The Mystery Of Why Sunflowers Turn To Follow The Sun — Solved

Scientists have answered a burning question central to the charm of sunflowers: Why do young flowers move their blooms to always face the sun over the course of a day?

And then: Once sunflowers reach maturity, why do they stop tracking the sun and only face east?

In a newly-published article in Science, the researchers say the young plant's sun-tracking (also called heliotropism) can be explained by circadian rhythms – the behavioral changes tied to an internal clock that humans also have, which follow a roughly 24 hour cycle. A young flower faces east at dawn and greets the sun, then slowly turns west as the sun moves across the sky. During the night, it slowly turns back east to begin the cycle again.
http://www.npr.org/sections/thetwo-way/2016/08/05/488891151/the-mystery-of-why-sunflowers-turn-to-follow-the-sun-solved?ft=nprml&f=1001