十万个为什么·科学阅读30篇（2）
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How Do Plants ‘Know’ Which Way Is Up When They Germinate Underground?
植物种子在地下萌发时，怎样‘知道’哪边是上方？
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Seedlings use gravity-sensing cells called statocytes, which contain starch-filled plastids called statoliths.幼苗依靠称为平衡细胞的重力感应细胞，其中含有淀粉质体——即平衡石。
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When a seed lands sideways, statoliths settle to the bottom of these cells due to gravity.当种子侧向落地时，平衡石因重力作用沉降到这些细胞底部。
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That physical signal triggers uneven distribution of auxin, a growth hormone, along the stem and root.这一物理信号引发生长素（一种生长激素）在茎和根中不均匀分布。
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Higher auxin concentration on the lower side slows cell elongation in shoots but speeds it in roots.较低一侧生长素浓度升高会抑制茎部细胞伸长，却促进根部细胞伸长。
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As a result, the shoot curves upward (negative gravitropism) and the root curves downward (positive gravitropism).结果，茎向上弯曲（负向重力性），根向下弯曲（正向重力性）。
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Light also guides young plants, but gravity acts first — even in total darkness or space experiments.光也引导幼苗生长，但重力作用优先——即使在完全黑暗或太空实验中亦然。
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NASA studies plant orientation in microgravity to prepare for future space farming missions.美国宇航局（NASA）在微重力环境下研究植物定向，为未来太空农业任务做准备。
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Mutant plants lacking functional statoliths grow randomly, proving how vital this mechanism is.缺乏功能性平衡石的突变植株随机生长，证明该机制至关重要。
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Root caps and coleoptiles house most gravity sensors, making them critical for early development.根冠和胚芽鞘集中了大部分重力感受器，对早期发育尤为关键。
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This built-in compass allows plants to anchor, breathe, and photosynthesize efficiently from day one.这种内置‘指南针’使植物从萌发首日便能高效固着、呼吸与光合作用。