Although iridescence might be expected to increase a flower’s attractiveness, it might also compromise identification, because the flower’s appearance will vary depending on the viewer’s perspective.  We found that bumblebees (Bombus terrestris) learn to disentangle flower iridescence from colour, and correctly identify iridescent flowers despite their continuously changing appearance.

We identified iridescence in hibiscus and tulip flowers, and demonstrated that their iridescence is generated through diffraction gratings – the same physical structure that makes compact discs iridescent.  Flowers were previously believed only to use chemical colours where a pigment absorbs all wavelengths except a few, giving them their apparent colour.  Iridescence can only be generated structurally, not through chemical colours.

Because most of the petal iridescence measured is at the ultraviolet end of the spectrum, which insects can see but humans cannot, this raises the intriguing possibility that many flowers are actually iridescent although they do not appear so to the human eye.

Our initial survey of plants suggests that iridescence may be very widespread.  From gardening to agriculture, flowers and their pollinators play an enormously important role in our daily lives, and it is intriguing to realise that they are signalling to each other with flashing multi-colours that we simply cannot see.

Paper

Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators by Heather M. Whitney, Mathias Kolle, Piers Andrew, Lars Chittka, Ullrich Steiner and Beverley J. Glover.  Science 2 January 2009 323: 130-133 [DOI: 10.1126/science.1166256]

The research was funded by the Natural Environment Research Council, the Engineering and Physical Sciences Research Council and the Cambridge University Research Exchange fund.