during my recent travels in southeast asia, i was blessed with the opportunity to visit moonriver lodge, a family owned & operated farmstay in the middle of Malaysia’s Sigar Highlands pristine bamboo jungle … i wrote this blog post for my gracious hosts ….
Ferns are intensely fascinating and beautiful plants. They’re among the oldest plants on Earth and they dominated many ecosystems around the world in prehistoric times.
If you’re a fern lover, you must visit the bamboo forest at Moonriver Lodge. You’ll find ferns big and small, including fern trees! And of course, whorled fiddleheads pepper the landscape with spectacular, subtle grace.
But by far the most stunning ferns you’ll see are the glow-in-the-light (or iridescent) ferns along the footpath of the bamboo forest trail. These ferns are humble in their beauty, so you have to pay attention to the fringes of the path in order to find them.
They’re colloquially known as the peacock fern or paku merak (which translates from Malay to English as “peacock nail”). The botanical name of the species is Selaginella willdenowii (synonym S. wallichiana) and they’re actually lycopods or club mosses (close relatives of ferns).
Once you discover them, you’ll be amazed by their exquisiteness. Their leaves glitter metallic blues, violets, and greens. Each leaf is subtly different and every angle catches the sunlight with a distinctive hue.
They’re scattered all along the trail, but you won’t tire of looking at them. Every single plant is a soothing sight to marvel at.
More details for those who are interested:
In 1896, a scientist by the name of Stahl published a paper describing the presence of reflection granules in the surface (epidermal) leaf cells of S. willdenowii. However, Lee and Lowry (1975) analyzed the leaves using a spectrophotometer and discovered that the leaf structure, and not reflective granules, was the source of iridescence (with peak light reflection at 405 nm).
They surmised that the planar orientation of cellulose moieties within the leaf cell walls caused refracted light, giving the leaves an iridescent sheen. They also suggested that the unique oval structure of the epidermal leaf cells and the peculiar arrangement of the photosynthetic organelles, known as chloroplasts, may also contribute to the glow-in-the-light effects.
However, they hypothesized that gross leaf anatomy doesn’t fully explain the iridescence because the leaves on the same plant will lose their glow under certain conditions:
- when immersed in water (although Gould and Lee, 1996, argue the opposite)
- when exposed to sunlight for extended periods of time
- as leaves age.
In contrast, leaves that grow in the shade of the forest understory maintain their striking iridescence. (So, I suppose it would be more accurate to call these plants glow-in-the-shade ferns!)
Lee and Lowry further postulated that the ecological and physiological significance of this iridescence was to compensate for the light-limiting environment in which shaded leaves grow.
Reflecting light spectra within the blue-green wavelengths of 400—500 nm, which are photosynthetically less active, increases the penetration of light within the range of the most photosynthetically active red spectra, 600—680 nm. This would help the ferns maximize the “quality” of light they absorb, since light is such a precious, limiting resource for them (they generally receive 0.1—1.9 % of the full sunlight becase they are understory plants and must compete for access to sunlight with taller canopy trees; Bazzaz & Pickett, 1980; Chazdon et al., 1996).
But this may not be the sole adaptive benefit of blue-green iridescence. Other possible advantages could be visual defence against predators (known as herbivores), photoprotection against the intensity of unpredictable sun-flecks and other potentially damaging sudden exposures to high light levels, and polarized filtration to improve the position of photosynthetic organelles within leaf cells (Thomas et al., 2010).
A few years after Lee and Lowry’s classic paper, Fox and Wells (1978) examined the fern leaves microscopically and discovered that the upper leaf surfaces of S. willdenowii are densely populated with innumerable areoles (tiny bumps) that variably reflect blue, blue-green, and violet hues, depending on the angle of incidence of light. The air trapped between the spaces of these projections reflects light of short wavelengths.
Moreover, the leaf epidermis contains two layers of cells that create multilayer interference because of their anatomical arrangement (Hébant and Lee, 1984), a property not observed in green, non-iridescent leaves of S. willdenowii.
Interestingly, Selaginella plants have a unique type of chloroplast pigment known as a bizonoplast, which may also help the plant adapt to low light conditions (Sheue et al., 2007).
Iridescence is known as “schemochromia”, which means that the visible colour of a surface (in this case, fern leaves) results from the presence of optical or structural components that contain no pigment but rather physically affect how light is reflected, refracted, and absorbed. Because the surface contains no pigments, the perceived hue changes at different angles of viewing (because the angle of incidence of the light source changes and so different wavelengths are reflected).
This is relatively uncommon for plants, as they most often contain chemical pigments, such as chlorophylls and anthocyanins, which impart colour.
Iridescent Selaginella spp. are therefore impressive not only for their unique aesthetic, but also for their myriad ecological adaptations and evolutionary persistence.
Bazzaz, F. A., and S. T. A. Pickett. 1980. Physiological ecology of tropical succession—a comparative review. Annual Review of Ecology and Systematics. 11: 287 – 310.
Fox, D. L., and J. R. Wells. 1978. Schemochromatic blue leaf-surfaces of Selaginella. American Fern Journal. 61(3): 137—139.
Glover, B. J., and H. M. Whitney, 2010. Structural colour and iridescence in plants: the poorly studied relations of pigment colour. Annals of Botany. 105(4): 505—511.
Gould, K. S., and D. W. Lee. 1996. Physical and ultrastructural basis of blue leaf iridescence in four Malaysian understory plants. American Journal of Botany. 83(1): 45—50.
Lee, D. W., and J. B. Lowry. 1975. Physical basis and ecological significance of iridescence in blue plants. Nature. 254(5495): 50—51.
Sheue, C. R., V. Sarafis, R. Kiew, H. Y. Liu, A. Salino, L. L. Kuo-Huang, Y. P. Yang, C. C. Tsai, C. H. Lin, J. W. H. Yong, and M. S. B. Ku. 2007. Bizonoplast, a unique chloroplast in the epidermal cells of microphylls in the shade plant Selaginella erythropus (Selaginellaceae). American Journal of Botany. 94(12): 1922—1929.
Thomas, K. R., M. Kolle, H. M. Whitney, B. J. Glover, and U. Steiner. 2010. Function of blue iridescence in tropical understory plants. Journal of the Royal Society Interface. 7(53): 1699—1707.