A humble understory shrub launches pollen using water pressure and physics.
Introduction
As spring-allergy sufferers know all too well, many plants rely on wind or animals to move pollen from flower to flower and help create the next generation of plants. This approach can seem like a pretty big gamble: How can you attract animals like honeybees with tasty pollen while not having too much of it eaten by those very same visitors? Or how can you make sure pollen you release gets caught by a breeze and doesn’t simply fall to the ground?
Creeping dogwood (Cornus canadensis) is an unshowy, carpet-like shrub growing across forest floors in upper latitudes of the northern hemisphere. It’s small: the biggest plant might not reach your kneecaps. The forest floor may not be a breezy place, but the plant’s flowers have solved the challenge of pollen dispersal in a unique and ingenious way.
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Creeping dogwood, a small, understory shrub, catapults its pollen far distances.
The Strategy
During the spring, the plant soaks up water through its roots and stores some of it in its flower buds. The water pressure continuously builds as the fluid fills the bud with nowhere to go. When the flower finally explodes open, its petals peel back at nearly seven meters (23 feet) per second, one of the fastest movements by any organism. This is faster than the jaws of a Venus flytrap, or club strike of a mantis shrimp––faster even than Superman’s proverbial speeding bullet. The once-constrained stamens, now free, unfold upward, accelerating at an astounding 24,000 meters (15 miles) per second squared––800 times the gravitational forces experienced by astronauts leaving Earth on a rocket ship.
The clincher lies at the tip of the stamen’s filaments, where pollen-filled sacs (anthers) rotate freely with hinge-like motion. Inside the small flower, the already-split pollen sacs are held by compression against one another, preventing the pollen’s escape. But when the petals spring apart and release the stamens upward, the sacs spread apart just as the stamen’s unfolding filaments reach their zenith. The anthers then teacup around the now fully vertical filaments, providing a secondary boost that flings the nearly weightless pollen dust straight up into the air, ten times higher than the height of the flower itself.
What happens next depends on who’s visiting. If it’s a bumble bee, whose weight is just right for triggering the flower’s explosion, the launched pollen envelops the bee’s furry body, making it more likely some of it will make it to another flower before being completely consumed. If it’s nothing but a breeze, the whisper-light pollen spreads out into a fine mist, where even the slightest passing air current transports it away.
At left, a closed dogwood flower with petals fused at the tips and the stamen filaments bent and protruding from between the petals. One petal has a trigger. At right, the flower has exploded and released most of its pollen. The stamens are extended up and the petals are completely bent back or reflexed.
The Potential
The flowers of creeping dogwood create surprising mechanical advantage out of a little water and plant tissue––seemingly powerless components. Running more of our inventions with clean, locally-available energy sources reduces our need for expensive energy infrastructure like wires or toxic batteries. The humble garden sprinkler, for instance, rotates back and forth watering your lawn using nothing but some clever mechanical design and water pressure. Utilizing the physical principles behind the creeping dogwood pollen release, a world of inventive opportunity can spring open.
