Conical cells on the petals of most flowering plants appear to play a much more grounding role for pollinating bees than scientists have previously assumed.

Bee landing on snapdragon Image: Don Manning and Beverley Glover University of Cambridge

According to a study to be published in the June 9 issue of Current Biology, these cone-shaped cells on the surface of petals appear to provide insect pollinators with secure footing while perched on the plant.

"Most people assumed that [conical cells] played a role primarily in visual communication," said David Baum, plant evolutionary biologist at the University of Wisconsin-Madison, who did not participate in the research. It was never entirely clear what conical cells were for; one hypothesis held that by modifying the spectral properties of the petal, the cells enabled the plant to appear brighter to pollinators. In this study, "they show the conical cells help provide more friction which allows bees to get better footing."

"I think this is an interesting and unique study," said Geraldine Wright, neuroethologist at Newcastle University, also not a co-author. "This is essentially a biophysical parameter that may have been overlooked before now."

By comparing two white snapdragon mutants -- a normal, conical-celled plant and a visually-identical, flat-celled form -- plant scientist Beverley Glover of the University of Cambridge and colleagues determined that bumblebees were able to detect these minute differences in petal surface structure (along the order of 10 micrometers). Indeed, when the textured flowers were stocked with 30% sucrose and the smooth flowers with a bitter quinine solution, the bees quickly learned to drink only from the plants with textured petals, reaching an accuracy of over 80% in just 20 visits.

To test if bees prefer the conical-celled petals when all the flowers were stocked with 30% sucrose, the researchers used two colored lines of snapdragons -- the bright magenta, wild-type with conical cells on the epidermis of the petals and a paler pink, mutant line with flat-celled petal surfaces. Interestingly, while the bees showed no preference for the conical-celled surfaces when the flowers were horizontal, they chose to land on the textured wild-type flower significantly more often when the flowers were presented vertically, suggesting that they only prefer the conical cells when it is difficult to land on the flower.

When the flower is vertical, Glover said, it's not very energetically efficient for bees to feed on the flat-celled flowers. "Their legs are scrambling as they do it, and their wings keep beating...like a mountain climber trying to find a hold on a vertical mountain side."

Furthermore, because flat cells reflect more white light, the color of petals lacking conical cells appears diluted. "We think the color difference is something that bees might be able to learn as a cue," Glover said.

"This is a great example of doing the hands-on manipulative experimentation to evaluate the adaptive value of an interesting trait," Baum agreed. The next step "is a phylogenetically structured survey to see if lineages that have lost conical cells tend to be those that switched to hovering animal pollinators."


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Flower power in motion
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