Deciduous Senescence - Senescence

Only the leaves senesce as in many trees. The senescence of leaves, or abscission occurs when at the base of a leaf a layer of cells is laid which is called abscission layer. The cells of this layer have high activity of cellulase, an enzyme which degrades cellulose of cell wall. Another enzyme that increases during abscission is polygalacturonase which hydrolyses pectin, a major component of the middle lamella region of the wall. So the cells get separated and the leaves fall. Below this layer, the plant makes a protective layer which has lot of lignified cells. When leaf falls, this layer protects the tissue from desiccation.