Friday, October 21, 2011
Weather-ology: Why the Leaves Change Color
Jaime McLeod of Farmers Almanac
Long before modern science began to understand the processes that create our weather, people made up their own explanations. Many of these accounts were fantastic in nature, with evil or benevolent gods, monsters, and spirits controlling the elements. In this series, we’ll explore some of these ancient myths and share the science behind them. Weather + mythology = weather-ology!
Among the Algonquin tribes who once inhabited much of North America, from New England all the way to Wyoming, there was a legend that explained why the leaves changed color in the fall. The story goes that there was once a great bear who roamed around terrorizing people. He ate the food they’d gathered, destroyed the homes they built, chased away their game, and even mauled women and children.
The bear was far too powerful for any one person to kill, so the bravest and strongest warriors from several tribes banded together to hunt it. When the bear saw the hunters coming for him, he did what any sensible bear would do; he took off running. The warriors chased the bear for many months, over all the earth, over mountains and across seas, all the while firing their arrows at him. Though they could never catch him, they did get close enough, once, for their arrows to reach him. One arrow pieced the bear’s side, not deeply enough to kill, but enough to draw blood. In his pain and rage, the bear reared up and took to the sky, running from his pursuers through the heavens. To this day, says the legend, those warriors continue to chase the bear in circles around and under the earth. In the fall, when he is rising above the horizon, the bear’s blood drips down onto the trees below, turning the leaves scarlet.
The bear is said to be the pan of the Big Dipper, coincidentally part of the constellation Ursa Major, or the Great Bear. The handle of the Dipper, known as the bear’s tail to the ancient Greeks, is said to be that band of warriors, still diligently chasing the bear after so many years.
Of course, we know today that the red of autumn leaves isn’t really the blood of a great bear, but then, why do leaves change colors in the fall? Actually, fall colors – the reds, oranges, and yellows – come from sugars produced by the tree. Each type of sugar has its own color: carotenoids are yellow or orange, while anthocyanins are red. The reason we don’t see these colors during the rest of the year is because they are masked by the green chlorophyll, which converts light from the sun into food for the tree.
As the days grow shorter and darker, less light from the sun is available, so the tree stops producing chlorophyll. This allows the brilliant colors of autumn to shine through. Eventually, these chlorophyll-stripped leaves cost the tree more to keep than they give back to it in energy, so it protects itself by producing a substance to seal off the leaves from the rest of the tree. Eventually, the nutrient-starved leaves fall off and die.
Without knowing about the complex chemical processes going on inside each intricate leaf, however, the early Americans were left to come up with their own explanation of how and why the leaves change colors every autumn.