Comments on scatterometry and optical modelling, and supplementary figures. from Functional optics of glossy buttercup flowers

Buttercup (<i>Ranunculus</i> spp.) flowers are exceptional because they feature a distinct gloss (mirror-like reflection) in addition to their matte yellow coloration. We investigated the optical properties of yellow petals of several <i>Ranunculus</i> and related species using (micro)spectrophotometry and anatomical methods. The contribution of different petal structures to the overall visual signal was quantified using a recently developed optical model. We show that the coloration of glossy buttercup flowers is due to a rare combination of structural and pigmentary coloration. A very flat, pigment-filled upper epidermis acts as a thin film reflector yielding the gloss, and additionally serves as a filter for light backscattered by the strongly scattering starch and mesophyll layers, which yields the matte yellow colour. We discuss the evolution of the gloss and its likely two functions: it provides a strong visual signal to insect pollinators and increases the reflection of sunlight to the reproductive organs, so to heat the reproductive organs.