When it comes to sowing seeds, some plants only have to drop them and let gravity take care of the rest. But seeds from a family of small flowering plants known as Geraniaceae, give themselves a helping hand: after bursting open from beak-shaped fruits, they literally drill themselves into the ground. Scientists have long known that this bursting and drilling results from hairy appendages on the seeds called awns, which coil up and straighten out with changes in humidity, slowly propelling the seeds downward. Now, researchers have figured out the structural changes occurring in the cells of these awns that generates the coiling mechanism. The scientists began by creating a mathematical model of an awn cell—a rod-shaped structure wrapped in rigid, helical fibers. They described these helices with two parameters, the so-called mean angle and tilt angle of the fibers, and found that if these didn't change as the cell shrunk due to lowering humidity, the cell would have to respond by taking on its own spiral shape. Likewise, the cell would straighten out as soon as humidity rose again, creating the drilling motion. To test its model, the team predicted values for the mean and tilt angles for two species of the Geraniaceae family—one commonly known as stork's bill (Erodium cicutarium, pictured) and the other as cranesbill (Geranium pusillum)—that would produce the observed coiling. The results of x-ray measurements of real awn cells closely matched the predictions, demonstrating that the coiling does indeed arise from the arrangement of the outside fibers. The researchers say their results, published this week in Physical Review Letters, could help engineers build materials that change their configuration with the environment, such as new types of catheters that automatically bunch up to unblock obstructed blood vessels.
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