Metallic blue fruit shines brightest of all

Don't judge a book by its cover. These fruit may be more intensely blue than anything in biology, but they contain no blue pigment at all. What's more, their inviting appearance hides a dry seed-filled centre that offers little nutritional value to hungry animals.

The berry-like fruit of Pollia condensata?? a forest plant from sub-Saharan Africa?? has an iridescence similar to that seen in some butterflies and scarab beetles.

All owe their colour not to pigments, but to the way light reflects off tiny structures on their surface, says Beverley Glover at the University of Cambridge. This structural colour is more intense in P. condensata than anything else.

"We measure intensity by looking at what proportion of the light arriving at the surface of the material is reflected back," says Glover. "Previously the Morpho butterfly won this competition, but our Pollia does better."

Because the colour comes from its structure, it remains vibrant long after the fruit has fallen. "Even [fruit] that is over 50 years old is as blue and shiny as ones growing today," says Glover.

Fresh or not, the fruit is barely worth eating. It contains no sweet and juicy pulp, just hard seeds. The intense colour may be an adaptation to trick birds into eating the fruit and dispersing the seeds, Glover's team suggests.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1210105109

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