1. 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 called abscission zone or abscission layer. An abscission layer is greenish-grey in colour.
2. The cells of abscission layer separate due to dissolution of middle lamella and primary wall of cells by the activity of enzymes pectinase and cellulase resulting in loosening of cells. Tyloses are also formed blocking the conducting vessels.
3. Degrading of chlorophyll occur leading to the change in the colour of leaves, leaf detachment from the plant and leaf fall.

Leopold (1961) has recognised four types of senescence:
1. Overall senescence
2. Top senescence
3. Deciduous senescence
4. Progressive senescence

1) Cells undergo changes in structure.
2) Vacuole of the cell acts as lysosome and secretes hydrolytic enzymes.
3) The starch content is decreased in the cells.
4) Photosynthesis is reduced due to loss of chlorophyll accompanied by synthesis and accumulation of anthocyanin pigments, therefore the leaf becomes red.

1) In most of the plants" there is sharp rise in respiration rate near·the end of the development of fruit, called climacteric rise.
2) Such fruits are called climacteric fruits. The ripening on demand can be induced in these fruits by using ethylene.
Eg: Tomato, Apples, Banana, Mango, etc.,

Agricultural role:
1. Formation of seedless fruits'without fertilization is induced by gibberellins Example: Seedless tomato, apple and cucumber.
2. It promotes the formation of male flowers in cucurbitaceae. It helps in crop improvement.
3. Uniform bolting and increased uniform seed production
4. Improves number and size of fruits in grapes
5. Promotes elongation of inter-node in sugarcane without decreasing sugar content.
6. Promotion of flowering in long day plants even under short day conditions.

Five major groups of hormones viz., auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to coordinate and regulate growth and development in plants.

lnternal Factors:
1. Genes are intracellular factors for growth.
2. Phytohormones are intracellular factors for growth. Example: auxin, gibberellin, cytokinin, etc.
3. C/N Ratio: The ratio of carbohydrates and nitrogenous compounds regulate the specific pattern of growth in plants

Stages in Growth rate:
The total period from initial to the final stage of growth is called the grand period of growth. The total growth is plotted against time and 'S' shaped sigmoid curve (Grand period curve) is obtained. It consists of four phases. They are:
I) Lag phase
2) Log phase
3) Decelerating phase
4) Maturation phase

1) Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.
2) Example: Heterophylly in cotton and coriander. In such plants, the leaves of the juvenile plant are different in shape from those in mature plants.
3) On the other hand, the difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to the environment. This phenomenon ofheterophylly is an example of plasticity.