Plant Growth and Development

Growth is an irreversible permanent increase in the size of an organ, its parts or even an individual cell. Plant growth is unique because plants retain meristems that can divide throughout life, giving an open form of growth. Growth can be measured; a maize root apical meristem forms over 17,500 new cells per hour. It describes the three phases of growth (meristematic, elongation and maturation), arithmetic and geometric growth with their equations, and the sigmoid growth curve. You will also compare absolute and relative growth rates and learn the conditions for growth: water, oxygen, nutrients and temperature. You will then study differentiation, dedifferentiation and redifferentiation, the plasticity of development shown by heterophylly in cotton, coriander and larkspur, and the five groups of plant growth regulators. These are auxins, discovered from the experiments of Charles and Francis Darwin and isolated by F.W. Went; gibberellins, discovered by E. Kurosawa from the bakanae disease of rice; cytokinins, discovered by Skoog and Miller; the gaseous hormone ethylene; and abscisic acid, the stress hormone.

Detailed Worksheet: Plant Growth and Development Section A - Definitions (10 marks) 1. Define growth. Why is plant growth called open growth? (2 marks) 2. Distinguish between dedifferentiation and redifferentiation with examples. (2 marks) 3. What is apical dominance? Which hormone causes it? (2 marks) 4. What is heterophylly? Give two examples. (2 marks) 5. Why is abscisic acid called the stress hormone? (2 marks) Section B - Calculations and Applications (15 marks) 6. In arithmetic growth, the length of a root increases according to Lt = L0 + rt. A root 2 cm long grows at a constant rate of 0.5 cm per day. Calculate its length after 10 days. Draw a sketch of the expected graph. (3 marks) 7. Leaf A grows from 5 square cm to 10 square cm and leaf B from 50 square cm to 55 square cm in the same time. Calculate the absolute growth and the relative growth rate (as a percentage) of each leaf. Which leaf has the higher relative growth rate? (3 marks) 8. In geometric growth, a cell population doubles every 2 hours. Starting with 1,000 cells, calculate the number after 12 hours. Why does geometric growth not continue indefinitely? (3 marks) 9. A maize root apical meristem produces about 17,500 new cells per hour. Calculate the number of new cells produced in a day and in a week. (3 marks) 10. Spraying sugarcane with gibberellins can increase its stem length and yield by up to 20 tonnes per acre. If a farmer's field of 5 acres normally yields 300 tonnes, calculate the possible total yield after spraying. Why does the stem elongate? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled sigmoid growth curve and mark the lag, log (exponential) and stationary phases. (4 marks) 12. Draw a labelled diagram of the Darwins' experiment on canary grass coleoptiles. (3 marks) 13. Draw labelled graphs comparing arithmetic and geometric growth. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain how auxins, gibberellins and cytokinins are used in agriculture and horticulture, with examples. (5 marks) 15. 'Ethylene and abscisic acid are mostly growth inhibitors, but they also have important positive roles.' Discuss this statement with examples. (5 marks) 16. Explain the concept of plasticity in plant development with examples. How do the environment and plant growth regulators control development? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw graphs and diagrams neatly with labels. Show the formula and steps in calculations.