Bent innards give orchid its kick

A flower mechanism for smacking pollen onto bees opens up diverse possibilities for floral architecture

SNOWBIRD, Utah — Like so many 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 load up on pollen by nuzzling against it, explains Daniel Fulop of Harvard University. When a bee lands, brushing a 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 attached to the pollen mass, Fulop reported July 27 at the Botany & Mycology 2009 meeting.

“It was just wonderful to see this mechanically complex problem dissected and explained,” said meeting organizer Wendy Silk of the University of California at Davis. She notes that the work is appropriate for the 150th anniversary of Darwin’s On the Origins of Species, in which he wrote briefly about Catasetum pollen delivery.

Based on Fulop’s calculations, if the blow a bee receives from the floral structure were scaled up to human size, it would be equivalent to getting hit with a lump weighing 6 to 16 kilograms, the heft of one or two bowling balls, moving at up to 8 kilometers per hour.

Bees seem to grow wary after such a thumping and avoid similar blossoms, according to earlier research. In Catasetum, helmet-shaped greenish female flowers don’t look like the flashy and wildly diverse male flowers, so the bees are still willing to deliver pollen to the female flowers.

Only a small percentage of orchid species fling their pollen, and the 150 or so species of Catasetum do so with particular finesse, Fulop reported. 

A Catasetum flower attracts male bees with floral scents that the bees collect for mating displays. Near the male flower’s pocket of volatile perfumes, along the middle axis of the bloom, lies a long, multipart structure holding two pollen balls. At one end of the structure lies a sticky foot and a broad strip of tissue, called the stipe, which curves over a bump in the middle of the flower and connects to the two balls and their long cap at the other end.

When a bee lands and follows the scents, it brushes a pair of long trigger hairs beside the pollen-holding structure. Fulop and Dumais found that the hairs react when displaced as little as 0.1 millimeters. And, with high-speed video, the team found that male flowers of C. pileatum need only 20 milliseconds to react before hammering the bee, which takes about another 25 milliseconds.

As soon as the hairs detect a touch, the pollen structure starts ripping loose from the flower at the end with the foot and the stipe, Fulop reported. The stipe abruptly unbends from the curved surface of the flower underneath. Pollen balls and stipe swing out and away from the plant.

Both would somersault beyond the bee, though, if it weren’t for a refinement at the other end of the structure. The last part to break loose, the end with the cap, gets pushed back against a floral spur. The spur gives a bit at first and then springs back, batting the departing structure — pollen, stipe and all — 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, according to Fulop and Dumais. When it finally tears loose from its bent position in the flower, it powers the pollen shot.

With the precise targeting ability of this pollen flinger, a flower does not need all the contours and other devices that some other orchids use to guide bees to rub against pollen. Fulop proposed that this method of pollen delivery could help explain how the genus has evolved such an extraordinary diversity of male flower shapes.

BENT OUT OF SHAPE The orchid Catasetum saccatum, like other members of its genus, slams its pollen (with glue) onto a visiting bee’s back. Researchers have now figured out how the pollen-hurling mechanism works.

VIOLENT SENDOFF A composite image shows a male flower of the Catasetum tabulare orchid firing its elongated pollen-holding structure. The energy for the pollen shot comes from the sudden unbending of tissue at the bottom. Jacques Dumais and Daniel Fulop

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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