Supporting Tissues in Plants, Their Mechanisms and Their Functions

SUPPORTING TISSUES IN PLANTS


[Photo credit: Emmanuel]

Plants generally are known to possess supporting tissues which give them definite shape, strength, rigidity and resistance against external forces such as wind and water to which they are continuously subjected.

Plants do not have to move about to obtain food. So, they do not need a highly developed skeletal system like animals. However, plants do need supporting tissues to remain upright, spread out their branches and hold their leaves in the best positions for trapping sunlight.

Supporting Tissues in Plants
The main supporting tissues in plants are:

• parenchyma
• collenchyma
• sclerenchyma (fibre)
• xylem (wood)

1. Parenchyma: Parenchyma is a tissue made up of living unspecialized plant cells that are roughly spherical in shape. A perenchymatous cell has a thin but fairly rigid cell wall composed mainly of cellulose, and a large vacuole containing cell sap. Cell sap is a concentrated solution and has a high osmotic pressure. It is the most common and abundant plant tissue

Location of Parenchyma

Parenchyma tissues are found in the cortex of stem, phloem, root, leaf, mesophyll, storage tissues and xylem.

Functions of Parenchyma

1. Parenchyma tissues can store food and water.
2. They are found in the leaf and mesophyll, functioning mainly in the synthesis of food.
3. When the vacuoles are filled with sap, parenchyma tissues give firmness and turgidity to the stems of herbaceous plants.

2. Collenchyma: This tissue consists of living cells that are thickened at the corners by the deposition of extra cellulose, to provide support and mechanical strength. They are usually polygonal elongated cells with tapering ends.

Location of Collenchyma

Collenchyma is found in the primary tissues of the plants. It is usually found below the epidermis in stem and in the midrib of leaves.

Functions of Collenchyma

1. Collenchyma supplements the strengthening effect of parenchyma.
2. Collenchyma tissues give flexibility and resilience to plants, i.e, enables plants to bend without breaking.

3. Sclerenchyma (Fibre): This tissue is composed of two types of cells, fibres and sclereids. Both are dead cells in which the cell walls are heavily thickened with deposits of ligmin, a hard substance that provides mechanical strength. Fibres are elongated cells with tapering ends. Sclereids, unlike fibres, are not too elongated but have great strength like fibres.

Location of Sclerenchyma 

Sclerenchyma tissues are found mainly in the pericycle in the vascular tissues and cortices of roots and stems.

Functions of Sclerenchyma (Fibre)

1. Sclerenchyma gives flexibility to plants and prevent them from braking up easily.
2. Sclerenchyma tissues provide strength, rigidity, hardness and support to plants.

4. Xylem (Wood): This is the water conducting tissue which also has a strengthening function, especially in plants that undergo secondary growth. It is made up of four types of cell: tracheids, vessels, fibres, and xylem parenchyma.

1. Tracheids: These are non-living, elongated, tapering cells with thickened, lignified walls which have piths that aid the passage of water and dissolved mineral salts.
2. Vessels: These are long tabular structures that are formed by the fusion of several elongated cells stacked one on top of another.
3. Fibres: These are similar to sclerenchyma fibres. They are narrow, elongated cells with very thick walls and tapering end walls.
4. Xylem parenchyma: These are similar to the parenchyma tissues. They are composed of cells with large vacuoles.

Functions of Xylem (Wood)

1. It gives strength, shape and support to plants.
2. It is also a conducting tissue as it helps to conduct water and dissolved mineral salts from the roots to the leaves.

Closely related with the major supporting tissues are the phloem tissues.

Phloem Tissues: The phloem tissues are closely related to the major supporting tissues are the phloem tissues. They are made up of four cells: sieve tubes, phloem parenchyma, phloem fibres, and companion cells.

Location of Phloem Tissues

Phloem tissues are found within the vascular bundles of all plants.

Functions of Phloem Tissues

1. The general function of the phloem is to conduct manufactured food from their area of production to area where they are needed.
2. They also assist to provide support to the entire plants.

MECHANISMS OF SUPPORTING TISSUES IN PLANTS

Support in plants is achieved through hardness, rigidity, flexibility and resilience. These quantities are due to:

• cell wall components and structure;
• turgidity of cells; and
• arrangement of the plant supporting tissues.

Cell wall: The cell walls of plants are composed mainly of cellulose fibrils (small, very slender fibres). The spaces between fibrils are filled with pectates and hemicellulose. The adjacent cells are cemented firmly together by calcium pectate. This cementing layer is the middle lamella. 

Turgidity: Turgid parenchyma is like an inflated car tyre. Air under pressure makes inflated tyres hard and strong so that they are able to support the weight of a car. In the case of turgid parenchyma, water pressure makes the cells strong and rigid so that they can give mechanical support to the plant parts in which they are present in large numbers.

Arrangement of Supporting Tissues: Individual sclerenchyma fibres, xylem fibres and xylem vessels are strong because of their lignified walls. In stems and roots, these cells are arranged in bundles or sheets which extend down the length of these organs to form solid rods or hollow cylinders (packed with other tissues) respectively. This arrangement enhances the strength and flexibility of the supporting tissues. The interlocking end walls of the fibres also add to the strength and rigidity of the tissues.

USES OF FIBRES TO PLANTS

Sclerenchyma fibres known simply as fibres provide flexibility and strength - two of the special functions of the strengthening tissues in plants. The fibres give mechanical functions, i.e, the necessary strength, rigidity, flexibility and elasticity to the plant body and also enable it to withstand various strains. Plants like hibiscus, jute and sisal are known to contain some of the strongest plant fibres hence they are used for making cloth, sacks, mats and ropes.

FUNCTIONS OF SUPPORTING TISSUES IN PLANTS

Supporting tissues provide the following functions to plants:

1. strengthening
2. rigidity
3. protection
4. resilience and flexibility
5. distinct shape
6. distinct shape
7. conduction

Categories: Biology, Tissue and Supporting System, Transport System