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Farms Appear in Cities 

      When you hear the word “farm”, you may imagine the countryside with cows and crops. But scientists say the farms of the future could be built in tall buildings in some large cities. 

     It may be difficult to believe but in fact the technology for growing crops indoors already exists. Farming in the city is already happening. Some vegetables are already grown in greenhouses. Even the scientists at the South Pole research station can enjoy fresh vegetables they grow in their own greenhouse.
     Experts say indoor farming solves many problems. First, traditional farming takes up a lot of land. Growing crops in tall buildings-called vertical farming（垂直农业）can solve the problem. Also, fruits and vegetables grown indoors would not face serious threats from insects and bad weather.
     Though vertical farms don't exist yet, experts have created a plan for recycling in such farms. The water from indoor fish ponds would be used to water crops. Gases from crop waste would be used to heat the building. Waste from chicken or pigs would be reused as a source of energy.
     But experts believe vertical farming is not going to be easy. They say it can be difficult to control climate conditions indoors. Besides, plants differ in their weather and lighting needs. For example, some plants like warm, sunny weather, but other plants prefer cooler temperatures.
     That’s not to say these difficulties won’t be overcome—but it will take time. Most experts suggest it may take about 5 to 15 years before the first vertical farms could be created.

It is still a dream for people to grow crops indoors.（ ）