Plant Growth and Development-2-NCERT

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Growth Definition	Growth is an irreversible, permanent increase in the size of an organ, its parts, or even an individual cell. This process involves metabolic activities (anabolism and catabolism) that utilize energy. For example, the expansion of a leaf is considered growth. However, the swelling of a piece of wood when placed in water is not growth because it lacks metabolic processes.
Uniqueness of Plant Growth	Plant growth is unique because plants have the capacity for unlimited growth throughout their life. This is due to the presence of meristems located in specific regions of the plant body. These meristematic cells continuously divide and contribute to the formation of new cells, which then lose their ability to divide and become part of the plant body. This form of growth is called open growth. If the meristems stop dividing, growth ceases. This happens in specific scenarios, such as when secondary growth begins in certain plants.
Types of Growth	Several growth types can be identified: – Determinate Growth: Ceases after reaching a certain size or maturity. – Indeterminate Growth: Persists throughout the plant’s life. – Open Growth: Presence of both growth types in a single plant. Examples: Determinate Growth: Sunflowers stop growing once mature. Indeterminate Growth: Vines growing continuously.
Types of Meristems	1. Apical Meristems: Found at the tips of roots and shoots, responsible for primary growth, which results in the elongation of the plant along its axis. 2. Lateral Meristems: Found in dicots and gymnosperms later in life, responsible for secondary growth, which increases the girth of organs. Examples include the vascular cambium and cork cambium.
Growth Measurement	Growth, at a cellular level, involves an increase in protoplasm, but it is difficult to measure directly. Instead, parameters like fresh weight, dry weight, length, area, volume, and cell number are used to measure growth. For example, one maize root apical meristem can produce more than 17,500 new cells per hour, while the cells in a watermelon can increase their size by 350,000 times. The growth of the pollen tube is measured in length, while the growth of a leaf is often measured by its increase in surface area.
Phases of Growth	Growth can be divided into three main phases: 1. Meristematic Phase: The cells located at the root and shoot apexes are continuously dividing. These cells are rich in protoplasm, have large nuclei, and their walls are thin, primary, and cellulosic, with many plasmodesmatal connections. 2. Elongation Phase: The cells adjacent to the meristematic zone undergo vacuolation, enlargement, and cell wall deposition. These cells expand in size and contribute to the increase in organ length. 3. Maturation Phase: Cells in this phase attain their maximum size, with well-developed thick cell walls and extensive protoplasmic changes. Most of the tissues and cells in this phase contribute to the plant’s functional body.
Growth Rates	The rate of growth refers to the increase in growth per unit time, and can be mathematically expressed: 1. Arithmetic Growth: This occurs when, after cell division, one daughter cell continues to divide, while the other differentiates and matures. Example: A root elongating at a constant rate follows arithmetic growth. Formula: Lt = L0 + rt. 2. Geometric Growth: Involves an initial slow phase (lag phase), followed by a rapid exponential increase (log phase), and eventually slows down due to nutrient limitations. Formula: W1 = W0e^rt.
Types of Growth Rates	– Absolute Growth Rate: Total growth of an organism or part per unit time. – Relative Growth Rate: Growth per unit time, expressed relative to the initial size or mass.
Conditions for Growth	1. Water: Required for cell enlargement, turgidity, and enzymatic activities. 2. Oxygen: Essential for cellular respiration. 3. Nutrients: Required for synthesizing protoplasm and other cellular components. 4. Temperature: Each plant has an optimal temperature range. 5. Light and gravity: Regulate various phases of growth.
Growth Measurement Parameters	1. Growth in Number: – Definition: Increase in the number of cells or organisms. – Example: The root apical meristem of maize can produce more than 17,500 new cells per hour. 2. Growth in Biomass: – Definition: Measured by changes in fresh weight or dry weight. – Example: Cells in a watermelon can increase in size by 350,000 times. 3. Growth in Length: – Definition: Increase in the linear dimensions of an organ or organism. – Example 1: Growth of the pollen tube is measured in length. – Example 2: Growth of a leaf is often measured by its increase in surface area.