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Tissues

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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 a 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. They are as follows:

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.
Intercalary meristems: These meristems occupy the 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.
Lateral meristems: This includes the meristematic tissues occupying the lateral regions of the stems and roots which bring about the 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 a large nucleus.

Permanent Tissue

The permanent tissues are composed of those cells which have lost their capability to divide.
They have a definite shape, size and thickness. The permanent tissue maybe dead or living.
The division & differentiation of the cells of meristematic tissues give rise to permanent tissues.
In cell differentiation, developing tissue and organs change from simple to more complex forms to become specialized for specific functions.
The cells of permanent tissue loose the capacity to divide and attain a permanent shape, size and function.

The permanent tissues are classified as

Simple tissues and
Complex tissues

Simple Permanent Tissue
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:
1. The cells of parenchyma have a thin cell wall. They are loosely packed; with a lot of intercellular spaces between them. They are living cells.
2. They are generally present in all organs of a plant.
3. The cell wall is made of cellulose and pectic materials.
4. Parenchyma makes the largest portion of a plant body.
5. Parenchyma mainly works are packing material in plant parts.
6. The main function of parenchyma is to provide support and to store food.
7. In aquatic plants, large air cavities are present in the parenchyma. This provides buoyancy to the plant, and then the parenchyma is known as aerenchyma.
Collenchyma:
1. The cells of collenchyma are polygonal in cross-section and have unevenly thickened walls.
2. These thickenings are due to the deposition of more cellulose, hemicellulose and pectin.
3. The thickening is confined to the corners of the cells.
4. They generally occur in the dicot stem in two or more layers below the epidermis.
5. It is absent in the roots. It also occurs in petiole and pedicel. Like Parenchyma, Collenchyma is also a living tissue.
6. The main function of Collenchyma is to provide strength and flexibility to the growing organs like a young stem.
Sclerenchyma:
1. 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.
2. Sclereids: Sclereids are stone cells which are commonly found in shells of the nut, pulp of certain fruits such as pear and Sapota.
3. Fibres: The fibres are elongated strands with simple pits throughout its length.

Complex Permanent Tissue
Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex 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.
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