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| Xylem |
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| It is a complex permanent tissue, which is specialized for the conduction of water and mineral substances in the plant body. Xylem is a heterogenous tissue made up of four different types of cellular elements. They are: |
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 Xylem tracheids |
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 Xylem tracheae |
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 Xylem fibers and |
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 Xylem parenchyma |
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| Of these, the tracheids and the tracheae are described as essential elements since they are directly involved in the translocation of water and mineral substances. Xylem fibers and xylem parenchyma are described as associated elements, since they are only supporting structures. The tracheids, the trachea and the xylem fibers are non-living components, while xylem parenchyma represents the only living component of the tissue. |
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| Xylem is commonly described as a dead, complex permanent tissue. |
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| They are found abundantly in pteridophytes, gymnosperms and primitive angiosperms. In these groups of plants, the tracheids represent the most active water conducting elements. In advanced angiosperms, the tracheids are found restricted to leaf margin and leaf tip. |
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| The tracheids are elongated, dead cells, with tapering ends. They are characterised by the presence of a thick cell wall consisting of primary wall and a secondary wall. The primary wall is composed of cellulose where as the secondary wall is made up of lignin. There is a spacious lumen that extends throughout the length of the tracheid. In some cases, due to the deposition of lignin, the primary wall develops numerous concave depressions called pits. When pits are present, the tracheid is described as pitted and when pits are absent, it is described as simple. |
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| Xylem Tracheid |
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| They are commonly known as xylem vessels. They are the most active water conducting elements in all higher angiosperms. The tracheae are found arranged parallel to each other, extending from one end of the plant body to another. |
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| The tracheae are long cylindrical dead cells. They are characterised by a thick cell wall consisting of a primary wall and a secondary wall. The primary wall is made up of cellulose where as the secondary wall is made up of lignin. There is a spacious lumen that extends throughout the length of the trachea. The deposition of lignin in the secondary wall is not always uniform. As a result, the xylem vessels exhibit different types of secondary thickenings. On this basis, xylem vessels can be distinguished into five types. |
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| Xylem Tracheae, vessel element in LS |
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 Annular vessels in which the secondary thickening is in the form of rings placed more or less at equal distance from each other. |
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 Spiral vessels in which the secondary thickenings are present in the form of a helix or coil. |
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 Scalariform vessels in which the secondary thickenings appear in the form of cross bands resembling the steps of a ladder. |
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 Reticulate vessels in which the secondary thickenings are irregular and appear in the form of a network. |
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 Pitted vessels in which the secondary thickenings result in the formation of depressions on the primary wall called pits. |
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| Xylem Tracheid |
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| B-G Tracheids in LS showing different types of thickenings |
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| They are represented by the dead sclerenchyma fibers that are found in between the vessels and the tracheids. They are meant for providing mechanical support to the essential elements. |
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| This is the only living component in the xylem tissue. It is represented by groups of parenchyma cells that are found in between the vessels and the fibers. They are meant for storage of reserve food. |
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| Xylem can be distinguished into two types namely |
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 Primary xylem and |
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 Secondary xylem |
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| Primary Xylem |
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| Primary xylem is the xylem that is formed during normal growth. It is a derivative of primary meristem. It occurs in both monocots and dicots. In the primary xylem, two types of xylem vessels can be distinguished, namely protoxylem and metaxylem. |
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| Secondary Xylem |
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| Secondary xylem is the xylem that is formed during secondary growth. It is derivative of secondary meristem. It is a characteristic feature of only dicots. Secondary xylem is commonly known as wood. It is of commercial importance since it is extensively used in the manufacturing of doors, windows and furniture. |
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