YoExam

Menu

Plant Growth and Development NCERT Highlights Line by Line for Class 11 & NEET

Understand the hormonal control of plant life. This resource gives you Plant Growth and Development NCERT Highlights Line by Line, covering all five major plant growth regulators and their physiological effects. Key lines on photoperiodism, vernalization, and seed dormancy are underlined to help you focus on high-priority conceptual topics.

Summary of Chapter: Plant Growth and Development NCERT Highlights Line by Line

Growth is defined as an irreversible permanent increase in size of an organ, part, or individual cell. Plant growth is generally indeterminate, meaning plants retain the capacity for unlimited growth due to the presence of meristems (the open form of growth).

Stages and Rates of Growth

Plant growth is divided into three phases:

  1. Meristematic Phase: Characterized by constant cell division in the root and shoot apices. Cells are rich in protoplasm and have large nuclei.

  2. Elongation Phase: Cells posterior to the meristematic region undergo rapid enlargement and formation of new cell wall material.

  3. Maturation Phase: Cells attain their maximal size and specific function, leading to differentiation.

Growth Rates: The increased growth per unit time.

  • Arithmetic Growth: Only one daughter cell continues to divide, while the other differentiates. Example is found in root elongating at a constant rate. A plot of length against time is a straight line.

  • Geometric Growth: Both progeny cells retain the ability to divide. The growth curve is Sigmoid (S-shaped), showing a Lag phase, a Log (Exponential) phase (where growth is fastest), and a Stationary phase.

Development, Differentiation, and Plasticity

  • Development: It is the sum of growth and differentiation. It is a sequence of events from seed germination to senescence.

  • Differentiation: Cells maturing to perform specific functions, often losing the capacity to divide (e.g., forming water-conducting tracheary elements).

  • Dedifferentiation: Living differentiated cells regaining the capacity of division (e.g., formation of interfascicular cambium).

  • Redifferentiation: Dedifferentiated cells losing the capacity to divide again and maturing (e.g., secondary cortex cells).

  • Plasticity: The ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures.

    • Example: Heterophylly in Buttercup (leaves in air are different from leaves in water) and in Cotton and Larkspur (juvenile leaves differ from mature ones).

Plant Growth Regulators (PGRs)

PGRs are small, simple molecules of diverse chemical composition. They are also called plant hormones or phytohormones.

PGR Key Functions and Discovery Highlights
Auxins Discovery: First isolated by F.W. Went from oat coleoptile tips, following the work of Charles and Francis Darwin on phototropism in canary grass.
  Functions: Initiate rooting in stem cuttings, promote flowering (pineapples), prevent fruit drop at early stages, and cause apical dominance (inhibition of lateral buds by the apical bud). 2, 4-D (a synthetic auxin) is used as a weedicide to kill dicot weeds.
Gibberellins (GAs) Discovery: Identified from the fungal disease ‘Bakanae’ (foolish seedling) of rice caused by the fungus Gibberella fujikuroi. GA3 is the most studied form.
  Functions: Cause elongation of the axis (increasing length of grape stalks), delay senescence, and promote bolting (internode elongation) in cabbages and beet. Increase sugarcane yield by up to 20 tonnes per acre.
Cytokinins Discovery: Active substance Kinetin was crystallized from autoclaved herring sperm DNA; it does not occur naturally in plants. Zeatin was isolated from corn-kernels and coconut milk.
  Functions: Synthesised where rapid cell division occurs (roots, developing shoots). Promote production of new leaves, help overcome apical dominance, and promote nutrient mobilisation (delaying leaf senescence).
Ethylene Discovery: Cousins confirmed a volatile substance from ripened oranges hastened the ripening of stored unripened bananas. It is a gaseous PGR.
  Functions: Promotes rapid fruit ripening (associated with Respiratory Climactic), breaks seed/bud dormancy, initiates germination (peanut seeds), and promotes root growth and root hair formation. Causes the triple response (horizontal growth, swelling of axis, apical hook formation) in dicot seedlings.
Abscisic Acid (ABA) Discovery: Known as inhibitor-B, abscission II, or dormin. It is often called the stress hormone.
  Functions: Stimulates the closure of stomata (tolerance to water stress), promotes seed dormancy (helps seeds withstand desiccation), and inhibits seed germination. It acts as an antagonist to GAs in most situations.

Photoperiodism and Vernalisation

  • Photoperiodism: The response of plants to the relative lengths of day and night.

    • Short Day Plants (SDP): Require a light period less than a critical duration to flower.

    • Long Day Plants (LDP): Require a light period exceeding a critical duration to flower.

    • Day Neutral Plants (DNP): Flowering is independent of light duration.

    • The site of light perception is the leaves. A hypothetical flowering hormone called Florigen is thought to migrate from leaves to shoot apices to induce flowering.

  • Vernalisation: The requirement for a period of low temperature for some plants to flower. It prevents pre-cocious reproductive development.

    • Examples: Winter varieties of Wheat, Barley, and Rye. Also seen in Biennials (like Sugarbeet, Cabbages, Carrots), which are monocarpic plants that usually flower and die in the second season. Exposing them to cold simulates flowering.