Issue Date: September 17, 2012
The brilliant blue hue of the Pollia condensata fruit does not arise from pigments, but rather from the layered structure of nanoscale cellulose fibrils in its skin’s cell walls, researchers report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1210105109). A team led by Ullrich Steiner of Cambridge University examined a sample of Pollia condensata collected in Ghana in 1974 and preserved at the Royal Botanic Gardens. The scientists were unable to extract any type of pigment from the fruit, so they studied its anatomy to figure out the origin of its unusual metallic blue color. They found that the fruit’s multilayered cell walls act like tiny reflectors, thanks to the helical cellulose fibers within them. Each cell reflects a specific color. Blue reflectance dominates, but there are also cells that reflect red or green, giving the fruit a pixelated appearance. Depending on the orientation of the helices, the fruit can reflect both left- and right-circularly polarized light. “The bright blue coloration of this fruit is more intense than that of any previously described biological material,” the researchers point out.
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