The sun part 1: the color of plants

For the sake of argument, light is made up of particles called photons; and although the sun emits photons of every conceivable color, if you could collect and sort them like so many M&Ms you would find a preponderance of "green" ones. In other words: the sun shines green more intensely than it does any other color.

(That the sun does not appear green may seem to belie my claim, but this is simply because we humans perceive sunlight as a blend of colors. In fact, sunlight is white - not yellow as artistic renditions had me believing. But it has been pointed out that if the sun really were yellow, then so too would be the clouds.)

It is unsurprising then, that the human eye is most sensitive to green light. After all, it has evolved specifically to take advantage of a green sun.

But it is much more surprising  that plants are usually green. I mean, seriously: why are plants green? Some readers might be quick to answer that it's because they contain chlorophyll - a green pigment essential to photosynthesis and thus the survival of most plants. But this is no answer at all, for it merely begs the next logical question: why is chlorophyll green?

Keep in mind that an object's color is determined by the light that it reflects. This means that green objects appear so because they are relatively poor at absorbing green light. Since plants must capture light to survive,  it is strange that those parts of them engaged in photosynthesis usually reflect the very color that is most prevalent in the sun's shine. Indeed, it's strange they aren't black (as astrobiologists suspect could be the case on other Earth-like planets): it would make little sense, for example, to construct a green solar panel. 

Could it be that our sun shines green light too intensely? that the green hue of  plants is meant to act as some sort of natural sunscreen?

Technically, plants can't get sunburns, but their DNA is not immune to damage from the sun. Plants can even get cancer-like diseases, but these appear to be far less of a concern for them, what with their non-essential "body" parts and rudimentary circulatory systems.

But the bottom line is that neither humans nor plants are in danger from green light, however intense. It is, of course, ultraviolet (UV) light that can damage DNA, and it does this not because it is abundant in sunshine but because it is sufficiently energetic: because, unlike visible light, UV is "ionizing radiation".

But wouldn't black plants get too hot?

It is true that black plants would absorb more light and thus would get hotter than green plants do, and it is possible that this could put them at a disadvantage, for example by increasing their rate of transpiration. But it seems to me that, by itself, this is probably insufficient to explain why it is that plants are green instead of black: for one thing, even in cold, damp, and poorly lit environments, plants tend to choose botanical fashion over function. 

Hmm. Are plants green... by accident?

Now I think we are getting closer to the truth: plants are green, not because the sun shines green light so intensely, but in spite of that fact. Certainly, there has been some sheer luck involved (for example, in plants chlorophyll resides within intracellular structures called "chloroplasts" which, incredibly, are thought to be descended from ancient bacteria [see "endosymbiosis"]). And, at this point, chlorophyll represents a well-established mode of phototrophism, so it is doubtful that it could be easily supplanted by an equally useful black pigment.

But even simpler microorganisms like green algae and cyanobacteria utilize chlorophyll, and taken together it's hard to chalk up these observations to chance. To me, they seem to imply that chlorophyll simply happens to have qualities that are able to compensate for any potential drawbacks associated with its hue.

However an "accident" can still have a casual factor. For example, one interesting idea known as the "purple Earth hypothesis" postulates that a purple photosynthetic pigment known as retinal (which absorbs green light particularly well) once dominated the Earth; and that chlorophyll-based photosynthesis co-evolved with and gradually out-competed retinal, probably by virtue of being more efficient. Since chlorophyll and retinal have complimentary absorption spectra, it is thought that chlorophyll might have evolved to target the blue and red light that the retinal-based competition was neglecting.

But the purple Earth hypothesis is not without critics, and in fact there appears to be no generally accepted, simple answer for those fool enough to wonder why we live on a blue and green planet, rather than on a blue and black one.

* * *

Will a simple answer present itself one day? Would we end up with black "plants" if the history of the Earth could be reset and rerun? What would life be like on such a planet? Might black "plants" arise sometime in Earth's distant future? And isn't it amazing, how science takes all the mystery out of life?