To take a drink of nectar, a bumblebee has to keep its balance on wobbly flower petals. The bees get some help from plants, according to a new study. Cone-shaped cells, found on 80% of flowering plants, offer firm footing and may have evolved to increase the odds of pollination.
Scientists have debated the function of these cells, known as conical epidermal cells, since the 1970s. Some thought they made flowers more attractive to bees by enhancing their color or by increasing the plant's temperature a degree or two, which might boost nectar production. But plant molecular biologist Beverley Glover of the University of Cambridge in the United Kingdom had a different theory: that the cells provided traction.
"In the wild, a flower isn't just a stationary dinner plate," says Glover. Cells that allow bees to latch on as a flower is waving in the wind make pollination more likely and would be an evolutionary plus, she says.
To rule out color and odor as functions of the cells, Glover and colleagues created white, odorless epoxy casts of snapdragon petals. The surfaces of the casts were either flat or lined with cone-shaped bumps simulating conical epidermal cells. Small tubes on the petals held sucrose.
When the casts were horizontal, the bees were equally attracted to flat and bumpy petals. However, when the casts were placed at an angle or vertically, the bees were far more likely to choose the petals with a rough surface, the researchers report  online today in Current Biology. Analysis of high-speed video showed that when bees landed on the bumpy petal casts, they latched onto the surface with all legs, stopped beating their wings, and dined with ease. However, bees attempting to feed on smooth petals slipped around, their middle legs scrambling on the petal surface while their wings beat furiously.
"Beating their wings costs bees a lot of energy," says Nickolas Waser, an emeritus evolutionary ecologist at the University of California, Riverside. "Animals want to gather as much energy as possible as quickly as possible, so it makes sense that bees would prefer a surface that allowed them to conserve energy while they ate."