- Permanent tissues are made up of mature cells that have undergone growth and differentiation.
- These tissues originate from meristematic tissues and become permanent at fixed positions in the plant body.
- The cells possess definite shape, size and function and normally have lost the power of division.
- Under special circumstances, these cells may regain their power of division and differentiation.
- Permanent tissues are further of two types:
- Simple permanent tissues
- Complex permanent tissues
Simple permanent tissues:
- They are composed of similar type of cells which have common origin and function.
- They are further grouped under three categories:
- Parenchyma
- Collenchyma
- Sclerenchyma
Parenchyma:
- The simplest and unspecialized tissue which is concerned mainly with the ordinary vegetative activities of the plant.
- The tissue consists of cells that may be spherical, oval, cylindrical, rectangular, stellate or long spindle like.
- The cells are living and consist of thin walls with intercellular spaces between them.
- The cell wall is made up of cellulose or calcium pectate.
- Each cell possesses a prominent nucleus and vacuolated cytoplasm.
- Parenchyma is distributed in almost all the parts of the plant body like epidermis, cortex, pith, pericycle, mesophyll of leaves, pulp of fruits, endosperm of seeds etc.
Functions of parenchyma:
- Some parenchyma cells containing chloroplast are called chlorenchyma which helps in synthesis and storage of food.
- Parenchyma also fills the space between other tissues and maintains the shape and firmness of the plant due to its turgidity.
- Aerenchyma, a type of parenchyma cells in which the cells are widely separated from one another by air spaces provide buoyancy or floatation in hydrophytes and also help in storage of oxygen for respiration.
- Storage of water in succulents like cactus
- They give rise to secondary meristem in the form of cork cambium and vascular cambium and also help in repair and replacement after injury.
Collenchyma:
- The cells of this tissue contain protoplasm and are living.
- They are generally elongated with oblique or rounded end walls.
- The cell walls show localized thickenings due to the presence of approximately 45% pectin, 35% hemicellulose and 20% cellulose.
- The cells are compact and intercellular spaces may or may not be present.
- These cells have a higher water absorbing capacity because of cellulose and pectin.
- The tissue is elastic, extensible and has capacity to expand and gives a tensile strength to the organ.
- Collenchyma occurs chiefly in the hypodermis of dicotyledonous stems and leaves (leaf veins) and is generally absent in monocots and roots.
Functions of collenchyma:
- It is the chief supporting tissue in young dicotyledonous stems.
- The tissue has capacity to expand and gives a tensile strength to the body.
- It is present at the margins of some leaves and resists tearing effect of the wind.
- Some collenchymatous cells possess chloroplasts and perform photosynthesis.
Sclerenchyma:
- Sclerenchyma consists of thick-walled dead cells.
- The cells vary in shape, size and origin.
- They possess hard and extremely thick secondary walls due to the uniform deposition of lignin and sometimes may be unlignified.
- In the beginning, the cells are living and have protoplast but later on become dead due to deposition of impermeable secondary walls.
- Sclerenchyma can broadly be divided into two main types:
- Fibers
- Sclereids
Fibers:
- They are sclerenchymatous tissues with long elongated cells having pointed ends.
- The cells are thick walled and dead. Thickening of walls is due to the deposition of lignin.
- The lumen inside the cells becomes narrow due to heavy deposition of secondary walls.
- The walls show simple and oblique pits.
- They mostly occur in hypodermis, pericycle, secondary xylem and secondary phloem. They may also occur in cortex and pith.
Sclereids:
- This tissue consists of cells which are short (usually shorter than fibers) and possess extremely thick lamellated lignified walls with long tubular simple pits.
- They originate from simple parenchyma cells by the deposition of secondary wall layers.
- The sclereids may be simple or branched and vary greatly in shape and size.
- They may be spherical, oval, cylindrical, T-shaped, dumbbell-shaped or even stellate.’
- They occur in harder parts of the plant body like endocarp of almond and coconut, hard seed coats, cortex and pith, pulp of some fruits as stone cells.
Types of sclereids:
- Brachy-sclereids or stone cells:
- These are small and more or less isodiametric in shape.
- They occur in the cortex, pith, phloem and pulp of fruits.
- Macro-sclereids or rod cells:
- These are rod shaped, elongated sclereids usually found in the leaves, cortex of stem and outer seed coats.
- Osteosclereids or bone cells:
- These are bone or barrel-shaped sclereids dilated at their ends. E.g. leaves of Hakea
- Astro-sclereids or stellate cells:
- These are star shaped sclereids with extreme lobes or arms. E.g. leaf of Nymphaea
- Tricho-sclereids or internal hairs:
- These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some hydrophytes.
Functions of sclerenchyma:
- They provide mechanical support and hardness to the plant parts.