We know that all the living organisms are made up of cells.
Tissues are a cluster of cells in a part of body that perform a specialized function. Some examples are, blood, muscle, phloem and others.
Thus, a group of cells that are similar in structure and or work together to achieve a particular function form a tissue.
Utility or importance of tissues in multicellular organisms
Tissues help in performing a specialized function in the living organisms. For example:
In unicellular organisms, all the life processes are carried out by the cell itself. For example: in Amoeba, single cell carries out movement, intake of food, gaseous exchange and excretion.
In multicellular organisms, there are millions of cells and specialized functions are performed by selective group of cells. For example: in humans muscle cells help in movement, nerve cells carry messages and in plants, vascular tissues conduct food and water from part of the plant to other parts.
- Meristematic tissues are responsible for growth of stem, roots and other parts of the plant.
- Cells of meristematic tissue are very active and have dense cytoplasm, thin cellulose walls and nuclei. They lack vacuoles.
- Cells are spherical polygonal.
- It is formed of immature and actively dividing cells.
- New cells produced by meristematic tissue change their characteristics once they become matured. Then they display different characteristics and become permanent tissues.
- Meristematic Tissues are classified as three types:
- Apical meristematic tissue: Apical meristem is found at the tip of root and stem and increases the length of root and stem.
- Lateral meristematic tissue (Cambium): It is responsible for the growth of the girth of the stem or root.
- Intercalary meristematic: It is located at the node of the plant.
- When the cells formed by meristematic tissue lose the ability to divide, they take up a specific role and become permanent tissue.
- The cell division happen into meristem tissue but once these cells get matured, the division stops and are differentiated for different functions.
- Differentiation leads to different types of permanent tissues.
- Types of permanent tissues are:
- Simple permanent tissues: These are made of only one type of cells.
- Complex permanent tissues: These are made of more than one types of cells.
Simple Permanent Tissues
- Simple permanent tissues are located under the epidermis layer (Epidermis is the top most layer on the plant like bark).
- These are classified into three types:
- Parenchyma cells
- Collenchyma cells
- Sclerenchyma cells
- It contains unspecialised cells with thin cell walls made up of cellulose.
- These are living cells with oval or polygonal shapes.
- There is large inter-cellular space between the cells.
- It helps in storage of food.
- When parenchyma contains chlorophyll and performs photosynthesis, then it is called chlorenchyma.
- In aquatic plants, large air cavities are present in the parenchyma and help them to float.
- These tissues provide mechanical support to the plants.
- The permanent tissue which provides flexibility to the plants is collenchyma tissue.
- It allows bending of various parts of plant like tendrils and stem of climbers without breaking.
- The cells are living, elongated and irregularly thickened at the corners.
- This tissue makes the plant hard and stiff.
- The cells of this tissue are dead. It provides strength to the plants.
- They are long and narrow and the walls are thickened due to lignin.
- There is no inter-cellular space.
- They are present in stems around the vascular bundles in the veins of leaves and in hard covering of seeds and nuts.
- For example: husk of the coconut is made up of sclerenchyma tissue.
- The outermost single layer of the cells on the plant is called epidermis.
- The cells are flat and there is no space between the cells.
- It protects the plants against the harsh environment and also protects against the water loss.
- Epidermal cells in the aerial parts of the plant secrete a waxy, water-resistance layer on the outer surface, that protects it against the water loss, mechanical injury and invasion of parasitic fungi.
- Epidermal cells of the roots have hair like parts that increase the surface area for water absorption.
- Epidermal cells of the desert plants have thick waxy coating of cutin that helps them to prevent the loss of water from the leaves through transpiration.
Complex Permanent Tissues
- These tissues are made of more than one type of cells which look alike.
- All these cells coordinate to perform specific function.
- Example of these tissues are:
- These are conducting tissues and form vascular bundle.
The constituents of the xylem are:
- They are longer and narrower and the cells have thick walls.
- The cells die upon maturity.
- Their end walls are not perforated.
- These help in the upward conduction of water and minerals.
- They are a series of cells joined end to end forming a cylindrical structure.
- The cells are open are both ends.
- Their walls have various types of thickenings.
- Xylem parenchyma
- Xylem parenchyma stores food.
- These cells are living cells.
- Xylem fibers
- The cells die upon maturity.
- Xylem fibres are supportive in nature.
The constituents of the phloem are:
- Sieve cells
- It transports food from the leaves to the other parts of the plant.
- Sieve tubes
- Sieve tubes are tubular cells with perforated walls.
- It is formed of cylindrical cells.
- The terminal wall of each sieve tube contains minute pores called as sieve plate.
- Through these pores food material passes from cell to cell.
- Companion cells
- These are associated with the sieve tubes. These are specialised parenchyma cells.
- They have nucleus and helps the sieve tube in conduction food materials.
- Phloem parenchyma
- The cells are thin walled.
- They help in the storage of food materials.
- Phloem fibres
- Phloem cells are living cells except the phloem fibres.
- They provide mechanical support.
Did You Know
- The study of tissue is called Histology.
- Xavier Bichat in 1801 introduced the term tissue in anatomy.
- The group of tissues form an organ.