Auxins play a role in the growth of plants. They are chemicals that act as hormones. Auxins allow plants to respond to changes in the environment, such as the availablity of light. Plants lack a nervous system but auxins help them to respond to stimuli. Auxins must be translocated to sites where needed. Phototropism is the directional growth of a plant toward the light. Cells farthest from the light have auxins that react when phototropism occurs. Auxins collect on the darker side of the stem. This causes growth at a much faster rate at the darker side, than the side in the light. The elongated cells on the side furthest from the light source, will cause a bending or curvature of the plant, towards the light.