present in leaves, the yellow and gold color remain fairly constant from year to year.
    The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.
What triggers leaf fall?
    In early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the processes leading up to their fall. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.
What does all this do for the tree?
    Winter is a certainty that all vegetation in the temperate zones must face each year. Evergreen plants, including trees, must have some sort of protection to survive freezing temperatures and other harsh wintertime influences. Stems, twigs, and buds are equipped to survive extreme cold so that they can reawaken when spring informs the start of another growing season. Tender leaf tissues, however, would freeze in winter, so plants must either toughen up and protect their leaves or dispose of them.
    The evergreens—pines, spruces, cedars, firs, and so on are able to survive winter because they have toughened up. Their needle-like or scale like leaf is covered with a heavy wax coating and the fluid inside their cells contains substances that resist freezing. Thus the leave of evergreens can safely withstand all but the severest winter conditions, such as those in the Arctic. Evergreen need les survive for some years but eventually fall because of old age.
The leaves of broad leaved plants, on the other hand, are tender and vulnerable to damage. These leaves are typically broad and thin and are not protected by any thick coverings. The fluid in ceils of these leaves is usually a thin, watery sap, that freezes readily. This means that be cells could not survive winter where temperatures fall below freezing. Tissues unable to endure winter must be sealed off and shed to ensure the plant’s continued survival. Thus leaf fall precedes each winter in the temperate zones.
When is the best time to see autumn color?
     Unfortunately, autumn color is not very predictable. But it generally starts in late September in New England and moves southward, reaching the Smoky Mountains by early November. It also appears about this time in the high mountains of the West. Remember that cooler high. positions will color up before the valleys, The Forest Service’s Fall Color Hotline (1-800-354-4595) can provide you with details as the autumn color display progresses.
选项 A、Y
B、N
C、NG
答案B
解析 由题干中的fall off the trees定位到文章首段第2句In those lucky places．．．is transformed into the vivid autumn palette of reds, oranges, golds, and browns before the leaves fall of the trees．题干中的change colors与原文中的transformed into the vivid autumn palette届同义转述；又因题干中的 fall off the trees before they change colors和原文中的is transformed into．．．before the leaves fall off the trees 相矛盾，因此题干表述错误。
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