全新版 大学英语 听说教程 第三册 听力原文

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,