環(huán)球英語—923:Growing Fruit in the Desert

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Welcome to Spotlight. I’m Liz Waid.
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And I’m Adam Navis. Spotlight uses a special English method of broadcasting. It is easier for people to understand - no matter where in the world they live.
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In the last fifteen years, farmers in Israel were experiencing a serious problem. They were finding it more and more difficult to grow their fruit crops. The earth was just too dry for their fruit trees. And it was hard for them to find extra supplies of water. Yosef Mizrahi is a farming expert. He told the BBC,
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“Farmers in the desert had lost all of their money. They were removing their fruit trees. What we did in the past is not working any more. We had an insane idea: why not take desert plants, and make them into crops?”
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The farmers needed to find a new way to grow crops in dry areas. So they turned to a desert plant, the cactus, for help. Cacti need far less water than most normal plants. They have physical and structural qualities that set them apart from all other plants.
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The farmers began using a special cactus that can produce fruit. It is called the nopal. It is also called the prickly pear. They added special fertilizers, or plant growth chemicals, to the nopal. The nopal began producing fruit for eleven months out of the year.
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So how are cactus plants able to grow so well in dry areas? How do they stay alive without a dependable water supply? Today’s Spotlight is on the cactus.
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Most people recognize the cactus plant by its sharp spines, or points. But there are other things that make the cactus plant different than other plants.
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Scientists think that long ago the cactus looked much like any other plant. It had a stem which grew out of the ground. On the stem there were leaves. And, sometimes there were even flowers. But, over the years the seeds of cactus plants started to spread. They moved into more difficult places, including hot, dry deserts.
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When cacti started to grow in dry areas they slowly began to change. Like all desert plants, cacti had to survive for long periods of time without water. Soon, the cacti’s stems started to grow thicker. The thick stem could store water.
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The stem of a cactus is made up of many special cells. These cells store the water. The walls around each cell are very thin. So, the walls can stretch. When it rains these cells fill up with water. They stretch wide to make sure they are completely full. After a good rain, cacti are made of ninety percent water!
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Over time there were other changes to the cactus plant. These changes helped the plant to survive even better in dry places. The stems became thicker and thicker. Soon, the chemical chlorophyll appeared in the stem.
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Chlorophyll is very important to both plants and trees. People can usually find chlorophyll in a plant’s leaves. Chlorophyll gives leaves their green colour. But chlorophyll has another important purpose. It creates the plant’s food. The plant’s energy to stay alive comes from chlorophyll.
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Once the chlorophyll moved to the cactus stem, the cactus no longer needed leaves. The leaves were actually a problem! The leaves were a barrier to the sun’s light. The chlorophyll in the stem could not receive enough sunlight through the leaves. Soon, cactus leaves became smaller and smaller. And today, there are usually no leaves growing from the cactus stem. The stem now produces food.
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The leaves may have disappeared for another reason too. All plants lose water. Water evaporates into the air from every part of a plant’s surface. So, a good way for a plant to survive in dry areas is to shrink its surface area. Then it loses less water. The cactus has a much better chance of surviving without the surface area of leaves.
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For these reasons, the cactus plant also makes food in a different way than all other plants. Most plants use energy from the sun, carbon dioxide from the air, and water from the earth. Special holes, called pores, cover the plant. During the day, the plant opens these pores to let in the sun and carbon dioxide. Immediately, the chlorophyll uses the sun’s energy and the carbon dioxide to make food. But the heat of the desert sun presents danger to the cactus plant. The heat could easily evaporate the plant’s water.
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So, the cactus performs a special trick. It does not open its pores during the day. Instead, the cactus opens up its pores at night. The temperature is cooler at night. So, the cactus loses less water. And, instead of using the carbon dioxide immediately, the cactus changes this gas into a liquid acid chemical. It stores the acid until morning. When the sun begins to rise, the cactus closes its pores again.
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The next day the cactus uses the acid to create food. The chlorophyll gains energy from the sun through closed pores. This energy helps the chlorophyll use the acid to create food. As the day goes on, the cactus uses all of its stored acid. By the end of the day the cactus has lost most of its stored acid. So, it must repeat this process every night.
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There are some animals that feed on desert cacti. But if an animal eats a cactus in the early morning, it will get a pain in its stomach! That is because they are eating the cactus when it is filled with acid!
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Some desert plants send down long roots to get water from deep inside the ground. But cacti have a different root system. Their roots do not go deep down into the ground. Instead, cacti roots spread in all directions. The roots gather water from a very large area. Even a small cactus plant may send its roots a distance of nine to twelve metres from the area where it grows. There are scientists who study the roots of the desert cacti. They say something surprising. If we looked at the ground underneath a desert, it would look like a thick forest of roots!
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Cacti are native to the Americas. Europeans first saw cactus plants when they discovered the New World. These travellers brought cacti home to Europe with them. Since then, people have planted cacti in many parts of the world. Now, you can find cacti growing in dry areas almost everywhere. They are a supply of food, water and beauty in many places.