Meristematic tissues
Meristos = divided. They 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:
(a) The apical meristem lies at the apex of the stem and the root and gives rise to primary permanent tissues while (b) The lateral meristem e.g.: Cambium lies among masses of permanent tissues and gives rise to secondary permanent tissues.
Permanent Tissues:
These 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.
Primary permanent simple tissues:
A simple tissue is made up of one type of cells forming a homogenous or uniform mass simple tissues are divided into following kinds:
(1) Parenchyma
Parenchyma consists of a collection of cells which are more or less iso-diametric that is equally expanded on all sides. Typical parenchymatous cells are oval, spherical or polygonal in shape. Their cells are thin and made of cellulose and are usually living. Parenchyma is universal occurance in all the 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 elongated cells with the corners or intercellular spaces much thickened with a deposit of cellulose and pectin. In a transverse section of the stem the cells look circular or oval. Their walls are provided with simple pits here and there. Collenchyma occurs in few layers under the skin (epidermis) of herbaceous dicotyledons e.g.: sunflower, gourd etc. it is absent from the root and monocotebdons 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
Schlerenchyma (scleros = hard) consists of very long, narrow, thick walled and lignified cells, usually pointed at both ends. They are fibres 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 a purely mechanical function i.e. they give strength and rigidity of the plant body and thus enable it to withstand various strains. Their average length is 1 to 3 mm but in the fibre yielding plants such as hemp (cannabis), Jute (Corchorus), these cells may be 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 iso-diametric or irregular in shape or slightly elongated. Stone cells occur in hard seeds, nuts and stony 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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