Plants teach big lessons about how to get going
Engineers who want to design devices that move quickly should consult with plants. Our green friends have evolved in simple ways to make some of the fastest movements known in nature.
Take the Venus' flytrap. It snaps up its prey in a few tenths of a second. The title of "fastest plant on Earth," however, goes to the bunchberry dogwood. When launching its pollen, this tiny plant moves more than 100 times as fast as the flytrap.
A fresh look at such familiar but poorly understood phenomena would uncover a treasure trove of examples to fire a designer's imagination, says Harvard mechanical engineer Lakshminarayanan Mahadevan. In January, Professor Mahadevan and several colleagues published in Nature the first detailed description of how the Venus' flytrap snaps. It's a simple system based on hydrologic pressure and release of pent-up elastic energy.
When resting, the two leaves of the hinged trap bend outward in a concave shape. The trap is open to admit unsuspecting insects. When an intruding fly trips trigger hairs inside the trap, movement of fluid within leaf cells builds up elastic strains in the leaf. When the leaves can no longer sustain the strains, they snap into their other stable shape, curving inward to form a concave enclosure. Voilá! A trapped insect and a tasty meal.
The system is a little like the pressure indicator on the lid of a jar of jam. As the jar is being sealed, the metal indicator is popped upward. But as the jar cools, internal pressure drops and the elastic strain in the top changes. At some point, the top snaps into its other stable position, inward, and stays that way until you open the jar and break the seal.
Canadian bunchberry dogwood is even trickier. Joan Edwards at Williams College in Williamstown, Mass., and colleagues have shown it to be the fastest-moving plant on the planet. It opens its petals and releases pollen in a few thousandths of a second. Their report last month in Nature explains that this action also involves hydrological and elastic forces with a twist: Its pollen launcher works like a trebuchet. That's a medieval catapult that carries its payload in a sling attached to the top of the catapult's arm. This design gives extra oomph to the missile tossed by the trebuchet.
Thanks to a similar scientific principle, the bunchberry's stamen tosses pollen grains 10 times as high as a tiny flower can. Internal hydrological pressures prepare the mechanical energy needed to open the flower petals explosively. It is the release of elastic energy that lofts the pollen.
Botanical engineers still have a lot to learn about the details of these and other rapid plant actions. They are making progress. Mahadevan and Jan Skotheim at the University of Cambridge in England have derived mathematical descriptions for hydraulically driven and elastically assisted plant motions. They explained last month in Science how the size of a plant - or plant part - determines when rapid motion needs an assist.
Whereas smaller plants can change their shape readily, larger plants, such as the Venus' flytrap, must rely on buckling or on the rapid release of stresses caused by internal fluids to snap shut.
The two scientists say their work provides a physical basis for classifying the hydrodynamics involved in plant motion. It also shows the limits on what speeds such motion can attain.
That's a helpful engineering guide. The challenge now for engineers is to use these processes in practical ways. As the two scientists note: "Nature has already implemented many such designs exquisitely; we simply need to follow her lead."