|Sunflowers - Wikipedia|
The Cliff Notes version is that young flowers face the sun in its daily east-west arc across the sky, growing larger than flowers in a lab with a fixed "sun." When those lab flowers are treated with an artificial "sun" which moves, they follow it and grow bigger. In their natural setting, incredibly, they then reset overnight, facing the east to get an early start on the day. When they get old, they stop moving (I can relate) and just face the east.
New studies shed more light (sorry!)* on the mechanism. During the day, the east side of the stems grow, slowly bending the heads west, then overnight the west sides grow, pushing them back facing east, ready for the rising sun. Meanwhile older flowers facing east all day heat up faster in the morning and attract more pollinators than west facing flowers. Apply heat to the west facing flowers and more pollinators arrive.
There is much more to the story and I highly recommend reading the NY Times story and the heliotropism link to get a better understanding of this fascinating piece of nature.
*Editor's note: Oh no, he's not.
Thanks to Amy and Steve of the Fishin' Magicians for sending the story.