A lowly dandelion is stuck in the ground. It can’t move. It seems hopelessly chained to earth by its roots. But one day, it will give its young marvelous wings that will let it soar above the landscape, to enjoy a brief but wondrous journey, traveling possibly for miles till it gently descends to a paradise its ancestors could never imagine: a land of fertile soils and fountains. Your lawn.
You have to grudgingly admire those pesky weeds that thrive so easily against the garden plants you want that require so much sweat and coaxing. Now, you will admire them more to hear that the little parachute-bearing seeds fly using a technique scientists didn’t even know about till now. Nature says:
Every child knows that blowing on a dandelion clock will send its seeds floating off into the air. But physicists wanted to know more. How does an individual seed manage to maintain such stable flight? Researchers at the University of Edinburgh studied the fluid dynamics of air flow around the seed and discovered a completely new type of flight. It’s based on a previously unknown kind of vortex which may even be common in the plant and animal kingdoms, now that we know where to look. [Emphasis added.]
Jeremy Kahn, also writing in Nature, calls it an “‘impossible’ method never before seen in nature.” A beautiful film clip describes how air flowing up through the bristles of the “pappus” as it is called creates a “separated vortex ring” above it that literally sucks the seed up into the air.
“Perhaps one day, even human technologies could be designed to fly as efficiently as the mighty dandelion seed,” the narrator says of this “completely new type of flight.”
The paper in Nature by Cummings et al. is titled, “A separated vortex ring underlies the flight of the dandelion.” The authors seem jazzed by what they found:
The porosity of the dandelion pappus appears to be tuned precisely to stabilize the vortex, while maximizing aerodynamic loading and minimizing material requirements. The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures.
The editors of Nature took note of this study, appreciating the broader implications by saying in their Editorial, “The floating of a seed shows how appreciating the wonders of the Universe can begin with a new look at the everyday.”
The English poet and artist William Blake was no fan of the reductionism of Isaac Newton. True discovery, and therefore knowledge, Blake insisted in his poem ‘Auguries of Innocence’, was to be found in the everyday, where a world could be seen in a grain of sand and “heaven in a wild flower”.
The editors emphasize that the dandelion’s flight trick depends on finely-tuned parts:
All falling objects, from feathers to cannon balls, create turbulence in their wake. But it takes a rare combination of size, mass, shape and, crucially, porosity for the pappus to generate this vortex ring. Size is also particularly important, because from the point of view of something as small as a pappus, the air is appreciably viscous. At such a scale, a parachute consisting of a bunch of bristles is as effective as the aerofoil found in larger seeds that disperse from taller plants — such as the winged seeds of the maple. In the same way, the tiniest insects do not fly with solid wings, but swim through the air using ‘paddles’ made of bristles.
Perhaps most surprisingly, the trick depends on the blank spaces between the parts. What goes on there depends on the solid materials and how they are arranged.
The key lies not in the bristles of the pappus, but in the spaces between them. If projected on to a disc, the bristles together occupy just under 10% of the pappus’s area, and yet create four times the drag that would be generated by a solid disc of the same radius. The study shows that air currents entrained by each bristle interact with pockets of air held by its neighbours, creating maximum drag for minimum expenditure of mass. The pappus’s porosity — a measure of the proportion of air that it lets pass — determines the shape and nature of the low-pressure vortex.
Isn’t it wonderful what blind, unguided processes designed? Evolution is a genius. Its productions are simply heavenly:
It’s an example of how evolution can produce ingenious solutions to the most finicky problems, such as seed dispersal. There are many things unknown that are smaller than atoms, or larger than galaxies, or billions of years away in time. But there are secrets held by things that we take for granted — things on a human or near-human scale — that seem all the more precious for it. Heaven in a wild flower, even.
Other Ingenious Seed Dispersal Mechanisms
There are seeds that can float across oceans (coconut, mangrove). The filaments on wild oat seeds respond to moisture, turning into outboard motors that drive the seeds along the ground and into the soil. Storksbill seeds actually fold into a drill for driving the seeds into the ground. We all know, too, how Velcro was inspired by cocklebur seeds catching rides on the fur of cows.
Some seeds prefer traveling indoors in comfort on natural transportation services. One particular tropical forest tree in Thailand, an article on Phys.org notes, seems to have a preference for elephants that eat the fruit and deposit the newly-fertilized seeds on the ground someplace else. Other seeds can fly as passengers in an airliner, hitching a ride through the “cabin” of a bird’s digestive tract, to disembark unharmed after landing miles away.
“Seed dispersal is an essential, yet overlooked process of plant demography,” says Utah State University ecologist Noelle Beckman in another article on Phys.org that begins:
Though mostly rooted in the ground, plants have a number of innovative ways to disperse their seeds and get on with the business of propagation. They drop seeds or release them to the wind. Or they fling seeds with a dramatic mechanical detonation. Or they rely on seed transport by water or hitching a ride on a traveling animal (including humans).
Beckman calls seed dispersal “a central process in ecology and evolution,” but is it not more empirically valid to stop at the word ecology? What’s evolution got to do with it?
The scientists found dispersal ability is related to fast life histories with maximum dispersal distances positively related to high reproductive rates, a long window of reproduction and a low likelihood of escaping senescence or growing old.
“The faster the life history, the farther distances seeds are dispersed,”Beckman says. “This may allow the species to take advantage of environments that vary unpredictably.”
While this statement gives the appearance of a scientific explanation, appealing to a law-like pattern, there are problems. Surely there are many exceptions to the alleged rule. More important, the explanation says nothing about the origin of the exquisitely engineered mechanisms used by seeds to disperse. This is evident when you consider the details. How could a blind process, that feels only the immediate environmental pressure and is incapable of aiming at distant goals, accomplish these engineering marvels?
- Carefully spaced filaments that generate a separated vortex ring
- Chemical coats that can survive an animal’s digestive tract without trapping the seed in a box it cannot escape from
- Propellers, such as those on a maple seed, of just the right length and curvature that can create lift in the breeze
- Projectile mechanisms that can fling a seed tens of feet away from the plant at high speed
- Cones that can insulate a seed with gases from a forest fire, then open up to drop the seeds after the fire has passed
- Seed filaments that turn into motors and drills
Students might become more motivated to choose science as a career when teachers cultivate the awe and wonder in everyday things, instead of dowsing it with vacuous appeals to sheer dumb luck. If scientists are just now discovering new methods of flight in a dandelion seed, what other worlds are there in a grain of sand, and heavens in a wild flower?
Photo credit: Cathal Cummins, University of Edinburgh, via EurekAlert!