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Some tropical plants produce color as insects do, using layered filters to create patterns of light interference. The filters' adaptive value remains a mystery
My first experience of the Malaysian tropical rain forest changed the direction of my scientific career. I had just taken up my first university position, as Lecturer in Experimental Taxonomy at the University of Malaya in Kuala Lumpur, when I was led on a walk of exploration in the forested mountains outside of the city. I was partly prepared for the bewildering diversity, the majesty of lofty trees festooned with epiphytes and lianes, the iridescent green-on-velvet-black wings of the Rajah Brooke birdwing butterfly-I had read about all that. But I was astonished by the leaf color of understory plants, especially the shimmering electric blue of Selaginella, a relative of the ferns.
We expect leaves to be green. The chlorophyll pigments in leaves absorb light at all visible wavelengths, but little in the green range; light scattered out of the leaf surfaces gives them their characteristic color. Leaf color other than green means that wavelengths normally captured by chlorophylls are absorbed by other pigments or reflected out of the leaf. This means that less energy is captured for photosynthesis and growth. Why did these spectacular blue plants, and other species I later found growing in such deeply shaded environments, reflect light that would otherwise add to their photosynthetic efficiency? How did they produce this spectacular color? When I left Southeast Asia in 1976 and eventually moved to Miami, I looked for iridescent blue plants in shady understory environments in the rain forests of Central and South America-and found them. The questions that arose about the nature and function of the color of these plants have stimulated two decades of research on iridescent blue plants in the rain forests of both the Asian and American tropics.
Structural Color
It was obvious that the basis of color production in the Selaginella plants was not pigmentation, because the leaves lost their color when immersed in water. Furthermore, only pigments found in the chloroplasts, the photosynthetic organelles found in plant cells, could be extracted. Blue color in flowers and fruits is almost always caused by anthocyanins modified from their association with metallic ions, or by...