iD magazine - - Questions & Answers -

“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.

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