Scatterings image

The glossy sheen of the buttercup, deriving from thin-film optics, may serve partly to enhance reflections on the flowers’ reproductive structures, helping to keep those structures warm and functional on cold days. [Image: Casper van der Kooi]

Spring in the northern hemisphere is still three weeks away—but when it comes, one of its most durable sights will be the flowers of the buttercup, various species of which bloom from April through August. Scientists in the Netherlands have dug into the optics behind the distinctive gloss of these familiar yellow flowers, and how that gloss helps the plants in the business of living and reproducing (J. Roy. Soc. Interf., doi: 10.1098/rsif.2016.0933).

A “butterfly’s wing” strategy?

The buttercup genus, Ranunculus, consists of some 600 species, many of which show a distinct sheen atop their matte yellow color. Scientists have studied and debated the sheen’s origins and function for many years, and have speculated that the flower’s colors may be structural—that is, based on nanoscale photonic structures, analogous to those of the butterfly’s wing.

That’s a very rare coloration strategy for flowers, which tend to rely instead on pigments in their cells that absorb light at certain wavelengths and scatter the rest. Surprisingly, however, the detailed optics underlying the buttercup’s sheen hasn’t previously been studied.

Casper J. van der Kooi and colleagues at the University of Groningen, Netherlands, sought to remedy that shortcoming. To do so, they obtained samples of three species of buttercup from the Groningen area and subjected them to detailed digital photography, photomicrography and cryo-electron microscopy to get at the flowers’ structural characteristics at a variety of length scales. They also used an integrating sphere and microspectrophotometry, under several different illumination conditions, to measure the buttercup samples’ reflectance and transmittance spectra.

Thin-film optics

Previous investigations had noted that the buttercup’s gloss likely traced to its flower petal’s cross-sectional structure, which includes a one-cell-thick epidermal layer, separated by an air chamber from a white starch layer and a still lower layer called the mesophyll. Kooi and colleagues focused their initial investigation on the epidermis, which they modeled optically as a thin film in air.

Putting that model together with the spectral measurements, the researchers found that the thin-film epidermis created the conditions for both the buttercup’s yellow color and its overlying gloss. When light hits the epidermis, it behaves as a thin-film reflector, analogous to a soap bubble, scattering part of the incident light back and imparting the glossy sheen.

The rest of the incident light is transmitted through the epidermis and the bounced off of the starch layer, where it is once again transmitted through the epidermis. And that’s where the yellow color comes from—it turns out that, in addition to acting as a thin-film reflector, the epidermis also includes pigments that filter blue wavelengths, giving the flowers their celebrated yellow color.

It’s all about reproduction

Hence the buttercup’s glossy yellow stems from a combination of pigments, responsible for the flat yellow, and structural color, which adds the gloss. But why do these flowers go to the trouble?

Van der Kooi and the team suggest two possible reasons, both related to the flowers’ reproductive drive. For one thing, the sheen likely serves as a signal to insects, increasing the chances that the flowers are pollinated. The researchers noted that the sheen depends heavily on the angle of incidence. Pollinators approaching from a distance, and thus at a large mirror angle, the authors suggest, would tend to “perceive the gloss as a flash,” creating a sort of long-distance signal guiding pollinators toward a field of the flowers.

Even more interesting is the other possible reproductive benefit of the gloss: keeping the reproductive organs of these diverse flowers (known as far north as the Arctic) sufficiently warm. It turns out that buttercups are heliotropic—that is, they follow the sun—and also that, as the temperature drops, the flowers close into a paraboloid shape.

The combination means that, on cold days, light reflected from the petals is focused in toward the reproductive structures in the center of the flower. The team speculates that the gloss from the epidermal thin film serves to enhance that light reflection—and, thereby, to increase the temperature of the reproductive organs, allowing seeds and pollen to mature faster and making the place a generally more hospitable hangout for the pollinators themselves. (Indeed, previous work has shown that the center of the flowers of the arctic buttercup, on cold days, can be several degrees warmer than the ambient air temperature.)