Maple seed fascination has a scientific side
By Paul G. Wiegman
Published: Sunday, May 25, 2008
On Mother's Day morning, the family was sitting at the dining room table enjoying a sumptuous "daughter-made, with able-granddaughter assistance" breakfast.
If you remember that day two weeks ago, it was overcast with leaden skies coming from the west. Rain punctuated the morning and, as a small storm neared, the wind picked up and was swirling.
At one point, my granddaughter's eyes widened and she exclaimed, "It's snowing." We all turned in unison toward the big windows facing the backyard and, indeed, even though it was May 11, it looked like a freak winter snow was occurring.
For a moment, there was astonished silence until again my granddaughter exclaimed, "Helicopters!" The wind had gusted and set adrift a storm of ripe seeds from a neighbor's sugar maple. The pale tan seeds were lifted above and away from the mother trees and were drifting down like big, fluffy snowflakes.
On first glance, it did, indeed, look like a freak spring snow.
But the flakes were twirling to the ground, not just drifting. The mass of sugar maple seeds was an armada of whirling helicopters landing all over the yard.
Almost everyone knows about maple seeds from childhood. About this time each spring, we would find them slowly twirling to earth from high in the canopy of maples. It was standard practice to gather a handful, toss them into the air and watch them spiral back down to the ground.
One neat trick involved carefully separating the bulbous end with the seed into halves without breaking the halves from the wings, and sticking the open halves on someone's nose. That was always good for a laugh. We called them polynoses.
Another common name for the twirling seeds falling from the maples is "whirligig." I guess that was more common before helicopters were invented.
Botanists, of course, have a special name for these flattened, papery wings attached to a seed. It is a samara and is a type of fruit. Most anything that a plant grows after fertilization takes place in a flower technically is a fruit.
Botanists think differently than grocers.
A samara is a dry fruit and is indehiscent. Dehiscent means that a fruit naturally opens along a seam. Think about a milkweed pod. When it is young, it is light green, soft and tightly closed. As it matures in the fall, the outer covering turns a light gray, hardens and, finally, splits along a seam from the tip to the base of the pod. The halves of the pod peel back, and the silky tufts of fine hairs, with a seed attached at the bottom, float away in the wind.
Maple seeds, being indehiscent, don't split along a seam. That was something we had to do for ourselves to be able to stick the halves on our noses.
Sugar maples aren't the only trees that have samara. It's a characteristic of all maples. Silver, red, mountain, stripped, black and Norway maples in this region all produce seeds with wings. Maples, the genus Acer, around the planet have winged seeds making it easy to recognize maple trees. Ash trees also have seeds with wings, as do elms, hoptrees and pines.
Why are whirligigs such popular seeds?
The tree that grew the seeds we watched on Mother's Day was two doors down the street. Many of the seeds, carried by the wind, ended up in our yard. That's a perfect example of seed dispersion by wind.
Trees stay in one place. Their seeds (which will produce their offspring) drop directly below the crown and sprout, grow and eventually begin to compete for water and food. Therefore, it's important to get those seeds away from the parent tree.
That process was easy to see two weeks ago.
Attaching a wing to the seed ensures that when it drops, especially on a windy day, it will be carried elsewhere, decreasing the likelihood it will have to compete for water and food.
The wind, however, is fickle when it comes to getting the seeds in the right habitat. We realize this as we spend time on ladders at the beginning of summer and clean gutters clogged with maple seeds. To make sure that at least some maple seeds get to a good place to grow, trees produce thousands and thousands of whirligigs. If only a small percentage of seeds gets to a good location to prosper, the species will proliferate.
The wind isn't the only means of dispersal. Squirrels carry acorns and bury them far from parent trees; birds feed on wild cherries and plant the undigested seeds far from where they gathered the cherries; rivers carry seeds long distances, where they are deposited on banks and sometimes covered with a layer of silt; and oceans carry large, buoyant coconuts to distant shores to grow new palms in new places.
Why do whirligigs whirl• The samara is heavy at the end with the bulbous seed and light on the fibrous wing. It would seem that if you held a mechanism like that and let it drop, it would just fall straight with the heavy side down. But that doesn't happen.
Gregory Vogt and Roger Storm of the NASA Glenn Research Center prepared a Web site explaining maple seed "helicopters."
Vogt and Storm explain that "because of the asymmetrical nature of maple seeds ... the center of mass of the seed is shifted well to one end while its center of lift is approximately in the middle. In a complicated process, the forces at work as the seed falls combine to begin a circular rotation of the seed about its center of mass. The rotation actually inscribes a cone around the axis of fall."
From that, it seems that what happens is that when the "complicated process" begins, the seed starts to spin, and once the spin starts, the fibrous blade actually becomes a wing, and as it circles, it works just like an airplane wing and creates lift making the wind and the seed float to the ground.
An interesting part of the Web site includes experiments with stiff paper and a paper clip to make such "autorotating helicopters." There also are instructions for an origami maple seed that works just like the real thing.
The origami model works well, but you can't stick it on your nose.
Paul g. Wiegman is a freelance writer, photographer and naturalist born and raised in western Pennsylvania.
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