Why do plants grow towards sunlight?

Plants have adapted to grow towards sunlight - an essential requirement for growth and survival. But how do they do this? An important factor in the control of plant growth is the hormone 'auxin', found within cells at the tip of the plant's stem. Plant cells that receive less light have higher levels of auxin, causing shaded plant cells to grow or elongate more than those in the sunlight. As the rate of cell elongation becomes uneven between the shaded and sunny sides of the stem tip, the stem begins to bend in the direction of the light source. The stem cannot continue growing upright, as the cells in sunlight are growing more slowly and so prevent the more rapidly elongating, shaded cells on the other side of the stem tip from continuing to grow straight upwards. Instead, the stem is forced to bend. If light is received evenly by the growing cells in the plant's stem, for example if the light source is placed directly above the plant, the cells will grow at an even rate and the stem will grow straight upwards.