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I am a former middle and high school science teacher pursuing a doctorate in Science Ed. at George Mason University, with a concentration in cognitive science and the evolution of cognition and learning. Postings on this blog represent my own views, not those of my employer or school. All writing displayed on this page is original work unless otherwise noted, and thus copyrighted.

07 June 2009

They live in Lotus Land, all right...


So, Wilhelm Bartholt examines lotus leaves, because they appear to be free of dirt at nearly all times, and discovers what he calls the "lotus effect", a combination of hydrophobic waxes and bumps on the leaves that cause water to bead up rapidly and run-off, cleaning the leaves as it goes. He's applied this useful phenomenon to fabrics, and hopes to do so to other materials. None of this is in contention, and is a commercially useful application of nanotechnology.
The issue arises when people attempt to explain the evolutionary reason for the lotus effect, and continue to focus on the self-cleaning aspect of the plants. Let's have a cursory examination of the ecology of the lotus: The water lotus grows in India, China, and Southeast Asia, and as you might gather from the name, it's aquatic. It also happens to be a perennial, meaning that the tubers under the sediment at the bottom of the water regrow into full plants yearly. It is known for purity because it grows only in clear water and its leaves are always clean. Some scientists have proposed that the self-cleaning adaptation in lotus' has evolved because there is an advantage to removing dust and dirt from the leaves, ostensibly for photosynthesis. While this sounds reasonable at first, shall we consider the environment of the lotus before we make such a pronouncement? How much dust would you say the average aquatic plant contends with? This isn't something that grows in a desert oasis, it's a subtropical to tropical (Burma) plant the grows in slow pools in rivers, naturally. So why decide that the lotus is trying to be clean? Well, there are insects that use a similar effect on their wings to keep them clean, dry, and ready for flight... and this seems to be reasonable as well until we understand that those are desert insects, who have to contend with immense amounts of dust in many cases. Indeed, dry seems to be the key here: aquatic plants, especially those that live on the surface, don't need to cleanse themselves of dust, they need to get water off of their leaves as fast as possible. Light attenuates with water depth, and even liquid water increases the albedo of a surface that, in this case, is supposed to be absorbing light. On top of that, the lotus grows only in clear pools, indicating that it has a low tolerance for low light levels (it also grows in areas that may already be shaded, which decreases available light intensity. The self cleaning effect of the hydrophobic leaf surfaces is a spandrel, a structure that happened to evolve along with a selected trait. The spandrel is the commercially viable aspect of that property, but that doesn't mean that the plant is trying to remain clean, it's simply trying to keep it's leaves on the surface and free of reflective water during heavy rains to maximise photosynthetic potential. A brief study in the light intensity reaching the leaves under a) conditions where the leaves have some dust on them and b) the leaves are free of dust but subject to pooling water or submersion would likely bury this silliness for good. Or, it may actually prove me wrong, but at least there would be some evidential basis for the adaptation story, which flies in the face of photo-physics and ecology.

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