Many 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
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