Phototropism, animation. Plant growth towards a light source is known as positive phototropism. At left the light source is directly above the growing shoot. The hormone auxin (red) is equally distributed throughout the shoot and it grows straight. At right the light source is coming from the right. This causes an increase in the concentration of auxin on the side of the shoot that receives less light (the left). The increased concentration of auxin causes the cells on the left to elongate, leading to a bending of the stem towards the light.
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