Plant stems bend toward the light because the growth hormone auxin, produced in the tip, is diverted to the shaded side of the plant where it stimulates markedly faster growth.
Auxin is any of a group of hormones that regulates plant growth. Auxins especially stimulate cell elongation in stems and inhibition in roots. They also control abscission and the plant's responses to light (see phototropism) and gravity (gravitropism).
Auxins accelerate plant growth by stimulating cell division and enlargement, and by interacting with other hormones. Actions include the elongation of cells (by increasing the elasticity of cell walls, allowing the cells to take up more water) in geotropism and phototropism, and fruit drop and leaf fall.
Natural auxins are derivatives of indole. Synthetic auxins are used for crop control, and in rooting powders and weedkillers.