Chapter Tissue - Class 9

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What is a Tissue?


A tissue is a group of similar cells that work together to perform a specific function. In multicellular organisms like plants and animals, tissues are essential because they allow for division of labor, meaning different tissues perform different functions, making the organism more efficient.

Types of Tissues

There are two main types of tissues based on the organisms:

  1. Plant Tissues
  2. Animal Tissues

Let's explore each type in detail.

 



 

Plant Tissues

Plant tissues are grouped into two main categories: Meristematic and Permanent tissues.

1. Meristematic Tissue

  • Function: Responsible for the growth of plants.
  • Characteristics: Cells are small, have thin walls, are densely packed, and have large nuclei.
  • Types:
    • Apical Meristem: Found at the tips of roots and shoots, helps in increasing the length.
    • Lateral Meristem: Found along the sides of roots and shoots, helps in increasing the girth.
    • Intercalary Meristem: Found at the base of leaves or internodes, helps in the growth of leaves and internodes.

 

Meristematic Tissue - Details

Definition: Meristematic tissue is a type of plant tissue that is made up of undifferentiated cells. These cells are responsible for the growth of plants. They continuously divide, leading to the formation of new cells, which eventually differentiate into various specialized tissues.

Features of Meristematic Tissue

  1. Cell Structure:
    • Small Cells: The cells are usually small and closely packed.
    • Thin Cell Walls: They have thin primary cell walls made of cellulose.
    • Dense Cytoplasm: The cells have a dense cytoplasm with a prominent nucleus.
    • No Vacuoles: They generally lack large vacuoles, which are prominent in mature plant cells.
  2. High Mitotic Activity:
    • The cells in meristematic tissue are constantly dividing through the process of mitosis.
  3. Lack of Specialized Structures:
    • These cells do not have specialized structures like chloroplasts or large vacuoles because their main function is to divide and create new cells.

Classification of Meristematic Tissue

 

 

Meristematic tissues are classified based on their location in the plant:

  1. Apical Meristem:
    • Location: Found at the tips of roots and shoots.
    • Function: Responsible for the increase in length of the plant. This type of growth is called primary growth.
    • Examples: Root tips and shoot tips.
  2. Lateral Meristem:
    • Location: Found along the sides of stems and roots.
    • Function: Responsible for the increase in girth (thickness) of the plant. This type of growth is called secondary growth.
    • Types:
      • Vascular Cambium: Produces secondary vascular tissues (xylem and phloem).
      • Cork Cambium: Produces the outer protective tissue (bark).
  3. Intercalary Meristem:
    • Location: Found at the base of leaves or internodes (the regions between the nodes).
    • Function: Responsible for the growth of leaves and internodes, helping in the elongation of these parts.
    • Examples: Grasses and monocots.



 

Summary

To summarize:

  • Meristematic tissue is vital for plant growth and is made of undifferentiated cells that continuously divide.
  • The features include small size, thin cell walls, dense cytoplasm, and high mitotic activity.
  • It is classified into apical, lateral, and intercalary meristems based on their location and function in the plant.

 

 

2. Permanent Tissue

  • Function: Formed from meristematic tissue, these cells have a specific role and do not divide.
  • Types:
    • Simple Permanent Tissue: Made of one type of cell.
      • Parenchyma: Stores nutrients and water. Example: in fruits.
      • Collenchyma: Provides flexibility and support. Example: in stems.
      • Sclerenchyma: Provides strength and rigidity. Example: in seeds.
    • Complex Permanent Tissue: Made of more than one type of cell.
      • Xylem: Transports water and minerals. Composed of tracheids, vessels, xylem parenchyma, and xylem fibers.
      • Phloem: Transports food. Composed of sieve tubes, companion cells, phloem fibers, and phloem parenchyma.

 

Permanent Tissue

Permanent tissues are a type of plant tissue that consists of cells that have lost the ability to divide. These cells have differentiated and matured to perform specific functions within the plant. Let's explore the features and classification of permanent tissues.

Features of Permanent Tissue

  1. Specialized Cells: The cells in permanent tissues are specialized for particular functions.
  2. Fixed Shape and Size: These cells have a fixed shape and size, unlike the cells in meristematic tissue (which are constantly dividing and changing).
  3. Mature Cells: The cells are fully mature and do not divide.
  4. Differentiated Cells: These cells are differentiated, meaning they have developed specific structures to perform specific functions.

 



Classification of Permanent Tissue

Permanent tissues can be classified into two main types: simple permanent tissues and complex permanent tissues.

 

Simple Permanent Tissues

Simple permanent tissues are composed of similar types of cells that perform the same function. They include:

  1. Parenchyma
  2. Collenchyma
  3. Sclerenchyma





1. Parenchyma

Parenchyma is the most common and versatile type of simple permanent tissue. It has several important roles, including storage, photosynthesis, and secretion.

Features:

  • Thin-walled cells
  • Large central vacuole
  • Living cells with the ability to store food
  • Can divide under certain conditions (healing wounds)

Types of Parenchyma:

  • Idioblasts: These are specialized parenchyma cells that contain unique substances like oils, resins, or crystals.
  • Chlorenchyma: Parenchyma cells that contain chloroplasts and are involved in photosynthesis.
  • Aerenchyma: Parenchyma with large air spaces to facilitate gas exchange, especially in aquatic plants.

2. Collenchyma

Collenchyma provides support and flexibility to growing parts of the plant, such as young stems and leaves.

Features:

  • Elongated cells with unevenly thickened cell walls
  • Living cells that can stretch and provide mechanical support
  • Found under the epidermis (outer layer) of stems and leaves

3. Sclerenchyma

Sclerenchyma cells provide strength and rigidity to mature parts of the plant. These cells are typically dead at maturity and have very thick cell walls.

Features:

  • Thick, lignified cell walls
  • Dead cells at maturity
  • Provides mechanical support and protection

Types of Sclerenchyma:

  • Fibres: Long, slender cells that are commonly found in stems and leaves, providing tensile strength.
  • Sclereids: Also known as stone cells, these are varied in shape and size and provide hardness to seed coats and nutshells.

Complex Permanent Tissues

Complex permanent tissues are composed of different types of cells working together to perform a specific function. The two main types are:

  1. Xylem: Conducts water and minerals from roots to other parts of the plant.
  2. Phloem: Transports nutrients, particularly sugars, throughout the plant.

 



Xylem

Xylem tissue includes several types of cells:

  • Tracheids and vessel elements: Conduct water and provide structural support.
  • Xylem fibres: Provide additional support.
  • Xylem parenchyma: Involved in storage and lateral transport.

Phloem

Phloem tissue includes:

  • Sieve tube elements: Main conducting cells for transporting sugars.
  • Companion cells: Assist sieve tubes in their function.
  • Phloem fibres: Provide structural support.
  • Phloem parenchyma: Involved in storage and lateral transport.

To summarize, permanent tissues are essential for the various functions and structure of plants. Simple permanent tissues (parenchyma, collenchyma, and sclerenchyma) perform roles ranging from storage and support to protection. Complex permanent tissues (xylem and phloem) are crucial for the transport of water, minerals, and nutrients.

 

 

Protective Tissue

Protective tissues in plants play a critical role in safeguarding the plant's inner tissues from physical damage, pathogens, and excessive water loss. These tissues include the epidermis and cork.

1. Epidermis

The epidermis is the outermost layer of cells covering the leaves, stems, roots, and other parts of the plant. It acts as a barrier between the plant and its external environment.

  • Structure: The epidermis is usually a single layer of closely packed cells. In most plants, it's covered by a waxy layer called the cuticle, which helps reduce water loss.
  • Stomata: These are tiny pores found mainly on the underside of leaves. Each stoma is surrounded by two guard cells that control its opening and closing.
    • Function of Stomata: Stomata are crucial for gas exchange and transpiration. They allow carbon dioxide to enter for photosynthesis and release oxygen. They also let water vapor escape, helping the plant cool down.
  • Chloroplasts: While the epidermis itself usually doesn't contain chloroplasts, the guard cells of stomata do.
    • Photosynthesis: Chloroplasts in guard cells help in photosynthesis by converting sunlight, carbon dioxide, and water into glucose and oxygen.

2. Cork (Phellum)

The cork or phellum is another type of protective tissue found in the outer layer of older stems and roots. It replaces the epidermis in mature plants and is part of the periderm.

  • Structure: Cork cells are dead at maturity and packed tightly without intercellular spaces. They have thick walls impregnated with suberin, a fatty substance.
  • Suberin: Suberin in cork cells makes them waterproof and resistant to microbial attacks. This prevents water loss and protects against pathogens.
  • Intercellular Spaces: These spaces are absent in cork tissues, which makes them an excellent protective barrier.

Role of Protective Tissues

  • Barrier Protection: Both the epidermis and cork act as barriers against physical damage and infection.
  • Water Regulation: The cuticle and suberin prevent water loss, helping the plant retain moisture.
  • Gas Exchange: Stomata regulate the exchange of gases, which is vital for photosynthesis and respiration.

Quick Summary

  • Epidermis: Outermost protective layer, contains stomata with guard cells that help in gas exchange and transpiration. Guard cells have chloroplasts for photosynthesis.
  • Cork (Phellum): Found in older stems and roots, made of dead cells with suberin, acts as a waterproof barrier without intercellular spaces.

 

Animal Tissues

Animal tissues are grouped into four main categories: Epithelial, Connective, Muscular, and Nervous tissues.

1. Epithelial Tissue

  • Function: Covers and protects the body surfaces and internal organs.
  • Types:
    • Squamous Epithelium: Thin and flat cells. Example: lining of the mouth.
    • Cuboidal Epithelium: Cube-shaped cells. Example: ducts of glands.
    • Columnar Epithelium: Tall and column-like cells. Example: lining of the intestine.
    • Ciliated Epithelium: Has hair-like structures called cilia. Example: respiratory tract.

2. Connective Tissue

  • Function: Supports, binds, and protects other tissues and organs.
  • Types:
    • Areolar Tissue: Binds skin to muscles.
    • Adipose Tissue: Stores fat.
    • Bone: Provides structure and support.
    • Cartilage: Flexible support. Example: ear and nose.
    • Blood: Transports nutrients and gases.

3. Muscular Tissue

  • Function: Enables movement.
  • Types:
    • Striated (Skeletal) Muscle: Voluntary muscles attached to bones.
    • Smooth Muscle: Involuntary muscles found in internal organs.
    • Cardiac Muscle: Found only in the heart, involuntary and striated.

4. Nervous Tissue

  • Function: Transmits impulses, controlling and coordinating body activities.
  • Components:
    • Neurons: Nerve cells that transmit signals.
    • Neuroglia: Support and protect neurons.

Histology

Histology is the study of the microscopic structure of tissues. It involves examining tissues under a microscope to understand their structure and function. Histology helps in identifying diseases and understanding how tissues operate in both health and disease.

Summary

  • Tissues are groups of similar cells performing specific functions.
  • Plant tissues are classified into meristematic and permanent tissues.
  • Animal tissues include epithelial, connective, muscular, and nervous tissues.
  • Histology is the study of tissues at the microscopic level.

 

 

 

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