Flowers pollinated by insects have evolved special cells on their petals to help bees stay put while they are feeding, UK researchers have found.
The bees can also adapt their behaviour to target flowers that are easier to hold on to.
The findings, published online today in the journal Current Biology, answer why these conical cells exist.
Lead author Dr Beverley Glover from the University of Cambridge, says scientists have long recognised that most flowers had surface cells on their petals that were shaped like little cones or pyramids.
Yet no one knew their purpose, she says.
“There were all these complicated ideas about how they might enhance light capture and make the petal look a brighter colour,” she said.
“Or that they enhance the temperature of the flower and therefore increase nectar secretion, or maybe affect the way scents are released.
“It turns out that they’re just providing a bit of grip to make life easier for pollinators.”
For the study, Dr Glover and her team tested the behaviour of bumblebees as they attempted to feed off fake epoxy snapdragon petals that had bitter and sweet nectars.
The only difference between the petals was the shape of the surface cells.
Velcro-like
In lab tests, the team found the bees, which had never seen a flower before, “learned” to recognise the shape of petal cells via touch and quickly began to prefer the conical-shaped models.
“To start with, they visit both flower types equally. But within 20 to 30 landings on flowers, they learn to target the conical-celled ones,” Dr Glover said.
She believes this learning would happen faster in the wild as most flowers have these conical cells.
Dr Glover says the special cells allow a “Velcro-like” grip between the pollinator’s middle feet and the flower.
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“That all makes it hard to keep the proboscis in the nectar and also wastes energy,” Dr Glover said.
She says about 80 per cent of flowers studied have conical cells and she expects the findings will apply to other bee and flower species.
“I strongly suspect that all pollinators that actually land on the flower [other bees, butterflies, flies, beetles] will prefer conical cells, while hovering pollinators [hummingbirds, moths] won’t care,” she said.
Plant adaptation
Bee researcher Dr Katja Hogendoorn from the University of Adelaide, says the findings are “beautiful” and open up a new dimension of morphological research.
In particular, Dr Hogendoorn says plants might adapt these conical cells so as to attract particular bee species.
“Bees range between 1.5 millimetres and 4 centimetres in length and can have wide and slender feet placed wide apart or close together,” she said.
“That is the equivalent of the variation between a rabbit and an elephant.”
Dr Hogendoorn says certain plants are highly adapted to certain bee species to encourage them to move between flowers of the same species for cross-pollination.
“Does the intricate structure of conical cells support this specialisation? Is its structure and placement variable between plants depending on the bee species, size and morphology?” she said.
Agricultural applications
Dr Glover agrees the findings do raise questions such as how these cells evolved and whether they target certain pollinators.
She believes the results might also have applications in agriculture.
“You can imagine that it might be possible to optimise petal cell shape to encourage pollinators to visit crop plants that rely on animal pollination, such as fruit trees,” she says.
“[But] this would need an analysis of what the petals currently look like and what pollinates them.”