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1、Why Leaves Turn Color in the Fall- Diane AckermanThe stealth of autumn catches one unaware. Was that a goldfinch perching in the early September woods, or just the first turning leaf? A red-winged blackbird or a sugar maple closing up shop for the winter? Keen-eyed as leopards, we stand still and sq
2、uint hard, looking for signs of movement. Early-morning frost sits heavily on the grass, and turns barbed wire into a string of stars. On a distant hill, a small square of yellow appears to be a lighted stage. At last the truth dawns on us: Fall is staggering in, right on schedule, with its baggage
3、of chilly nights, macabre holidays, and spectacular, heart-stoppingly beautiful leaves. Soon the leaves will start cringing on the trees, and roll up in clenched fists before they actually fall off. Dry seedpods will rattle like tiny gourds. But first there will be weeks of gushing color so bright,
4、so pastel, so confetti-like, that people will travel up and down the East Coast just to stare at it a whole season of leaves. Where do the colors come from? Sunlight rules most living things with its golden edicts. When the days begin to shorten, soon after the summer solstice on June 21, a tree rec
5、onsiders its leaves. All summer it feeds them so they can process sunlight, but in the dog days of summer the tree begins pulling nutrients back into its trunk and roots, pares down, and gradually chokes off its leaves. A corky layer of cells forms at the leaves slender petioles, then scars over. Un
6、dernourished, the leaves stop producing the pigment chlorophyll, and photosynthesis ceases. Animals can migrate, hibernate, or store food to prepare for winter. But where can a tree go? It survives by dropping its leaves, and by the end of autumn only a few fragile threads of fluid-carrying xylem ho
7、ld leaves to their stems. A turning leaf stays partly green at first, then reveals splotches of yellow and red as the chlorophyll gradually breaks down. Dark green seems to stay longest in the veins, outlining and defining them. During the summer, chlorophyll dissolves in the heat and light, but it
8、is also being steadily replaced. In the fall, on the other hand, no new pigment is produced, and so we notice the other colors that were always there, right in the leaf, although chlorophylls shocking green hid them from view. With their camouflage gone, we see these colors for the first time all ye
9、ar, and marvel, but they were always there, hidden like a vivid secret beneath the hot glowing greens of summer. The most spectacular range of fall foliage occurs in the northeastern United States and in eastern China, where the leaves are robustly colored, thanks in part to a rich climate. European
10、 maples dont achieve the same flaming reds as their American relatives, which thrive on cold nights and sunny days. In Europe, the warm, humid weather turns the leaves brown or mildly yellow. Anthocyanin, the pigment that gives apples their red and turns leaves red or red-violet, is produced by suga
11、rs that remain in the leaf after the supply of nutrients dwindles. Unlike the carotenoids, which color carrots, squash, and com, and turn leaves orange and yellow, anthocyanin varies from year to year, depending on the temperature and amount of sunlight. The fiercest colors occur in years when the f
12、all sunlight is strongest and the nights are cool and dry (a state of grace scientists find vexing to forecast). This is also why leaves appear dizzyingly bright and clear on a sunny fall day: The anthocyanin flashes like a marquee. Not all leaves turn the same colors. Elms, weeping willows, and the
13、 ancient ginkgo all grow radiant yellow, along with hickories, aspens, bottle brush buckeyes, cottonweeds, and tall, keening poplars. Basswood turns bronze, birches bright gold. Water-loving maples put on a symphonic display of scarlets. Sumacs turn red, too, as do flowering dogwoods, black gums, an
14、d sweet gums. Though some oaks yellow, most turn a pinkish brown. The farmlands also change color, as tepees of cornstalks and bales of shredded-wheat-textured hay stand drying in the fields. In some spots, one slope of a hill may be green and the other already in bright color, because the hillside
15、facing south gets more sun and heat than the northern one. An odd feature of the colors is that they dont seem to have any special purpose. We are predisposed to respond to their beauty, of course. They shimmer with the colors of sunset, spring flowers, the tawny buff of a colts pretty rump, the shu
16、ddering pink of a blush. Animals and flowers color for a reason adaptation to their environment but there is no adaptive reason for leaves to color so beautifully in the fall any more than there is for the sky or ocean to be blue. Its just one of the haphazard marvels the planet bestows every year.
17、We find the sizzling colors thrilling, and in a sense they dupe us. Colored like living things, they signal death and disintegration. In time, they will become fragile and, like the body, return to dust. They are as we hope our own fate will be when we die: Not to vanish, just to sublime from one be
18、autiful state into another. Though leaves lose their green life, they bloom with urgent colors, as the woods grow mummified day by day, and Nature becomes more carnal, mute, and radiant. We call the season “fall,” from the Old English feallan, to fall, which leads back through time to the Indo-Europ
19、ean phol, which also means to fall. So the word and the idea are both extremely ancient, and havent really changed since the first of our kind needed a name for falls leafy abundance. As we say the word, were reminded of that other Fall, in the garden of Eden, when fig leaves never withered and scal
20、es fell from our eyes. Fall is the time when leaves fall from the trees, just as spring is when flowers spring up, summer is when we simmer, and winter is when we whine from the cold. Children love to play in piles of leaves, hurling them into the air like confetti, leaping into soft unruly mattress
21、es of them. For children, leaf fall is just one of the odder figments of Nature, like hailstones or snowflakes. Walk down a lane overhung with trees in the never-never land of autumn, and you will forget about time and death, lost in the sheer delicious spill of color. Adam and Eve concealed their n
22、akedness with leaves, remember? Leaves have always hidden our awkward secrets. But how do the colored leaves fall? As a leaf ages, the growth hormone, auxin, fades, and cells at the base of the petiole divide. Two or three rows of small cells, lying at right angles to the axis of the petiole, react
23、with water, then come apart, leaving the petioles hanging on by only a few threads of xylem. A light breeze, and the leaves are airborne. They glide and swoop, rocking in invisible cradles. They are all wing and may flutter from yard to yard on small whirlwinds or updrafts, swiveling as they go. Fir
24、mly tethered to earth, we love to see things rise up and fly soap bubbles, balloons, birds, fall leaves. They remind us that the end of a season is capricious, as is the end of life. We especially like the way leaves rock, careen, and swoop as they fall. Everyone knows the motion. Pilots sometimes d
25、o a maneuver called a “falling leaf,” in which the plane loses altitude quickly and on purpose, by slipping first to the right, then to the left. The machine weighs a ton or more, but in one pilots mind it is a weightless thing, a falling leaf. She has seen the motion before, in the Vermont woods wh
26、ere she played as a child. Below her the trees radiate gold, copper, and red. Leaves are falling, although she cant see them fall, as she falls, swooping down for a closer view. At last the leaves leave. But first they turn color and thrill us for weeks on end. Then they crunch and crackle underfoot
27、. They shush, as children drag their small feet through leaves heaped along the curb. Dark, slimy mats of leaves cling to ones heels after a rain. A damp, stucco-like mortar of semi-decayed leaves protects the tender shoots with a roof until spring, and makes a rich humus. An occasional bulge or ripple in the leafy mounds signals a shrew or a field mouse tunneling out of sight. Sometimes one finds in fossil stones the imprint of a leaf, long since disintegrated, whose outlines remind us how detailed, vibrant, and alive are the things of this earth that perish.
