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Structure and Function of Plant Tissues

In the higher plants there are different types of cells. These cells vary in shape, size, origin and functions. A group of cells, alike in size and shape, having the same origin and performing a particular function, is called a tissue. Tissues perform specialized functions in the plant body. Some of these are responsible for the growth of the plant while others are meant to carry out photosynthesis to prepare food and for other metabolic functions.

Tissues are mainly classified into two groups :-

1. Meristematic Tissue

2. Permanent Tissue


Meristematic Tissue

The meristematic tissue consists of cells which are not differentiated and which are capable of cell division. The cells of these tissues are comparatively small, polygonal or cubical in shape and are compactly arranged without intercellular spaces. They have dense cytoplasm with small or no vacuoles and large nuclei. The cell wall is very thin and is made up of cellulose.

Depending on the position they occupy in plants, meristems is classified as apical, lateral and intercalary meristems.


Apical Meristems

They occur in the growing regions, that is, at the apex of the stem and the root. Due to the activity of their cells, roots and stems increase in length.

Meristematic Tissues

Intercalary Meristems : -
They occur between regions of permanent tissues. They are found in the nodes of the stem and at leaf bases. They help in the elongation of the axis.

They are mostly seen in monocots.

Intercalary meristems at the nodes of bamboo allow for rapid stem elongation.

Lateral Meristems :-
Permanent tissues are formed by differentiation of the cells produced by the apical meristem. The cells have lost the power of division and have acquired a permanent shape and structure. Permanent tissue is classified into simple permanent tissue and complex permanent tissue.

Vascular cambium: produces secondary xylem and phloem and cork cambium : gives rise to the bark of a tree.

Parenchyma : -
Parenchyma is the simplest of all tissues. The cells of this tissue are mostly living. The cells are round or oval in shape and are of the same size, that is, they are isodimetric. The cells are closely packed and are mostly found in the soft parts of the plant. The cell wall is thin and there may be intercellular spaces between the cells. Its function is mainly the storage of food material. The parenchyma cells, which contain chloroplasts, are called chlorenchyma. Its function is to manufacture food material. Presence of air cavities are called aerenchyma.


Collenchyma : -
Collenchyma cells are slightly elongated. They look circular, oval or polygonal in a transverse section of the stem. The corners, where three or four cells meet, of the cell walls are thickened. The thickening is due to the deposition of cellulose with pectin materials. The cells give support and mechanical strength to many non-woody plants.


Sclerenchyma : -
Sclerenchyma is made up of dead cells which have very thick walls. The walls are thickened due to the deposition of cutin and lignin (it is an abundant organic material present in the integral part of the cell wall and is used as an adhesive for linoleum) Such tissue gives mechanical strength to the plant. On the basis of the shape and size sclerenchyma may be classified into sclerenchyma fibres and sclereids.

Sclerenchyma fibres are highly elongated cells with tapering ends. Sclereids are also called stone cells. They are mostly irregular in shape.



Epidermal Tissue : -
It consists of flattened irregular cells forming a layer called epidermis. This layer covers the surface of leaves, stem and root and is coated with a waxy substance called cuticle to prevent water loss. The epidermis protects the underlying layers. The epidermal layer also has openings called stomata, through which exchange of gases takes place between the plant and the atmosphere. Presence of trichomes on the epidermis, specialized for absorption of water and mineral nutrients.


Complex Permanent Tissue

The complex permanent tissue is made up of more than one type of cells working together as a unit. There are two types of complex permanent tissues in plants, namely, the xylem and phloem. They are conducting or vascular tissues.


The Xylem : -
The xylem is made up of elongated dead cells. They conduct water and mineral salts from the roots to the shoots. It consists of different kinds of cell, namely, tracheids, trachea vessels, xylem fibres and xylem parenchyma.


Tracheids : -
In plants like fern and in gymnosperms, the tracheids are the chief water-conducting element. The cells are long and have tapering ends. They have a thick and lignified wall. These are dead cells. They are generally seen in gymnosperms and pteridophytes.


Tracheae/Vessels : -
Trachea is otherwise known as vessel. They are also water-conducting elements and form the bulk of the conducting tissues in flowering plants. They are present in angiosperms but absent in pteridophytes and gymnosperms. The vessel is a long, vertical tube consisting of elongated tubular cells arranged end to end. The cross-walls between the vessel members are perforated so that there is an end. The cross-walls between the vessel members are perforated so that there is a long continuous passage for the easy conduction of water. The walls of the vessel cells become thickened due to heavy deposition of lignin. According to the mode of deposition of secondary walls, the vessels may be:

Annular - in the form of a ring;
Spiral - spirally coiled structure;
Scalariform (ladder-like) or pitted- uniformly thick except for the pits.


Xylem Fibres (libriform fibres)
Xylem fibres are also associated with vessels and tracheids. They resemble sclerenchyma fibres and give mechanical support.



Xylem Parenchyma : -
The parenchyma cells associated with the xylem are known as xylem parenchyma. They are thin–walled living cells. They assist in conduction of water. They store water and food.


Xylem Parenchyma

The Phloem : -
The phloem is the food-conducting tissue. It conducts the food prepared by the leaves to all parts of the plant, especially to the storage organs of the plant. It is composed of four types of elements, namely

  • sieve tubes,
  • companion cells,
  • phloem parenchyma and
  • phloem fibres.


Sieve Tubes : -
The sieve tubes are the conducting elements of the phloem. Their end walls are transverse and oblique. The end walls contain a number of pores, which look like sieve and are called sieve plates. The sieve elements are attached end-to-end forming vertical tubes. Each cell of the sieve tube has a thin layer of cytoplasm with no nucleus.

Companion Cells : -
Associated with the sieve tube, there are elongated, thin-walled living cells called companion cells. Each companion cell has dense cytoplasm and a prominent nucleus. These are present only in angiosperms. They assist the sieve tubes in conduction of food.

Phloem parenchyma : -
Thin- walled parenchyma cells occurring in the phloem are called phloem parenchyma. They are live cells and their function is storage of food substances.

Phloem Fibres (bast fibres) : -
Sclerenechyma cells occurring in the phloem are called phloem fibres. They provide protection and mechanical support to the softer tissues of the phloem.


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