Way back when the Scribbler was a feisty young field biologist staking out nests of strange Neotropical antpittas, I often chewed grass stems to keep myself awake. Occasionally, I would panic to find that a seed head was working itself stubbornly down my throat, pointy end first. When I tried to spit it out, little hairs would anchor it to my cheek, and every time I swallowed it would slide farther back toward the soft and presumably important parts of my throat. If you’ve never tried this, don’t: Choking to death on a hayseed is both frightening and kind of pathetic at the same time.
If only I had realized there was a Science paper in there somewhere, I might not have felt so humiliated.
Rivka Elbaum and colleagues noticed the same sort of thing happening with wild wheat, but they went the extra cogitational mile and realized that
From a mechanistic point of view, we have discovered a device for movement that is composed of passive elements.
What they found was that the wheat awns – those are the long, stiff hairs that poke up off the wheat seed – are asymmetrical in cross section. When they’re moist, the awns are more or less straight, but when they’re dry one side of the awn shortens more than the other, causing the awn to curl.
Combine that with the natural cycle of humidity in a day and the action of those little side-hairs that got stuck in my throat, and the awns can actually propel the seeds over the ground or down into the soil. No, really.
As the day heats up and the seed dries, the awns bend and the stiff little side hairs grip the soil. When night falls, the cool air moistens and lengthens the awns. They straighten, but the hairs allow that straightening to happen only in one direction, pushing the pointy seed forward or down.
Voila: Wiggles in the humidity record induce wiggles in the wheat awns (which the authors note, in a nice touch, “resembles the swimming stroke of frog legs”), and the seed buries itself. Next day: more wiggles, more progress for the wheat seed.
(No measurements yet on the force with which the point is driven into the ground. Or whether the power involved could be, uh, harvested, to help those poor underpowered krill mix the oceans.)
Yo-
I thought humidity increased at night, and decreased during the heat of the day; what gives?
Nice catch. I went back, re-read a certain figure caption, and got things straightened out. Should read better now. Thanks for the peer review!
you’ve sparked an interesting thought (you often do) by mentioning awns. i had never actually heard of an awn except in it’s longer version: awning. then it occurred to me that awnings increase in length as moisture increases, as in hauling them out from the wall to cover diners in posh sidewalk cafes, when it starts to rain.
any connection, would you say or am i simply being silly?
the mummbler