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Tissues




1. Introduction

A group of cells similar in structure that work together to perform a particular function forms a tissue.

All types of tissues have two basic components:

  1. Cells: having common origin and function.
  2. Intercellular substances: Are nonliving, fibrous, jelly-like substances.

2. Importance of tissues:

  1. Formation of tissues has brought about division of labour in multicellular organisms.
  2. Tissues become organized to form organs and organs into organ systems.
  3. Workload of individual cell ahs decreased due to origin of tissues.
  4. As a result of improved organization and higher efficiency, multicellular organisms have higher survival.

3. Classification of tissues:

Tissues are broadly classified as animal tissues and plant tissues.

Plant Tissues

Plant tissues can be broadly divided into two main types. These are Meristematic tissue and Permanent tissue.

Meristematic Tissues:

  • A meristematic tissue constitutes a group of actively dividing cells present in the growing region of plant, e.g., the tips of roots and stems.
  • These tissues are responsible for increasing the length and girth of the plant.

Characteristics of meristematic tissues:

  • The cells of the meristematic tissue are similar in structure and have thin cellulose cell walls.
  • The cells may be spherical, oval, polygonal or rectangular in shape.
  • The cells of tissue are compactly arranged and do not have intercellular space.
  • The cells have dense protoplasm with prominent nuclei.
  • Vacuoles in these cells are either small or absent.

On the basis of their position in the plant body, meristematic tissues are classified into three types: - Apical, Lateral and Intercalary.

Apical meristems:

  • These are present at the tips of roots, shoots, branches and leaves.
  • It brings about the elongation of the root and stem. It results in increase in the hight of the plant, which is called primary growth.

Lateral meristems:

  • These are present along the lateral side of the stems and roots. For example: cork cambium.
  • It causes the organ (stem or root) to increase in diameter and girth. This is called secondary growth.

Intercalary meristems:

  • They are located at the base of leaves or internodes, e.g., stems of grasses and other monocots and below the nodes (e.g., mint).
  • It produces an increase of length of organ such as leaves and internodes.

Functions:

  • Meristematic tissue acts as a parent tissue from which other tissues develop.
  • These tissues take part in growth by formation of new cells.
  • The place of injury in plants is healed up by the formation of new cells by meristems.

Permanent tissues:

  • A permanent tissue is a group of cells, which is derived from the meristematic tissues, but these cells have lost the power of division temporarily or permanently.

Note: The development process by which cells which have been derived from meristematic tissue, take up a permanent shape, size and function is called differentiation.

  • Permanent tissues are of two types: - simple and complex permanent tissue.

Simple permanent tissues:

  • These tissues are composed of cells which are structurally and functionally similar. These tissues are of three types: parenchyma, collenchymas and sclerenchyma.

Parenchyma:

  • Parenchyma forms the bulk of plant body. It consists of thin walled living cells.
  • The cells are isodiametric, i.e., equally expanded on all sides.
  • The cell wall is thin and encloses a dense cytoplasm which contains a small nucleus and a large central vacuole.
  • The intercellular spaces are abundant.
  • The parenchyma is present in all the organs of the plants, i.e., roots, stems, leaves, flowers, fruit and seeds.

Function:

  • The main function of parenchymatous tissue is storage of food, e.g., starch in the parenchyma of cortex of potato tuber.
  • Parenchyma forms the framework of all the plant organs and tissues like cortex. Pith etc.
  • Parenchyma serves as packing tissue to fill the spaces between other tissues.
  • It stores waste materials of plants such as gum, crystals etc.
  • The intercellular air spaces of parenchyma cells allow gaseous exchange.
  • If chloroplast is present, the parenchyma tissue is called chlorenchyma and it performs photosynthesis.
  • In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.

Collenchyma:

  • Collenchyma is usually found in 3-4 layers beneath epidermis in stem, petioles and leaves of herbaceous dicot plants.
  • The cells of this tissue are living, elongated and irregularly thickened at the corner.
  • In collenchymas, intercellular spaces are generally absent.

Functions:

  • It provides the mechanical support, protection, flexibility and elasticity to the plants organs.
  • It allows easy bending in various parts of the plant (leaf, stem) without breaking.
  • When cells of collenchymas contain some chloroplasts, they manufacture sugar and starch.

Sclerenchyma:

  • Sclerenchyma cells are dead cells and they are devoid of protoplasm.
  • They are long and narrow as the walls are thickened due to lignin, such cell walls are called lignified.
  • The cells of sclerenchyma are closely packed without intercellular spaces.
  • Cells of sclerenchyma are of two types: fibers and sclereids.
  • Fibers consist of very long, narrow, thick and lignified cells. Sclereids are irregular shaped.
  • This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. Husk of coconut is made of sclerenchymatous tissue.

Functions:

  • The sclerenchyma is mainly mechanical and protective in function.
  • It gives strength, rigidity, flexibility and elasticity to the plant body and, thus, enables it to withstand various strains.

Complex Permanent Tissues:

  • The complex tissue consists of more than one type of cell having a common origin. All these cells coordinate to perform a common function.
  • Complex tissues are of two types: Xylem or wood and phloem or bast.
  • Xylem and phloem are both conducting tissues and also known as vascular tissues; together both them constitute vascular bundles.

Xylem:

  • Xylem is a vascular and mechanical tissue.
  • Xylem is composed of cells of four different types:
  • 1. Tracheids   2. Vessels or tracheae   3.  Xylem parenchyma    4. Xylem sclerenchyma (or fibers).
  • Except xylem parenchyma, all other elements are dead and bounded by thick lignified wall.
  • Tracheids and vessels are tubular structures.

Functions:

  • The main function of xylem is to carry water and mineral salts upward from the root to different parts of shoots, hence also called water conducting tissue.
  • Since walls of tracheids, vessels and sclerenchyma of xylem are lignified, they give mechanical strength to the plant body.
  • The parenchyma stores food and helps in the sideway conduction of water.

Phloem:

  • Phloem (bast) is a living conducting tissue. It also contains tubes just like xylem but does not perform mechanical function.
  • Phloem is composed of following four elements or cells:
  • 1. Sieve tubes   2. Companion cells   3. Phloem parenchyma   4. Phloem fibers.
  • Sieve tubes are slender, tube like structures with perforated walls.
  • Companion cells are living parenchymarous cells lying on the sides of the sieve tubes.
  • Sieve tube and companion cells have close cytoplasmic connection with each other through fine pits.
  • Phloem fibres are thick walled fibres with simple pits.
  • Phloem parenchymas are thin walled, living cell of parenchyma of phloem.

Function:

  • Phloem transports (conducts) photosynthetically prepared food materials from the leaves to the storage organs and later from storage organs to the growing regions of the plant body.

Protective tissues:

  • Protective tissues are a part of plant tissue system. Protective tissues include epidermis and cork.

Epidermis:

  • It is the outermost protective layer of plant organs.
  • The epidermis is usually made of a single layer of cells.
  • Cells of epidermis are elongated and flattened, without intercellular space. They are living cells but their inner contents are similar to parenchyma cells.
  • In leaves, epidermis bears small pores called stomata.
  • In some plants living in very dry habitats, the epidermis may be thicker since protection against water loss is critical.

Functions:

  • The function of epidermis is the protection of plant from injury and infection.
  • Cuticle of epidermis also helps to reduce water loss by evaporation to prevent dessication.
  • Stomata present in the epidermis allow gaseous exchange to occur during photosynthesis and respiration.
  • It also facilitates transpiration.

Cork (or phellem):

  • Cork cells are dead cells without having intercellular spaces.
  • They appear at the periphery of roots and stems when they grow older and increase in girth.
  • They also have a chemical called suberin in their walls that makes them impervious to gases and water.

Functions:

  • The function of cork in plant body is to provide protection. It protects plants from external injury and infection.
  • It also prevents dessication.
  • Since cork does not catch fire easily, it is used for insulation, shock-absorber, linoleum.
  • It is also used for making sports goods, such as shuttle-cock, table tennis paddles, crcket balls, etc.

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