How does the sunflower track the sun?
By Chelsie Vandaveer
May 1, 2003
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In French, the plant is called tournesol, in Spanish, girasol, both names refer to the heliotropism or sun tracking of the sunflower (Helianthus annuus Linnaeus). The inflorescence turns with the movement of the sun, its 'face' held obliquely to the sun's rays. Domesticated sunflowers seldom face the sun directly except in the early morning and late afternoon. Once the seeds are set, the inflorescence droops and no longer receives sunlight. Often the direction of droop is toward the east. (personal observation)
The leaves of the sunflower also track the sun turning slowly through the day to keep their broad surface perpendicular to (facing) the incoming rays. This positive or diaheliotropism maximizes the solar radiation available for photosynthesis. Some plants do not benefit from direct sunlight and display negative or paraheliotropism, turning their leaves edge-on to the incoming light. (Biology of Plants, Raven, Evert, and Eichhorn, 5th edition, 1992)
Heliotropism is not to be confused with phototropism. Phototropism is a change in the plant's growth in response to light and is not a reversible condition. Heliotropism is caused by differential turgor pressures.
The peduncle (stem of the inflorescence) and the petiole (leaf's stem) contain an aggregate of specialized cells called a pulvinus. Pulvini are capable of altering the amount of water present and like a balloon can either be rigid or limp.
Cells within the pulvinus reduce their osmotic potential in response to the blue wavelengths present in sunlight. When the osmotic potential decreases, water flows into the cells. The increase in turgor pressure expands the size of the cells. The expansion of the pulvinus reorients the leaf or flower.
Stan Herd is probably one of the most unusual artists of our time, an earth works creator. He designed and planted a 'painting' of a vase of sunflowers using sunflowers as well as clover, soybeans, and plowed earth. To view this wonderful earthwork, click on the link:
http://www.stanherdart.com/PF_sunflower.htm
Series: | 1 | | 2 | | 3 | | 4 |
killerPlants Tendrils: ~~1~~2~~3~~4~~5~~
Suggested Reading:
How does hoarding benefit sunflowers? Weird Plants - May 8, 2003
Why did Mennonites plant sunflowers? Plants that Changed History - May 13, 2003
Why did settlers plant sunflowers? Herbal Folklore - April 28, 2003
Black-eyed Susan (Rudbeckia hirta) Plant of the Week - January 23, 2006
Sunflower (Helianthus annuus) Plant of the Week - April 28, 2003
What crop grows best irrigated with sea water? Weird Plants - June 13, 2002
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Gurney's Seed and Nursery -$20 off—>Click here.
Henry Fields Seed and Nursery -$20 off—>Click here.
Spring Hill Nursery -$20 off—>Click here.
Gardens Alive! -$20 off—>Click here.
Michigan Bulb -$20 off—>Click here.
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Maximillian Sunflower
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Helianthus maximilliani—Makes a bold statement wherever it is planted! Strong, 4-7' tall stems are covered with dozens of sunny yellow, 2-3" blooms from midsummer to frost.
A favorite of birds and butterflies! Long-lived plants thrive in heat, drought and even poor soil. A natural for cutting.
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Heliopsis, Summer Sun
Springhill Nursery®
Summer Sun Heliopsis plants are loaded with dozens of orange-yellow, double flowers with green hearts. Blooms from midsummer until frost. With daisy like charm, bushy plants send up floral sunbursts on 3-4’ stems that need no staking. Showy, disease free. Space plants 8-12” apart.
#1 Field-Grown Plants. Heliopsis helianthoides var. 'Summer Sun' Zone 4 to 9
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Black Eyed Susan
Springhill Nursery®
Multitudes of large, golden, daisylike flowers with glistening black centers start blooming in midsummer. These Black-Eyed Susans are one of the few perennials to remain in bloom until frost stops them. Hardy, easily grown plants multiply fast, reach 18-30" tall. Plant 12-16" apart.
#1 Field-Grown Plant. Deer tend to avoid. Rudbeckia fulgida var. speciosa Zone 4 to 9
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