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　　[2015·黑龙江大庆质检二]We tend to think of plants as the furniture of the natural world. They don't move, they don't make sounds and they don't seem to respond to anything－at least not very quickly. But as is often the case, our human view of the world misses quite a lot. Plants talk to each other all the time. And the language is chemical.

　　Over the years, scientists have reported that different types of plants, from trees to tomatoes, release compounds (化合物) into the air to help neighboring plants. These chemical warnings all have the same purpose－to spread information about one plant's disease or infestation so other plants can defend themselves. But exactly how plants receive and act on many of these signals is still mysterious.

　　In this week's Proceedings of the National Academy of Sciences, researchers in Japan offered some explanations. They had identified one chemical message and traced it all the way from release to action.

　　The scientists looked at tomato plants infested by a common pest, the cutworm. The researchers studied leaves from exposed and unexposed plants. They found one compound showed up more often in the exposed plants. The substance is called HexVic. When the scientists fed HexVic to cutworms, it knocked down their survival rate by 17%. The scientists identified the source of HexVic, and sprayed it lightly over healthy plants. Those plants were then able to start producing the cutworm­killing HexVic. Researchers confirmed that uninfested plants have to build their own weapons to fight off bugs and disease. How do they know when to play defense? They are warned first by their friendly plant neighbors.

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