This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
One of the obscure points of science is the cause of the harmony of colors always observed in flowers. An exchange states that when two colors are found, they are generally complements of each other. The wild asters of Autumn generally have purple rays and yellow disk flowers. The pansy is yellow and purple, and the blue violet has its stamens yellow and its petals a reddish blue. In fact yellow and purple generally go together in flowers.
A splendid example is afforded by the large Iris Germanica, the popular flower-de-luce of our gardens. From the white base of its petals the colorless sap passes into its petals, which become of a gorgeous purple, while the beard of the petals becomes at the tip a very rich yellow, though the lower part of each separate filament is not of the purest white, what chemical or physical law determines the arrangement of color, if there be any such secondary cause, is not yet discovered.
Two French chemists, Fremy and Cloez, say that the tints of flowers are due to cyanin xanthin, and xanthein. Cyanin is reddened by acids. A supply of vegetable acid developed in a flower would then turn the blue to rose color, while a scarcely sensible quantity might produce a purple. Xanthin is a yellow from the sun-flower, and xanthein the yellow of the dahlia. There are probably other coloring sub-stanoes. - Household.