The metallic blue color on the fruit wall in Pollia condensata (Commelinaceae) has been found to be more intense than any other biological material, according to a study published in the Proceedings of the National Academy of Sciences of the United States of America. The structure of the wall of the fruit in Pollia, a pantropical genus of Commelinaceae, has always been of interest to Robert Faden, one of the authors of the study, because the indehiscent fruits, although showy and often metallic blue on the surface, like Christmas tree ornaments, entirely lack flesh but instead are fully packed with dry seeds. Faden has always thought it likely that these fruits mimic truly fleshy berries and drupes and are dispersed when they are eaten in error by birds. It has also been suggested that some birds and mammals decorate their nests with such baubles, but that has yet to be demonstrated in Pollia.
Three species of Pollia occur in mainland Africa and P. condensata is the most widespread, ranging from West Africa (Ivory Coast) to Ethiopia and south to Angola and Mozambique. Like other species of this tropical and warm temperate genus it is a forest understory herb that can spread rapidly by means of stolons. (Pollia japonica is a hardy temperate species because it spreads by means of underground rhizomes.) The erect shoots are usually up to 1-meter tall, unbranched and terminate in a single, very dense inflorescence. The white flowers have three fertile stamens on one side and three minute staminodes on the other side of the flower. The infructescences are very dense and contain up to 40 spherical, metallic blue fruits.
Pollia fruits maintain their color indefinitely, in nature when the shoots dry up or the fruits fall to the ground, or as pressed and dried herbarium specimens. It has always seemed logical to Faden that they must have a structural color, not one based on a pigment. He suggested that the structure of the Pollia fruit wall might make a good study to Paula Rudall, Jodrell Laboratory, Royal Botanic Gardens Kew in 2009, when he was working in the Kew herbarium. At first the study was going to include ornithologists, but the laws of the United Kingdom, with regard to the use of birds in experiments, were so onerous that that idea was abandoned.
After it was determined that the fruits lacked pigment, the structure of the fruit wall was studied by transmission electron microscopy in the United Kingdom and was found to be very complex and unique in nature. The color varies from cell to cell, producing a pixilated effect at high magnification, according to lead authored Silvia Vignolini from the University of Cambridge’s Department of Physics. Developmental studies of the fruit wall in P. condensata have yet to be conducted and the structure of fruit walls of other Pollia species, particularly those that are not metallic blue at maturity, has not been studied.