Facing of sunflower towards the sun is known as phototropic movement. Sunflower plant contains a growth hormone named Auxin which is sensitive to the sun-rays. This chemical accumulates on the shaded part of stem and boosts the growth of cells on the shaded part of stem. So the sunflower automatically leans in the opposite direction i.e. towards the Sun. As the direction of shade changes with the direction of the Sun, Auxin hormones also shift their own position on the stem to remain away from the sun-rays. This keeps sunflower always facing the Sun.