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Monday, 17 December 2018

Mechanism of Support

Support in plants is achieved through hardness, rigidity (firmness), flexibility and resilience. These qualities are due to

• Cell wall components and structure
• Turgidity of cells
• Arrangements 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 pectates. This cementing layer is the middle lamella.

   In a young plant cell, the cellulose fibrils are laid down randomly to form the primary cell wall which can stretch ad the cell grows. More cellulose fibrils are laid down on the inside of the primary wall as the cell grows, to form the secondary wall. In the secondary wall, the cellulose fibril are more closely packed and arranged in an orderly manner in each layer. The different layers of fibrils in the secondary wall are orientated at different angles. This structure makes the cell wall fought, relatively rigid and slightly elastic.

   In collenchyma, the thickening of the corners of the cell walls with Cellulose give the cells added strength and rigidity or firmness. the cells are also flexible and extensive , enabling the stems in which they occur to grow. In sclerenchyma and xylem, the cellulose cell wall becomes thickened with lignim, a hard substance which can withstand a considerable amount of stress and compression without breaking or buckling. The cell walls of these tissues give strength , flexibility and resilience to the plant organs.

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 of hollow cylinders (packed with other tissues) respectively. This arrangement enhances the strength and flexibility of the supporting tissues. The interlocking end wall of the fibres also add to the strength and rigidity of the tissues.

     Like fibres, individual sclereids are strong because of their lignified walls. In groups, sclereids give firmness to various parts of the plant. I. Seed coats, shells of nuts and stones of fruits, they form solid layers which give hardness and strength to these structures.
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