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.
Sunflowers in their blooming stage lose their heliotropic capacity. The stem becomes 'frozen', typically in an eastward orientation. The stem and leaves lose their green color.
The wild sunflower, when at full-bloom, will generally face itself towards the south as it is in this location that the sun is strongest.
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