Phototropism, also called heliotropism, is growth of a plant in response to the stimulus of light, which increases cell growth on the shaded side of the plant, resulting in curvature towards the source of light. Auxin hormones are involved in this process. They are produced in the tip of the shoot, and when light shines on the shoot, auxin travels preferentially down the shaded side of the shoot, where it promotes cell elongation, causing the shoot to bend towards the light. Leaves and stems respond positively to light and roots negatively or not at all. Indoor plants lean towards windows; leaves usually grow at right-angles to light and are positioned to ensure that overlapping occurs as little as possible.