Directional scattering from the glossy flower of Ranunculus : how the buttercup lights up your chin

Abstract

The bright and glossy appearance of the flowers of Ranunculus repens was investigated spectroscopically and the optical results were correlated with the layered anatomy of the petal. The highly directional reflected light arises from the partially transparent, pigment-bearing epidermal layer, while a more diffused yellow colour is the result of scattering from the lower starch layer. This directionality of the light reflections causes the unusually intense gloss of the buttercup flower and the strong yellow reflection evident when holding the flower under the chin.

A nanostructural basis for gloss of avian eggshells

“…Iridescence can be produced by diffraction gratings, or when light passes through multiple semi-transparent materials that differ in refractive index, causing light to phase-shift and cancel out particular wavelengths at particular viewing angles [4,10,11]. Gloss, which is loosely defined as the specular or mirror-like component of light reflection, is a common component of animal coloration and is present in invertebrates, vertebrates and plants [2,[12][13][14]. Gloss is often produced by smooth or polished surfaces.…”

Functional optics of glossy buttercup flowers

“…The wavenumbers of the extrema (k e ¼ 2p/l e ) hence are a linear function of u: k e ¼ su, with slope s ¼ p/(2nd). We assumed an upper epidermis with a refractive index of n ¼ 1.4 [24], which faced air on both sides and contained b-carotene with peak absorbance A ¼ 1.4 (measured at 448 nm; see below). In our modelling, we considered a number of cases where the thin film had a Gaussian-distributed varying thickness with mean 2.7 mm and standard deviation s ¼ 0, 25, 50, 75, 100 and 125 nm.…”

“…Previous studies suggested that the gloss is due to the very flat and smooth epidermal layer that acts as a mirror [19,22], possibly enhanced by an air layer immediately below the epidermis [24]. Vignolini et al [24] applied optical multilayer theory to model the reflectance spectra, but stated that an optical model of the whole complex petal structure was missing. A comprehensive overview that quantifies the contribution of different petal components to the visual signal of Ranunculus flowers has not yet been developed, leaving the complex nature of the flowers' coloration unknown.…”

“…a mirror-like reflection) in addition to their overall yellow coloration [19,22,23]. Previous studies suggested that the gloss is due to the very flat and smooth epidermal layer that acts as a mirror [19,22], possibly enhanced by an air layer immediately below the epidermis [24]. Vignolini et al [24] applied optical multilayer theory to model the reflectance spectra, but stated that an optical model of the whole complex petal structure was missing.…”

“…Inspired by the likely structural origin of the buttercup's glossiness [19,23,24], we present here an in-depth study of the reflection characteristics of Ranunculus flowers. The genus Ranunculus comprises more than 500 species with a virtually cosmopolitan distribution [25].…”