Have you ever tried to photograph lesser celandine or buttercup flowers on a sunny day? The petals are so shiny, like little cups of mirrors, that the reflected sunlight flares and obscures the details of the flower; if you are trying to photograph a celandine in close up, you have to do it in the shade.
Research has shown that the glossy yellow of a lesser celandine (Ficaria verna) is due to a rare combination of structure and pigment that is a lot more common in butterflies and birds than it in flowers. It is what makes the iridescence on a peacock’s tail feathers or on the wings of a small tortoiseshell butterfly.
Below is a diagram that shows what happens in buttercup flowers which have the same structure as celandine. There is a thin and very flat film of yellow pigment with a layer of starch cells beneath, and between these two layers is air. The thin top film creates the gloss and the starch and air layers diffuse and scatter the light.
Why would a flower need to be so glossy and reflective? Firstly, the cup shape of the flower concentrates the reflection of the sunlight into the centre of the flower where the reproductive organs are; researchers have found arctic buttercups with the temperature inside the flower measurably warmer than the temperature outside. This could perhaps hasten the ripening of seed or just be a nice warm place for pollinators to hang out.
Secondly, at certain angles, light reflects from the petals as a flash; a strong visual signal to flying pollinators. This time of year, nectar is a scarce resource for insects: the queen bumblebees just out of hibernation and the gall wasps in the oaks looking for somewhere to lay their eggs must welcome the flash of a celandine.
Header picture Suzanne Humphries