As plants take up water from their roots into their cells, the pressure builds inside the cells until it is like being between 50 meters and 100 meters under the sea. Inside plants there is another type of nanostructure. This is called “ superhydrophobicity” or the “lotus effect”. When water lands on this surface, it can’t stick to it at all and so it forms spherical drops that roll across the leaf picking up dirt along the way until they fall off the edge. These are in turn coated with fractal patterns of wax at even smaller scales. The wax is arranged in an array of cone-like structures about five thousandths of a millimeter in height. For example, the waxes that protect the petals and leaves of all plants repel water, a property known as “hydrophobicity.” But in some plants, such as the lotus, this property is enhanced by the shape of the wax coating in a way that effectively makes it self-cleaning. Plants have evolved many ways to use these kind of structures, effectively making them nature’s first nanotechnologists. Fascinatingly, it seems that many different types of flowering plant may have evolved this structure separately, each using nanostructures that give slightly off-kilter iridescence to strengthen their signals to bees. They found that although the bees learned to associate the iridescent fake flowers with sugar, they learnt better and quicker with the blue halos. The other had flawed arrangements replicating the blue halos from different real flowers. One had petals made using perfectly aligned gratings that gave normal iridescence. The researchers tested this by training bees to associate sugar with two types of artificial flower. Humans can occasionally see these patterns but they are usually invisible to us against red or yellow pigmented backgrounds that look much darker to bees. The question was whether the flaws in the wax patterns were “designed” to generate the intense blues, violets and ultra-violets that bees see most strongly. The color perception of bees is shifted towards the blue end of the spectrum compared to ours. These imperfections meant that instead of giving a rainbow as a CD does, the patterns worked much better for blue and ultra-violet light than other colours, creating what the researchers called a “blue halo.” There was good reason to suspect that this wasn’t a coincidence. And they weren’t quite perfect in very similar ways in all of the types of flowers that they looked at. The spacing and alignment of the grooves weren’t quite as perfect as expected. But researchers investigating the way that some flowers use iridescence to attract bees to pollinate have noticed something odd. Many flowers use grooves between one and two thousandths of a millimeter apart in the wax coating on their surface to produce iridescence in a similar way. As your viewing angle shifts, the amplified colors change to give the shimmering, morphing color effect that you see.īees can see a blue halo around the purple region. It’s caused by interactions between light waves bouncing off the closely spaced microscopic indentations in its surface, which means some colors become more intense at the expense of others. The shifting rainbow colours you can see on a CD are an example of iridescence. But some flowers also use iridescence, a different type of color produced when light reflects from microscopically spaced structures or surfaces. Most flowers appear colorful because they contain light-absorbing pigments that reflect only certain wavelengths of light. And, what’s more, by studying these systems we might be able to put them to use in our own technologies. Plants are actually full of this kind of “nanotechnology," that enables them to do all kinds of amazing things, from cleaning themselves to generating energy. This fascinating phenomenon shouldn’t come as too much of a surprise to scientists. Nanoscale patterns on the petals reflect light in a way that effectively creates a “blue halo” around the flower that helps attract the bees and encourages pollination. And new research has just given us a greater insight into how this signal works. Flowers have a secret signal that’s specially tailored for bees so they know where to collect nectar.
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