Plant Growth and Development
Q1. What do you understand by growth?
Answer: Growth is defined as a permanent and irreversible increase in size, mass, volume or number of cells in an organism. In plants, growth occurs throughout life due to the presence of meristematic tissues.
Q2. Why is growth considered irreversible?
Answer: Growth involves cell division, elongation and differentiation, which permanently increase the size or number of cells. These changes cannot be reversed, hence growth is irreversible.
Q3. What are the phases of plant growth?
Answer: Plant growth occurs in three phases:
- Meristematic phase: Active cell division
- Elongation phase: Rapid increase in cell size
- Maturation phase: Cells differentiate and attain permanent shape
Q4. What is arithmetic growth? Give an example.
Answer: Arithmetic growth is linear growth where only one daughter cell continues to divide, while the other differentiates. Example: Growth of root or shoot in length.
Q5. What is geometric growth?
Answer: Geometric growth is exponential growth where both daughter cells continue to divide, resulting in rapid increase in cell number.
Q6. What are plant growth regulators (PGRs)?
Answer: Plant growth regulators are small organic compounds that influence physiological processes of plants at very low concentrations. They can promote or inhibit growth.
Q7. Name the five major classes of plant growth regulators.
Answer:
- Auxins
- Gibberellins
- Cytokinins
- Ethylene
- Abscisic acid (ABA)
Q8. Write two physiological effects of auxins.
Answer:
- Promote cell elongation in stems
- Maintain apical dominance
Q9. How do gibberellins help in seed germination?
Answer: Gibberellins stimulate synthesis of α-amylase in the aleurone layer, which converts starch into sugars needed for embryo growth.
Q10. What is bolting? Which hormone causes it?
Answer: Bolting is rapid elongation of internodes in rosette plants. It is caused by gibberellins.
Q11. What are cytokinins? Mention their functions.
Answer: Cytokinins are plant hormones that promote cell division. Functions:
- Delay senescence
- Promote lateral bud growth
- Help in nutrient mobilisation
Q12. What is Richmond–Lang effect?
Answer: Delay of senescence of leaves by cytokinins is called Richmond–Lang effect.
Q13. Why is ethylene called a gaseous hormone?
Answer: Ethylene is the only plant hormone that exists and functions in gaseous form.
Q14. Explain the triple response of ethylene.
Answer: Triple response includes:
- Reduced stem elongation
- Radial swelling of stem
- Horizontal growth due to exaggerated apical hook
Q15. What is abscisic acid? Write its functions.
Answer: Abscisic acid is a growth-inhibiting hormone. Functions:
- Maintains seed dormancy
- Induces stomatal closure during drought
- Helps plants tolerate stress
Q16. What is photoperiodism?
Answer: Photoperiodism is the response of plants to relative length of day and night, especially for flowering.
Q17. Which part of the plant perceives photoperiodic stimulus?
Answer: Leaves perceive photoperiodic stimulus through phytochrome.
Q18. What is vernalisation?
Answer: Vernalisation is the induction of flowering by exposure to low temperature for a specific period.
Q19. What is devernalisation?
Answer: Devernalisation is the reversal of vernalisation effect by exposure to high temperature.
Q20. What is seed dormancy? Why is it important?
Answer: Seed dormancy is failure of seed to germinate under favourable conditions. Importance:
- Prevents germination during unfavourable season
- Ensures survival and proper timing of growth
Q21. Name two methods of breaking seed dormancy.
Answer:
- Scarification
- Stratification
Q22. Which hormones regulate seed dormancy and germination?
Answer:
- ABA maintains dormancy
- Gibberellins break dormancy and promote germination
Q23. Distinguish between photoperiodism and vernalisation.
Answer: Photoperiodism depends on day–night length, while vernalisation depends on low temperature exposure.
Q24. Why is night length more important than day length in flowering?
Answer: Because flowering depends on uninterrupted dark period, as shown by night-break experiments.