Chapter 15: Plant Growth and Development – Short Answer Type Questions
CBSE Class 11 Biology Chapter 15 – Plant Growth and Development | Short 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: Short Answer Type Questions
Answer Length: 60–80 words
Syllabus: Strictly as per NCERT Biology Textbook
Section A: Growth in Plants (Q1–Q12)
Q1. Define growth in plants and state its main characteristics.
Answer:
Growth in plants refers to a permanent and irreversible increase in size, mass, volume, or number of cells. It is an energy-dependent process involving cell division, enlargement, and differentiation. Plant growth is usually indeterminate because meristematic tissues remain active throughout life. It is influenced by genetic factors, plant hormones, and environmental conditions such as light, temperature, water, and nutrients.
Q2. Why is plant growth considered indeterminate?
Answer:
Plant growth is considered indeterminate because plants retain actively dividing meristematic tissues throughout their life. These meristems, present at root and shoot apices, continuously produce new cells. Unlike animals, plants do not have a fixed body plan, allowing them to grow indefinitely and adapt their growth patterns according to environmental conditions and availability of resources.
Q3. Describe the three phases of plant growth.
Answer:
Plant growth occurs in three phases: meristematic, elongation, and maturation. In the meristematic phase, cells divide actively. During elongation, cells increase in size due to vacuole expansion and wall loosening. In the maturation phase, cells differentiate into specialized tissues, attain permanent shape, and perform specific functions, marking the completion of growth.
Q4. Explain the meristematic phase of growth.
Answer:
The meristematic phase is characterized by active cell division. Cells in this phase have thin primary cell walls, dense cytoplasm, prominent nuclei, and little or no vacuoles. This phase occurs in regions like root tips, shoot tips, and cambium. Rapid mitotic activity in this phase ensures continuous formation of new cells required for plant growth.
Q5. What changes occur during the elongation phase of growth?
Answer:
During the elongation phase, cells formed in the meristematic region increase rapidly in size. This occurs due to cell wall loosening, increased water uptake, vacuole enlargement, and synthesis of new cell wall materials. Metabolic activity is high, and this phase contributes significantly to the overall increase in length of roots and shoots.
Q6. Describe the maturation phase in plant growth.
Answer:
The maturation phase marks the final stage of growth where cells lose the capacity to divide and differentiate into specialized tissues. Cells attain definite shape, size, and function. Thickening of cell walls and development of specific structures occur. This phase ensures functional specialization of tissues like xylem, phloem, and epidermis.
Q7. Differentiate between arithmetic and geometric growth.
Answer:
Arithmetic growth involves an increase by a constant amount per unit time and produces a linear growth curve. Geometric growth occurs when growth rate is proportional to existing size, resulting in exponential growth. In plants, arithmetic growth is seen in root elongation, while geometric growth occurs when resources are unlimited and cells divide continuously.
Q8. What is growth rate? How is it expressed?
Answer:
Growth rate is 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 compare growth efficiency under different environmental conditions and is influenced by internal factors like hormones and external factors such as nutrients and temperature.
Q9. List the internal factors affecting plant growth.
Answer:
Internal factors affecting plant growth include genetic constitution and plant hormones. Genes determine the inherent growth potential and developmental pattern, while plant hormones regulate processes such as cell division, elongation, differentiation, dormancy, and senescence. Interaction among different hormones also plays a crucial role in controlling overall plant growth.
Q10. Mention the external factors required for plant growth.
Answer:
External factors necessary for plant growth include light, temperature, water, oxygen, and nutrients. Light influences photosynthesis and flowering, temperature affects enzyme activity, water maintains turgidity, oxygen supports respiration, and nutrients provide essential elements for synthesis of biomolecules required for growth and development.
Q11. Explain the role of water in plant growth.
Answer:
Water plays a vital role in plant growth by maintaining cell turgidity, enabling cell expansion, and acting as a solvent for metabolic reactions. It facilitates nutrient transport, photosynthesis, and enzymatic activities. Water deficiency restricts cell elongation and division, thereby reducing growth rate and overall plant productivity.
Q12. Why is temperature important for plant growth?
Answer:
Temperature influences enzyme activity and metabolic reactions involved in plant growth. Each plant has an optimum temperature range for growth. Extremely low or high temperatures inhibit enzyme function, reduce photosynthesis, and slow down growth processes. Proper temperature ensures efficient physiological activities essential for normal plant development.
Section B: Differentiation and Development (Q13–Q20)
Q13. What is differentiation in plants?
Answer:
Differentiation is the process by which meristematic cells develop into specialized cells with specific structure and function. It involves changes in cell size, shape, wall thickness, and metabolic activity. Through differentiation, simple cells form complex tissues such as xylem and phloem, enabling efficient transport and support functions in plants.
Q14. Explain dedifferentiation with an example.
Answer:
Dedifferentiation is the process in which mature, differentiated cells regain the capacity to divide. This occurs under certain conditions, allowing plants to form new meristematic tissues. For example, cortical cells dedifferentiate to form cork cambium during secondary growth, enabling the plant to increase girth and repair damaged tissues.
Q15. What is redifferentiation?
Answer:
Redifferentiation is the process by which dedifferentiated cells lose their ability to divide and again become specialized. After forming new tissues, these cells acquire specific structures and functions. For instance, cells formed by cambium differentiate into secondary xylem and phloem, contributing to conduction and mechanical support.
Q16. Define development in plants.
Answer:
Development in plants is the sum total of growth and differentiation occurring throughout the life cycle. It includes processes such as seed germination, vegetative growth, flowering, fruit formation, and senescence. Development is regulated by genetic factors, plant hormones, and environmental conditions like light and temperature.
Q17. Why are plants considered developmentally plastic?
Answer:
Plants are considered developmentally plastic because they can modify their growth and development in response to environmental conditions. The same plant can show different forms under different habitats. This plasticity allows plants to survive and adapt by altering growth patterns, flowering time, and structural organization.
Q18. Explain the relationship between growth and development.
Answer:
Growth and development are closely related processes. Growth provides the increase in size and number of cells, while development involves differentiation and maturation of these cells. Growth alone does not ensure functional specialization; development integrates growth with differentiation to form functional tissues and organs in plants.
Q19. How do genes influence plant development?
Answer:
Genes control the synthesis of enzymes and proteins required for growth and differentiation. They determine the developmental pattern, timing of flowering, and response to environmental signals. Although genes provide the blueprint for development, their expression is influenced by hormones and external factors, resulting in coordinated plant development.
Q20. State the importance of differentiation in plants.
Answer:
Differentiation is essential for forming specialized tissues and organs that perform specific functions. It allows efficient transport of water and nutrients, mechanical support, and photosynthesis. Without differentiation, plants would remain as undifferentiated cell masses, incapable of performing complex physiological processes necessary for survival.
Section C: Plant Hormones (Q21–Q35)
Q21. What are plant hormones?
Answer:
Plant hormones are small organic compounds produced in minute quantities that regulate growth, development, and physiological responses. They are synthesized in one part of the plant and transported to another where they exert their effect. Examples include auxins, gibberellins, cytokinins, ethylene, and abscisic acid.
Q22. Explain the role of auxins in plant growth.
Answer:
Auxins promote cell elongation, especially in stems, by increasing cell wall plasticity. They maintain apical dominance by suppressing lateral bud growth and help in root initiation. Auxins also regulate tropic movements and delay leaf abscission. Indole-3-acetic acid (IAA) is a common natural auxin.
Q23. Describe the functions of gibberellins.
Answer:
Gibberellins promote stem elongation by stimulating cell division and elongation. They induce bolting in rosette plants, break seed dormancy, and promote germination by activating enzymes. Gibberellins also influence flowering in some plants and increase fruit size, making them agriculturally significant hormones.
Q24. What are cytokinins and their functions?
Answer:
Cytokinins are plant hormones that promote cell division and delay senescence. They stimulate lateral bud growth, counteract apical dominance, and mobilize nutrients to growing regions. Cytokinins also help maintain chlorophyll and protein content in leaves, thereby prolonging photosynthetic activity.
Q25. Why is ethylene considered a unique plant hormone?
Answer:
Ethylene is unique because it is the only gaseous plant hormone. It regulates fruit ripening, senescence, and leaf abscission. Ethylene also exhibits the triple response in seedlings, including reduced elongation, increased lateral swelling, and horizontal growth, distinguishing it from other plant hormones.
Q26. Explain the triple response of ethylene.
Answer:
The triple response of ethylene includes three distinct effects on seedlings: inhibition of stem elongation, lateral swelling of the hypocotyl, and horizontal growth. This response helps seedlings navigate through soil obstacles and is a characteristic feature of ethylene action in plants.
Q27. Describe the role of abscisic acid in plants.
Answer:
Abscisic acid (ABA) is a growth-inhibiting hormone that induces dormancy in seeds and buds. It promotes stomatal closure during water stress, reducing transpiration. ABA also accelerates senescence and abscission, helping plants survive unfavorable environmental conditions, hence it is known as a stress hormone.
Q28. Why is ABA called a stress hormone?
Answer:
ABA is called a stress hormone because it helps plants cope with adverse conditions such as drought and cold. It induces stomatal closure to reduce water loss and inhibits growth to conserve energy. ABA also promotes dormancy, enabling plants to survive unfavorable periods.
Q29. Classify plant hormones based on their function.
Answer:
Plant hormones are classified into growth promoters and growth inhibitors. Growth promoters include auxins, gibberellins, and cytokinins, which stimulate growth processes. Growth inhibitors include abscisic acid and ethylene, which inhibit growth, promote dormancy, senescence, and abscission under certain conditions.
Q30. Mention two agricultural uses of plant hormones.
Answer:
Plant hormones are used to promote rooting in cuttings, induce flowering, increase fruit size, and regulate fruit ripening. Auxins are used as weed killers, gibberellins improve crop yield, and ethylene is used for artificial ripening of fruits in agricultural practices.
Q31. How do hormones regulate plant growth collectively?
Answer:
Plant growth is regulated by the combined action of different hormones rather than a single hormone. Hormonal balance, interaction, and concentration determine specific responses. For example, auxins and cytokinins together regulate apical dominance, while gibberellins and ABA have antagonistic effects on growth and dormancy.
Q32. Name the hormone responsible for delaying senescence.
Answer:
Cytokinins are responsible for delaying senescence in plant tissues. They maintain protein and chlorophyll levels by promoting nutrient mobilization. This delay helps leaves remain green and functional for a longer time, enhancing photosynthetic efficiency and plant productivity.
Q33. How does ethylene influence fruit ripening?
Answer:
Ethylene accelerates fruit ripening by increasing respiration rate and activating enzymes that convert starch into sugars. It promotes softening of fruit tissues, development of color, and characteristic flavor. Ethylene action ensures synchronized ripening, which is important for seed dispersal.
Q34. Compare auxins and cytokinins briefly.
Answer:
Auxins primarily promote cell elongation and maintain apical dominance, while cytokinins promote cell division and lateral bud growth. Auxins suppress lateral buds, whereas cytokinins counteract this effect. Together, they regulate balanced growth and branching patterns in plants.
Q35. What is the importance of hormonal balance in plants?
Answer:
Hormonal balance ensures coordinated growth and development. Excess or deficiency of a hormone can disrupt normal physiological processes. Balanced interaction among hormones regulates growth rate, differentiation, flowering, dormancy, and stress responses, enabling plants to adapt effectively to environmental conditions.
Section D: Photoperiodism and Vernalisation (Q36–Q50)
Q36. Define photoperiodism.
Answer:
Photoperiodism is the physiological response of plants to the relative length of day and night, particularly affecting flowering. Plants detect day length using light-sensitive pigments and initiate flowering only when specific photoperiodic conditions are met.
Q37. What is the critical photoperiod?
Answer:
Critical photoperiod is the specific duration of light or darkness required by a plant to initiate flowering. It varies among plant species and determines whether a plant behaves as a short-day, long-day, or day-neutral plant.
Q38. Explain short-day plants with an example.
Answer:
Short-day plants flower when the day length is shorter than the critical photoperiod and nights are longer. They require uninterrupted darkness for flowering. Examples include rice and chrysanthemum. Interruption of the dark period can prevent flowering in these plants.
Q39. Explain long-day plants with an example.
Answer:
Long-day plants flower when the day length exceeds the critical photoperiod and nights are shorter. They require longer exposure to light for flowering. Examples include wheat and spinach, which typically flower during late spring or early summer.
Q40. What are day-neutral plants?
Answer:
Day-neutral plants flower irrespective of day length and are not influenced by photoperiod. Their flowering depends mainly on age or internal factors rather than light duration. Examples include tomato and cotton.
Q41. What is phytochrome?
Answer:
Phytochrome is a light-sensitive pigment involved in regulating photoperiodic responses like flowering. It exists in two interconvertible forms, Pr and Pfr, which absorb red and far-red light. The balance between these forms controls plant responses to light.
Q42. Explain the two forms of phytochrome.
Answer:
Phytochrome exists as Pr, which absorbs red light and is inactive, and Pfr, which absorbs far-red light and is biologically active. Conversion between these forms enables plants to measure day length and regulate processes like flowering and seed germination.
Q43. Define vernalisation.
Answer:
Vernalisation is the induction of flowering by exposure to low temperature for a specific period. It occurs in certain plants to ensure flowering takes place during favorable seasons, preventing premature flowering before winter conditions end.
Q44. Why is vernalisation important for some plants?
Answer:
Vernalisation ensures timely flowering by shortening the vegetative phase. It allows plants to flower only after experiencing cold conditions, preventing damage to reproductive structures and ensuring better seed production during suitable environmental conditions.
Q45. Which plants require vernalisation?
Answer:
Biennial plants and winter annuals commonly require vernalisation. These plants complete vegetative growth during one season and flower only after exposure to prolonged cold, ensuring flowering occurs under favorable conditions.
Q46. What is devernalisation?
Answer:
Devernalisation is the reversal of the effect of vernalisation by exposure to high temperatures. It prevents flowering even after cold treatment, indicating that vernalisation effects are temperature-dependent and can be modified by environmental conditions.
Q47. How does vernalisation differ from photoperiodism?
Answer:
Vernalisation depends on exposure to low temperature, while photoperiodism depends on day length. Both regulate flowering but respond to different environmental cues. Vernalisation ensures seasonal timing, whereas photoperiodism synchronizes flowering with appropriate light conditions.
Q48. Explain the ecological significance of photoperiodism.
Answer:
Photoperiodism ensures flowering occurs at a time favorable for pollination, seed formation, and dispersal. It synchronizes reproductive events with seasonal changes, increasing survival and reproductive success of plant species in different climatic regions.
Q49. How do environmental factors influence flowering?
Answer:
Environmental factors such as light duration and temperature influence flowering through photoperiodism and vernalisation. These factors interact with plant hormones and genetic controls to regulate the transition from vegetative to reproductive phase.
Q50. Why are photoperiodism and vernalisation important in agriculture?
Answer:
Understanding photoperiodism and vernalisation helps farmers manipulate flowering time and crop yield. Controlled light exposure and temperature treatment improve crop adaptation, ensure synchronized flowering, and enhance productivity in different climatic conditions.