All plant cells are covered with a rigid outer covering called the cell wall. In young cell it is thin elastic, transparent and colourless and is formed of protoplasm. Cell wall is non living part of cell. It maintains the shape of cell protects the protoplasm from external injuries and is permeable to water. It is made up of cellulose.
The cell wall consists of (1) Middle lamella (2) Primary wall and (3) Secondary wall and (4) Tertiary wall.
(1) MIDDLE LAMELLA
It is outmost layer of plant cell wall. It has two adjacent cells and is also called intercellular substance. It is made up of pectin substance and can be dissolved by strong acids.
(2) PRIMARY WALL
It consists of layers present on either side of middle lamella. It is thin and merges with secondary wall.
(3) SECONDARY WALL
It lies below primary wall. It is much thicker than primary wall and consists of several layers. It forms the main bulk of mature cell wall and helps in maintaining cell shape. It gives mechanical strength. It is made up of cellulose.
(4) TERTIARY WALL
It lies inner to secondary wall. This is a dried up residue of degenerated plasma lining and is rarely present.
In meristematic tissues the cells are compact. Their walls lose contact with advancement of differentiation. It results in the formation of intercellular spaces.
Secondary thickening is uniform in all round the cell. Thickening takes place only after the cell has attained its full size. Thickenings are as under:
(1) Annular or ring like: In this case the rings of lignin are deposited one above the other in the interior of the original cell wall.
(2) Spiral: The thickening takes place in the form of spiral bands.
(3) Scalari form or ladder like: The lignin is deposited in the form of steps of a ladder.
(4) Reticulate or netted type: The thickening is irregular and a network is formed.
(5) Pitted: Entire inner surface of the cell wall is thickened leaving small un-thickened areas here and there called pits. Pits are present in pairs lying against each other on opposite sides of the wall. The portion of the original cell wall that separates two pits is called closing membrane or pit membrane. Pits are simple pits and bordered pits.
Chemical changes in the cell wall
(1) Lignification: Deposition of lignin on the cell wall or the conversion of cellulose into lignin is called lignification. Lignin makes the cell hard, thick walled and dead. The wall still remains permeable but serves mechanical purpose e.g. fibres of xylem, phloem, tracheids and vessels.
(2) Cutinization: Transference of cellulose to cutting is called cutinisation. It forms definite impermeable membrane on the cell wall known as cuticle.
(3) Suberization: In the walls of cork cells deposition of suberin is called suberization. Suberin makes the cell wall impervious to water and prevents its evaporation.
(4) Mucilagenous changes: The cellulose is sometimes changed to mucilage which can absorb and retain water to form a viscous mass. On drying it becomes very hard. It is found in the leaves of Indian aloe, flowers of China rose, fruits of lady’s finger and seeds of linseed.
(5) Mineralization: It takes place in the cell wall either by infiltration or by deposition of inorganic salts on the walls; the main minerals being silica and oxalate and carbonates of calcium. Silica is found in superficial cell walls of grasses and sedges and firm ness to the cell wall by silicification and calcification cell wall becomes harder and brittle. In some plants like Indian rubber tree the crystals of calcium carbonate are deposited in the form of a bunch and are called cryslalith.
ULTRA STRUCTURE OF CELL WALL
Under the electron microscope the cell wall shows two parts (i) Matrix, the non cellulosic part, (ii) fibrils embedded in the matrix and made up of cellulose. The largest fibril could be seen by a light microscope and is called macrofibril; while the smaller ones are seen under the electron microscope and are called microfibrils.
With the increase in resolving power, smaller and smaller fibrils are visualized as subunits of microfibrils. These are called micelles which in turn are further divided to form cellulose molecules. Microfibrils display a dense textile like pattern in electron microscope preparation.
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