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    Plants are very important. This is because plants can make food from air, water and sunlight. Animals and man cannot make food from air, water and sunlight. Animals can get their food by eating plants and other animals. Man gets his food by eating plants and animals, too. So animals and man need plants in order to live. This is why there are so many plants around us.

    There are two kinds of plants: flowering plants and non-flowering plants. Almost all the trees around us are flowering plants. You can know some trees from their flowers and fruit. Non-flowering plants do not grow flowers. You cannot see many non-flowering plants around you.

    If you look carefully at the plants around you, you will find that there are many kinds of plants. Some plants are large and some are small. Most of them are green. Thanks to the plants around us we can live on earth.

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    Have you ever wondered how and why a fall leaf changes color? We first have to understand what leaves are and what they do.

    Plants are the world's food factories. Plants take water from the ground and take CO₂ from the air. Plants use sunlight to turn water and CO₂ into glucose(葡萄糖). Glucose is a kind of sugar. Using it as food, plants get energy and grow. Plants using sunlight to turn water and CO₂ into sugar is called photosynthesis(光合作用). ChlorophyII helps photosynthesis. It gives plants their green color.

    As summer ends and autumn comes, the days get shorter and shorter, and 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 on the food they store during the summer. They begin to shut down their food — making factories. As the green chlorophyII disappears from the leaves, we begin to see yellow and orange leaves. These colors have been in the leaves all the time. We just can't see them in the summer, because they are covered up by the green chlorophyII.

    The bright reds and purples we see in leaves are made mostly in autumn. In some trees, 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 is made from waste left in the leaves. It is mixture(混合物) of all these things. It makes the beautiful leaves during autumn.

C

    If you go into the forest with your friends, stay with them. If you don't, you may get lost. If you get lost, this is what you should do. Sit down and stay where you are. Don't try to find your friends—let them find you. You can help them find you by staying in one place. There is another way to help your friends or other people to find you. You can shout or whistle (吹口哨) three times. Stop. Then shout or whistle three times again. Any signal (信号) given three times is a call for help.

    Keep up shouting or whistling. Always three times together. When people hear you, they will know that you are not just making a noise for fun. They will let you know that they have heard your signal. They will give you two shouts or two whistles. When a signal is given twice. it is an answer to a call for help.

    If you don't think that you will get help before night comes, try to make a little house with branches (树枝). Make yourself a bed with leaves and grass.

    When you need some water, you have to leave your little house to look for it. Don't just   walk away. Pick off small branches and drop (扔下) them when you walk to go back again easily. When you are lost, the most important thing to do is to stay in one place.

    Since the 1970s, scientists have been searching for ways to link the brain with computers. Brain-computer interface (BCI) technology could help people with disabilities send commands to machines.

    Recently, two researchers, Jose Millan and Michele Tavella from the Federal Polytechnic School in Lausanne, Switzerland, demonstrated(展示) a small robotic wheelchair directed by a person's thoughts.  In the laboratory, Tavella operated the wheelchair just by thinking about moving his left or right hand. He could even talk as he watched the vehicle and guided it with his thoughts.

    "Our brain has billions of nerve cells~ These send signals through the spinal cord(脊髓) to the muscles to give us the ability to move. But spinal cord injuries or other conditions can prevent these weak electrical signals from reaching the muscles," Tavella says. "Our system allows disabled people to communicate with external (外部的) world and also to control devices."

    The researchers designed a special cap for the users. This head cover picks up the signals from the scalp(头皮) and sends them to a computer. The computer interprets (translate) the signals and commands the motorized wheelchair. The wheelchair also has two cameras that identify (find or discover) objects in its paths. They help the computer react to commands from the brain.

    Professor Millan, the team leader, says scientists keep improving the computer software that interprets brain signals and turns them into simple commands. "The practical possibilities that BCI technology offers to disabled people can be grouped in two categories: communication and controlling devices. One example is this wheelchair. "

    He says his team has set two goals. One is testing with real patients, so as to prove that this is a technology they can benefit from. And the other is to guarantee that they can use the technology over long periods of time.