Sunflowers (Helianthus annuus) are annuals with long stems that keep the blossoms standing tall and broad leaves for capturing sunlight used in photosynthesis. Bristles along the stem stave off water loss and help defend the plant from predation by animals. The plants have existed since ancient times; carbon dating of North American clay samples has revealed sunflower seeds that date back 3,000 years. Sunflowers require full sun, at least six hours each day, to thrive.
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To help maximise sun exposure, sunflowers have adapted a means of moving their large, flowered heads to track the movement of the sun. This adaptation is called heliotropism. Sunflowers face east in the morning to greet the rising sun, then follow the sun's movement across the sky until it sets in the west. During the evening hours, the sunflower head returns to face eastward once again.
No one is certain why heliotropism is important to sunflowers. The most prevalent theory is that insects are attracted to the sun's light and, by keeping the plant's large head facing the sun, the sunflower increases its chances of attracting insects that pollinate the surface of the head. Another theory is that seed growth is enhanced with the added warmth gained by constantly facing the sun.
Mature Sunflower Plants
Supporting both the pollination and seed growth theories, only younger sunflowers with heads not yet pollinated engage in heliotropism. Once the plant is mature and pollination has been completed, the sunflower faces east and does not move westward with the sun.
Mechanics of Motion
To achieve the movement necessary to follow the sun, sunflowers have motor cells in a joint-like thickened part of the stem called the pulvinus. Located just below the head, the pulvinus is flexible enough in sunflowers to rotate the head. Motor cells expand and shrink based on turgor pressure inside the cells. Turgor pressure is water-induced pressure that is exerted against cell walls. Motion is created in the pulvinus as pressure increases on one side while decreasing on the other.
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