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

Buttercup (Ranunculus 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 Ranunculus 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.