Why and how does the sunflower always face the Sun?

Certain flowers such as sunflower are attracted to the sun strongly. They begin the day facing east and then follow the sun. This is because of a phenomenon called phototropism.

         Phototropism is a growth-mediated response of a plant to stimulation by visible light. The response is stimulated by a hormone called auxin present in the stem.

   Auxins promote lengthwise growth of plants. The auxin, beta-indylacetic acid (IAA), is formed either from the amino acid, tryptophan, or from the breakdown of carbohydrates known as glycosides.

They promote growth by acting on the chemical bonds of carbohydrates on the cell wall. In positively phototropic plants when one side of the plant is shaded, greater quantities of auxin are produced on the darker side. This causes that side of the plant to grow fast. In the case of sunflower, the phenomenon is pronounced so as to make the flower turn towards the sun.