茎不向上生长，根也不向下生长。

The stems weren't growing upward and the roots weren't growing downward.

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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.

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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".

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Let's look at 2py, which is vertical.

我们看看2py，垂直向上的那根。

麻省理工公开课 - 固态化学导论课程节选