vessels. Degrading of chlorophyll occur leading to the change in the colour of leaves, leaf detachment from the plant and leaf fall. After abscission, outer layer of cells becomes suberized by the development of periderm (Figure . ).
Figure . : L.S of petiolar base showing abscission layer Abscission layer Cortex Pholoem Xylem . Hormones influencing abscission All naturally occurring hormones influence the process of abscission. Auxins and cytokinins retard abscission, while abscisic acid (ABA) and ethylene induce it.
. Significance of abscission . Abscission separates dead parts of the plant, like old leaves and ripe fruits. .
It helps in dispersal of fruits and continuing the life cycle of the plant. . Abscission of leaves in deciduous plants helps in water conservation during summer. .
In lower plants, shedding of vegetative parts like gemmae or plantlets help in vegetative reproduction. to conduct water for transport. In aquatic plants, aerenchyma is normally formed in different parts of the plant such as roots and stems which encloses large air spaces that are created through PCD. In the development of unisexual flowers, male and female flowers are present in earlier stages, but only one of these two completes its development while other aborts through PCD (Figure .
). Mitochondria Vacuole Nucleus Plastid Figure . : Programmed cell death . Abscission Abscission is a physiological process of shedding of organs like leaves, flowers, fruits and seeds from the parent plant body.
When these parts are removed the plant seals off its vascular system to prevent loss of water and nutrients. Final stage of senescence is abscission. In temperate regions all the leaves of deciduous plants fall in autumn and give rise to naked appearance, then the new leaves are developed in the subsequent spring season. But in evergreen plants there is gradual abscission of leaves, the older leaves fall while new leaves are developed continuously throughout the year.
. Morphological and Anatomical changes during abscission Leaf abscission takes place at the base of petiole which is marked internally by a distinct zone of few layers of thin walled cells arranged transversely. This zone is