Question
What are permanent tissues? Describe the different types of simple permanent tissues.
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Answer
Permanent tissues are those in which, growth has stopped either completely or for the time being. At times, they become meristematic partially or wholly. Permanent tissues are of two types namely
Simple tissue are homogeneous-composed of structurally and functionally similar cells eg., Parenchyma, Collenchyma, and Sclerenchyma.
$i)$ Parenchyma
Types of Prenchyma
Parenchyma are simple permanent tissue composed of living cells. Parenchyma cells are thin-walled, oval, rounded or polygonal in shape with well-developed spaces among them. In aquatic plants. Parenchyma possesses intercellular air spaces and is named as Aerenchyma. When exposed to light, parenchyma cells may develop chloroplasts and are known as Chlorenchyma.
Functions: Parenchyma may store water in many succulent and xerophytic plants. It also serves the functions of storage of food reserves, absorption, buoyancy, secretion etc.
$ii)$ Collenchyma
Collenchyma is a living tissue found beneath the epidermis.
Cells are elongated with unevenly thickened non-lignified walls.
Cells have rectangular oblique or tapering ends and persistent protoplasts. They possess thick primary non-lignified walls.
Functions: They provide mechanical support for growing organs.
$iii)$ Sclerenchyma
Sclerenchyma consists of thick-walled cells which are often lignified. Sclerenchyma cells do not possess living protoplasts at maturity.
Sclerenchyma cells are grouped into
- simple tissue and
- complex tissue.
Simple tissue are homogeneous-composed of structurally and functionally similar cells eg., Parenchyma, Collenchyma, and Sclerenchyma.
$i)$ Parenchyma
Types of Prenchyma
Parenchyma are simple permanent tissue composed of living cells. Parenchyma cells are thin-walled, oval, rounded or polygonal in shape with well-developed spaces among them. In aquatic plants. Parenchyma possesses intercellular air spaces and is named as Aerenchyma. When exposed to light, parenchyma cells may develop chloroplasts and are known as Chlorenchyma.
Functions: Parenchyma may store water in many succulent and xerophytic plants. It also serves the functions of storage of food reserves, absorption, buoyancy, secretion etc.
$ii)$ Collenchyma
Collenchyma is a living tissue found beneath the epidermis.
Cells are elongated with unevenly thickened non-lignified walls.
Cells have rectangular oblique or tapering ends and persistent protoplasts. They possess thick primary non-lignified walls.
Functions: They provide mechanical support for growing organs.
$iii)$ Sclerenchyma
Sclerenchyma consists of thick-walled cells which are often lignified. Sclerenchyma cells do not possess living protoplasts at maturity.
Sclerenchyma cells are grouped into
- fibres and
- Sclereids.
- Fibres are elongated sclerenchymatous cells, usually with pointed ends. Their walls are lignified. Fibres are abundantly found in many plants. The average length of fibres is $1$ to $3$ mm, however in plants like Linum usitatissimum (flax), Cannabis sativa (hemp) and Corchorus capsularis (jute), fibres are extensively longer ranging from $20$ mm to $550$ mm.
- Sclereids are widely distributed in plant body. They are usually broad, may occur in single or in groups. Sclereids are isodiametric, with liginified walls. Pits are prominent and seen along the walls. Lumen is filled with wall materials. Sclereids are also common in fruits and seeds.
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