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  3. We tend to think of memory as unique to animals. But it isn’t. Plants also have a form of memory. Yes: they, too, are shaped by

We tend to think of memory as unique to animals. But it isn’t. Plants also have a form of memory. Yes: they, too, are shaped by

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问题     We tend to think of memory as unique to animals. But it isn’t. Plants also have a form of memory. Yes: they, too, are shaped by what happens to them, and alter their responses to future events based on their experiences in the past.
    For example, like all plants, wild tobacco,(also called Nicotiana sylvestris)can’t move to escape from its enemies—the caterpillars(毛虫)and other animals that enjoy eating its leaves. It can, however, act to stop them. It can detect damaged leaves; in response, it produces nicotine. The nicotine travels from the roots, through the sap(树液)and into the leaves. Nicotine apparently doesn’t taste good: caterpillars fed on leaves from plants that recently experienced damage—and so are high in nicotine—eat much less than caterpillars fed on leaves from previously undamaged plants.
    But here’s the interesting part. Tobacco plants attacked for the first time take longer to mount their defense than tobacco plants that have previously experienced an attack. This isn’t because the previously attacked plants keep on producing a higher level of nicotine—they don’t. Nicotine is expensive for a plant to make, so they only do it when necessary. And plants that have been attacked twice are faster to respond than plants that have only been damaged once. Somehow, they remember.
    The physical basis of plant memory is still being figured out. Of course trees don’t have conscious memory. But by now it’s clear that wild tobacco is not the only plant with the capacity for memory, nor is caterpillar attack the only stress that produces such an effect. Drought, cold and altered salt levels in the soil all do so; likewise, exposure to bacteria.
    If plants remember—can they also forget? As far as I can tell, no one knows the answer to this yet. Nor does anyone know how many different kinds of stresses a plant can keep track of at once. But the subject is important, as the stresses plants are exposed to can affect how well they grow. Being able to prime them to respond to pests, or enable them to forget about a drought, could have big implications for agriculture.
Besides pests’ attacks, natural disasters as well as ______ in the soil can also be a stress for plants to produce a memory.
选项
答案altered salt levels
解析 文章出现的drought、cold均属于natural disasters范围，根据题干中明显的提示语in the soil，可确定答案为altered salt levels。
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