One of the most fascinating aspects of growing sunflowers is to observe the flowers turning with the movement of the sun, their ‘faces’ held obliquely to the sun’s rays [heliotropism].
The default direction of the sunflower head is to point east towards sunrise. During the day motor cells [called pulvinus cells] in the sunflower stem tilt the flower bud to receive a maximum amount of sunlight. By evening, the sunflower head is pointing west, towards sunset. Overnight, the sunflower will reset to its original eastward positioning and wait for the morning, ready to follow the sun’s path once again.
Once mature, sunflowers no longer exhibit heliotropic behavior, and the stem is frozen into an eastward-facing position, so that they are forever greeting the dawn with their nodding heads.
[Pulvinus cells are capable of altering their turgor pressure in response to blue light present in sunlight, and like a balloon can either be rigid or limp. Hence, expansion and contraction of the motor cells are the driving forces behind heliotropism.]