We all enjoy the colors of autumn leaves. Did you ever wonder how and why a fall leaf changes color? Where do the yellows and oranges e from? To answer those questions, 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. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose, which 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, which means putting together with lightA chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.
As summer ends and autumn es, the days get shorter and shorter. This is how the trees know to begin getting ready for winter.
During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during 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 colors. Small a-mounts of these colors have been in the leaves all along. Covered up by the green chlorophyll, we just can't see them in summer.
The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
It is the bination of all these things that make the beautiful colors we enjoy in the fall.
【小题1】The writer asked two questions in the beginning in order to .A．persuade readers to believe somethingB．introduce the topic of the passageC．get the readers excitedD．offer something to think over【小题2】 Which of the following is True according to the passage?A．Trees don't change colours with seasons.B．Trees can still perform photosynthesis well in winter.C．Trees have colours like yellow and orange even in summer.D．Trees don't need food in winter.【小题3】Photosynthesis is a way that .A．plants change water and carbon dioxide into sugarB．plants turn water and carbon dioxide into sugar with the help of sunlightC．plants use glucose as food for energy and growingD．chlorophyll is a great help【小题4】 Which is the best title for this passage?A．Colorful trees in autumnB．Mysteries of tree colorsC．Do you enjoy tree colors?D．Wonderful colors in autumnB
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(答案→)B
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