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  Plant cells help bees get a grip
Flowers pollinated by insects have evolved special cells on their petals to help bees stay put while they are feeding, say UK researchers.

Plus, bees can learn to prefer flowers that are easier to hold on to.

The findings, published online today in the journal Current Biology, settles the debate over why these conical cells exist.

Lead author Dr Beverley Glover of the Department of Plant Sciences at the University of Cambridge, says scientists had long recognised that most flowers have surface cells on their petals that are shaped like little cones or pyramids.

Yet no one knew what they were for, she says.

"There were all these complicated ideas about how they might enhance light capture and make the petal look a brighter colour, or enhance the temperature of the flower and therefore increase nectar secretion, or maybe affect the way scents are released," she says.

"It turns out that they're just providing a bit of grip to make life easier for pollinators."

For the study, Glover and her colleagues 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.

In laboratory 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," says Glover.

She says the special cells allow a "Velcro-like" grip between the pollinator's middle feet and the flower.

On flowers without the cells the bees cannot get a foothold and they are "continually scrabbling while the bee drinks, and the wings continue beating".

"That all makes it hard to keep the proboscis in the nectar and also wastes energy," says Glover.

Glover says about 80% of flowers studied have these 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."
Plant adaptation

Bee researcher, Dr Katja Hogendoorn, of the University of Adelaide, says the findings are "beautiful" and open up a whole new dimension of morphological research.

In particular, 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 says.

"That is the equivalent of the variation between a rabbit and an elephant."

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 asks.

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."
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