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The big event’s timing is determined by changing chemistry in the tiny abscission zone, a narrow band of cells at the base of each petiole, or leaf stalk, where it attaches to the stem or branch.

Not abscisic acid, the one that “abscission zone” would seem to imply.

“The restriction of ethylene’s destructive effects only to cells in the abscission zone illustrates the precise control plants exercise over their hormone systems,” Capon wrote.