We all enjoy the beautiful show of colors as leaves change each autumn. But did you ever wonder how and why this happens? To answer the question, first you 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 they take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose (葡萄糖). Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing.

The way plants turn water and carbon dioxide into sugar is called photosynthesis (光合作用). That means, “putting together with light”.

A chemical called chlorophyll (叶绿素) helps photosynthesis. Chlorophyll is what gives plants their green color. As summer ends and autumn comes, the days get shorter and shorter. This is how trees “know” to begin getting ready for winter.

During winter, there is not enough light or water for photosynthesis. Trees rest during this time and live off the food they stored during the summer. They begin to shut down their food-making factories.

The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange leaves. Small amounts of these colors have been in the leaves all along. We just can’t see them in the summer, because they are covered up by the green chlorophyll.

The bright reds and yellows we see in leaves are made mostly in autumn. In some trees, like maples, glucose is caught in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red color. The brown color of trees like oaks (橡树) is made from waste left in the leaves.

It is the mixture of all these things that makes the beautiful colors we enjoy during autumn.

36. Which of the following about chlorophyll is NOT true?

A. Without it photosynthesis couldn’t happen.

B. It is the reason why leaves have their green color.

C. It turns the leaves red during autumn.

D. It is a chemical that’s important for trees’ food making.

37. From the text we can see that photosynthesis _____.

A. can produce glucose, which can provide energy for trees’ growth

B. can happen whenever there is light and water

C. is the process of putting sunlight together with chlorophyll

D. doesn’t stop during the trees’ lifetime even in winter

38. What’s the author’s purpose of writing the passage?

A. To explain the process of trees’ photosynthesis.

B. To encourage readers to protect the environment.

C. To analyze the reasons why leaves change colors.

D. To help readers understand what leaves and trees are.