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Tuesday, 10 January 2017

Inside The Rainbow

A rainbow over Agios Ioannis
I see that you’ve got your wet weather gear on this morning which is just as well looking at those clouds covering the mountains. I’m not sure we’ll get down to where three valleys meet but never mind; seeing that beautiful rainbow pouring itself into the village of Agios Ioannis gives me an idea. We’ll see if we can find all the colours of the rainbow reflected in nature. I say ‘reflected’ advisedly for the colours that we see are mere reflections which we should be able to illustrate as we go along but first, have you ever stopped to consider how we perceive colour? It’s all down to cone cells in our eyes. Back in 1672 Sir Isaac Newton first discovered that light was made up of different colours when he recreated a rainbow with a prism. If you could slow light down you could see that it came in waves, like the sea crashing onto the shore, but whereas the distance between the crests of the waves in the sea may be several metres or more, waves of light are infinitely smaller (about 4-7 ten millionths of a metre) and the different colours have different wave lengths. This is where the cones come in. We have three different sizes: one for catching the longer red, orange and yellow wavelengths; one for the medium green; and one for the smaller blues, indigo and violets. These send electrical signals to the rest of the brain telling how much of each they’ve collected and the brain then combines these signals, like an artist with a palette, to give us the colours that we visualise. 

Why do leaves change colour?
Let’s see if we can give the big cones some exercise and find some reds, oranges and yellows. If you’ve ever wondered why leaves change colour at the end of the summer it’s because they’ve stopped photosynthesizing. During photosynthesis they use all of the energy in the light waves to produce sugars with the exception of the middle green wavelengths which they reflect back. Hence we see plants as green. As light levels decrease in the darker months they shut down photosynthesis and the green reflecting chlorophyll is broken down and reabsorbed into the roots for use in the spring. The unwanted parts of the leaves now reflect the long wavelengths of red, orange and yellow back at us. Now these little fungi (Calocera cornea) don’t photosynthesize and their colour comes from the nutrients that they absorb from the soil. They still use light to process these nutrients efficiently (except the yellow wavelengths) as scientists found out when they grew some of their relatives in the dark. They didn’t grow nearly so well and were completely devoid of colour.

Why is moss green?
Last week we were looking at some bryophytes (mosses, liverworts and hornworts) and we have some more here on this rock. If you remember the gametophytes – the green leafy bits that sexually reproduce – give birth to the sporophytes which reproduce asexually. Now, from the colour you can see that the gametophytes, like other plants, use chlorophyll to photosynthesize but the sporophytes, which appear brown, obviously don’t. The question is; where do they get their food from? And the answer is; they sponge off their parents. The sporophytes remain attached to the gametophytes for their entire life cycle. They do make some contribution to the household budget however. As you can see they are quite good at collecting water droplets.

Why is the sky blue?
I think that we’re going to have a bit of trouble getting down into the valley from here. We’ve walked to within view of the Milonas Gorge and there still seems to be no route to the bottom. A lovely view of the sea and sky though from up here.  And here comes the answer to the age old puzzler often asked by children: why is the sky blue? Simply, as the light passes through the atmosphere it collides with the oxygen and nitrogen molecules (see Every Breath You Take). This doesn’t affect the longer wavelengths much but the shorter wavelengths get scattered and wherever you look you are seeing this scattered blue light which comes at you from all directions. The large and medium cones are receiving red to green in one direction only but the small cones are receiving blue to indigo from everywhere and so the brain concludes that the sky must be blue. It’s totally wrong of course, the sky contains all the wavelengths of light and is in fact white.

Why are some leaves indigo/violet?
The indigo violet hues on the underside of this Oxalis leaf are a bit of a puzzle. The reason that some plants have leaves of this colour, top and bottom, is due to the preponderance of chemicals called anthocyanins that help to protect the plant against overexposure to ultraviolet wavelengths (even shorter than violet and too small for our cones – we cannot see in the ultraviolet but some animals can). Anthocyanins are present in most plants but they outweigh the chlorophyll in indigo violet plants. They absorb the green wavelengths but aren’t so good with other wavelengths that they reflect back to us. Why the underside of this oxalis leaf contains so many anthocyanins (or conversely so little chlorophyll) is the mystery. If you take a look under the other Oxalis leaves nearby they’re a regular green. My guess is that the genes that program the underside of the leaf to make chloroplasts (that manufacture the chlorophyll) aren’t working. A genetic mutation in other words. In theory, if this confers an evolutionary advantage, such as animals preferring to eat the all green leaves of the other plants, then the mutants would survive and all Oxalis would have undersides this colour eventually. We could be watching evolution in action (if I hadn’t plucked the leaf to show it to you).

I promise not to destroy the environment  so much next week.

The Extra Bit

If you look back over your shoulder to where we started out a few weeks back there’s a lovely view of the mountains covered with snow (which scatters all the wavelengths to such a degree that we see it as white). 

For more detail on how we see colour this is a good article: 




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LINKS:
Naturalists (the facebook page that accompanies this blog)
See detailed pictures at http://www.inaturalist.org/login  (search - people-stevedaniels-observations)
The Nature of Crete  (Flipboard Magazine)

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