HOW DO LEAVES KNOW THAT IT’S FALL?
“Leaves have an internal timer that measures the daily duration of light,” explains cell biologist Seth Davis. In order to be able to perceive the light, leaves possess sensors for red and blue as well as ultraviolet light. In autumn every day is four minutes shorter than the previous one. That means leaves have four fewer minutes of sunlight they can use to convert light, water, and carbon dioxide into energy. A point is eventually reached when the effort isn’t worth it any longer— the trees then shed their leaves. By the way: Trees that grow near a street light keep their leaves green longer than those growing in a forest. Trees are not picky about where light comes from. If a tree is close enough to a street light, it experiences a type of extended summer.