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Monday, 17 December 2018

Main Features of Supporting Tissues

Turgid parenchyma : parenchyma is a tissue made up of living unspecialized plant cell that are roughly spherical in shape. A parenchymatous 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. Water (absorbed by the roots) enters the cell and passes into the vacuole by osmosis. The cell vacuole increase in volume causing the cytoplasm to press against the cell wall. The cell wall only expands slightly and resists the internal water pressure. This is known as turgor pressure. Eventually, turgor pressure builds high enough to stop the flow of water into the cell ( although the cell sap may still be the cell). Such a cell is said to be fully turgid.

   Turgid parenchymatous cells press against one another and become tightly packed, providing support in the organs where they are found. In herbaceous plants, turgid parenchyma provides the main support.

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.

    Collenchyma is found in the primary tissues of the plant. It is , therefore, an important tissue in young plants, herbaceous plants, and plants or plant organs, such as leaves, in which secondary growth does not occur. In these plants, it supplements the strengthening effect of the turgid parenchyma. Collenchyma is usually found below the epidermis in stems and in the midrib of leaves.

Sclerenchyma : This tissue is composed of two types of cells, fibres and sclereids. Both are heavily thickened with deposits of lignim, a hard substance that provides mechanical strength. (A dead cell has a cell wall enclosing a space or lumen instead of protoplasm.)

    Sclerenchyma fibres are narrow. Polygonal, elongated cells with tapering end walls. The lumen is small because of the thickened cell walls. The end walls of adjacent fibres are arranged in bundles or sheets in the outer regions of the cortex and pericycles of stems, and in vascular tissue.

   Sclereids or stone cells are heavily lignified cells which are roughly spherical and slightly elongated. They are scattered singly, in groups or in solid layers in almost all parts of the plant, especially in the cortex, pith, phloem, fruits and seed coat.

Xylem : This is the water conducting tissue which also has a strengthening function, especially in plants that undergo secondary growth. Four types of cells make up the xylem tissue: the tracheids, vessels, fibres and parenchyma. Of these, the first three are lignified and so provide mechanical support.

   Tracheids are single elongated cells with tapering end walls , which may grow to a  width of 0.03mm. They are arranged in strands just like sclerenchyma fibres. Mature tracheids are dead with empty lumens. They are the only xylem tissue in less advance vascular plants like ferns. The xylem of angiosperms contains few tracheids.

    Vessels are more abundant in the  vascular tissues of angiosperms. They are long tubular structures, that are formed by the fusion of several elongated cells stacked one on top of another. They are wider than tracheids and are dead. Xylem vessels have distinctive wall patterns, formed by the way lignim was deposited on the primary cell wall.

    Xylem fibres are similar to sclerenchyma fibres. They are narrow, elongated cells with very thick Wales and tapering end walls. Their sole function is to provide support.

    In plants where secondary growth occurs secondary xylem or wood takes over the mechanical function from sclerenchyma and collenchyma ( the main supporting tissues in the  primary plant body).
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