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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.
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When a seed lands sideways, statoliths settle to the bottom of these cells due to gravity.
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That physical signal triggers uneven distribution of auxin, a growth hormone, along the stem and root.
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Higher auxin concentration on the lower side slows cell elongation in shoots but speeds it in roots.
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As a result, the shoot curves upward (negative gravitropism) and the root curves downward (positive gravitropism).
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Light also guides young plants, but gravity acts first — even in total darkness or space experiments.
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NASA studies plant orientation in microgravity to prepare for future space farming missions.
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Mutant plants lacking functional statoliths grow randomly, proving how vital this mechanism is.
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Root caps and coleoptiles house most gravity sensors, making them critical for early development.
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This built-in compass allows plants to anchor, breathe, and photosynthesize efficiently from day one.