It’s surprisingly difficult — by puffing from any one direction — to send all of a dandelion’s delicate white seed tufts wafting away from their stem. A clump almost always clings on the opposite side of the stem no matter which direction the wind blows. Understanding a plant’s selective seed release has been tricky too.
After about three years of off-and-on rethink, retest and repeat, a team of U.S. and Australian researchers has found a structural quirk that lets batches of seeds catch upward puffs. That allows the plant to game the vagaries of wind, says fluid dynamicist Chris Roh of Cornell University.
While a dandelion parent doesn’t have a muscle to move in giving her offspring the best start in life, the newfound structure gives the plant a considerable range of force that lowers the chance of seeds catching a doomed downward flight, he and colleagues report September 10 in Journal of the Royal Society Interface.
How the dandelion seeds windsurf, fluffy end up, is already known. A little above the seed tuft, a vortex of swirling air “like a very chubby ring-doughnut with barely any hole” creates a low-pressure zone that keeps the tuft aloft, Roh says.
For his focus on the journey’s beginning, “I need to give credit to my 4-year-old daughter,” Roh says. “On a walk, we take out a dandelion and blow on it. And that’s how this curiosity really started.”
As Cornell biophysicist Jena Shields remembers, “Chris comes in [to the lab]. He’s like, ‘Our fingers are FORCE SENSORS! Look at this dandelion!’”
Pulling out newly ripe tufts at different angles, he showed her that even humans can feel how much easier it is to tug off a seed by pulling slantwise upward instead of down-slanting. Then came his vision: “Now go measure it!”
Shields attached a force sensor to individual tufts on a dandelion with its newly ripe fluffball and confirmed what her own 10 sensors had experienced. Overall, pulling loose the upslanting seeds took an order of magnitude less force than plucking them downward. (Pulling a seed straight out was the most difficult, a result that’s inspiring a variety of speculations.) As far as Shields knows, these are the first formal measurements of the force needed to release dandelion seeds.
What creates the differences could be the microscopic architecture where seed and mom connect. They’re tethered by an off-center, skinny strand, imaging by a team headed by biomechanist Sridhar Ravi at University of New South Wales in Canberra shows. That tether is surrounded by horseshoe-shaped shielding. When wind or science grabs a seed to pull, that U-curl steadies the heavy seed in place — unless the force tilts the seed body toward the open side. Without the U’s support, the whole tuft weight overwhelms the tether. Lift-off!
Dandelions make “a great example of science that was just right under our noses this whole time,” Shields says. She hopes this bit of dandelion science will inspire more of us to stop and wonder “Why is this thing doing this weird thing?”
