Permanent Tissues in Plants

1.0Permanent Tissue

They are formed by division and differentiation of meristematic tissue.

They are composed of those cells which have lost the power of division (temporarily or permanent) and attain a permanent shape, size and function. Cells may be living or dead.

Permanent tissues are of two types, simple permanent tissue and compound or complex permanent tissue.

Note: Differentiation is the development process in which cells take up a permanent shape, size and perform a specific function.

2.0Simple Permanent Tissue

These tissues are made up of similar types of cells, that perform a common function. They are protective and supportive in nature. Simple tissues are of three types –

(i) Parenchyma

(ii) Collenchyma

(iii) Sclerenchyma

Parenchyma

It is a living and basic packing tissue which consists of relatively unspecialised cells.

Cells of this tissue have thin cell wall which is made up of cellulose. Cells of this tissue have cytoplasm with small nucleus and large vacuole. The cells may either be closely packed or may have intercellular spaces. 

Parenchyma is the first evolved permanent tissue which is present in all soft parts of plant (therefore called as universal tissue).

Functions

This tissue provides support to the plant and help in storage of food.

Modification of Parenchyma

Chlorenchyma

Chlorenchyma is the type of parenchyma in which abundant quantity of chloroplasts (containing chlorophyll) are found.

They are present in mesophyll of leaves.

Functions

Synthesis of food (Photosynthesis). 

Aerenchyma

Parenchyma is made up of rounded cells which surrounds the large air cavities. It is found in aquatic plants or hydrophytes.

Function

It provides buoyancy to the aquatic plants to help them float.

Collenchyma (Flexible tissue)

Cells of this tissue are living, elongated or vary in structure.

Cells of this tissue are irregularly thickened at the corners due to the deposition of pectin. It is present below the epidermis in stem of some plants and also present at leaf stalk and leaf margin.

Intercellular spaces are usually very little between cells of this tissue.                                                             

Functions 

It provides mechanical support (tensile strength) and elasticity.

It allows easy bending in various parts of plant (tendrils and stems of climbers) without breaking. It provides flexibility to plant.

Sclerenchyma 

Sclerenchyma cells are dead, narrow and long cells and they are devoid of protoplasm. The walls of cells of sclerenchyma are greatly thickened with deposition of lignin. Such cell walls are called lignified. Lignin is a complex polymer which acts as a cement and hardens cell wall. Lignin makes the cell wall impermeable, so important substances are unable to pass through it. The cells of sclerenchyma are closely packed without intercellular spaces.

They are found in stems of some plants (around the vascular bundle), orchid roots, veins of leaves, husk of coconut, hard coverings of seed and nuts.

Functions

It is the main mechanical tissue which provides mechanical support. It makes the plant hard and stiff.

Sclerenchymatous cells are of two types in structure

1. Sclerenchyma fibres: They constitute the major mechanical tissue of the plants and are abundantly found in plants. Commercial fibres obtained from plants (e.g. jute, flax, hemp) usually are sclerenchymatous fibres. Husk of coconut is made up of sclerenchymatous tissue.
2. Sclereids: They are highly thickened and irregularly shaped dead cells. They are found in various parts of the plant such as cortex, pith, phloem, hard seed coats. Sclereids provide strength to seed covering and grittiness to the pulp of many fruits, such as guava, apple and pear.

3.0Comparison between Parenchyma, Collenchyma and Sclerenchyma

4.0Compound or Complex Permanent Tissue

The complex tissues consist of more than one type of cells. All these cell coordinate to perform a common function. Complex tissues are of the following two types:

(i)  Xylem or wood

(ii)  Phloem or bast

Vascular bundles consist of xylem and phloem and both are known as conducting tissue. They transport water, mineral and food materials respectively from one part to another part of the plant body.

Xylem or wood

Xylem is made up of four types of cells:

(d) Xylem parenchyma - Living element 

(a) Tracheids: Tracheids are elongated cells with tapering ends. Since tracheids do not have open ends like the vessels, so the water has to pass from cell to cell via the pits. Tracheids are the chief water conducting elements in non flowering plants. i.e. gymnosperms.

(b) Vessels or tracheae: Very long tube like structures formed by a row of cells placed end to end. Vessels are present in flowering plants i.e. angiosperms only.

The transverse walls between the vessels are completely dissolved to form continuous channels or water-pipes. 

Tracheids and vessels help in long distance conduction of water and minerals vertically upward from the root system to various parts of plant.

Tracheids and vessels also provide mechanical support.

(c) Xylem fibre: These are dead and lignified sclerenchymatous cells which are mainly supportive in function.

(d) Xylem parenchyma: It is formed of living parenchymatous cells which helps in storage of food and radial conduction of water and minerals.

Phloem

It is also made up of four types of cells

(d) Phloem fibres - dead element 

(a) Sieve tubes: Sieve tubes are slender, tube like structures composed of elongated thin-walled cells, placed end to end.

Their end walls are perforated by numerous pores and are called sieve plates. Wall of sieve tube is perforated. The nucleus of each sieve tube degenerates at maturity, however cytoplasm persists in the mature sieve tube. Thus, nuclei are absent in mature sieve tube elements. 

(b) Companion cells: These are associated with sieve tubes. These are smaller cells having dense cytoplasm and prominent nucleus. The companion cells help the sieve tubes in the conduction of food material.

(c) Phloem parenchyma: These are living and thin walled cells which helps in sideways conduction of food. It stores various materials like resin, latex and mucilage.

(d) Phloem fibre: Phloem fibres are dead sclerenchymatous cells. Phloem or bast fibres of some plants are source of commercial fibres e.g. Jute, Hemp, Flax.

Function of phloem

Phloem transport photosynthetically prepared food materials in both direction from the leaves to the storage organs and later when required from storage organs to the growing regions of the plant body.

5.0Differences between Xylem and Phloem

Vascular bundles consist of xylem and phloem and both are known as conducting tissue.

6.0Differences between Meristematic tissue and Permanent tissue