If you live in certain parts of the world, you will see something magical happen each fall. It is the changing of the color of the leaves on many different trees. Each year deciduous (落叶的) trees go through a process in which their green leaves become bright yellow, gold, orange and red before turning brown and falling onto the ground.
Photosynthesis (光合作用) is the process for trees to make their food taking energy from the sun, water from the ground, and carbon dioxide from the air, they make sugar to “eat” so they can grow into strong, healthy trees. The leaves of a tree are where photosynthesis happens because the chlorophyll (叶绿素) in the leaves is what makes photosynthesis possible. Chlorophyll also has another job—it is what makes leaves green. When the seasons change in places where deciduous trees grow and the days get shorter and the weather gets colder, it is harder for the chlorophyll in the leaves to make the food needed to stay green. So instead of making more food, the leaves start using food they have stored away for this time of year.
As the leaves use the food that has been stored away, cells form at the bottom of each leaf. These cells act like a door between the leaf and the rest of the tree—a door that closes very slowly and doesn’t shut until all the leaf’s food is gone. While this is happening, the colors in the leaves of the trees are able to show through. In fact, the red, yellow, gold and orange colors are hiding in the leaves all summer long. The colors just can’t be seen in the summer because of all the chlorophyll in the leaves.
You might be wondering how the rest of the tree keeps growing when the weather turns cold and the leaves die and fall. Doesn’t the rest of the tree need the food made by the tree’s leaves?
Not really. The tree trunk and branches get food from the roots of the tree. The roots supply water, vitamins and minerals they get from the ground. Trees need sunshine and warm weather to grow, but they also need time to rest like bears that sleep during the winter.
8. Why do leaves change colors?
A. They save some energy to stay alive.
B. The chlorophyll stores some food away.
C. They have to get used to the cold weather.
D. They can’t get enough energy from the sun.
9. In which situation do leaves fall off deciduous trees?
A. When there is no sunlight any longer .
B. When the food stored away in the leaves is use up.
C. When the chlorophyll in the leaves stops working.
D. When cells start to form at the bottom of each leaf .
10. How do trees survive in winter?
A. By getting food from the ground.
B. By storing food in their branches.
C. By storing food in their roots.
D. By making the most use of the dead leaves.
11. What is the purpose of the text?
A. To study how trees survive in winter.
B. To tell about the importance of photosynthesis.
C. To explain why leaves change colors.
D. To show the changes of trees in the whole year.
【答案】8. D    9. B    10. A    11. C