SUNFLOWER

Sunflower (Helianthus annuus)

Observation
I captured the picture of this sunflower while driving my children to school on 25 September 2011. At first glance, I thought the was flower was artificial. As I got closer, I realized that the flower was real. Someone planted it along with a few others in a huge pot that sits on a corner in Northwest Philadelphia, PA. As I observed the flower a bit longer, I noticed that it was facing the sun. I admire the flower for its tasty seeds, deep orange and yellow colors and its height.

Questions

Does the sunflower follow the movement of the sun? At what growth stage does the sunflower stop following the sun? What causes the sunflower's movement?

Scientific Research
Schaffner, John, H. Observations on the Nutation of Helianthus Annuus, Botanical Gazette, Vol. 25, No. 6 (Jun., 1898), pp. 395-403.

Keller initially believed that the head of the sunflower following the sun was just a myth. After performing some observations on the movement of the sunflower, he suggested that the head of the sunflower does go through a process of nutation. His observations suggest that the head of an immature sunflower would nutate in the morning toward the east. As the sun rises, the plant would gradually begin to stand tall and face the sun. In the afternoon, the head would begin to slowly nutate towards the west and its stance would change from horizontal to vertical. By the evening the plant is mostly in a vertical stance. The leaves are faced downward and the plant is in a drooping or sleeping position. In the very early morning hours, the leaves and stem would slowly begin to face the east and rise until the sun appeared.

It is suggested that the sunflower goes through this process to help increase the development of pollen. This process of following the sun ceases after the sunflower is pollinated or reaches maturation. The sunflower's movement is caused by motor cells in the flexible part of the stem called pulvinus. These cells grow and shrink due to pressure from water against the cell walls.When the pressure from the water increases or decreases the stem responds by becoming stiff or limp.

Further Information
EHOW
EPOD
Wikipedia