NCERT Solutions for Class 11 Biology Chapter 13 Plant Growth and Development

Short Note:

• Plant Growth: An irreversible permanent increase in size, volume, or mass of an organ or its individual parts, accompanied by metabolic processes.

• Open Form of Growth: New cells are continuously added to the plant body by the activity of meristems (apical and lateral meristems).

• Phases of Growth:

  - Meristematic Phase: Cells actively dividing, thin cellulosic walls, abundant plasmodesmata, and dense cytoplasm.

  - Elongation Phase: Increased vacuolation, cell enlargement, and new cell wall deposition.

  - Maturation Phase: Cells attain maximal size, wall thickening, and structural modifications.

• Types of Growth Rates:

  - Arithmetic Growth: Only one daughter cell divides while the other differentiates (e.g., root elongation). Equation: Lt = L0 + rt.

  - Geometric Growth: Both daughter cells retain division capability, producing an S-shaped (Sigmoid) growth curve with Lag, Log (Exponential), and Stationary phases. Equation: W1 = W0ert.

Short Note:

• Differentiation: Process where meristematic cells undergo structural and functional changes to mature (e.g., loss of protoplasm to form tracheary elements).

• Dedifferentiation: Process by which mature, living, differentiated cells regain the capacity to divide (e.g., formation of interfascicular cambium and cork cambium from parenchyma).

• Redifferentiation: Process where cells produced by dedifferentiated meristems lose division power again to perform specific mature functions (e.g., secondary xylem, secondary phloem, cork).

• Development & Plasticity: Development includes all changes from seed germination to senescence. Plasticity is the ability of plants to follow different pathways in response to environment or life phases to form different structures (e.g., Heterophylly in Cotton, Coriander, Larkspur, and Buttercup).

Short Note:

• Plant Growth Promoters:

  - Auxins (IAA, IBA, NAA, 2,4-D): Discovered by F.W. Went (Avena curvature test). Promotes apical dominance, root initiation, parthenocarpy, and acts as selective herbicide (2,4-D kills broad-leaved weeds).

  - Gibberellins (GA₃): Discovered by E. Kurosawa in rice seedlings infected by Gibberella fujikuroi. Causes stem bolting (internode elongation) in rosette plants, breaks seed dormancy, and increases sugarcane yield.

  - Cytokinins (Zeatin, Kinetin): Discovered by Skoog and Miller. Promotes cell division, lateral shoot growth, delays leaf senescence (Richmond-Lang effect).

• Plant Growth Inhibitors:

  - Ethylene (Gaseous hormone): Promotes fruit ripening, triple response in seedlings, root hair formation, and female flowering in cucumbers.

  - Abscisic Acid (ABA): Known as the Stress Hormone. Promotes stomatal closure during water stress, induces seed dormancy, and inhibits cell growth.

Short Note:

• Photoperiodism: Response of plants to the relative duration of day and night light exposure for flowering.

  - Long-Day Plants (LDP): Require light duration above critical photoperiod.

  - Short-Day Plants (SDP): Require light exposure below critical photoperiod.

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

• Vernalisation: Quantitative or qualitative dependence of flowering on low-temperature treatment (chilling effect, e.g., winter vs. spring varieties of wheat, barley, and rye).

• Seed Dormancy: State where seeds fail to germinate even under favorable environmental conditions due to endogenous factors (e.g., hard seed coat, chemical inhibitors like ABA, para-ascorbic acid).