Botanic Superlatives: Brightest Berry Joins Glitterati of the Natural World

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Botanic Superlatives: Brightest Berry Joins Glitterati of the Natural World

November 19, 2012
12:34pm
Photo by: PNAS

What's brighter than a peacock and shinier than a butterfly? A tiny, hard, fruit that grows in the forests of Tanzania, Mozambique, Ethiopia, and other African countries. According to biologists, the tiny fruits of Pollia condensata, a meter-tall perennial herb commonly known as the marble berry, surpass anything other living organism in their capacity to dazzle with a rainbow of iridescent light. The vibrant blue berries have been awarded the stylish & superlative title of Nature's Brightest Bauble, or Brightest Living Thing on Earth, according to a study published in the Proceedings of the National Academy of Sciences. As to how the 5mm berry became so bright, the birds will tell you: while they're no more edible than a jewel, they can really bring a nest together.

Not only is the fruit without precedent in its iridescence, it produces its color in a very surprising way, particularly for a plant. Most colors in the natural world, and especially among flora, are produced by pigments that absorb selective wavelengths of light. Those not absorbed are reflected, and these are the colors we see. For example, the pigment chlorophyll: For most of their life cycle, leaves are reliably green due to chlorophyll, which absorbs most wavelengths of color except green. Chlorophyll degrades with autumn's waning light, and we see red and yellow instead of green.

Yet this blue jewel of African forests lacks pigments. Instead, it shines because of structural coloration, a phenomenon that has been observed in animals, but never in plants. In the case of P. condensata, iridescence is produced as light hits epidermal cells in the organism's skin, which contain stacks of microscopic, rod-shaped cellulose fibers. Arrayed in spiraled layers, the rods in the fruit's epidermal cells are spaced at staggered intervals. As light hits and scatters, wavelengths of different colors of light are reflected. Structural coloration manifest a little differently in every species of animal (eg. peacock, scarab beetle), and never before in a plant. “The optics are impressive,” says Silvia Vignolini, a physicist at the University of Cambridge and one of the authors of the study. “There are no previous examples of this in nature.”

Like a sapphire, they are extremely hard, and timeless—P. condensata berries will retain their vibrance for decades after the plant has died. One specimen, studied by researchers from Kew, the Univeristy of Cambridge, and the Smithsonian Natural History Museum, has been radiant and bright since collected in Ghana in 1974.

Initially, scientists were puzzled by the berry's iridescence. Why did it evolve? Often, a unique trait can be attributed to an advantage it confers to the survival of the species. For the Morpho butterfly, iridescence confuses and scares predators by appearing to change color. In the peacock, a shimmering color makes for a very impressive strut. Similarly, fruit color is a often ploy to attract seed dispersers, but no bird would eat this stone. P. condensata is very hard, and very inedible— the berry is effectively a husk and seed, with no soft, nutritious flesh. So what function does iridescence serve in the obdurate berry?

There are a couple ideas. First, to trick. P. condensata berries might have evolved to mimic another blue berry that grows in the same region, such as Psychotria peduncularulis. Birds will pluck the hard berries before the inedibility is realized, and by that time, the fruits have already achieved their dispersal goal.

The second theory is especially fun—to play matchmaker by bedazzling a nest. Beverley Glover, a plant scientist at the University of Cambridge and a co-author of the study, agrees that the glittering fruits attract birds, but not for food. She suspects that birds pluck the inedible, shiny berries to decorate nests to attract mates. Plants have evolved many incredible features to disperse their seeds, but Glover says that “this strategy is brilliant as the plant does not waste any precious energy on providing food for birds."

And so the peackock got its strut, the Morpho butterfly eluded its predator, and the African berry dispersed its way into a bird's nest.

 

Anna Laurent is a writer & photograher.