Xylem and phloem

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Xylem and phloem

  1. 1.  Vascular tissue is a complex conducting tissue, formed of more than one cell type.  Found in vascular plants.  The primary components of vascular tissue are the xylem and phloem.
  2. 2.  These two tissues transport fluid and nutrients internally.  There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium.  All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.
  3. 3.  Xylem cells are elongated cells.  Has lignified secondary thickening of the cell walls.  Xylem cells are specialized for conduction of water.  The possession of xylem defines the vascular plants or Tracheophytes.  Xylem tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops or reticulate networks.
  4. 4.  More complex tracheids with valve-like perforations called bordered pits characterise the gymnosperms.  The ferns and other pteridophytes and the gymnosperms only have xylem tracheids.  The angiosperms also have xylem vessels.
  5. 5.  Vessel members are hollow xylem cells aligned end-to-end,  Without end walls that are assembled into long continuous tubes.  The bryophytes lack true xylem cells, but their sporophytes have a water conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.
  6. 6.  The bryophytes lack true xylem cells.  However, their sporophytes have a water conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.
  7. 7.  Xylem  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.
  8. 8.  They are: Xylem tracheids (1) Xylem tracheids (2) Picture of tracheids(1), (2) Xylem tracheae (1), (2), (3 ), (4) Xylem vessel (1), (2) Xylem fibers and Xylem parenchyma
  9. 9.  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.  Xylem is commonly described as a dead, complex permanent tissue.
  10. 10.  Xylem Tracheids 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.
  11. 11.  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.
  12. 12.  Xylem Tracheae 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.
  13. 13.  Xylem Tracheae 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.
  14. 14.  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.
  15. 15.  On this basis, xylem vessels can be distinguished into five types. Annular vessels in which the secondary thickening is in the form of rings placed more or less at equal distance from each other. Spiral vessels in which the secondary thickenings are present in the form of a helix or coil.
  16. 16. Scalariform vessels in which the secondary thickenings appear in the form of cross bands resembling the steps of a ladder. Reticulate vessels in which the secondary thickenings are irregular and appear in the form of a network. Pitted vessels in which the secondary thickenings result in the formation of depressions on the primary wall called pits.
  17. 17.  Xylem Fibres 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.  Xylem Parenchyma 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.
  18. 18.  Types of Xylem  Xylem can be distinguished into two types namely Primary xylem and Secondary xylem
  19. 19.  Primary Xylem 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.
  20. 20.  Secondary Xylem 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 Animation
  21. 21.  Phloem is a specialised tissue for food conduction in higher plants.  Phloem consists of two cell types, the sieve tubes and the intimately-associated companion cells.  The sieve tube elements lack nuclei and ribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells.
  22. 22.  Sieve tubes are joined end to end with perforate end-plates between known as sieve plates, which allow transport of photosynthetic between the sieve elements.  The companion cells, connected to the sieve tubes via plasmodesmata, are responsible for loading the phloem with sugars.  The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous function known as the leptome.
  23. 23.  Phloem is a complex permanent tissue, which is specialized for the conduction of food and other organic substances.  Phloem is also a heterogenous tissue, made up of four different types of cellular elements, namely, Sieve tubes Companion cells Phloem parenchyma and Phloem fibres
  24. 24.  Of these, the sieve tubes and the companion cells are directly involved in the translocation of the organic substances.  Hence, they are commonly described as essential elements.  Phloem parenchyma and phloem fibres are described as associated elements since they play only a supporting role in the process.
  25. 25.  The sieve tubes, the companion cells and the phloem parenchyma represent the living components of the tissue while phloem fibres represent the only nonliving component of the tissue.  Phloem is commonly described as a living, complex permanent tissue.
  26. 26.  Sieve Tubes They represent the most active food conducting elements in the phloem tissue. The sieve tubes are found arranged parallel to one another from one end of the plant body to another.  Each sieve tube is formed by a series of hollow, cylindrical cells called sieve tube cells arranged one above the other. The sieve cells are separated from each other by horizontal perforated plates called sieve plates.
  27. 27. The sieve cells communicate with each other through the sieve plates. Each sieve cell has a thin cell wall, which is composed of only cellulose. The cell has a central mass of dense cytoplasm. The granular cytoplasm forms a number of projections called cytoplasmic strands extending towards the sieve plate. The nucleus is absent.
  28. 28.  Companion Cells They are more or less spindle shaped cells associated with the sieve tube cells.  Each companion cell is found attached to any one lateral surface of a sieve cell. The companion cell and the neighbouring sieve cell together represent a pair of sister cells. The companion cell has a granular cytoplasm, prominent nucleus and one or two small vacuoles.  The nucleus of the companion cell is said to be capable of exerting its influence on the adjacent sieve cells.
  29. 29.  Phloem Parenchyma Phloem parenchyma is represented by a group of living parenchyma cells that are found in-between the sieve tubes.  They are meant only for storage of organic food.  Phloem Fibres Phloem fibres are represented by the dead sclerenchyma fibres that are found in between the sieve tubes.  They are meant only for providing mechanical support.
  30. 30.  Types of Phloem Phloem can be distinguished into two types namely Primary phloem Secondary phloem
  31. 31.  Primary Phloem Primary phloem is the phloem that is formed during normal growth in the plant body.  It is a derivative of primary meristem.  It is found in both monocots and dicots.  The primary phloem is further composed of protophloem and metaphloem. The sieve tubes and the companion cells, which appear earlier during normal growth, represent protophloem, while metaphloem is represented by the sieve tubes and companion cells that appear later. However, there is no significant morphological difference between protophloem and metaphloem.
  32. 32.  Secondary Phloem Secondary phloem is the phloem that is formed during secondary growth. It is a derivative of secondary meristem. Secondary phloem is characteristic feature of only dicots.  It is also known as bast. It is also of commercial importance since it yields bast fibers. The following table summarise the characteristics of and difference between xylem and phloem
  33. 33. a) xylem cuticle b) phloem c) guard cells d) palisade parenchyma
  34. 34. a) Stems b) roots c) Leaves d) stems and leaves e) roots and leaves
  35. 35. a) center ... Center b) surface ... Surface c) surface ... Center d) center ... surface e) top ... bottom
  36. 36. a) providing a site for photosynthesis b) providing cells for secondary growth insulation and waterproofing c) providing cells for primary growth d) regulating the opening and closing of stomata
  37. 37. a) by the differentiation of secondary xylem b) by the differentiation of secondary phloem c) by the differentiation of cork by the division of its cells d) by the differentiation of apical meristem
  38. 38. The end

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