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in Plant Growth by (46.2k points)
Write notes on the following:

1. Phases of Growth.

2. Growth Kinetics.

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1. Phases of Growth: 

Plant growth is completed in three phases. 

  • The phase of Cell Division: This is the initial phase. The meristematic cells divide mitotically and add new cells continuously. This is also called a phase of formation. 
  • The phase of cell elongation: In this phase, the newly formed cells elongate in a particular direction. The thin and elastic cell wall stretches and a large vacuole begins to develop in the centre of the cell. New protoplasm is synthesized & dry weight of cell increases. This phase is also called a phase of enlargement. 
  • The phase of Cell differentiation: In this phase the cells mature, secondary wall material is deposited and cells stop dividing. Depending upon the function they are to perform, the cells undergo physiological and biochemical changes. This phase is also called a phase of maturation. 

Phases of Growth: 

  • Meristematic cells divide mitotically to form new cells. 
  • Some of the newly formed cells continue to divided and add new cells whereas some begin to elongate and undergo differentiation to form permanent tissue and become part of mature tissue. 
  • The whole process of growth may be divided into the following three phases. 

1. A phase of Cell Division: This is the first step of growth. In this phase, the number of cells increases by the division of meristematic cells. 

  • The meristematic cells continuously divide by mitosis and form new cells exactly similar to parent cells. 
  • The meristematic cells have dense cytoplasm, a conspicuous nucleus placed in the centre with other cell organelles and have very small vacuoles.
  • The cell wall is thin and elastic and is made up of cellulose. 
  • These cells are metabolically highly active. 
  • This phase of growth is also called a phase of formation. 

2. A phase of cell Elongation: This is the second phase of growth in which the cells stop dividing and begin to elongate in size. 

  • The increase in size is accompanied by the formation of a large central vacuole. 
  • The cytoplasm is pushed as a thin layer between the cell wall and the periphery of the vacuole.
  • The nucleus is now situated in the cytoplasm near the cell wall. 
  • The new cell wall material is synthesized to cope up with the enlargement.
  • In this phase, there is an irreversible increase in size and dry weight of the cell. This phase is also called a phase of enlargement. 

3. A phase of Differentiation: In this phase, the cells differentiate and develop into specified permanent tissue. 

  • The cells begin to differentiate according to the function performed by these and according to their shape and size, they undergo conspicuous changes.
  • The cell wall of the cells of mechanical tissue becomes highly thickened. 
  • Cell wall shows deposition of cellulose, suberin, lignin etc. and the cells mature, become strong and form permanent tissue.
  • Xylem and phloem tissues are differentiated and cells undergo differentiation to form the components tissue of xylem & phloem. 
  • The cells show structural, physiological and biochemical differentiation by changes in their physiological and biochemical reactions.

 2. Growth Kinetics: Growth of an organism or part of an organism is expressed in the form of growth rate, for example – increase in a number of cells per unit time or increase in dry weight per unit time. The growth rate of an organism may be expressed arithmetically or geometrically. 

  •  The growth rate is defined as an increase in the growth of an organism or a part of the organism per unit time. For example, an increase in the number of cells per unit time or increase in dry weight per unit time.
  • Growth rate can be expressed arithmetically or geometrically. 
  • Geometrically, the two daughter cells formed by one cell again divide and hence after each division the number of cells becomes double.Example: 1-2-4-8-16 and so on.
  • This geometric growth is seen in initial divisions of a zygote. 
  • During arithmetic growth, out of the two cells formed by each division, one differentiates into permanent tissue and one continues to divide.
  • Hence in arithmetic growth, new cells are continuously added to the body resulting in the continuous growth. This can be expressed as 1-1-1-1.
  • Such divisions occur in root and shoot apex.

Growth Curve: 

  • Keeping all the factors controlling growth at optimum, the pattern of growth of the cell or organ or the whole plant is not the same at all times. 
  • When a graph is plotted taking growth rate on one axis and time on the other, an ‘S’ shaped or sigmoid curve is obtained. This is called a growth curve. 
  • The whole growth curve can be divided into four parts.

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