Queen of Night tulips are known for their ultra-deep purple hue, with a touch of shimmer to enhance the jewel tones, but they don't get that striking colour from the usual pigment molecules. According to a new paper in The Journal of Chemical Physics, what you're seeing is actually a result of how the plant's cellulose structure interacts with light.
This is a quality the regal tulip shares with cicadas, butterfly wings, beetles, opals, oyster shells, and the brightly coloured feathers of male peacocks, all of which get their shimmery iridescent colour from a type of structure known as a photonic crystal.
Manmade photonics crystals are engineered with a highly precise lattice structure that causes light to reflect off the surface in such a way as to create the perception of colour in the human eye. They block certain frequencies of light and lets others through, so what colour you see depends on the angle of reflection. Naturally occurring photonics crystals — like those found in butterfly wings or the Queen of Night tulip — don't have the same dependence on viewing angle. They scatter light selectively, much like a diffraction grating. The effect is similar to what happens when light hits the tiny grooves etched into a CD, causing flashes of rainbow colour.
The secret to the Queen of Night's colour is cellulose, a key element in the cell walls of green plants. The layers of fibres in cellulose make a plant's structure stronger, reinforcing cell walls in whatever direction they are oriented. Usually they line up in just one direction, but cellulose can also self-assemble into a twisting pattern known as a "cholesteric phase" (mostly because scientists first noticed it while studying cholesterol molecules). And as you rotate those fibres (see schematic below), you get added mechanical stiffness in several directions.
So cellulose serves as a kind of reinforcing biological armour. Now it seems that protective armour also confers an element of style: shiny iridescent hues.
Much prior research on cellulose has focused on its mechanical properties. Alejandro Rey, a chemical engineer at McGill University, and his collaborators were more interested in whether that twisting structure might explain the iridescence of the Queen of Night tulip and similar plants.
So they devised a computer model to simulate how this particular type of cellulose behaves, and found that over time, that twisting gave rise to microscopic parallel ridges along the surface of their model system — exactly the kind of microscopic pattern one finds on the Queen of Night's petals. Those ridges split white light into colours just like a prism, giving the tulip that gorgeous iridescent sheen.
But it's not just the twisting pattern that influences colour in some plants: how much water is stored in those cellulose layers can also affect the optics. The more water there is, the less tightly those layers twist, so the resulting ridges are spaced further apart. And those spaces determine which wavelengths of light get diffracted (the "pitch"). For instance, varying humidity can cause wavelengths to shift from 460 nm (visible blue light) to 520 nm (green light).
This is why dipping certain Malaysian rainforest plants into water changes their bright blue leaves to green. A similar effect can be seen in a type of spike moss (Selaginella willdenowii), which sports shimmery blue leaves when young, or growing in a shaded area. But that colour fades as it ages, or finds itself exposed to bright light — likely due to changing water levels in the cell walls of those leaves.
According to Shu Yang, a materials scientist at the University of Pennsylvania, the ability to change colour in response to shifts in humidity confers added sensing and camouflage abilities to such plants, upping their odds of survival. This latest paper is significant, she says, because it's the first model to look closely at the optical and water-sensitive properties of this twisty phase in cellulose.
Yang also pointed out that with more development, the model might be used to design various types of materials that shimmer and shift in hue. This would have potential applications in colour displays, camouflage, and optical sensors in fabric and buildings, for example, that change colour when there is a change in humidity, mimicking those Malaysian plants. Her own research includes a light-bending, water-repelling spray coating inspired by butterfly wings that might one day be used to create better solar panels, among other uses.
As always, further research is needed; this is a computer simulation, and the team still must prove that naturally occurring diffractive surfaces work exactly the same way. But Rey said their model provides a solid foundation for better understanding how structural colour works in nature. "The optics [of cholesteric cellulose] are just as exciting as the mechanical properties," he said.
[Image credits: (top) S. Vignolini. (bottom) Abigail Malate. Both via American Institute of Physics.]
Rofouie, P. et al. (2015) "Tunable nano-wrinkling of chiral surfaces: structure and diffraction optics," The Journal of Chemical Physics. [Preprint: September 15, 2015]
Vignolini, S. (2013) "Structural colour and iridescence in transarent sheared cellulosic films," Macromol. Chem. Phys.