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Falling Leaves

© Irene Taylor

Here in the northern hemisphere we have just entered the lovely season of fall. This is a wonderful time of the year - filled with pretty sights and cool, crisp weather.

Fall is the perfect time to teach your students all about the reasons for seasons and all about the wonderful changes we see in the fall.

If you live where the seasons change, you've probably made use of those changes to teach your students about what is really happening to the Earth on its trip around the sun. My article, Sunrise, Sunset has a good overview of the reasons for seasons. Take a look for some basic information.

Along with the meteorological change to fall - the passing of the Earth through the vernal equinox and the start of the march toward winter - comes the start of another "season" in the north. It is a spectacular sight that millions look forward to each year - the Fall Foliage Season. For a time, our trees are painted with brilliant hues, a treat for our eyes, and a warm glow that lasts well into the bleak whiteness of the coming winter.

But what causes that spectacular display? It all has to do with the way trees with green leaves make their food. In the summer months, deciduous trees (trees that lose their leaves) have green leaves because of a green pigment called chlorophyll that the tree uses in food production. Chlorophyll captures energy from the sun and allows the tree to use water and carbon dioxide to create sugar - the food the tree needs (Think sugar maple and the syrup from the sap of the tree). All through the summer while the tree needs food, the leaves stay green and chlorophyll continues to be used as an energy source.

Okay - so that's why the leaves are green! That doesn't tell us why they change into the beautiful colors we love to see in the fall.

As the days start to grow shorter with the approaching equinox, and as the nights turn cooler, the chlorophyll production begins to slow and the tree prepares for its winter survival. The tree knows that in order to survive, it can't continue to support the leaves that have sustained it all summer, because those leaves will not be able to make the needed food in the cold and "dark" winter months. They will also freeze in the cold winter temperatures. So, it begins a process toward self preservation, and we get to see a landscape painted with beautiful colors.

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