Born to Shed
Catch a falling leaf on the first day of autumn, according to folklore, and you won’t get a cold all winter. We can’t verify if that’s true, but we do know that the annual tree-leaf drop, or abscission, is the final step in a complex sequence of events. The process allows the tree to discard its disposable leaves and seal off dead and dying tissues.
Freezing temperatures, drought, and diminishing daylight weaken the leaf tissue so that it stops photosynthesizing. Hormone actions shifts; the production of auxin, a growth regulator, wanes, and ethylene, an again hormone, is stimulated. Fracture lines develop in the spaces between cell walls at the base of leaf petioles (the abscission layer) and protective materials fill in the fractures, causing corky scarring that shields the wounds from infection. A stiff breeze or a rain shower will finish the job. Trees such as oaks and beeches hang on to some of their leaves. These marcescent leaves dangle, brown and tattered, until expanding buds push them off their branches in spring.
