Pollia condensata, colloquially called the marble berry, is a perennial herbaceous plant with stoloniferous stems and shiny, metallic blue berries found in forested regions of Africa. The berries are of particular interest, as their blue coloration is the most intense of any known biological material. The structural coloration is created by Bragg reflection from helicoidally stacked cellulose microfibrils. The distance over which the fibrils have the same orientation defines the periodicity of the helicoid and it is this which determines the wavelength of the light reflected by the stack. Each stacked layer of cells acts as an individual curved micro-Bragg reflector and variation in each of these stacks results in the color being reflected varying from cell to cell. This facilitates the perception of a high level of gloss but causes the fruit to have a somewhat pixelated appearance.
The surface of the Pollia berry has a particularly smooth and transparent cuticular surface layer which reflects light in the same way a mirror does (specular reflection) and as a result makes the berry appear to have a high level of gloss. Beneath the surface layer lies a special microscopic structure which reflects only light within a narrow range of wavelengths. This structure is composed of layers of thick-walled cells, with each layer consisting of fibrils (small fibers) that are aligned in parallel to each other. Each layer has a slightly different orientation, and when these layers are stacked on top of each other they form a helicoid. This spiral acts as a micro-Bragg reflector, with the specific wavelength reflected being determined by the height of the stack. Variability in the stack height allows different wavelengths (or different wavelengths ranges) to be reflected at different locations on the berry which allows more overall light to be reflected and the systematic nature of this variability assists with the presentation of a very high level of gloss.
In addition to simply reflecting light of a specific wavelength, the helicoid structure also causes causes light of other wavelengths to be modified so that the wavelength converges to within a narrow range before being reflected, which appears to amplify the light and produces the perception of a very intense colour. Physically, this process is called "constructive interference", and the result is a berry whose coloration is more intense than that of any other existing living organism. Total reflectivity is about 30%, which compares with the reflectivity of a standard silvered glass mirror and is higher than that of any other known biological material. As a result the berry is attractive to certain types of birds despite having no nutritional value. These birds will sometimes decorate their nest with the berries, which in time helps to spread the seeds the berry contains within it.
According to Ullrich Steiner of Cambridge University, who co-authored the PNAS paper,
- Structural colors come about not by pigments that absorb light, but the way transparent material is arranged on the surface of a substance ... light bounces off the interface ... between each of these layers ... 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. ... 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.
Images (by Vignolini et al./PNAS) may be seen alongside the interview, and at various other news sources reporting the PNAS paper findings.
- S. Vignolini et al. (2012), Pointillist structural color in Pollia fruit, Proceedings of the National Academy of Sciences. Retrieved 15 September 2012
- "The Plant List: A Working List of All Plant Species".
- Vignolini, Silvia; Paula J. Rudall, Alice V. Rowland, Alison Reed, Edwige Moyroud, Robert B. Faden, Jeremy J. Baumberg, Beverley J. Glover, Ullrich Steinera (September 10, 2012). Pointillist structural color in Pollia fruit. Retrieved 11 September 2012.
- Angier, Natalie (22 October 2012). "True Blue Stands Out in an Earthy Crowd". New York Times. Retrieved 24 October 2012.
- "Berry Bling Shines on Forever : Discovery News". News.discovery.com. 2011-06-21. Retrieved 2012-09-12.
- "Super Blue Berry: The Natural World’s Most Intense Color". wired.com. 2012-09-11. Retrieved 2012-09-11.
- "Pollia condensata". Encyclopedia of Life. Retrieved 11 September 2012.
- A Berry So Shiny, It's Irresistible (And Inedible) at npr.org