You probably noticed by now that the days are getting
shorter and the air is a little chillier. Fall has fallen! The sun gives life
to everything on Earth through heat and energy for plant photosynthesis. It
also triggers changes in life cycles, some of which are evident to almost
everyone right now.
The most obvious cycle that is affected by the sun is
the current situation with our tree leaves changing colors and falling off.
Because Earth’s axis is tilted 23 degrees, we experience seasonal variances in
the amount of sunlight we get. As we move through fall and into winter, the sun
sinks lower in the sky and light that hits us is less direct.
Nature loves efficiency, and trees lose their leaves
this time of year because with decreasing sunlight, they would expend more energy
performing photosynthesis (turning sunlight, water, and carbon dioxide into energy) than they would get from it. Chlorophyll, the active photosynthesizing
pigment, dries up and the cells in the leaf stem die, weakening it to the point
that it falls off the tree. Until spring arrives, trees and other plants live
off of starches stored in their roots. That starch is the byproduct of
photosynthesis.
Other seasonal cycles are the leafing out of trees,
hibernation, and mating seasons. Some cycles run by the sun can be daily, not
just seasonal. Dandelions and other flowers close their petals at night. Daily
sleep cycles are also affected by sunlight. Animals don’t use an alarm clock,
but they do have an internal clock.
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| Bigleaf maple |
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| Even shrubs change colors |
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