In vascular plants, abscission is the dropping of leaves, flowers, fruits, or stems at the end of a growing season, as the result of formation of a layer of specialized cells (the abscission layer) at the base of the stalk and the action of the hormone abscisic acid and other growth-inhibiting substances. In the abscission layer, the parenchyma cells become separated from one another through dissolution of the middle lamella. Wind and rain help dislodge the parts separated by abscission layers.