Super Blue Berry: The Natural World’s Most Intense Color

Pollia condensata Berries

The most intense color in the biological world belongs to a tiny African berry. Iridescent blue and metallic, it literally outshines any other plant or animal substance in the world.

The plant itself is called Pollia condensata, and researchers have now explained the material magic underlying its marvelous hues: layers of cells that refract light in a manner usually seen in butterfly wings and beetle shells.

“Structural colors come about not by pigments that absorb light, but the way transparent material is arranged on the surface of a substance,” said physicist Ullrich Steiner of Cambridge University. “This fruit is one of the first known examples in plants. We compared it with some other structural colors, such as the morpho butterfly wing, which is often described as the strongest structural color. This is stronger.”

On the following pages, Wired talks to Steiner about the findings, which were co-authored with fellow Cambridge physicist Silvia Vignolini and published Sept. 10 in Proceedings of the National Academy of Sciences.

Image 1:
Pigments fade, but structural colors remain largely untouched by time. Berries used in the new study came from botanical garden specimens collected in 1974, “and this one here is as bright and shiny blue as it was 40 years ago,” Steiner said.

If nature is any guide, they’ll last far longer than that. Structural colors are still vivid in the fossils of beetles that lived 50 million years ago.

An Invisible Marvel

Image 2:
When light hits the berry’s deepest surface layer, some is absorbed. The rest is reflected. These photons, which have linear, up-and-down wavelengths, can be seen by the naked human eye. Some reflected photons, however, have spiral wavelengths.

Known as circular polarized light, or CPL, it’s invisible to our eyes. Instead we rely on technological imaging to detect and represent it, as in the picture above. With eyes that could see CPL, the berries of P. condensata would look something like this.

P. condensata’s berries not only reflect CPL, but two types of it — photons that spiral to the left, or to the right. This has never before been observed in any living tissue, writes Steiner’s team, and may be just as remarkable as the berries’ singularly intense hues.

Structural Color

Image 3:
At first glance, P. condensata berry surfaces look smooth (figure A). Cross-sectioned and placed under more magnification, though, a three-level structure emerges (figure B). The outermost is made from layers of thick-walled cells. The walls themselves are made from densely packed stacks of cellulose fibers (figures C and D, captured by transmission electron microscope.)

It’s these stacks that guide light reflected from the innermost level, producing each berry’s visible colors.

“The light bounces off the interface of between each of these layers,” Steiner said. “The more layers you stack up, the better defined the color is. The brightness and color purity we see in the fruit comes from the fact that many, many layers add up to produce these very strong reflective characteristics of just one wavelength.”

As some layers don’t add up to blue, flecks of other hues can be seen on the berries, which because of the cells’ configurations have a pixellated appearance, with each cell representing a single pixel.

A Precise Arrangement

Image 4:
As illustrated, the cellulose fiber layers are not arranged haphazardly, but in precisely aligned configurations. These generate the berries’ circular polarizations.

“The planes of the fibers rotate around,” Steiner said. “Sometimes they rotate clockwise, at other times counterclockwise.”

The Mystery of Circular Polarization

Image 5:
As seen (in the image), the same cross-section of a berry produces circular polarized light that spirals to the left (figure A) and to the right (figure B). (In figure C, they’re combined.) Why a berry should produce any circular polarized light at all, however, is an open question.

As best as scientists know, only one creature — strange, sea-dwelling crustaceans called mantis shrimps — can see CPL wavelengths. Their appearance in this terrestrial berry could hint at as-yet-unidentified powers of perception: Perhaps insects or birds or some other African animal can perceive CPL, and for whatever reason the berries benefit from detection. Or maybe CPL is simply a side effect of cellulose layers arranged to generate optimal shades of visible colors.

“We don’t know the answer,” Steiner said.

The Meaning of Blue

Image 6:
If P. condensata’s circular polarized properties have no obvious function, their visible coloration is rather less mysterious. It’s likely an evolutionary adaptation to catch the eyes of birds, which eat the berries and disperse their seeds. Though the berries are actually near-fleshless and nutrient-poor, the bright color may make them appear juicy and rewarding.

Steiner’s team is now looking to learn how P. condensata coloration evolved and whether other examples of structural color can be found among plants.

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