Why does sunflower always face towards the Sun?

The property of vegetation to turn towards the changing direction of Sun is called phototropism. Sunflower plant contains a 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.

Such characteristic is not seen in most of the other flowers. Then why has Nature made sunflower different? A number of flowers grow on other plants and at least some of the flowers clustering the plant receive the sunlight directly to display their colors and attract the insects engaged in providing pollination service. But sunflower plant has only one flower on its stem. As pollination is entirely dependent upon the flower the sunflower plant having only one flower has to display it conspicuously in the sunlight! Nature has made this special arrangement for sunflower only for this purpose.

Additional reading:
Sunflower (Wikipedia)

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