Phototropism is the growth and response to a light stimulus. Auxins are plant hormones that make some parts of a plant stem grow faster than others. The result is that the plant stem bends towards the light.

A houseplant grows towards the window and turns its leaves towards the light. It does this because light coming from the window side of the plant destroys the auxins in that side of the stem, so growth on that side slows down. On the shaded side of the plant there is more auxin. So growth on this side speeds up. The result is that the shoots and leaves are turned towards the light for photosynthesis.

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