Auxin
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State
Auxin drives cell elongation behind phototropism and gravitropism, controls apical dominance, and promotes root formation, moving through the plant to the side away from a stimulus.
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Auxin accumulating on a stem's shaded side elongates those cells faster, bending the stem toward the light source.
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Synthetic auxins serve as herbicides at high concentration, though naturally occurring auxin at normal levels drives ordinary growth.