CBSE Class 11 Biology Chapter 15: 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 organ, an organism, or even a single cell. It is a fundamental characteristic of all living beings.
2. Differentiation: This is a process where cells, originating from meristems (like apical meristem in roots and shoots, and cambium), undergo structural modifications in their cell walls and protoplasm to become mature and specialized for performing specific functions.
3. Development: Development encompasses all the changes that an organism undergoes throughout its entire life cycle, starting from the germination of a seed until its senescence (aging).
4. Dedifferentiation: This is a fascinating process where mature, specialized plant cells lose their unique characteristics and regain the capacity to divide. This often occurs when cells are induced by certain conditions.
5. Redifferentiation: Following dedifferentiation, these newly formed, undifferentiated cells undergo further changes to mature again and become specialized, often losing their ability to divide once more.
6. Determinate growth: This refers to growth that has a limit. Once an organ or organism reaches a certain size or maturity, its growth ceases. Examples include the growth of animal limbs and the leaves of plants.
7. Meristem: In plants, growth is primarily confined to specific regions where cells actively divide. These regions of active cell division are called meristems. Plants have three main types: apical meristems (at shoot and root tips), lateral meristems (like cambium), and intercalary meristems (in between mature tissues).
8. Growth rate: Growth rate quantifies the increase in growth per unit of time. It measures how quickly growth is occurring.

Demonstrating plant growth accurately requires considering multiple parameters because growth involves an increase in the amount of protoplasm, which is a complex process. Relying on a single measurement is insufficient to capture the full picture of growth throughout a plant's life cycle. For instance:

• Fresh weight can be affected by water content, which fluctuates.
• Dry weight provides a better measure of accumulated matter but might not reflect the dynamic changes in cell division and expansion.
• Length, area, or volume changes can indicate growth but don't directly measure the increase in cellular material or metabolic activity.
• Cell number increases through division, but cell size also changes significantly during growth.

Therefore, a combination of these parameters, measured over time, is necessary to adequately demonstrate and quantify growth in a flowering plant.

Here's a brief description of each concept:

1. Arithmetic growth: In this type of growth, after mitotic cell division, one daughter cell continues to divide, maintaining the meristematic activity, while the other daughter cell differentiates and matures. This results in a linear increase in size over time. A classic example is the elongation of plant roots under optimal conditions, where the rate of growth remains constant.

   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: In geometric growth, both daughter cells produced by mitosis continue to divide, leading to an exponential increase in size. Initially, the growth rate is slow, but it accelerates rapidly as the number of dividing cells increases. However, this rapid growth is often limited by factors such as nutrient availability. The growth rate slows down as resources become scarce.

   Mathematically, it can be 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 time.

3. Sigmoid growth curve: The growth of most living organisms, when studied in a nutrient-rich, natural environment, typically follows a characteristic S-shaped curve known as the sigmoid growth curve. This curve represents three distinct phases:
   • Lag phase: An initial period of slow growth as the organism adapts to the new environment.
   • Log phase (or Exponential phase): A period of rapid, accelerated growth where the organism increases in size exponentially.
   • Stationary phase: Growth slows down and eventually stops as resources become limited, and waste products accumulate, reaching the carrying capacity of the environment.

   The formula $W\_1\; =\; W\_0\; e^\{rt\}$ describes the exponential growth phase within the sigmoid curve.

   

   The curve typically shows size/weight on the y-axis and time on the x-axis, illustrating the transition from lag to log to stationary phases.

4. Absolute and relative growth rates:
   • Absolute growth rate (AGR): This refers to the increase in size per unit time, considering the total size. For example, if a leaf grows from 5 sq cm to 10 sq cm in a week, its absolute growth rate is 5 sq cm/week.
   • Relative growth rate (RGR): This measures the increase in size per unit time, expressed as a proportion of the initial size. It is often more informative as it accounts for the initial size. For the same leaf example, if the initial size was 5 sq cm and it grew to 10 sq cm (an increase of 5 sq cm) in a week, the RGR would be (5 sq cm / 5 sq cm) / week = 1 per unit initial size per week. This indicates that the growth rate is proportional to the initial size.

   Comparing the relative growth rates of different plants or organs can provide a better understanding of their growth efficiency.