Senescence has been defined as those changes or deteriorative processes, which lead to the death of an organism or some part of it. Another term, which often creeps into the literature of this subject, is aging. The whole plant may senesce and dies at one time as occurs in many annuals after fruiting is complete. Alternatively, there may be a progressive senescence of parts as the whole plant ages. Cotyledons of seeds undergo senescence and death following a depletion of the storage reserves during germination. During the process of tissue maturation, certain cells such as xylem vessels and tracheids or sclerenchyma tissue may senesce and die although the plant as whole is in a state of vigorous growth. The term of abscission is used to describe the processes involved in the shedding of plant structures, characterized by the degradation of cell walls at the point of weakening. Present evidence suggests that cells surrounding the fracture line produce and secrete cell wall degrading enzymes, which hydrolyze the central region of the wall, allowing the cells to separate and the fracture to occur. The fall of leaves from deciduous plants is a distinctive phenomenon of periodic occurrence. The leaf fall is usually associated with the formation of the so-called abscission zone which is made up of one or more layer of cells extending across the petiole near its base. Separation of cells of the abscission layer usually results from the dissolution of the middle lamella and sometimes also the cellulose wall of abscission layer cells. The petiole remains attached only by the vascular elements.