当光照充足时，植物的茎向上生长，根向下生长。
When the plants had more light, the stems turned up and the roots went down.
在地球上，重力有助于植物的茎“向上”生长，根“向下”生长。
On Earth, gravity helps a plant's stems and roots to find "up" and "down".
当植物在地面上生长时，它们的茎生长得方向与重力相反（重力是让你的脚留在地面上的力量），因此它们总是向上生长。
When plants shoot on the earth, their stems grow away from the gravity (the force that keeps your feet on the ground), and so they always shoot upwards.
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