What do peacocks, CDs and certain plants have in common? They all have multi-coloured parts – feathers, surfaces or petals – which change their hue depending on the angle you look at them. This physical phenomenon in which an ordered repeating surface structure rather than a pigment gives an object its colour is called iridescence.
Iridescence has evolved multiple times in plants and occurs in a lot of land plant families, from angiosperms to algae and ferns. It can impact on how insects and animals see plants. Dr Heather Whitney, a plant scientist from Bristol University, was awarded the President’s Medal of the Society of Experimental Biology (SEB) last week for her novel and interdisciplinary work. Heather studies how plant surfaces become iridescent and how iridescence influences plant-animal interactions.
Heather started her presentation by talking about how she became interested in her study field. When she went to the Botanic Garden she noticed that even though most flowers of Hibiscus trionum (pictured below) were creamy white, their centre had an oily sheen. So she decided to look at the petals with an electron microscope and realised that the surface looked very structured: The oily sheen on the petals is caused by iridescence.
One way to proof that a flower’s colour is created by iridescence is to replicate the petal structure in epoxy resin, which makes the clear resin shine blue when looked at from a certain angle. This is why iridescence is also called a “structural colour”.
One function of iridescence in plants is to make them more appealing to pollinators. An example is the “sexually deceptive orchid”, Ophrys speculum (pictured right). It pretends to be an animal by mimicking the wings of a female wasps. Similarly, Moraea villosa copies the iridescence of pollinating beetles.
1. Peacock Head by Cuatrok77 at Flickr. This image licensed under a Creative Commons by licence. 2. Hibiscus trionum flower closeup by la la means I love you. This image licensed under a Creative Commons by-sa licence. 3. Ophrys speculum (plant) by Hans Hillewaert. This image licensed under a Creative Commons by-sa licence. 4. Iridescent Tulip by Anne Hornyak at Flickr. This image licensed under a Creative Commons by-sa licence.