Falling off at maturity. the process is controlled by the plant hormone auxin. as light levels and temperature drops, the flow if auxin to the leaves slows and the levels of other hormone, whenever, rise. this signals the cells at the vase of the leaf to weaken their cell walls , at the same time as other cells explains to break the connection between the weakened cells. the result is like tearing perforated paper and the leaf drops to the ground
