Fig. 4.

Various types of visual signals are not due to backscattering by the flower’s interior. (A) Fluorescence is the emission of light by pigments that absorbed light, for example absorption of ultraviolet light leads to emission of long-wavelength light such as blue. (B) Periodically structured striations of the flower’s cuticle may cause incident light to be diffracted, yielding a visual signal that is dependent on the angle of illumination and observation, a phenomenon known as iridescence. (C) A very flat and smooth flower surface may cause specular reflection when the angle of illumination is identical to the angle of observation (denoted by α). Light that is reflected by the surface will not be modified by floral pigments inside the flower, so the gloss will generally be perceived as (achromatic) white. (D) Unpolarized light illuminating the flower becomes linearly horizontally polarized when it is reflected by the smooth surface of a flower; the degree of polarization depends on the angle of incidence. In glossy areas and under large angles the ratio of surface versus subsurface reflection – and thus the polarization effect – will be maximal.