Most plants and fungi can only move themselves by slowly growing, but a few have lightning reflexes to trap unwary flies or to disperse their seeds with a bang. Now, researchers say they can classify plants and fungi based on how they move, which is ultimately determined by the physics of water flow through their tissues.
Moving without muscles requires creative engineering. The fruit of the Mediterranean squirting cucumber (Ecballium elaterium), for instance, swells until it explodes off the vine, scattering its seeds. And the Canada dogwood (Cornus canadensis) builds up pressure in their tiny flowers until they pop open, ejecting pollen skyward. In the May 27 issue of Science, two applied mathematicians have now categorized the motions by their speed and scale.
Inspired by a Venus flytrap, Lakshminarayanan Mahadevan of Harvard University in Cambridge, Massachusetts, and Jan Skotheim at Rockefeller University, New York City, found that diverse species all move in three basic ways. Swelling and shrinking of tissues works well for smaller, slower movements, as in the nooselike loops of the fungus Arthrobotrys dactyloides, which tighten to lasso tiny nematode worms. But some plants adopt different strategies. The Venus flytrap uses energy stored in the leaves' elastic tissue, causing them to buckle and snap shut, while the Canada dogwood relies on pressure built up in its tissue for an explosive fracture. The researchers find that the dividing line between these movements-swelling versus snapping and exploding-is apparently determined by how fast fluid can flow through tissues.
"The study really elegantly summarizes how two very different groups, plants and fungi, share a common toolbox of mechanisms for movements," says botanist Joan Edwards of Williams College in Williamstown, Massachusetts. Biomechanist Steven Vogel of Duke University in Durham, North Carolina, says he would not have guessed that the rate of flow would limit such a diversity of movements. "But I'm nearly persuaded," he adds.