What causes unequal growth in plant stems allowing them bend toward light?

Plants need light because it is one of the ingredients to perform photosynthesis-process of making food. Now, when light is not present or when it is blocked, some hormones of plants react and respond so that plants can obtain this resource. A hormone called auxin causes stem elongation which lets the plant bend to where the light is and this action of the plan is described as phototropism-movement towards sunlight.