Bent tissue strip gives orchid kick; study dissects how plants hit bees with pollen masses.

Like some people, Catasetum orchids get rough because they're bent out of shape.

Male flowers in this tropical genus don't wait for a visiting bee to nuzzle the pollen, explains Daniel Fulop of Harvard University. When a bee lands, brushing the flower's long trigger hairs, a floral structure slams a pollen mass onto the bee's back.

After studying 16 species in the genus, Fulop and Harvard colleague Jacques Dumais have now figured out how the pollen smacker works. Its power comes from the sudden release of a bent strip of tissue, called a stipe, attached to the pollen mass, Fulop reported July 27.

"It was just wonderful to see this mechanically complex problem dissected and explained," said Wendy Silk of the University of California, Davis.

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A Catasetum flower uses scents to attract male bees. Near the male flower's pocket of volatile perfumes sits a long, multipart structure holding two pollen balls. At one end of the structure lies a sticky foot and the stipe, which curves over a bump in the middle of the flower and connects to the two balls and to the cap at the other end.

When a bee brushes against a pair of long trigger hairs, the pollen structure starts ripping loose from the flower at the end with the foot, Fulop reported. The stipe abruptly unbends and, along with the pollen balls, swings out and away.

The structure would somersault beyond the bee, though, if it weren't for a refinement. The end with the cap, the last part to break loose, gets pushed against a floral spur. The spur gives a bit and then springs back, batting the departing structure toward the bee. The sticky foot on the structure fastens the pollen balls in place.

What puts the zing into the action is the stipe, Fulop said. When it tears loose and straightens, it powers the pollen shot.