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.
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Their thick stems hold water like natural tanks.
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Waxy skin stops moisture from escaping into dry air.
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Spines protect stored water from thirsty animals.
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Roots spread wide and shallow to catch every drop.
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They open pores only at night to save water.
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Stems swell after rain and shrink during droughts.
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This design helps them survive over a year without rain.