PHOTOTROPISM (heliotropism), the bending of growing plant organs under the influence of unilateral illumination, the resulting increase in the growth of the CELLS on the shady side of a plant and consisting in the bending of plants towards light source. This process involves the hormones AUXINS, which are produced at the tip of the escape. When light shines on the shoot, auxin moves largely on its shadow side, where it promotes cell elongation, causing the escape to bend toward the light. The leaves and stems are usually detected positive phototropism and roots negative, or none at all. Houseplants lean toward the Windows. Their leaves usually grow at right angles to the light and located so that they cover each other as little as possible.