Plant Tissues – Short Notes
Plant Tissues
A tissue is a group of cells having a common origin, similar structure, and performing a specific function. Plant tissues are classified into two major categories based on their capacity for cell division:
1. Meristematic Tissues
2. Permanent Tissues
1. Meristematic Tissues
1.1 Key Characteristics & Structure
- Mitotic Activity: Composed of actively dividing, undifferentiated cells responsible for plant growth.
- Cell Wall: Thin primary cell wall made mostly of cellulose.
- Cytoplasm & Nucleus: Dense cytoplasm (cell fluid) with a large, prominent central nucleus.
- Vacuolation: Vacuoles (fluid-filled storage sacs) are absent or extremely small to allow unhindered cell division.
- Arrangement: Cells are small, spherical, oval, or polygonal, arranged compactly with no intercellular spaces (gaps between cells).
1.2 Classification by Location
1. Apical Meristem:
- Location: Tips of growing shoots (apical bud) and roots (root tip).
- Role: Responsible for primary growth, extending the height of the plant and depth of roots.
2. Lateral Meristem (Vascular Cambium and Cork Cambium):
- Location: Arranged laterally parallel to the long axis of stems and roots.
- Role: Responsible for secondary growth, increasing the girth (thickness/diameter) of stems and roots.
3. Intercalary Meristem:
- Location: Situated at the base of nodes or leaves (predominantly in monocots/grasses).
- Role: Enables rapid stem elongation and regrowth of leaves eaten by herbivores.
2. Permanent Tissues
Permanent tissues develop from meristematic tissues that have differentiated, specialized, and lost the capacity to divide.
Permanent tissues are subdivided into:
- Simple Permanent Tissues (composed of only one type of cell)
- Complex Permanent Tissues (composed of multiple cell types working together)
3. Simple Permanent Tissues
3.1 Parenchyma Tissue
- Cell Nature: Living cells at maturity.
- Cell Wall: Thin, flexible primary cell wall composed of cellulose.
- Structure: Isodiametric shape (equally wide in all directions), containing a large central vacuole (fluid storage sac).
- Intercellular Spaces: Prominent intercellular spaces are present.
- Locations: Found throughout soft plant organs — stem and root cortex, leaf mesophyll, pith, and fruit pulp.
- Functions:
- Storage: Stores starch, water, and essential nutrients.
- Photosynthesis (making food using sunlight): Parenchyma containing chloroplasts (green organelles) in leaves is termed Chlorenchyma.
- Buoyancy: In aquatic plants, parenchyma with large air spaces is termed Aerenchyma, facilitating gas exchange and floating.
- Wound Healing: Retains potential to divide during tissue repair.
3.2 Collenchyma Tissue
- Cell Nature: Living cells at maturity.
- Cell Wall: Unevenly thickened primary cell wall with deposits of pectin and cellulose, particularly concentrated at the corners where cells meet.
- Structure: Elongated cells with small or no intercellular spaces.
- Locations: Located beneath the epidermis (outer protective layer) of young stems, petioles (leaf stalks), and leaf midribs of dicots.
- Functions:
- Provides mechanical support and tensile strength to young, growing plant stems and petioles without restricting growth.
- Gives flexibility to plant parts, allowing bending under wind pressure without breaking.
3.3 Sclerenchyma Tissue
- Cell Nature: Dead cells at maturity (lack protoplasm/living contents).
- Cell Wall: Secondary cell wall uniformly thickened with lignin (a hard, water-impermeable organic polymer).
- Intercellular Spaces: Absent.
- Types of Sclerenchyma:
- Sclerenchyma Fibers: Long, narrow, tapered cells with pointed ends (e.g., coconut husk/coir, jute).
- Sclereids (Stone Cells): Short, irregularly shaped, extremely hard cells (e.g., shell of coconut, seed coats, gritty texture in pear fruit).
- Locations: Vascular bundles, hard seed coats, nut shells, and stems.
- Functions:
- Provides maximum rigidity, structural support, and hardness to mature plant structures.
- Protects internal soft tissues from mechanical stress.
4. Complex Permanent Tissues
4.1 Xylem Tissue
Xylem conducts water and dissolved mineral salts from the roots up to the leaves (unidirectional transport) and provides mechanical strength.
Components of Xylem:
1. Xylem Vessels:
- Continuous, hollow, tube-like structures formed by long rows of dead cells joined end-to-end.
- End walls are completely perforated or dissolved.
- Walls heavily thickened with lignin. Main water-conducting channels in angiosperms.
2. Tracheids:
- Elongated, narrow dead cells with tapering, overlapping ends.
- Cell walls are lignified with pits (thin areas in the wall) for lateral water movement.
3. Xylem Parenchyma:
- The only living component in xylem.
- Functions in short-distance storage of food and lateral conduction of water.
4. Xylem Fibers:
- Highly lignified dead sclerenchymatous fibers providing mechanical reinforcement.
4.2 Phloem Tissue
Phloem translocates synthesized organic food substances (sucrose/sugar) from leaves (photosynthetic sources) to other non-photosynthetic parts (sinks) such as roots, stems, fruits, and seeds (bidirectional transport).
Components of Phloem:
1. Sieve Tube Elements:
- Living, elongated tubular cells connected end-to-end.
- End walls feature sieve plates containing tiny pores for food transiting.
- Mature sieve tube elements lack a nucleus, ribosomes, and large vacuoles to allow unobstructed movement of food solution.
2. Companion Cells:
- Specialized living parenchymatous cells with a prominent nucleus and dense cytoplasm, connected to sieve tubes via plasmodesmata (cytoplasmic channels).
- Regulate metabolic activities and energy supply (ATP) for adjacent sieve tube elements.
3. Phloem Parenchyma:
- Living cells involved in food storage and lateral transfer.
4. Phloem Fibers:
- Dead sclerenchymatous fibers that provide structural support to the phloem tissue.
5. Summary Table: Comparison of Plant Tissues
| Tissue Type | Living/Dead at Maturity | Cell Wall Thickness & Composition | Main Function | Primary Locations |
| Meristematic | Living | Thin; cellulose | Active cell division & growth | Shoot & root tips, cambium |
| Parenchyma | Living | Thin; cellulose | Storage, photosynthesis | Cortex, pith, leaf mesophyll |
| Collenchyma | Living | Unevenly thickened; pectin & cellulose | Flexible mechanical support | Sub-epidermis of young stems, petioles |
| Sclerenchyma | Dead | Thick; heavily lignified | Rigid structural protection | Coconut coir, nut shells, seed coats |
| Xylem | Mostly Dead (except parenchyma) | Thick; lignified | Conducts water & minerals | Vascular bundles |
| Phloem | Mostly Living (except fibers) | Primary cellulose walls | Translocates sucrose/food | Vascular bundles |
Grade 11 Science English Medium – Plant Tissues