They're lenses because they're convex vessels, and when they're full of water, they act as a lens and you see the horizon in them, upside down and backwards.
Water surges from the cells on the inside of the leaf to those on the outside, causing the leaf to rapidly flip in shape from convex to concave, like a soft contact lens.
水从叶子内部的细胞涌向外面的细胞，导致叶子迅速地从凸向凹翻转，就像一片柔软的隐形眼镜。
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They're lenses because they're convex vessels, and when they're full of water they act as a lens and you see the horizon in them, upside down and backwards.