| 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 move to track 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.
The wild sunflower typically does not turn toward the sun; its
flowering heads may face many directions when mature. However, the
leaves typically exhibit some heliotropism.
Sunflowers in the blooming stage are not heliotropic anymore. The
stem has frozen, typically in an eastward orientation. The stem and
leaves lose their green color.
|
