All my life, I thought leaves departed trees in the fall because they got too old to hang onto the branch and gravity bested their grasp. It does not work that way. Trees literally release their leaves from the limb. Here is how a leaf becomes the prey of gravity.

As days get colder and shorter, trees create a hormone that sends out a chemical message and triggers the creation of cells that appear at the spot where the leaf stem meets the branch. These abscission (root word for scissors) cells...