Why leaves ‘should’ be purple

I’ve just finished writing a scene in Renegade Legion where our ragged band of Human Legion renegades are watching a woodland clearing, checking the area is secure before moving in.

The peace is disturbed when a bird hoots an alarm and erupts through the tree canopy and into the air in a snap of panicked wingbeats.

Our main character is tracking the bird with his eyes, but his suit AI shifts his visor display to a point much higher in the sky where the bird has attracted something… something incoming at speed…

In terms of scene setting, there’s a lot to do in the background here simply because we aren’t on Earth. It might look like a bird that’s taken to the sky, but it isn’t related to anything you might have eaten last Thanksgiving (or so you’d better hope). And the trees and other plants haven’t evolved on Earth. How do they look? What eats them? And come to think of it, what do humans eat when they are fighting an interstellar campaign? If their supply chain stretches over tens of light years, and interstellar cruising speed maxes out at about 0.7 lightspeed, they’re going to need a very large supply of tinned beans!

How to keep your troops fed on campaign is something that I’ll cover later in several Infopedia entries.

I want to mention just one aspect of world-building this time. The color of leaves.

In the experience of our Human Legion characters, plant life is largely purple. They’ve seen crops transplanted from Earth that are green, but that’s unnatural cultivation; natural plants are purple. Earth plants are weird.

There’s a reason for this.

I first used the idea of purple foliage when I wrote a colonization short story some years back. While I was writing the story, I read an article in New Scientist. This presented a hypothesis to explain why the plants around us are green, despite that being such an (apparently) inefficient color. The answer, so the article suggested, was that plant leaves are green because they aren’t purple!

I’ll explain that gobbledygook in just a moment.

As far as I can tell, it seems this hypothesis has not gained much acceptance. That’s the scientific method for you, I’m afraid. So our children (probably) won’t be seeing this idea in biology textbooks. But I don’t know that it has been completely disproved, and I do like the idea of challenging the easy assumption that life on other planets looks just like it does on Earth, just with pointier ears.

So here’s my scientifically shaky explanation of why the woodland clearing in Renegade Legion is lit in dappled shades of purple and lilac.

To understand, we first need to know a little about color.

Imagine a scene in the city of London. What do you see on every street corner? A red London bus, of course. But what makes it red?

Sunlight is a mix of many colors, and we perceive this as white light. That’s what Newton was doing with his prism and raindrops do when they make a rainbow: they split the white into its component colors.

If you shine white sunlight on a London bus, the red paint pigments absorb much of the light but not red light. We see the bus as red because that’s the color left over and reflected back after the bus has absorbed all the other wavelengths.

They're on every London street corner, and they take away the blue, green, and yellow light, leaving just red. Image by Jon Bennett, sourced from Wikipedia.

They’re on every London street corner, and they take away the blue, green, and yellow light, leaving just red. Image by Jon Bennett, sourced from Wikipedia.

Shift to New York and shine sunlight on a medallion taxi. Now the paintwork absorbs red, blue, violet, and green light. It doesn’t absorb yellow. That’s reflected back, which is why we perceive the cab as yellow.

The scientists who wrote the paper that spawned the New Scientist article were trying to answer the question of why plant leaves are green. If our eyes see the leaf as green, it means that is the color left over — the wavelength that the leaf does not absorb. The chlorophyll in the leaf is feasting on the blue and red light, leaving green and yellow behind. But it turns out that the color of sunlight that carries the most energy is green light. On the face of it, plants should be feasting on the green light leaving behind red and blue. We perceive red and blue mixtures as purple. Therefore leaves ‘should’ be purple.

One has only to see New England in the fall to appreciate the effect that removing chlorophyll has on Earth forests. But why is chlorophyll (and its green leaves) so dominant when leaves should be purple?

The hypothesis suggested that leaves originally evolved to be purple (because that’s most effective color to absorb sunlight’s energy), and green leaves evolved later because that was the most efficient color to absorb the light left over after purple leaves had stripped out the good wavelengths. Green leaves, and the chlorophyll inside, were specialist scavengers feasting on scraps.

There have been several great species diebacks in Earth’s history. And if the dominant purple-leaf plants were wiped out in one, green leaves might emerge from the carnage rather like mammals emerged from the shadow of the dinosaurs.

And so, my reasoning goes, it is only an unlikely roll of the evolutionary dice that means we think foliage is naturally green. On a planet where life has evolved to extract energy efficiently from the local sun, the norm is purple (for stars with similar emission spectra to ours).

As I said, this was an idea a few years back that had a little traction briefly, but hasn’t won many adherents for reasons that I’m not biologist enough to understand. [Counter-arguments include: (1) that early plant life evolved in the deep oceans, and most light penetrating to such depths is blue, not green, and (2) green light is reflected by leaves because it is too energetic to feed off safely.]

But like the best pseudo-science, it just has enough of a toehold on plausibility to overcome strict scientific accuracy for my storytelling purposes. And I guess that after all that pontificating about wavelengths, for me to swap green leaves for purple is no more of a difference from Earth biology than giving people pointy ears. Tough! I like the idea 🙂

So in Renegade Legion, when that bird breaks cover in the woodland scene, it emerges through a tree canopy of the lushest purple.

It’s going to look great, if only someone would make the movie.

2 Comments

  1. […] I’m making good progress writing the third book of the Human Legion series. As an example of the strange thoughts sf authors get that seem briefly to be of vital importance to us, I’ve written a short piece on why our characters are fighting in a forest where the trees are purple. […]

  2. SGT Mike says:

    It’s right because you’re the God of your Universe! Nuff said, carry on!!

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