October 13, 2000

     Why Leaves Fall

The forest areas are starting to look bare.  At the end of the growing season deciduous tree leaves fall, in contrast to the evergreens, which keep their leaves (needles) for several seasons.  A chemical called auxin, a growth-regulating substance produced by plants, creates a process that makes deciduous plants lose their leaves at about the same time each year.

When leaves unfold in the spring, each new leaf makes large amounts of auxin.  In some way auxin signals the leaf stem to grip its branch tightly, but as the leaf gets older it produces less auxin.  At about this time, two thin layers of cells called the abscission layer grow across the base of the leaf stem where it is connected to the branch, probably initiated by the decreasing daily light period.

As the supply of auxin slows to a trickle, the cells in the abscission layer separate from one another.  The leaf stem's hold on the branch grows weaker, a gust of wind blows the leaf free and it falls to the ground.