Plant Growth and Development – Class 11 Biology Notes
Introduction to Plant Growth and Development
Plant growth refers to the irreversible increase in size, volume, and mass of a plant, which results from the division and expansion of cells. Development in plants includes the processes of growth, differentiation, and morphogenesis, which lead to the formation of new tissues and organs and the overall transformation of the plant.
Plants grow and develop in response to both internal signals (like hormones) and external environmental factors (such as light, temperature, and water). Growth and development are highly coordinated processes that involve cell division, elongation, and differentiation.
1. Plant Growth
1.1. Definition of Growth
- Growth in plants is defined as the permanent increase in size, weight, and volume due to the activity of cells, tissues, and organs.
- Growth is a continuous and irreversible process in plants that occurs in three distinct phases: Cell Division, Cell Elongation, and Cell Maturation.
1.2. Phases of Growth
- Cell Division (Meristematic Activity):
- Meristems are regions of active cell division that lead to growth. They are located at the tips of roots and shoots (apical meristems) and in the cambium (lateral meristems).
- Apical Meristem: Responsible for primary growth, which increases the length of the plant.
- Lateral Meristem: Responsible for secondary growth, which increases the girth (width) of the plant.
- Cell Elongation:
- After cell division, cells elongate, leading to an increase in plant size. This process is regulated by plant hormones like auxins and gibberellins.
- Cell Maturation:
- The cells that have elongated become mature and take on specific functions, leading to the formation of specialized tissues such as xylem, phloem, and parenchyma.
1.3. Measurement of Growth
- Growth is often measured in terms of:
- Increase in length
- Increase in weight (dry or fresh weight)
- Increase in volume
2. Plant Development
Plant development refers to the series of changes that occur as a plant grows, from seed germination to the formation of flowers, fruits, and seeds. It includes:
- Cell differentiation
- Morphogenesis
- Developmental phases
2.1. Differentiation
- Differentiation is the process by which cells become specialized in structure and function to form different tissues and organs (e.g., xylem, phloem, epidermis).
- Differentiation is influenced by genetic and environmental factors, including light and temperature.
2.2. Morphogenesis
- Morphogenesis refers to the process by which cells and tissues organize to form the overall shape and structure of the plant.
- This process includes the development of organs like leaves, stems, roots, and flowers.
2.3. Developmental Stages of a Plant
- Germination: The process by which a seed develops into a new plant.
- Vegetative Growth: The period when the plant grows in size and develops leaves, stems, and roots.
- Reproductive Growth: When the plant develops flowers and fruits for reproduction.
3. Factors Affecting Plant Growth and Development
3.1. External Factors
- Light: Light affects photosynthesis and also influences various developmental processes such as photoperiodism and flowering. Blue light plays an important role in regulating plant growth.
- Temperature: Plant growth is temperature-dependent. Each plant has an optimal temperature range for growth. Extreme temperatures can reduce growth or even damage plant tissues.
- Water: Water is essential for nutrient transport and maintaining turgidity (cell pressure). It also plays a role in biochemical reactions within the plant.
- Nutrients: Plants require various minerals and nutrients (nitrogen, phosphorus, potassium, etc.) for growth. Deficiency or excess of any nutrient can lead to stunted growth or other abnormalities.
- Gravity: Plants exhibit gravitropism or geotropism, where roots grow downward (positive gravitropism) and stems grow upward (negative gravitropism) in response to gravity.
3.2. Internal Factors
- Hormones (Plant Growth Regulators): Plant growth and development are controlled by several hormones. Some of the key plant hormones include:
- Auxins: Promote cell elongation and root formation.
- Cytokinins: Promote cell division and influence shoot and root growth.
- Gibberellins: Stimulate growth, especially stem elongation and seed germination.
- Abscisic Acid (ABA): Inhibits growth and promotes dormancy.
- Ethylene: Involved in fruit ripening and leaf abscission.
- Genetic Factors: The genetic makeup of a plant determines its growth potential and limits. Plants with specific genes may show different growth rates, flower types, or resistance to diseases.
4. Plant Hormones and Their Role
4.1. Auxins
- Function: Stimulate elongation of cells, promote root formation, and are involved in phototropism (growth toward light) and gravitropism.
- Example: Indole Acetic Acid (IAA) is a natural auxin.
4.2. Gibberellins
- Function: Stimulate stem elongation, promote seed germination, and increase fruit size.
- Example: Gibberellic Acid (GA) is a well-known gibberellin.
4.3. Cytokinins
- Function: Stimulate cell division and differentiation, delay senescence (aging) in leaves, and promote lateral bud growth.
- Example: Kinetin is a common cytokinin.
4.4. Abscisic Acid (ABA)
- Function: Inhibits growth, promotes seed dormancy, and helps in water stress responses.
- Example: ABA is involved in closing stomata during water shortage.
4.5. Ethylene
- Function: Involved in fruit ripening, leaf abscission, and stress responses.
- Example: Ethylene gas is produced during fruit ripening.
5. Important Processes in Plant Development
5.1. Germination
- Germination is the process in which a seed absorbs water, swells, and breaks the seed coat to begin growing into a seedling.
- The process involves the activation of enzymes that break down stored nutrients, which fuel the early growth of the seedling.
5.2. Photoperiodism
- Photoperiodism is the response of plants to the length of day and night, affecting flowering and other developmental processes.
- Short-day plants: Flower when the night length exceeds a critical period.
- Long-day plants: Flower when the night length is shorter than a critical period.
- Day-neutral plants: Flower independently of the photoperiod.
5.3. Vernalization
- Vernalization is the process by which exposure to low temperatures induces flowering in certain plants, such as winter wheat.
MCQs – Plant Growth and Development
1. Which of the following hormones is responsible for promoting cell division?
a) Gibberellins
b) Cytokinins
c) Abscisic acid
d) Ethylene
Answer: b) Cytokinins
2. The process by which a seed develops into a seedling is known as:
a) Germination
b) Photoperiodism
c) Growth
d) Differentiation
Answer: a) Germination
3. The hormone that helps in the formation of roots is:
a) Gibberellins
b) Cytokinins
c) Auxins
d) Ethylene
Answer: c) Auxins
4. Which of the following hormones is involved in the ripening of fruits?
a) Auxins
b) Cytokinins
c) Gibberellins
d) Ethylene
Answer: d) Ethylene
5. Which of the following is an example of a short-day plant?
a) Rice
b) Wheat
c) Chrysanthemum
d) Tomato
Answer: c) Chrysanthemum
6. The process by which low temperatures induce flowering is called:
a) Photoperiodism
b) Germination
c) Vernalization
d) Respiration
Answer: c) Vernalization
7. Which of the following hormones promotes seed germination?
a) Auxins
b) Abscisic acid
c) Gibberellins
d) Cytokinins
Answer: c) Gibberellins
8. The increase in size due to the enlargement of cells is called:
a) Differentiation
b) Growth
c) Germination
d) Morphogenesis
Answer: b) Growth
9. Which of the following factors does NOT affect plant growth?
a) Light
b) Temperature
c) Soil pH
d) Genetic mutations
Answer: d) Genetic mutations
10. The primary growth of plants occurs due to the activity of:
a) Lateral meristems
b) Apical meristems
c) Cambium
d) Xylem
Answer: b) Apical meristems