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1、Unit 6Part BText 1Why Do Leaves Change Color?In some places, as days shorten and temperatures become crisp, the quiet green of summer foliage is transformed into the vivid autumn of reds, oranges, yellows and browns before the leaves fall off the trees. In special years, the colors are truly breatht
2、aking.But have you ever wondered how and why this happens? To answer that question, we first have to understand what leaves are and what they do. Leaves are Natures food factories. Plants take water from the ground through their roots, and carbon dioxide from the air. Then they turn water and carbon
3、 dioxide into a kind of sugar, using sunlight and something called chlorophyll. This process is called photosynthesis. As chlorophyll is green, leaves are therefore also green in color.During winter, there is not enough light or water to help plants produce sugar as their food for energy and as a bu
4、ilding block for growing. The trees will rest, and live off the food they stored during the summer. 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 colors have been in the leaves all along. We just can
5、t see them in summer, because they are covered up by the green chlorophyll. The bright reds and purples we see in leaves are made mostly in fall. In some trees, like maples, sugar, which is produced in the leaves during warm, sunny days, is kept from moving out of the leaves after photosynthesis sto
6、ps. Sunlight and the cool nights of fall turn the sugar into a red color. The brown color of trees like oaks is made from wastes left in the leaves. It is the combination of all these things that makes the beautiful colors we enjoy in fall. Questions:1. What is the passage mainly about? 2. Which of
7、the following plays a major role in making leaves change color? 3. Why cant we see yellow and orange colours in leaves during summer? 4. Which of the following best describes the speakers attitude toward his subject? Text 2Timing of Color Change in TreesMany trees and shrubs change color in fall. Fo
8、r years, scientists have worked to understand the changes that happen to them. They find that three factors influence falls colorful farewell - leaf pigments, length of night, and weather. The timing of the color change is mainly regulated by the increasing length of night. None of the other environ
9、mental influences, such as temperature, rainfall, food supply, are as unchanging as the steadily increasing length of night during fall. As days grow shorter, and nights grow longer and cooler, biochemical processes in leaves begin to paint the landscape with an explosion of colors. And Nature puts
10、on one of its most spectacular displays of beauty.The timing of the color change varies by species. Some species 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 thei
11、r leaves. These differences in timing among species seem to be genetically inherited, for a particular species, whether on a high mountain or in warmer lowlands, will change color at the same time.However, some species are evergreens. Pines, for example, are green all the year round because they hav
12、e toughened up. They have developed over the years a needle-like or scale-like foliage, which is covered with a heavy wax coating. And the liquid inside their cells contains cold- resistant elements. So the leaves of evergreens can safely withstand all but the most severe winter conditions, such as
13、those in the Arctic. Questions:1. What does the speaker mainly tell us? 2. What are the two major kinds of trees that the speaker differentiates? 3. By what is the timing of the color change mainly regulated? 4. Why do some species of trees remain evergreen?Part CThe Missing Monarchs (Part One)The m
14、onarch butterfly has rich orange-gold wings outlined in black and decorated with small dots of white. It looks like a stained-glass window that has come alive as it flutters through the summer sunshine.Across most of the United States and Canada monarchs take a long journey southward when the cold s
15、eason sets in. Monarchs from the western United States travel to a winter home on the California coast. But until recently, no one had ever seen the winter home of the eastern monarchs. For more than forty years, a Toronto-based Canadian zoologist, Fred Urquhart, tried to solve the puzzling mystery
16、of the missing monarch butterflies. His first step was to mark the butterflies. It took a long time to find a way to attach tags so the tag would stay in place and the butterfly could still fly. Many people volunteered to help. They caught, tagged, and set free again thousands of butterflies. Each tag bore a code to indicate the exact place where the butterfly had been tagged. A message also asked anyone who found the tagged butterfly to send the infor
