The Bottom Line

Issue date: 10/11/06 Section: The Voltage Gate

Media Credit: Kaiti Robinson, Jason Thomas

Midterms are looming for all of us. More and more I find myself staring at books and monitors, reviewing notes, highlighting lines of text and typing papers and articles. The workload is increasing exponentially. We could all use a break.

Bundle up and step outside. Follow the path through the Arboretum, along Sand Spring Run, or make use of that waste of pavement above the Appalachian Lab. Step into the forest. Kick some leaves. Gather your thoughts. Breathe. The leaves have changed.

Poets and artists have pondered the autumn palette, and there is no better place in the country, perhaps, to take in the beauty of October.

It is fleeting, however, as Frost notes:

Nature's first green is gold,
Her hardest hue to hold,
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.

But exactly why leaves change color in the fall is a mystery deeper than any poem has grasped. It also could change the way we think of fall.

For most of the twentieth century, it was believed that autumn leaves were nothing more than a biological wastebasket of sorts. As the light and temperature in our hemisphere begins to decrease, the tree begins to shut down, reabsorbing nutrients from its leaves. A woody cork membrane grows between branch and leaf, cutting off the circulation of nutrients, and the trees stops producing chlorophyll. (Retaining leaves during the winter is not energy efficient for broad-leafed trees.)

Think back to BIO 101. Chlorophyll is a pigment not only responsible for the green color of plants, but also for the production of sugars from captured sunlight. Chlorophyll is also a very unstable compound, constantly broken down by the energy from the sun. So when the trees stops circulating nutrients, the chlorophyll breaks down completely, allowing the tree's other pigments to reflect reds (anthocyanins) or yellows (carotenoids; see sidebar).

Carl Zimmer says it best: "In other words, autumn leaves were a tree's gray hair."

Zimmer says "were" because that notion has changed somewhat over the last decade. Nature, specifically natural selection, allows very little waste, and most traits are expressed through environmental pressures. Colorful fall leaves are no exception. New evidence has birthed two new theories, both provocative in their own right, both controversial among evolutionary biologists.

Continued...

Page 1 of 2 next >

Article Tools