Tissue is group cells of the same type or of mixed type having common origin and performing an identical function. Tissues may be meristematic and permanent.
A simple tissue is made up of one type of cells forming homogenous or wniform mass.
Simple tissue is of three kinds i.e. Parenchyma, Collenchyma and Sclerenchyma.
(1) Parenchyma
Parenchyma consists of a collection of cells which are more or less equally expanded on all sides. Typical parenchymatous cells are oval, spherical or polygonal in shape. Their cells are thin and made of cellulose. They are usually living. Parenchyma is of universal occurance in all soft parts of plants. Its function is mainly storage of food material.
Parenchyma containing chloroplasts often called chlorenchyma, manufactures sugar and starch. Star like parenchyma with radiating arms leaving a lot of air cavities is called acrenchyma, as in the petiole of banana and canna and also in many aquatic plants.
(2) Collenchyma
This tissue consists of somewhat elongated cells with corners or intercellular spaces much thickened with a deposit of cellulose and pectin. In T.S of stem the 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, gourd etc. It is absent from the root and monocotyledons except in special cases. The cells are living and often contain some chloroplasts. Being flexible in nature collenchyma gives tensile strength to the stem. Containing chloroplasts it also manufactures sugar and starch. Its functions are both mechanical and vital.
(3) Schlerenchyma
scleros = hard.
Schlerenchyma consists of very long, narrow, thick walled and lignified cells, usually pointed at both ends. They are fibre like in appearance and hence they are also called schlerenchymatous fibres or simply fibres. They have simple often oblique pits in their walls. The middle Lamella is conspicuous in schlerenchyma. Schlerenchymatous cells are found abundantly in plants and occur in patches or definite layers. They are dead cells and serve purely mechanical function. They give strength and rigidity to the plant body and thus enable it to withstand some strains. Their average length is 1 to 3 mm but in fibre yielding plants such as hemp and Jute these cells may be of excessive length ranging from 20 mm to 550 mm. Such fibres are of commercial importance.
Sometimes here and there in the plant body special types of schlerenchyma may be developed. These are known as stone or sclerotic cells. The cells are vary thick walled and strongly lignified and are mostly irregular in shape or slightly elongated. Stone cells occur in hard seeds, nuts and stone fruits. They contribute to the firmness and hardness of the part concerned. The flesh of pear is gritty because of the presence of stone cells in it.
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