Wednesday, June 20, 2012

Weird Wednesday ~ Heliotropism

*** This post is in answer to a question ask by a faithful reader. ***

"Heliotropism is the diurnal motion of plant parts (flowers or leaves) in response to the direction of the sun. It is found in some members of family Malvacea e.g. Malva or Lavetara. Heliotropism was first described by Leonardo da Vinci (along with gravitropism) in his botanical studies. The term heliotropism is for movement, not growth.
Heliotropic flowers track the sun's motion across the sky from east to west. During the night, the flowers may assume a random orientation, while at dawn they turn again toward the east where the sun rises. The motion is performed by motor cells in a flexible segment just below the flower, called a pulvinus. The motor cells are specialized in pumping potassium ions into nearby tissues, changing their turgor pressure. The segment flexes because the motor cells at the shadow side elongate due to a turgor rise. Heliotropism is a response to blue light.
Leaf heliotropism is the solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicularly to the sun's rays in the morning (diaheliotropism), and others have those that orient themselves parallel to these rays at midday (paraheliotropism). Floral heliotropism is not necessarily exhibited by the same plants that exhibit leaf heliotropism."

West Facing Sunflowers
Many people believe that the flower heads of many plants track the sun. This is a misconception. Immature flower buds of the sunflower will track the sun across the sky from east to west and by dawn the buds will have returned to face east. But once the flower bud matures and blossoms, the stem stiffens and the flower becomes fixed facing East.
Snow Buttercup
But there are some plant species which truly are solar trackers - the Snow Buttercup (Ranunculus adoneus). By facing the sun throughout the day, the flowers of the Snow Buttercup are able to collect heat. This heat is thought to help pollination - insects which fly into the Buttercup will become warm and thus fly better to deliver pollen to other flowers.
Some plant species orient their leaves perpendicular to the sun's rays in the mornings and evenings to maximize photosynthesis. These plants have higher photosynthetic rates throughout the day. Leaves parallel to the sun's rays have reduced leaf temperatures and transpirational water losses. No doubt a mechanism to survive in hot, dry areas.

1 comment:

Debra She Who Seeks said...

A perfect post for the longest day of the year! Summer Solstice blessings to you!