Plant a field of sunflowers, and watch them grow. All summer you will see something amazing. In the morning, all the sunflowers will face east in the direction of the rising sun. As the sun moves, so will the sunflower heads to follow it. But as summer turns to autumn and the sunflowers get big and ripe and heavy with seeds, they will stop moving. Until they are harvested, they will face only east.
This has to do with the changing needs of the sunflowers as they grow, said Stacey Harmer. She’s a plant biologist at the University of California.
“When they’re holding their seed heads, the stems have to become super-rigid and reinforced to support their weight, to stop them from falling over,” she says. To do that, “They have to make lignin, which is the material in tree trunks” that makes them stiff.
Harmer and members of her lab had already discovered that bees prefer flowers that face east. The researchers wondered whether there was any other advantage to the flowers facing in the same direction.
To find out, they planted genetically identical sunflowers and let some face east when they were mature. They turned the pots of other sunflowers 180 degrees, so they were forced to face west. Bees swarmed to the east-facing flowers and ignored the west-facing ones. The researchers found that the east-facing flowers also had heavier seeds than the west-facing ones.
“That was an important finding, because it means the mother plants put more resources into the east-facing flowers,” Harmer says.
She points out that sunflowers release pollen in the morning, which is also appealing to the bees that are foraging at that time.
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