Tissues [Plant and Animal Tissues]
Tissues – A group of cells having similar structure and performing similar functions is called a tissue.
Plant Tissues –
1:- Meristematic Tissues – It consists of rapidly dividing cells. These cells lead to rapid growth in the plant and are thus located in actively growing regions namely, the root tip and the shoot tip. This tissue keeps on dividing throughout the life of the plant. The cells of this tissue are thin-walled and closely packed without any spaces in between. The cells are almost alike irrespective of their location and may be round, oval, or polygonal in shape.
Meristematic tissue divides rapidly and forms a large number of cells. These cells undergo gradual changes and acquire specific functions and form the permanent tissue. As the meristematic tissue keeps on adding new cells to the plant body, it results in the growth of the plant.
In an older seedling, the meristematic tissue is present at the tip of the root (radicle) and the tip of the newly emerged shoot.
2:- Permanent Tissue – It consists of non-dividing cells. These cells have lost the power of dividing and have become specialised to perform specific function. They have diverse shapes and sizes that suit the function they perform. Cells of permanent tissues are derived from the meristematic tissue and may be thin-walled or thick-walled, living or dead. Since this tissue has acquired a permanent structure, it is called a permanent tissue.
Permanent tissues are of three types –
1:- Dermal Tissue –
- The term ‘dermal’ is derived from the Greek word ‘derma‘ meaning skin.
- It forms the outermost layer covering the entire surface of the plant body.
- It usually consists of a one-cell thick layer of closely packed cells having no space in between.
- The dermal tissue possesses several special structure to suit the part of the plant body it covers.
- In the leaves the dermal tissue possesses several minute pores called stomata which help in exchange of gases.
- The dermal tissue may also possess soft or stiff hairs.
- Functions – (a) It protects the inner parts of the plant. (b) It reduces the evaporation of water from the plant surface. (c) It helps in exchange of gases (oxygen, carbon-dioxide and water vapour) between the plant body and atmosphere through stomata. (d) In the roots, dermal tissue bears certain hairs that help in the absorption of water and mineral salts.
2:- Ground Tissue – It forms the main bulk of the plant body. It provides mechanical support to the plant. It extends from beneath the dermal tissue to the centre of the plant.
Ground tissue can be of three types :-
- Parenchyma – It consists of thin-walled rounded or polygonal living cells. It is generally found in all soft parts of plants. Its main function is storage of food. Parenchyma also serves as packing tissue. It contains chloroplasts in leaves and helps in photosynthesis.
- Collenchyma – Cells of collenchyma are somewhat elongated and have thickened corners. These are living cells which give strength and flexibility to the plant body. Collenchyma allows the bending of plant parts without breaking. It is generally not found in monocots.
- Sclerenchyma – It is composed of dead, thick-walled cells. These cells are long and narrow, fibre-like (thread-like) in appearance. They give strength and rigidity to the plant body. Sclerenchyma is found in all the hard parts of the plant body such as hard seed-coats, nuts and stony fruits. Fibre-yielding plants such as hemp and jute contain long fibres of sclerenchyma.
3:- Vascular Tissue – The term vascular is derived from a Latin word meaning tubes and vessels. It is a conducting tissue which transports water, minerals and food to different parts of the plant. This tissue consists of long cells placed end to end to form long pipes or tubes.
Vascular tissue is of two types:-
- Xylem – The word xylem is derived from a Greek word meaning “wood“.
- It is mainly made of long, thick-walled dead cells placed end to end forming long pipes and running from roots to the leaf tips.
- The partitions between the cells dissolve to allow formation of continuous pipes.
- These cells carry water and mineral salts from roots to other parts of the plant and provide mechanical strength to the plant body.
- Phloem – The word phloem is derived from a Greek word meaning “bark“.
- It consists of tubular cells with sieve-like partitions in between.
- Phloem cells are living and serve to transport the food produced by green leaves to other parts of the plant body.
- Xylem and Phloem are often closely associated with each other forming structures called Vascular Bundles.
Animal Tissues – In multicellular animals four main types of tissues are found – Epithelial tissue, Muscular tissue, Nervous tissue and Connective tissue.
1:- Epithelial Tissue –
- The term was coined by Ruysch.
- It is a fundamental animal tissue which is made of one or a few layers of compactly arranged cells overlying a basement membrane that covers the external and internal surfaces of various body parts.
- Based on their shapes they are categorised into- (a) Squamous epithelium – having flattened cells. (b) Cubodial epithelium – having cubodial cells. (c) Columnar epithelium – having tall column-like cells.
- Functions – (a) It protects the body and its internal organs. (b) The epithelial cells lining the stomach wall secrete digestive juice. (c) Epithelial cells in intestine help in the absorption of digested food. (d) Epithelial cells present in the skin help in the removal of wastes as sweat. (e) Epithelial cells lining the nose and trachea produce a sticky secretion called mucus which traps dust particles and harmful bacteria. (f) In trachea and oesophagus the epithelial cells bear tiny projections called cilia which prevent entry of dust particles. (g) It determines polarity, metabolism, cell division, repair and movement of other tissues.
2:- Muscular Tissue – It causes movements of limbs and internal organs and also locomotion of the organism. It is found in every part of the body which shows movement. Muscle tissue is a bundle of muscle cells which contract forcefully and again return to the relaxed state. This ability of the muscle cells to contract and relax is called contractility. Muscle cells are thin and elongated and are often called muscle fibres. In higher animals some muscles are attached to the skeleton while some others form walls of internal organs like heart, stomach, intestine, urinary bladder and blood vessels.
The heart is largely made of muscle tissue which gives it ability to contract and relax repeatedly. Food moves forward in our stomach and intestine because of the repeated contractions of their muscular walls. Based on their structure and function muscle tissues can be classified to three types-
(a) Skeletal muscles or striated muscles – These muscles are attached to the skeleton and hence the name skeletal muscle. The skeletal muscle is made of bundles of long, unbranched, cylindrical cells that show transverse striations (stripes or bands) which give them their other name, strirated muscles.
The striated muscles can be moved at our will. Therefore, they are also called voluntary muscles. This is the only type of muscle which is in our control.
For example, the muscles involved in locomotion are voluntary muscles.
(b) Smooth Muscles – Smooth muscle fibres are long and spindle shaped. They do not show any stripes or striations and are therefore called smooth muscles. They are involuntary muscles, i.e., the movement of these muscles is not under our control. They are found in the walls of most internal organs like stomach, intestine, urinary bladder and blood vessels. These muscles bring about movement of food in the alimentary canal, movement of urine in the urinary ducts and in the urinary bladder and movement of blood in the blood vessels.
(c) Cardiac muscles – are found only in heart. They are involuntary muscles. They go on contracting rapidly but rhythmically throughout one’s life without tiring. Cardiac muscles fibres have striations like the striated muscle fibres but are branched forming junctions with neighbouring cells.
3:- Nervous Tissue – It is found in the brain, spinal cord and nerves. Nervous tissue is made of specialized cells called neurons. A typical nerve cell has three parts.
- A cell body which contains nucleus.
- Short processes called dendrites emerging from the cell body.
- A long cylindrical process called axon emerging from the cell body.
Several nerve cells are collected into bundles and covered by a connective tissue covering to form nerves. These nerves carry messages from one part of the body to the other. Nervous tissue is not present in all animals. Lower animals like hydra have very few nerve cells.
4:- Connective Tissue – The prime function of connective tissue is to bind together other tissues and support various organs of the body.It fills up the space between the organs, thereby forming a packing around organs.
Connective Tissue is different from other tissues because while all other tissues have tightly packed cells, in connective tissue, cells are separated from one another by a non-living substance called matrix. Matrix forms the main bulk of the connective tissue and the cells are few and wide apart. Composition of the matrix varies in different types of connective tissues. Blood, bones, cartilage, tendon and ligament are different types of connective tissues.
Bone – is a hard connective tissue. The hardness is due to the presence of calcium and other minerals in the matrix. Bones support the body and provide framework for body movement. Bones protect internal organs like brain and heart.
Cartilage – is a flexible tissue found in ear lobes, nose tip, trachea and between vertebrae. Cartilage provides support and flexibility to a region where it is present.
Tendon – Muscles are connected to bones by means of bands of strong and flexible connective tissue called tendons.
Ligament – Bones are connected to each other at the joints by tough, yet flexible connective tissue bands called ligaments.