A group of cells which are similar in origin; similar in structure and similar in function are called simple permanent tissue.
Simple permanent tissues are of three types :
(i) Parenchyma: (a) Parenchyma is a simple permanent living tissue which is made up of thin-walled similar isodiametric cells,
(b) They may be oval, rounded or polygonal in outline,
(c) The cell wall is made up of cellulose,
(d) Cells may be closely packed or have small intercellular spaces for exchange of gases. Internally each cell encloses a large central vacuole and a peripheral cytoplasm containing nucleus,
(e) The adjacent parenchyma cells are connected with one another by plasmodesmata.
(f) The typical parenchyma is meant for the storage of food, slow conduction of various substances and for providing turgidity to the softer parts of the plants.
(ii) Collenchyma : (a) Collenchyma is a simple permanent tissue of refractile non-lignified living cells which possess pecto-cellulose thickenings in specific areas of their walls,
(b) The cells are often elongated. They are circular, oval or angular in transverse section. Each cell possesses a large central vacuole and a peripheral cytoplasm. Wall possesses uneven longitudinal thickenings in specific areas.
(iii) Sclerenchyma : (a) Sclerenchyma is a simple supportive tissue of highly thick-walled cells with little or no protoplasm,
(b) The cell cavities are narrow. The thickening of the wall is made up of cellulose or lignin or both,
(c) Sclerenchyma is of two types- Sclerenchyma fibres and Sclereids.
(i) Sclerenchyma fibres : The sclerenchyma fibres are highly elongated, narrow and spindle-shaped thick-walled cells with pointed or oblique end walls. The adjacent fibres possess simple oblique pits. Sclerenchyma fibres constitute the major mechanical tissue of the plants because they can bear compression, pull, bending and shearing.
(ii) Sclereids : They are highly thickened dead sclerenchyma cells with very narrow cavities. Sclereids are broader as compared to the fibres being isodiametric polyhedral, short or cylindrical.
