You've probably learned that leaves are green because they're full of a green molecule called
chlorophyll - it's the reason for the greenness of plants, from moss-green moss to grass-green
grass to ever-green evergreens.
And the reason chlorophyll is so incredibly popular--showing up in nearly every plant
on Earth--is that it's good at soaking up sunlight.
The more light a plant gets, the more energy it has to make sugar and grow tulips and kumquats
and do all the other things plants do.
But wait a second: doesn't the fact that chlorophyll is green mean that it's actually bad at absorbing
green light?
I mean, sunlight is composed of the whole rainbow of colors, each a separate wavelength,
and the colors we see when we look at a leaf are the wavelengths that leaf isn't absorbing
-- that is, the ones that bounce off the leaf and reach our eyes.
Chlorophyll is great at absorbing reds and blues, but if it were really awesome, wouldn't
it absorb all colors of light, green included, giving us a world full of black moss and black
grass and everblack trees?
So why aren't leaves black?
And since they apparently don't have to be black, what's stopping them from being red
or blue instead?
Is green just Nature's favorite color, or is there something special about those wavelengths
of light?
The short answer is that we don't know for sure.
Some scientists speculate it's serendipity that plants today are green.
Long ago, Earth's oceans were filled with purple single-celled organisms called archaea.
Being purple, they absorbed green light and let red and blue pass through.
The theory is that this "minus-green" light was the only source of solar energy in the
deep water below, so bacteria living there got really good at absorbing everything else
but green.
And the molecule that did the light-absorbing?
Chlorophyll!
Maybe.
In the long run, what we do know is that Chlorophyll, as wasteful as its failure to absorb green
light may be, turned out to have other biochemical advantages over the purple competition.
And perhaps as a result of this ancient biological rivalry, the plant world today is green, and
not purple or red or blue.
Of course, you might still wonder why plants didn't develop black leaves once they emerged
from the depths.
I mean, they changed a lot of other things going from bacteria to mighty redwood!
Well, maybe it's simply that nature isn't about perfection, and life doesn't need "flawless"
black to get by - just "good enough" green.
Or maybe plants were even more clever and turned the weakness of chlorophyll into a
strength?
But we'll have to leave that story for later...