Group of cells is called tissue. Group of cells may be similar or dissimilar with common origin and perform identical function.
Meristematic tissue
Meristos = divided. These are composed of cells that are in a state of division or retain the power of dividing. These cells are essentially alike being either spherical, oval or polygonal in shape without any intercellular spaces, their walls thin and homogenous; the protoplasm abundant and active with large nuclei and the vacuoles small or absent.
Meristematic tissues may be apical and lateral. The apical meristem lies at the apex of the stem and the root and gives rise to primary permanent tissues while the lateral meristem e.g.: Cambium lies among masses of permanent tissues and gives rise to secondary permanent tissues.
Permanent Tissues are composed of cells that have lost the power o dividing, having attained their definite form and size. They may be living or dead and thin walled or thick walled. Permanent tissues are formed by differentiation of the cells of the meristems (apical and lateral) and may be primary and secondary.
The primary permanent tissues are derived from the apical meristems of growing region and the secondary permanent tissues from lateral meristems.
(1) Parencnhyma
It is kind of simple tissue. It consists of collection of cells which are more or less are equally expanded on all sides. Typical parenchymatous cells are oval, spherical or polygonal in shape. Their walls are thin and made up of cellulose. These are living. Parenchyma is of universal occurence in all soft parts of plants. Its function is storage of food material.
It manufactures sugar and starch.
(2) Collenchyma
This tissue consists of somewhat elongated cells with the corners or intercellular spaces much thickened with a deposit of cellulose and pectin. In T.S of stem cells look circular or oval. Their walls are provided with simple pits. Collenchyma occurs in few layers under the skin (epidermis) of herbaceous dicotyledons e.g.: sunflower and gourd. It is absent from root and monocots. The cells are living and contain some chloroplasts. Being flexible in nature collenchyma gives tensile strength to the stem. It manufactures sugar and starch. Its functions are mechanical and vital.
(3) Schlerenchyma
Scleros = hard.
Schlerenchyma consists of very long, narrow, thick walled and lignified cells pointed at both ends. They are fibre like and are called schlerenchymatous fibres or simply fibres. They have simple often oblique pits in their walls. These cells are many in number in plants and occur in patches. They are dead cells and serves as mechanical function and give strength and rigidity to plant body and thus enable it to withstand various strains. Their average length is 1 to 3 mm but in fibre yielding plants such as hemp, Indian hemp, Decan hemp, jute. Such fibres are of commercial importance. Sometimes here and there in the plant body special types of sclerenchyma may be developed. These are known as stone or sclerotic cells. Cells are thick walled and are irregular in shape or slightly elongated. Some cells occur in hard seeds, nuts and stony fruits.
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