十万个为什么·科学阅读30篇（3）
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Why Do Cacti Store Water in Their Stems?
仙人掌为什么把水储存在茎里？
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Cacti grow where rain is rare and unpredictable.仙人掌生长在雨水稀少且难以预测的地区。
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Their thick stems hold water like natural tanks.它们粗厚的茎像天然水箱一样储水。
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Waxy skin stops moisture from escaping into dry air.蜡质表皮防止水分蒸发到干燥空气中。
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Spines protect stored water from thirsty animals.尖刺保护储存的水分不被口渴的动物取食。
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Roots spread wide and shallow to catch every drop.根系横向延展、分布浅层，以捕捉每一滴雨水。
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They open pores only at night to save water.它们只在夜间打开气孔以节约水分。
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Stems swell after rain and shrink during droughts.降雨后茎部膨大，干旱时则收缩。
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This design helps them survive over a year without rain.这种结构使它们能在无雨条件下存活一年以上。