May

7

Is Green Universal?

Posted by Christopher Waldrop

May 7, 2008 |

A common question children ask is, why is the sky blue? I know this question stumps most parents, even though the answer is pretty simple: sunlight is made up of all colors of the spectrum (which is why rainbows sometimes appear) and blue, having a longer wavelength, gets zinged around more than any other color when it hits Earth’s atmosphere. The shorter wavelengths of red and orange mean they get spread out more when they hit larger particles such as dust, which is why those colors are prominent at sunset and sometimes at sunrise.

A less common question is, why is grass green? Why, for that matter, are most plants green? Of course the color of anything is determined by the colors it absorbs and reflects, but why do plants go green? In the article The Color of Plants on Other Worlds, Nancy Y. Kiang explains:

The energy spectrum of sunlight at Earth’s surface peaks in the blue-green, so scientists have long scratched their heads about why plants reflect green, thereby wasting what appears to be the best available light. The answer is that photosynthesis does not depend on the total amount of light energy but on the energy per photon and the number of photons that make up the light.

Whereas blue photons carry more energy than red ones, the sun emits more of the red kind. Plants use blue photons for their quality and red photons for their quantity. The green photons that lie in between have neither the energy nor the numbers, so plants have adapted to absorb fewer of them.

So, logically, plants on other planets–especially planets that circle stars unlike our Sun–might have different colors. It’s something for exobiologists to keep in mind as their telescopes zero in on other planets: they may go green, or blue, or red, or purple, or whatever color in the spectrum is most advantageous.

In fact even on Earth not all plants are green. Kiang adds, “Some land plants have red leaves, and underwater algae and photosynthetic bacteria come in a rainbow of colors.” Plant leaves adapt to their conditions. While she doesn’t mention them specifically, both rainforest plants and succulents often have wildly colored leaves. Some species of the genus Aeonium have very dark purple or even black leaves, while lithops–the “living stones” of sub-Saharan Africa–come in a variety of colors ranging from black to various shades of brown, gray, or cream. Other desert plants, such as fenestraria, have evolved “windows”, clear spots so that intense sunlight doesn’t damage them. Yes, even for plants there is such a thing as too much sun.


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