Chapter - 6 Tissues

Tissue is a group of cells that are similar in structure and are organised together to perform a specific task. Organs are then formed by the functional grouping together of multiple tissues.

In unicellular organisms, a single cell performs all the basic functions such as respiration, movement, excretion, digestion, etc.

Whereas in multicellular organisms, cells are grouped to form tissues. These tissues are specialised to carry out a particular function at a definite place in the body. For example, the muscle cells form muscular tissues which helps in movement, nerve cells form the nervous tissue which helps in transmission of messages.

This is known as division of labour in multicellular organisms. It is because of this division of labour that multicellular organisms are able to perform all functions efficiently.

Simple permanent tissues are of three types: Parenchyma, Collenchyma, andSclerenchyma. Parenchyma tissue is of further two types - aerenchyma and chlorenchyma.

Apical meristem is present at the growing tips of stems and roots. Their main function is to initiate growth in new cells of seedlings, at the tip of roots, and shoots.

The husk of a coconut is made up of sclerenchyma tissue.

Phloem is the food conducting tissue in plants. It is made up of four components:

1. Sieve tubes
2. Companion cells
3. Phloem parenchyma
4. Phloem fibres

The muscular tissue is responsible for movement in our body.

A neuron consists of a cell body with a nucleus and cytoplasm. It has two

important extensions known as the axon and dendrites. An axon is a long thread-like extension of nerve cells that transmits impulses away from the cell body.

Dendrites, on the other hand, are thread-like extensions of cell body that receive nerve impulses. Thus, the axon transmits impulses away from the cell body,whereas the dendrite receives nerve impulses. This coordinated function helps intransmitting impulses very quickly.

Three features of cardiac muscles are:

1. Cardiac muscles are involuntary muscles that contract rapidly, but do not get fatigued.
2. The cells of cardiac muscles are cylindrical, branched, and uninucleate.
3. They control the contraction and relaxation of the heart.

Functions of areolar tissue:

1. It helps in supporting internal organs.
2. It helps in repairing the tissues of the skin and muscles.

The group of cells similar in structure that work together to achieve a particular function forms a tissue. This group of cells has a common origin.

Xylem is a complex tissue. It is made up of following four kinds of cells or elements:

1. Tracheids
2. Vessels
3. Xylem parenchyma
4. Xylem fibres.

Simple tissues are made up of one type of cells which coordinate to perform a common function.
Complex tissues are made up of more than one type of cells. All these coordinate to perform a common function.

The differences between cell walls of parenchyma, collenchyma and sclerenchyma are given in the following table:

The small pores present in the epidermis of the leaf are stomata. Stomata are enclosed by two kidney shaped cells called guard cells.

Functions of Stomata::

1. Exchange of gases, particularly CO2 and O2, with atmosphere.
2. Loss of water in the form of vapour during transpiration.

Cardiac muscles are involuntary muscles which show characteristics of both smooth and striated muscles. These muscles occur in the walls of the heart.

Functions of Cardiac Muscles

1. Cardiac muscles contract and relax rapidly, rhythmically and tirelessly throughout life. They contract endlessly from early embryonic stage until death.
2. The contraction and relaxation of heart muscles help to pump and distribute blood to various parts of body.

Striated, Unstriated and Cardiac muscles are three types of muscle tissues. Their different characteristics are as follows:

1. Squamous epithelium
2. Tendon
3. Phloem
4. Adipose tissue
5. Blood
6. Nervous tissue.

• Skin - Stratified Squamous epithelium
• Bark of tree - Cork protective tissue
• Bone - Connective tissue
• Lining of kidney tubule - Cuboidal epithelium (Cuboidal epithelial tissue)
• Vascular bundle - Complex permanent tissue (Xylem and Phloem)

Parenchyma is a simple permanent tissue of angiospermic plants. It is present in cortex and pith of stem and roots. It is also present in mesophyll of leaves. When it contains chlorophyll, it is called Chlorenchyma, found in green leaves.

Epidermis is a protective tissue of angiospermic plants. It provides protections to underlying tissues. Epidermis forms outer covering of various plant organs such as roots, stem, leaves, and flowers and remains in direct contact with the environment. Any substance whether solid, liquid or gas can enter into the plant
or move outside only after passing through this layer. Epidermis helps in absorption, secretion, gaseous exchange and transpiration. It helps in preventing the entry of pathogens.

The cork cells are dead cells and do not have any intercellular spaces. The cell wall of the cork cells are coated with suberin (a waxy substance). Suberin makes these cells impermeable to water and gases. Cork is protective in function; it protects underlying tissues from desiccation, infection and mechanical injury.