Why do plants grow in some places and not in others? Why does some land have so much growing on it, while other land has almost no plants growing on it at all?

    To grow,  plants need several things. One is warmth. In very cold places almost nothing grows. Plants also need water. In very dry parts of the Earth only a few unusual plants can grow. That's why dry deserts(沙漠) everywhere are almost not covered by trees or grass.

    Plants must also have a place in which to put down their roots and grow. They find it difficult to grow on hard land. The town is built on hard land. The plants here have only the soil(土壤) found between the cracks of the stones to grow in.

    Another thing plants must have before they can grow is food.

    What will happen if we try to make things grow on the sandy beach? A few plants, such as beach grass, will grow in sand, but most plants won't. Even if the weather is warm enough and we water the plants each day, many of them will die because the sand on this beach has almost no plant food.

    In 1901, H. G. Wells, an English writer, wrote a book describing a trip to the moon. When the explorers (探险者) landed on the moon, they discovered that the moon was full of underground cities. They expressed their surprise to the "moon people" they met. In mm, the "moon people" expressed their surprise. "Why are you travelling to outer space when you don't even use your inner space?" they asked.

    H. G. Wells could only imagine the travel to the moon. In 1969, human beings really did land on the moon. People today know that there are no underground cities on the moon. However, the question that the "moon people" asked is still an interesting one. A growing number of scientists are seriously thinking about it.

    Underground systems (体系) are already in place. Many cities have underground car parks. In some cities, such as Tokyo, Seoul and Montreal, there are large underground shopping areas. The "Chunnel", a tunnel connecting England and France, is now completed.

    But what about underground cities? Japan's Taisei Corporation is designing a network of underground systems, called "Alice Cities". The designers imagine using surface space for public parks and using underground space for flats, offices, shopping and so on. A solar dome (太阳能穹顶) would cover the whole city.Supporters of underground development say that building down rather than building up is a good way to use the earth's space. The surface, they say, can be used for farms, parks, gardens, and wilderness. H. G. Wells's "moon people" would agree. Would you?

    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.

    •Plant flowers, grass or trees.

    •Whenever you visit a park or beach, take away what you bring there—keep rubbish in a bag until you can put it in a dustbin.

    •Turn off the lights and TV sets when you leave the room. This can save a lot of electricity.

    •Turn off the tap when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.

    •Keep the doors and windows closed in winter to keep warm air in.

    •Give your old books and magazines to a library instead of throwing them away.

    •Give your old clothes to poor children you know instead of throwing them away.

    •Use both sides of paper.

    •Stop pouring dirty water into the rivers or lakes nearby.

    •Encourage all your friends to do the same things you do to help protect the earth. You don't have to wait until Earth Day to do these things. Make every day Earth Day. If everyone makes a contribution to protecting the environment, the world will become much more beautiful.

    When I was a little girl, my family lived in the countryside. There was a beautiful river near my home. The water was clean and cool. I liked to go fishing there with my mother. We would catch fish, look for clams(蛤)and play in the water. There were also a lot of birds near the river. We would spend all day watching the birds.

    Now my family live in the city. Last Sunday, my daughter asked me to take her to see the river I was always talking about. "I want to go fishing there so much with you and Grandma," she said.

    When we went to the river, we only saw a factory and a mountain of waste. My mother was surprised, my daughter was disappointed, and I was sad. The river was my best friend. I grew up with it. Now there are no fish in it, and the birds have gone too. I hear them crying for help. But what can I do?