Leaves don’t simply break off at the end of the growing
season. If that were the case, there would be a tiny open wound at the site where each
leaf petiole broke off, inviting disease and insects. Instead, at the end of the growing
season, once the nutrients have been drawn out and the leaves have begun to dry up, the
plant actively severs the leaves by destroying cell walls across a section of the leaf
stalk. This area is called the abscission zone,
and—you guessed it—auxin is believed to play a role in the process. Once this layer of cells is destroyed, a corky layer begins to form
between the base of the leaf stalk and the branch, protecting living tissues. When this
layer is complete, the only thing attaching the leaf petiole to the limb is the vascular
tissue, and a gentle breeze can easily break these brittle cells. If you carefully examine
a twig, you should be able to see the shield-shaped leaf scars where leaves have dropped.
Look closely, and you’ll see a line of dots along the bottom. These are the remnants
of the vascular bundles.
Deciduous Twig
after leaves have dropped
A similar abscission process takes place in fruit stems,
allowing the fruit to drop without damaging the plant.
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