Plant Growth and Development: Class 11 NCERT Biology Guide

Understanding Growth: What Does Plant Growth Mean?

Growth in plants refers to an irreversible and permanent increase in size and volume of an organ or the whole organism. It involves:

• Increase in cell number (cell division)
• Increase in cell size (cell enlargement)
• Cell differentiation into specialized types

Growth is measured using parameters like height, fresh weight, dry weight, and leaf area. However, no single parameter fully represents growth throughout a plant’s life because growth is complex and varies across organs and stages.

Types of Growth

• Arithmetic Growth: Increase by a constant amount over equal time intervals, resulting in a straight line graph.
• Geometric Growth: Increase proportional to the current size, leading to exponential growth.

Sigmoid Growth Curve

Plant growth often follows an S-shaped (sigmoid) curve with three phases:

1. Lag Phase: Slow initial growth 2. Exponential Phase: Rapid growth 3. Stationary Phase: Growth ceases or slows down

Understanding these growth patterns is essential for studying plant development.

Cellular Differentiation: Specialization in Plant Growth

Differentiation is the process where unspecialized cells from meristems mature into specialized cells with distinct structures and functions.

• During differentiation, cells modify their cell walls and protoplasm.
• Example: Tracheary elements lose protoplasm and develop thick lignified walls to conduct water.
• Differentiated cells usually lose the ability to divide.

However, plants can reverse this process through dedifferentiation, where mature cells regain the ability to divide under certain conditions. These dedifferentiated cells can then redifferentiate into new tissues, such as:

• Interfascicular cambium
• Cork cambium

This flexibility allows plants to repair and grow new tissues even after maturity. The position of cells within an organ influences their final differentiation, contributing to complex tissue organization.

Development: From Germination to Maturity

Development in plants is the sum of all changes an organism undergoes from germination to maturity. It includes:

• Growth (increase in size and volume)
• Differentiation (specialization of cells)
• Morphogenesis (formation of shape and structure)
• Reproduction (formation of flowers, fruits, seeds)

Development is a continuous and coordinated process regulated by genetic and environmental factors. It ensures that plants form organs like roots, stems, leaves, and flowers in the correct sequence and pattern.

Meristems and Development

Meristems are regions of actively dividing cells responsible for growth and development:

Understanding meristems helps explain how plants grow and develop new organs.

Plant Growth Regulators: Hormones Controlling Development

Plant growth regulators (PGRs) or plant hormones are natural substances that influence growth and development at low concentrations. The five main groups are:

1. Auxins

• Promote cell elongation, root initiation
• Example: Indole-3-acetic acid (IAA)

2. Gibberellins

• Stimulate stem elongation, seed germination

3. Cytokinins

• Promote cell division, delay leaf senescence

4. Ethylene

• Regulates fruit ripening, leaf abscission

5. Abscisic Acid (ABA)

• Inhibits growth, induces dormancy

These hormones interact to regulate processes like seed germination, flowering, fruit development, and responses to environmental stress.

Example: Role of Auxin in Phototropism

Auxin accumulates on the shaded side of a plant shoot, causing cells there to elongate more and the shoot to bend toward light.

Measuring Growth: Absolute and Relative Growth Rates

To quantify plant growth, scientists use two important parameters:

• Absolute Growth Rate (AGR): The increase in size or weight per unit time.

$$ AGR = \frac{W_2 - W_1}{T_2 - T_1} $$

Where $W_1$ and $W_2$ are weights at times $T_1$ and $T_2$.

• Relative Growth Rate (RGR): The increase in size relative to the initial size per unit time.

$$ RGR = \frac{\ln W_2 - \ln W_1}{T_2 - T_1} $$

Where $\ln$ is the natural logarithm.

Worked Example

If a plant weighs 10 g at day 1 and 20 g at day 3, then:

• $AGR = \frac{20 - 10}{3 - 1} = 5$ g/day
• $RGR = \frac{\ln 20 - \ln 10}{2} = \frac{2.9957 - 2.3026}{2} = 0.3465$ per day

These measures help compare growth rates under different conditions or treatments.

Differentiation, Dedifferentiation, and Redifferentiation Explained

These three cellular processes are key to understanding plant development:

• Differentiation: Cells become specialized, losing the ability to divide.
• Dedifferentiation: Specialized cells revert to a less specialized state and regain division ability.
• Redifferentiation: Dedifferentiated cells develop into new specialized cells or tissues.

For example, in woody dicot stems, parenchyma cells dedifferentiate to form the cork cambium, which then redifferentiates to produce protective cork cells.

This ability allows plants to adapt, repair, and continue growing even after initial specialization, unlike most animal cells.