May 11, 2005

Catapult Plant is World Record-Breaker

PARIS (AFP) -- A tiny Canadian shrub is the quickest-moving thing in the plant world, using a catapult mechanism to eject its pollen at a speed hundreds of times faster than a launched rocket, scientists have found.

The plant, bunchberry dogwood (Cornus canadensis), grows in thick carpets in the vast swampy, spruce-fir forests of the North American taiga.

Growing to a height of only 20 centimetres (eight inches), the bunchberry needs the explosive push to get its pollen into the forest breeze so that it maximises its chance of fertilising other shrubs.

It does so by using its star-shaped flower like a trebuchet -- a giant mediaeval catapult that had a wooden throwing arm with a strap attached to its tip.

In the trebuchet's case, a rock would be put in the strap's pouch, the catapult was wound up, and a combination of stored mechanical energy and slingshot effect would then hurl the payload in a low arc at besieged castles or enemy troops.

Using a high-speed camera capable of recording at 10,000 frames per second, the researchers found the bunchberry's secret lies in its folded petals and a vegetal catapult inside.

Initially, the bunchberry's four petals are fused together at the tips.

They suddenly spring open and flip back, unleashing the tensed stalks of the stamen, the plant's male organs.

These stalks, also called filaments, are the equivalent of the trebuchet's throwing arm.

Attached to them is attached a thin, flexible strand, like the trebuchet's strap. And at the end of strand is a "payload" of pollen, in a tiny sac called the anther.

"After the petals open, the bent filaments unfold, releasing elastic energy," the authors, led by Joan Edwards of Williams College, Massachusetts, report on Thursday in Nature, the British weekly science journal.

"The tip of the filament follows an arc, but the rotation of the anther about the filament tip allows it to accelerate pollen upwards to its maximum vertical speed, and the pollen is released only as it starts to accelerate horizontally."

The whole process takes 0.5 milliseconds -- half a thousandth of a second -- from start to finish and is the fastest movement ever recorded in a plant.

The modest-looking bunchberry is thus at least five times faster than the previous record-holder, Impatiens pallida, which takes between 2.8 and 5.8 milliseconds to open its fruits.

And by comparison, the Venus flytrap is a slouch, snapping its jaws together to catch an insect prey at a mere 100 milliseoncds.

During the first 0.3 milliseconds, the bunchberry stamens accelerate at up to 2,400 G, or about 800 times the forces that astronauts experience during takeoff.

Mini but mighty, the catapult can propel its pollen 2.5 centimetres (0.8 of an inch) into the air above the plant.

In laboratory conditions, the dust-like spores settled more than 22 centimetres away (nine inches), but in the wild, a steady wind would take them more than a metre (3.25 feet) away, thus enhancing the chance of fertilising fellow shrubs, Edwards' team believes.

Another possibility may come from insects. A relatively large insect, such as a bumblebee, could trigger the flower opening, and pollen could stick to its hairs, thus enabling cross-pollination when the creature visits other plants.


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Williams College