Why do plants grow in some places but not in others? Why does some land have so many plants 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 there are almost no trees or grass in dry deserts.

    Plants must also have a place to put down their roots and grow. They find it difficult to grow on hard land. The town is built(被建立) on hard land. The plants there have only the soil 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 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 food for plants.

    We need animals. Every animal has its place in the balance(平衡) of nature. We should try to help save the animals.

    Some people think zoos are good places. The zoos give the animals the comfortable and safe living places. If the animals get hurt or ill, people will take care of them. The zoo is a good place to know the rules in the wild world. We can be much closer to nature. We can know more about the animals in the zoos by watching, listening even smelling instead of just looking at the pictures in books.

    Some people don't think animals feel happy in the zoos. Animals like pandas, wolves and bears usually live in the wild. They play and find food by themselves. They are free and happy in the wild. If they are in the zoos, food is enough and rooms are comfortable. They do not need to do their best to find food to eat or places to stay in. They just walk around, sleep day and night, but they are not free at all.

    I think we should build national parks as wild life nature reserves to protect animals. If we do nothing, animals will get fewer and fewer.

    A study of sleep in sparrows suggests the relationship between sleep and the ability to learn may be more complicated (复杂) than realized．

    Niels C. Rattenborg of the University of Wisconsin—Madison wanted to find out how the sparrows were able to deal with getting so much less sleep．He and his team brought eight wild birds into a lab and watched them for one year．They invented a game to check how well the birds could learn．In the game, the sparrows had to peck (啄) three buttons (按钮) in a certain order to get a food treat．The scientists discovered that the birds' ability to learn the right button order depended on (依靠) two things, the time of year and the

amount of sleep the birds had．

    During the migration (迁徙) season, the sparrows were restless at night and got much less sleep than usual．Even so, they were able to understand how to get the food quickly as if they'd had a regular night of sleep．Outside the migration season, they found the sparrows that got less sleep than usual had much more difficulty learning how to get the food treats than birds that had a regular sleep．

    The results show that the sparrows can deal with much less sleep during the migration season than usual．If scientists can find out why, they may learn from sparrows and find ways of helping people deal with getting much less sleep．Until scientists fully understand the relationship between sleep and learning, it's better to get plenty of sleep when preparing for an exam．

    There are millions of stars and planets in space. What's the difference between stars and planets? Stars can give out heat and light, but planets cannot.

    The sun is one of the stars. Some planets are balls of rock and some are balls of gas. We live on the planet earth. The earth spins around once every day to give us day and night. The earth goes around the sun once every year to give us different seasons.

    There are eight planets moving around the sun. We call the family of the sun and its planets the solar system. Mercury is the nearest to the sun, so it is very hot. It is the smallest planet. Venus is very bright and hot. It is the nearest to the earth. Mars is covered with orange-red desi and rocks, so it looks like a red star in the sky. It has seasons like the earth. Jupiter has more than 16 moons （卫星）and it is the biggest and heaviest planet. Saturn has more than 22 moons. The other planets, such as Uranus and Neptune, are all very far from the sun and very cold. We need a telescope（望远镜）to see them.

    Populations of white-tailed deer (白尾鹿) are growing rapidly in many parts of the United States. As populations grow, food becomes a problem. Many deer die of hunger. Others grow up small and unhealthy. In search of food, hungry deer move closer to where humans live. They eat farm crops, vegetables and even trees. Besides, increased numbers of deer near roads can cause traffic accidents.

    People admire the grace and fast speed of deer. Most people don't want these animals to be hungry and ill. Should people take action to control the growing deer populations?

Should People Take Direct Action?

    Many people think that hunting (打猎) is the best way to control the animal populations. Wildlife managers will see if there's enough food in an area and decide its carrying capacity (容量). Then hunters with licenses are sent to help control the number of deer. Hunting is usually not allowed in cities or suburbs, however.

    Some people have the idea to catch the deer and move them to other places. But this method is expensive and requires finding another place that can accept the deer without breaking the balance of ecosystem.

    Scientists are also working to develop chemicals to control the birth rate in deer populations. But this plan works for only one year at a time.

Should People Take Indirect Action?

    Some suggest bringing in natural enemies of deer, such as wolves, lions and bears, to areas with too many deer. But these animals could also hurt dogs, cats, and even humans. Other communities have built tall fences (篱笆) around areas to keep out the deer. However, this is impossible for farmers.

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    Some people are against any kind of action. They support leaving the deer alone. Animal populations in an area naturally go up and down over time. Doing nothing means that some deer will die of hunger or illness. But finally, the population will reach a size within the carrying capacity of the environment. So, let nature go its own way.