Biological communication by means of structural color has existed for at least 500 million years. Structural color is commonly observed in the animal kingdom, but has been little studied in plants. We present a striking example of multilayer-based strong iridescent coloration in plants, in the fruit of Pollia condensata. The color is caused by Bragg reflection of helicoidally stacked cellulose microfibrils that form multilayers in the cell walls of the epicarp. We demonstrate that animals and plants have convergently evolved multilayer-based photonic structures to generate colors using entirely distinct materials. The bright blue coloration of this fruit is more intense than that of any previously described biological material. Uniquely in nature, the reflected color differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or pointillist appearance. Because the multilayers form with both helicoidicities, optical characterization reveals that the reflected light from every epidermal cell is polarized circularly either to the left or to the right, a feature that has never previously been observed in a single tissue.
Faden, R. B. & Hunt, D. R.: The classification of the Commelinaceae. ‐ Taxon 40: 19–31. 1991. ‐ ISSN 0040‐0262.
A new classification of the Commelinaceae above the generic level is presented. Two subfamilies are recognized, the Cartonemaloideae, comprising two unigeneric tribes, Cartonemateae and Triceratelleae, and the Commelinoideae, containing the remaining 38 genera. Subfamily Commelinoideae is divided into two tribes, Tradescantieae, which is split into seven subtribes, each confined to either the Old or the New World, and Commelineae, which is not divided into subtribes.
Chromosome counts are reported for 32 taxa (31 species and 1 subspecies) belonging to 10 genera of Commelinaceae from seven African and Asiatic countries. Counts for 13 species and 1 subspecies are recorded for the first time. Published chromosome numbers for Anhicopsis and Polyspatha are confirmed. It is suggested that Pdisota, Pollia and Stanfieldidla each has a single basic number (x = 20, 16 and 11, respectively). The known cytological diversity in Floscopa is extended. The third continental African species of Coleolrype is found to have the same chromosome number (2n = 36) as the other two. The preponderance of the basic number x = 15 in Commelina is supported. The uncommon basic number x= 13 is reported in four taxa of Cyanotis together with karyotypic differences. The basic number x= 6 is found in a second species of Murdannia. Karyotypic data in addition to chromosome numbers are presented for 24 of the 32 taxa investigated. Karyotypes are found to be useful in assessing relationships in the family, and evolutionary trends in the karyotype are noted.
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