全新版 大学英语 听说教程 第三册 听力原文Tap(2)

全新版 大学英语 听说教程 第三册 听力原文

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