I’m in Maine, which gives me the opportunity to photograph wildflowers I don’t see in Arkansas. It feeds a diverse sense of enjoying beauty in nature. What excites me is seeing a rare plant that I’m seeing for the first time or even a weed I don’t see at home, especially one for which I wish I had more photographs.
Such is the case with the ubiquitous European weed commonly called tall buttercup (Ranunculus acris). This is the classic buttercup of European lore, common throughout the Northeastern U.S., but not found in Arkansas. We have other species of buttercups in Arkansas that do have shiny surfaces like the petals of the tall buttercup.
Turns out glossy buttercups are among the most challenging flowers to photograph because of their functional optics. A Dutch research group headed by computational physicist Casper J. van der Kooi shows why in a 2017 paper. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5332578/pdf/rsif20160933.pdf].
Buttercups have a mirror-like, distinct gloss due to a pigmented thin reflective layer of cells on the surface of the flower petals, which serves like a flashing light filter for matte-yellow cell layers beneath. This is a very rare combination of structural coloration (in the very thin glossy epidermal layer) and pigment coloration in the petal’s cell layers below.
Structural pigmentation usually occurs in highly iridescent animal coloration like the flash of the brilliant red neck patch of a ruby-throated hummingbird, or the blue of morpho butterflies. It is rare in plants to have color produced by both structural and pigmented organs.
The gloss of the buttercup flower reflects in a highly directional manner rather than absorbs light, while the pigmented cell layers beneath, absorb and scatter light providing the yellow coloration that we see with the naked eye. The high gloss layer is believed to provide a visual signal to pollinators, and reflects sunlight to the flower’s reproductive organs providing them heat.
Each cellular layer within the buttercup’s petals has specific light transmitting and oscillating reflective spectra. Overall, it looks like shiny yellow to us, but certain insects are adapted to see this visual signal like the flash of a lighthouse to hone in on the plant’s reproductive organs. Modern camera sensors and film before them, sense the upper reflective cells as blown-out light values, while the pigmented layers beneath are sensed as flat color.
That’s why buttercups are so tough to photograph and get the exposure right.