Phototropism and the distribution of auxin
Phototropism is the growth of plants in response to light. When the sun is positioned almost directly over the plant, the hormone auxin (pink dots) in the plant stem is evenly distributed. As the sun moves, the auxin is repositioned on the other side of the plant. This overload of auxin next to these cells causes them to start to grow or elongate, tipping the growth of the stem toward the light.
Source
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"Phototropism Diagram."
http://en.wikipedia.org/wiki/File:Phototropism_Diagram.svg
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