全新版大学英语听说教程第三册 Unit 06（在线收听）

Unit 6

Part B

Text 1

Why 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 breathtaking.

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 Nature's food factories. Plants take water from the ground through their roots, and carbon dioxide from the air. Then they turn water and carbon 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 building 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'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 stops. 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 the following plays a major role in making leaves change color?

3. Why can't we see yellow and orange colours in leaves during summer?

4. Which of the following best describes the speaker's attitude toward his subject?

Text 2

Timing of Color Change in Trees

Many trees and shrubs change color in fall. For years, scientists have worked to understand the changes that happen to them. They find that three factors influence fall's 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 environmental 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 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 their 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 have 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 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 C

The Missing Monarchs (Part One)

The monarch 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 season 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 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 information to an address in Toronto, where it would reach the zoologist. Thus, the tags were to serve as the scientist's clues.

Their detective work paid off. Over the years they learned a great deal about the migration of the eastern monarch butterflies. These seemingly fragile creatures have been known to cover eighty miles in a single day! They can fly ten miles an hour, and some have been clocked at thirty miles an hour! The butterflies travel and eat during the day. When it cools off at night, they rest in trees. The morning sun warms them, and they continue their migration.

Some of the butterflies were traced south across Florida. Many were traced through Texas into Mexico. But there the trail was lost.

Statements:

1. The monarch butterflies have orange-gold, black and white colors on their wings.

2. According to the passage, scientists failed to find the winter home of monarchs from western United States.

3. The Canadian zoologist, Fred Urquhart, spent several decades trying to solve the mystery of the missing monarch butterflies.

4. Urquhart and many volunteers marked thousands of butterflies by attaching tags to them.

5. Each tag bore a code name and the address of Urquhart's Toronto home.

6. The butterflies can fly ten miles an hour and some can even go at eighty miles an hour.

7. The butterflies are actually tougher than we expected.

8. The scientist lost the trail of the butterflies, though some were traced south across Florida and many, through Texas into Mexico.

Part D

The Missing Monarchs (Part Two)

For years Mr. Urquhart and his colleagues wondered where the migratory monarchs spent the winter. Despite their hopes, fieldwork in Florida and along the Gulf Coast discovered no large groups of wintering monarchs. Then in late 1972, his wife Norah wrote to newspapers in Mexico about the project, asking for volunteers to report sightings of the butterfly and help with tagging. Finally, in response came a letter, dated February 26, 1973, from a man called Kenneth Brugger in Mexico City, who offered to help find the butterfly hideaway.

Traveling in his motor home, Brugger drove back and forth across the Mexican countryside, looking for clues. He was especially watchful at dusk, when the butterflies would be moving about looking for a place to sleep.

At last, one day was successful. On the evening of January 9, 1975, Brugger called from Mexico. "I have found them -- millions of monarchs -- in evergreens beside a mountain clearing," he said, unable to control the excitement in his voice.

High in a range of volcanic mountains that crosses central Mexico, he came upon hundreds of evergreen trees, each entirely hidden by sleeping butterflies. Some of the insects wore tags that Mr. Urquhart and his helpers had put on them in Canada and the northern United States. The mystery was solved! The monarchs' winter home is well suited to their needs. Throughout the winter the temperature stays near freezing. It is not cold enough to kill the visiting insects, but it is chilly enough to keep them from moving about. The butterflies survive on the stored fat from their summer foods.

In spring the butterflies awaken and fly north again. Tagged butterflies, which were marked in Mexico, have been found in the United States.

So one mystery is solved. But another remains. How do the butterflies find their way? Those that migrate south in the fall were born sometime during the summer or early fall. They have never been to Mexico. Yet they somehow seek out the same resting places. The mystery of how they find their way is left for future scientists to solve.

Questions:

1. What did Mr. Urquhart and his colleagues do in order to find where the monarchs spent the winter?

2. Which of the following is a key condition for the butterflies' winter home?

3. Where do the butterflies sleep?

4. What can we infer about Mr. Urquhart's project according to the passage?