Sunflowers Follow the Sun
August 01, 2010
Common sunflowers (Helianthus annuus) exhibit heliotropism -- they follow the Sun’s daily round. Not to be confused with phototropism, which is a plant’s growth response to any light, heliotropism refers to the plant’s response to the daily cycle of the Sun. Heliotropism most likely helps to increase the development of pollen -- once pollinated the sunflower head remains facing east. This daily dance with the Sun results from motor cells in a flexible segment of the stem just below the bud known as the pulvinus. These cells enlarge or shrink according to the turgor pressure of the water against the cell walls. As pressure increases on one side and decreases on the other the stem responds by drooping or stiffening.
Sunflowers are native to the North American prairies, and some seeds have been carbon dated to be over 4,000 years old. Sunflowers were taken to Europe in the sixteenth century and proved to be a big hit in Russia where research and selective breeding created a plant with much higher oil content. Now Russia is a major producer of sunflower seeds and oil. Often mistaken for a flower, the head of Helianthus annuus is made of many tiny flowers. The florets on the circumference of the rim are sterile while only the inside florets can become seeds. It's not unusual to see sunflowers, such as the beauty above, photographed in a garden in Alexandroupolis, Greece, exceed heights of 7 ft (2.13 m) in mid-summer. Photo taken on July 24, 2010.