Learning Objectives

• Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem, and growth rate.
• Explain the significance of meristems in plant growth.
• Describe the phases of growth: lag, log, and senescent.
• Differentiate between arithmetic and geometric growth.
• Identify the roles of intrinsic and extrinsic factors in plant growth and development.
• Discuss the physiological effects of plant growth regulators (PGRs).
• Analyze the relationship between cell structure and function in plant tissues.
• Illustrate the process of seed germination and the conditions necessary for growth.
• Explain the concept of plasticity in plant development.

Plant Growth and Development

13.1 Growth

• Growth is defined as an irreversible increase in size of an organ or its parts or even of an individual cell.
• Plant growth is unique due to the presence of meristems, allowing for unlimited growth throughout their life.
• Growth can be categorized into two types:
  • Arithmetic Growth: Only one daughter cell continues to divide after mitotic division.
  • Geometric Growth: Both daughter cells retain the ability to divide.

13.1.1 Conditions for Growth

• Essential conditions include:
  • Water
  • Oxygen
  • Nutrients
  • Optimum temperature range

13.1.4 Growth Rates

• Growth rates can be expressed mathematically:
  • Arithmetic Growth:
    • Formula: Lₜ = L₀ + rt
      • Lₜ = length at time 't'
      • L₀ = length at time 'zero'
      • r = growth rate / elongation per unit time.
  • Geometric Growth:
    • Formula: W₁ = W₀e^(rt)
      • W₁ = final size (weight, height, number, etc.)
      • W₀ = initial size at the beginning of the period
      • r = growth rate
      • t = time of growth
      • e = base of natural logarithms.

13.2 Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: The process where cells mature to perform specific functions.
• Dedifferentiation: Living differentiated cells regain the capacity to divide under certain conditions.
• Redifferentiation: Cells that lose the capacity to divide mature to perform specific functions again.

13.3 Development

• Development encompasses all changes an organism undergoes during its life cycle from germination to senescence.
• Development is influenced by both intrinsic factors (like plant growth regulators) and extrinsic factors (like light and temperature).

Important Diagrams

Diagram of Cell Division Phases

• Arithmetic Phase: One cell divides, one differentiates.
• Geometric Phase: All cells divide, increasing cell number exponentially.

Diagram of Carpel Structure

• Stigma: Top part where pollen lands.
• Style: Tube connecting stigma to ovary.
• Ovary: Contains ovules that develop into seeds after fertilization.

Diagram of Plant Cell Life Cycle

• Stages: Meristematic cell → Cell Division → Plasmatic Growth → Differentiation → Maturation → Senescence → Death.

Common Mistakes and Exam Tips in Plant Growth and Development

Common Pitfalls

• Misunderstanding Growth Phases: Students often confuse the different phases of growth (lag, log, and senescent). It's crucial to understand that growth is not sustained at a high rate throughout the life of a plant.
• Confusing Differentiation with Dedifferentiation: Differentiation is the process where cells mature to perform specific functions, while dedifferentiation refers to mature cells regaining the ability to divide. Be clear on these definitions.
• Ignoring the Role of Environmental Factors: Many students overlook how extrinsic factors like light and temperature influence plant growth and development. Remember that these factors can interact with intrinsic factors like plant growth regulators (PGRs).

Exam Tips

• Define Key Terms Clearly: Be prepared to define terms such as growth, differentiation, dedifferentiation, and redifferentiation. Clear definitions can earn you easy marks.
• Use Diagrams: When applicable, use diagrams to illustrate your answers, especially for processes like seed germination or the structure of plant organs. Labeling diagrams correctly can enhance your responses.
• Understand PGR Functions: Familiarize yourself with the five main groups of PGRs (auxins, gibberellins, cytokinins, abscisic acid, and ethylene) and their physiological effects. Be ready to explain how they interact in various growth processes.
• Practice Application Questions: Be prepared for scenario-based questions, such as predicting the effects of applying specific PGRs to plants or understanding the consequences of environmental changes on plant growth.
• Review Growth Rates: Understand the differences between arithmetic and geometric growth, and be able to apply the formulas for calculating growth rates in different contexts.