🌿 Plant Tissues – Meristematic and Permanent
Plant tissues are groups of similar cells performing specific functions. They are classified into meristematic (actively dividing) and permanent (differentiated) tissues.
🌱 Meristematic Tissues
These tissues consist of actively dividing cells responsible for plant growth.
🔹 Characteristics
- Cells small, thin-walled, rich in cytoplasm.
- No intercellular spaces.
- Large nucleus, dense protoplasm.
- Vacuoles are absent or very small.
🔸 Classification of Meristems
- Based on Origin:
- Pro-meristem: Found in embryo; gives rise to primary meristem.
- Primary meristem: Causes primary growth (root and shoot apices).
- Secondary meristem: Causes secondary growth (cork cambium, vascular cambium).
- Based on Location:
- Apical meristem: At root and shoot tips; increases length.
- Intercalary meristem: Between mature tissues; causes elongation (e.g., grass nodes).
- Lateral meristem: In cambium; increases girth.
🌾 Permanent Tissues
These tissues are formed when meristematic cells stop dividing and specialize for specific functions.
🔹 Types of Permanent Tissues
- Simple Permanent Tissue: Made of one type of cell.
- Parenchyma: Living cells with thin walls, stores food and water.
- Collenchyma: Living cells with thickened corners, provides mechanical support and flexibility.
- Sclerenchyma: Dead, lignified cells; provides rigidity (fibres and sclereids).
- Complex Permanent Tissue: Made of more than one type of cell working together.
- Xylem: Conducts water and minerals upward.
- Tracheids, Vessels, Xylem parenchyma, Xylem fibres.
- Phloem: Conducts food from leaves to other parts.
- Sieve tubes, Companion cells, Phloem parenchyma, Phloem fibres.
🌼 Differences Between Xylem and Phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Function | Water transport | Food transport |
| Cell type | Mostly dead | Mostly living |
| Direction | Upward | Bidirectional |
| Main elements | Tracheids, vessels | Sieve tubes, companion cells |
🍃 Complex Tissue Arrangement
- Xylem and phloem together form vascular bundles.
- Found in roots, stems, and leaves for efficient conduction.
Vascular Bundle Types
- Radial: Xylem and phloem alternate (root).
- Conjoint: Xylem and phloem together (stem and leaf).
- Collateral: Phloem outside xylem (e.g., sunflower stem).
- Bicollateral: Phloem on both sides (e.g., cucurbita).
- Concentric: One surrounds the other (e.g., fern).
💡 Questions and Answers
Q1. Differentiate between apical and lateral meristem.
A1. Apical meristem increases length (found at tips), while lateral meristem increases thickness (found in cambium).
Q2. Why is xylem called a complex tissue?
A2. Because it contains multiple cell types (tracheids, vessels, fibres, and parenchyma) performing different functions.
Q3. What are the functions of collenchyma?
A3. Provides mechanical strength and elasticity to young stems and petioles.