The following points highlight the five phenomenon that takes place because of the chemical changes in the cell wall. The phenomenon are: 1. Lignification 2. Cutinization 3. Suberinization 4. Mineralization 5. Mucilaginous Change.
Phenomenon # 1. Lignification:
In this phenomenon lignin is the substance of impregnation. Lignin is a phenolic polymer and contains three subunits namely-coumaryl, guiacyl and sinapyl-propane. It is found in sclerenchyma, which includes fibres and sclereids, and in the tracheids and vessels of xylem. It sometimes infiltrate in response to attack of microorganisms.
Wood contains about 30% lignin. It is usually absent from growing walls or may be present in very little amounts. It is deposited over the middle lamella and primary walls that have developed secondary walls. Lignin deposits to fill up the spaces present between the macromolecules of cell walls.
Thus, it cements the wall components and forms a very strong structure. As a result, the cell wall loses the capability of plastic extension and hence growth ceases. Moreover, lignified walls form a waterproof system of xylem and resist the pathogens. It forms the mechanical cells of plants.
Phenomenon # 2. Cutinization:
In this phenomenon the impregnation of cell wall with cutin occurs. Cell wall impregnated with cutin is described as cutinised wall. Cutin is an indigestible polymer and composed of polyester of hydroxy-fatty acids. Cutin may deposit on outer surface of the cells as a separate layer.
This is cuticle and the phenomenon of its formation is cuticularization. Cuticle forms a continuous layer over the aerial parts of higher plants interrupted only by stomata and lenticels. Cutinised walls and cuticle contain the phenol – ferulic acid.
Cutin is found in the outer epidermal walls of leaves, stems and fruits. The presence of cutin is also reported from roots and root hairs. Cutinised walls may be smooth or rough.
In aerial parts, the chief function of cutin is to prevent the excessive water loss. It protects the cell against mechanical injury and from the attack of microorganisms. Cutin has high chemical stability and is preserved as such in fossil materials. This property is utilized in the identification of fossil species.
Phenomenon # 3. Suberinization:
It refers the phenomenon of impregnation of cell wall with suberin. It is found in certain specialized cell walls, e.g. cork, seed coats and the casparian strip of endodermis. It is also found in exodermis. It may occur in association with cellulose or cutin. It is polyester and contains more phenolic materials than cutin. Suberin impregnated wall layers are protective tissues.
This layer is impervious to water and gases. In cork cells suberin occurs in between the middle lamella and the inner cellulose layer of cell walls. Thus, suberin makes cork an excellent protection for the stem against excessive loss of water and mechanical injury. It makes the cork resistant to acid also.
Phenomenon # 4. Mineralization:
It refers to the phenomenon of the deposition of inorganic substances in a cell wall. The substances are anhydrides of silica and calcium salts. Silica salt is a common wall component of grasses, horsetails (Equisetum), sedges, and many species of Cyperaceae, Arecaceae, Moraceae, Aristolochiaceae and Magnoliaceae. It is also noted inside the cells.
Calcium oxalate crystals in the form needles (raphides) occur in Pistia, Arum etc. Calcium oxalate crystal in the form of star (sphaeraphides) occurs in Pistia. Calcium carbonate crystals occur in the epidermal cells of leaves of Ficus sp. (cystolith). Cystoliths are commonly noted in Moraceae and Urticaceae.
Phenomenon # 5. Mucilaginous Change:
The cell wall may undergo changes due to deposition of mucilage. It is the polymer of galactan and related to pectic compounds. They are found in seed coats (ex. Salvia sp., Ocimum sp., Plantago sp. etc.) and outer cell layers of plant bodies of many aquatic plants.
The cell walls of the families Moraceae, Malvaceae, Rhamnaceae, Thymelaeaceae and Euphorbiaceae may become mucilaginous. In seeds, the outer walls of epidermal cells become mucilaginous, e.g. Linum usitatissimum and species of Alyssum. The epidermal cells of nectaries also become mucilaginous at the time of nectar secretion.
Mucilage apparently increases the water holding capacity of the wall. It also hinders the entry of oxygen into the cell. In Sinapis arvensis, the dormancy of seed is considered to be due to a specific growth inhibitor, which is produced at low oxygen concentration. Mucilage is present in the testa and it is thought that it hinders the entry of oxygen in the seed.