Chapter 15: Plant Growth and Development – Long Answer Type Questions
CBSE Class 11 Biology Chapter 15 – Plant Growth and Development | Long Answer Type Questions (NCERT)
Course & Examination Details
Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 15 – Plant Growth and Development
Board: Central Board of Secondary Education (CBSE)
Question Type: Long Answer Type Questions
Answer Length: 120–150 words
Syllabus: Strictly as per NCERT Biology Textbook
Section A: Growth in Plants (Q1–Q7)
Q1. Define growth in plants and explain its main characteristics.
Answer:
Growth in plants is defined as a permanent and irreversible increase in size, mass, volume, or number of cells. It is a fundamental property of living organisms and results from metabolic activities such as cell division and enlargement. One major characteristic of plant growth is that it is generally indeterminate due to the presence of meristematic tissues that remain active throughout life. Growth is energy-dependent and requires nutrients, water, and suitable environmental conditions. It is measurable in terms of length, area, volume, or dry weight. Plant growth is also influenced by internal factors like genetic makeup and plant hormones, as well as external factors such as light, temperature, oxygen, and water. Unlike animals, plants can continuously form new organs, making growth a lifelong process.
Q2. Why is plant growth considered indeterminate? Explain with suitable examples.
Answer:
Plant growth is considered indeterminate because plants continue to grow throughout their life due to the presence of meristematic tissues. These tissues, such as apical meristems in roots and shoots and lateral meristems like cambium, retain the ability to divide continuously. For example, root tips keep producing new cells that allow roots to grow deeper into the soil, while shoot apical meristems enable continuous elongation of stems. Even secondary growth in woody plants occurs due to lateral meristems. This indeterminate nature allows plants to adapt their growth to environmental conditions and replace damaged parts. Unlike animals, plants do not have a fixed body plan, which explains their continuous and flexible growth pattern.
Q3. Describe the three phases of growth in plants.
Answer:
Plant growth occurs in three distinct phases: meristematic, elongation, and maturation. In the meristematic phase, cells actively divide and are characterized by thin cell walls, dense cytoplasm, prominent nuclei, and absence of vacuoles. This phase occurs at root and shoot apices. The elongation phase follows, during which cells increase in size due to cell wall loosening, vacuole expansion, and synthesis of new cell wall materials. This phase contributes significantly to the increase in length of plant organs. The final maturation phase involves differentiation of cells into specialized tissues. Cells attain permanent shape, size, and function, completing the growth process. These phases together ensure proper development of plant organs.
Q4. Explain arithmetic and geometric growth patterns in plants.
Answer:
Arithmetic growth is a type of growth where an organism increases by a constant amount per unit time. It produces a linear growth curve and is expressed mathematically as Lt = L₀ + rt. This type of growth is commonly seen in root elongation. In contrast, geometric growth occurs when the growth rate is proportional to the existing size, resulting in exponential growth. It follows the equation Wt = W₀eʳᵗ and produces a sigmoid growth curve under natural conditions. Geometric growth is observed when resources are unlimited and cells divide continuously. However, due to environmental limitations, geometric growth eventually slows down, forming a sigmoid curve. Both growth patterns help in understanding plant growth dynamics.
Q5. What are the internal and external factors affecting plant growth?
Answer:
Plant growth is influenced by both internal and external factors. Internal factors include genetic constitution and plant hormones. Genes determine the inherent growth potential and developmental pattern, while hormones regulate processes such as cell division, elongation, differentiation, dormancy, and senescence. External factors include light, temperature, water, oxygen, and nutrients. Light affects photosynthesis and flowering, temperature regulates enzyme activity, water maintains cell turgidity and metabolic reactions, oxygen supports respiration, and nutrients provide essential elements for biosynthesis. The interaction between internal and external factors ensures coordinated growth and development of plants under varying environmental conditions.
Q6. Explain the role of water and temperature in plant growth.
Answer:
Water plays a crucial role in plant growth by maintaining cell turgidity, enabling cell expansion, and acting as a medium for biochemical reactions. It facilitates nutrient transport, photosynthesis, and enzymatic activities. Lack of water reduces cell elongation and growth rate. Temperature is equally important as it affects enzyme activity and metabolic processes. Each plant species has an optimum temperature range for growth. Low temperatures slow metabolic reactions, while excessively high temperatures can denature enzymes. Proper temperature ensures efficient physiological processes, enabling normal growth and development of plants.
Q7. What is growth rate? Explain its significance.
Answer:
Growth rate refers to the increase in size, mass, area, or volume of a plant or its parts per unit time. It can be expressed as absolute growth rate or relative growth rate. Growth rate helps in comparing growth under different environmental conditions and understanding the efficiency of growth processes. It reflects the influence of internal factors like hormones and external factors such as nutrients and temperature. Measurement of growth rate is important in agriculture to assess crop productivity, optimize nutrient supply, and improve yield by managing environmental conditions effectively.
Section B: Differentiation and Development (Q8–Q13)
Q8. What is differentiation in plants? Explain its significance.
Answer:
Differentiation is the process by which meristematic cells develop into specialized cells with distinct structure and function. During differentiation, cells undergo changes in size, shape, wall thickness, and metabolic activity. This process results in the formation of various tissues such as xylem, phloem, epidermis, and cortex. Differentiation is essential for functional specialization, allowing plants to perform complex activities like conduction, support, protection, and photosynthesis. Without differentiation, plants would remain as undifferentiated cell masses and would not be able to carry out essential physiological processes necessary for survival and growth.
Q9. Explain dedifferentiation and redifferentiation with examples.
Answer:
Dedifferentiation is the process by which mature, differentiated cells regain the capacity to divide. This usually occurs under specific conditions and leads to the formation of secondary meristems. For example, cortical cells dedifferentiate to form cork cambium during secondary growth. Redifferentiation follows dedifferentiation, where newly formed cells lose their ability to divide and specialize again to perform specific functions. An example is the differentiation of cambial cells into secondary xylem and phloem. These processes highlight the developmental plasticity of plants and their ability to adapt to changing conditions.
Q10. Define development in plants and explain the factors regulating it.
Answer:
Development in plants is the sum total of growth and differentiation that occurs throughout the life cycle, from seed germination to senescence. It includes vegetative growth, flowering, fruiting, and aging. Development is regulated by internal factors such as genes and plant hormones, which control the timing and pattern of growth. External factors like light, temperature, water, and nutrients also play a significant role. The interaction between internal and external factors ensures proper coordination of developmental processes and allows plants to respond effectively to environmental changes.
Q11. Why are plants described as developmentally plastic?
Answer:
Plants are described as developmentally plastic because they can alter their growth and development patterns in response to environmental conditions. The same plant species may show different forms when grown under different conditions. For example, buttercup plants produce different leaf shapes in water and on land. This plasticity allows plants to adapt to changing environments, optimize resource utilization, and survive under diverse ecological conditions. Developmental plasticity is made possible by the presence of meristems and the ability of cells to dedifferentiate and redifferentiate.
Q12. Explain the relationship between growth and development in plants.
Answer:
Growth and development are closely interrelated processes in plants. Growth refers to an increase in size or mass, while development includes differentiation and maturation of cells. Growth provides the raw material in the form of new cells, whereas development organizes these cells into functional tissues and organs. Growth alone does not ensure functionality; differentiation is essential for specialization. Together, growth and development lead to the formation of a complete and functional plant body capable of carrying out physiological processes efficiently.
Q13. Discuss the role of genes in plant development.
Answer:
Genes play a crucial role in plant development by controlling the synthesis of proteins and enzymes required for growth and differentiation. They determine the basic developmental pattern, organ formation, and timing of flowering. Gene expression is influenced by environmental factors and plant hormones, allowing flexibility in development. While genes provide the blueprint, the actual developmental outcome depends on interactions with external conditions. Thus, plant development is genetically controlled but environmentally modulated.
Section C: Plant Hormones (Q14–Q21)
Q14. What are plant hormones? Classify them based on function.
Answer:
Plant hormones are small organic substances produced in minute quantities that regulate growth, development, and physiological responses. Based on function, they are classified into growth promoters and growth inhibitors. Growth promoters include auxins, gibberellins, and cytokinins, which stimulate cell division, elongation, and differentiation. Growth inhibitors include abscisic acid and ethylene, which inhibit growth, induce dormancy, promote senescence, and regulate stress responses. Hormones often act in combination, and their balance determines the overall growth response.
Q15. Explain the functions of auxins in plant growth and development.
Answer:
Auxins are growth-promoting hormones that primarily stimulate cell elongation in stems by increasing cell wall plasticity. They maintain apical dominance by suppressing the growth of lateral buds and promote root initiation in cuttings. Auxins also regulate tropic movements such as phototropism and geotropism. Additionally, they delay leaf abscission and help in vascular differentiation. Indole-3-acetic acid (IAA) is the most common natural auxin found in plants.
Q16. Describe the role of gibberellins in plants.
Answer:
Gibberellins are plant hormones that promote stem elongation by stimulating cell division and elongation. They induce bolting in rosette plants and break seed dormancy, facilitating germination. Gibberellins activate enzymes like amylase during germination, which mobilize stored food. They also influence flowering in some plants and increase fruit size. Due to these functions, gibberellins are widely used in agriculture to improve crop yield and quality.
Q17. Explain the functions of cytokinins.
Answer:
Cytokinins promote cell division and play a key role in delaying senescence. They stimulate lateral bud growth, counteracting apical dominance maintained by auxins. Cytokinins also mobilize nutrients to growing regions and help maintain chlorophyll and protein content in leaves. This prolongs photosynthetic activity and enhances plant productivity. Zeatin is a naturally occurring cytokinin found in developing fruits and seeds.
Q18. Why is ethylene considered a unique plant hormone? Explain its functions.
Answer:
Ethylene is unique because it is the only plant hormone that exists in gaseous form. It regulates fruit ripening, senescence, and abscission. Ethylene also induces the triple response in seedlings, characterized by reduced stem elongation, lateral swelling, and horizontal growth. During fruit ripening, ethylene increases respiration rate and activates enzymes responsible for softening, color development, and flavor formation. Its gaseous nature allows rapid diffusion and action.
Q19. Discuss the role of abscisic acid (ABA) in plants.
Answer:
Abscisic acid is a growth-inhibiting hormone that plays a crucial role in stress responses. It induces dormancy in seeds and buds, preventing germination under unfavorable conditions. ABA promotes stomatal closure during water stress, reducing transpiration and conserving water. It also accelerates senescence and abscission. Due to its role in helping plants survive adverse conditions, ABA is known as the stress hormone.
Q20. Explain the importance of hormonal balance in plant growth.
Answer:
Plant growth and development are regulated by the combined action of different hormones rather than a single hormone. The balance and interaction among hormones determine specific responses. For example, auxins and cytokinins together regulate apical dominance, while gibberellins and ABA have antagonistic effects on growth and dormancy. An imbalance in hormone levels can disrupt normal development. Therefore, hormonal balance ensures coordinated growth, proper differentiation, and adaptive responses to environmental conditions.
Q21. Describe the agricultural importance of plant hormones.
Answer:
Plant hormones are widely used in agriculture to improve crop yield and quality. Auxins are used to promote rooting in cuttings and as selective weed killers. Gibberellins increase fruit size, induce malting in barley, and improve crop productivity. Ethylene is used for artificial ripening of fruits, while ABA helps in stress management. Application of plant hormones allows controlled manipulation of growth and flowering, making them valuable tools in modern agriculture.
Section D: Photoperiodism and Vernalisation (Q22–Q25)
Q22. Explain photoperiodism and its significance.
Answer:
Photoperiodism is the physiological response of plants to the relative length of day and night, particularly in relation to flowering. Plants are classified as short-day, long-day, or day-neutral based on their photoperiodic response. Photoperiodism ensures that flowering occurs at an appropriate season, improving chances of pollination and seed formation. It is regulated by the pigment phytochrome, which helps plants perceive light duration and quality.
Q23. Describe phytochrome and its role in flowering.
Answer:
Phytochrome is a light-sensitive pigment involved in regulating photoperiodic responses such as flowering. It exists in two interconvertible forms: Pr, which absorbs red light, and Pfr, which absorbs far-red light and is biologically active. The ratio of these two forms helps plants measure day length. Changes in phytochrome form trigger biochemical reactions that lead to flowering or inhibition of flowering depending on photoperiod.
Q24. What is vernalisation? Explain its importance.
Answer:
Vernalisation is the induction of flowering by exposure to low temperature for a specific period. It occurs in certain plants, particularly biennials and winter annuals, to ensure flowering happens at a favorable time. Vernalisation shortens the vegetative phase and prevents premature flowering before winter. This process helps plants synchronize flowering with suitable environmental conditions, enhancing reproductive success.
Q25. Differentiate between photoperiodism and vernalisation.
Answer:
Photoperiodism depends on the duration of light and darkness, while vernalisation depends on exposure to low temperature. Both regulate flowering but respond to different environmental cues. Photoperiodism ensures seasonal timing based on day length, whereas vernalisation ensures flowering only after cold exposure. Together, they help plants flower under optimal conditions, improving survival and reproduction.