When a pollinator is at your front steps about to come in for a drink of nectar, you'd be foolish to let a gust of wind blow her away. That's why most flowers have installed velcro doormats. Pointy cells give their petals an extra-grippy surface that encourages bees, even in the middle of a windstorm, to stop and stay a while.
Flowers such as roses, tomatoes, and petunias have cone-shaped cells in the surface of their petals. In fact, about 80 percent of flowers with traditional petals have these conical cells, says University of Cambridge plant scientist Beverley Glover. The cones make a flower's color appear more vibrant by focusing sunlight onto pigment held in the center of the cell. They also help bees latch on. Since the cells are about the same size as the tiny claws on bees' feet, the claws slide in between the cones for a tight grip.
Previous research has shown that conical cells help bumblebees get the nectar out of snapdragons. These flowers keep their goods hidden inside what looks like a hinged door (or apparently, if you're the person who named this flower, a dragon's mouth). Snapdragons with pointed petal cells let bees comfortably land, pry open the door, and drink the nectar. When bees visit mutant snapdragons with flat petal cells, they struggle to maintain their grip.
But snapdragons, with their elaborate structure, are an unusual case. So Glover's team set out to see what conical cells are doing in simpler flowers. They started with the ordinary petunia.
When the team presented bumblebees with both standard petunias and mutant (flat-celled) petunias, the bees preferred to visit the more velcro-esque flowers. The difference in color between the two purple flowers was too slight for a bee's eyes to detect, so something else about the conical cells must have been attracting them.
To find the attractive quality in grippy petunia petals, the researchers swapped their standard petunias for a grippy petunia that's unattractive. This genetic mutant has the usual cone-shaped cells, but a much darker purple color that makes the flower difficult for bees to see. Now the bees preferred the flat-celled flowers, since they were easier to find and fly to.
Then the team created a little wind to see whether it gave the upper hand back to cone-celled flowers. Or rather, they created the appearance of wind by placing their petunias on a laboratory shaking machine. This device has a platform that jostles its contents around, keeping tubes and beakers of things well-mixed during experiments. When loaded up with petunias, it waved the flowers as if they were outside in a stiff breeze. (To add to the illusion, the shaker was covered with green tissue paper.)
At first, the bees still flew to the lighter-colored flowers with flat cells. But over the course of the 100 flower visits researchers let each bee make, the bees learned to favor the conical-celled flowers, even though they were still harder to see. By the end of the experiment, bees flew to these flowers a majority of the time.
Not only do conical cells help bees grip flowers, but they become even more important when those flowers are moving. The bumblebees' desire for a comfy place to wedge their feet overcame their dislike of difficult-to-see colors.
If pointed cells on your petals is so great, why do flowers such as lilies, tulips, and magnolias have flat cells instead? Glover says this is a question she's still trying to answer. "In a couple of plant groups we've been studying, we do think we see an association between switching to moth or bird pollination and losing your conical cells," she wrote in an email. If your pollinator is an insect or hummingbird that hovers over you rather than landing, there may be no advantage to keeping a sticky doormat.
Another possibility is that slippery-petaled flowers encourage bees to ignore the doorstep altogether and fly straight in the window. In flowers such as the flat-celled woody nightshade, Glover says, "the bee grasps the anthers and vibrates them to get the pollen out." But, she adds, "we don't have data to support this idea yet." When it comes to pollination etiquette, we're still learning the rules.
Alcorn, K., Whitney, H., & Glover, B. (2012). Flower movement increases pollinator preference for flowers with better grip Functional Ecology DOI: 10.1111/j.1365-2435.2012.02009.x
Images: Bee on flower by Martin Cathrae/Flickr; experimental diagram by Alcorn et al.