The falling of a leaf is the final step in an ordered series of events referred to as senescence. This process allows trees to conserve resources, prepare for a dormant period, and shed inefficient tissues. When leaves become unable to produce food due to a lack of chlorophyll, a process of shutting-down and sealing-off begins. Leaves are shed through a number of biological actions which take place at the base of the leaf’s stem, or petiole. The walls of some cells weaken, while those of other adjacent cells expand. The expansion of the latter causes pressure against the weaker-walled cells, resulting in these two groups of cells tearing away from each other, causing the leaf to fall. The tree forms a protective barrier on the wound where the leaf had been attached to the branch, sealing it off from pests and the environment and leaving a leaf scar.