Flowers that glow and reflect

• Wikipedia (CC BY-SA 4.0)

An artist asked me recently whether there were any luminescent plants, plants that glowed in the dark. I gather she wanted to include them in an installation that would attract attention night and day.  

There are certainly luminescent fungi, I said, such as ghost fungus, Omphalotus nidiformis. Then there are the famously phosphorescent dinoflagellates, planktonic alga that glisten and glow when disturbed by a boat, a wave or if you thrash about in the ocean. I mentioned the turquoise flowers of the jade vine, but admitted these were vivid rather than glowing.

Later, I remembered that a few liverworts fluoresce, but only when exposed to ultraviolet light. As we’ve seen already, flowers present differently to animals that perceive ultraviolet light, so I wondered too if that might be a similar kind of phenomenon. Maybe my artist friend could set us some mood lighting at night to give visitors an insect-eye view of her floral installations. Would that be called luminescence? Apparently not.

Luminescence is defined as the production and emission of light due to a chemical reaction but not an increase in the object’s temperature – which is incandescence, as we see in hot metal or an open fire. Bioluminescence is when luminescence happens in a living thing, which it does in more than 10,000 species of bacteria, fungi, protists and animals, but apparently no flowering plants. 

Fluorescence and phosphorescence are forms of luminescence, occurring when light of one colour is absorbed but another emitted. In fluorescence, the light is absorbed and emitted as different colour almost immediately; in phosphorescence, the light is emitted over a longer time. Think fluorescent marker as an example of the first, and a glow-in-the dark toy for the second. Both can happen in daylight, but more often when exposed to ultraviolet light, which then mimics what some animals will see under normal light. 

A good example of fluorescence is the green chlorophyll colouring the leaves of most flowering plants. If illuminated with ‘long-wave’ ultraviolet light, the chlorophyll in most plants (particularly those with a thin waxy ‘cuticle’) will glow red. Other parts of flowering plants also fluoresce, including various parts of a flower, including pollen and nectar. While assumed to attract pollinators such as bees, recent studies of pollination in pineapple lily (Eucomis) and hedgehog lily (Massonia) by mice and elephant-shrews, failed to demonstrate fluorescence being any more attractive than scent and other visual attractants such as colour.  

If floral fluorescence isn’t part of any pollination strategy, it has been mooted as either a chemical by-product of some other process providing a reflective or ultraviolet absorbing attractant; a means of protecting genetic material in pollen from ultraviolet damage; or perhaps making the nectar bitter to attract only certain pollinators.

Finally, there is iridescence, ‘a form of structural colouration … in which hue is dependent on viewing angel’. Think fish scales and the pearly insides of some shellfish. Iridescence ‘can provide some of the most saturated and colourful displays in nature’.  Which brings me to a fascinating presentation I heard at the Twentieth International Botanical Congress in Madrid, in 2024. Beverly Glover, director of Cambridge Botanic Garden gave an excellent plenary lecture on the ‘evolution and development of iridescent petals’. Her prime example was Hibiscus trionum, also known as flower-of-the-hour or bladder ketmia (ketmia being a Latinisation of French and Arabic words for hibiscus). 

Bladder ketmia is a shrubby annual or perennial, about a metre high and wide, native to Africa, Europe, western Asia and, until 2011, possibly Australia. Turns out there are three similar looking species of Hibiscus in Australia, two of which have become weedy in agricultural land, but no true Hibiscus trionum.  In any case, most flowers on these apparently closely related species in Australia seem to share the iridescence studied by Beverly Glover and her colleagues.

From: Craven LA, de Lange PJ, Lally TR, Murray BG, Johnson SB (2011) A taxonomic re-evaluation of Hibiscus trionum (Malvaceae) in Australasia. New Zealand Journal of Botany 49, 27-40. doi:10.1080/0028825X.2010.542762.

That iridescence is physical rather than chemical, but at a microscopic scale. Early in her career, Beverely Glover wondered why the surface cells in petals are often cone-shaped. She found that in addition to providing a better grip for crawling, or clinging, insects, this lumpy surface intensifies flower colour by focussing light into fluid-filled spaces within the cell called vacuoles. Mutant flowers without conical surface cells look paler, to us and to some potential pollinators (although bees are more interested in getting a firm grip on the petal).

Surface texture can vary within a flower, with the petals of a bladder ketmia typically dark purple at their base, without any purchase for an insect but – at certain angles – a distinctive sheen. This is iridescence, a ‘structural colour’, which Glover compares to a soap bubble which only has colour when it interferes with light. Blue butterflies are perhaps the best-known example of this in the animal kingdom, gaining their blue colour solely from iridescence.

Iridescence is less common in flowers but occurs across the flowering plant tree of life. Good examples are the maroon to almost black flowers of the queen of the night tulip and the metallic spots in some South African daisies. In the tulip, at least, the iridescence results from a regular set of grooves about one micron apart, not unlike the surface of a CD (or at a larger scale, a corrugated iron roof). Bees will find an iridescent flower sooner than one that isn’t, in the same we find a dropped iridescent object (like a CD) before a plain one. 

Our bladder ketmia has conical cells on the white part – to help those poor bees get a grip – but a corrugated outer waxy layer (the cuticle) above non-conical surface cells in the dark area. These ‘nano-wrinkles’ or striations in the cuticle create the iridescence observed by insects.  In her published work I see that while the nano-wrinkles in bladder ketmia are not perfectly aligned, as they might be in a CD or perhaps a tulip, that is more attractive to the pollinators studied than a more machine-made ‘diffraction grate’.  As often seems to be the case, the attractiveness of a flower to a pollinator is the sum of its parts.

Note: Other than the Craven et al. (2011) reference, all other sources for this article will be provided in the final version of this essay, when including in my book about flowers.