Sunflowers (Helianthus annuus) are one of several plant species that make optimum use of light by turning to face the sun, known as heliotropism or phototropism. 

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A flexible segment of the stem just below the flower responds to pressure within the motor cells, causing the sunflower to turn toward light. The plant tracks light to enhance photosynthesis, the process of creating food from light. Heliotropism also increases the flower temperature, which attracts bees and other pollinators. The rise in temperature increases the rate of pollen germination. Mature sunflowers with stiffer stems become stationary, facing the rising eastern sun.

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Heirloom varieties of sunflower may exhibit more pronounced sun-tracking characteristics than newer, hybrid versions.

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Mathematical biologists love sunflowers. The giant flowers exhibit demonstrations of a hidden mathematical rule shaping the patterns of life: the Fibonacci sequence, a set in which each number is the sum of the previous two (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, ...), found in everything from pineapples to pine cones. In sunflowers, the sequence is illustrated in  the number of different seed spirals on the sunflower's face. Counting the clockwise and counterclockwise spirals that reach the outer edge usually results in a pair of numbers from the sequences: 34 and 55, or 55 and 89, or with very large sunflowers, 89 and 144. 

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A study published in 2016 in Royal Society Open Science reported that nearly one in five of the flowers had either non-Fibonacci spiraling patterns or patterns more complicated than has ever been reported, including near-Fibonacci sequences and other mathematical patterns that compete and clash across the flower's face. 

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This tartan designed by Carol A.L. Martin picks up the deeper shades seen in the seedhead of the sunflower.

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For more illustrations of the golden ratio and Fibonacci sequence in nature, click the sunflower!