AbstractThe blue-grey sheen of dried specimens ofSphagnum subnitensis attributed to thin film interference in the walls of the hyaline cells, this being produced by reflection from upper and lower film surfaces. The reflectance is large in air and small in water, so that the colour is visible only in dried specimens.S. rubellurnlacks the blue-grey sheen because only a small part of the total area of the hyaline cells is thin enough to generate interference colour.