CBSE Class 9 Science Tissues Notes

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TISSUES

The body of plants and animals is made up of different types of cells. These cells originate from a single cell by repeated divisions and get differentiated during development. In unicellular organisms all the body functions are performed by a single cell. But in multicellular organisms, different functions are performed by different groups of cells. The groups of cells having a common origin and performing similar functions are called tissues. Several tissues are organized to form tissue system and the tissue systems form the organs and several organs into organism.

Study of tissues is called Histology

Tissue and Division of Labour: In complex organisms, different tasks are carried out by different organs and organ systems. Tissues are the first step towards division of labour in complex organisms.

INTEXT QUESTIONS

Q1. What is a tissue?

Answer: A group of cells that are similar in structure and/or work together to achieve a particular function is called tissue.

Q2. What is the utility of tissues in multi-cellular organisms?

Answer: In multicellular organisms, the body system is based on the division of labour. It means the cells performing a specific function are grouped together to form a particular tissue.

The different tissues are organized in a way to provide highest efficiency in functioning of the body.

PLANT TISSUES

Plant tissues are of two main types, viz. meristematic tissue and permanent tissue.

MERISTEMATIC TISSUE

The growth of plants occurs only in certain specific regions. This is because the dividing tissue also known as meristematic tissue (Meristos – divisible) is located only at these points.

The meristematic tissues are made up of group of similar and immature cells, which can divide and form new cells. Meristematic cells divide continuously and thus help in increasing the length and thickness of the plant. Depending upon the position, meristematic tissues are of three types.

i) Apical meristems: Apical meristem is present at the growing tips of stems and roots and increases the length of the plant body. They are responsible for growth in length, i.e. primary
growth.
ii) Intercalary meristems: These meristems occupy base of the leaves and the base of the internodal regions in plants such as grasses (mostly in monocotyledonous plants). These help
in elongation of the internodes.
iii) Lateral meristems: This includes the meristematic tissues occupying the lateral regions of the stems and roots which bring about increase in the width of the plant body. (e.g. Cork cambium and Vascular cambium).

Characteristic features of Meristematic tissues
The meristematic cells may be round, oval, polygonal or rectangular in shape.
Their cell walls are thin, elastic and made up of cellulose.
They are closely arranged without intercellular spaces.
They have dense cytoplasm with large nucleus.

PERMANENT TISSUE:
Once the cells of meristematic tissue divide to a certain extent, they become specialized for a particular function. This process is called differentiation. Once differentiation is accomplished, the cells lose their capability to divide and the tissue becomes permanent tissue.
Some cells produced by meristematic tissues stop dividing and form a permanent tissue. They have definite structure and function. They are differentiated into various types to perform different functions.

The permanent tissues are classified as
i) Simple tissues and
ii) Complex tissues

SIMPLE TISSUES
A tissue with the cells of similar structure (one type of cells) and function is called simple
tissue. It is of three types.
1. Parenchyma
2. Collenchyma
3. Sclerenchyma

PARENCHYMA
The cells of parenchyma have thin cell wall. They are loosely packed; with lot of intercellular spaces between them. They are living cells. They are generally present in all organs of a plant.
They are oval or spherical or rectangular or cylindrical in shape. The cell wall is made of cellulose and pectic materials. Parenchyma makes the largest portion of a plant body.
Parenchyma mainly works are packing material in plant parts. The main function of parenchyma is to provide support and to store food. In some plant parts, parenchyma has chlorophyll as well. In that case, parenchyma carries out photosynthesis and is then termed as chlorenchyma. In aquatic plants, large air cavities are present in parenchyma. This provides buoyancy to the plant, and then the parenchyma is known as aerenchyma.

COLLENCHYMA
The cells of collenchyma are polygonal in cross section and have unevenly thickened walls.
These thickenings are due to the deposition of more cellulose, hemi-cellulose and pectin. The thickening is confined to the corners of the cells. They generally occur in the dicot stem in two or more layers below the epidermis. It is absent in the roots. It also occurs in petiole and pedicel. Like Parenchyma, Collenchyma is also a living tissue. The main function of Collenchyma is to provide strength and flexibility to the growing organs like young stem.

SCLERENCHYMA
It is a dead tissue. The cells are thick with lignified walls. They give mechanical support to the organs. This has two types of cells – Sclereids and Fibres.
Sclereids
Sclereids are stone cells which are commonly found in shells of the nut, pulp of certain fruits such as Pear and Sapota.
Fibres
The fibres are elongated strands with simple pits throughout its length.

COMPLEX PERMANENT TISSUES
XYLEM
Xylem is mainly concerned with the transport of nutrients, water and minerals upwards in the plant body. It forms a continuous tube through the roots, stems, leaves, flowers and fruits by the fusion of elongated cells.

It is composed of different kinds of cells namely,
1. Tracheids
2. Xylem vessels.
3. Xylem fibres
4. Xylem parenchyma.

Tracheids
Tracheids are elongated, tapering cells with blunt ends. They have lignified secondary wall.
They are the chief water conducting elements in Pteridophytes and Gymnosperms.

Xylem vessels
Xylem vessels have perforations at the end and are placed one above the other like a long pipe line. They are seen in the xylem of angiosperms. They conduct water, mineral nutrients and also provide mechanical strength to the plant body.

Xylem Fibres
The fibres of Sclerenchyma associated with the xylem are known as xylem fibres. They give additional mechanical strength to the plant. They are also called wood fibres.

Xylem Parenchyma
The parenchyma cells associated with xylem are known as xylem parenchyma. It is the only living tissue amongst xylem cells. They store food reserves in the form of starch and fat. They also help in conduction of water.

PHLOEM
Phloem conducts food materials from leaves to the other parts of the plant. It is made up of
four types of cells.
1. Sieve elements
2. Companion cells
3. Phloem fibres
4. Phloem parenchyma
Sieve elements
Sieve elements are the conducting elements of the phloem. Sieve elements are of two types –sieve cells and sieve tubes.

Sieve cells are present in Pteridophytes and Gymnosperms where as sieve tubes are present in Angiosperms.

Companion cells
Companion cells are thin walled elongated specialized Parenchyma cells. They are associated with sieve elements. They have a prominent nucleus and cytoplasm. They help the sieve tube in conduction of food materials in angiosperms.

Phloem fibres
The fibres of sclerenchyma associated with phloem are called phloem fibres.
They are also called bast-fibres. They give mechanical support to the plant. Among the four types of phloem cells, phloem fibres are the only dead tissues.

Phloem parenchyma
The parenchyma cells associated with phloem are called phloem parenchyma. They store starch and fats.

ANIMAL TISSUES
Animal tissues are of four types, viz. epithelial tissue, connective tissue, muscular tissue and nervous tissue.

EPITHELIAL TISSUE:
The epithelial tissue forms the covering or lining of most of the organs. The cells of epithelial tissue are tightly packed and form a continuous sheet. There is small amount of cementing
materials between the cells and no intercellular space is present. Permeability of the epithelial tissue plays a great role in exchange of materials among various organs it also plays an
important role in osmoregulation. All epithelial tissues are separated by the underlying tissue by an extracellular fibrous basement membrane.

Epithelial tissues are of following types:
1. Simple Epithelium
2. Cuboidal Epithelium
3. Columnar Epithelium
4. Stratified Epithelium

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