Tissue : The group of cells with more or less similar structure and performing a definite function, is called a tissue.
Organs : In most of the complex living beings group of tissues are combined to form organs. Stomach of vertebrate animals is an example of an organ. An organ, therefore, consists of a group of tissues that work togther to perform some special functions.
System : In mammals like very complex organisms, groups of organs make up various systems. For example, the stomach is an organ of the digestive system. Other organs of the digestive system are small intestine, large intestine and various glands. They together form digestive system.
Meristematic tissue : The meristematic tissue or meristem comprises actively dividing cells that develop and differentiate into yet other tissues and organs.
Permanent tissue : Permanent tissues are composed of cells. They have a definite form and size and are no longer capable of further division.
Parenchyma : A tissue, composed of thin-walled, loosely packed, unspecialized cells, is called parenchyma tissue.
Function of Parenchyma tissue : Parenchyma has the following functions:
(i) The shape of the organ is maintained by it.
(ii) In the green parts of the plants, food is stored.
(iii) Water and food materials are laterally conducted by the parenchyma.
Collenchyma : Collenchyma is a simple permanent tissue consisting of arranged, living and somewhat elongated cells with different wall thickness.
Functions of Collenchyma tissue : (i) It provides mechanical strength and support to young stems, leaves and floral stalks. (ii) Collenchyma tissues containing chloroplast may take part in photosynthesis. (iii) Collenchyma cells may also store food.
Sclerenchyma : The sclerenchyma is simple permanent tissue consisting of uniformally thick-walled lignified dead cells.
Functions of Sclerenchyma tissue : It is the mechanical tissue of plant and gives strength. It gives toughness and rigidity to different parts of the plant.
Complex permanent tissue : A tissue is said to be complex when it is composed of cells of different type, but they perform together as a unit.
Xylem : The Xylem is a complex permanent vascular tissue through which upward movement of water and minerals takes place.
Components of Xylem : It has following four components (a) tracheids, (b) vessels, (c) parenchyma cells and
(d) sclerenchyma fibres.
Tracheids : Tracheids are long tubular dead cells with tapering ends and do not possess any protoplasmic matter. Tracheids contribute significant strength and support to the stems of gymnosperms.
Vessels : Vessels are composed of elongated cells placed vertically one above another that are untied after the transverse partition walls of the cells have dissolved. It constitutes a long continuous open tube or pipe-like structure through which water and dissolved minerals are transported from roots to the leaves.
Xlem parenchyma : The elongated living parenchymatous cells which are arranged in vertical files are known as xylem parenchyma. It occurs as a part of xylem in almost all higher plants.
Sclerenchyma fibre of xylem fibre of wood fibre : The thick-walled sclerenchyma cells associated with tracheids and vessels are called sclerenchyma fibre or wood fibre.
Functions of Xylem : Xylem is responsible for the upward transport of water and dissolved minerals from roots to leave, lignified walls of cells of xylem provide mechanical strength to different parts of the plant body.
Phloem : Phloem is the complex permanent tissue through which organic food and other minerals form leaves are transported to different parts of the plant body.
Components of Phloem : (a) sieve tubes, (b) companion cells, (c) phloem parenchyma cells (d) phloem fibres.
Sieve tubes : Sieve tubes arranged end to end are long, slender thin walled cells.
Companion cells : Companion cells are long, slender, elongated living cells which lie adjacent to the sieve tube.
Phloem paranchyma : The living parenchyma cells associated with the phloem tissue are called phloem parenchyma.
Phloem fibre (bast fibre) : The sclerenchyma fibres associated with the phloem tissue are known as phloem fibres. Phloem fibre constitute the dead part of the phloem tissue.
Functions of phloem : It carries the following functions :
(i) Phloem helps in the transport of organic food and metabolites from leaves to different parts such as stems, roots, storage tissue, fruits, flowers, etc.
(ii) Phloem also serves as a store house of organic food materials.
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