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Why Do Sunflowers Move with the Course of Sun?

来源:中国数字科技馆

Why Do Sunflowers Move with the Course of Sun?

Source of pictures: Pixabay.com

Having heard that the sunflowers in the suburb were in bloom, Daniel's mom took him to watch the sunflowers early in the morning this weekend. Daniel was too excited to shut his mouth all the way and kept asking his mom when they would arrive. After a three-hour ride, they got off the car and walked on the dusty country road. Daniel looked around and saw a large golden yellow field in the distance. He excitedly asked Mom, "Is it there?" Mom nodded and walked toward the field.

After walking for more than ten minutes, Daniel arrived at the sunflower field and was stunned by what he saw. Many 'smiling faces' were greeting him, more dazzling than the sun in the sky. He ran excitedly into the field and darted among the flowers higher than himself. Mom warned him from behind, "Be careful. Don't step on the flowers."

They were so tired after playing all day that they sat by the roadside where they got off in the morning. Daniel looked at the evening glow and the 'smiling' sunflowers and found the flowers were facing a different direction. The sunflowers were facing them when they came, but now with the sun setting the sunflowers turned their faces away from them. "Mom, can sunflowers move?" He was puzzled and asked, "They look different from what we saw in the morning."

Mom smiled, "You are so observant. Sunflowers have the ability to follow the sun. Their heads face the east as the sun rises and turn west when the sun sets. During the night when there is no sunlight, they straighten up slowly, and turn east in the next morning to begin the cycle again. Once they produce seeds and the stems harden, the blooms are fixed and no longer move with the sun."

Daniel was wondering what magic power the sun has to make sunflowers move with the course of it.

Mom explained, "It's because sunflowers contain a plant hormone called auxin, known to promote the growth of cells of sunflowers' stems. The auxins are sensitive to sunlight and do everything they can to seek shade. As the sunlight comes from the east in the morning, the auxins migrate to the west part of the sunflower. With the auxins distributing unevenly in the stem, the cells grow faster in the west part than in the east part, so the stems bend to the east. That's why the sunflowers always face the sun. As the sun moves across the sky, the sunflower heads continue to change the direction they face."

Daniel kept asking, "But there is no sun at night. In which direction do they face?"

"Without sunlight, sunflowers' stems will grow upright because the auxins are evenly distributed. In the next morning, they once again face the eastern sky as the sun rises. Such heliotropism of sunflowers will disappear when they get mature. Then auxins are no longer needed to stimulate cell growth in the stems. Mature sunflowers will remain facing east, because they can enjoy gentle sunlight in the morning and reduce the strong direct sunlight at noon to dry dew collecting at night and prevent mildew."

After saying that, Mom stood up and hold Daniel's hand to walk toward the car.

Daniel looked back at those 'docile' sunflowers and felt they were so amazing. Although their flowers look like a little sun, the auxins in the plant fear sunlight and make them move with the course of the sun.

"Good thing we don't need auxins, or we would have to move with the sun and grow crooked," Daniel whispered.

Author: Huang Jing

本文来自:中国数字科技馆
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