الوضع الليلي
11:34:53 
2023-05-30 
1293
Why dandelion seeds are so good at spreading widely
You don’t need a dandelion to know which way the wind blows. But it can help.
On any given dandelion, some seeds are destined to go north, while others are fated to fly east, south or west, and every direction in between. In effect, each dandelion seed is programmed to release for a wind coming from other direction and resists winds from other directions, according to research to be presented at the American Physical Society’s Division of Fluid Dynamics in Indianapolis on November 20.
Dandelion seeds are susceptible to different wind directions depending on where they are on the seed head, says Jena Shields, a biophysicist at Cornell University. The feathery seeds on the side facing a breeze will let go most easily; the others hold on tens to hundreds of times tighter — until the wind shifts.
It’s a phenomenon that Shields set out to study after her adviser noticed the way dandelions responded as his toddler played with the flowers.
Shields measured the force it takes to pluck dandelion seeds by supergluing a fine wire to the tufted ends and pulling them from the seed heads at various angles. This seed-by-seed study mimicked what happens when wind, or a child’s breath, pushes them over. Because each seed is most susceptible to winds from distinct directions, it helps prevent seeds from all going the same way, Shields says, and may explain why the plants are so successful at spreading. Once blown off a dandelion, the umbrella-like tuft on a seed carries it on the breeze that pulled it away.
“But a strong, turbulent wind can still send all the seeds flying in the same direction,” Shields says, so the effect can’t guarantee that a powerful gust or exuberant child won’t blow off all the seeds at once.
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