The compound cyanidine is responsible for the colors of the flowers on the cover of this book-both the red of the poppy and the blue of the cornflowers. The difference in the colors results from a variation in the pH of the sap of the flowers. The model above represents a cyanidine molecule as it exists in the acidic sap of the poppy; in the basic sap of the cornflower, one proton (shown in blue) is removed. The loss of this simple proton causes a significant shift in the compound's absorption of visible light-producing a different color.

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