Auxin is a hormone found in the stem, buds, and root tips of a plant. This hormone stimulates cell elongation/lengthening, and it is created at the tips of the stems. This hormone also causes the curvature of the stem of the plant. This is due because the hormone is attracted to the dark, thus cells in the darker spots grow larger producing a bent stem shape. This action is called phototropism. Auxin also participates in apical dominance.