Conical Epidermal Cells Allow Bees to Grip Flowers and Increase Foraging Efficiency

The plant surface is by default flat, and development away from this default is thought to have some function of evolutionary advantage. Although the functions of many plant epidermal cells have been described, the function of conical epidermal cells, a defining feature of petals in the majority of insect-pollinated flowers, has not. The location and frequency of conical cells have led to speculation that they play a role in attracting animal pollinators. Snapdragon (Antirrhinum) mutants lacking conical cells have been shown to be discriminated against by foraging bumblebees. Here we investigated the extent to which a difference in petal surface structure influences pollinator behavior through touch-based discrimination. To isolate touch-based responses, we used both biomimetic replicas of petal surfaces and isogenic Antirrhinum lines differing only in petal epidermal cell shape. We show that foraging bumblebees are able to discriminate between different surfaces via tactile cues alone. We find that bumblebees use color cues to discriminate against flowers that lack conical cells--but only when flower surfaces are presented at steep angles, making them difficult to manipulate. This facilitation of physical handling is a likely explanation for the prevalence of conical epidermal petal cells in most flowering plants.