Selaginella willdenowii is probably one of the best known ‘blue’ plants (Figure 1). It can have strikingly iridescent blue leaves in common with another Selaginella, S. uncinata. Both plants grow in dense shade on the floor of tropical jungles. The mechanism of iridescence is known as thin-film interference, though the advantage of this to the plant is not clearly understood. Both of these Selaginella and other iridescent ferns have two or more very thin layers of material in the outer epidermis of the photosynthetic cells. These thin layers have a different refractive index to the surrounding tissues which causes light to be reflected back to the surface of the leaf. If the layer reflects the particular wavelength of light back, so that light reflected off the surface of the leaf is in synchrony with the light reflected off the thin layer, then that wavelength of light will be reinforced or doubled. If the wavelength is such that the two elements of light are out of synchrony, then they will cancel each other out and that wavelength will not be reflected by the leaf. In blue iridescent plants it is the blue end of the spectrum that is reinforced by this mechanism and the red end which is eliminated. In this way the leaves appear to be blue even though they contain no blue pigment.
In Selaginella willdenowii, the thin reflective layers are only produced in leaves growing in shade. If the plant is moved to a sunnier location, the blue iridescence will fade over two to three weeks and the leaves will appear green. Similarly, if the leaves are wetted or submerged, the refractive indices change and the blue reflection is lost until the leaf dries.
Selaginella willdenowii has been sold as an underwater aquarium plant (Willdenow’s Spikemoss), and as a greenhouse plant. It is tropical and won’t tolerate temperatures below 15 °C in my experience. I have had this plant for several years, originally in a pot of soil-less houseplant compost near a shallow tub of rainwater in a conservatory. This has a winter minimum of 20 °C and supplementary lighting for fourteen hours per day. The humidity is boosted with an ultrasonic humidifier using deionised water. The aim is a minimum of 70 %RH, but it is often higher than that. In these conditions, the leaves were blue and it grew well. In fact, it grew so well that part of the plant fell into the adjacent tub of rainwater and grew aquatic roots (Figure 2). Eventually, the part in the pot died off. So, this was now growing as an emergent aquatic plant. It did remarkably well, climbing the electric wires to the lights in the apex of the conservatory (where it was green, not blue) and reaching a height of about ten feet (Figure 3). At the same time, it produced leaves under the surface of the water which showed no iridescence. Sadly, it suffered in the very hot summer when the conservatory temperature rose to 38 °C over several days. Much of the emerged plant died off and had to be cut away, whilst the submerged plant and parts just above the water surface survived to regrow. I am about to take cuttings and start some in pots of light compost again.
It is an easy Selaginella to grow, so long as it is kept moist and warm. However, if you want to see the blue iridescence at its best, the plant needs to be kept in the shade.
Further reading: ‘Iridescent Ferns and Their Shady Behaviour’, in Robbin
Moran’s A Natural History of Ferns (Timber Press, 2004, p. 147–53).
PETER BLAKE
Peter became interested in tropical ferns whilst making regular trips to India in the
1990s and 2000s. He retired from the NHS in 2012 and was able to develop his interest in growing tropical ferns from spores. He now lives in Norwich where he can provide much more space for their culture. He maintains a particular interest in the ferns of South India and Sri Lanka.
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