1.1 Organisation of Plant Tissues
Plant tissues are categorized into two main types: Meristematic Tissues and Permanent Tissues.
1. Meristematic Tissues
Undifferentiated, actively dividing tissues through mitosis.
- Apical Meristems: Located at the tips of shoots and roots; responsible for primary growth (length/height).
- Lateral Meristems: Located along the sides of stems and roots (e.g., vascular cambium and cork cambium); responsible for secondary growth (girth/diameter).
2. Permanent Tissues
Mature tissues that have differentiated or are differentiating to perform specific functions.
A. Dermal Tissue (Epidermis)
- Outermost layer covering stems, roots, and leaves.
- Cuticle layer reduces water loss via transpiration.
- Modified epidermal cells include guard cells (regulate stomatal opening) and root hair cells (increase surface area for water absorption).
B. Ground Tissues
- Parenchyma Tissue: Thin primary cell walls, large central vacuole. Functions: photosynthesis, food/starch storage, gas exchange, and healing/regeneration.
- Collenchyma Tissue: Unevenly thickened primary cell walls (thickened with cellulose and pectin). Provides flexible structural support to young, herbaceous plants and growing stems.
- Sclerenchyma Tissue: Uniformly thickened secondary cell walls with lignin. Dead at maturity. Provides rigid mechanical support and protection to mature plant parts.
C. Vascular Tissues
- Xylem: Made of vessel elements and tracheids (dead at maturity, walls thickened with lignin). Transports water and inorganic mineral ions from roots to leaves and provides mechanical support.
- Phloem: Made of sieve tubes (lacks nuclei at maturity) and companion cells. Transports organic substances (sucrose and amino acids) from leaves to other parts via translocation.
1.2 Structural Adaptations of Tissues for Photosynthesis and Transport
- Palisade Mesophyll: Closely packed columnar cells with high chloroplast density to maximize light absorption.
- Spongy Mesophyll: Loosely arranged irregularly shaped cells with extensive intercellular air spaces to facilitate gas exchange ($CO_2$ and $O_2$).
- Xylem Vessels: Hollow, continuous tubes without cross walls or cytoplasm, allowing unimpeded water flow driven by transpiration pull.
- Sieve Tubes: Connected end-to-end with perforated sieve plates, allowing efficient translocation of organic nutrients assisted by metabolic energy from companion cells.
1.3 Zone of Cell Growth
Growth at root and shoot tips occurs across three distinct zones:
- Zone of Cell Division: Occurs at the apical meristem. Cells actively divide by mitosis to increase cell count.
- Zone of Cell Elongation: Cells absorb water by osmosis and undergo vacuolation; cells elongate and expand rapidly.
- Zone of Cell Differentiation: Cells reach full size and undergo structural changes to form specialized permanent tissues (e.g., xylem, phloem, epidermis).
1.4 Types of Growth
1. Primary Growth
- Occurs in all woody and non-woody plants.
- Driven by apical meristems at shoot and root tips.
- Increases plant height and root length to absorb light and reach soil minerals.
2. Secondary Growth
- Occurs mainly in eudicots (dicots) and gymnosperms; minimal in monocots.
- Driven by lateral meristems: Vascular Cambium and Cork Cambium.
- Increases stem and root diameter (girth) to provide structural support and additional transport tissues.
Process of Secondary Growth in Stems:
- Vascular cambium divides inward to form secondary xylem (wood) and outward to form secondary phloem.
- Accumulation of secondary xylem creates pressure that ruptures the primary epidermis.
- Cork cambium forms beneath the epidermis and divides to produce cork cells containing suberin (waterproof material) on the outside, forming bark.
3. Annual Rings
In temperate regions, seasonal variations affect vascular cambium activity:
- Spring (Springwood): Abundant water/light produces large xylem vessels with thin walls (light-colored ring).
- Summer/Autumn (Autumnwood): Limited water produces small xylem vessels with thick walls (dark-colored ring).
- One light ring + one dark ring = 1 Annual Ring (used to estimate tree age).
1.5 Growth Curves
Plant growth is measured by parameter changes over time (e.g., dry mass, height, or fresh mass).
- Sigmoid Growth Curve: Standard growth curve shape (S-shaped) for annual plants and individual organs.
- Lag Phase: Slow growth rate (cell division and adaptation).
- Exponential Phase: Rapid growth rate (active elongation and division).
- Decelerating Phase: Growth slows down due to aging or limited resources.
- Stationary Phase: Growth rate reaches zero (maturity).
- Senescence Phase: Negative growth rate (aging and death).
- Annual Plants: Complete life cycle in 1 year; single sigmoid curve followed by death.
- Biannual Plants: Complete life cycle in 2 years; two sigmoid curves (Year 1: vegetative growth; Year 2: flowering/reproduction).
- Perennial Plants: Live for many years; series of continuous stepwise sigmoid curves (growth increases every spring/summer).