Tag Archives: nature

why are leaves green?

Why are leaves green? The most obvious answer is that they contain green pigments, the most abundant being chlorophyll and that chlorophyll absorbs the short and long wavelengths in the visible spectrum leaving the middle wavelengths to be reflected and scattered. However, the deeper question is why should chlorophyll absorb in the short and long wavelengths of the visible spectrum when there is more light available in the middle of the spectrum?

The spectral irradiance of sunlight varies with the time of day, the weather conditions, the time of year, and the latitude/longitude. However, I think it would be reasonable to say that by and large, in most situations, the peak irradiance is in the middle of the spectrum (that which we would normally associate with being green and yellow).

So if one assumes that evolution has produced a perfect engineering solution to this aspect of nature in particular then I think one may expect plants to absorb mainly in the middle part of the spectrum (and this would result in the bluish and reddish wavelengths being reflected and a purplish colour).

So why don’t we have a chlorophyll equivalent that is purple? I have come across a number of arguments.

1. One could go further and say that if a plant wanted to be really efficient it would absorb all wavelengths of the visible spectrum and would therefore appear black. So black, rather than purple, would be the perfect engineering solution. Given that most plants are neither black nor purple then I think we can assume that evolution did not find the perfect engineering problem or that the problem is more complex than we think. For example, it could be that a plant that is black would absorb too much light and overheat. Or it could be that chlorophyll evolved from some earlier light-sensitive chemical and that genetic mutations could lead more easily to chlorophyll than to purple or black pigments.

2. Taking this point further, I have heard it suggested that most plants evolved from earlier plants that lived under water and that absorbed mainly short wavelengths of light (long wavelengths – red – cannot penetrate much more than 1 m of water). These earlier cousins of the modern plant would most likely have been brownish. Indeed, if one looks today ay plants in seawater, green plants are only seen on the surface or at very low depths. So the ancestor of chlorophyll could have been a brown pigment which mutated into green chlorophyll more easily than it could have mutated into a purple pigment.

3. I have also come across the ‘early purple earth’ hypothesis. This suggests that originally most plant life on land was indeed optimally purple and that chlorophyll absorbed to take advantage of those wavelengths that were not already being gobbled up by the dominant species. Subsequently, chlorophyll proved more successful than its purple companion.

4. It could be argued that optimally absorbing light (and being purple) is not the most important thing and that there are other aspects of the problem that are more important. Green chlorophyll could be the optimal solution to this more complex problem.

In short, the real answer is … I don’t know. I am not overwhelmingly convinced by any of the above arguments.

If you enjoyed this post you may like to look at my special christmas post on carrots and why they are orange.

blue appetite suppressant

It is said that blue is an appetite suppressant and that red stimulates appetite. But is this really true? I would be interested if anyone knows of any studies into this.

I have also read that the reason that blue is an appetite suppressant is that blue food is very rare. I think blue food is less frequent than, say, green or red. But there is, of course, blueberries. And I just came across a type of mushroom that is naturally blue. It’s called Lactarius Indigo. I’ve also come across blue food more commonly in other places such as Japan.

living pigments

Recent research from Prof Pettigrew – University of Queensland, Australia – has shown that one of the reasons that some cave paintings have apparently retained their colour for so long is that the original pigments in the paint have been replaced by living organisms that are themselves coloured. This makes dating cave paintings difficult and also makes it hard to know what the original paint colours were. The story is reported by the BBC – http://www.bbc.co.uk/news/science-environment-12039203

chicken colour vision

Human colour vision under normal lighting levels is mediated by three cones (light-sensitive cells) in the retina. Each class of cone has peak sensitivity at a different wavelength and thus the cones are known as L (long-wavelength sensitive), M (medium-wavelength sensitive) and S (short-wavelength sensitive) cones or (sometimes) as red, green and blue cones. Both colour and luminance are captured by the same cone mechanism. The L and M cone responses are combined to give luminance and various cone responses are compared to give rise to hue and chroma. Interestingly, the distribution of L, M and S cones in the retina is not uniform but is random.

A recent paper – http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0008992 – by scientists at Washington University School of Medicine (St Louis, USA) reveals that chickens have five types of cone. Interestingly, one of these types of cones (so-called double cones) seems to encode luminance, whereas the other four cones (red, green, blue and violet) give rise to tetrachromatic vision. The cones are very regulary spaced in the retina.

The spacing of cones in the human retina may result from a compromise – the same cones need to encode colour and luminance. The avian colour vision system seems to be more sophisticated. One can only wonder at what benefit was bestowed in avians by separating the processing of colour and luminance information.

where do the stars go in the daytime?

Galloway Forest Park in Scotland has picked up an international award as one of the best places for stargazing in the world. The 300sq mile Forestry Commission site has been commended for its dark skies and named one of the best places in the world for stargazing.



So where do the stars go in the daytime? The answer is that they are still there of course. It is a nice reminder that our colour vision is more about contrast than about absolute intensities. With old TV sets when they were switched off the screens were grey; but when they were turned on it would be possible to see some really dark blacks. But the screen cannot emit less light than when it is turned off! So the black looks black because of contrast, not simply because of the amount of light emitted or not emitted.

Against the dark skies of night the stars contrast and appear bright – during the day, the skies are much brighter because of the sun and though the light from the stars is just as intense as it is at night the contrast conditions are very different.

Cryptic coloration

Cryptic coloration is the most common form of camouflage, found to some extent in the majority of species. The simplest way is for an animal to be of a colour similar to its surroundings.


The female Misumena vatiaspider switches her body colour over the course of days depending on the flower where she lurks. This is often cited as an example of cryptic coloration. However, according to a new paper in Proceedings of the Royal Society B, a white spider on a white flower doesn’t catch more prey than a white spider moved to a yellow flower. Nor does a yellow spider on a yellow flower get a colour-coordination bonus. The study may shatter the myth of crypsis by colour matching in crab spiders. For further details see http://www.sciencenews.org/view/generic/id/49079/title/Textbook_case_of_color-changing_spider_reopened

The colour of the universe

The colour of the universe is …..






Astrophysicists Karl Glazebrook and Ivan Baldry took light measurements from more than 200,000 galaxies, broke them down into their constituent colours and then averaged the colours out to produce a single shade visible to the human eye. The result was beige. See http://www.guardian.co.uk/theguardian/2009/nov/04/pass-notes-the-universe

Staying on the space theme, One week ago, the MESSENGER spacecraft transmitted to Earth the first high-resolution image of Mercury by a spacecraft in over 30 years. MESSENGER’s Wide Angle Camera is equipped with 11 narrow-band color filters, in contrast to the two visible-light filters and one ultraviolet filter that were on Mariner 10’s vidicon camera. By combining images taken through different filters in the visible and infrared, the MESSENGER data allow Mercury to be seen in a variety of high-resolution color views not previously possible. MESSENGER’s eyes can see far beyond the color range of the human eye, and the colors seen in the accompanying image are somewhat different from what a human would see.

The color image was generated by combining three separate images taken through WAC filters sensitive to light in different wavelengths; filters that transmit light with wavelengths of 1000, 700, and 430 nanometers (infrared, far red, and violet, respectively) were placed in the red, green, and blue channels, respectively, to create this image.


What colour were the dinosaurs?

Have you ever wondered what colour the dinosaurs were? Probably not, but when you think about it – how could we know. Fossils are …. well, … fossil coloured. The movies portray most of the dinosaurs as greyish, probably because the biggest land mammal we know – the elephant – is grey. Dinosaurs are also sometimes portrayed to be similar in colour to many lizzards and reptiles that are alive today – and I suppose that makes some sense. But is there any way we can work out what colour the dinosaurs really were? Probably not ….


Until Jakob Vinther’s work that is. Jakob is currently working towards a PhD in paleontology at Yale University – http://www.jakobvinther.com/.

Part of Jakob’s interest is the preservation of malanin in fossils. He has discovered that melanin granules survive in their original shapes and patterns, which can be compared with existing feathers to determine their original color. One possible application of this work is that we may be able to make a very good guess at what colour many dinosaurs really were based on an analysis of their fossil remains.

new white roof coating

A new white coating that reflects 85% of the heat that hits it is being trialled in Los Angeles. The new coating, developed by former military scientists Ronald Savin, reduces the surface temperature of the roof by as much as 50 C and thus reduces the amount of energy required to cool the interior of the building. Suffice to say, as I write this in rainy Cornwall, there is no requirement as yet for this coating in the UK!

white roof

For full story see http://seattletimes.nwsource.com/html/realestate/2009578076_roofs02.html

sheep change colour

A few weeks ago I wrote about the moths in the UK that are now changing colour as a result of changes in their environment. The species that darkened in colour in response to the industrial revolution is now becoming lighter again – http://colourware.wordpress.com/2009/07/01/moths-change-colour-again/.

Today I came across a story about sheep changing colour; this time in response to global warming, apparently. According to Dr Maloney at the University of Western Australia,  in colder environments, mammals with darker coats absorb more solar radiation and so need to expend less food energy keeping warm than do their lighter counterparts. He has found fewer dark coloured Soay sheet over the last 20 years and links this to changing temperatures. H expects the proportion of dark sheep to decrease further over the coming years. His work was reported in the Royal Society journal Biology Letters but a summary is availale online at The Telegraph – http://www.telegraph.co.uk/earth/earthnews/5879613/Bye-bye-black-sheep—climate-change-making-sheep-change-colour.html.