Friday, January 1, 2016

Helicopter Seeds

A few snowflakes tumbled and drifted their way down as I opened my bedroom window. The fresh air felt good, but that wasn’t my goal. I was feeding the birds. In order to give my chickadees consistency throughout bear season, I have a small feeder suction-cupped to my second story window. I can tilt out the pane of glass and reach over the top to fill the feeder with sunflower seeds. Even while the bears are asleep, I like the alarm clock services that the chickadees – as they peck noisily on seeds and gargle at each other – provide.

As I swung the window shut, the motion dislodged a maple seed that must have been stuck on the outside of window frame. Already feeling child-like and giddy from wrapping Christmas gifts, I pressed my nose to the window and eagerly watched the helicopter spin its way to the ground. The sight brought back a flood of memories – not just from the handfuls of maple seeds we tossed into the air as kids, but also of the very playful college freshmen who studied the seeds during a botany lab I taught in graduate school.

Maple seeds are exquisitely designed for both human play and the trees’ reproductive fitness. Their spinning flight is not only fun to watch, but it also allows the seeds to disperse up to a mile away from their parent tree, out from under its oppressive shade.

These magical seeds develop from tiny spring flowers. Nose-to-nose, with their rooster-tail wings arching away, two seeds start off in a pair. Each seed holds the key to future generations, and it also holds the key to flight. The heavy seed carries enough energy to produce a root that can penetrate through the maple’s own thick leaf litter.

Without much wind, my windowsill maple seed spun almost straight down and disappeared into the soft snow. With a terminal velocity of about 3 meters per second (about 6.7 miles per hour) it was a quick trip. At the end of its flight, the structure of the seed and wing becomes more like a dart – drilling the seed into the substrate to find it a secure place for germination.

But the center of that useful mass is not at the geographical center of the seed. This asymmetrical distribution of mass is one of the essential ingredients in the helicopter seed’s flight. Even broken samaras (samara is the technical name for these winged seeds) with barely any wing left can spin and generate some lift.

When you add the wing off to one side, the center of balance becomes even more lopsided. Part of the seed’s role is to counterweight the papery wing and make sure that the wing is oriented for maximum lift as it spins. The wing itself is elegantly designed with a slight pitch (like a fan blade), that facilitates the spin. As the wing – tapered at both ends – spins around the weight of the seed, the air rushes fastest over the wide main blade, increasing the lift, while the narrower outer tip cuts through the air with less drag.

As the wing spins, its leading edge generates a horizontal, tornado-like vortex that lowers the air pressure over the upper surface of the wing, effectively counteracting gravity and doubling the lift in comparison to non-spinning seeds. The spiral motion is resistant to disturbance by wind, so the lift it generates can allow the wind to help disperse the seed farther.

Some helicopter seeds are better at sustaining flight than others, though. That’s what we tested in the botany lab. Students calculated the surface area and weight of each seed. The ratio between the two measurements is known as “wing loading.” Then students stood on a table and dropped the seeds one by one, while a research partner timed the descent. Seeds with lower wing loadings tended to stay aloft longer. Students with poorer balance tended to descend sooner and with more laughter. While smaller seeds might disperse farther, though, they would not have as much energy to germinate. Over the years, maple seeds have found a sweet spot within the larger range of variability that allows them to achieve sufficiency in both travel and growth.

Even with my nose still pressed up against the window, a hungry chickadee couldn’t resist swooping in for a fresh snack. Seeing my face startled it, though, and it backed up in a short hover before landing again. I smiled to see the shiny black eye so close to my brown ones, and admired the delicate detail of the feathers. Using a leading-edge vortex to generate lift isn’t just used by maple seeds (and similar samaras from ash, elm, and boxelder trees). Insects, bats, hummingbirds, and maybe even my chickadees all use this little bit of physics to help them hover, too.

What fun in just a little seed! As we all watch kids open oodles of expensive toys this holiday season, I can’t help but be grateful for the simple toys that nature provides. Nature’s toys aren’t free either, though. It’s just that they require our stewardship instead of our money. 

Sugar maple seeds. Photo by Steve Hurst @ USDA-NRCS PLANTS Database

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