When you go leave the Big Smoke, and you cruise along the coastal fringe, your eyes are soothed by all the greenery around you. But why are trees green? After all, if plants wanted to absorb the maximum amount of sunlight, they'd be black.
You can see the answer in some of our salt lakes that have a purple tinge to them. Purple is a tricky colour - it's actually a mixture of red and blue. Complimentary colours are tricky. Purple is white light with the green colour removed. Green is white light with the purple colour removed. I think the French impressionists were the first to paint the shadow under green trees as purple, not black.
Andrew Goldsworthy, a British botanist, has worked out a theory. Modern green plants appeared on the planet late in the scene, and he reckons that the best colour had already been picked. So plants are green because something else isn't.
The original non-green plants can still be found in some of our Australian salt lakes. While sea water is only 4% salt, some salt lakes can be reach 20%. In many of these salt lakes you'll find a bacteria called Halbacterium halobium, a living fossil. It thrives, where other creatures would die from the high salt levels and heart attacks. But if the salt level falls low enough for other creatures to grow - it dies.
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Halobacterium lives in a salty world by itself - a world free of competition - so evolution has passed Halobacterium by. Some scientists are sure that Halobacterium , or at least its very close relatives, have been found in fossils 3,000 million years old - so it's been around a very long time.
Halobacterium was one of the first solar powered creatures. It discovered photosynthesis - being able to get energy out of sunlight.. Halobacterium does not have any green chlorophyll in it. Instead, it is loaded up with a purple chemical - but why is purple a better colour for solar energy collectors?
White sunlight is made up of many different colours - from red to green to blue. But the Sun puts out most of its energy in the middle of the visible spectrum - green. So for a solar-powered leaf to work at top efficiency it should suck out most of green light. When you suck away the green out of white light, you end up reflecting red and blue light which mix to make purple.
So purple Halobacterium spread across the planet, because of its free energy supply. But it had one great weakness. It could not use carbon dioxide from the atmosphere to make simple sugars. So even if it had a free energy supply, it had to eat up big if it wanted to get bigger.
Andrew Goldsworthy's theory says a new creature evolved to fill this ecological niche. Not only did it have a free energy supply from the Sun, it could turn air into new leaves. Green photosynthesis kick-started the oxygen atmosphere, stabilised the temperature, and made life as we know it possible.
This creature turned out to be the mother of all plants - the green algae - but why green ?. Think back to our Halobacterium floating on the surface of the water, absorbing all the green light. The mother of all plants, the new intruder, was floating down below, waiting in the wings. Only blue and the red light would have filtered down through the water. It wasn't the light with the most energy, but it was all there was at the time, and the new plants had to make the most of it. So they invented chlorophyll - which is green.
But why didn't the green algae choose to be black. After all, black is the most efficient absorbing colour of all. Well there's another factor - temperature control. The best temperature for leaves is between 10 and 30°C. A tree can control the temperature of its leaves using a number of tactics - absorbing or evaporating water, the shape of the leaf, the depth of the canopy and the colour of the leaves. When you look at a tree, you'll see that the leaves at the bottom of the canopy are much darker in colour than the leaves at the top. They are not at risk for overheaing, because they are shaded by the upper leaves.
So that's probably why the plants didn't undergo a further evolution to be black - because they'd overheat in summer. Just think if things had been different - it wouldn't be Greenpeace out there saving the whales, it would be Purplepeace!
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