CBSE Class 11 Biology Chapter 17: Plant Growth and Development NCERT Solutions

Here are the definitions of the key terms related to plant growth and development:

1. Growth: Growth is defined as an irreversible and permanent increase in the size of an organism, an organ, or even a single cell. It is a fundamental characteristic of life.
2. Differentiation: This is a process where cells, originating from meristems (like apical meristem or cambium), undergo structural and biochemical changes to become specialized and mature, enabling them to perform specific functions.
3. Development: Development encompasses all the changes an organism undergoes throughout its entire life cycle, starting from seed germination and continuing until senescence (aging).
4. Dedifferentiation: This is a fascinating process where mature, differentiated plant cells lose their specialized characteristics and regain the capacity to divide, often in response to specific stimuli or conditions.
5. Redifferentiation: Following dedifferentiation, these newly formed, undifferentiated cells mature again, acquiring new specialized structures and functions, and typically lose their ability to divide.
6. Determinate Growth: This refers to growth that has a limit. Once an organism or organ reaches a certain size or maturity, growth ceases. Examples include the growth of animal bodies and the cessation of leaf growth in plants.
7. Meristem: In plants, growth is localized in specific regions called meristems, which are characterized by actively dividing cells. These regions are responsible for producing new cells. Major types include apical meristems (at root and shoot tips) and lateral meristems (vascular cambium, cork cambium).
8. Growth Rate: Growth rate quantifies the increase in growth per unit of time. It measures how quickly growth is occurring.

Demonstrating growth in a flowering plant throughout its life cycle requires considering multiple parameters because growth is a complex process. While an increase in protoplasm is the fundamental aspect of growth, measuring it directly is challenging. Therefore, indirect measures are used, such as:

• Fresh weight: The weight of the tissue sample when it is fresh.
• Dry weight: The weight of the tissue sample after all water has been removed.
• Length, Area, Volume, and Cell Number: These are geometric parameters that can be measured over time.

Relying on only one of these parameters can be misleading. For instance, fresh weight can be affected by water content, and cell number alone doesn't account for cell size increase. A comprehensive understanding of growth requires observing changes across several of these measurable aspects over the plant's lifespan.

Here's a brief description of each concept:

1. Arithmetic Growth: In arithmetic growth, after mitotic cell division, one daughter cell differentiates and matures, while the other daughter cell continues to divide. This results in a constant rate of growth over time. A classic example is the elongation of plant roots, where the rate of increase in length remains steady. Mathematically, it can be represented as \(L_t = L_0 + rt\), where \(L_t\) is the length at time \(t\), \(L_0\) is the initial length, and \(r\) is the growth rate.
2. Geometric Growth: Geometric growth is characterized by an initial slow growth phase, followed by a period of rapid, exponential growth. In this type of growth, both daughter cells produced by mitosis retain the ability to divide. However, as resources become limited, the growth rate slows down. This pattern is often seen in young, actively growing tissues or organisms under favorable conditions. The increase in size is multiplicative.
3. Sigmoid Growth Curve: The growth of most living organisms in a natural environment typically follows an S-shaped curve, known as the sigmoid growth curve. This curve represents three distinct phases:
   • Lag Phase: A period of slow initial growth as the organism adapts to the new environment or conditions.
   • Log (Exponential) Phase: A period of rapid, exponential growth where the organism increases in size at its maximum rate.
   • Stationary Phase: Growth slows down and eventually stops as the organism reaches its carrying capacity, resource limitations, or maturity.
   The exponential growth phase can be mathematically expressed as \(W_1 = W_0 e^{rt}\), where \(W_1\) is the final size, \(W_0\) is the initial size, \(r\) is the growth rate, and \(t\) is the time.
4. Absolute and Relative Growth Rates:
   • Absolute Growth Rate (AGR): This refers to the increase in size (e.g., length, weight) per unit time. It measures the raw increase in size. For example, if a leaf grows from 5 cm to 10 cm in a day, its AGR is 5 cm/day.
   • Relative Growth Rate (RGR): This measures the increase in size per unit time relative to the initial size of the object. It provides a better comparison between the growth of two different objects or the same object at different times. For example, if two leaves of different initial sizes grow by 5 cm in a day, the one with the smaller initial size will have a higher RGR. It is calculated as \(\frac{1}{W_0} \frac{dW}{dt}\), where \(W_0\) is the initial size and \(\frac{dW}{dt}\) is the absolute growth rate.