修改时间：2022-01-20 浏览次数：65 类型：单元试卷
Hundreds of years ago, no dandelions (蒲公英) grew in North America. Then people came from Europe. The seeds fell from their clothes onto the ground. Since then, dandelions have spread all over.
Most people believe that dandelions are weeds (杂草). They do not want them in their gardens. But some people think that dandelion flowers are pretty. They cook dandelion leaves or put them into a fresh salad. The leaves must be picked before the flowers come out or they will not taste good.
Dandelions do not die easily. If the winter is not very cold, their leaves may stay green. When spring arrives, their flowers come out.When the sun shines the next day, they open up again. Dandelion flowers are very unusual. They do not have to get pollen (花粉) from another dandelion flower in order to form seeds. So after several days, the yellow flower turns white and soft. A tiny brown seed forms at the bottom of each white petal (花瓣). When the wind blows, the petals float away. Each white petal carries a seed away. New dandelions grow where the seeds land.
| A. And others think that dandelions taste good. B. They had dandelion seeds on their clothes. C. Each night their bright yellow flowers close up. D. This lets the dandelions spread their seeds all over. E. Some people make dandelion flowers into wine. |
Do you know who invented paper? A Chinese man1Cai Lun invented paper almost 2,000 years ago. He made paper2wood.
Now we still use wood3paper. We use a lot of paper every day. If we keep on4so much paper, there will not be any trees left on the earth. If5no trees, there will be no paper. Every day, people6about 2,800 tons of paper. We use 17 trees to make one ton of paper. This means we are cutting nearly 48,000 trees every day.
Since it7more than 10 years for a tree to grow, we must start using8paper now. So how can we save paper? We can use both sides of9of paper, especially when we are making notes. We can choose drinks in bottles instead of those in paper packets. We can also use cotton handkerchiefs (手帕). When we go shopping, we can use fewer paper bags. If the shop assistant gives us a paper bag, we can save it and reuse it later.
Everyone can help to save paper. If we all think carefully, we can help10trees. But we should do it now, before it is too late.
There are many kinds of trees. All trees have roots. The roots hold trees in the ground. The roots soak up ( 吸收 ) water and food. Then trees get water and food from the roots. You can't see inside a tree. But you can study how water moves through trees.
First, get a stalk ( 茎 ) of celery ( 芹菜 ) and cut off its end. Put red colour in a cup of water. Then put the stalk of the celery into the red water. Then we can see the red water moving up in the celery easily.
After half an hour, take the celery out. Cut it in half. Your celery should be red. This is because the water moved up the stalk. Water moves through trees in the same way as it moved through the celery.
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