Heliotropism

Today, while solving science problems, discussed "Heliotropism" with my father. The wiki entry says:

Sunflowers in the bud stage exhibit heliotropism. At sunrise,
the faces of most sunflowers are turned towards the east. Over the course of the
day, they follow the sun from east to west, while at night they return to an
eastward orientation. This motion is performed by motor cells in the pulvinus, a
flexible segment of the stem just below the bud. As the bud stage ends, the stem
stiffens and the blooming stage is reached.