Bumpy petals help bees get a grip on flowers

Blogging on Peer-Reviewed ResearchMany plants depend so heavily on visits from bees that they go to great lengths to attract them, using brightly coloured flowers baited with sweet nectar. But some of their tricks are much subtler and are designed not to attract six-legged visitors, but to make their stay more convenient.

The majority of flowering plants have evolved special conical cells that line the surface of their petals and are found nowhere else. These cells provide the flower with a rougher texture that is indistinguishable to human fingers, but that provide just enough purchase for the claws of landing insects. Heather Whitney from the University of Cambridge found that these conical cells turn the petal into a more conducive landing pad, and bees can tell if a petal has these bonus features or not by the way it reflects light.

About 80% of flowering plants possess these conical cells, but some develop mutations that do away with them. The snapdragon can develop a fault in the MIXTA gene, which prevents petal cells from developing into a conical shape. The lack of cones means that more white light reflects from the flowers' surface, giving them a paler pink colour and making them stand out from the rich magenta of their peers. Honeybees tend to ignore these paler flowers, even though they smell the same as the normal variety.


Whitney showed that the bees were ignoring the mutant snapdragons because they didn't feel right. She gave bees a choice between two snapdragon flowers - a white one with conical cells (the nivea strain), and a white one with flat cells (a nivea/mixta cross-breed). Neither human nor insect eyes can tell the difference between the two, but insect feet can. At first, the bees visited both flowers equally, but after about 20 trials, they chose the one with the conical cells, on 82% of their visits.

They felt the same way about fake snapdragons that mimicked the textures of their wild counterparts. Whitney created "biomimetic epoxy casts" that imitated the surface of conical and flat-celled flowers in remarkable detail. These casts allowed her to study the effect of texture, devoid of the confusing influences of colour, smell or any other floral cue. After several visits, bees learned to choose the bumpier surface with almost perfect accuracy - strong proof that the feel of a flower's petal is part of its attraction.

These microscopic bumps give the bee something to latch onto when it searches for nectar and pollen, and Whitney showed that bees are fussier about the texture of their petals if they're more likely to slip off. Bees will happily land on flowers with either flat or conical cells if they are laid out horizontally. But when Whitney presented the flowers at a vertical angle, the conical-celled ones drew the biggest buzz, no matter what colour they are painted. 

High-speed videos revealed why. When bees landed on flat-celled flowers, they scrabbled about with their legs, beating their wings to stay upright and trying in vain to get a foothold. When they landed on the conical-celled flowers, they found surer footing and soon settled into a resting position with folded wings. 

The conical cells probably have other purposes too, for they're also found on flowers that are pollinated by moths, which hover over flowers and never touch down on them. Perhaps the conical cells also affect the colour, shape and temperature of flowers but for now, Whitney's study clearly show that in some cases, they are an adaptation that makes it easier for pollinators to get a grip.

Reference: Whitney, H., Chittka, L., Bruce, T., & Glover, B. (2009). Conical Epidermal Cells Allow Bees to Grip Flowers and Increase Foraging Efficiency Current Biology DOI: 10.1016/j.cub.2009.04.051

More on plants and insects:

i-77217d2c5311c2be408065c3c076b83e-Twitter.jpg i-3a7f588680ea1320f197adb2d285d99f-RSS.jpg

More like this

Many living things, from chameleons to fish to squid, have the ability to change their colour. But flowers? Yes, over 450 species of flower have the ability to shapeshift, altering their colour and positions over the course of a day. The goal, as with many aspects of a flower's nature, is…
Sometimes a picture can tell you a lot about evolution. This particular picture has a story to tell about how two species--in this case a fly and an orchid--can influence each other's evolution. But the story it tells may not be the one you think. Coevolution, as this process is now called, was one…
The partnerships between flowering plants and the animals that pollinate them are some of the most familiar in the natural world. The active nature of animals typically casts the plants as the passive partners in this alliance, but in reality, they're just as involved. That becomes particularly…
Photo by: Leif Richardson, Dartmouth College I came across this neat press release from the University of Massachusetts: AMHERST, Mass. – Researchers studying the interaction between plants, pollinators and parasites report that in recent experiments, bees infected with a common intestinal…

That's an interesting study and write-up, but is it really safe to assume that "flowers use bumpy petals to help bees get a grip?"

I would rather say that Whitney's study shows that the conical cells are a feature that bees use. Whether they were actually selected on that basis is a much tougher question to answer, IMO.

Not that it's improbable. I grant that it's a very reasonable scenario and may well be true. I'm just averse to over-interpretation.

An entirely fair point Qetzal. See revised headline.