B

    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.

    We have known for a long time that flowers of different plants open and close at different time of day. Yet no one really understands why flowers open and close like this at special times. It is not as simple as we might think, as new experiments(实验) have shown. In one experiment, flowers were kept in darkness. We might expect that the flowers, without any information about the time of the day, did not open as they usually do. In fact, they continued to open at their usual time. This shows that they have some mysterious (神秘的) way of knowing the time.

    Their sense of time does not depend on information from the outside world; it is, so to speak, inside them, a kind of “inner clock”. This discovery may not seem to be very important. However, it was later found that not just plants but also animals including man have this “inner clock”which controls(控制) working of their bodies and their activities.

    Human beings, then, are also controlled by this mysterious power. Whether we wish it or not, it affects such things in our life as our need for sleep, our need for food. And our ability to concentrate(集中).

    Hong Kong has about forty public beaches. Some of the beaches are the best in the world. People can go there for a swim. You can go to most of them by bus, but if you want to go to some beaches you must take a boat. There are toilets, changing rooms and places to buy food and drink on most of the beaches.

    You can swim there without danger if you remember these instructions(指令).

⒈Never swim alone.

⒉Never swim after a meal or when you feel hungry or tired.

⒊Do not stay in the water too long.

    Remember: A red flag(旗帜)means that it is dangerous for anybody to go into the water. A blue flag means that it is dangerous for children.

    Did you know that each day one person in the USA throws away 4.4 pounds of rubbish? Almost every month, we throw away our own rubbish in packaging alone. Packaging (including boxes, bags, etc.) comes to our homes with the things we have bought. Several weeks ago, the TV news said that in a city in Italy tons of rubbish were piled up in the streets, and the smell was rather terrible. What can we do to help? We should get to know the following Rs.

    Reduce To reduce means to make less rubbish. That is one of the first things we all should do for the environment. Reduce your rubbish by doing things like packing your lunch in reusable containers(可再利用的容器), taking smaller portions(份) at dinner so you don't waste food when you have had enough, writing on both sides of paper, and using less water when taking a shower.

    Reuse to reuse is to use things again. Think about the bags you get from the shop. They can be reused to carry your lunch, to hold your rubbish, or to collect used cans. You can even take them back to the school next time to carry your things instead of getting a new bag!

    Recycle(回收利用) Recycling is an important part of keeping our planet clean and green. It can also save energy. Be sure that you don't throw things in the rubbish bins that can be recycled to make something else Paper, plastic, cans and bottles can all be recycled. But food, electrical waste and mobile phones cant. When you are not sure whether something can be recycled, go to your parents and teachers for advice.

Follow the three Rs, and every one of us can do something to save the Earth. It is also the best way to save ourselves.

    Like you, plants need good things like food, water, and minerals in order to grow. Usually they get these important nutrients(营养) from the rich soil(土壤) in which they're planted. But there's another way to grow plants that doesn't use soil. It's called hydrogenic gardening.

    "Hydrogenic" means "working with water." In a hydrogenic garden, plants grow with their roots in water, not in dirt. The water is filled with special food for the plants that gardeners have carefully measured and prepared.

    Hydrogenic plants grow quickly. In ordinary gardens, plants need time and energy to grow long roots that search the soil for food and water. In a hydrogenic garden, plants can use their energy to grow tall instead. And because there is no soil, there are no weeds and few insect pests(害虫). Gardeners don't need to spray(喷洒) plants with dangerous chemicals. Both the plants and the environment stay healthy. 

    Scientists hope that, one day, hydrogenic gardens may be able to help hungry people all over the world grow enough food-even in countries that have bad soil, or weather that is too hot, too cold, or too dry. And astronauts hope to take hydrogenic gardens with them to the spaceship they fly to Mars, so that they will always have fresh vegetables, and the fun of growing a garden without dirt.