Why Sunflowers Follow the Sun ... Article...!!!

Sun-tracking or heliotropic flowers face the Sun as it moves across the sky each day.
Sunflowers are heliotropic:

How do these flowers turn so they face the Sun during the day??

Common sunflowers (Helianthus annuus) exhibit heliotropism -- they follow the Sun’s daily round. Not to be confused with phototropism, which is a plant’s growth response to any light, heliotropism refers to the plant’s response to the daily cycle of the Sun. Heliotropism most likely helps to increase the development of pollen -- once pollinated the sunflower head remains facing east. This daily dance with the Sun results from motor cells in a flexible segment of the stem just below the bud known as the pulvinus. These cells enlarge or shrink according to the turgor pressure of the water against the cell walls. As pressure increases on one side and decreases on the other the stem responds by drooping or stiffening.