''Pollia condensata'', sometimes called the marble berry, is a

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

herbaceous plant Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

with stoloniferous stems and hard, dry, shiny, round, metallic blue fruit. It is found in forested regions of

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. The

blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...

colour of the fruit, created by

structural coloration Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...

, is the most intense of any known biological material.

Description

The plant was originally described from

Angola , national_anthem = " Angola Avante"() , image_map = , map_caption = , capital = Luanda , religion = , religion_year = 2020 , religion_ref = , coordina ...

, in southern Africa. It has large, smooth, narrow leaves, and pale pink or whitish flowers on a stem about 60 cm high. The fruit capsule is about 4 mm in diameter.

Structural coloration

The surface of the ''Pollia'' fruit has an especially smooth and transparent cuticle which reflects light as a mirror does (

specular reflection Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...

). Beneath this glossy surface lies a special layer of cells which have an elaborate but unpigmented microstructure, whose function is to reflect light within a narrow range of wavelengths. This

is created by

Bragg reflection In physics and chemistry , Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposition of wave ...

from spirally stacked

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...

in the walls of these cells. The wavelength reflected depends on the height of the stack, which varies from cell to cell. Variability in the stack height allows more overall light to be reflected and this further enhances the glossy appearance, but it makes the fruit appear somewhat

pixelated Pixelization (British English, pixelisation) or mosaic processing is any technique used in editing images or video, whereby an image is blurred by displaying part or all of it at a markedly lower resolution. It is primarily used for censorship ...

. In addition to simply reflecting light of a specific wavelength, the helicoid structure also causes light of other wavelengths to be modified so that the wavelength converges to within a narrow range before being reflected, which acts to amplify the light at that specific wavelength. This process of

constructive interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

produces the most intense coloration of any living organism. The total reflectivity is about 30% of that of a silvered glass mirror, and is the highest of any known biological material. The intense coloration makes the fruit attractive to some birds despite having no nutritional value. The birds sometimes decorate their nest with the berries, which over the long term helps to disperse the seeds.

According to materials scientist Ullrich Steiner, who led the team which carried out the original research on structural color in plants,
Structural color Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...
s 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 The morpho butterflies comprise many species of Neotropical butterfly under the genus ''Morpho''. This genus includes more than 29 species problem, accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central ...
wing... This is stronger.

References

External links

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A Berry So Shiny, It's Irresistible (And Inedible)
at

npr.org National Public Radio (NPR, stylized in all lowercase) is an American privately and state funded nonprofit media organization headquartered in Washington, D.C., with its NPR West headquarters in Culver City, California. It differs from othe ...

The Fruit with a tale in the twist
Cambridge university microscope video {{Taxonbar, from=Q7225666 Commelinaceae Flora of Africa Plants described in 1881 Taxa named by Charles Baron Clarke