Why Leaves Fall Off the Trees: Mini Science Lesson

Sheri Reed
fall leaf

Photo by Cafe Sheri

Why do leaves fall off trees? I found myself completely fascinated by this little science and nature lesson on NPR's All Things Considered the other day.

Turns out, leaves don't really "fall" off trees. According to Peter Raven, president of the Missouri Botanical Garden and a renowned botanist, trees actually throw their leaves off. Funny huh?

Autumn's shorter days and colder weather trigger a hormone in trees that, per Raven, says, "Time to go! Let's part company!"

If you had a microscope up in the tree's branches and a lot of time on your hands, you'd see a small bumpy line of cells called "abscission" cells appear where the leaf's stem meets the branch. That line of cells slowly pushes the leaves away from the tree branches until they're pretty much dangling.

"With that very slender connection, [the leaves are] sort of ready to be kicked off," says Raven, and then an autumn breeze comes along and causes the leaves to fall to the ground.

Nature is so cool.

Find out more about fascinating tree behaviors over NPR.


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